JP2019036880A - Optical transmission line and optical transmission system - Google Patents

Abstract

To provide an optical transmission line optimized to transmit a low linearity, low DMD, high efficiency Raman amplification multimode, and capable of expanding transmission capacity and transmission distance, while reducing the DSP load at a reception end compensating for inter-mode crosstalk.SOLUTION: An optical transmission line is constituted of more than one kind of large Aeff fiber and small Aeff fiber, where the DMD is 50 ps or more up to first half 40 km, at least second half 20 km or more is the small Aeff fiber, and includes a mode transducer converting the exciting light subjected to Raman amplification by the optical fiber into one propagation mode, before being incident on the optical fiber, and a mode multiplexer for multiplexing signal light from more than one transmitters as propagation modes different from each other, and connecting to one end of the optical fiber, and demultiplexing the propagation modes of the signal light.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to an optical transmission path that enables Raman optical amplification by mode multiplexing transmission and an optical transmission system including the optical transmission path.

  The “single mode optical fiber” and “multimode (several mode) optical fiber” described in this specification are an optical fiber that propagates in a single mode and a light that propagates in a multimode, respectively, at the wavelength of an optical signal to be transmitted. Means fiber.

  A number mode optical fiber using a plurality of propagation modes has been proposed as a technique for expanding the transmission capacity. In particular, mode multiplex transmission using a plurality of propagation modes is attracting attention as a new large-capacity transmission system because the transmission capacity can be improved several times the number of modes.

  In transmission using this number mode optical fiber, crosstalk between modes occurs in the transmission path, and therefore, a MIMO (Multiple-Input Multiple-Output) equalizer is used at the receiving end as compensation means. However, if the group delay difference between modes (hereinafter referred to as DMD) is large at the receiving end, the load of digital processing (DSP) related to MIMO increases, and in order to realize long-distance transmission, the DSP load is large. Reduction is an issue (see, for example, Non-Patent Document 1). In order to alleviate the influence of DMD, an optical transmission system using an optical fiber having a small DMD as a transmission path has been proposed (see, for example, Non-Patent Document 2).

  In mode multiplex transmission, in order to compensate for the signal-to-noise ratio of the transmission line, a technique using distributed Raman amplification has been studied as in the case of the single mode transmission line, and experiments and calculation studies have been performed (for example, non-patent literature). (See 3, 4). In examining the optical amplification technique in mode multiplex transmission, it is important to reduce the difference in gain between modes (Diffferential Modal Gain: DMG). However, the signal light propagating through the multimode fiber has a different electric field distribution for each mode, and DMG is generated due to the difference in the magnitude of the overlap with the electric field distribution of the excitation light for each mode. A study of Raman amplification using a transmission line (GI type fiber) having a graded index refractive index profile has been conducted, and the propagation mode of pumping light can be set to LP21 mode, and DMG can be reduced to 1.0 dB. (For example, refer nonpatent literature 5).

S. O. Arik, D.D. Askarov, J. et al. M.M. Kahn, “Effect of mode coupling on signal processing complexity in mode-division multiplexing”, J. Am. Lightwave Technol. 31 (3) (2013) 423-431. T.A. Mori, T .; Sakamoto, M .; Wada, T .; Yamamoto, and F.A. Yamamoto, “Few-mode fibers supporting more than two LPs for mode-division-multiplexed transmission with MIMO DSP,” J. Yamamoto. Lighttw. Technol. , Vol. 32, pp. 2468-2479 2014. R. Ryf, A.A. Sierra, RJ. Essiambre, and S.M. Randel, A.D. H. Gnauck, C.I. Boll, M.M. Esmaelpour, P.M. J. et al. Winzer, R.A. Delbue, P.A. Pupalaikise, A.M. Sureka, D.A. W. Peckham, A.D. McCurdy, and R.M. Lingle, Jr “Mode-Equalized Distributed Raman Amplification in 137-km Few-Mode Fiber”, ECOC, paper Thr. 13. K. 5. 2011. R. Ryf, R.A. Essiambre, J. et al. Hoyningen-Huene, and P.M. Winzer, “Analysis of Mode-Dependent Gain in Raman Amplified Few-Mode Fiber”, in Optical Fiber Communication Conference, OSA Technical Digest. 2. 2012. M.M. Wada, T .; Sakamoto, T .; Mori, T .; Yamamoto and K.K. Nakajima, "4-LP mode distributed Raman amplification technique with graded-index multimode fiber transmission line", 2016 21st OptoElectronics and Communications Conference (OECC) held jointly with 2016 International Conference on Photonics in Switching (PS), Niigata, Japan, 2016, pp. 1-3. C. Koebel, M.C. Salsi, G.M. Charles and S.C. Bigo, “Nonlinear Effects in Mode-Division-Multiplexed Transmission Over Few-Mode Optical Fiber,“ 11 IEEE Photonics Technology Letters, 13 p.13. P. Genevaux et al. “6-mode Spatial Multiplexer with Low Loss and High Selectivity for Transmission over Few Mode Fiber”, Proc. OFC, W1A. 5, Los Angeles (2015). S. Randel, C.I. Schmidt, R.A. Ryf, R.A. -J. Essiambre, P.M. J. et al. Winzer, “MIMO-based signal processing for mode-multiplexed transmission”, in: Proc. Photonics Society Summer Topic Meeting Series, 2012, paper MC4.1, 2012.

  In order to realize the extension of repeaterless transmission, it is necessary to reduce the influence of the nonlinear phenomenon. In order to suppress the nonlinear phenomenon, the effective area of the mode propagating through the optical fiber may be increased. On the other hand, increasing the effective area decreases the Raman amplification efficiency. That is, there is a trade-off relationship between the suppression of nonlinear phenomena and the Raman amplification efficiency in the effective cross-sectional area of the mode propagating through the optical fiber, and there is a problem that it is difficult to achieve both. In the case of mode multiplex transmission, a problem has been reported in which signal quality deteriorates due to non-linear effects between modes when the DMD is small (see Non-Patent Document 6, for example).

  Accordingly, in order to solve the above-described problems, the present invention provides an optical transmission line and an optical transmission system that can prevent degradation of Raman amplification efficiency while suppressing signal degradation due to nonlinear phenomena and prevent degradation of signal quality during mode multiplexing transmission. The purpose is to provide.

  In order to achieve the above object, the optical transmission line according to the present invention uses an optical fiber having a large effective area in the first half of the optical fiber transmission line and an optical fiber having a small effective area in the second half. The sign was changed.

Specifically, the optical transmission line according to the present invention is an optical transmission line in which at least two types of optical fibers capable of propagating a propagation mode of m (m is an integer of 2 or more) or more are connected in series,
Among the optical fibers, the front optical fiber into which the signal light is incident has a larger effective cross-sectional area of the fundamental mode than the rear optical fiber that outputs the signal light, and the front optical fiber and the rear optical fiber are in the inter-mode group. The sign of the delay difference is different.

  This optical transmission line suppresses nonlinear phenomena for the front optical fiber, increases the effective area of the fundamental mode to such an extent that DMG can be reduced, and increases the effective area of the fundamental mode to the extent that the Raman amplification efficiency can be increased for the rear optical fiber. Is reduced to achieve both suppression of nonlinear phenomena and Raman amplification efficiency. Furthermore, this optical transmission line has different DMD codes for the front optical fiber and the rear optical fiber, so that the DMD approaches zero as the entire optical transmission line, thereby preventing signal quality degradation during mode multiplex transmission.

  Therefore, the present invention can provide an optical transmission line that can prevent a decrease in Raman amplification efficiency while suppressing signal deterioration due to a nonlinear phenomenon, and can prevent a decrease in signal quality during mode multiplex transmission.

The optical fibers adjacent to each other in the optical transmission line according to the present invention are characterized in that the difference in effective cross-sectional area of the fundamental mode is ± 30 μm ² or less. Loss generated at the connection point between optical fibers can be reduced.

  When the propagation distance is within 40 km, the influence of the nonlinear phenomenon is larger than the propagation loss. Therefore, in the optical transmission line according to the present invention, it is preferable that the optical fiber within 40 km from the incident end where the signal light is incident has an absolute value of the inter-mode group delay difference of 50 ps or more.

  The back optical fiber of the optical transmission line according to the present invention is 20 km or more. In the region where the length of the rear optical fiber is 20 km or more, the light intensity (gain) can be 50% of the saturation intensity regardless of the Raman gain coefficient.

  The optical fibers adjacent to each other in the optical transmission line according to the present invention are connected by a tapered fiber having a tapered core according to the effective area of each fundamental mode. Loss generated at the connection point between optical fibers can be reduced.

An optical transmission system according to the present invention includes:
The optical transmission line;
A mode converter that converts light from a light source into one of propagation modes that propagate through the optical transmission line, enters the optical transmission line, and Raman-amplifies the optical fiber in the optical transmission line;
A mode multiplexer that multiplexes signal light from two or more transmitters as mutually different propagation modes and couples them to the front optical fiber of the optical transmission line;
A mode demultiplexer for demultiplexing the signal light output from the rear optical fiber of the optical transmission path for each propagation mode;
Is provided.
Since this optical transmission system uses the optical transmission path, it is possible to prevent a decrease in Raman amplification efficiency while suppressing a signal deterioration due to a non-linear phenomenon, and to prevent a decrease in signal quality during mode multiplex transmission.

  The present invention can provide an optical transmission line and an optical transmission system capable of preventing a decrease in Raman amplification efficiency while suppressing signal degradation due to a nonlinear phenomenon, and preventing signal quality from being degraded during mode multiplexing transmission.

It is a figure explaining the optical transmission system concerning the present invention. It is a figure explaining an example of the composition of the optical transmission line concerning the present invention. It is a figure explaining the electric field distribution of the mode which propagates the optical transmission line concerning the present invention. It is a figure explaining the relationship between the effective area difference and the loss in the connection point between the optical fibers of the optical transmission line concerning this invention. It is a figure explaining the relationship between the transmission distance and maximum phase shift amount in the optical fiber of the optical transmission line concerning this invention. It is a figure explaining the light intensity change of the transmission line longitudinal direction accompanying Raman amplification. It is a figure explaining the example of the refractive index of the optical fiber which concerns on this invention. It is a figure explaining the optical fiber structure of the optical transmission line concerning this invention. It is a figure explaining the relationship between the transmission distance and the effective area of a fundamental mode in the optical transmission line concerning this invention. It is a figure explaining the relationship between the transmission distance and DMD in the optical transmission line concerning this invention. It is a figure explaining the impulse response of the optical transmission line concerning the present invention. It is a figure explaining the optical fiber transmission experiment system using back Raman amplification performed with the optical transmission system concerning the present invention. It is a figure explaining the optical fiber transmission experiment result using back Raman amplification performed with the optical transmission system concerning the present invention. It is a figure explaining an example of the composition of the optical transmission line concerning the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. These embodiments are merely examples, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art.

(Embodiment 1)
FIG. 1 is a diagram illustrating an optical transmission system 301 according to this embodiment. The optical transmission system 301 is
An optical transmission line 11 of a multimode optical fiber;
A mode converter 12 that converts light from the light source 10 into one of propagation modes propagating through the optical transmission line 11, enters the optical transmission line 11, and Raman-amplifies the optical fiber of the optical transmission line 11;
A mode multiplexer 14 that multiplexes signal lights from two or more transmitters 13 as different propagation modes and couples the light beams to the front optical fiber of the optical transmission line 11;
A mode demultiplexer 15 for demultiplexing the signal light output from the rear optical fiber of the optical transmission line 11 for each propagation mode;
Is provided.

  The multimode optical fiber of the optical transmission line 11 is an optical fiber having a core refractive index profile capable of propagating two or more propagation modes and having a GI type (details will be described later). The mode converter 12 converts the excitation light for Raman amplification with the optical fiber into one propagation mode included in the mode group M, and enters the optical fiber. The mode multiplexer 14 combines signal lights from two or more transmitters 13 as different propagation modes and couples them to one end of the optical fiber. At least two of the signal light propagation modes are propagation modes included in the mode group M. However, the mode group M is a group of propagation modes where M = 2p + l−1 and 3 or more when the propagation mode is expressed as LPlp. For example, when the multimode optical fiber is a 4LP mode GI fiber in which LP01, LP11, LP21, and LP02 modes propagate, the number of mode groups is three.

  The N types of signals emitted from the N transmitters 13 are multiplexed by the mode multiplexer 14. The combined signal light enters the optical fiber of the optical transmission line 11 and is demultiplexed to the M port by the mode demultiplexer 15 installed on the output side. As the refractive index distribution of the optical fiber used here, at least a core having a graded index type is used (for example, see Non-Patent Document 2). Further, it has a light source 10 for pumping light for distributed Raman amplification, and is converted into a desired mode by a mode converter 12 as necessary, and then incident on an optical fiber of the optical transmission line 11. The optical transmission system 301 in FIG. 1 shows an example in which the excitation light is incident from the receiver side, but the same effect can be obtained even if it is incident from the transmitter side or from both directions.

FIG. 2 is a diagram for explaining the configuration of the optical transmission line 11. The optical transmission line 11 is an optical transmission line in which at least two types of optical fibers capable of propagating a propagation mode of m (m is an integer of 2 or more) or more are connected in series,
Among the optical fibers, the front optical fiber 21 into which the signal light is incident has a larger effective cross-sectional area of the fundamental mode than the rear optical fiber 29 that outputs the signal light, and the front optical fiber 21 and the rear optical fiber 29 are mutually in mode. The sign of the intergroup delay difference is different.

  The optical transmission line 11 in FIG. 2 is configured by two types of optical fibers, and light is incident on an optical fiber (front optical fiber 21) having a large effective area (Aeff) with respect to the input. Signal light is emitted from an optical fiber having a small length (rear optical fiber 29) (FIG. 2B).

  Also, DMD is compensated to reduce the MIMO signal processing load at the receiving end. The optical transmission line 11 is characterized in that the DMD codes of the front optical fiber 21 and the rear optical fiber 29 are different in order to compensate for DMD (FIG. 2C).

[Examination of effective area]
FIG. 3 shows the electric field distribution of the mode propagating through the 4LP mode GI fiber. Four LP modes of LP01, LP11, LP21, and LP02 modes propagate. Here, when fibers having different Aeff are connected, loss at the connection point occurs. FIG. 4 shows the relationship between the effective area difference and the loss. At this time, the electric field distribution for the LP01, LP11, LP21, and LP02 modes was obtained by the finite element method, the coupling efficiency η was obtained by overlap integration, and the loss (1-η) was calculated.
Ei is the electric field distribution on the input fiber side, and Eo is the electric field distribution on the output fiber side.

When the core radius is changed by ± 5 μm using the 4LP mode GI fiber described in Non-Patent Document 2, in order to make the loss less than 0.1 dB at one connection point from FIG. It turns out that it is necessary to make it 30 μm ² or less. In other words, the optical fibers adjacent to each other in the optical transmission line 11 have a difference in effective cross-sectional area of the fundamental mode of ± 30 μm ² or less.

[Examination of group delay difference between modes]
Next, the relationship between the size of the effective area and nonlinear deterioration was examined. The maximum phase shift amount φmax due to self-phase modulation that occurs in an independent mode, which is one of nonlinear phenomena, is calculated by the following equation.
P ₀ is the input intensity, Aeff is the effective area, α is the propagation loss, L is the fiber length, n ₂ is the nonlinear refractive index, c is the speed of light (constant: 29792458 m / s), and ω ₀ is each frequency.

FIG. 5 shows the relationship between φmax and transmission distance. The input intensity P ₀ was 20 dBm, the propagation loss α was 0.2 dB / km, the nonlinear refractive index n ₂ was 2.66 × 10 ⁻²⁰ m ² / W, and the wavelength was 1550 nm. ω ₀ was calculated from the wavelength.

  FIG. 5 shows that the maximum phase shift amount φmax is about 80% of the saturation value at 35 km or more. That is, as the transmission distance increases, the propagation loss increases and the light intensity decreases, so the influence of self-phase modulation becomes invisible. Although the loss is 0.2 dB / km this time, the maximum phase shift amount φmax varies depending on the loss and input intensity. If the loss is 0.23 dB / km, the transmission distance that is 80% of the saturation value is about 30 km, and if the loss is 0.17 dB / km, the transmission distance that is 80% of the saturation value is about 40 km. . For example, the transmission loss of the GI fiber described in Non-Patent Document 2 is about 0.23 dB / km. A multimode GI fiber with a transmission loss of 0.17 dB / km or less has not been reported so far, and if it is about 40 km, it becomes 80% of the saturation value. That is, it is considered that the non-linear phenomenon does not greatly affect the transmission distance of 40 km or more.

  In addition, the non-linear effect (cross phase modulation) between modes has an effect when DMD is 50 ps or less (see Non-Patent Document 6). Therefore, DMD is set to 50 ps or more until it exceeds 40 km. That is, the optical transmission line 11 is characterized in that the optical fiber within 40 km from the incident end where the signal light is incident has an absolute value of the group delay difference between modes of 50 ps or more.

[Examination of Raman amplification]
FIG. 6 shows a change in the light intensity in the longitudinal direction of the transmission line accompanying Raman amplification when the light intensity at the incident end is 1. As parameters for calculating the distributed Raman gain, a propagation loss αs of signal light wavelength of 1550 nm = 0.2 dB / km, a propagation loss αs of pumping light wavelength of 1450 nm = 0.25 dB / km, an excitation light intensity of 50 mW, and a Raman gain coefficient. Was variable from 3.0 × 10 ^{−14 to} 15.0 × 10 ⁻¹⁴ m / W. From FIG. 6, it can be confirmed that the light intensity accompanying the distributed Raman amplification is saturated at about 60 km regardless of the Raman gain coefficient.

  The result when the light intensity (gain) required for the rear optical fiber 29 is 50% of the saturation intensity is shown by a dotted line in FIG. It can be confirmed that the above characteristics are satisfied at least in the region of 20 km or more regardless of the Raman gain coefficient. That is, the rear optical fiber 29 of the optical transmission line 11 is 20 km or more.

(Embodiment 2)
In this embodiment, a specific optical transmission system 302 will be described.
FIG. 7 shows refractive index distribution examples of an optical fiber having a large Aeff (120 μm ² ) (FIG. 7A) and an optical fiber having a small Aeff (80 μm ² ) (FIG. 7B). The refractive index distribution is expressed by the following equation.
The structural parameters are as shown in FIG. a _{1 to} a ₃ are distances in the radial direction of the cross section of the optical fiber. n _{1 to} n ₃ are refractive indexes. Δ1 and Δ2 are relative refractive indexes with respect to the cladding (n ₃ ). α is an alpha parameter. The large Aeff fiber of FIG. 7A has an LP01 mode Aeff of 120 μm ² , and the small Aeff fiber of FIG. 7B has an LP01 mode Aeff of 80 μm ² . Four LP modes can propagate. Then, the value and sign of the DMD can be adjusted by adjusting the α parameter.

FIG. 8 shows a fiber configuration of the optical transmission line 11 of the optical transmission system 302. It is composed of a total of nine types of optical fibers (21 to 29). FIG. 9 shows the LP01 mode Aeff of each optical fiber (21 to 29), and the LP01 mode Aeff with respect to the transmission distance. For example, the forward optical fiber 21, Aeff is 5.4km in length by 128 .mu.m ^2, the backward light fiber 29, Aeff length in 81Myuemu ² is 44.4. Each optical fiber (21 to 29) is connected with an effective area difference of 20 μm ² or less.

  FIG. 10 is a DMD with respect to the transmission distance of the optical transmission line 11. As described with reference to FIG. 5, since the influence of the nonlinear effect is small in the region where the transmission distance is 40 km or more, it is expected that the influence of the nonlinear effect between modes as shown in Non-Patent Document 6 can be sufficiently suppressed. Is done. Based on the prediction, the optical transmission line 11 of the present embodiment is designed so that the DMD difference is 1 ns or more in order to suppress the non-linear effect between modes up to a point of 40 km.

  FIG. 11 shows an impulse response of the optical transmission line 11 (total fiber length 103.4 km of the optical fibers 21 to 29). Impulse response is 4 ns or less (37 ps / km) due to compensation of DMD. By using such an optical transmission line 11, it is possible to reduce the load of the MIMO signal processing performed by the DSP 17 at the receiving end.

  FIG. 12 is a diagram illustrating the configuration of an optical transmission system 302 that is a 12 × 12 optical MIMO transmission experimental system. A 1.25 Gb / s QPSK signal was used as the signal light output from the optical transmitter 13, and independent polarization multiplexed signal and mode multiplexed signal channels were created using delay lines. As the mode multiplexer / demultiplexer (14, 15), a spatial device (Non-patent Document 7) was used. From the subsequent stage of the optical transmission line 11, a signal obtained by mode-converting the pumping light from the Raman amplification pumping light source 10 by the mode converter 12 is input. The signal light that has passed through the optical transmission line 11 is demultiplexed by the mode demultiplexer 15, the optical signal is converted into an electric signal by the coherent receiver 16, and MIMO signal processing is performed by the DSP 17.

  FIG. 13 is a constellation map of each signal for explaining a transmission experiment result in the transmission experiment system of FIG. FIG. 13A shows the result without Raman amplification, and FIG. 13B shows the result with Raman amplification. The mode dependent loss (MDL) was calculated by the formula described in Non-Patent Document 8. From FIG. 13, the signal quality Q value was improved by about 2 dB by using Raman amplification. In addition, MDL deteriorates only by about 0.1 dB and realizes low DMG.

(Embodiment 3)
FIG. 14 is a diagram for explaining the connection between the optical fibers of the optical transmission line 11. The optical fibers adjacent to each other in the optical transmission line 11 of the present embodiment are connected by a tapered fiber 31 whose core is tapered according to the effective cross-sectional area of each fundamental mode. The connection loss can be reduced by connecting the large Aeff fiber (for example, the front optical fiber 21) and the small Aeff fiber (for example, the front optical fiber 29) with the tapered fiber 31.

(Effect of the invention)
The present invention achieves low nonlinearity and high-efficiency Raman amplification by optimally designing the effective cross-sectional area and mode group delay difference (DMD) of an optical transmission line, and increases the DSP load in mode multiplexing transmission MIMO processing. It is possible to provide a transmission system that can relax the transmission distance and increase the transmission distance.

  The present invention can realize a large capacity and a long distance of optical fiber transmission by using a higher-order mode in the fiber.

10: Light source 11: Optical transmission line 12: Mode converter 13: Optical transmitter 14: Mode multiplexer 15: Mode duplexer 16: Optical receiver 17: DSP
21: Front optical fibers 22 to 28: Optical fiber 29: Rear optical fiber 31: Tapered fiber 301, 302: Optical transmission system

Claims (6)

An optical transmission line in which at least two types of optical fibers capable of propagating a propagation mode of m (m is an integer of 2 or more) or more are connected in series,
Among the optical fibers, the front optical fiber into which the signal light is incident has a larger effective cross-sectional area of the fundamental mode than the rear optical fiber that outputs the signal light, and the front optical fiber and the rear optical fiber are in the inter-mode group. An optical transmission line characterized in that the signs of the delay differences are different. ^2. The optical transmission line according to claim 1, wherein the adjacent optical fibers have a difference in effective area of a fundamental mode of ± 30 μm ² or less.   3. The optical transmission line according to claim 1, wherein the optical fiber within 40 km from the incident end where the signal light is incident has an absolute value of a group delay difference between modes of 50 ps or more.   The optical transmission line according to any one of claims 1 to 3, wherein the rear optical fiber is 20 km or more.   The optical transmission line according to any one of claims 1 to 4, wherein the adjacent optical fibers are connected by a tapered fiber having a tapered core according to the effective area of each fundamental mode. An optical transmission line according to any one of claims 1 to 5,
A mode converter that converts light from a light source into one of propagation modes that propagate through the optical transmission line, enters the optical transmission line, and Raman-amplifies the optical fiber in the optical transmission line;
A mode multiplexer that multiplexes signal light from two or more transmitters as mutually different propagation modes and couples them to the front optical fiber of the optical transmission line;
A mode demultiplexer for demultiplexing the signal light output from the rear optical fiber of the optical transmission path for each propagation mode;
An optical transmission system comprising: