UNCOUPLED MULTICORE OPTICAL FIBER WITH ALKALI DOPED CORES

Abstract

Embodiments include a multicore optical fiber including: a common cladding having a refractive index .sub.CC and an outer radius of 70 m to 95 m; a coating having an outer radius of 120 m to 127.5 m; and a plurality of core portions; wherein at least one core portion comprises: an alkali-doped core region having a relative refractive index .sub.1 and a trench region having a relative refractive index .sub.3, wherein .sub.1>.sub.CC>.sub.3; an attenuation less than 0.165 dB/km, and an effective area of 75 m.sup.2 to 135 m.sup.2 at a wavelength of 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm; wherein the multicore optical fiber has a core multiplicity factor of 0.028 to 0.045; and wherein a counter-propagating crosstalk at 1550 nm between adjacent core portions is less than 40 dB per 100 km.

Claims

1. An uncoupled multicore optical fiber for counter-propagation transmission, comprising: a common cladding having a refractive index .sub.CC and an outer radius R.sub.CC greater than or equal to 70 m and less than or equal to 95 m; a coating encircling and directly contacting the common cladding and extending from the outer radius R.sub.CC to an outer radius R.sub.C that is greater than or equal to 120 m and less than or equal to 127.5 m; and a plurality of core portions disposed within the common cladding; wherein at least one core portion of the plurality of core portions comprises: a central axis; a core region extending from the central axis, the core region comprising a relative refractive index .sub.1 relative to pure silica, wherein the core region comprises an alkali dopant; a trench region encircling the core region, the trench region comprising a relative refractive index .sub.3 relative to pure silica, wherein .sub.1>.sub.CC>.sub.3; an attenuation of less than 0.165 dB/km at a wavelength of 1550 nm; an effective area greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2 at a wavelength of 1550 nm; and a cable cutoff wavelength less than or equal to 1530 nm; wherein the common cladding directly contacts the trench region; wherein the uncoupled multicore optical fiber has a core multiplicity factor ranging from about 0.028 to about 0.045; and wherein a counter-propagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to 40 dB per 100 km of the uncoupled multicore optical fiber.

2. The uncoupled multicore optical fiber of claim 1, wherein a radiation loss of the at least one core portion of the plurality of core portions is less than 0.01 dB/km at a wavelength of 1550 nm.

3. The uncoupled multicore optical fiber of claim 1, wherein the coating comprises: a primary coating encircling and directly contacting the common cladding and extending from the outer radius R.sub.CC to an outer radius R.sub.Cp; and a secondary coating encircling and directly contacting the primary coating extending from the outer radius R.sub.Cp to an outer radius R.sub.Cs.

4. The uncoupled multicore optical fiber of claim 3, wherein a ratio of a thickness of the secondary coating to a thickness of the primary coating ranges from about 0.7 to about 1.3, or from about 0.8 to about 1.2.

5. The uncoupled multicore optical fiber of claim 3, wherein at least one of a thickness of the primary coating or a thickness of the secondary coating is greater than or equal to 14 m and less than or equal to 35 m.

6. The uncoupled multicore optical fiber of claim 3, wherein the in-situ modulus of the primary coating is greater than or equal to 0.15 MPa and less than or equal to 0.4 MPa, and wherein the in-situ modulus of the secondary coating is greater than or equal to 1,600 MPa and less than or equal to 2,400 MPa.

7. The uncoupled multicore optical fiber of claim 1, wherein the outer radius R.sub.CC is greater than or equal to 70 m and less than or equal to 90 m, or greater than or equal to 75 m and less than or equal to 90 m.

8. The uncoupled multicore optical fiber of claim 1, wherein the plurality of core portions comprises at least 6 core portions, or 6 to 10 core portions, or 6 to 8 core portions.

9. The uncoupled multicore optical fiber of claim 1, wherein the trench region has a trench volume greater than or equal to 5% micron.sup.2 and less than or equal to 60% micron.sup.2.

10. The uncoupled multicore optical fiber of claim 1, wherein the effective area of the at least one core portion of the plurality of core portions is greater than or equal to 100 m.sup.2 and less than or equal to 125 m.sup.2 at a wavelength of 1550 nm.

11. The uncoupled multicore optical fiber of claim 1, wherein a counter-propagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to 50 dB per 100 km of the uncoupled multicore optical fiber.

12. The uncoupled multicore optical fiber of claim 1, wherein the dispersion at 1550 nm of the at least one core portion of the plurality of core portions is greater than or equal to 18 ps/nm/km and less than or equal to 22 ps/nm/km.

13. The uncoupled multicore optical fiber of claim 1, wherein central axes of adjacent core portions of the plurality of core portions are separated from one another by a minimum separation distance that is greater than or equal to 32 m and less than or equal to 50 m, or greater than or equal to 34 m and less than or equal to 45 m, or greater than or equal to 34 m and less than or equal to 40 m.

14. The uncoupled multicore optical fiber of claim 1, wherein the at least one core portion of the plurality of core portions is configured for transmission over at least one of C band or L band.

15. A bidirectional transmission system, comprising: an uncoupled multicore optical fiber of claim 1; a first transceiver optically coupled to a first end of the uncoupled multicore optical fiber; and a second transceiver optically coupled to a second end of the uncoupled multicore optical fiber opposite the first end of the uncoupled multicore optical fiber; wherein the first transceiver and the second transceiver are configured to transmit signals from the first end of the uncoupled multicore optical fiber to the second end of the uncoupled multicore optical fiber via a first plurality of core portions of the uncoupled multicore optical fiber; wherein the first transceiver and the second transceiver are further configured to transmit signals from the second end of the uncoupled multicore optical fiber to the first end of the uncoupled multicore optical fiber via a second plurality of core portions of the uncoupled multicore optical fiber; and wherein the first plurality of core portions of the uncoupled multicore optical fiber is different from the second plurality of core portions of the uncoupled multicore optical fiber.

16. The bidirectional transmission system of claim 15, wherein at least one of the first plurality of core portions or the second plurality of core portions is configured for transmitting signals over C band and L band simultaneously.

17. A method of bidirectional transmission over a multicore optical fiber having a first plurality of core portions and a second plurality of core portions, the method comprising: transmitting a first optical signal over at least one core portion of the first plurality of core portions in a first direction, wherein at least one core portion of the first plurality of core portions comprises: a core region comprising an alkali dopant; and an attenuation of less than 0.165 dB/km at a wavelength of 1550 nm; transmitting a second optical signal over at least one core portion of the second plurality of core portions in a second direction opposite the first direction, wherein at least one core portion of the second plurality of core portions comprises: a core region comprising an alkali dopant; and an attenuation of less than 0.165 dB/km at a wavelength of 1550 nm; wherein the first plurality of core portions and the second plurality of core portions are disposed within a common cladding having an outer radius R.sub.CC greater than or equal to 70 m and less than or equal to 95 m; wherein the multicore optical fiber further comprises a coating encircling and directly contacting the common cladding, wherein an outer radius R.sub.C of the coating is greater than or equal to 120 m and less than or equal to 127.5 m; wherein the multicore optical fiber has a core multiplicity factor ranging from about 0.028 to about 0.045; and wherein a counter-propagating crosstalk at 1550 nm between two adjacent core portions is less than or equal to 40 dB per 100 km of the uncoupled multicore optical fiber.

18. The method of claim 17, further comprising at least one of: transmitting a third optical signal over the at least one core portion of the first plurality of core portions in the first direction; or transmitting a fourth optical signal over the at least one core portion of the second plurality of core portions in the second direction; wherein: one of the first optical signal and the third optical signal is transmitted over C band; the other one of the first optical signal and the third optical signal is transmitted over L band; one of the second optical signal and the fourth optical signal is transmitted over C band; and the other one of the second optical signal and the fourth optical signal is transmitted over L band.

19. The method of claim 17, wherein the first plurality of core portions and the second plurality of core portions are arranged in an alternating manner.

20. The method of claim 17, wherein at least one of the first plurality of core portions or the second plurality of core portions comprises at least 3 core portions, or 3 to 5 core portions, or 3 to 4 core portions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the detailed description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures.

[0010] FIG. 1 schematically depicts a simplified diagram of an exemplary optical system for bidirectional transmission over an uncoupled-core multicore optical fiber, according to some embodiments.

[0011] FIG. 2 schematically illustrates a radial cross section of an exemplary multicore optical fiber viewed along line II-II of FIG. 1.

[0012] FIG. 3 schematically depicts a radial cross section of an exemplary core portion of a multicore optical fiber viewed along line II-II of FIG. 1, according to some embodiments.

[0013] FIG. 4 depicts an exemplary relative refractive index profile of a core portion and surrounding common cladding of a multicore optical fiber, according to some embodiments.

[0014] FIG. 5 depicts another exemplary relative refractive index profile of a core portion and surrounding common cladding of a multicore optical fiber, according to some embodiments.

[0015] FIG. 6 schematically depicts a radial cross section of another exemplary core portion of a multicore optical fiber viewed along line II-II of FIG. 1, according to some embodiments.

[0016] FIG. 7 depicts another exemplary relative refractive index profile of a core portion and surrounding common cladding of a multicore optical fiber, according to some embodiments.

[0017] FIG. 8 illustrates impact of crosstalk on SNR performance of bidirectional (or counterpropagating) transmission, according to some embodiments.

[0018] FIG. 9 illustrates inter-core crosstalk for co-propagating transmission and inter-core crosstalk for counter-propagating transmission, according to some embodiments.

[0019] FIG. 10 illustrates the counter-propagating crosstalk for exemplary core portion designs, according to some embodiments.

[0020] FIG. 11 schematically illustrates a radial cross section of another exemplary multicore optical fiber.

[0021] FIG. 12 schematically illustrates a radial cross section of another exemplary multicore optical fiber.

[0022] FIG. 13 schematically illustrates a system for measuring co-propagating crosstalk and counter-propagating crosstalk, according to some embodiments.

DETAILED DESCRIPTION

[0023] Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. The claims as set forth below are incorporated into and constitute part of this detailed description.

[0024] In this document, relational terms, such as first and second, top and bottom, and the like, are used to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions.

[0025] It will be understood by one having ordinary skill in the art that construction of the described apparatus and/or components is not limited to any specific material. Exemplary embodiments disclosed herein may be formed from a wide variety of materials, unless described otherwise herein.

[0026] In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:

[0027] As used herein, the term about means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. When the term about is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to. Whether or not a numerical value or end-point of a range in the specification recites about, the numerical value or end-point of a range is intended to include two embodiments: one modified by about, and one not modified by about. It will be further understood that the end-points of each of the ranges are significant both in relation to the other end-point, and independently of the other end-point.

[0028] A multicore optical fiber, also referred to as a multicore fiber or MCF, is considered for the purposes of the present disclosure to include two or more core portions disposed within a common cladding. Each core portion can be considered as having a higher index core region surrounded by one or more lower index cladding regions. As used herein, the term inner core portion refers to the higher index core region.

[0029] Radial position and/or radial distance, when used in reference to the radial coordinate r refers to radial position relative to the centerline (r=0) of each individual core portion in a multicore optical fiber. Radial position and/or radial distance, when used in reference to the radial coordinate R refers to radial position relative to the centerline (R=0, central fiber axis) of the multicore optical fiber.

[0030] The length dimension micrometer may be referred to herein as micron (or microns) or m.

[0031] The refractive index profile is the relationship between refractive index or relative refractive index and radial distance r from the core portion's centerline for each core portion of the multicore optical fiber. For relative refractive index profiles depicted herein as relatively sharp boundaries between various regions, normal variations in processing conditions may result in step boundaries at the interface of adjacent regions that are not sharp. It is to be understood that although boundaries of refractive index profiles may be depicted herein as step changes in refractive index, the boundaries in practice may be rounded or otherwise deviate from perfect step function characteristics. It is further understood that the value of the relative refractive index may vary with radial position within the core region and/or any of the cladding regions. When relative refractive index varies with radial position in a particular region of the fiber (core region and/or any of the cladding regions), it may be expressed in terms of its actual or approximate functional dependence or in terms of an average value applicable to the region. Unless otherwise specified, if the relative refractive index of a region (core region and/or any of the cladding regions) is expressed as a single value, it is understood that the relative refractive index in the region is constant, or approximately constant, and corresponds to the single value or that the single value represents an average value of a non-constant relative refractive index dependence with radial position in the region. Whether by design or a consequence of normal manufacturing variability, the dependence of relative refractive index on radial position may be sloped, curved, or otherwise non-constant.

[0032] The relative refractive index or relative refractive index percent as used herein with respect to multicore optical fibers and fiber cores of multicore optical fibers is defined according to the following equation:

[00001]$\%=100\frac{n^2\left(r\right)-n_c^2}{2n^2\left(r\right)}$

where n(r) is the refractive index at the radial distance r from the core's centerline at a wavelength of 1550 nm, unless otherwise specified, and n.sub.e is 1.444, which is the refractive index of undoped silica glass at a wavelength of 1550 nm. As used herein, the relative refractive index is represented by (or delta) or % (or delta %) and its values are given in units of % or %, unless otherwise specified. Relative refractive index may also be expressed as (r) or (r)%. When the refractive index of a region is less than the reference index n.sub.e, the relative refractive index is negative and can be referred to as a trench. When the refractive index of a region is greater than the reference index n.sub.e, the relative refractive index is positive, and the region can be said to be raised or to have a positive index.

[0033] The average relative refractive index of a region of the multicore optical fiber can be defined according to the following equation:

[00002]$\%=\frac{{}_{r_{\mathrm{inner}}}^{r_{\mathrm{outer}}}\left(r\right)\mathrm{dr}}{\left(r_{\mathrm{outer}}-r_{\mathrm{inner}}\right)}$

where r.sub.inner is the inner radius of the region, r.sub.outer is the outer radius of the region, and (r) is the relative refractive index of the region.

[0034] The term a-profile (also referred to as an alpha profile) refers to a relative refractive index profile (r) that has the following functional form:

[00003]$\left(r\right)=\left(r_0\right)\left\{1-{\left[\frac{.Math.r-r_0.Math.}{\left(r_1-r_0\right)}\right]}^{}\right\}$

where r.sub.o is the point at which (r) is maximum, r.sub.1 is the point at which (r) is zero, and r is in the range r.sub.irr.sub.f, where r.sub.i is the initial point of the -profile, r.sub.f is the final point of the -profile, and is a real number. In some embodiments, examples shown herein can have a core alpha of 1 100. In practice, an actual optical fiber, even when the target profile is an alpha profile, some level of deviation from the ideal configuration can occur. Therefore, the alpha parameter for an optical fiber may be obtained from a best fit of the measured index profile, as is known in the art.

[0035] The term graded-index profile refers to an -profile, where <10. The term step-index profile refers to an -profile, where 10.

[0036] Unless otherwise noted herein, optical properties (such as dispersion, dispersion slope, etc.) are reported for the LP01 mode.

[0037] Chromatic dispersion, herein referred to as dispersion unless otherwise noted, of an optical fiber is the sum of the material dispersion, the waveguide dispersion, and the intermodal dispersion. Material dispersion refers to the manner in which the refractive index of the material used for the optical core affects the velocity at which different optical wavelengths propagate within the core. Waveguide dispersion refers to dispersion caused by the different refractive indices of the core and cladding of the optical fiber. In the case of single mode waveguide fibers, the inter-modal dispersion is zero. Dispersion values in a two-mode regime assume intermodal dispersion is zero. The zero-dispersion wavelength (.sub.0) is the wavelength at which the dispersion has a value of zero. Dispersion slope is the rate of change of dispersion with respect to wavelength. Dispersion and dispersion slope are reported herein at a wavelength of 1310 nm or 1550 nm, as noted, and are expressed in units of ps/nm/km and ps/nm.sup.2/km, respectively. Chromatic dispersion is measured as specified by the IEC 60793-1-42:2013 standard, Optical fibresPart 1-42: Measurement methods and test proceduresChromatic dispersion.

[0038] The cutoff wavelength of an optical fiber is the minimum wavelength at which the optical fiber will support only one propagating mode. For wavelengths below the cutoff wavelength, multimode transmission may occur, and an additional source of dispersion may arise to limit the fiber's information carrying capacity. Cutoff wavelength will be reported herein as a cable cutoff wavelength. The cable cutoff wavelength is based on a 22-meter cabled fiber length as specified in TIA-455-80: FOTP-80 IEC-60793-1-44 Optical FibresPart 1-44: Measurement Methods and Test ProceduresCut-off Wavelength (21 May 2003), by Telecommunications Industry Association (TIA).

[0039] The bend resistance of an optical fiber, expressed as bend loss herein, can be gauged by induced attenuation under prescribed test conditions as specified by the IEC-60793-1-47:2017 standard, Optical fibresPart 1-47: Measurement methods and test proceduresMacrobending loss. For example, the test condition can entail deploying or wrapping the fiber one or more turns around a mandrel of a prescribed diameter, e.g., by wrapping 1 turn around either a 15 mm, 20 mm, or 30 mm or similar diameter mandrel (e.g. 115 mm diameter bend loss or the 120 mm diameter bend loss or the 130 mm diameter bend loss) and measuring the increase in attenuation per turn.

[0040] The term attenuation, as used herein, is the loss of optical power as the signal travels along the optical fiber. Attenuation is measured as specified by the IEC 60793-1-40:2019 standard entitled Optical fibresPart 1-40: Attenuation measurement methods.

[0041] As used herein, the multicore optical fiber can include a plurality of core portions, wherein each core portion can be defined as an i.sup.th core portion (i.e., 1.sup.st, 2.sup.nd, 3.sup.rd, 4.sup.th, etc. . . . ). Each i.sup.th core portion can have an outer radius r.sub.Ci. In embodiments, the outer radius r.sub.Ci of each core portion may correspond to an outer radius of an outer cladding region of that core portion. In some embodiments, the outer radius r.sub.Ci of each core portion may correspond to an outer radius of a trench region of that core portion. Each i.sup.th core portion is disposed within a cladding matrix of the multicore optical fiber, which defines a common cladding of the multicore optical fiber. The common cladding includes a relative refractive index .sub.CC and an outer radius R.sub.CC.

[0042] According to one aspect of the present disclosure, the core region forms the central portion of each core portion within the multicore optical fiber and is substantially cylindrical in shape. When two regions are directly adjacent to each other, the outer radius of the inner of the two regions coincides with the inner radius of the outer of the two regions. For example, in some embodiments in which an inner cladding region surrounds and is directly adjacent to a core region, the outer radius of the core region coincides with the inner radius of the inner cladding region.

[0043] An up-dopant is a substance added to the glass of the component being studied that has a propensity to raise the refractive index relative to pure undoped silica. A down-dopant is a substance added to the glass of the component being studied that has a propensity to lower the refractive index relative to pure undoped silica. Examples of up-dopants include GeO.sub.2 (germania), Al.sub.2O.sub.3, P.sub.2O.sub.5, TiO.sub.2, Cl, Br, and alkali metal oxides, such as K.sub.2O, Na.sub.2O, Li.sub.2O, Cs.sub.2O, Rb.sub.2O, and mixtures thereof. Examples of down-dopants include fluorine and boron.

[0044] As used herein, the term substantially free, when used to describe the concentration and/or absence of a particular up-dopant or down-dopant in a particular portion of the fiber, means that the constituent component is not intentionally added to the fiber as an up-dopant or a down-dopant. However, the fiber may contain traces of the constituent component as a contaminant or tramp in amounts. In some embodiments, the fiber may contain traces of the constituent component as a contaminant or tramp in amounts of less than or equal to 0.15 wt. %. In some embodiments, depending on the constituent component, the fiber may contain traces of the constituent component as a contaminant or tramp in amounts that may be less than or equal to 1 wt. %, less than or equal to 0.9 wt. %, less than or equal to 0.8 wt. %, less than or equal to 0.7 wt. %, less than or equal to 0.6 wt. %, less than or equal to 0.5 wt. %, less than or equal to 0.4 wt. %, less than or equal to 0.3 wt. %, less than or equal to 0.2 wt. %, less than or equal to 0.15 wt. %, less than or equal to 0.1 wt. %, less than or equal to 0.05 wt. %, or less than or equal to 0.01 wt.

[0045] The term crosstalk in a multi-core optical fiber is a measure of how much power leaks from one core portion to another, adjacent core portion. As used herein, the term adjacent core portion refers to the core that is nearest to the reference core portion. In some embodiments, all core portions may be equally spaced from one another, meaning that all core portions are adjacent one another. In other embodiments, the core portions may not be equally spaced from one another, meaning that some core portions will be spaced further from the reference core portion than adjacent core portions are spaced from the reference core portion.

[0046] As used herein, the term uncoupled-core multicore optical fiber (or uncoupled multicore optical fiber) refers to a multicore optical fiber that has crosstalk values between adjacent core portions less than or equal to 40 dB, or even less than or equal to 50 dB per 100 km of the multicore optical fiber. For example, for long-haul transmission in an uncoupled-core multicore fiber, the crosstalk should be less than or equal to 30 dB, less than or equal to 40 dB, or even less than or equal to 50 dB per 100 km.

[0047] The system to measure co-propagating crosstalk and counter-propagating crosstalk, as disclosed herein, is shown in FIG. 13. In order to measure the disclosed crosstalk between adjacent core portions (e.g., as described herein, two core portions having centerlines separated by a minimum core-to-core separation distance), multicore fibers with a length between 20 km and 25 km on a standard shipping spool are tested. As shown in FIG. 13, the system comprises a tunable laser source (TLS) with a linewidth of 200 kHz, a tap to monitor the laser output power, and a multicore fiber fan-in/fan-out (FIFO 1) spliced to the fiber to inject the source light into one of two adjacent core portions of the fiber while directing backward propagating light in the other of the two adjacent core portions. A fan-out (FIFO 2) is spliced at the far end of the fiber, and its outputs are connected to optical receivers to measure forward propagating light out of each core. The optical receivers #1, #2, and #3, as shown in FIG. 13, are each a high sensitivity detector with 109 dBm noise sensitivity and linearity error with <20% deviation over the entire power measured range (+5 to 75 dBm). All three optical receivers were calibrated and read out the same power at one power level. The multicore fiber used to fabricate FIFOs had the same mode field diameter and core-to-core pitch as the transmission multicore fiber under test. Co-propagating crosstalk can then be calculated using the measured power P3 and P4, and the counter-propagating crosstalk can then be calculated using the measured P3 and P2. Further details about the measurement method and setup can be found in P. Tandon, et al., Record Low Loss 0.144 dB/km 2-Core Optical Fiber for Submarine Transmission, Journal of Lightwave Technology, Vol. 42, No. 12, Jun. 15, 2024, the content of which is incorporated herein by reference in its entirety.

[0048] The phrase coupling coefficient , as used herein, is related to the overlap of electric fields when the two cores are close to each other. The square of the coupling coefficient, .sup.2, is related to the average power in core m as influenced by the power in other cores in the multicore optical fiber. The coupling coefficient can be estimated using the coupled power theory, with the methods disclosed in M. Koshiba, K. Saitoh, K. Takenaga, and S. Matsuo, Analytical Expression of Average Power-Coupling Coefficients for Estimating Intercore Crosstalk in Multicore fibers, IEEE Photonics J., 4(5), 1987-95 (2012); and T. Hayashi, T. Sasaki, E. Sasaoka, K. Saitoh, and M. Koshiba, Physical Interpretation of Intercore Crosstalk in Multicore fiber: Effects of Macrobend, Structure Fluctuation, and Microbend, Optics Express, 21(5), 5401-12 (2013), the contents of which are incorporated by reference herein in their entirety.

[0049] The effective area can be defined as:

[00004]$A_{\mathrm{eff}}=\frac{2{\left[{}_0^{}{\left(f\left(r\right)\right)}^2\mathrm{rdr}\right]}^2}{{}_0^{}{\left(f\left(r\right)\right)}^4\mathrm{rdr}}$

where f(r) is the transverse component of the electric field of the guided optical signal and r is radial position in the fiber. Effective area or A.sub.eff depends on the wavelength of the optical signal. Specific indication of the wavelength will be made when referring to Effective area or A.sub.eff herein. Effective area is expressed herein in units of m.sup.2, square micrometers, square microns or the like.

[0050] The mode field diameter (MFD) is measured using the Petermann II method and was determined from:

[00005]$\mathrm{MFD}=2w$$w=\frac{{}_0^{}{\left(f\left(r\right)\right)}^2}{{}_0^{}{\left(\frac{\mathrm{df}\left(r\right)}{\mathrm{dr}}\right)}^2\mathrm{rdr}}$

where f(r) is the transverse component of the electric field distribution of the guided light and r is the radial position in the fiber. Unless otherwise specified, mode field diameter or MFD refers to the mode field diameter at 1310 nm.

[0051] Trench volume is defined as:

[00006]$V_{\mathrm{Trench}}=.Math.2{}_{r_{\mathrm{Trench},\mathrm{inner}}}^{r_{\mathrm{Trench},\mathrm{outer}}}{}_{\mathrm{Trench}}\left(r\right)\mathrm{rdr}.Math.$

where r.sub.Trench,inner is the inner radius of the trench region of the refractive index profile, r.sub.Trench,outer is the outer radius of the trench region of the refractive index profile, .sub.Trench(r) is the relative refractive index of the trench region of the refractive index profile, and r is radial position in the fiber. Trench volume is in absolute value and a positive quantity and will be expressed herein in units of %micron.sup.2, %-micron.sup.2, %-m.sup.2, or %m.sup.2, whereby these units can be used interchangeably herein. A trench region is also referred to herein as a depressed-index cladding region.

[0052] Directional terms as used hereinfor example up, down, right, left, front, back, top, bottomare made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0053] While the techniques and/or procedures are depicted and/or described in a certain order for purpose of illustration, it should be appreciated that certain techniques and/or procedures may be re-ordered and/or omitted within the scope of various embodiments.

[0054] Regardless of whether shown or described in combination or separately, the various features (both structural and methodological) are intended to be selectively rearranged, included, or omitted to produce an example or embodiment with a particular set of features.

[0055] As used herein, the singular forms a, an and the include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0056] A great variety of multicore optical fibers for space division multiplexing (SDM) have been demonstrated. As the industry focus shifts to product realization, practical factors, such as mechanical properties, bending loss, simplicity of operation, minimum change in infrastructure, etc., all become increasingly important.

[0057] The inventors have developed multicore optical fibers with form factors, including but not limited to coating diameter, that are similar to or the same as those of existing fibers, such as submarine fibers. The multicore optical fiber disclosed herein can be processed in existing cable designs. The multicore optical fibers described herein may be incorporated into transmission systems for C band and/or L band transmission in bidirectional mode. Further, the multicore optical fiber disclosed herein may be uncoupled in that the crosstalk values at 1550 nm between adjacent core portions may be less than or equal to 30 dB, less than or equal to 40 dB, or less than or equal to 50 dB per 100 km of the multicore optical fiber for bidirectional transmission.

[0058] In some embodiments, the uncoupled multicore optical fiber described herein for bidirectional transmission may include greater than or equal to 6 core portions. An outer diameter of the common cladding of the multicore optical fiber may be greater than or equal to 140 microns and less than or equal to 180 microns. A coating diameter of the multicore optical fiber may be greater than or equal to 240 microns and less than or equal to 255 microns.

[0059] In some embodiments, the multicore optical fiber may include trench assisted designs as will be discussed in more detail below. The core portions may each have an effective area greater than or equal to 75 micron.sup.2 and less than or equal to 135 micron.sup.2. An attenuation of each core portion of the multicore optical fiber at a wavelength of 1550 nm may be less than or equal to less than 0.165 dB/km. A radiation loss of each core portion of the multicore optical fiber may be less than or equal to 0.01 dB/km at a wavelength of 1550 nm. A cable cutoff wavelength of the multicore optical fiber may be less than or equal to 1530 nm. The multicore optical fiber may include a core multiplicity factor greater than or equal to 0.028 and less than or equal to 0.045. A counter-propagating crosstalk at 1550 nm between two adjacent core portions may be less than or equal to 40 dB per 100 km of the multicore optical fiber.

[0060] The multicore optical fiber designs described herein represent a higher fiber capacity compared to existing fiber designs. In some embodiments, the multicore optical fibers described herein may demonstrate 3 to 8 times capacity increase compared to a single core optical fiber. Furthermore, despite the increase in capacity, the form factors of the multicore optical fibers described herein are similar to those of existing submarine fibers and can be readily processed in existing cable designs.

[0061] FIG. 1 depicts a simplified schematic diagram of an exemplary optical system 100 for bidirectional (or counterpropagating) transmission over an uncoupled-core multicore optical fiber 110 (or multicore optical fiber 110) having a plurality of core portions. In some embodiments, the optical system 100 may include a first transceiver 180 and a second transceiver 190 optically coupled by the multicore optical fiber 110. The multicore optical fiber 110 may include a first end 112 optically coupled to the first transceiver 180 and a second end 114 optically coupled to the second transceiver 190. The multicore optical fiber 110 may be configured for transmission at one or more wavelengths in C band and/or L band, such as one or more wavelengths between 1535 nm and 1625 nm.

[0062] In some embodiments, the first transceiver 180 may include a first plurality of light sources configured to produce optical signals for transmitting over the multicore optical fiber 110. The optical signals from the first plurality of light sources of the first transceiver 180 may be transmitted from the first end 112 of the multicore optical fiber 110 to the second end 114 of the multicore optical fiber 110 via a first plurality of core portions of the multicore optical fiber 110. In some embodiments, the first transceiver 180 and the first end 112 of the multicore optical fiber 110 may be optically coupled such that each of the first plurality of light sources of the first transceiver 180 may transmit photons into an individual core portion of the first plurality of the core portions of the multicore optical fiber 110.

[0063] In some embodiments, the second transceiver 190 may include a second plurality of light sources configured to produce optical signals for transmitting over the multicore optical fiber 110. The optical signals from the second plurality of light sources of the second transceiver 190 may be transmitted from the second end 114 of the multicore optical fiber 110 to the first end 112 of the multicore optical fiber 110 via a second plurality of core portions of the multicore optical fiber 110. The second plurality of core portions of the multicore optical fiber 110 may be different from the first plurality of core portions of the multicore optical fiber 110. In some embodiments, the second transceiver 190 and the second end 114 of the multicore optical fiber 110 may be optically coupled such that each of the second plurality of light sources of the second transceiver 190 may transmit photons into an individual core portion of the second plurality of the core portions of the multicore optical fiber 110.

[0064] Depending on applications, the first transceiver 180 and/or the second transceiver 190 may be configured such that transmission of optical signals from any one light source of the first and/or second plurality of light sources to a corresponding core portion of the multicore optical fiber 110 may be independent from, or dependent upon, transmission of optical signals from any other light source(s) of the first and/or plurality of light sources to corresponding core portion(s) of the multicore optical fiber 110. Transmission of optical signals from any one light source of the first and/or second plurality of light sources to a corresponding core portion of the multicore optical fiber 110 may be performed simultaneously with transmission of optical signals from any other light source(s) of the first and/or second plurality of light sources to corresponding core portion(s) of the multicore optical fiber 110.

[0065] In some embodiments, the first transceiver 180 may further include a first plurality of detectors, such as photodetectors, configured to detect optical signals from the multicore optical fiber 110. For example, the first plurality of detectors may be configured to receive optical signals transmitted from the second transceiver 190 from the second end 114 of the multicore optical fiber 110 to the first end 112 of the multicore optical fiber 110 via the second plurality of core portions of the multicore optical fiber 110. In some embodiments, the first transceiver 180 and the first end 112 of the multicore optical fiber 110 may be optically coupled such that each of the first plurality of detectors of the first transceiver 180 may receive optical signals from an individual core portion of the second plurality of core portions of the multicore optical fiber 110.

[0066] In some embodiments, the second transceiver 190 may further include a second plurality of detectors, such as photodetectors, configured to detect optical signals from the multicore optical fiber 110. For example, the second plurality of detectors may be configured to receive optical signals transmitted from the first transceiver 180 from the first end 112 of the multicore optical fiber 110 to the second end 114 of the multicore optical fiber 110 via the first plurality of core portions of the multicore optical fiber 110. In some embodiments, the second transceiver 190 and the second end 114 of the multicore optical fiber 110 may be optically coupled such that each of the second plurality of detectors of the second transceiver 190 may receive optical signals from an individual core portion of the first plurality of core portions of the multicore optical fiber 110.

[0067] As mentioned above, the multicore optical fiber 110 may be configured for transmission at one or more wavelengths in C band and/or L band to further increase transmission capacity. C band refers to the wavelength range from 1535 nm to 1565 nm, and L band refers to the wavelength range from 1565 nm to 1625 nm. In some embodiments, one or more of the first plurality of core portions and/or one or more of the second plurality of core portions may be configured for transmission over at least one of C band or L band. In some embodiments, each of the first plurality of core portions and/or each of the second plurality of core portions may be configured for transmission over at least one of C band or L band. In some embodiments, each of the first plurality of core portions and/or each of the second plurality of core portions may be configured for transmission over both C band and L band (or C+L band). In some embodiments, the transmission over C band and L band may be performed simultaneously. In some embodiments, the transmission over C band and L band may be performed simultaneously over a common core portion.

[0068] More specifically, in some embodiments, optical signals may be transmitted at one or more wavelengths of C band via one or more core portions of the first plurality of core portions in a first direction from the first transceiver 180 to the second transceiver 190. Optical signals may also be transmitted at one or more wavelengths of L band via one or more core portions of the first plurality of core portions in the first direction. In some embodiments, the multicore optical fiber 110 described herein may allow the transmission of signals in the first direction over C band and L band to be performed over a common core portion of the first plurality of core portions, and in some embodiments, the transmission of signals in the first direction over C band and L band may be performed over the common core portion simultaneously.

[0069] Similarly, in some embodiments, optical signals may be transmitted at one or more wavelengths of C band via one or more core portions of the second plurality of core portions in a second direction from the second transceiver 190 to the first transceiver 180. Optical signals may also be transmitted at one or more wavelengths of L band via one or more core portions of the second plurality of core portions in the second direction. In some embodiments, the transmission of signals in the second direction over C band and L band may be performed over a common core portion of the second plurality of core portions, and in some embodiments, the transmission of signals in the second direction over C band and L band may be performed over the common core portion simultaneously.

[0070] Although transmission over C band and L band over the same core portion in the same direction is described an example, in some embodiments, the transmission over C band and L band may be conducted over the same core portion in opposite directions. In some embodiments, the transmission over C band and L band may be conducted over different core portions in the same direction. In some embodiments, the transmission over C band and L band may be conducted over different core portions in different directions.

[0071] Further, although bidirectional (or counterpropagating) transmission is described herein as an example utilizing the multicore optical fiber 110, in some embodiments, the multicore optical fiber 110 may also be utilized for unidirectional transmission where the first plurality of core portions and the second plurality of core portions may be configured to transmit optical signals in the same direction. For example, in some embodiments, one or more of the first plurality of core portions and one or more of the second plurality of core portions may be configured to transmit over different wavelengths of C band and/or L band to minimize crosstalk for unidirectional (or copropagating) transmission.

[0072] FIG. 2 schematically illustrates a radial cross section of an exemplary embodiment of the multicore optical fiber 110 viewed along line II-II of FIG. 1. In some embodiments, the multicore optical fiber 110 may include a glass fiber portion 10 and a non-glass, polymer coating portion 20. In some embodiments, the coating portion 20 may include a primary coating layer and a secondary coating layer as will be discussed in more detail below. The multicore optical fiber 110 may include a central fiber axis 12, which defines radial position R=0. In some embodiments, the central fiber axis 12 of the multicore optical fiber 110 may correspond to the centerline of the glass fiber portion 10 and/or the non-glass coating portion 20.

[0073] In some embodiments, the glass fiber portion 10 may include a common cladding 19 and a plurality of core portions C.sub.i (described in more detail below) disposed within the common cladding 19. The common cladding 19 may include an outer radius R.sub.CC. In some embodiments, the outer radius R.sub.CC of the common cladding 19 may be greater than or equal to 70 m, greater than or equal to 75 m, greater than or equal to 80 m, greater than or equal to 85 m, or greater than or equal to 90 m. In some embodiments, the outer radius R.sub.CC of the common cladding 19 may be less than or equal to 95 m, less than or equal to 90 m, less than or equal to 85 m, less than or equal to 80 m, less than or equal to 75 m. In some embodiments, the outer radius R.sub.CC of the common cladding 19 may be greater than or equal to 70 m and less than or equal to 95 mincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius R.sub.CC of the common cladding 19 may be greater than or equal to 70 m and less than or equal to 90 m, greater than or equal to 70 m and less than or equal to 85 m, greater than or equal to 75 m and less than or equal to 95 m, greater than or equal to 75 m and less than or equal to 90 m, or greater than or equal to 75 m and less than or equal to 85 m.

[0074] In some embodiments, such as in the embodiment depicted in FIG. 2, the outer radius R.sub.CC of the common cladding 19 may correspond to the outer radius of the glass fiber portion 10. A glass diameter of the glass fiber portion 10 (2R.sub.CC) may be greater than or equal to 140 m and less than or equal to 190 mincluding all sub-ranges and values there-between. For example, in some embodiments, the glass diameter of the glass fiber portion 10 (2R.sub.CC) may be greater than or equal to 150 m and less than or equal to 190 m, greater than or equal to 160 m and less than or equal to 190 m, greater than or equal to 170 m and less than or equal to 190 m, greater than or equal to 180 m and less than or equal to 190 m, greater than or equal to 140 m and less than or equal to 180 m, greater than or equal to 150 m and less than or equal to 180 m, greater than or equal to 160 m and less than or equal to 180 m, greater than or equal to 170 m and less than or equal to 180 m, greater than or equal to 140 m and less than or equal to 170 m, greater than or equal to 150 m and less than or equal to 170 m, greater than or equal to 160 m and less than or equal to 170 m, greater than or equal to 140 m and less than or equal to 160 m, greater than or equal to 150 m and less than or equal to 160 m, or greater than or equal to 140 m and less than or equal to 150 m.

[0075] In some embodiments, the common cladding 19 may include a relative refractive index .sub.CC. The relative refractive index .sub.CC of the common cladding 19 may be in a range from about 0.2% to about 0.4%including all sub-ranges and values there-between. For example, in some embodiments, the relative refractive index .sub.CC of the common cladding 19 may be in a range from about 0.2% to about 0.38%, from about 0.2% to about 0.35%, from about 0.2% to about 0.32%, from about 0.2% to about 0.3%, from about 0.2% to about 0.28%, from about 0.2% to about 0.25%, from about 0.2% to about 0.22%, from about 0.22% to about 0.4%, from about 0.22% to about 0.38%, from about 0.22% to about 0.35%, from about 0.22% to about 0.32%, from about 0.22% to about 0.3%, from about 0.22% to about 0.28%, from about 0.22% to about 0.25%, from about 0.25% to about 0.4%, from about 0.25% to about 0.38%, from about 0.25% to about 0.35%, from about 0.25% to about 0.32%, from about 0.25% to about 0.3%, from about 0.25% to about 0.28%, from about 0.28% to about 0.4%, from about 0.28% to about 0.38%, from about 0.28% to about 0.35%, from about 0.28% to about 0.32%, from about 0.28% to about 0.3%, from about 0.3% to about 0.4%, from about 0.3% to about 0.38%, from about 0.3% to about 0.35%, from about 0.3% to about 0.32%, from about 0.32% to about 0.4%, or from about 0.32% to about 0.38%, from about 0.32% to about 0.35%, from about 0.35% to about 0.4%, or from about 0.35% to about 0.38%, or from about 0.38% to about 0.4%. In some embodiments, the relative refractive index .sub.CC may be preferably constant or approximately constant within the common cladding 19.

[0076] In some embodiments, the coating portion 20 may encircle and directly contact the common cladding 19 and may extend from the outer radius R.sub.CC of the common cladding 19 to an outer radius R.sub.C of the coating portion 20. In some embodiments, the outer radius R.sub.C of the coating portion 20 may be greater than or equal to 120 m, greater than or equal to 122 m, greater than or equal to 124 m, or greater than or equal to 125 m. In some embodiments, the outer radius R.sub.C of the coating portion 20 may be less than or equal to 127.5 m, less than or equal to 127 m, less than or equal to 126 m, or less than or equal to 125 m. In some embodiments, the outer radius R.sub.C of the coating portion 20 may be greater than or equal to 120 m and less than or equal to 127.5 mincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius R.sub.C of the coating portion 20 may be greater than or equal to 122.5 m and less than or equal to 127.5 m, greater than or equal to 124 m and less than or equal to 126 m, or in some embodiments, the outer radius R.sub.C of the coating portion 20 may be about 125 m. In some embodiments, the multicore optical fiber 110 may include form factors, such as coating diameter (2R.sub.C), that may be similar to or the same as those of existing submarine fibers. Thus, the multicore optical fiber 110 described herein can be readily processed in existing cable designs while providing improved performance (as discussed below) than the existing submarine fibers.

[0077] In some embodiments, the coating portion 20 may include a primary coating 22 and a secondary coating 24. In some embodiments, the primary coating 22 may encircle and directly contact the common cladding 19 and may extend from the outer radius R.sub.CC of the common cladding 19 to an outer radius R.sub.Cp of the primary coating 22. In some embodiments, the outer radius R.sub.Cp of the primary coating 22 may be greater than or equal to 170 m, greater than or equal to 185 m, greater than or equal to 200 m, or greater than or equal to 215 m. In some embodiments, the outer radius R.sub.Cp of the primary coating 22 may be less than or equal to 220 m, less than or equal to 205 m, less than or equal to 190 m, less than or equal to 175 m. In some embodiments, the outer radius R.sub.Cp of the primary coating 22 may be greater than or equal to 170 m and less than or equal to 220 mincluding all sub-ranges or values therebetween. For example, in some embodiments, the outer radius R.sub.Cp of the primary coating 22 may be greater than or equal to 170 m and less than or equal to 210 m, greater than or equal to 170 m and less than or equal to 200 m, greater than or equal to 170 m and less than or equal to 185 m, greater than or equal to 180 m and less than or equal to 220 m, greater than or equal to 180 m and less than or equal to 210 m, greater than or equal to 180 m and less than or equal to 200 m, greater than or equal to 190 m and less than or equal to 220 m, greater than or equal to 190 m and less than or equal to 210 m, greater than or equal to 190 m and less than or equal to 200 m, or greater than or equal to 210 m and less than or equal to 220 m.

[0078] In some embodiments, a thickness T.sub.Cp of the primary coating 22, as defined by a difference between the outer radius R.sub.Cp of the primary coating 22 and the outer radius R.sub.CC of the common cladding 19 (i.e., T.sub.Cp=R.sub.CpR.sub.CC), may be greater than or equal to 14 m, greater than or equal to 17 m, greater than or equal to 20 m, greater than or equal to 23 m, greater than or equal to 27 m, greater than or equal to 30 m, or greater than or equal to 33 m. In some embodiments, the thickness T.sub.Cp of the primary coating 22 may be less than or equal to 35 m, less than or equal to 32 m, less than or equal to 29 m, less than or equal to 26 m, less than or equal to 22 m, less than or equal to 19 m, or less than or equal to 16 m. In some embodiments, the thickness T.sub.Cp of the primary coating 22 may be greater than or equal to 14 m and less than or equal to 35 mincluding all sub-ranges and values there-between. For example, in some embodiments, the thickness T.sub.Cp of the primary coating 22 may be greater than or equal to 18 m and less than or equal to 30 m, or greater than or equal to 22 m and less than or equal to 26 m.

[0079] In some embodiments, the secondary coating 24 may encircle and directly contact the primary coating 22 and may extend from the outer radius R.sub.Cp of the primary coating 22 to an outer radius R.sub.Cs of the secondary coating 24. In some embodiments, such as in the embodiment depicted in FIG. 2, the outer radius R.sub.Cs of the secondary coating 24 may correspond to the outer radius R.sub.C of the coating portion 20.

[0080] In some embodiments, the outer radius R.sub.Cs of the secondary coating 24 may be greater than or equal to 120 m, greater than or equal to 122 m, greater than or equal to 124 m, or greater than or equal to 125 m. In some embodiments, the outer radius R.sub.Cs of the secondary coating 24 may be less than or equal to 127.5 m, less than or equal to 127 m, less than or equal to 126 m, or less than or equal to 125 m. In some embodiments, the outer radius R.sub.Cs of the secondary coating 24 may be greater than or equal to 120 m and less than or equal to 127.5 mincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius R.sub.Cs of the secondary coating 24 may be greater than or equal to 122.5 m and less than or equal to 127.5 m, greater than or equal to 124 m and less than or equal to 126 m, or in some embodiments, the outer radius R.sub.Cs of the secondary coating 24 may be about 125 m.

[0081] In some embodiments, a thickness T.sub.Cs of the secondary coating 24, as defined by a difference between the outer radius R.sub.Cs of the secondary coating 24 and the outer radius R.sub.Cp of the primary coating 22 (i.e., T.sub.Cs=R.sub.CsR.sub.Cp), may be greater than or equal to 14 m, greater than or equal to 17 m, greater than or equal to 20 m, greater than or equal to 23 m, greater than or equal to 27 m, greater than or equal to 30 m, or greater than or equal to 33 m. In some embodiments, the thickness T.sub.Cs of the secondary coating 24 may be less than or equal to 35 m, less than or equal to 32 m, less than or equal to 29 m, less than or equal to 26 m, less than or equal to 22 m, less than or equal to 19 m, or less than or equal to 16 m. In some embodiments, the thickness T.sub.Cs of the secondary coating 24 may be greater than or equal to 14 m and less than or equal to 35 mincluding all sub-ranges and values there-between. For example, in some embodiments, the thickness T.sub.Cs of the secondary coating 24 may be greater than or equal to 18 m and less than or equal to 30 m, or greater than or equal to 22 m and less than or equal to 26 m.

[0082] In some embodiments, a ratio of the thickness T.sub.Cs of the secondary coating 24 to the thickness T.sub.Cp of the primary coating 22 may be greater than or equal to 0.7, greater than or equal to 0.8, greater than or equal to 0.9, greater than or equal to 1.0, greater than or equal to 1.1, or greater than or equal to 1.2. In some embodiments, the ratio of the thickness T.sub.Cs of the secondary coating 24 to the thickness T.sub.Cp of the primary coating 22, may be less than or equal to 1.3, less than or equal to 1.2, less than or equal to 1.1, less than or equal to 1.0, less than or equal to 0.9, or less than or equal to 0.8. In some embodiments, the ratio of the thickness of the secondary coating 24 to the thickness T.sub.Cp of the primary coating 22 may be greater than or equal to 0.7 and less than or equal to 1.3including all sub-ranges and values there-between. For example, in some embodiments, the ratio of the thickness T.sub.Cs of the secondary coating 24 to the thickness T.sub.Cp of the primary coating 22 may be greater than or equal to 0.8 and less than or equal to about 1.3, greater than or equal to 0.9 and less than or equal to 1.2, greater than or equal to 0.9 and less than or equal to 1.1, greater than or equal to 0.8 and less than or equal to 1.0, greater than or equal to 1.0 and less than or equal to 1.2, or greater than or equal to 1.0 and less than or equal to 1.1. The inventors have recognized that the thickness ratio of the secondary coating 24 to the primary coating 22 described herein may allow for reduction in microbending loss of the optical fiber when deployed in an optical fiber cable.

[0083] In some embodiments, an in-situ modulus of the primary coating 22 may be less than an in-situ modulus of the secondary coating 24. In some embodiments, the in-situ modulus of the primary coating 22 may be greater than or equal to 0.15 MPa, greater than or equal to 0.21 MPa, greater than or equal to 0.27 MPa, greater than or equal to 0.33 MPa, or greater than or equal to 0.39 MPa. In some embodiments, the in-situ modulus of the primary coating 22 may be less than or equal to 0.4 MPa, less than or equal to 0.34 MPa, less than or equal to 0.28 MPa, less than or equal to 0.22 MPa, or less than or equal to 0.16 MPa. In some embodiments, the in-situ modulus of the primary coating 22 may be greater than or equal to 0.15 MPa and less than or equal to 0.4 MPaincluding all sub-ranges and values there-between. For example, in some embodiments, the in-situ modulus of the primary coating 22 may be greater than or equal to 0.2 MPa and less than or equal to 0.35 MPa, greater than or equal to 0.25 MPa and less than or equal to 0.35 MPa, greater than or equal to 0.2 MPa and less than or equal to 0.3 MPa, or greater than or equal to 0.25 MPa and less than or equal to 0.3 MPa.

[0084] In some embodiments, the in-situ modulus of the secondary coating 24 may be greater than or equal to 1,600 MPa, greater than or equal to 1,800 MPa, greater than or equal to 2,000 MPa, greater than or equal to 2,200 MPa, or greater than or equal to 2,300 MPa. In some embodiments, the in-situ modulus of the secondary coating 24 may be less than or equal to 2,400 MPa, less than or equal to 2,200 MPa, less than or equal to 2,000 MPa, less than or equal to 1,800 MPa, or less than or equal to 1,700 MPa. In some embodiments, the in-situ modulus of the secondary coating 24 may be greater than or equal to 1,600 MPa and less than or equal to 2,400 MPaincluding all sub-ranges and values there-between. For example, in some embodiments, the in-situ modulus of the secondary coating 24 may be greater than or equal to 1,750 MPa and less than or equal to 2,250 MPa, greater than or equal to 1,750 MPa and less than or equal to 2,100 MPa, greater than or equal to 1,900 MPa and less than or equal to 2,250 MPa, or greater than or equal to 1,900 MPa and less than or equal to 2,100 MPa.

[0085] With further reference to FIG. 2, the multicore optical fiber 110 may include a plurality of core portions C that may be uncoupled. The core portions C may be individually denoted C.sub.i, such as individually denoted C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8 in the exemplary embodiment shown in FIG. 2, and collectively referred to as core portions C. Each core portion C.sub.1 may generally extend through a length of the multicore optical fiber 110 parallel to the central fiber axis 12. Although 8 core portions are shown in the exemplary embodiment in FIG. 2, the multicore optical fiber 110 may include more or less than 8 core portions that are uncoupled. In some embodiments, the multicore optical fiber 110 may include greater than or equal to 6 core portions, greater than or equal to 7 core portions, greater than or equal to 8 core portions, greater than or equal to 9 core portions, or greater than or equal to 10 core portions. In some embodiments, the multicore optical fiber 110 may include less than or equal to 10 core portions, less than or equal to 9 core portions, less than or equal to 8 core portions, less than or equal to 7 core portions, less than or equal to 6 core portions. In some embodiments, the multicore optical fiber 110 may include 6 to 10including all sub-ranges and values there-betweencore portions. For example, in some embodiments, the multicore optical fiber 110 may include 6 to 10 core portions, 6 to 8 core portions, 8 to 10 core portions, or in some embodiments, the multicore optical fiber 110 may include 6 core portions, 8 core portions, or 10 core portions.

[0086] Each core portion C.sub.i may include a central axis or centerline CL.sub.i (which defines radial position r=0 for each core portion) and an outer radius r.sub.Ci. A position of each CL.sub.i within the multicore optical fiber 110 can be defined using Cartesian coordinates with the central fiber axis 12 defining the origin (0,0) of an x-y coordinate system coincident with the coordinate system defined by the radial coordinate R. The position of centerline CL.sub.i can be defined as (x.sub.i,y.sub.i) (e.g., the position of centerline CL.sub.1 being defined as (x.sub.1,y.sub.1), the position of centerline CL.sub.2 being defined as (x.sub.2,y.sub.2), etc.).

[0087] In some embodiments, each of the core portions C.sub.i may be separated from a nearest core portion (e.g., the core portion C.sub.i having a central axis CL.sub.i that is closest to the central axis of that core portion) by a minimum core-to-core separation distance (or minimum separation distance). Core portions having central axes that are most proximate to one another (i.e., there is no other core portion having a central axis that is more proximate to a core portion than an adjacent core portion) may be referred to as adjacent core portions. The minimum separation distance thus may refer to the separation distance between a core portion and its adjacent (or nearest) core portion. The minimum separation distance between two adjacent core portions C.sub.m and C.sub.n may be defined as the following:

[00007]$D_{\left(C_m,C_n\right)}=\sqrt{{\left(x_m-x_n\right)}^2+{\left(y_m-y_n\right)}^2}$

where (x.sub.m, y.sub.m) denotes the positional coordinates of the central axis CL.sub.m of core portion C.sub.m, and (x.sub.n, y.sub.n) denotes the positional coordinates of the central axis CL.sub.n of core portion C.sub.1.

[0088] In some embodiments, a core portion C.sub.i may be separated from more than one core portion by the minimum separation distance. As a non-limiting example, in the exemplary embodiment as depicted in FIG. 2, the core portions C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8 may be arranged annularly and form a ring about the central fiber axis 12. Thus, in some embodiments, the core portions C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8 may be arranged along the circumference of a circle that has its center located at the central fiber axis 12 (or radial position R=0) and a radius (or circumradius) R.sub.D defined by the distance between the central fiber axis 12 and the respective center lines CL.sub.1, CL.sub.2, CL.sub.3, CL.sub.4, CL.sub.5, CL.sub.6, CL.sub.7, and CL.sub.8 of the core portions C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8. Accordingly, the core portions C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8 are positioned at an equal radial distance R.sub.D from the central fiber axis 12.

[0089] In some embodiments, the radius (or circumradius) R.sub.D may be greater than or equal to 30 m, greater than or equal to 35 m, greater than or equal to 40 m, or greater than or equal to 45 m. In some embodiments, the radius (or circumradius) R.sub.D may be less than or equal to 50 m, less than or equal to 45 m, less than or equal to 40 m, or less than or equal to 35 m. In some embodiments, the radius (or circumradius) R.sub.D may range from about 30 m to about 50 mincluding all sub-ranges and values there-between. For example, in some embodiments, the radius (or circumradius) R.sub.D may be greater than or equal to 30 m and less than or equal to 50 m, greater than or equal to 35 m and less than or equal to 50 m, greater than or equal to 40 m and less than or equal to 50 m, greater than or equal to 45 m and less than or equal to 50 m, greater than or equal to 30 m and less than or equal to 45 m, greater than or equal to 35 m and less than or equal to 45 m, greater than or equal to 40 m and less than or equal to 45 m, greater than or equal to 30 m and less than or equal to 40 m, greater than or equal to 35 m and less than or equal to 40 m, or greater than or equal to 30 m and less than or equal to 35 m.

[0090] Further, in the exemplary embodiment as depicted in FIG. 2, the center lines CL.sub.1, CL.sub.2, CL.sub.3, CL.sub.4, CL.sub.5, CL.sub.6, CL.sub.7, and CL.sub.8 of the core portions C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8 are equally spaced apart along the circumference of the circle. Thus, in this exemplary arrangement, each of the core portions C.sub.1, C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, and C.sub.8 is separated from two adjacent core portions by the minimum separation distance.

[0091] In some embodiments, the minimum separation distance between two adjacent core portions may be greater than or equal to 32 m, greater than or equal to 33 m, greater than or equal to 34 m, greater than or equal to 35 m, greater than or equal to 36 m, greater than or equal to 37 m, greater than or equal to 38 m, greater than or equal to 39 m, or greater than or equal to 40. In some embodiments, the minimum separation distance between two adjacent core portions may be less than or equal to 50 m, less than or equal to 46 m, less than or equal to 42 m, less than or equal to 40 m, less than or equal to 38 m, less than or equal to 36 m, less than or equal to 34 m, or less than or equal to 33 m.

[0092] In some embodiments, the minimum separation distance between two adjacent core portions may be greater than or equal to 32 m and less than or equal to 50 mincluding all sub-ranges and values there-between. For example, in some embodiments, the minimum separation distance between two adjacent core portions may be greater than or equal to 32 m and less than or equal to 45 m, greater than or equal to 32 m and less than or equal to 42 m, greater than or equal to 32 m and less than or equal to 38 m, greater than or equal to 32 m and less than or equal to 36 m, greater than or equal to 33 m and less than or equal to 50 m, greater than or equal to 33 m and less than or equal to 45 m, greater than or equal to 33 m and less than or equal to 40 m, greater than or equal to 33 m and less than or equal to 36 m, greater than or equal to 34 m and less than or equal to 50 m, greater than or equal to 34 m and less than or equal to 45 m, greater than or equal to 34 m and less than or equal to 42 m, greater than or equal to 34 m and less than or equal to 40 m, greater than or equal to 34 m and less than or equal to 36 m, greater than or equal to 36 m and less than or equal to 50 m, greater than or equal to 36 m and less than or equal to 45 m, greater than or equal to 36 m and less than or equal to 42 m, greater than or equal to 36 m and less than or equal to 40 m, greater than or equal to 40 m and less than or equal to 50 m, or greater than or equal to 40 m and less than or equal to 45 m.

[0093] In some embodiments, edges of the plurality of core portions C.sub.i may also be spaced apart from an outer surface 16 of the glass fiber portion 10 of the multicore optical fiber 110 by at least a minimum core edge to glass fiber edge distance D.sub.E as measured from the edge of each of the core portions to the outer surface 16 of the glass fiber portion 10. As depicted in FIG. 2, the minimum core edge to glass fiber edge distance D.sub.E is the minimum distance from a point along the outer circumference of a core portion C.sub.i(e.g., a point on the outer circumference that is closest to the outer surface 16) to a nearest point along the circumference of the outer surface 16 of the glass fiber portion 10, as determined by a line segment between the point along the outer circumference of the core portion C.sub.i and the nearest point along the circumference on the outer surface 16 in a plan perpendicular to the fiber axis 12. The outer circumference of each core portion C.sub.i may be defined by the outer radius R.sub.Ci of each core portion C.sub.i which may correspond to an outer radius of a trench region of each core portion C.sub.i as will be discussed in more detail below.

[0094] Without intending to be bound by any particular theory, it is believed that the extent of signal loss due to tunneling, such as radiation loss of each core portion C.sub.i, may be influenced by the minimum value for D.sub.E. In some embodiments, the minimum core edge to glass fiber edge distance D.sub.E may be greater than or equal to 10 m, greater than or equal to 15 m, greater than or equal to 20 m, or greater than or equal to 25 m. In some embodiments, D.sub.E may be less than or equal to 40 m, less than or equal to 35 m, less than or equal to 25 m, less than or equal to 20 m. In some embodiments, D.sub.E may range be in a range from about 10 m to about 30 mincluding all sub-ranges and values there-between. For example, in some embodiments, D.sub.E may be in a range from about 10 m to about 25 m, from about 10 m to about 20 m, from about 10 m to about 15 m, from about 15 m to about 40 m, from about 15 m to about 35 m, from about 15 m to about 25 m, from about 20 m to about 30 m, from about 20 m to about 25 m, or from about 25 m to about 40 m.

[0095] The minimum core edge to glass fiber edge distance D.sub.E may also influence the crosstalk between adjacent core portions of the multicore optical fiber 110. Specifically, as the values for the minimum core edge to glass fiber edge distance D.sub.E increase, core-to-core separation distance D may be reduced, which may lead to increased crosstalk between adjacent core portions. To minimize radiation leakage loss while also limiting crosstalk between adjacent core portions, the inventors have discovered that a core multiplicity factor (CMF) in the range from about 0.028 to about 0.045 may be selected to yield superior performance that achieve both low radiation loss and low crosstalk between adjacent core portions of the multicore optical fiber 110 as described herein.

[0096] The core multiplicity factor (CMF) of the multicore optical fiber can be determined in accordance with the following equation:

[00008]$\mathrm{CMF}=\frac{{.Math.}_{i=1}^m{A}_{{\mathrm{eff}}_i}}{{R_{\mathrm{CC}}}^2}$

wherein m is the number of core portions C with the multicore optical fiber 110, A.sub.eff.sub.i is the effective area of each core portion C.sub.i, and R.sub.CC is the outer radius of the common cladding 19 of the multicore optical fiber 110 which also corresponds to the outer radius of the glass fiber portion 10 of the multicore optical fiber 110. In some embodiments, the effective areas A.sub.eff.sub.i of the core portions C are substantially the same, the core multiplicity factor of the multicore optical fiber 110 can be determined in accordance with the following equation:

[00009]$\mathrm{CMF}=\frac{m{A}_{\mathrm{eff}}}{{R_{\mathrm{CC}}}^2}$

[0097] In some embodiments, the designs of the multicore optical fiber 110 described herein may provide a CMF greater than or equal to 0.028 that may achieve both low radiation loss and low crosstalk. For example, in some embodiments, the CMF of the multicore optical fiber 110 may be greater than or equal to 0.03, greater than or equal to 0.033, greater than or equal to 0.035, greater than or equal to 0.037, greater than or equal to 0.04, or greater than or equal to 0.043. In some embodiments, the CMF of the multicore optical fiber 110 may be less than or equal to 0.045, less than or equal to 0.043, less than or equal to 0.04, less than or equal to 0.037, less than or equal to 0.035, less than or equal to 0.033, less than or equal to 0.032, or less than or equal to 0.03.

[0098] In some embodiments, the multicore optical fiber 110 described herein may provide a CMF ranging from about 0.028 to about 0.045including all sub-ranges and values there-between. For example, in some embodiments, the CMF of the multicore optical fiber 110 may range from about 0.028 to about 0.043, from about 0.028 to about 0.040, from about 0.028 to about 0.037, from about 0.028 to about 0.035, from about 0.028 to about 0.033, from about 0.03 to about 0.045, from about 0.03 to about 0.043, from about 0.03 to about 0.04, from about 0.03 to about 0.037, from about 0.03 to about 0.035, from about 0.03 to about 0.033, from about 0.033 to about 0.045, from about 0.033 to about 0.043, from about 0.033 to about 0.040, from about 0.033 to about 0.037, from about 0.033 to about 0.035, from about 0.035 to about 0.045, from about 0.035 to about 0.043, from about 0.035 to about 0.041, from about 0.035 to about 0.039, from about 0.035 to about 0.037, from about 0.037 to about 0.045, from about 0.037 to about 0.043, from about 0.037 to about 0.041, or from about 0.037 to about 0.039.

[0099] The arrangement or configuration of core portions C within in the common cladding 19, as well as the associated CMF described herein, may reduce crosstalk, radiation loss, and/or attenuation while increasing transmission capacity. For example, with the configuration of the core portions C within the common cladding 19 and/or the CMF values described herein, each core portion C.sub.i may have an attenuation of less than 0.165 dB/km and a radiation loss of less than 0.01 dB/km at a wavelength of 1550 nm. Further, a counter-propagating crosstalk between two adjacent core portions may be less than or equal to 40 dB per 100 km, thereby allowing for uncoupled bidirectional transmission using the multicore optical fiber 110 described herein.

[0100] For bidirectional (or counterpropagating) transmission, such as discussed above with reference to FIG. 1, a first plurality of core portions and a second plurality of core portions may be utilized for transmission in opposite directions over the multicore optical fiber 110. In some embodiments, adjacent core portions may be configured for transmission in opposite directions. In some embodiments, the first plurality of core portions and the second plurality of core portions may be arranged in an alternating manner. For exempla, in the exemplary embodiment shown in FIG. 2, the first plurality of core portions C.sub.1, C.sub.3, C.sub.5, and C.sub.7 of the multicore optical fiber 110 may be configured for transmission in a first direction, and the second plurality of core portions C.sub.2, C.sub.4, C.sub.6, and C.sub.8 may be configured for transmission in a second direction opposite the first direction. At the first end 112 of the multicore optical fiber 110, the first plurality of core portions C.sub.1, C.sub.3, C.sub.5, and C.sub.7 may be coupled to the plurality of light sources of the first transceiver 180, and the second plurality of core portions C.sub.2, C.sub.4, C.sub.6, and C.sub.8 may be coupled to the plurality of detectors of the first transceiver 180. At the second end 114 of the multicore optical fiber 110, the first plurality of core portions C.sub.1, C.sub.3, C.sub.5, and C.sub.7 may be optically coupled to the plurality of detectors of the second transceiver 190, and the second plurality of core portions C.sub.2, C.sub.4, C.sub.6, and C.sub.8 may be optically coupled to the plurality of light sources of the second transceiver 190.

[0101] In some embodiments, the first plurality of core portions may include greater than or equal to 3 core portions, greater than or equal to 4 coating portions, or greater than or equal to 5 core portions. In some embodiments, the first plurality of core portions may less than or equal to 5 coating portions, less than or equal to 4 core portions, or less than or equal to 3 core portions. In some embodiments, the first plurality of core portions may include greater than or equal to 3 core portions and less than or equal to 5 core portions, greater than or equal to 3 core portions and less than or equal to 4 core portions, or greater than or equal to 4 core portions and less than or equal to 5 core portions. In some embodiments, the second plurality of core portions may include greater than or equal to 3 core portions, greater than or equal to 4 coating portions, or greater than or equal to 5 core portions. In some embodiments, the second plurality of core portions may less than or equal to 5 coating portions, less than or equal to 4 core portions, or less than or equal to 3 core portions. In some embodiments, the second plurality of core portions may include greater than or equal to 3 core portions and less than or equal to 5 core portions, greater than or equal to 3 core portions and less than or equal to 4 core portions, or greater than or equal to 4 core portions and less than or equal to 5 core portions. In some embodiments, the first plurality of core portions and the second plurality of core portions may include equal number of core portions. In some embodiments, the first plurality of core portions and the second plurality of core portions may include different number of core portions, depending on the application.

[0102] In some embodiments, such as the exemplary embodiment shown in FIG. 2, the core portions may be arranged around the central fiber axis 12, and no core portion may be positioned at or aligned with the central fiber axis 12 of the multicore optical fiber 110. In some embodiments, the multicore optical fiber 110 may include a core portion with its core centerline aligned with the central fiber axis 12. In some embodiments, all of the core portions may be arranged at an equal radial distance from the central fiber axis 12 of the multicore optical fiber 110. In some embodiments, one or more of the core portions may be positioned at a radial distance from the central fiber axis 12 that may be different from the radial distance at which another one or more of the core portions may be positioned. In some embodiments, two or more core portions may be arranged along a circle having an axis aligned with the central fiber axis 12, and the two or more core portions may be equally spaced apart along the circumference of the circle in some embodiments. In some embodiments, the two or more core portions may be spaced apart along the circumference of the circle by different distances. Although exemplary arrangement of core portions within the common cladding 19 are shown and described herein, these embodiments are not intended to be limiting, and other arrangement of the core portions within the common cladding 19 may be contemplated.

[0103] FIG. 3 schematically depicts a radial cross section of an exemplary core portions C.sub.i viewed along line II-II of FIG. 1, according to some embodiments. In some embodiments, each of the core portions C.sub.i may include a core region 150 centered on a central axis CL.sub.i and a cladding region 155 encircling and directly contacting the core region 150. In some embodiments, the core region 150 and the cladding region 155 may be concentric such that the cross section of the core portion C.sub.i may be generally circular symmetric with respect to the centerline CL.sub.i having an overall radius r.sub.Ci. The cladding region 155 may include an inner cladding region 160 (also referred to herein as an inner cladding layer) encircling and directly contacting the core region 150, and a trench region 170 encircling and directly contacting the inner cladding region 160. The common cladding 19 may encircle and direct contract the trench region 170 of each core portion C.sub.i and extend from the outer radius r.sub.3 of the trench region 170. Referring back to FIG. 2, the common cladding 19 may extend from the outer radius r.sub.3 of the trench region 170 to the outer radius R.sub.CC of the common cladding 19. The common cladding 19 may also extend from the outer radius r.sub.3 of the trench region 170 to the central axis 12 of the multicore optical fiber 110 and the other core portions such that the core portions may be embedded in the common cladding 19 of the multicore optical fiber 110.

[0104] The core region 150 may extend from the central axis CL.sub.i to an outer radius r.sub.1. The inner cladding region 160 may extend from the outer radius r.sub.1 of the core region 150 to an outer radius r.sub.2. The outer radius r.sub.1 of the core region 150 may coincide with the inner radius of the inner cladding region 160. The trench region 170 may extend from the outer radius r.sub.2 of the inner cladding region 160 to an outer radius r.sub.3. The outer radius r.sub.2 of the inner cladding region 160 may coincide with the inner radius of the trench region 170. The outer radius r.sub.3 of the trench region 170 may coincide with the radius r.sub.Ci associated with each core portion C.sub.i.

[0105] FIGS. 4 and 5 graphically depict exemplary relative refractive index profiles (relative to pure silica) of a core portion C.sub.1 and the surrounding common cladding 19, according to some embodiments. The relative refractive index profiles of each core portion C.sub.1 are plotted as a function of radial distance r from the centerline CL.sub.i of the core portion C.sub.i. Each of the relative refractive index profiles depicted in FIGS. 4 and 5 extend radially outward from a centerline CL.sub.i of the core portion Ci and into a portion of the common cladding 19.

[0106] As depicted in FIGS. 4 and 5, the core region 150 has a relative refractive index .sub.1 relative to pure silica. In some embodiments, the relative refractive index .sub.1 may vary with radial coordinate (radius) r and be represented as .sub.1(r). The relative refractive index .sub.1 of the core region 150 may be greater than the relative refractive indices of other regions of the core portion C.sub.i, such as the relative refractive index .sub.2 of the inner cladding region 160 and/or the relative refractive index .sub.3 of the trench region 170 of the core portion C.sub.1 as will be discussed in more detail below. In some embodiments, the relative refractive index .sub.1 may also be greater than the relative refractive index .sub.CC of the common cladding 19 of the multicore optical fiber 110.

[0107] In some embodiments, the core region 150 may include silica-based glass having an up-dopant. In some embodiments, the core region 150 may include silica-based glass doped with at least one alkali. In some embodiments, the core region 150 may include silica-based glass doped with one or more alkali chosen from a group comprising lithium, sodium, potassium, rubidium or a combination thereof. In some embodiments, the dopant consists of lithium, sodium, potassium, rubidium or a combination thereof. In some embodiments, the dopant consists essentially of lithium, sodium, potassium, rubidium or a combination thereof. In some embodiments, the core region 150 may include silica-based glass doped with sodium and potassium.

[0108] The average concentration of the alkali dopant in the core region 150 is defined as:

[00010]$C_{\mathrm{alkali},\mathrm{avg}}=\frac{8{}_0^{\mathrm{MFD}/2}{C}_{\mathrm{alkali}}\left(r\right)\mathrm{rdr}}{\mathrm{MFD}}$

where C.sub.alkali(r) is the concentration of the alkali as a function of radial distance from the center of the core region 150 and MFD is the mode field diameter of the core region 150 at the wavelength of interest (here considered as 1550 nm). In some embodiments, the average concentration of alkali in the core region 150 may be in the range from 10 ppm to 500 ppm, or from 20 ppm to 500 ppm, or from 25 ppm to 400 ppm, or from 50 ppm to 300 ppm. Because the core region 150 may include a maximum average alkali concentration of 500 ppm, the alkali dopant may only slightly increase the refractive index of the core region 150. Therefore, the core region 150 may still be able to effectively achieve a refractive index of about 0% even with such alkali up-doping. When core region 150 includes two or more alkali dopants (such as both potassium and rubidium), the average concentration of alkali dopant is the sum of the average concentration of each of the individual alkali dopants. In some embodiments, the silica glass of core region 150 may be free or substantially free of germanium and/or chlorine, and the core region 150 may include silica glass that may lack germanium and/or chlorine.

[0109] In some embodiments, the relative refractive index .sub.1(r) of the core region 150 may include a maximum relative refractive index .sub.1max. In some embodiments, the maximum relative refractive index .sub.1max of the core region 150 may be in a range from about 0.20% to about 0.20%, or in a range from about 0.15% to about 0.15%, or in a range from about 0.10% to about 0.10%, or in a range from about 0.05% to about 0.05%. In some embodiments, the relative refractive index .sub.1max may be about 0.0%. The relative refractive index .sub.1max may be preferably constant or approximately constant.

[0110] In some embodiments, the relative refractive index .sub.1(r) follows a graded index profile. The relative refractive index of each core region 150 may be described by an a-profile with an a value that may be in a range of about 7.0 or less, or about 6.0 or less, or about 5.0 or less, or about 4.0 or less, or about 3.0 or less, or about 2.0 or less, or about 1.0 or less. In some other embodiments, the a value may be about 8.0 or greater, or about 9.0 or greater, or about 10.0 or greater, or about 11.0 or greater, or about 12.0 or greater, or about 13.0 or greater. In some embodiments, the core region value may be about 10, or about 12, or about 20. Therefore, core region 150 can have either a graded-index profile or a step-index profile. For example, in some embodiments, the maximum relative refractive index .sub.1max may occur at r=0 (e.g., at the centerline CL.sub.i) and decrease with an alpha profile until reaching the radius r.sub.1. In some embodiments, the relative refractive index .sub.1 (r) may follow a step index profile. For example, in some embodiments, the relative refractive index .sub.1 (r) may remain substantially equal to the maximum relative refractive index .sub.1max until the radius r.sub.1.

[0111] In some embodiments, the outer radius r.sub.1 of the core region 150 may be greater than or equal to 3.0 microns and less than or equal to 7.0 micronsincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius r.sub.1 of the core region 150 may be greater than or equal to 3.5 microns and less than or equal to 6.5 microns, greater than or equal to 3.5 m and less than or equal to 6 m, greater than or equal to 4.0 m and less than or equal to 6.0 m. Providing a core radius r.sub.1 within this range facilitates each core portion C.sub.1 having a mode field diameter at 1550 nm greater than or equal to 9.5 m and less than or equal to 13.5 m.

[0112] Referring still to FIGS. 4 and 5, the inner cladding region 160 may extend from radius r.sub.1 to radius r.sub.2 such that the inner cladding region 160 may have a radial thickness T.sub.2=r.sub.2-r.sub.1. In some embodiments, the inner cladding region 160 may be formed from silica-based glass and have a relative refractive index .sub.2. In some embodiments, the inner cladding region 160 may include silica-based glass doped with down-dopants such that the relative refractive index .sub.2 may be less than 0. For example, in some embodiments, inner cladding region 160 may be down-doped with fluorine or boron. In some embodiments, the relative refractive index .sub.2 of the inner cladding region 160 may be similar to or the same as the relative refractive index .sub.CC of the common cladding 19. In some embodiments, a difference between the relative refractive index .sub.2 of the inner cladding region 160 and the relative refractive index .sub.CC of the common cladding 19 may range from about 0.05% to about 0.05%.

[0113] Without wishing to be bound by theory, it is believed that the value of r.sub.2 (and hence the radial thickness T.sub.2 of the inner cladding region 160) may in part determine the dispersion properties of each of the core portions C.sub.i and ability to achieve high mode field diameter with acceptable cutoff and bending properties. In some embodiments, the outer radius r.sub.2 of the inner cladding region 160 may be greater than or equal to 7 m and less than or equal to 15 mincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius r.sub.2 of the inner cladding region 160 may be greater than or equal to 7 m and less than or equal to 14 m, or greater than or equal to 8 m and less than or equal to 14 m, or greater than or equal to 9 m and less than or equal to 13 m, or greater than or equal to 10 m and less than or equal to 12 m, or greater than or equal to 8 m and less than or equal to 13 m, or or greater than or equal to 8 m and less than or equal to 11 m.

[0114] The relative refractive index .sub.2 of inner cladding region 160 may be in a range from about 0.20% to about 0.40%, or in a range from about 0.20% to about 0.30%, or in a range from about 0.21% to about 0.38%, or in a range from about 0.22% to about 0.36%, or in a range from about 0.25% to about 0.33%. In some embodiments, the relative refractive index .sub.2 may be about 0.25%, or about 0.26%, or about 0.31%, or about 0.34%, or about 0.35%. The relative refractive index .sub.2 may be preferably constant or approximately constant.

[0115] With further reference to FIGS. 4 and 5, the trench region 170 may extend from the radius r.sub.2 to the radius r.sub.3 such that the trench region 170 may have a radial thickness T.sub.3=r.sub.3r.sub.2. Without wishing to be bound by theory, it is believed that the value of r.sub.3 (and hence the radial thickness T.sub.3 of the trench region 170) may in part determine bend loss of each of the core portions C.sub.1 and inter-core crosstalk between the core portions. To achieve such optical properties, in some embodiments, the outer radius r.sub.3 of the trench region 170 may be greater than or equal to 7 m and less than or equal to 20 mincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius r.sub.3 of the trench region 170 may be greater than or equal to 7 m and less than or equal to 17 m, greater than or equal to 7 m and less than or equal to 13 m, greater than or equal to 7 m and less than or equal to 10 m, greater than or equal to 10 m and less than or equal to 20 m, greater than or equal to 10 m and less than or equal to 17 m, greater than or equal to 10 m and less than or equal to 13 m, greater than or equal to 13 m and less than or equal to 20 m, greater than or equal to 13 and less than or equal to 17 m, or greater than or equal to 17 m and less than or equal to 20 m.

[0116] The trench region 170 has a relative refractive index .sub.3. In some embodiments, the relative refractive index .sub.3 may be less than or equal to the relative refractive index .sub.2 of the inner cladding region 160 throughout the trench region 170. The relative refractive index .sub.3 may also be less than or equal to the relative refractive index .sub.CC of the common cladding 19. Accordingly, in some embodiments, .sub.2>.sub.3 and .sub.CC>.sub.3 such that the trench region 170 may form a depressed-index trench in a relative refractive index profile of each core portion between r.sub.2 and r.sub.3.

[0117] The term trench, as used herein, refers to a region of the core portion that is, in radial cross section, surrounded by regions of the multicore fiber (e.g., the inner cladding region 160 and the common cladding 19) having relatively higher refractive indexes. In some embodiments, the trench region 170 may include silica glass having one or more down-dopants (e.g., fluorine). In some embodiments, the down-dopant concentration within the trench region 170 may vary as a function of radial distance from the centerline CL.sub.i of the core portion C.sub.i.

[0118] In some embodiments, the relative refractive index .sub.3 may be substantially constant throughout the trench region 170. In other words, the relative refractive index .sub.3 of the trench region 170 may remain substantially constant from the radius r.sub.2 to the radius r.sub.3 such that the trench region 170 may have a substantially rectangular shape. In other embodiments, the relative refractive index .sub.3 may vary with radial coordinate r (radius) and be represented as .sub.3(r).

[0119] Still referring to FIGS. 4 and 5, in some embodiments, the relative refractive index .sub.3(r) of the trench region 170 may include a minimum relative refractive index .sub.3min. In some embodiments, the trench region 170 may include a minimum relative refractive index .sub.3min at radium r.sub.3. In some embodiments, a minimum relative refractive index .sub.3min may be maintained or substantially maintained throughout the trench region 170, such as from the outer radius r.sub.2 of the inner cladding region 160 to the outer radius r.sub.3 of the trench region 170.

[0120] In some embodiments, the relative refractive index .sub.3 or the minimum relative refractive index .sub.3min of the trench region 170 may be in a range from about 0.25% to about 0.7%including all sub-ranges and values there-between. For example, in some embodiments, the relative refractive index .sub.3 or the minimum relative refractive index .sub.3min of the trench region 170 may be in a range from about 0.25% to about 0.6%, from about 0.25% to about 0.5%, from about 0.25% to about 0.4%, from about 0.25% to about 0.3%, from about 0.3% to about 0.7%, from about 0.3% to about 0.6%, from about 0.3% to about 0.5%, from about 0.3% to about 0.4%, from about 0.4% to about 0.7%, from about 0.4% to about 0.6%, from about 0.4% to about 0.5%, from about 0.5% to about 0.7%, from about 0.5% to about 0.6%, or from about 0.6% to about 0.7%.

[0121] In some embodiments, each core portion C.sub.i may be fabricated such that the relative refractive index .sub.3 of the trench region 170 may be determined by a down-dopant concentration D that varies with radial coordinate r, i.e., D=D(r). In some embodiments, the down-dopant may include fluorine and D(r) may be expressed as a radially-dependent fluorine concentration F(r). As such, F(r) within the trench region 170 may vary between a minimum value F.sub.min and a maximum value F.sub.max. In some embodiments, F.sub.min may be at the radial position r.sub.2 and F.sub.max may be at the radial position r.sub.3. In some embodiments, F.sub.max may be greater than or equal to 1.2 wt. % and less than or equal to 2.2 wt. %. In some embodiments, F.sub.max may be greater than or equal to 1.5 wt. % and less than or equal to 2.2 wt. %. The values of the down-dopant concentrations (e.g., F.sub.max and F.sub.min) within the trench region 170 may determine the refractive index profile therein, and therefore the trench volume of the trench region 170 in FIGS. 3, 4, and 5.

[0122] Without wishing to be bound by theory, it is believed that the trench volume may influence one or more of cable cutoff, mode field diameter, effective area, dispersion, bend loss, and/or inter-core crosstalk of the core portions C.sub.i. For example, the core designs as described herein may provide a trench volume within the trench region 170 of each core portion C.sub.1 to be greater than or equal to 5% m.sup.2 and less than or equal to 60% m.sup.2 to achieve effective area of between 75 m.sup.2 and 135 m.sup.2, cable cutoff of less than 1530 nm, dispersion at 1550 nm of between 18 ps/nm/km and 22 ps/nm/km, and/or crosstalk less than 40 dB/100 km.

[0123] In some embodiments, the trench region 170 may include a trench volume greater than or equal to 5% micron.sup.2, greater than or equal to 10% micron.sup.2, greater than or equal to 15% micron.sup.2, greater than or equal to 20% micron.sup.2, greater than or equal to 25% micron.sup.2, greater than or equal to 30% micron.sup.2, greater than or equal to 35% micron.sup.2, greater than or equal to 40% micron.sup.2, greater than or equal to 45% micron.sup.2, greater than or equal to 50% micron.sup.2, or greater than or equal to 55% micron.sup.2. In some embodiments, the trench region 170 may include a trench volume less than or equal to 60% micron.sup.2, less than or equal to 55.5% micron.sup.2, less than or equal to 50% micron.sup.2, less than or equal to 45% micron.sup.2, less than or equal to 40% micron.sup.2, less than or equal to 35% micron.sup.2, less than or equal to 30% micron.sup.2, less than or equal to 25% micron.sup.2, less than or equal to 20% micron.sup.2, or less than or equal to 15% micron.sup.2.

[0124] In some embodiments, the trench region 170 may include a trench volume greater than or equal to 5% micron.sup.2 and less than or equal to 60% micron.sup.2including all sub-ranges and values there-between. For example, in some embodiments, the trench region 170 may include a trench volume greater than or equal to 5% micron.sup.2 and less than or equal to 55.5% micron.sup.2, greater than or equal to 5% micron.sup.2 and less than or equal to 45% micron.sup.2, greater than or equal to 5% micron.sup.2 and less than or equal to 35% micron.sup.2, greater than or equal to 5% micron.sup.2 and less than or equal to 25% micron.sup.2, greater than or equal to 5% micron.sup.2 and less than or equal to 15% micron.sup.2, greater than or equal to 15% micron.sup.2 and less than or equal to 60% micron.sup.2, greater than or equal to 15% micron.sup.2 and less than or equal to 55.5% micron.sup.2, greater than or equal to 15% micron.sup.2 and less than or equal to 45% micron.sup.2, greater than or equal to 15% micron.sup.2 and less than or equal to 35% micron.sup.2, greater than or equal to 15% micron.sup.2 and less than or equal to 25% micron.sup.2, greater than or equal to 25% micron.sup.2 and less than or equal to 60% micron.sup.2, greater than or equal to 25% micron.sup.2 and less than or equal to 55.5% micron.sup.2, greater than or equal to 25% micron.sup.2 and less than or equal to 45% micron.sup.2, greater than or equal to 25% micron.sup.2 and less than or equal to 35% micron.sup.2, greater than or equal to 35% micron.sup.2 and less than or equal to 60% micron.sup.2, greater than or equal to 35% micron.sup.2 and less than or equal to 55.5% micron.sup.2, greater than or equal to 35% micron.sup.2 and less than or equal to 45% micron.sup.2, greater than or equal to 45% micron.sup.2 and less than or equal to 60% micron.sup.2, or greater than or equal to 45% micron.sup.2 and less than or equal to 55.5% micron.sup.2.

[0125] FIG. 6 schematically depicts a radial cross section of another exemplary core portions C.sub.i viewed along line II-II of FIG. 1, according to some embodiments. FIG. 7 graphically depicts an exemplary relative refractive index profile (relative to pure silica) of a core portion C.sub.i (such as the exemplary core portion C.sub.i described herein with reference to FIG. 6) and the surrounding common cladding 19, according to some embodiments. The relative refractive index profile of the core portion C.sub.i is plotted as a function of radial distance r from the centerline CL.sub.i of the core portion C.sub.i, and extends radially outward from the centerline CL.sub.i of the core portion C.sub.i and into a portion of the common cladding 19.

[0126] Similar to the core portion C.sub.i described with reference to FIGS. 3, 4, and 5, in some embodiments, the core portion C.sub.i depicted in FIG. 6 may include a core region 150 centered on a central axis CL.sub.i and a cladding region 155 encircling and directly contacting the core region 150. In some embodiments, the core region 150 and the cladding region 155 may be concentric such that the cross section of the core portion C.sub.i may be generally circular symmetric with respect to the centerline CL.sub.i having the overall radius r.sub.Ci.

[0127] Different from the core portion C.sub.i described with reference to FIGS. 3, 4, and 5, the cladding region 155 of the exemplary embodiment of FIG. 6 may include only one region, i.e., the trench region 170 that may encircle and directly contact the core region 150. In other words, in some embodiments, the core portion C.sub.i, such as the core portion C.sub.i shown in FIG. 6, may not include an inner cladding region, such as the inner cladding region 160 described above with references to FIGS. 3, 4, and 5. The common cladding 19 may encircle and direct contact the trench region 170 (as shown in FIG. 7).

[0128] With further reference to FIGS. 6 and 7, the core region 150 may have an outer radius r.sub.1. The trench region 170 may extend from the outer radius r.sub.1 of the core region 150 to an outer radius r.sub.3 such that the outer radius r.sub.1 of the core region 150 may coincide with the inner radius of the trench region 170. The outer radius r.sub.3 of the trench region 170 may define an outer radius of the core portion C.sub.i such that the outer radius r.sub.3 of the trench region 170 may correspond to the radius r.sub.Ci associated with each core portion C.sub.i.

[0129] The outer radius r.sub.1 of the core region 150 may be greater than or equal to 3.0 microns and less than or equal to 7.0 micronsincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius r.sub.1 of the core region 150 may be in a range from about 3.5 m to about 6.5 m, from about 3.5 m to about 6 m, from about 4.0 m to about 6.0 m.

[0130] In some embodiments, the outer radius r.sub.3 of the trench region 170 may be greater than or equal to 7 m and less than or equal to 20 mincluding all sub-ranges and values there-between. For example, in some embodiments, the outer radius r.sub.3 of the trench region 170 may be greater than or equal to 7 m and less than or equal to 17 m, greater than or equal to 7 m and less than or equal to 13 m, greater than or equal to 7 m and less than or equal to 10 m, greater than or equal to 10 m and less than or equal to 20 m, greater than or equal to 10 m and less than or equal to 17 m, greater than or equal to 10 m and less than or equal to 13 m, greater than or equal to 13 m and less than or equal to 20 m, greater than or equal to 13 and less than or equal to 17 m, or greater than or equal to 17 m and less than or equal to 20 m.

[0131] The trench region 170 may have a thickness T.sub.3=r.sub.3r.sub.1 in the radial direction. The thickness T.sub.3 may be greater than or equal to 6 m and less than or equal to 16 mincluding all sub-ranges and values there-between. For example, in some embodiments, the thickness T.sub.3 of the trench region 170 may range from about 6 m to about 14 m, from about 6 m to about 12 m, from about 6 m to about 10 m, from about 6 m to about 8 m, from about 8 m to about 16 m, from about 8 m to about 14 m, from about 8 m to about 12 m, from about 8 m to about 10 m, from about 10 m to 16 m, from about 10 m to about 14 m, from about 10 m to about 12 m, from about 12 m to about 16 m, from about 12 m to about 14 m, or from about 14 m to about 16 m.

[0132] The core region 150 may include a refractive index similar to that of the core region 150 described above with reference to FIGS. 3, 4, and 5. In some embodiments, the core region 150 may include a silica-based glass having one or more up-dopants, such as one or more alkali chosen from a group comprising lithium, sodium, potassium, rubidium or a combination thereof, and have a relative refractive index .sub.1 relative to pure silica. The relative refractive index .sub.1 of the core region 150 may be greater than the relative refractive index .sub.3 of the trench region 170 of the core portion C.sub.i. In some embodiments, the relative refractive index .sub.1 may also be greater than the relative refractive index .sub.CC of the common cladding 19 of the multicore optical fiber 110. In some embodiments, the relative refractive index .sub.1 may vary with radial coordinate (radius) r and be represented as .sub.1(r). In some embodiments, the relative refractive index .sub.1(r) of the core region 150 may include a maximum relative refractive index .sub.1max. In some embodiments, the maximum relative refractive index .sub.1max of the core region 150 may be in a range from about 0.20% to about 0.20%, or in a range from about 0.15% to about 0.15%, or in a range from about 0.10% to about 0.10%, or in a range from about 0.05% to about 0.05%.

[0133] In some embodiments, the trench region 170 may be formed from silica-based glass doped with down-dopants (e.g., fluorine or boron) such that the trench region 170 may have a relative refractive index .sub.3 less than 0. The relative refractive index .sub.3 of the trench region 170 may include a minimum relative refractive index .sub.3min. The minimum relative refractive index .sub.3min of the trench region 170 depicted in FIG. 7 may be greater than the minimum relative refractive index .sub.3min of the trench region 170 described with reference to FIGS. 3, 4, and 5 but less than the relative refractive index .sub.CC of the common cladding 19.

[0134] In some embodiments, the difference between the refractive index .sub.CC of the common cladding region 19 and the minimum relative refractive index .sub.3min of the trench region 170 may be in a range from about 0.02% to about 0.5%including all sub-ranges and values there-between. For example, in some embodiments, the difference in the refractive index of the common cladding region 19 and the minimum relative refractive index .sub.3minimum of the trench region 170 may be in a range from about 0.02% to about 0.45%, from about 0.02% to about 0.4%, from about 0.02% to about 0.35%, from about 0.02% to about 0.3%, from about 0.02% to about 0.25%, from about 0.02% to about 0.2%, from about 0.02% to about 0.1%, from about 0.02% to about 0.05%, from about 0.05% to about 0.5%, from about 0.05% to about 0.45%, from about 0.05% to about 0.4%, from about 0.05% to about 0.35%, from about 0.05% to about 0.3%, from about 0.05% to about 0.25%, from about 0.05% to about 0.2%, from about 0.05% to about 0.1%, from about 0.05% to about 0.05%, from about 0.1% to about 0.5%, from about 0.1% to about 0.45%, from about 0.1% to about 0.4%, from about 0.1% to about 0.35%, from about 0.1% to about 0.3%, from about 0.1% to about 0.25%, from about 0.1% to about 0.2%, from about 0.15% to about 0.5%, from about 0.15% to about 0.45%, from about 0.15% to about 0.4%, from about 0.15% to about 0.35%, from about 0.15% to about 0.3%, from about 0.15% to about 0.25%, from about 0.15% to about 0.2%, from about 0.2% to about 0.5%, from about 0.2% to about 0.45%, from about 0.2% to about 0.4%, from about 0.2% to about 0.35%, from about 0.2% to about 0.3%, from about 0.2% to about 0.25%, from about 0.25% to about 0.5%, from about 0.25% to about 0.45%, from about 0.25% to about 0.4%, from about 0.25% to about 0.35%, from about 0.25% to about 0.3%, from about 0.3% to about 0.5%, from about 0.3% to about 0.45%, from about 0.3% to about 0.4%, from about 0.3% to about 0.35%, from about 0.35% to about 0.5%, from about 0.35% to about 0.45%, from about 0.35% to about 0.4%, from about 0.4% to about 0.5%, from about 0.4% to about 0.45%, or from about 0.45% to about 0.5%. The relative refractive index .sub.3, may be preferably constant or approximately constant. The performance of the relative refractive profiles depicted in FIGS. 4, 5, and 7, as well as specific values associated therewith, will be described in more detail below.

[0135] In some embodiments, the trench region 170 may include a trench volume greater than or equal to 3% micron.sup.2, greater than or equal to 15% micron.sup.2, greater than or equal to 30% micron.sup.2, greater than or equal to 45% micron.sup.2. In some embodiments, the trench region 170 may include a trench volume less than or equal to 60% micron.sup.2, less than or equal to 50% micron.sup.2, less than or equal to 35% micron.sup.2, or less than or equal to 20% micron.sup.2. In some embodiments, the trench region 170 may include a trench volume greater than or equal to 3% micron.sup.2 and less than or equal to 60% micron.sup.2including all sub-ranges or values therebetween. For example, in some embodiments, the trench region 170 may include a trench volume greater than or equal to 3% micron.sup.2 and less than or equal to 45% micron.sup.2, greater than or equal to 3% micron.sup.2 and less than or equal to 30% micron.sup.2, greater than or equal to 3% micron.sup.2 and less than or equal to 15% micron.sup.2, greater than or equal to 20% micron.sup.2 and less than or equal to 60% micron.sup.2, greater than or equal to 20% micron.sup.2 and less than or equal to 45% micron.sup.2, greater than or equal to 20% micron.sup.2 and less than or equal to 30% micron.sup.2, greater than or equal to 35% micron.sup.2 and less than or equal to 60% micron.sup.2, greater than or equal to 35% micron.sup.2 and less than or equal to 45% micron.sup.2, or greater than or equal to 50% micron.sup.2 and less than or equal to 60% micron.sup.2.

[0136] In some embodiments, the various designs of the core portion C.sub.1 described herein may produce core portions C.sub.1 with cable cutoff values of less than or equal to 1530 nm to allow for single-mode operation at 1550 nm. In some embodiments, the cable cutoff of each core portion C.sub.1 may be less than or equal to 1530 nm, less than or equal to 1500 nm, less than or equal to 1450 nm, less than or equal to 1400 nm, or less than or equal to 1350 nm. In some embodiments, the cable cutoff of each core portion C.sub.1 may be greater than or equal to 1300 nm and less than or equal to 1530 nmincluding all sub-ranges and values there-between. For example, in some embodiments, the cable cutoff of each core portion C.sub.1 may be greater than or equal to 1300 nm and less than 1500 nm, greater than or equal to 1300 nm and less than 1450 nm, greater than or equal to 1300 nm and less than or equal to 1400, greater than or equal to 1350 nm and less than 1530 nm, greater than or equal to 1350 nm and less than 1500 nm, greater than or equal to 1350 nm and less than 1450 nm, greater than or equal to 1350 nm and less than or equal to 1400 nm, greater than or equal to 1400 nm and less than 1530 nm, greater than or equal to 1400 nm and less than 1500 nm, or greater than or equal to 1400 nm and less than 1450 nm.

[0137] In some embodiments, each core portion C.sub.i of the multicore optical fiber 110 may have an effective area A.sub.eff at 1550 nm wavelength greater than or equal to 75 m.sup.2, greater than or equal to 80 m.sup.2, greater than or equal to 85 m.sup.2, greater than or equal to 90 m.sup.2, greater than or equal to 95 m.sup.2, greater than or equal to 100 m.sup.2, greater than or equal to 105 m.sup.2, greater than or equal to 110 m.sup.2, greater than or equal to 115 m.sup.2, greater than or equal to 120 m.sup.2, greater than or equal to 125 m.sup.2, or greater than or equal to 130 m.sup.2. In some embodiments, each core portion C.sub.i of the multicore optical fiber 110 may have an effective area A.sub.eff at 1550 nm wavelength less than or equal to 135 m.sup.2, less than or equal to 130 m.sup.2, less than or equal to 125 m.sup.2, less than or equal to 120 m.sup.2, less than or equal to 115 m.sup.2, less than or equal to 110 m.sup.2, less than or equal to 105 m.sup.2, less than or equal to 100 m.sup.2, less than or equal to 95 m.sup.2, less than or equal to 90 m.sup.2, less than or equal to 85 m.sup.2, or less than or equal to 80 m.sup.2.

[0138] In some embodiments, each core portion C.sub.i of the multicore optical fiber 110 may have an effective area A.sub.eff at 1550 nm wavelength greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2including all sub-ranges and values there-between. For example, in some embodiments, each core portion C.sub.i of the multicore optical fiber 110 may have an effective area A.sub.eff at 1550 nm wavelength greater than or equal to 80 m.sup.2 and less than or equal to 135 m.sup.2, greater than or equal to 90 m.sup.2 and less than or equal to 135 m.sup.2, greater than or equal to 100 m.sup.2 and less than or equal to 135 m.sup.2, greater than or equal to 110 m.sup.2 and less than or equal to 135 m.sup.2, greater than or equal to 75 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 80 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 85 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 90 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 95 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 100 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 110 m.sup.2 and less than or equal to 130 m.sup.2, greater than or equal to 75 m.sup.2 and less than or equal to 125 m.sup.2, greater than or equal to 80 m.sup.2 and less than or equal to 125 m.sup.2, greater than or equal to 90 m.sup.2 and less than or equal to 125 m.sup.2, greater than or equal to 100 m.sup.2 and less than or equal to 125 m.sup.2, greater than or equal to 110 m.sup.2 and less than or equal to 125 m.sup.2, greater than or equal to 80 m.sup.2 and less than or equal to 120 m.sup.2, greater than or equal to 80 m.sup.2 and less than or equal to 110 m.sup.2, greater than or equal to 90 m.sup.2 and less than or equal to 120 m.sup.2, greater than or equal to 90 m.sup.2 and less than or equal to 110 m.sup.2, or greater than or equal to 100 m.sup.2 and less than or equal to 120 m.sup.2, greater than or equal to 75 m.sup.2 and less than or equal to 115 m.sup.2, greater than or equal to 80 m.sup.2 and less than or equal to 115 m.sup.2, greater than or equal to 90 m.sup.2 and less than or equal to 115 m.sup.2, greater than or equal to 100 m.sup.2 and less than or equal to 115 m.sup.2, or greater than or equal to 110 m.sup.2 and less than or equal to 115 m.sup.2. The effective area is determined individually for each core portion C.sub.i of the multicore optical fiber 110 without consideration of the effects of crosstalk between the core portions C.sub.i of the multicore optical fiber 110.

[0139] In some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength greater than or equal to 9.5 m, greater than or equal to 10.5 m, greater than or equal to 11.5 m, or greater than or equal to 12.5 m. In some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength less than or equal to 13.5 m, less than or equal to 13 m, less than or equal to 12 m, less than or equal to 11 m, or less than or equal to 10 m. In some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength greater than or equal to 9.5 m and less than or equal to 13.5 mincluding all sub-ranges or values therebetween. For example, in some embodiments, the core region 150 disclosed herein may have a mode field diameter at 1550 nm wavelength greater than or equal to 10 m and less than or equal to 13.5 m, greater than or equal to 11 m and less than or equal to 13.5 m, greater than or equal to 12 m and less than or equal to 13.5, greater than or equal to 13 m and less than or equal to 13.5 m, greater than or equal to 10 m and less than or equal to 12.5 m, greater than or equal to 11 m and less than or equal to 12.5 m, greater than or equal to 12 m and less than or equal to 12.5, greater than or equal to 10 m and less than or equal to 11.5 m, greater than or equal to 11 m and less than or equal to 11.5 m, or greater than or equal to 10 m and less than or equal to 10.5 m.

[0140] In some embodiments, the core portion C.sub.i described herein may have a dispersion at 1550 nm of greater than or equal to 18 ps/nm/km and less than or equal to 22 ps/nm/kmincluding all sub-ranges and values there-between. For example, in some embodiments, the core portion C.sub.i described herein may have a dispersion at 1550 nm of greater than or equal to 18 ps/nm/km and less than or equal to 21 ps/nm/km, greater than or equal to 18 ps/nm/km and less than or equal to 20 ps/nm/km, greater than or equal to 18 ps/nm/km and less than or equal to 19 ps/nm/km, greater than or equal to 19 ps/nm/km and less than or equal to 22 ps/nm/km, greater than or equal to 19 ps/nm/km and less than or equal to 21 ps/nm/km, greater than or equal to 19 ps/nm/km and less than or equal to 20 ps/nm/km, greater than or equal to 20 ps/nm/km and less than or equal to 22 ps/nm/km, or greater than or equal to 20 ps/nm/km and less than or equal to 21 ps/nm/km.

[0141] In some embodiments, a co-propagating crosstalk at a wavelength of 1550 nm between two adjacent core portions may be less than or equal to 40 dB per 100 km of the multicore optical fiber 110, may be less than or equal to 45 dB per 100 km of the multicore optical fiber 110, or may be less than equal to 50 dB per 100 km of the multicore optical fiber 110. In some embodiments, a counter-propagating crosstalk at a wavelength of 1550 nm between two adjacent core portions may be less than or equal to 40 dB per 100 km of the multicore optical fiber 110, may be less than or equal to 45 dB per 100 km of the multicore optical fiber 110, or may be less than or equal to 50 dB per 100 km of the multicore optical fiber 110. In some embodiments, the counter-propagating crosstalk at a wavelength of 1550 nm between two adjacent core portions may be in the range from 40 dB to 100 dB per 100 km of the multicore optical fiber 110, from 45 dB to 100 dB per 100 km of the multicore optical fiber 110, from 50 dB to 100 dB per 100 km of the multicore optical fiber 110, or from 60 dB to 100 dB per 100 km of the multicore optical fiber 110.

[0142] As discussed above, the configuration of the multicore optical fiber 110 described herein may allow for transmission, such as counter-propagating transmission, at one or more wavelengths of C band (from 1535 nm to 1565 nm) and/or L band (from 1565 nm to 1625 nm) without inducing unacceptable level of crosstalk. In some embodiments, a counter-propagating crosstalk between two adjacent core portions at one or more wavelength in the C band and/or L band may be less than or equal to 40 dB per 100 km of the multicore optical fiber 110, may be less than or equal to 45 dB per 100 km of the multicore optical fiber 110, or may be less than or equal to 50 dB per 100 km of the multicore optical fiber 110. In some embodiments, the counter-propagating crosstalk between two adjacent core portions at one or more wavelength in the C band and/or L band may be in the range from 40 dB to 100 dB per 100 km of the multicore optical fiber 110, from 45 dB to 100 dB per 100 km of the multicore optical fiber 110, from 50 dB to 100 dB per 100 km of the multicore optical fiber 110, or from 60 dB to 100 dB per 100 km of the multicore optical fiber 110.

[0143] The average attenuation of the multicore optical fiber 110 is determined by measuring the attenuation for each core portion C.sub.i of the multicore optical fiber 110 at a wavelength of 1310 nm or 1550 nm and then calculating an average attenuation for the entire uncoupled-core multicore optical fiber 110 based on the individual attenuation measurements of each core portion C.sub.i. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.165 dB/km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.16 dB/km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.155 dB/km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be less than 0.15 dB/km. In some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be in a range from about 0.13 dB/km to about 0.165 dB/kmincluding all sub-ranges and values there-between. For example, in some embodiments, the average attenuation at 1550 nm of the multicore optical fiber 110 may be in a range from about 0.13 dB/km to about 0.16 dB/km, from about 0.13 dB/km to about 0.155 dB/km, from about 0.13 dB/km to about 0.15 dB/km, from about 0.13 dB/km to about 0.145 dB/km, from about 0.13 dB/km to about 0.14 dB/km, from about 0.14 dB/km to about 0.165 dB/km, from about 0.14 dB/km to about 0.16 dB/km, from about 0.14 dB/km to about 0.155 dB/km, from about 0.14 dB/km to about 0.15 dB/km, from about 0.14 dB/km to about 0.145 dB/km, from about 0.145 dB/km to about 0.165 dB/km, from about 0.145 dB/km to about 0.16 dB/km, from about 0.145 dB/km to about 0.155 dB/km, from about 0.145 dB/km to about 0.15 dB/km, from about 0.15 dB/km to about 0.165 dB/km, from about 0.15 dB/km to about 0.16 dB/km, from about 0.15 dB/km to about 0.155 dB/km, or from about 0.155 dB/km to about 0.16 dB/km.

[0144] The individual attenuation of each core portion C.sub.i at 1550 nm may be less than 0.165 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i at 1550 nm may be less than 0.16 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i at 1550 nm may be less than 0.155 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i at 1550 nm may be less than 0.15 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i at 1550 nm may be in a range from about 0.13 dB/km to about 0.165 dB/kmincluding all sub-ranges and values there-between. For example, in some embodiments, the individual attenuation of each core portion C.sub.i at 1550 nm may be in a range from about 0.13 dB/km to about 0.16 dB/km, from about 0.13 dB/km to about 0.155 dB/km, from about 0.13 dB/km to about 0.15 dB/km, from about 0.13 dB/km to about 0.145 dB/km, from about 0.13 dB/km to about 0.14 dB/km, from about 0.14 dB/km to about 0.165 dB/km, from about 0.14 dB/km to about 0.16 dB/km, from about 0.14 dB/km to about 0.155 dB/km, from about 0.14 dB/km to about 0.15 dB/km, from about 0.14 dB/km to about 0.145 dB/km, from about 0.145 dB/km to about 0.165 dB/km, from about 0.145 dB/km to about 0.16 dB/km, from about 0.145 dB/km to about 0.155 dB/km, from about 0.145 dB/km to about 0.15 dB/km, from about 0.15 dB/km to about 0.165 dB/km, from about 0.15 dB/km to about 0.16 dB/km, from about 0.15 dB/km to about 0.155 dB/km, from about 0.155 dB/km to about 0.165 dB/km, or from about 0.155 dB/km to about 0.16 dB/km.

[0145] As discussed above, the configuration of the multicore optical fiber 110 described herein may allow for transmission at one or more wavelengths of C band (from 1535 nm to 1565 nm) and/or L band (from 1565 nm to 1625 nm) without inducing unacceptable level of attenuation. In some embodiments, the individual attenuation of each core portion C.sub.i or the average attenuation thereof at one or more wavelength in the C band and/or L band may be less than 0.165 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i or the average thereof at one or more wavelength in the C band and/or L band may be less than 0.16 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i or the average thereof at one or more wavelength in the C band and/or L band may be less than 0.155 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i or the average thereof at one or more wavelength in the C band and/or L band may be less than 0.15 dB/km. In some embodiments, the individual attenuation of each core portion C.sub.i or the average thereof at one or more wavelength in the C band and/or L band may be in a range from about 0.13 dB/km to about 0.165 dB/kmincluding all sub-ranges and values there-between. For example, in some embodiments, the individual attenuation of each core portion C.sub.i or the average thereof at one or more wavelength in the C band and/or L band may be in a range from about 0.13 dB/km to about 0.16 dB/km, from about 0.13 dB/km to about 0.155 dB/km, from about 0.13 dB/km to about 0.15 dB/km, from about 0.13 dB/km to about 0.145 dB/km, from about 0.13 dB/km to about 0.14 dB/km, from about 0.14 dB/km to about 0.165 dB/km, from about 0.14 dB/km to about 0.16 dB/km, from about 0.14 dB/km to about 0.155 dB/km, from about 0.14 dB/km to about 0.15 dB/km, from about 0.14 dB/km to about 0.145 dB/km, from about 0.145 dB/km to about 0.165 dB/km, from about 0.145 dB/km to about 0.16 dB/km, from about 0.145 dB/km to about 0.155 dB/km, from about 0.145 dB/km to about 0.15 dB/km, from about 0.15 dB/km to about 0.165 dB/km, from about 0.15 dB/km to about 0.16 dB/km, or from about 0.155 dB/km to about 0.165 dB/km.

[0146] The radiation loss, as discussed herein, represents mode leakage loss that may be due to a smaller core edge to glass fiber edge distance and overlap of mode field intensity with high index primary coating. Methods for determining radiation loss can be found in P. Tandon, et al., Impact of multicore fiber (MCF) opticals, cross-talk, radiative leakage loss, splice loss and propagation configuration on the system transmission performance, Optics Communications, Volume 539, 15 Jul. 2023, 129483, and P. Tandon, et al., Record Low Loss 0.144 dB/km 2-Core Optical Fiber for Submarine Transmission, Journal of Lightwave Technology, Vol. 42, No. 12, Jun. 15, 2024, J. Hu, et al., Understanding leaky modes: slab waveguide revisited, Adv. Opt. Photonics 1 (2009) 58-106, the contents of which are all incorporated herein by reference in their entirety. In some embodiments, the radiation loss of each core portion C.sub.i may be less than about 0.01 dB/km at a wavelength of 1550 nm, less than about 0.009 dB/km at a wavelength of 1550 nm, less than about 0.007 dB/km at a wavelength of 1550 nm, less than about 0.005 dB/km at a wavelength of 1550 nm, or less than about 0.003 dB/km at a wavelength of 1550 nm. In some embodiments, the radiation loss of each core portion C.sub.i may be in the range from 0.0001 dB/km to 0.01 dB/km at a wavelength of 1550 nm, from 0.0001 dB/km to 0.009 dB/km at a wavelength of 1550 nm, from 0.0001 dB/km to 0.007 dB/km at a wavelength of 1550 nm, from 0.0001 dB/km to 0.005 dB/km at a wavelength of 1550 nm, or from 0.0001 dB/km to 0.003 dB/km at a wavelength of 1550 nm.

[0147] As discussed above, the configuration of the multicore optical fiber 110 described herein may allow for transmission at one or more wavelengths of C band (from 1535 nm to 1565 nm) and/or L band (from 1565 nm to 1625 nm) without inducing unacceptable level of radiation loss. In some embodiments, the radiation loss of each core portion C.sub.i at one or more wavelengths in the C band and/or L band may be less than about 0.01 dB/km, less than about 0.009 dB/km, less than about 0.007 dB/km, less than about 0.005 dB/km, or less than about 0.003 dB/km. In some embodiments, the radiation loss of each core portion C.sub.i at one or more wavelengths in the C band and/or L band may be in the range from 0.0001 dB/km to 0.01 dB/km, from 0.0001 dB/km to 0.009 dB/km, from 0.0001 dB/km to 0.007 dB/km, from 0.0001 dB/km to 0.005 dB/km, or from 0.0001 dB/km to 0.003 dB/km.

[0148] The uncoupled-core multicore optical fibers of the present disclosure can be made using any suitable method for forming a multicore optical fiber. See, for example, U.S. Pat. No. 11,370,689 B2, the entire content of which is incorporated herein by reference. An exemplary method that is used to form the multicore optical fiber (or any of the alternative embodiments thereof) described herein includes forming a glass blank for common cladding. Formation of the glass blank may involve first forming a soot body via an outside vapor deposition (OVD) process, a soot pressing method, a vapor axial deposition (VAD) process, or any other known method and then dehydrated and consolidated to fully densified glass. The soot body may be formed of a glass precursor material. In some embodiments, the soot body is formed of silica-based material. Multiple holes are then drilled along the length of the glass blanks for the core canes to be inserted in them. In some embodiments, the common cladding glass blank is doped with a down dopant such as fluorine.

[0149] Next, a core region of a core cane may be formed. In some embodiments, the core region is comprised of at least one alkali in concentration between 0.1 wt % and 5 wt %. In some embodiments, the core is doped with potassium. In other embodiments, the core is doped with two alkali components selected from the group comprising potassium, sodium, rubidium, and cesium. The core cane may be formed by methods disclosed in U.S. Pat. No. 7,536,076 B2, the entire content of which is incorporated herein by reference.

[0150] Next, one or more clad layers may be deposited on the core region. In some embodiments, soot overclad layer(s) of silica-based soot is formed on the core region via an OVD or VAD process. The overclad layer(s) has a composition corresponding to the depressed-index cladding of the core portion of the multicore optical fiber. For example, the overlayers may include separate layers having a composition corresponding to trench region. The overcladded core region is positioned within a consolidation furnace and consolidation of the overcladded core region is initiated. For example, the overcladded core region may be heated to a peak sintering temperature to initiate consolidation.

[0151] During the consolidation, the overcladded core region is exposed to a down-dopant for a period T after initiation of the consolidation. A soot preform resulting from the completion of the consolidation process includes a core region and an overcladding layer surrounding the core region. The core region (e.g., in an unconsolidated or partially consolidated state) and overcladding layer may be placed in an interior of a consolidation furnace. The consolidation furnace may be heated to a peak sintering temperature of the overcladding layer to initiate consolidation.

[0152] A gas source is in fluid communication with the interior of the consolidation furnace provides a gas containing a down-dopant into the interior. The down-dopant (e.g., fluorine) then diffuses through the overcladding layer during consolidation. In some embodiments, the rate of diffusion of the down-dopant through the overcladding layer is dependent on the compositional and material properties of the overcladding layer (e.g., porosity, density, etc.). As the overcladding layer is consolidated, the porosity of the overcladding layer is diminished such that a rate of diffusion of the down-dopant may decrease as the overcladding layer consolidates.

[0153] In some embodiments, a region of the core portion encircling and directly contacting the core corresponds to the trench region. In some embodiments, trench region possess a concentration of the down-dopant.

[0154] After the core region is consolidated into a glass preform, the glass preform is inserted into the holes drilled into the glass blank formed during the step. After each core cane is inserted into the glass blank, the fiber preform is assembled by thermally closing the gap between the inserted cane and the drilled hole. The assembled preform is then drawn into a multicore optical fiber. The multicore optical fiber may then be coated with one or more coatings, such as a primary coating, a secondary coating, etc. Example methods of forming a cane-based optical fiber preform are discussed in U.S. Pat. No. 11,370,689, the entire content of which is incorporated by reference herein. Example coating materials and methods are discussed in U.S. Pat. No. 9,057,817, the entire content of which is incorporated by reference herein.

Examples

[0155] Provided below are exemplary embodiments of the multicore optical fibers disclosed herein. The below exemplary embodiments are intended to be exemplary and are not intended to limit the scope of the disclosure.

[0156] Table 1 below provides three exemplary core portion designs (Design A, Design B, and Design C) of the multicore optical fibers according to some embodiments described herein. Design A has a relative refractive index profile similar to that depicted in FIG. 4, Design B has a relative refractive index profile similar to that depicted in FIG. 7, and Design C has a relative refractive index profile similar to that depicted in FIG. 5.

TABLE-US-00001 TABLE 1 Design A Design B Design C Core Region Refractive 0 0 0 Index (.sub.1%) Core Radius r.sub.1 (m) 5.55 6.1 4.67 Core Alpha 20 20 20 Inner Cladding Refractive 0.245 0.34 Index (.sub.2%) Inner Cladding Region 10 10 Radius r.sub.2 (m) Trench Region Refractive 0.555 0.33 0.61 Index, .sub.3% Trench Region Radius, 16.5 16.5 17.63 r.sub.3 (m) Trench Volume (% m.sup.2) _53.4_ 20 .sub.56.9 Common Cladding 0.245 0.245 0.34 Refractive Index (.sub.4%) Cable Cutoff (nm) 1386 1423 1383 Effective Area A.sub.eff at 110.39 110.23 82.53 1550 nm (m.sup.2) Dispersion at 1550 nm 21.056 20.9 19.09 (ps/nm/km) Dispersion Slope at 6.43E02 6.09E02 6.27E02 1550 nm (ps/nm.sup.2/km)

[0157] FIG. 8 illustrates the impact of crosstalk on the SNR performance of bidirectional (or counterpropagating) transmission system, based on calculation as further described in P. Tandon, et al., Record Low Loss 0.144 dB/km 2-Core Optical Fiber for Submarine Transmission, Journal of Lightwave Technology, Vol. 42, No. 12, Jun. 15, 2024, P. Tandon, et al., Impact of multicore fiber (MCF) optical, cross-talk, radiative leakage loss, splice loss and propagation configuration on the system transmission performance, Opt. Commun., vol. 539, July 2023, Art. no. 129483, doi: 10.1016/j.optcom.2023.129483, M. Ohashi, et al., Optical fibers for space-division multiplexing, in Space Division Multiplexing in Optical Communication Systems, M. Nakazawa, et al., Eds. Cham, Switzerland: Springer, 2022, Ch. 2, pp. 40-170, T. Hayashi, et al., Uncoupled multi-core fiber enhancing signal-to-noise ratio, Opt. Exp., vol. 20, pp. B94-B103, 2012, J. M. Gene and P. J. Winzer, A universal specification for multicore Fiber crosstalk, IEEE Photon. Technol. Lett., vol. 31, no. 9, pp. 673-676, May 2019, T. Hayashi, et al., Low loss multi-core fibers for submarine transmission, in Proc. SubOptic, 2019, Paper OP10-3, the contents of which are all incorporated herein by reference in their entirety.

[0158] In FIG. 8, the sensitivity of transmission system SNR or SNR between single-core fiber (SCF) and multicore optical fiber (MCF) on crosstalk was calculated for: number of spans=20, span length=100 km, WDM bandwidth=20 THz, and amplifier noise figure=4-6 dB. It has been observed that SNR may be negatively impacted when crosstalk between adjacent core portions becomes greater than about 45 dB/100 km (or about 65 dB/km).

[0159] FIG. 9 illustrates the calculated inter-core crosstalk for co-propagating transmission and inter-core crosstalk for counter-propagating transmission per 100 km for multicore optical fibers having an effective area A.sub.eff of 112 m.sup.2. It has been observed that utilizing the multicore optical fibers described herein for bidirectional (or counterpropagating) transmission may reduce crosstalk appreciably when compared to co-propagating mode of transmission, thereby allowing a greater number of core portions to be accommodated in the multicore optical fiber to achieve increased transmission capacity. Details of crosstalk calculations can be found in P. Tandon, et al., Record Low Loss 0.144 dB/km 2-Core Optical Fiber for Submarine Transmission, Journal of Lightwave Technology, Vol. 42, No. 12, Jun. 15, 2024, M. Koshiba, et al., Analytical expression of average power-coupling coefficients for estimating intercore CrossTalk in multicore fibers, IEEE Photon. J, vol. 4, no. 50, pp. 1987-1995, October 2012, F. Ye, et al., Simple analytical expression for crosstalk estimation in homogeneous trench-assisted multi-core fibers, Opt. Exp., vol. 22, no. 19, pp. 23007-23018, 2014, T. Hayashi, et al., Design and fabrication of ultra-low crosstalk and low-loss multi-core fiber, Opt. Exp., vol. 19, pp. 16576-16592, 2011, T. Hayashi, et al., Uncoupled multi-core fiber design for practical bidirectional optical communications, in Proc. Opt. Fiber Commun. Conf, 2022, Paper M1E.1, and A. Sano, et al., Crosstalk-managed high capacity long haul multicore Fiber transmission with propagation direction interleaving, J. Lightw. Technol., vol. 32, no. 16, pp. 2771-2779, August 2014, the contents of which are all incorporated herein by reference in their entirety.

[0160] FIG. 10 illustrates the calculated counter-propagating crosstalk for core portion Designs A, B, and C of Table 1. Without intending to be bound by any particular theory, to achieve low crosstalk between adjacent core portions, the core-to-core separation distance may need to be greater than a certain distance for the core portions to be uncoupled. However, increasing the core-to-core separation distance to help with reducing the crosstalk may position the core portions closer to the outer radius of the glass fiber, which may lead to increased attenuation due to radiation leakage loss from overlap of mode field with the high refractive index coating. To minimize radiation leakage loss while also maintaining low crosstalk between adjacent core portions, the inventors have designed the multicore optical fibers described herein. Table 2 below provides six exemplary multicore optical fiber designs (Examples 1-6) utilizing various core designs in accordance with some embodiments described herein. Examples 1, 3, and 5 each have a core arrangement similar to that shown in FIG. 2, Examples 2 and 4 each have a core arrangement similar to that shown in FIG. 11, and Example 6 has a core arrangement similar to that shown in FIG. 12. In the examples shown in FIGS. 11 and 12, the core portions are arranged annularly about the central fiber axis, and the core portions in the respective examples are positioned at an equal radial distance from the central fiber axis.

TABLE-US-00002 TABLE 2 Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Core Design Design A Design A Design B Design B Design C Design C Number of Cores 8 6 8 6 8 10 Inter-Core Distance (m) 36 36 40 40 33 33 Circumradius (m) 47 36 52 40 43 53 Glass Fiber Radius (m) 84 73 89 78 80 90 Glass Fiber Diameter (m) 168 146 178 156 160 180 Coating Diameter (m) 250 250 250 250 250 250 Effective Area (m.sup.2) 110 110 110 110 82 82 Number of Cores per 0.000361 0.000358 0.000321 0.000314 0.000398 0.000393 Glass Fiber Area* (m.sup.2) Core Multiplicity Factor 0.039699 0.039423 0.035363 0.034531 0.032627 0.032224 *Number of Cores per Glass Fiber Area = Number of Cores/( Glass Fiber Radius.sup.2)

[0161] As shown in Table 2 above, the various designs of the multicore optical fibers described herein, in particular the CMF values described herein, may allow for a large number of core portions to be accommodated within a multicore optical fiber with standard form factors, such as standard coating diameter, thereby enabling the processing/incorporation of the multicore optical fibers described herein in existing cables while achieving increased transmission capacity, low crosstalk, low attenuation, low radiation loss, etc.

[0162] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus, it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.

EXEMPLARY CLAIM SET

[0163] Exemplary claim 1. An uncoupled multicore optical fiber for counter-propagation transmission, comprising: [0164] a common cladding having a refractive index .sub.CC and an outer radius R.sub.CC greater than or equal to 70 m and less than or equal to 95 m; [0165] a coating encircling and directly contacting the common cladding and extending from the outer radius R.sub.CC to an outer radius R.sub.C that is greater than or equal to 120 m and less than or equal to 127.5 m; and [0166] a plurality of core portions disposed within the common cladding; [0167] wherein at least one core portion of the plurality of core portions comprises: [0168] a central axis; [0169] a core region extending from the central axis, the core region comprising a relative refractive index .sub.1 relative to pure silica, wherein the core region comprises an alkali dopant; [0170] a trench region encircling the core region, the trench region comprising a relative refractive index .sub.3 relative to pure silica, wherein .sub.1>.sub.CC>.sub.3; [0171] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0172] an effective area greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2 at a wavelength of 1550 nm; and [0173] a cable cutoff wavelength less than or equal to 1530 nm; [0174] wherein the common cladding directly contacts the trench region; [0175] wherein the uncoupled multicore optical fiber has a core multiplicity factor ranging from about 0.028 to about 0.045; and [0176] wherein a counter-propagating crosstalk between two adjacent core portions is less than or equal to 40 dB per 100 km of the uncoupled multicore optical fiber.

[0177] Exemplary claim 2. The uncoupled multicore optical fiber of exemplary claim 1, wherein the coating comprises: [0178] a primary coating encircling and directly contacting the common cladding; and [0179] a secondary coating encircling and directly contacting the primary coating.

[0180] Exemplary claim 3. The uncoupled multicore optical fiber of exemplary claim 2, wherein an outer radius R.sub.Cs of the secondary coating is greater than or equal to 120 m and less than or equal to 127.5 m.

[0181] Exemplary claim 4. The uncoupled multicore optical fiber of any of exemplary claims 1 to 3, wherein an outer radius R.sub.Cp of the primary coating is greater than or equal to 170 m and less than or equal to 220 m.

[0182] Exemplary claim 5. The uncoupled multicore optical fiber of any of exemplary claims 1 to 4, wherein each core portion of the plurality of core portions comprises: [0183] a central axis; [0184] a core region extending from the central axis, the core region comprising a relative refractive index .sub.1 relative to pure silica, wherein the core region comprises an alkali dopant; [0185] a trench region encircling the core region, the trench region comprising a relative refractive index .sub.3 relative to pure silica, wherein .sub.1>.sub.CC>.sub.3; [0186] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0187] a radiation loss of less than 0.01 dB/km at a wavelength of 1550 nm; [0188] an effective area greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2 at a wavelength of 1550 nm; and [0189] a cable cutoff wavelength less than or equal to 1530 nm.

[0190] Exemplary claim 6. An uncoupled multicore optical fiber for counter-propagation transmission, comprising: [0191] a common cladding having a refractive index .sub.CC and an outer radius R.sub.CC greater than or equal to 70 m and less than or equal to 95 m; [0192] a coating comprises: [0193] a primary coating encircling and directly contacting the common cladding and extending from the outer radius R.sub.CC to an outer radius R.sub.Cp; and [0194] a secondary coating encircling and directly contacting the primary coating and extending from the outer radius R.sub.Cp to an outer radius R.sub.Cs, wherein the outer radius R.sub.Cs is greater than or equal to 120 m and less than or equal to 127.5 m; and [0195] a plurality of core portions disposed within the common cladding, wherein at least one core portion of the plurality of core portions comprises: [0196] a central axis; [0197] a core region extending from the central axis, the core region comprising a relative refractive index .sub.1 relative to pure silica, wherein the core region comprises an alkali dopant; [0198] a trench region encircling the core region, the trench region comprising a relative refractive index .sub.3 relative to pure silica, wherein the common cladding directly contacts the trench region, and wherein .sub.1>.sub.CC>.sub.3; [0199] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0200] an effective area greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2 at a wavelength of 1550 nm; and [0201] a cable cutoff wavelength less than or equal to 1530 nm; [0202] wherein the uncoupled multicore optical fiber has a core multiplicity factor ranging from about 0.028 to about 0.045; and [0203] wherein a counter-propagating crosstalk between two adjacent core portions is less than or equal to 40 dB per 100 km of the uncoupled multicore optical fiber.

[0204] Exemplary claim 7. The uncoupled multicore optical fiber of any one of exemplary claims 5 to 6, wherein the at least one core portion of the plurality of core portions further comprises: [0205] an inner-cladding region between the core region and the trench region, the inner cladding region comprising a relative refractive index .sub.2 relative to pure silica.

[0206] Exemplary claim 8. The uncoupled multicore optical fiber of exemplary claim 7, wherein: [0207] the core region extends from the central axis to an outer radius r.sub.1; [0208] the inner-cladding region encircles and directly contacts the core region and extends from the outer radius r.sub.1 to an outer radius r.sub.2; and [0209] the trench region encircles and directly contacts the inner-cladding region and extends from the outer radius r.sub.2 to an outer radius r.sub.3, wherein the common cladding directly contacts the trench region and extends from the outer radius r.sub.3 to the outer radius R.sub.CC.

[0210] Exemplary claim 9. The uncoupled multicore optical fiber of exemplary claim 8, wherein an outer radius r.sub.1 of the core region is greater than or equal to 3.5 m and less than or equal to 6 m.

[0211] Exemplary claim 10. The uncoupled multicore optical fiber of any one of exemplary claims 8 to 9, wherein an outer radius r.sub.2 of the inner-cladding region is greater than or equal to 6 m and less than or equal to 14 m.

[0212] Exemplary claim 11. The uncoupled multicore optical fiber of any one of exemplary claims 8 to 10, wherein an outer radius r.sub.3 of the trench region is greater than or equal to 7 m and less than or equal to 20 m.

[0213] Exemplary claim 12. The uncoupled multicore optical fiber of any of exemplary claims 8 to 11, wherein the relative refractive index .sub.3 of the trench region remains substantially constant from the outer radius r.sub.2 to the outer radius r.sub.3 such that the trench region has a substantially rectangular shape.

[0214] Exemplary claim 13. The uncoupled multicore optical fiber of any of exemplary claims 7 to 12, wherein the refractive index .sub.2 of the inner cladding region is in the range of about 0.2% to 0.3%.

[0215] Exemplary claim 14. The uncoupled multicore optical fiber of any of exemplary claims 7 to 13, wherein .sub.1>.sub.2>.sub.3.

[0216] Exemplary claim 15. The uncoupled multicore optical fiber of any of exemplary claims 7 to 13, wherein .sub.1>.sub.2=.sub.3.

[0217] Exemplary claim 16. The uncoupled multicore optical fiber of any one of exemplary claims 3 to 15, wherein the outer radius R.sub.Cs of the secondary coating is greater than or equal to 124 m and less than or equal to 126 m.

[0218] Exemplary claim 17. The uncoupled multicore optical fiber of any one of exemplary claims 2 to 16, wherein a ratio of a thickness of the secondary coating to a thickness of the primary coating ranges from about 0.7 to about 1.3, or from about 0.8 to about 1.2.

[0219] Exemplary claim 18. The uncoupled multicore optical fiber of any one of exemplary claims 2 to 17, wherein at least one of a thickness of the primary coating or a thickness of the secondary coating is greater than or equal to 14 m and less than or equal to 35 m.

[0220] Exemplary claim 19. The uncoupled multicore optical fiber of any one of exemplary claims 2 to 18, wherein the in-situ modulus of the primary coating is greater than or equal to 0.15 MPa and less than or equal to 0.4 MPa.

[0221] Exemplary claim 20. The uncoupled multicore optical fiber of any one of exemplary claims 2 to 19, wherein the in-situ modulus of the secondary coating is greater than or equal to 1,600 MPa and less than or equal to 2,400 MPa.

[0222] Exemplary claim 21. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 20, wherein the outer radius R.sub.CC is greater than or equal to 70 m and less than or equal to 90 m, or greater than or equal to 75 m and less than or equal to 90 m.

[0223] Exemplary claim 22. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 21, wherein the plurality of core portions comprises at least 6 core portions, or 6 to 10 core portions, or 6 to 8 core portions.

[0224] Exemplary claim 23. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 22, wherein the alkali dopant comprises at least one of sodium, potassium, rubidium, and lithium.

[0225] Exemplary claim 24. The uncoupled multicore optical fiber of any of exemplary claims 1 to 23, wherein an average concentration of the alkali dopant in the core portion is greater than or equal to 10 ppm or greater than or equal to 20 ppm and less than or equal to 500 ppm.

[0226] Exemplary claim 25. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 25, wherein the refractive index .sub.1 of the core region is in the range of about 0.05% to about 0.05%.

[0227] Exemplary claim 26. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 25, wherein the refractive index .sub.3 of the trench region is in the range of about 0.25% to about 0.7%.

[0228] Exemplary claim 27. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 26, wherein the refractive index .sub.CC of the common cladding is in the range of about 0.2% to about 0.4%.

[0229] Exemplary claim 28. The uncoupled multicore optical fiber of any of exemplary claims 1 to 27, wherein the trench region has a trench volume greater than or equal to 5% micron.sup.2 and less than or equal to 60% micron.sup.2.

[0230] Exemplary claim 29. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 20, wherein the cable cutoff wavelength of the at least one core portion of the plurality of core portions is greater than or equal to 1300 nm.

[0231] Exemplary claim 30. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 29, wherein the effective area of the at least one core portion of the plurality of core portions is greater than or equal to 100 m.sup.2 and less than or equal to 125 m.sup.2 at a wavelength of 1550 nm.

[0232] Exemplary claim 31. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 30, wherein a co-propagating crosstalk between two adjacent core portions is less than or equal to 50 dB per 100 km of the uncoupled multicore optical fiber.

[0233] Exemplary claim 32. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 31, wherein a counter-propagating crosstalk between two adjacent core portions is less than or equal to 50 dB per 100 km of the uncoupled multicore optical fiber.

[0234] Exemplary claim 33. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 32, wherein the dispersion at 1550 nm of the at least one core portion of the plurality of core portions is greater than or equal to 18 ps/nm/km and less than or equal to 22 ps/nm/km.

[0235] Exemplary claim 34. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 33, wherein the uncoupled multicore optical fiber is configured for transmission over at least one of C band or L band or over one or more wavelengths between 1535 nm and 1625 nm.

[0236] Exemplary claim 35. The uncoupled multicore optical fiber of any one of exemplary claims 1 to 34, wherein central axes of adjacent core portions of the plurality of core portions are separated from one another by a minimum separation distance that is greater than or equal to 32 m and less than or equal to 50 m, or greater than or equal to 34 m and less than or equal to 45 m, or greater than or equal to 34 m and less than or equal to 40 m.

[0237] Exemplary claim 36. A bidirectional transmission system, comprising an uncoupled multicore optical fiber of any one of exemplary claims 1 to 13.

[0238] Exemplary claim 37. The bidirectional transmission system of exemplary claim 36, further comprising: [0239] a first transceiver optically coupled to a first end of the uncoupled multicore optical fiber; and [0240] a second transceiver optically coupled to a second end of the uncoupled multicore optical fiber opposite the first end of the uncoupled multicore optical fiber; [0241] wherein the first transceiver and the second transceiver are configured to transmit signals from the first end of the uncoupled multicore optical fiber to the second end of the uncoupled multicore optical fiber via a first plurality of core portions of the uncoupled multicore optical fiber; and [0242] wherein the first transceiver and the second transceiver are further configured to transmit signals from the second end of the uncoupled multicore optical fiber to the first end of the uncoupled multicore optical fiber via a second plurality of core portions of the uncoupled multicore optical fiber.

[0243] Exemplary claim 38. The bidirectional transmission system of exemplary claim 37, wherein the first plurality of core portions of the uncoupled multicore optical fiber is different from the second plurality of core portions of the uncoupled multicore optical fiber.

[0244] Exemplary claim 39. The bidirectional transmission system of any one of exemplary claims 37 to 38, wherein: [0245] the first transceiver comprises: [0246] a first plurality of light sources optically coupled to the first plurality of core portions of the uncoupled multicore optical fiber; and [0247] a first plurality of detectors optically coupled to the second plurality of core portions of the uncoupled multicore optical fiber; and [0248] the second transceiver comprises: [0249] a second plurality of light sources optically coupled to the second plurality of core portions of the uncoupled multicore optical fiber; and [0250] a second plurality of detectors optically coupled to the first plurality of core portions of the uncoupled multicore optical fiber.

[0251] Exemplary claim 40. A method of bidirectional transmission over a multicore optical fiber having a first plurality of core portions and a second plurality of core portions, the method comprising: [0252] transmitting a first optical signal over at least one core portion of the first plurality of core portions in a first direction, wherein the at least one core portion of the first plurality of core portions comprises: [0253] a core region comprising an alkali dopant; and [0254] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0255] transmitting a second optical signal over at least one core portion of the second plurality of core portions in a second direction opposite the first direction, wherein the at least one core portion of the second plurality of core portions comprises: [0256] a core region comprising an alkali dopant; and [0257] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0258] wherein the first plurality of core portions and the second plurality of core portions are disposed within a common cladding having an outer radius R.sub.CC greater than or equal to 70 m and less than or equal to 95 m; [0259] wherein the multicore optical fiber further comprises a coating encircling and directly contacting the common cladding, wherein an outer radius R.sub.C of the coating is greater than or equal to 120 m and less than or equal to 127.5 m; [0260] wherein the multicore optical fiber has a core multiplicity factor ranging from about 0.028 to about 0.045; and [0261] wherein a counter-propagating crosstalk between two adjacent core portions is less than or equal to 40 dB per 100 km of the uncoupled multicore optical fiber.

[0262] Exemplary claim 41. The method of exemplary claim 40, wherein the outer radius R.sub.C of the coating is greater than or equal to 120 m and less than or equal to 127.5 m, or greater than or equal to 124 m and less than or equal to 126 m.

[0263] Exemplary claim 42. The method of any of exemplary claims 40 to 41, wherein the coating comprises: [0264] a primary coating encircling and directly contacting the common cladding; and [0265] a secondary coating encircling and directly contacting the primary coating.

[0266] Exemplary claim 43. The method of exemplary claim 42, wherein an outer radius R.sub.Cs of the secondary coating is greater than or equal to 120 m and less than or equal to 127.5 m, or greater than or equal to 124 m and less than or equal to 126 m.

[0267] Exemplary claim 44. The method of any of exemplary claims 42 to 43, wherein an outer radius R.sub.Cp of the primary coating is greater than or equal to 170 m and less than or equal to 220 m.

[0268] Exemplary claim 45. The method of any of exemplary claims 42 to 44, wherein a ratio of a thickness of the secondary coating to a thickness of the primary coating ranges from about 0.7 to about 1.3, or from about 0.8 to about 1.2.

[0269] Exemplary claim 46. The method of any of exemplary claims 42 to 45, wherein at least one of a thickness of the primary coating or a thickness of the secondary coating is greater than or equal to 14 m and less than or equal to 35 m.

[0270] Exemplary claim 47. The method of any of exemplary claims 42 to 46, wherein the in-situ modulus of the primary coating is greater than or equal to 0.15 MPa and less than or equal to 0.4 MPa.

[0271] Exemplary claim 48. The method of any of exemplary claims 42 to 47, wherein the in-situ modulus of the secondary coating is greater than or equal to 1,600 MPa and less than or equal to 2,400 MPa.

[0272] Exemplary claim 49. The method of any of exemplary claims 40 to 48, wherein each core portion of the first plurality of core portions comprises at least one of: [0273] a core region comprising an alkali dopant; [0274] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0275] a radiation loss of less than 0.01 dB/km at a wavelength of 1550 nm; or [0276] an effective area that is greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2 at a wavelength of 1550 nm, or greater than or equal to 100 m.sup.2 and less than or equal to 125 m.sup.2 at a wavelength of 1550 nm.

[0277] Exemplary claim 50. The method of any one of exemplary claims 40 to 49, wherein each core portion of the second plurality of core portions comprises at least one of: [0278] a core region comprising an alkali dopant; [0279] an attenuation of less than 0.16 dB/km at a wavelength of 1550 nm; [0280] a radiation loss of less than 0.01 dB/km at a wavelength of 1550 nm; or an effective area that is greater than or equal to 75 m.sup.2 and less than or equal to 135 m.sup.2 at a wavelength of 1550 nm, or greater than or equal to 100 m.sup.2 and less than or equal to 125 m.sup.2 at a wavelength of 1550 nm.

[0281] Exemplary claim 51. The method of any of exemplary claims 40 to 50, wherein each core portion of the first plurality of core portions and the second plurality of core portions further comprises: [0282] an inner-cladding region encircling and directly contacting the core region; and [0283] a trench region encircling and directly contacting the inner cladding region, wherein the common cladding directly contacts the trench region.

[0284] Exemplary claim 52. The method of exemplary claim 51, wherein the trench region has a trench volume greater than or equal to 5% micron.sup.2 and less than or equal to 60% micron.sup.2.

[0285] Exemplary claim 53. The method of any one of exemplary claims 51 to 52, wherein the core region comprising a relative refractive index .sub.1 relative to pure silica, wherein the inner cladding region comprising a relative refractive index .sub.2 relative to pure silica, wherein the trench region comprising a relative refractive index .sub.3 relative to pure silica, and wherein .sub.1>.sub.2.sub.3.

[0286] Exemplary claim 54. The method of any one of exemplary claims 51 to 52, wherein the core region comprising a relative refractive index .sub.1 relative to pure silica, wherein the trench region comprising a relative refractive index .sub.3 relative to pure silica, wherein the common cladding has a refractive index .sub.CC, and wherein .sub.1>.sub.CC>.sub.3.

[0287] Exemplary claim 55. The method of any one of exemplary claims 51 to 54, wherein a refractive index .sub.2 of the inner cladding region is in the range of about 0.2% to about 0.3%.

[0288] Exemplary claim 56. The method of any one of exemplary claims 51 to 55, wherein a refractive index .sub.3 of the trench region is in the range of about 0.25% to about 0.7%.

[0289] Exemplary claim 57. The method of any one of exemplary claims 51 to 56, wherein the relative refractive index .sub.3 of the trench region remains substantially constant within the trench region.

[0290] Exemplary claim 58. The method of any one of exemplary claims 40 to 57, wherein a refractive index .sub.1 of the core region is in the range of about 0.05% to about 0.05%.

[0291] Exemplary claim 59. The method of any one of exemplary claims 40 to 58, wherein a refractive index .sub.CC of the common cladding is in the range of about 0.2% to about 0.4%.

[0292] Exemplary claim 60. The method of any one of exemplary claims 40 to 59, wherein the core region extends from a central axis to an outer radius r.sub.1, and wherein the outer radius r.sub.1 of the core region is greater than or equal to 3.5 m and less than or equal to 6 m.

[0293] Exemplary claim 61. The method of exemplary claim 60, wherein the inner-cladding region extends from the outer radius r.sub.1 to an outer radius r.sub.2, and wherein the outer radius r.sub.2 of the inner-cladding region is greater than or equal to 6 m and less than or equal to 14 m.

[0294] Exemplary claim 62. The method of exemplary claim 61, wherein the trench region extends from the outer radius r.sub.2 to an outer radius r.sub.3, wherein the outer radius r.sub.3 of the trench region is greater than or equal to 7 m and less than or equal to 20 m.

[0295] Exemplary claim 63. The method of any one of exemplary claims 40 to 62, wherein the outer radius R.sub.CC is greater than or equal to 70 m and less than or equal to 90 m, or the outer radius R.sub.CC is greater than or equal to 75 m and less than or equal to 90 m.

[0296] Exemplary claim 64. The method of any one of exemplary claims 40 to 63, wherein a co-propagating crosstalk between two adjacent core portions is less than or equal to 50 dB per 100 km of the uncoupled multicore optical fiber.

[0297] Exemplary claim 65. The method of any one of exemplary claims 40 to 64, wherein a counter-propagating crosstalk between two adjacent core portions is less than or equal to 50 dB per 100 km of the uncoupled multicore optical fiber.

[0298] Exemplary claim 66. The method of any one of exemplary claims 40 to 65, wherein the dispersion at 1550 nm of the at least one core portion is greater than or equal to 18 ps/nm/km and less than or equal to 22 ps/nm/km.

[0299] Exemplary claim 67. The method of any one of exemplary claims 40 to 66, wherein the uncoupled multicore optical fiber is configured for transmission over at least one of C band or L band or over one or more wavelengths between 1535 nm and 1625 nm.

[0300] Exemplary claim 68. The method of any one of exemplary claims 40 to 67, wherein central axes of adjacent core portions of the plurality of core portions are separated from one another by a minimum separation distance that is greater than or equal to 32 m and less than or equal to 50 m, or greater than or equal to 34 m and less than or equal to 45 m, greater than or equal to 34 m and less than or equal to 40 m.

[0301] Exemplary claim 69. The method of any of one of exemplary claims 40 to 68, wherein the first plurality of core portions and the second plurality of core portions are arranged in an alternating manner.

[0302] Exemplary claim 70. The method of any one of exemplary claims 40 to 69, wherein the alkali dopant comprises at least one of sodium, potassium, rubidium, and lithium.

[0303] Exemplary claim 71. The method of any one of exemplary claims 40 to 70, wherein an average concentration of the alkali dopant in the core portion is greater than or equal to 10 ppm or greater than or equal to 20 ppm and less than or equal to 500 ppm.

[0304] Exemplary claim 72. The method of any one of exemplary claims 40 to 71, wherein a combination of the first plurality of core portions and the second plurality of core portions comprises at least 6 core portions, or 6 to 10 core portions, or 6 to 8 core portions.

[0305] Exemplary claim 73. The method of any one of claims 40 to 72, wherein at least one of the first plurality of core portions or the second plurality of core portions comprises at least 3 core portions, or 3 to 5 core portions, or 3 to 4 core portions.

[0306] Exemplary claim 74. The method of any one of claims 40 to 73, wherein the first plurality of core portions and the second plurality of core portions comprise equal number of core portions.