Polarization-maintaining highly elliptical core fiber with stress-induced birefringence

Abstract

An optical fiber comprises a core having an elliptical cross section and a cladding having a circular cross section. The core has an aspect ratio between 2 and 40. The core and the cladding have a common central axis with the core being enclosed by the cladding. The difference of a refractive index of the cladding to a refractive index of the core is between 110.sup.2 and 1.510.sup.1. A trench is located between the core and the cladding. The trench has a uniform width and encircles the core. The refractive index of the trench is lower than the refractive index of the cladding.

Claims

1. An optical fiber comprising: a core having an elliptical cross section, the core having an aspect ratio between 2 and 40; and a cladding, having a circular cross section, enclosing the core, wherein a difference of a refractive index of the cladding to a refractive index of the core is between 110.sup.2 and 1.510.sup.1.

2. The optical fiber of claim 1 wherein the core and the cladding have a common central axis.

3. The optical fiber of claim 1 further comprising a trench located between the core and the cladding, the trench having a uniform width and encircling the core, a refractive index of the trench being lower than the refractive index of the cladding.

4. The optical fiber of claim 1 wherein a width of the core along a y-axis allows for single mode transmission.

5. The optical fiber of claim 1 wherein a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

6. The optical fiber of claim 5 wherein each of the plurality of mode pairs is comprised of two orthogonal linear polarizations.

7. The optical fiber of claim 6 wherein the plurality of mode pairs have an effective index separation between adjacent vector modes greater than 110.sup.4.

8. The optical fiber of claim 6 wherein the effective index separation is caused by thermal stress induced during the manufacture of the optical fiber and the elliptical shape of the core.

9. The optical fiber of claim 1 wherein the core is doped with rare earth ions.

10. An optical fiber amplifier comprising: a first WDM coupler receiving an input signal and an output from a first pump optical source; an optical fiber receiving an output from the first WDM coupler, the optical fiber comprising a core having an elliptical cross section, the core having an aspect ratio between 2 and 40, and a cladding, having a circular cross section, enclosing the core, the optical fiber further comprising a trench located between the core and the cladding, the trench having a uniform width and encircling the core, a refractive index of the trench being lower than the refractive index of the cladding; and a second WDM coupler receiving the output of the optical fiber and an output of a second pump optical source, the second WDM coupler outputting an amplified optical signal.

11. The optical fiber amplifier of claim 10 wherein the core and the cladding have a common central axis.

12. The optical fiber amplifier of claim 10 wherein the core is doped with rare earth ions.

13. The optical fiber amplifier of claim 10 wherein a width of the core along a y axis allows for single mode transmission and a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

(2) FIG. 1 illustrates a cross sectional view of an optical fiber according an embodiment.

(3) FIG. 2 illustrates a refractive index profile along the x-axis of an optical fiber according an embodiment.

(4) FIG. 3 illustrates a cross sectional view of an optical fiber and the compressive thermal stress that is induced into the core along the normal direction of the core/cladding border during the fiber drawing process.

(5) FIG. 4 illustrates a cross sectional view of an optical fiber according another embodiment.

(6) FIG. 5 illustrates a refractive index profile along the x-axis of an optical fiber according an embodiment.

(7) FIG. 6 illustrates a combined WDM and MDM optical communications system.

(8) FIG. 7 illustrates the mode multiplexing of data onto an optical fiber according to an embodiment.

(9) FIG. 8 illustrates the mode demultiplexing of data onto an optical fiber according to an embodiment.

(10) FIG. 9 illustrates an optical fiber amplifier utilizing an optical fiber according to an embodiment.

(11) FIG. 10 illustrates an optical fiber sensing apparatus utilizing an optical fiber according to an embodiment.

(12) FIG. 11 illustrates an MDM transmitter utilizing an optical fiber according to an embodiment.

(13) FIG. 12 illustrates an MDM receiver utilizing an optical fiber according to an embodiment.

(14) FIG. 13 illustrates an MDM transceiver utilizing an optical fiber according to an embodiment.

(15) FIG. 14 illustrates a method of manufacturing an HECF according to an embodiment.

(16) FIG. 15 is a flowchart of the method of manufacturing an HECF according to an embodiment.

(17) It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

(18) Embodiments of the invention comprise a polarization-maintaining highly elliptical core fiber (HECF). By combining the highly elliptical core shape and the thermal stress induced during the fiber fabrication, the birefringence values are elevated to be higher than 110.sup.4, which reduces the mode coupling and in turn makes this fiber suitable for use in MIMO-free MDM transmission systems.

(19) FIG. 1 illustrates a cross-section view of a highly elliptical core fiber (HECF) 100 according to a first embodiment of the invention. The optical fiber comprises a cladding 102, which is typically the industry standard, 125 m in diameter. At the center of the fiber is a solid core 104 surrounded by a trench 106. Though not shown in FIG. 1, in most embodiments, the cladding is further surrounded by a buffer, which is in turn covered by a jacket as is known in the art.

(20) The cross-section profile of the core 104 is elliptical in shape. The core's cross section 104 has an x-axis 110 and a y-axis 108. In one embodiment the core 104 has approximate cross-sectional dimensions of 38.5 m 202 along the x-axis 110 and 6.1 m along the y-axis 108. The dimension along the short, y-axis 108, is selected to allow for single mode operations, n=1 for TE.sub.m,n/TM.sub.m,n modes. The dimension of the x-axis 110 is selected to allow for multiple spatial modes to be distributed in a one-dimensional array in the x-axis 110. In other embodiments, the aspect ratio, , of the core, defined as the ratio of the cross-sectional length in the x-direction to the cross-sectional length in the y-direction, is between 2 and 40. Those skilled in the art will appreciate that the x-axis 110 may also be referred to as the major axis of the elliptical core, while the y-axis 108 may be referred to as the minor axis of the elliptical core. Rotation of the fiber so that the major axis is no longer horizontal should not be considered to be changing the alignment of the X and Y-axes. Those skilled in the art will also appreciate that the aspect ratio, , may also be considered as the ratio of the width of the core along the x-axis 110 to the width of the core along the y-axis 108.

(21) The core 104 may be thought of as elliptic cylinder, such that the length of the x-axis 110 is greater than the length of the y-axis and therefore, the x-axis 110 forms the major axis of the ellipse and the y-axis 108 forms the minor axis of the ellipse. The x-axis 110 and the y-axis 108 intersect at a right angle and the intersection of the x-axis 110 and the y-axis 108 defines the center of the elliptical cross-section. The core 104 has the shape of an elliptic cylinder with a central axis, also known as a longitudinal axis, running through the center of each elliptical cross-section of the core 104. The cladding 102 may be viewed as having a circular cross section with a center and has a cylindrical shape with a central axis running through the center of each circular cross-section of the cladding 102. The core 104 and the cladding 102 have a common central axis, such that the core 104 is centered within the cladding 102. Accordingly, the HECF is formed so that the cladding 102 and the core 104 share a common central axis.

(22) Optical fiber is manufactured using a silica glass material comprised mainly of SiO.sub.2 and the cladding 102 is comprised of a substantially pure silica glass material. The core 104 is manufactured by doping the SiO.sub.2 material with GeO.sub.2. In some embodiments, the core 104 is comprised of SiO.sub.2 doped with 7.3 mol % of GeO.sub.2.

(23) The trench 106 surrounds the core 104 and, in some embodiments is approximately 4.7 m wide 206. The trench is made of SiO.sub.2 doped with F, fluorine. In some embodiments the trench is doped with 2 mol % of F. The bending loss experienced by the HECF increases as the aspect ratio increases, however the trench serves to reduce the bending loss of higher-order modes in the optical fiber.

(24) FIG. 2 shows the refractive index profile along the x-axis 110 for the optical fiber 100. The core 104, trench 106, and cladding 102 have differing refractive indexes determined by their respective materials. In one embodiment the difference in refractive index between the core 104 and cladding 102, n 204, is approximately 1.110.sup.2. In embodiments, the refractive index difference between the core 104 and the cladding 102 is within the range 110.sup.2n1.510.sup.1. In one embodiment the difference in refractive index between the trench 106 and the cladding 102, n.sub.trench 208, is approximately 5.710.sup.3.

(25) The geometry of the highly elliptical core leads to geometry-enhanced birefringence in the optical fiber. This source of birefringence is related to the geometric structure of the core, where a higher core aspect ratio will cause a higher value of birefringence.

(26) Referring to FIG. 3, during manufacture of the optical fibre, inconstant, compressive thermal stress 302 is induced into the core 104, mainly along the y-axis 108 direction. During the fiber drawing and annealing process, the fiber becomes stressed due to different thermal expansion coefficients of the core 104 and the cladding layers 102. Different amounts of stress are applied along long, x-axis 110 and short, y-axis 108 due to the asymmetry of its core structure. With the increase of this stress, the isotropic glass starts to become anisotropic, with a consequent change of the refractive indices along the principal, x and y stress directions. The thermal stress has the effect to increase the effective index difference (n.sub.eff) between the two polarizations of the same spatial mode.

(27) Optical fiber according to an embodiment of the invention may transmit data on five spatial mode groups with two-fold polarization degeneracy to obtain ten MDM channels. The supported modes are labeled as TE.sub.1n, and TM.sub.1n, depending on the polarization state (respectively along the x-axis or the y-axis). In particular, the subscript 1 indicates the single mode operation along the short-axis (y-axis), while n is an integer indicating the number of the dark lines of the mode profile along the long-axis (x-axis).

(28) The elliptical shape of the core combined with thermal stress created during manufacturing produces a relatively large birefringence so that the modal effective index separation, n.sub.eff, is greater than 110.sup.4. This reduces the mode coupling inside the HECF so that the fiber modes can propagate without significant crosstalk up to a fiber length of approximately 1 km. This allows for transmission systems to be designed that do not require MIMO DSPs.

(29) In some embodiments the core may be doped with rare earth ions such as one or both of erbium and ytterbium in order to produce an optical active fiber.

(30) FIG. 4 illustrates an optical fiber 400 according to another embodiment with an alternative structure. In comparison to the embodiment depicted in FIG. 1, this embodiment comprised a cladding 102 and core 104, but lacks a trench 106 between the core and cladding. The trench 106 helps to reduce the bending loss of higher-order modes in the fiber. However, in some embodiments, the bending loss may also be reduced by increasing the refractive index difference 204, n, to approximately 1.510.sup.1.

(31) FIG. 5 shows the refractive index profile along the x-axis 110 for the optical fiber 400. The core 104 and cladding 102 have differing refractive indexes determined by their respective materials. In one embodiment the difference in refractive index between the core 104 and cladding 102, n 204, is approximately 1.110.sup.2.

(32) FIG. 6 illustrates a system incorporating embodiments of the invention. The invention can be used in the MIMO-free mode-division multiplexing system. The outputs from WDM transmitters 602 are input to WDM couplers 604 and are multiplexed into a single output fiber. Each of the outputs from the WDM couplers 604 are then multiplexed by the mode multiplexer 606 and converted into a corresponding mode in the HECF 608 and. After transmission through the HECF 608, the fiber modes are received and separated by the mode demultiplexer 610 following by a number of WDM couplers 612 to separate the wavelength channels and finally be detected by the receivers 614. Due to the low crosstalk among the modes inside the fiber, a MIMO-DSP is not required.

(33) FIG. 7 illustrates an embodiment of a mode division multiplexer utilizing HECF 702. Optical inputs of different modes TE.sub.11, TE.sub.12, . . . , TE.sub.1n may be multiplexed onto the HECF 702 using fused fiber mode couplers 704.

(34) FIG. 8 illustrates an embodiment of a mode division demultiplexer utilizing HECF 802. Fused fiber mode couplers 804 pass light of a predetermined mode to output optical fibers 806 in order to receive the signals from TE.sub.11, TE.sub.12, . . . , TE.sub.1n.

(35) FIG. 9 illustrates an optical fiber amplifier (OFA) 900 comprising an HECF 902 according to an embodiment. The HECF 902 has a core that is doped with rare earth ions such as erbium and connects two WDM couplers 904. Each of the WDM couplers receives the output of pump diodes 908. The optical power of the input 906 is amplified by the OFA and output on fibre 910.

(36) FIG. 10 illustrates a sensing system 1000 according to another embodiment. The HECF 1002 passes through sensing elements 1004. The signal source and data acquisition module are co-located in device 1006. In some embodiments, sensing elements 1004 are fiber Bragg gratings (FBG). When light from the optical signal source 1006 passes through the sensing elements 1004, light of a predetermined fiber mode is back reflected to the source and received by the data acquisition module 1006.

(37) FIG. 11 illustrates an optical transmitter 1100 with an integrated mode-division multiplexing module. The transmitter is typically contained in a housing to enable it to be easily integrated into optical transmission equipment. A transmitter 1100 may be packaged on its own or be integrated with a receiver to form a transceiver. In some embodiments a light source 1102, which may be a diode or laser, outputs at a continuous wavelength or mode. Data is applied to the light source 1102 by a modulator 1104 which is then multiplexed with other modulated light sources to be transmitted on the HECF 1108. In some embodiments, the light source and modulator may be integrated together.

(38) FIG. 12 illustrates an optical receiver 1200 with an integrated mode-division de-multiplexing module. The receiver is typically contained in a housing to enable it to be easily integrated into optical transmission equipment. A receiver 1100 may be packaged on its own or be integrated with a transmitter 1100 to form a transceiver. Mode multiplexed data is received from the HECF 1108 and split into its modes in the mode demultiplexer 1206. Light of each mode is then received optical detectors 1202 which may be PIN diodes or APDs.

(39) FIG. 13 illustrates an optical transceiver 1300 with an integrated mode-division multiplexer/demultiplexer module. The transceiver is typically contained in a housing to enable it to be easily integrated into optical transmission equipment. The transceiver 1300 combines the functionality of a transmitter 1100 and a receiver 1200. In some embodiments a light source 1102, which may be a diode or laser, outputs at a continuous wavelength or mode. Data is applied to the light source 1102 by a modulator 1104 which is passed through a circulator 1312 and then multiplexed with other modulated light sources to be transmitted on the HECF 1108. In some embodiments, the light source and modulator may be integrated together. Received light passes through the mode MUX/DeMUX 1310 and into the circulator 1312 that sends the demultiplexed received light to optical detectors 1202 which may be PIN diodes or APDs, to receive the data.

(40) As shown in FIG. 14, the HECF 1400 may be fabricated using from a preform prepared using a modified chemical vapor deposition (MCVD) process. The preform has a cylindrical shape and includes a coaxial inner core 104 and outer cladding 102. The inner core 104 has a circular cross section. Opposing, longitudinal sides 1402, 1404 are cut from the preform resulting in two opposing, flat, parallel sides 1406 1408. The cut preform is then heated so that the flat surfaces disappear due to the surface tension and the flow of material. Consequently, the circular core 104 becomes elliptical during the heating process. The cut preform with an elliptical core is pulled to form the optical fiber. The resulting optical fiber includes a core 104 having an elliptical cross section having an aspect ratio between 2 and 40, and a cladding 102 having a circular cross section. The core 104 and the cladding 102 have a common central axis with the core 104 being enclosed by the cladding 102. Since the preform was fabricated through MCVD, it was able to include a trench 106 surrounding the core.

(41) Those skilled in the art will appreciate that it may be possible to make a plurality of cuts that are not necessarily aligned with each other (e.g. without creating longitudinal sides) but that taken as a whole, after the heating induced reshaping, result in the elliptical core discussed above.

(42) FIG. 15 is a flowchart illustrating a method 1500 of manufacturing a HECF as shown in FIG. 14. In step 1502, a cylindrical preform having an inner core 104 and an outer cladding 102 is formed. The inner core 104 has a circular cross section profile. The cylindrical preform may be formed using an MCVD process. In step 1504, two opposing sides 1402 1404 of the cylindrical preform are cut along a length of the cylindrical preform to produce a cut preform with opposing parallel surfaces 1406 1408 along a longitudinal axis of the cylindrical preform. In various embodiments, the cutting of the preform is done in a manner that avoids cutting the inner core 104. In step 1506, the cut preform is heated. The surface tension of the exterior surface of the cut preform, subjected to the softening caused by the heating, allows the cut preform to distort until the exterior of the preform is circular again. The surface tension causes a distortion in the entire preform, so that when the cut preform assumes an outer circular profile, the inner core 104 has been distorted and result in an elliptical profile for the core 104. As discussed above, this generally elliptical core has an aspect ratio of between 2 and 40. The outer cladding, after enclosing (or enveloping) the elliptical core has a generally circular cross section. In step 1508, the cut and heated preform is pulled to form an optical fiber. In some embodiments, the core 104 and cladding 102 are generally co-axial, having a common central axis.

(43) An aspect of the disclosure includes an optical fiber including a core having an elliptical cross section and an aspect ratio between 2 and 40. The optical fiber also includes a cladding, having a circular cross section, enclosing the core.

(44) An aspect of the disclosure includes the core and the cladding have a common central axis.

(45) An aspect of the disclosure includes an optical fiber wherein a difference of a refractive index of the cladding to a refractive index of the core is between 110.sup.2 and 1.510.sup.1.

(46) An aspect of the disclosure includes an optical fiber wherein a ratio of a refractive index of the cladding to a refractive index of the core is between 110.sup.2 and 1.510.sup.1.

(47) An aspect of the disclosure includes an optical fiber comprising a trench located between the core and the cladding. The trench has a uniform width and encircles the core. The refractive index of the trench is lower than the refractive index of the cladding.

(48) An aspect of the disclosure includes an optical fiber wherein a width of the core along a y-axis allows for single mode transmission.

(49) An aspect of the disclosure includes an optical fiber wherein a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

(50) An aspect of the disclosure includes an optical fiber wherein each of the plurality of mode pairs have two orthogonal linear polarizations.

(51) An aspect of the disclosure includes an optical fiber wherein the plurality of mode pairs have an effective index separation between the adjacent vector modes greater than 110.sup.4.

(52) An aspect of the disclosure includes an optical fiber wherein the effective index separation is caused by thermal stress induced during the manufacture of the optical fiber and the elliptical shape of the core.

(53) An aspect of the disclosure includes an optical fiber wherein the core is doped with rare earth ions.

(54) An aspect of the disclosure includes an optical fiber amplifier (OFA) including a first WDM coupler receiving an input signal and an output from a first pump optical source. An optical fiber receiving an output from the first WDM coupler. The optical fiber includes a core having an elliptical cross section having an aspect ratio between 2 and 40. The optical fiber also includes a cladding, having a circular cross section, enclosing the core. The OFA also includes a second WDM coupler receiving the output of the optical fiber and an output of a second pump optical source. The second WDM coupler outputs an amplified optical signal.

(55) An aspect of the disclosure includes an OFA wherein the core and the cladding have a common central axis.

(56) An aspect of the disclosure includes an OFA wherein the core is doped with rare earth ions.

(57) An aspect of the disclosure includes an OFA wherein the optical fiber includes a trench located between the core and the cladding. The trench has a uniform width and encircles the core. The refractive index of the trench is lower than the refractive index of the cladding.

(58) An aspect of the disclosure includes an OFA wherein a width of the core along a y-axis allows for single mode transmission and a width of the core along an x-axis allows for the transmission of a plurality of mode pairs.

(59) An aspect of the disclosure includes an optical sensor detecting an optical transmission of an optical mode. The optical sensor includes a plurality of sensing elements disposed along an optical fiber. The optical fiber includes a core having an elliptical cross section and an aspect ratio between 2 and 40. The optical fiber includes a cladding having a circular cross section. The core is enclosed by the cladding. The optical fiber receives an input signal including a plurality of vector modes. Each of the plurality of sensing elements reflects one of the plurality of vector modes. A data acquisition module receives one of the plurality of the vector modes reflected by one of the plurality of sensing elements. In some embodiments, the core and the cladding have a common central axis.

(60) An aspect of the disclosure includes an optical transmitter module. The module includes a plurality of light sources. Each of the plurality of light sources outputs a constant light signal at one of a plurality of vector modes. A plurality of optical modulators receive one of the constant light signals and output a modulated light signal. A mode multiplexer receives each of the modulated light signals to produce a mode multiplexed optical output. An optical fiber receives the mode multiplexed optical output. The optical fiber includes a core having an elliptical cross section having an ellipticity between 2 and 40. The optical fiber also includes a cladding having a circular cross section, with the core being enclosed by the cladding. In some embodiments. the core and the cladding have a common central axis.

(61) An aspect of the disclosure includes an optical receiver module including an optical fiber. The optical fiber receives a mode multiplexed optical signal. The optical fiber includes a core having an elliptical cross section having an aspect ratio between 2 and 40. The optical fiber also includes a cladding. The core is enclosed by the cladding. The optical receiver module also includes a mode demultiplexer receiving the mode multiplexed optical signal from the optical fiber and outputting a plurality of modulated light signals. Each of the plurality of modulated light signals includes light at one of a plurality of vector modes. A plurality of optical detectors each receive one of the plurality of modulated light signals. In some embodiments. the core and the cladding have a common central axis.

(62) An aspect of the disclosure includes an optical transceiver including a transmit path and a receive path. The transmit path includes a plurality of light sources outputting a constant light signal at one of a plurality of vector modes. A plurality of optical modulators receives one of the constant light signals and outputs a modulated light signal. A plurality of circulators receives one of the modulated light signals and outputs one of the modulated light signals. A mode multiplexer receives each of the modulated light signals from the plurality of circulators to produce a mode multiplexed optical output. An optical fiber receives the mode multiplexed optical output. The optical fiber includes a core having an elliptical cross section and an aspect ratio between 2 and 40. The optical fiber also includes a cladding having a circular cross section, with the core being enclosed by the cladding. In some embodiments. the core and the cladding have a common central axis.

(63) The receive path includes a mode demultiplexer receiving the mode multiplexed optical signal from the optical fiber and outputting a plurality of received modulated light signals. Each of the plurality of received modulated light signals includes light at one of the plurality of vector modes. A plurality of optical detectors each receives one of the plurality of received modulated light signals via the plurality of circulators.

(64) An aspect of the disclosure includes a method for manufacturing an optical fiber. The method includes preparing a cylindrical preform having a cross section comprising an inner core and an outer cladding. The inner core has a circular profile. Cutting two opposing sides of the cylindrical preform along a length of the cylindrical preform to produce a cut preform with opposing parallel surfaces along a longitudinal axis of the cut preform. Heating the cut preform until the cut preform has a circular profile and an inner core of the cut preform has an elliptical profile. Pulling the cut preform to form the optical fiber having a core with an elliptical profile. The core has an aspect ratio between 2 and 40, and a cladding has a circular cross section. The core being enclosed by the cladding. In some embodiments. the core and the cladding have a common central axis.

(65) An aspect of the disclosure includes a trench portion situated between the core and the cladding. The trench portion is present in the optical fiber and isolates the core from the cladding.

(66) An aspect of the disclosure includes a cylindrical preform that is fabricated using a modified chemical vapor deposition (MCVD) process.

(67) In another aspect of the disclosure the preform further includes a trench portion situated between the core and the cladding. The trench portion is present in the optical fiber and isolates the core from the cladding.

(68) In another aspect of the disclosure, the elliptical profile is formed due to surface tension and the flow of material during the heating.

(69) Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

(70) Although the present invention has been described with reference to specific features and embodiments thereof, it is evident that various modifications and combinations can be made thereto without departing from the invention. The specification and drawings are, accordingly, to be regarded simply as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present invention.