Optical transmission system and optical transmission method

Abstract

The objective of the present disclosure is to further reduce the signal crosstalk between cores in a multicore optical fiber transporting.　The present disclosure is an optical transporting system for transporting signal lights by using, as transporting paths, a multicore optical fiber having two or more transporting cores. Two or more transmitters send out signal light to adjacent ones of the cores in the opposite directions. Two or more excitation light oscillators emit excitation light into the ones of the adjacent cores that are the same as the transporting cores from which the signal light is sent out such that the excitation light propagates in the same directions or opposite directions as or to the directions of the signal light, thereby performing a distributed Raman amplification of the signal light using the excitation light. In this way, the signal light gains of the signal light sent out from the transmitters are set such that the ratio of the signal strength to the strength of crosstalk noise flowing in from the adjacent transporting cores is higher.

Description

[Technical Field]

[0001] The present disclosure relates to an optical transmission system that uses a multicore fiber as a transmission medium. [Background technology]

[0002] Space division multiplexing (SDM) technology has been attracting attention as a high-capacity transmission line technology. In multi-core optical fiber transmission, which is one of the core technologies, signal crosstalk between cores (hereinafter sometimes referred to as XT) is known to be a limiting factor in expanding transmission capacity (see, for example, Non-Patent Document 1).

[0003] Therefore, optical fiber design methods for suppressing signal crosstalk between cores have been proposed (see, for example, Non-Patent Documents 2 to 4). Also, a bidirectional communication system has been proposed in which the transmission direction is switched for each core (see, for example, Non-Patent Document 5). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] PJ Winzer et al., ECOC Tu5.B.7 (2011) [Non-patent document 2] T. Matsui et al., ECOC M.1.D.5 (2019) [Non-patent document 3] Y. Sagae et al., IEICE Trans. Commun. E103-B.11 1199 (2020) [Non-patent document 4] S. Nozoe et al., ECOC (2017) [Non-Patent Document 5] A. Sano et al., JLT 32. 16. 2771 (2014) Summary of the Invention [Problem to be solved by the invention]

[0005] To further expand transmission capacity, it will be necessary to further reduce XT by combining fiber design and transmission methods.

[0006] The present disclosure aims to further reduce signal crosstalk between cores in multi-core optical fiber transmission. [Means for solving the problem]

[0007] This disclosure proposes an optical transmission system that applies distributed Raman amplification in an optical fiber, thereby suppressing the XT noise light intensity and achieving low XT.

[0008] Specifically, the optical transmission system and the optical transmission method of the present disclosure include: An optical transmission system that transmits signal light using a multi-core optical fiber having two or more transmission cores as a transmission line, two or more transmitters transmit signal light to adjacent cores in opposite directions; two or more pumping light oscillators input pumping light into the same core as the transmission core from which the signal light is sent out, among the adjacent cores, so that the pumping light propagates in the same direction or in the opposite direction to the signal light, and perform distributed Raman amplification of the signal light by the pumping light; The signal light gain of the signal light sent from the transmitter is set so that the ratio of signal strength to crosstalk noise strength flowing in from the adjacent transmission core is high. [Effects of the Invention]

[0009] The present disclosure can further reduce signal crosstalk between cores in multi-core optical fiber transmission, thereby achieving an effect of increasing the transmission capacity of an optical transmission system. [Brief explanation of the drawings]

[0010] [Figure 1] 1 shows a schematic of a transmission system that uses distributed Raman amplification to improve the signal-to-crosstalk noise intensity ratio. [Figure 2] This is a diagram showing the configuration of a bidirectional transmission, backward-pumped DRA that achieves XT reduction. [Figure 3] This is a diagram of a multi-stage configuration of a bidirectional transmission, back-pumped DRA that achieves XT reduction. [Figure 4] This is a diagram showing the repeater configuration of a bidirectional transmission, backward pumping DRA that achieves XT reduction. [Figure 5] This is an example of the dependence of XT change on signal light gain in bidirectional transmission and backward pumping. [Figure 6] This is an example of the transmission distance dependence of the maximum signal light gain that achieves XT reduction in backward pumping. [Figure 7] This is a diagram showing the configuration of a bidirectional transmission, forward-pumped DRA that achieves XT reduction. [Figure 8] This is a diagram of a multi-stage configuration of a bidirectional transmission, back-pumped DRA that achieves XT reduction. [Figure 9] This is a diagram showing the configuration of a bidirectional transmission, forward-pumped DRA that achieves XT reduction. [Figure 10] This is an example of the dependency of the XT reduction amount on the signal light gain (transmission distance 100 km). [Figure 11] This is an example of the gain difference dependency of the maximum and minimum signal light gains that achieves XT reduction (transmission distance 100 km). [Figure 12] This is an example of the dependency of the XT reduction amount on the signal light gain when there is a signal light gain difference between cores (transmission distance 150 km). [Figure 13] This is an example of the gain difference dependency of the maximum and minimum signal light gains that realizes XT reduction (transmission distance 150 km). [Figure 14] This is an example of the dependency of K1 on transmission distance. [Figure 15] This is an example of the dependence of K2 on transmission distance. [Figure 16] This is an example of the transmission distance dependency of K3. [Figure 17] This is an example of the transmission distance dependency of K4. [Figure 18] This is an example of the transmission distance dependency of K5. [Figure 19]This is an example of the transmission distance dependence of signal light gain for minimizing XT in backward-pumped and forward-pumped DRAs. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0012] The present disclosure provides an optical transmission system (using a multi-core optical fiber) using distributed Raman amplification, which has the following features. The transmitter sends signal light to adjacent cores in opposite directions. The pump light is injected into adjacent cores by a pump light oscillator so that it propagates in the same direction or counter-propagating direction as the signal light, achieving distributed Raman amplification of the signal light. The signal light gain of the signal light sent from the transmitter is designed so that the ratio of signal strength to crosstalk noise strength is high. The present disclosure makes it possible to suppress the effects of crosstalk noise and increase the transmission capacity of an optical transmission system. This will be explained in detail below.

[0013] FIG. 1 shows a conceptual diagram of an optical transmission system according to the present disclosure. As an example, this figure shows a system in which forward-pumped distributed Raman amplification (DRA) of signal light is performed in a bidirectional transmission system in which the signal propagation directions are opposite between adjacent cores C1 and C2, numbered 1 and 2, in an MCF transmission line. A signal is sent from transmitter 71 to core C1, and a signal is sent from transmitter 72 to core C2 in the opposite direction. In this way, in the present disclosure, signal light is sent from opposite directions to different adjacent cores. The signal light in core C1 is received by receiver 81, and the received light intensity is P s1A part of the signal light in the core C2 is coupled to the core C1, and the optical intensity P XT1 The crosstalk noise light (XT light) is detected.

[0014] In FIG. 1, an example is shown in which pumping light oscillators 91 and 92 that realize DRA are incident in the same direction as the signal light in each core. The pumping light oscillators 91 and 92 incident pumping light into the same transmission core of the adjacent cores C1 and C2 from which the signal light is sent. This performs distributed Raman amplification of the signal light by the pumping light. Here, the pumping light may be incident so as to propagate in the same direction as the signal light, or may be incident so as to propagate in the opposite direction to the signal light. P s1 and P XT1 is amplified, but P s1 The amplification in P XT1 As a result, the signal to XT light intensity ratio P s1 / P XT1 will improve.

[0015] FIG. 2 shows a configuration diagram of an optical transmission system according to the present disclosure, where an MCF having N+M cores is used as the transmission path. Here, where n is an integer between 1 and N, cores Cun numbered un are not adjacent to each other. Similarly, where m is an integer between 1 and M, cores Cdm numbered dm are not adjacent to each other. Signals generated by transmitters 71u1-71uN are transmitted to cores Cu1-CuN in the MCF 83 via optical coupler 84#1 and received by receivers 81u1-81uN via optical coupler 84#2. Meanwhile, signals generated by transmitters 72d1-72dM are transmitted to cores Cd1-CdM in the MCF 83 via optical coupler 84#2 and received by receivers 82d1-82dM via optical coupler 84#1.

[0016] In this configuration, the signals propagating in adjacent cores propagate in opposite directions. Pumping light oscillators 93u1-93uN and pumping light oscillators 94d1-94dM emit pumping light in the direction opposite to the signal propagation direction in each core. That is, the signal light propagating in cores Cu1-uN and cores Cd1-dM is amplified by the counter-pumping DRA.

[0017] FIG. 3 shows a configuration diagram of an optical transmission system according to the present disclosure, in which the transmission system shown in FIG. 2, which uses an MCF with N+M cores as a transmission path, is connected to K stages. K is an integer greater than or equal to 2. When n and m are integers from 1 to N and 1 to M, respectively, each stage is connected by optical connectors 73un,1 to 73un,K-1 and optical connectors 73dm,1 to 73dm,K-1. Furthermore, at each stage, pumping light emitted from pumping light oscillators 74dn,1 to 74dn,K and 74dm,1 to 74dm,K realizes backward pumping DRA of the signal light. This configuration is preferable because it allows for the construction of a planar network by placing routers or the like at the connection points of each stage.

[0018] FIG. 4 shows an optical transmission system according to the present disclosure, in which K MCFs 83 are connected by K−1 amplifiers 85. The transmitters 71 and 72, receivers 81 and 82, and pumping light oscillators 93 and 94 each have the configuration shown in FIG. 2, and perform back-pumping DRA of signal light in MCFs 83#1 to MCFs 83#K. When k is an integer between 1 and K−1, the MCF 83#k is connected to the amplifier 85#k by the optical coupler 84#2k and the optical coupler 84#2k+1. This configuration is preferable because it allows for compatibility with long-distance transmission paths, since the amplifiers 85 compensate for loss caused by signal light propagating through the MCFs.

[0019] Figure 5 shows the signal gain dependence of the XT change ΔXT for the optical transmission system according to the present disclosure in the configuration shown in Figure 2. The XT change can be calculated as ΔXT = XT1 - XT, where XT without DRA is XT0 (dB) and XT with DRA is XT1 (dB). A two-core transmission line is used as an example. The solid, dashed, dashed, and two-dot chain lines show the ΔXT characteristics for transmission distances of 50, 75, 100, and 200 km when the same signal gain is achieved in each core. ΔXT varies depending on the signal gain of the back-pumping DRA, and XT reduction is achieved in the region where ΔXT < 0. This characteristic can also be achieved with the device configurations shown in Figures 3 and 4.

[0020] FIG. 6 shows the maximum signal light gain G that realizes XT reduction in the configuration shown in FIG. 2 , that is, realizes ΔXT≦0 in FIG. 3 , for the optical transmission system according to the present disclosure. max This figure shows the dependence of the transmission distance on the transmission distance. When the transmission distance is L, the solid line in the figure can be expressed by equation (1). (Number 1) G max =12.1-18.1×10 3 L -1.9 (1)

[0021] As a result, the signal light gain G s of G s ≦12.1-18.1×10 3 L -1.9 By doing so, XT can be reduced.

[0022] FIG. 7 shows a configuration diagram of an optical transmission system according to the present disclosure, where an MCF having N+M cores is used as the transmission path. Here, where n is an integer between 1 and N, cores Cun numbered un are not adjacent to each other. Similarly, where m is an integer between 1 and M, cores Cdm numbered dm are not adjacent to each other. Signals generated by transmitters 71u1-71uN are transmitted to cores Cu1-CuN in the MCF via optical coupler 84#1 and received by receivers 81u1-81uN via optical coupler 84#2. Meanwhile, signals generated by transmitters 72d1-72dM are transmitted to cores d1-dM in the MCF via optical coupler 84#2 and received by receivers 82d1-82dM via optical coupler 84#1.

[0023] In this configuration, the propagation directions of signals propagating through adjacent cores are opposite. Pumping light oscillators 91u1-91uN and pumping light oscillators 92d1-92dM emit pumping light in the same direction as the signal propagation direction in each core. In other words, the signal light propagating through cores Cu1-CuN and cores Cd1-CdM is amplified by the co-pumping DRA. Each of the pumping light oscillators 91 and 92 independently determines the pumping light intensity and controls the amplification gain of the signal light so that the signal light propagating through each core reaches the receiver with the desired intensity.

[0024] FIG. 8 shows a configuration diagram of an optical transmission system according to the present disclosure, in which the transmission system shown in FIG. 7, which uses an MCF with N+M cores as a transmission path, is connected to K stages. K is an integer greater than or equal to 2. When n and m are integers from 1 to N and 1 to M, respectively, each stage is connected by optical connectors 73un,1 to 73un,K-1 and optical connectors 73dm,1 to 73dm,K-1. Furthermore, in each stage, signal light propagating through each core is amplified by forward pumping DRA using pump light from pump light oscillators 74dn,1 to 74dn,K and 74dm,1 to 74dm,K. This configuration is preferable because it allows for the construction of a planar network by placing routers or the like at the connection points between each stage.

[0025] FIG. 9 shows an optical transmission system according to the present disclosure, in which K MCFs 83 are connected by K−1 amplifiers 85. The transmitters 71 and 72, receivers 81 and 82, and pumping light oscillators 91 and 92 each have the configuration shown in FIG. 7 and perform forward pumping DRA of signal light in MCFs 83#1 through MCFs 83#K. When k is an integer between 1 and K−1, MCF-83#k and MCF-83#k+1 are connected by an optical coupler 84#2k, an optical coupler 84#2k+1, and an amplifier 85#k. This configuration is preferable because it allows for compatibility with long-distance transmission paths, since the amplifiers 85 compensate for loss caused by signal light propagating through the MCFs.

[0026] Figure 10 shows the dependence of signal light gain on ΔXT at a transmission distance of 100 km, according to the present disclosure. The solid lines represent the results when the difference ΔG between the signal light gain G1 in the transmission core and the signal light gain G2 in the adjacent core is set to -5.1 dB, -3.4 dB, 0 dB, 3.4 dB, and 6.9 dB. When the signal light gain G1 in the transmission core is larger than the signal light gain G2 in the adjacent core, i.e., when ΔG > 0, there is a maximum signal light gain that reduces XT, i.e., ΔXT ≦ 0. On the other hand, when the signal light gain G1 in the transmission core is smaller than the signal light gain G2 in the adjacent core, i.e., when ΔG < 0, there are minimum and maximum values of signal light gain that satisfy ΔXT ≦ 0.

[0027] Figure 11 shows the ΔG dependence of the maximum and minimum signal light gain G1 that realizes XT reduction. The solid line indicates the maximum signal light gain G max The dashed line represents the minimum signal gain G min represents the XT reduction effect. Furthermore, if ΔG is greater than 7.6 dB (gray), the XT reduction effect cannot be achieved. XT reduction is achieved when the signal light gain is smaller than the solid line in the figure and larger than the dashed line. Here, the solid line can be expressed by equation (2), and the dashed line can be expressed by equation (3). G max =19.4-0.9ΔG+0.03ΔG 2 (2) G min =0.04ΔG+0.2ΔG 2 (3)

[0028] Therefore, for a transmission distance of 100 km, At ΔG<7.6dB 0≦G1≦19.35-0.85ΔG+0.03ΔG 2 And 0≦ΔG<7.6 0.04ΔG+0.2ΔG 2 ≦G1≦19.4-0.9ΔG+0.03ΔG 2 This makes it possible to reduce XT.

[0029] Figure 12 shows the signal light gain dependence of ΔXT at a transmission distance of 150 km according to the present disclosure. The solid lines represent the results when the difference ΔG between the signal light gain G1 in the transmission core and the signal light gain G2 in the adjacent core is set to -10.3 dB, -6.9 dB, 0 dB, 3.4 dB, and 6.9 dB. When the signal light gain G1 in the transmission core is larger than the signal light gain G2 in the adjacent core, i.e., when ΔG > 0, there is a maximum signal light gain that achieves XT reduction, i.e., ΔXT ≦ 0. On the other hand, when the signal light gain G1 in the transmission core is smaller than the signal light gain G2 in the adjacent core, i.e., when ΔG < 0, there are minimum and maximum signal light gain values that satisfy ΔXT ≦ 0.

[0030] Figure 13 shows the ΔG dependence of the maximum and minimum signal light gain G1 that realizes XT reduction. The solid line indicates the maximum signal light gain G max and the dashed line represents the minimum signal gain G min It represents the XT reduction effect. Furthermore, if ΔG is greater than 12.0 dB (gray), the XT reduction effect cannot be achieved. XT reduction is achieved when the signal light gain is smaller than the solid line in the figure and larger than the dashed line. Here, the solid line can be expressed by equation (4), and the dashed line can be expressed by equation (5). G max =29.0-0.6ΔG+0.02ΔG 2 (4) G min =0.2ΔG+0.1ΔG 2 (5)

[0031] Therefore, for a transmission distance of 150 km, when ΔG<12.0 dB, 0≦G1≦29.01-0.61ΔG+0.02ΔG 2 and, When 0≦ΔG<12.0, 0.19ΔG+0.11ΔG 2 ≦G1≦29.01-0.61ΔG+0.02ΔG 2 This makes it possible to reduce XT.

[0032] Here, when equations (2), (4) and equations (3), (5) are coefficients K1(L), K2(L), K3(L), K4(L), and K5(L) that depend on the transmission distance L, they can be expressed as the following equations, respectively. G max =K1(L)+K2(L)ΔG+K3(L)ΔG 2 G min =K4(L)ΔG+K5(L)ΔG 2

[0033] Figure 14 shows the dependence of K1 on L. The solid line in the figure can be expressed by equation (6). (Number 6) K1=-2.0+0.2L (6)

[0034] Figure 15 shows the dependence of K2 on L. The solid line in the figure can be expressed by equation (7). (Number 7) K2=-0.4-140.5L -1.3 (7)

[0035] Figure 16 shows the dependence of K2 on L. The solid line in the figure can be expressed by equation (8). (Number 8) K3=-0.04+1.1×10 -4 L (8)

[0036] From the above, the maximum signal light gain that realizes XT reduction can be expressed by the following equation using ΔG and L. (Number 9) G max =-2.0+0.2L +(-0.4-140.5L -1.3 )ΔG +(-0.04+1.1×10 -4 )ΔG 2

[0037] Figure 17 shows the dependence of K4 on L. The solid line in the figure can be expressed by equation (10). (Number 10) K4=-0.40-443.8L -1.5 (10)

[0038] Figure 18 shows the dependence of K5 on L. The solid line in the figure can be expressed by equation (11). (Number 11) K5=-0.1-8.0×10 6 L -4 (11)

[0039] From the above, the minimum signal light gain that realizes XT reduction can be expressed by the following equation using ΔG and L: G min =(-0.40-443.8L -1.5 )ΔG +(-0.10-8.0×10 6 L -4 )ΔG 2

[0040] Therefore, the signal light gain G1 for ΔG and L is G1≦-2.0+0.2L+(-0.4-140.5L -1.3 )ΔG +(-0.04+1.1×10 -4 )ΔG 2 And 0≦ΔG (-0.4-443.8L -1.5 )ΔG +(-0.1-8.0×10 6 L -4 )ΔG 2 ≦ G1 By doing so, the XT reduction effect can be obtained.

[0041] FIG. 19 shows the signal light gain G that minimizes ΔXT in the configuration in which the backward pumping and forward pumping DRA shown in FIGS. 2 and 7 are implemented for the optical transmission system according to the present disclosure. s This figure shows the transmission distance dependence of . As an example, a two-core transmission line is assumed, with the same signal optical gain realized in each core. The solid and dashed lines in the figure represent the cases of backward-pumped and forward-pumped DRA, and when the transmission distance is L, the curves can be expressed by equations (12) and (13) on the right. (Number 12) G s =5.6-6.2×10 4 L -2.4 (12) (Number 13) G s =3.9-0.2L (13)

[0042] By designing the transmission system so as to realize the signal light gains shown in equations (12) and (13), the maximum XT reduction effect can be obtained. [Industrial Applicability]

[0043] The present disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0044] 73: Optical connector 74: Pumped optical oscillator 83:MCF 84: Optical coupling part 85: Optical amplifier 71, 72: Transmitter 81, 82: Receiver 91, 92, 93, 94: Pumped optical oscillator

Claims

1. An optical transmission system that transmits signal light using a multi-core optical fiber having two or more cores as a transmission line, two or more transmitters transmit signal light to adjacent cores among the two or more cores in opposite directions; two or more pumping light oscillators input pumping light into the same core as the core from which the signal light is transmitted, among the adjacent cores, so that the pumping light propagates in the same direction or in the opposite direction to the signal light; performing distributed Raman amplification of the signal light using the pump light so that the signal light gain is the same in each core; Optical transmission system.

2. An optical transmission system that transmits signal light using a multi-core optical fiber having two or more cores as a transmission path, comprising: two or more transmitters transmit signal light to adjacent cores in opposite directions; two or more pumping light oscillators input pumping light into the same core as the transmission core from which the signal light is sent out, among the adjacent cores, so that the pumping light propagates in the same direction or in the opposite direction to the signal light, and perform distributed Raman amplification of the signal light by the pumping light; The signal light gain G ｓ is relative to the transmission distance L of the adjacent core. G ｓ ≦12.1-18.1×10 ３ L －１．９ An optical transmission system characterized by satisfying the above.

3. The signal light gain of each signal light in the adjacent cores is G １ and G ２ and G １ G ２ The signal light gain difference ΔG is the difference between the signal light gain G and the transmission distance L. １ but G １ ≦-2.0+0.2L +(-0.4-140.5L －１．２７ )ΔG +(-0.044+1.1×101 －４ )ΔG ２ And, when ΔG≧0 G １ ≧(-0.40-443.8L －１．５４ )ΔG +(-0.10-8.0×10 ６ L －４ )ΔG ２ 3. The optical transmission system according to claim 2, wherein the following is satisfied:

4. the transmitter transmits the signal light with a signal light gain that maximizes a ratio of signal intensity to crosstalk noise intensity flowing in from the adjacent core; 3. The optical transmission system according to claim 2.

5. An optical transmission method for transmitting signal light using a multi-core optical fiber having two or more cores as a transmission line, comprising: two or more transmitters transmit signal light to adjacent cores among the two or more cores in opposite directions; two or more pumping light oscillators input pumping light into the same core as the core from which the signal light is transmitted, among the adjacent cores, so that the pumping light propagates in the same direction or in the opposite direction to the signal light; performing distributed Raman amplification of the signal light using the pump light so that the signal light gain is the same in each core; Optical transmission method.

6. An optical transmission system that transmits signal light using a multi-core optical fiber having two or more cores as a transmission path, comprising: two or more transmitters transmit signal light to adjacent cores in opposite directions; two or more pumping light oscillators input pumping light into the same core as the transmission core from which the signal light is sent out, among the adjacent cores, so that the pumping light propagates in the same direction or in the opposite direction to the signal light, and perform distributed Raman amplification of the signal light by the pumping light; The signal light gain G ｓ is relative to the transmission distance L of the adjacent core. G ｓ ≦12.1-18.1×10 ３ L －１．９ An optical transmission method characterized by satisfying the above.

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