High bandwidth multimode optical fiber optimized for multimode and single-mode transmissions

Abstract

It is proposed a home optical data network formed of an optical fiber comprising an optical core and an optical cladding surrounding the optical core, the optical core having a refractive graded-index profile with a minimal refractive index n.sub.1 and a maximal refractive index n.sub.0, said optical fiber being such that it has a numerical aperture NA and an optical core radius a satisfying a criterion C of quality of optical communications defined by the following equation: $C=\mathrm{NA}-0.02a$$\mathrm{where}\text{:}$$\mathrm{NA}=\sqrt{n_0^2-n_1^2}=n_0.Math.\sqrt{2}\mathrm{with}=\frac{n_0^2-n_1^2}{2n_0^2},$
is the normalized refractive index difference,
and in that said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that NA>0.20, a>10 m and |C|<0.20.

Claims

1. An apparatus comprising: a home optical data network formed of an optical fiber comprising: an optical core; and an optical cladding surrounding the optical core, the optical core having a refractive graded-index profile with a minimal refractive index n.sub.1 and a maximal refractive index n.sub.0; and said optical fiber having a numerical aperture NA and an optical core radius a satisfying a criterion C of quality of optical communications defined by the following equation: $C=\mathrm{NA}-0.02a$ $\mathrm{where}\text{:}$ $\mathrm{NA}=\sqrt{n_0^2-n_1^2}=n_0.Math.\sqrt{2}\mathrm{with}=\frac{n_0^2-n_1^2}{2n_0^2},$ is the normalized refractive index difference, and wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that NA>0.20, a>10 m and |C|<0.20; and a data transceiver coupled to the home optical data network.

2. The apparatus according to claim 1, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that |C|<0.10.

3. The apparatus according to claim 1, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that |C|<0.05.

4. The apparatus according to claim 1, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that C<0.

5. The apparatus according to claim 1, wherein said optical core radius is such that a>14 m.

6. The apparatus according to claim 1, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 are chosen such that NA>0.25.

7. The apparatus according to claim 1, wherein the refractive graded-index profile is a single alpha graded-index profile n(r) defined by the following equation: $n\left(r\right)=n_0.Math.\sqrt{1-2.Math..Math.{\left(\frac{r}{a}\right)}^{}}ra$ where: r is a variable representative of the radius of said optical fiber, 1, being a non-dimensional parameter that defines the index profile shape of the optical core.

8. The apparatus according to claim 1, wherein the optical cladding comprises a depressed trench surrounding the optical core.

9. The apparatus of claim 1, further comprising: a multimode light source coupled to the optical fiber; and a receiver coupled to the optical fiber.

10. A method comprising: forming a home optical data network with an optical fiber comprising: an optical core; and an optical cladding surrounding the optical core, the optical core having a refractive graded-index profile with a minimal refractive index n.sub.1 and a maximal refractive index n.sub.0; and said optical fiber having a numerical aperture NA and an optical core radius a satisfying a criterion C of quality of optical communications defined by the following equation: $C=\mathrm{NA}-0.02a$ $\mathrm{where}\text{:}$ $\mathrm{NA}=\sqrt{n_0^2-n_1^2}=n_0.Math.\sqrt{2}\mathrm{with}=\frac{n_0^2-n_1^2}{2n_0^2},$ is the normalized refractive index difference, and wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that NA>0.20, a>10 m and |C|<0.20; and transmitting data over the optical fiber by a data transceiver.

11. The method of claim 10, wherein transmitting data comprises transmitting a multimode optical data signal over the optical fiber.

12. The method of claim 10, wherein transmitting data comprises transmitting a single mode optical data signal over the optical fiber.

13. The method of claim 10, wherein transmitting data comprises transmitting both single mode and multimode optical data signals over the optical fiber.

14. The method according to claim 10, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that |C|<0.10.

15. The method according to claim 10, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that |C|<0.05.

16. The method according to claim 10, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 and said optical core radius a are chosen such that C<0.

17. The method according to claim 10, wherein said optical core radius is such that a>14 m.

18. The method according to claim 10, wherein said minimal and maximal refractive indexes n.sub.1, n.sub.0 are chosen such that NA>0.25.

19. The method according to claim 10, wherein the refractive graded-index profile is a single alpha graded-index profile n(r) defined by the following equation: $n\left(r\right)=n_0.Math.\sqrt{1-2.Math..Math.{\left(\frac{r}{a}\right)}^{}}ra$ where: r is a variable representative of the radius of said optical fiber, 1, being a non-dimensional parameter that defines the index profile shape of the optical core.

20. The method according to claim 10, wherein the optical cladding comprises a depressed trench surrounding the optical core.

21. The method according to claim 10, wherein transmitting data over the optical fiber comprises transmitting by single-mode optical data transmission at a wavelength of 1310 nm or 1550 nm.

22. The method according to claim 10, wherein transmitting data over the optical fiber comprises transmitting by multi-mode optical data transmission at a wavelength of 850 nm.

Description

LIST OF FIGURES

(1) Other features and advantages of embodiments of the invention shall appear from the following description, given by way of an indicative and non-exhaustive examples and from the appended drawings, of which:

(2) FIG. 1A graphically provides the refractive index profile of an optical fiber according to a first embodiment of the invention;

(3) FIG. 2A graphically provides the refractive index profile of an optical fiber according to a second embodiment of the invention;

(4) FIGS. 1B and 2B depict each a differential-mode-delay measurement carried out on the optical fibers of FIGS. 1A and 2A respectively;

(5) FIG. 3 graphically depicts the signal-to-incoherent noise ratio at a wavelength of 1550 nm as a function of numerical aperture and core radius of a graded-index optical fiber;

(6) FIG. 4 graphically depicts the signal-to-coherent noise ratio at a wavelength of 1550 nm as a function of numerical aperture and core radius of a graded-index optical fiber;

(7) FIG. 5 graphically depicts the signal-to-incoherent noise and signal-to-coherent ratios at wavelengths of 1550 nm and 1310 nm as a function of a criterion of quality of optical communications set in accordance with the invention;

(8) FIG. 6 graphically depicts the cumulative connection losses as a function of numerical aperture and core radius of a graded-index optical fiber;

(9) FIG. 7 illustrates a schematic diagram used for measuring cumulative connection losses under multimode launch conditions defined in the Encircled Flux standard (IEC 61280-4-1);

(10) FIG. 8 graphically illustrates the Encircled Flux template used for implementing the schematic diagram of FIG. 7.

DETAILED DESCRIPTION

(11) The general principle of the invention is to propose an optical fiber for which the values of numerical aperture and core diameter are adapted to support multimode operation up to a wavelength of 1550 nm with a high modal bandwidth at a wavelength 850 nm for a 10 Gbps operation over long distances (a few tens to a few hundreds of meters) and with reduced modal noises when said optical fiber is coupled with standard single-mode fiber for reliable high speed transmission with single-mode transmission systems.

(12) FIG. 1A depicts the refractive index profile n(r) of an optical fiber according to a first embodiment of the invention. It describes the relationship between the refractive index value n and the distance r from the center of the optical fiber.

(13) In that first embodiment, the optical fiber is a graded-index optical fiber having a refractive index profile n(r) defined as follow:

(14) $\begin{array}{cc}n\left(r\right)=\{\begin{array}{cc}{n}_0.Math.\sqrt{1-2.Math..Math.{\left(\frac{r}{a}\right)}^{}}& ra\\ n_0.Math.\sqrt{1-2.Math.}& ra\end{array}& \left(\mathrm{III}\right)\end{array}$
where:
r is a variable representative of the radius of the optical fiber,
a is the optical core radius,
is the normalized refractive index difference, with

(15) $=\frac{n_0^2-n_1^2}{2n_0^2}$
n.sub.1 is the minimal refractive index of the optical core,
n.sub.0 is the maximal refractive index of the optical core,
is a non-dimensional parameter that defines the index profile shape of the optical core, which is chosen between 1.9 and 2.2 so as to provide the largest bandwidth at the target operating wavelength.

(16) The optical fiber comprises, for 0ra, an optical core implementing a single alpha graded-index profile and, for ar, an optical cladding directly surrounding the optical core and having a standard constant refractive index. The alpha refractive index profile of the optical core allows reducing intermodal dispersion of the optical fiber.

(17) According to the invention, the optical core has a graded-index profile for which the values of numerical aperture NA and core radius a (expressed in micrometers) are tuned so that they satisfy the following equation:
C=NA0.02a(IV)
where:
NA is linked univocally to the normalized refractive index difference and the optical core's maximal refractive index n.sub.0 as follows:
NA={square root over (n.sub.0.sup.2n.sub.1.sup.2)}=n.sub.0.Math.{square root over (2)}(V)
a>10 m,
|C|<0.20, C being a real number, which represents a criterion of quality of optical communications.

(18) By adapting the values of numerical aperture NA and core diameter a in order to satisfying the above equation (IV), the invention provides a graded-index optical fiber optimized for effectively reduce incoherent and coherent modal noises at single-mode wavelength of 1550 nm, while keeping a high modal bandwidth at multimode wavelength of 850 nm.

(19) The inventors established that equation (IV) corresponds to a predetermined criterion of quality of optical communications that ensures supporting both single-mode and multimode transmissions with an adequate trade-off in terms of optical properties for high-data rate application. This criterion of quality has been obtained through a numerical assessment of the signal-to-incoherent noise ratio

(20) ${\mathrm{SNR}}_{\mathrm{incoherent}}=\frac{{.Math..Math.}^4}{\underset{i}{}{.Math.{}_i.Math.}^4}$
and the signal-to-coherent noise ratio

(21) 0${\mathrm{SNR}}_{\mathrm{coherent}}=\frac{\left({.Math..Math.}^4+\underset{i}{.Math.}{.Math.{}_i.Math.}^4\right)}{{}_{\mathrm{coherent}}}$
at a wavelength of 1550 nm as a function the core radius a and the numerical aperture NA, as depicts in FIGS. 3 and 4.

(22) The left-hand y-axis depicts the numerical aperture of the optical core (NA) and the x-axis depicts the optical core radius (a). The values of SNR.sub.incoherent (FIG. 3) and of SNR.sub.coherent (FIG. 4) corresponding to a given pair of parameters (NA, a) are illustrated in shades of gray in the right-hand y-axis.

(23) The inventors discovered that the core radius and numerical aperture of a graded-index optical fiber are strongly correlated to both signal-to-coherent noise and signal-to-incoherent noise ratios at both single-mode and multimode transmission wavelengths. Based on this principle, SNR.sub.incoherent and SNR.sub.coherent have been simulated with various values of numerical aperture and core radius to establish a relationship allowing a significant reduction of modal noises at single-mode wavelength of 1550 nm, while delivering the broadest modal bandwidth at multimode wavelength of 850 nm. The criterion of quality has been derived from those numerical assessments assuming that, for values of core radius larger than 10 m, SNR.sub.incoherent and SNR.sub.coherent shall be larger than 0 dB, and more preferentially SNR.sub.incoherent shall be approximately larger than 20 dB and SNR.sub.coherent shall be approximately larger than 10 dB at the wavelength of 1550 nm.

(24) It appears especially that decreasing core radius (a) and increasing numerical aperture (NA) lead to promote higher SNR.sub.incoherent and SNR.sub.coherent at 1550 nm. It further appears that the greater the numerical aperture is, the more the core radius to set can be relatively high: by doing this, multimode optical transmissions can be optimized to meet the demands of high-bandwidth applications (typically 10 Gbps) over long distances (a few tens to a few hundreds of meters), such as in the Ethernet high speed transmission networks.

(25) As a strictly illustrative example (and therefore of a non-limiting nature), the optical core radius a illustrated in FIG. 1 is about 19 m and the numerical aperture NA is about 0.297, thereby satisfying the criterion of quality of optical communications established in complying with the invention. The parameter of the optical core's index profile is about 2.065 and the normalized refractive index difference is about 2% (n.sub.1 being approximately equal to 1.457 and n.sub.0 approximately equal to 1.487).

(26) The advantages of the invention will be more evident by comparing optical fibers of the prior art with an exemplary optical fiber according to the invention. Table 1 below shows values of the core radius and numerical aperture of a standard graded-index optical fibers and value of the criterion C of quality that would be obtained by using the above equation (IV). That prior art fibers are subjected to an optical signal of a wavelength of 850 nm for the high-speed networks.

(27) TABLE-US-00001 TABLE 1 a (m) NA C (a, NA) 25 0.200 0.30 31.25 0.275 0.35 40 0.290 0.51 25 0.290 0.21

(28) The graph of FIG. 5 depicts the signal-to-incoherent noise and signal-to-coherent ratios at wavelengths of 1550 nm and 1310 nm as a function of the quality criterion C discussed above in relation with FIGS. 1, 3, 4 and applied both to graded-index optical fibers of prior art and optical fibers of the invention. The y-axis depicts SNR.sub.incoherent and SNR.sub.coherent (in dB) and the x-axis depicts different values of the criterion C of quality comprised between 0.60 and 0.20.

(29) It can be observed that none of the optical fibers of prior art owns a core index profile that allows meeting the criterion C of quality of the invention |C|<0.20, which consequently is reflected by lower values of SNR compared to those resulting from the invention. This graph shows that the model according to the invention leads to the establishment of a good quality criterion.

(30) In addition, in order to further improve SNR.sub.incoherent and SNR.sub.coherent, the criterion of quality can be set advantageously such as |C|<0.10 (i.e. |NA0.02a|<0.10), and more advantageously such as |C|<0.05 (i.e. |NA0.02a|<0.05), preferably with C<0. It can be seen that these signal-to-noise radios are maximized when the value of C is close to 0.

(31) According to one advantageous characteristic, the index profile of the optical fiber of FIG. 1 can comprise a depressed-index portion (not shown on FIG. 1) located between the graded-index core and the cladding. This depressed-index portion, also called a depressed trench, has a negative refractive index difference with respect to the optical fiber cladding, and its position and size are designed so as to improve bend-loss resistance of multimode fiber.

(32) FIG. 2A graphically provides the refractive index profile n(r) of an optical fiber according to a second embodiment of the invention.

(33) In that second embodiment, the optical fiber exhibits an optical core consisted of two portions, an inner optical core and an outer optical core surrounding the inner optical core, and the refractive graded-index profile is a twin alpha graded-index profile n(r) defined by the following equation:

(34) $\begin{array}{cc}n\left(r\right)=\{\begin{array}{cc}{n}_1^{.Math.}.Math.\sqrt{1-2.Math.{}_1.Math.{\left(\frac{r}{a}\right)}^{{}_1}}& 0r{r}_t\\ n_2^{}.Math.\sqrt{1-2.Math.{}_2.Math.{\left(\frac{r}{a}\right)}^{{}_2}}& r_{t}ra\\ n_1^{}.Math.\sqrt{1-2.Math.}& a<r\end{array}\mathrm{where}\text{:}{}_1=\frac{{}_2{\left(\frac{r_t}{a}\right)}^{{}_2-{}_1}}{{}_1+\left({}_2-{}_1\right){\left(\frac{r_t}{a}\right)}^{{}_2}}{}_2=\frac{{}_1}{\left(1-2\right).Math.\left({}_2-{}_1\right).Math.{\left(\frac{r_t}{a}\right)}^{{}_2}+{}_1}{n}_1^{}=\frac{n_1}{\sqrt{1-2}}{n}_2^{}=n_1.Math.\sqrt{\frac{\left(1-2\right).Math.\left({}_1-{}_2\right).Math.{\left(\frac{r_t}{a}\right)}^{{}_2}-{}_1}{\left(1-2\right).Math.\left(\left({}_1-{}_2\right).Math.{\left(\frac{r_t}{a}\right)}^{{}_2}-{}_1\right)}}& \left(\mathrm{VI}\right)\end{array}$
r is a variable representative of the radius of said optical fiber,
a is the optical core radius comprising both inner and outer optical cores,
r.sub.t is the radius of the inner optical core,
n.sub.1 is the maximal refractive index of the inner optical core,
n.sub.2 is the maximal refractive index of the outer optical core,
.sub.1 is the normalized refractive index difference relative to the inner optical core,
.sub.2 is the normalized refractive index difference relative to the outer optical core,
.sub.11, .sub.1 being a non-dimensional parameter that defines the index profile shape of the inner optical core,
.sub.21, .sub.2 being a non-dimensional parameter that defines the index profile shape of the outer optical core.

(35) The respective parameters .sub.1, .sub.2 and n.sub.1, n.sub.2 ensure the continuity of the refractive index profile and its first derivative at the transition from the inner core to the outer core.

(36) That particular twin alpha index profile offers the advantage of being able to improve even more the modal bandwidth of the optical fiber at multimode wavelengths.

(37) All that has been said so far in relation with FIG. 1A, 3 to 5 about the criterion of quality applies mutatis mutandis to that second embodiment of the invention. Also, in order to further improve the modal bandwidth of the optical fiber, the fiber according to second embodiment can comprise a depressed trench as described above in accordance with explanation provided in FIG. 5.

(38) As a strictly illustrative example (and therefore of a non-limiting nature), the optical core radius a illustrated in FIG. 2A is about 19 m and the numerical aperture NA is about 0.297, thereby satisfying the criterion of quality of optical communications established in complying with the invention. The parameters .sub.1 and .sub.2 of the optical core's index profile are respectively about 2.0851 and 2.0433. The radius of the inner optical core (r.sub.t) is about 0.5 m.

(39) FIGS. 1B and 2B graphs depict each a differential-mode-delay measurement (hereafter called DMD measurement) (e.g. as set forth in the FOTP-220 standard) carried out on the optical fibers of FIGS. 1A and 2A respectively. This kind of graph is obtained by successively injecting into the multimode optical fiber a light pulse having a given wavelength with a radial offset between each successive pulse. Delay of each pulse is then measured after a given length of fiber. Multiple identical light pulses are injected with different radial offsets with respect to the center of the optical core's core. The y-axis depicts the radial offset (noted radial launch on the Figure) in micrometers with respect to the center of the optical core's core and the x-axis depicts the time in nanoseconds. From these DMD measurements, it is possible to determine the effective modal bandwidth of the optical fiber. It appears from graphs 1B and 2B that the optical fibers of the invention present a time lag between the pulses propagating along different radial offsets which is relatively low, resulting in broad modal bandwidth. Furthermore, one can see the advantage of the twin alpha graded-index profile (FIG. 2B), which depicts differential-mode-delay measurements narrower than that of the single alpha graded-index profile (FIG. 1B), therefore has better modal bandwidths.

(40) It should be noted that the DMD measurements carried out with a radial offset upper than 18 m are not relevant. In particular it can be observed a few multiple pulses on the left-hand graph caused by cladding effect.

(41) FIG. 6 depicts the cumulative connection losses as a function of numerical aperture and core radius of a single alpha graded-index optical fiber.

(42) The left-hand y-axis depicts the numerical aperture of the optical core (NA) and the x-axis depicts the optical core's radius (a). The values of cumulative connection losses (expressed in dB) corresponding to a given pair of parameters (NA, a) are illustrated in shades of gray in the right-hand y-axis.

(43) Cumulative connection losses are measured at a wavelength of 850 nm under multimode launch conditions for measuring attenuation defined in the known Encircled Flux standard (IEC 61280-4-1). Principle of launch conditions defined by the EF is reminded in FIG. 8. EF defines the integral of power output of the optical fiber over the radius of the fiber.

(44) As illustrated in FIG. 7, to characterize cumulative connection losses in accordance with EF standard conditions, an optical fiber 70 according to the invention is subject to a spot of a multimode light source 71 coupled to thereon. The near field pattern of the spot is then observed at the output of the optical fiber by a receiver 72 and post-processed to assess the cumulative connection losses at P1 and P2 levels. In other words, cumulative connection losses means losses measured cumulatively at connections P1 and P2.

(45) It appears from FIG. 6 that, for acceptable cumulative losses, the optical core radius a shall be upper than 20 m. If one choose the criterion of quality C be such that |C|<0.10 for example, the numerical aperture shall be upper than 0.30. With such values, the numerical aperture NA and optical core radius a satisfy the criterion C of quality as defined according to the above equation (IV). To complete the illustration of FIG. 6, a few complementary values of core radius a, numerical aperture NA and criterion C applied for optical fibers in compliance with the invention are showed in the Table 2 below and compared with cumulative losses measured.

(46) TABLE-US-00002 TABLE 2 Cumulative a (m) NA C (a, NA) Loss (dB) 24 0.28 0.20 1.0 19 0.28 0.10 2.2 16.5 0.28 0.05 3.1 27.5 0.35 0.20 1.0 22.5 0.35 0.10 1.4 20 0.35 0.05 1.8

(47) It becomes apparent that, for values of NA of 0.35, the optical fibers of the invention allow larger core radius than that for which values of NA is 0.28, which enables to obtain reduced cumulative losses.

(48) Finally, in addition to improve the signal-to-noise ratios, increasing numerical aperture of the optical fiber leads to obtain a higher number of optical modes at multimode wavelengths. The number of optical modes guided in the fiber is function of the numerical aperture and optical core radius. In particular the number of guided optical modes can been determined by means of the following equation:

(49) $N=\frac{}{+2}.Math.a^2.Math.{\left(\frac{2}{}\right)}^2.Math.n_0^2.Math.$
wherein:
a is the optical core radius,
is the normalized refractive index difference, with

(50) $=\frac{n_0^2-n_1^2}{2n_0^2}$
is a non-dimensional parameter that defines the index profile shape of the optical core, comprised between 1.9 and 2.2,
N is the number of optical modes.

(51) An exemplary embodiment of the present application overcomes the different drawbacks of the prior art.

(52) More specifically, an exemplary embodiment provides an optical fiber optimized for supporting both single-mode and multimode transmissions with an adequate trade-off in terms of optical properties for high-data rate applications.

(53) An exemplary embodiment provides an optical fiber that offers the broadest modal bandwidth for multimode transmission over long distances and that sustains a fundamental mode similar to that required for single-mode transmission.

(54) An exemplary embodiment provides an optical fiber that significantly reduces modal noises at wavelengths of 1310 nm and 1550 nm, while delivering a broad modal bandwidth at a wavelength of 850 nm.

(55) An exemplary embodiment provides an optical fiber that is simple to manufacture and costs little.

(56) Although the present disclosure has been described with reference to one or more examples, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the disclosure and/or the appended claims.