Optical transmission path and optical transmission system

Abstract

An optical transmission path 10 comprises a multicore fiber 11 and multicore fibers 12 connected to both ends of the multicore fiber 11. The multicore fibers 12 have characteristics such that crosstalk between cores is reduced compared to the multicore fiber 11.

Description

Optical transmission line and optical transmission system

[0001] The present disclosure relates to an optical transmission line and an optical transmission system.

[0002] Multicore fiber (MCF) is known to increase the transmission capacity of optical fibers. When MCF is used as an optical transmission line in an optical transmission system, crosstalk (XT) between cores causes signal quality degradation.

[0003] In order to suppress inter-core crosstalk, an MCF with an appropriately designed core spacing has been proposed (Non-Patent Document 1).

[0004] An optical transmission system has been proposed in which the transmission directions of signal light to adjacent cores are made different from each other to mitigate the effects of crosstalk (Non-Patent Document 2).

[0005] A trench-type MCF has been proposed in which the core has a trench structure to reduce crosstalk between cores (Non-Patent Document 3).

[0006] T. Matsui, et.al. "Weakly Coupled Multicore Fiber Technology, Deployment, and Systems," in Proceedings of the IEEE, vol. 110, no. 11, pp. 1772-1785, Nov. 2022. T. Ito, et al., "Reduction of influence of inter-core cross-talk in MCF with bidirectional assignment between neighboring cores," OFC2013, OTh3K.2. Masanori Koshiba and seven others, "Multicore Fiber for Spatial Multiplexing Transmission," Fujikura Technical Report, Fujikura Ltd., 2011 Vol.2, No. 121, pp. 1-7.

[0007] The effects of crosstalk can be mitigated if the entire optical transmission line is constructed using trench-structured MCF, but trench-structured MCF has the problem of being expensive, and depending on the transmission distance, the crosstalk performance may exceed the required transmission system requirements.

[0008] The present disclosure has been made in view of the above, and aims to provide an optical transmission line that achieves both quality and cost.

[0009] An optical transmission path according to one aspect of the present disclosure includes a first multicore fiber and a second multicore fiber connected to both ends of the first multicore fiber, and the second multicore fiber has a characteristic of reducing crosstalk between cores more than the first multicore fiber.

[0010] An optical transmission system according to one aspect of the present disclosure is an optical transmission system that transmits signal light bidirectionally, and includes the above-described optical transmission line, a first transmitter that transmits first signal light to a core of the optical transmission line, a first receiver that receives the first signal light via the optical transmission line, a second transmitter that transmits second signal light to the core of the optical transmission line, and a second receiver that receives the second signal light via the optical transmission line, and the first signal light and the second signal light have different transmission directions.

[0011] According to the present disclosure, an optical transmission line that achieves both quality and cost can be provided.

[0012] FIG. 1 is a diagram showing an example of the configuration of an optical transmission line. FIG. 2 is a diagram showing an example of the cross section of a multicore fiber. FIG. 3 is a diagram showing an example of the relationship between the length ratio of low-crosstalk multicore fibers and crosstalk. FIG. 4 is a diagram showing an example of the relationship between the length ratio of low-crosstalk multicore fibers and the amount of crosstalk improvement when the crosstalk characteristic difference is 20 dB. FIG. 5 is a diagram showing an example of the relationship between the length ratio of low-crosstalk multicore fibers and the amount of crosstalk improvement when the crosstalk characteristic difference is 10 dB. FIG. 6 is a diagram showing an example of the relationship between the length ratio of low-crosstalk multicore fibers and the amount of crosstalk improvement when the crosstalk characteristic difference is 30 dB. FIG. 7 is a diagram showing an example of the relationship between the length ratio of low-crosstalk multicore fibers and the amount of crosstalk improvement when the crosstalk characteristic difference is 40 dB. FIG. 8 is a diagram showing the relationship between the total transmission distance and the length ratio of low-crosstalk multicore fibers when the crosstalk improvement is 5 dB. FIG. 9 is a diagram showing the relationship between the total transmission distance and the length ratio of low-crosstalk multicore fibers when the crosstalk improvement is 10 dB. Fig. 10 is a diagram showing the relationship between the total transmission distance and the length ratio of the low-crosstalk multi-core fiber when the crosstalk improvement amount is 15 dB. Fig. 11 is a diagram showing an example of the configuration of an optical transmission path. Fig. 12 is a diagram showing an example of the configuration of an optical transmission system. Fig. 13 is a diagram showing an example of the configuration of an optical transmission system.

[0013] [Optical Transmission Path] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0014] FIG. 1 is a diagram showing an example of the configuration of an optical transmission line 10 according to this embodiment. The optical transmission line 10 in FIG. 1 is an optical transmission line configured with multicore fibers 11 and 12 having multiple cores. Multicore fibers 12 are connected to both ends of the multicore fiber 11. The multicore fiber 12 has lower crosstalk characteristics between adjacent cores than the multicore fiber 11. The crosstalk characteristics are an index for evaluating the interference and influence of signal light between adjacent cores. Low crosstalk characteristics can suppress degradation of signal quality due to crosstalk. The multicore fibers 11 and 12 are connected by fusion splicing or an optical connector with their core axes aligned and their cores positioned. Hereinafter, the multicore fiber 11 may also be referred to as a high XT-MCF, and the multicore fiber 12 may also be referred to as a low XT-MCF.

[0015] The following table shows an example of the characteristics of high XT-MCF and low XT-MCF.

[0016]

[0017] The difference in crosstalk characteristics between high XT-MCF and low XT-MCF is 10 -2 km -1 The difference between the co-directional XT and return XT characteristics of the high XT-MCF and the low XT-MCF is 10 -2 km -1 (-20dB / km). The high XT-MCF and low XT-MCF have a propagation loss of 0.2dB / km.

[0018] For example, a high XT-MCF is a step-index MCF, and a low XT-MCF is a trench-assisted MCF. Step-index MCFs are easier to fabricate than trench-assisted MCFs, and can reduce costs.

[0019] Considering the splice loss between high XT-MCF and low XT-MCF, it is desirable that the core spacing of the MCFs be the same. The high XT-MCF and low XT-MCF are uncoupled MCFs, and the core spacing is desirably about 40 μm.

[0020] The optical transmission line 10 is a transmission line premised on bidirectional transmission. Opposing signal light is input to each adjacent core. When a low XT-MCF is connected to one end of a high XT-MCF, the signal light passes through multicore fibers with different crosstalk characteristics in different orders depending on the input direction of the signal light, resulting in differences in signal quality. In this embodiment, by connecting low XT-MCFs to both ends of the high XT-MCF, a crosstalk reduction effect can be achieved during bidirectional transmission, regardless of the input direction of the signal light.

[0021] Fig. 2 shows an example of a cross-sectional view of the multicore fibers 11 and 12. The multicore fibers 11 and 12 have a plurality of cores 102 in a cladding 101. Fig. 2 shows an example of a multicore fiber having an even number of cores, such as 2 cores, 4 cores, 6 cores, and 8 cores. The refractive index difference and core radius of each core may be different.

[0022] The four-core and six-core MCFs have the cores 102 arranged in a circular ring. The four-core and six-core MCFs may have the cores 102 arranged in a linear manner, as in the two-core MCF. The eight-core MCF has the cores 102 arranged in a double circular ring.

[0023] For MCFs with more than two cores, the adjacent core crosstalk is the crosstalk between the nearest cores.

[0024] [Length ratio of low XT-MCF to total length of optical transmission line] Next, we will explain how to determine the length ratio of low XT-MCF to the total length of the optical transmission line. Increasing the proportion of low XT-MCF in the optical transmission line can further mitigate the effects of crosstalk. However, this increases costs and, depending on the communication method and transmission distance, there is a possibility that the XT performance may be over-specified. Therefore, we will explain how to determine the length ratio of low XT-MCF that is appropriate for the transmission system requirements.

[0025] Figure 3 shows the crosstalk value versus the length ratio r of low XT-MCF for optical transmission lines with total transmission distances L of 20, 40, 80, and 100 km. The horizontal axis represents the length ratio r of low XT-MCF, and the vertical axis represents the crosstalk value. When r=0, the optical transmission line is entirely composed of high XT-MCF, and when r=1, the optical transmission line is entirely composed of low XT-MCF. The proportion of low XT-MCF increases as you move to the right of the diagram, which means higher costs. The impact of crosstalk increases as you move up the diagram. The difference in crosstalk characteristics between high XT-MCF and low XT-MCF is 10 -2 km -1 (-20dB / km).

[0026] For example, to keep the quality penalty caused by crosstalk in a PDM-QPSK signal below 1 dB, the crosstalk must be below -17 dB. Figure 3 shows that when the total transmission distance L is 80 km, r must be 0.35 or greater. The longer the total transmission distance, the larger r must be.

[0027] To suppress the quality penalty due to crosstalk in PDM-16QAM signals to 1 dB or less, the crosstalk must be -24 dB or less. Figure 3 shows that when the total transmission distance L is 80 km, r = 0.6 or more is sufficient.

[0028] To suppress the quality penalty due to crosstalk in PDM-64QAM signals to 1 dB or less, the crosstalk must be -30 dB or less. Figure 3 shows that when the total transmission distance L is 80 km, r = 0.9 or more is sufficient.

[0029] By determining the length ratio r of the low XT-MCF depending on the communication method (allowable crosstalk value) of the transmission system and the total transmission distance L, an optical transmission path that combines quality and cost can be realized.

[0030] Next, the determination of the length ratio r of the low XT-MCF based on the crosstalk improvement amount will be described.

[0031] Figures 4 to 7 show the crosstalk improvement versus the length ratio r of low XT-MCF, with the crosstalk value when the entire optical transmission line is made of high XT-MCF (r = 0) taken as the reference, for optical transmission lines with total transmission distances L of 20, 40, 80, and 100 km. The horizontal axis represents the length ratio r of low XT-MCF, and the vertical axis represents the crosstalk improvement. Note that Figures 4 to 7 show different differences in crosstalk characteristics between high XT-MCF and low XT-MCF. Each figure will be explained below.

[0032] Figure 4 shows the crosstalk improvement XT when the crosstalk characteristic difference ΔXT between high XT-MCF and low XT-MCF is 20 dB / km. R The crosstalk characteristics of the high XT-MCF are shown in Fig. -2 km -1 The crosstalk characteristic of low XT-MCF is 10 -4 km -1 is.

[0033] Crosstalk Improvement XT R To achieve 10 dB or more, r should be 0.3 or more when L = 100 km, r should be 0.42 or more when L = 80 km, r should be 0.75 or more when L = 40 km, and r should be 0.9 or more when L = 20 km.

[0034] Crosstalk Improvement XT R To achieve 5 dB or more, r should be 0.15 or more when L = 100 km, r should be 0.2 or more when L = 80 km, r should be 0.4 or more when L = 40 km, and r should be 0.6 or more when L = 20 km.

[0035] Figure 5 shows the crosstalk improvement XT when the crosstalk characteristic difference ΔXT between high XT-MCF and low XT-MCF is 10 dB / km. R The crosstalk characteristics of the high XT-MCF are shown in Fig. -2 km -1 The crosstalk characteristic of low XT-MCF is 10 -3 km -1 is.

[0036] Crosstalk Improvement XT RTo achieve 10 dB or more, r should be 0.4 or more when L = 100 km, r should be 0.58 or more when L = 80 km, r should be 0.92 or more when L = 40 km, and r should be 1 or more when L = 20 km.

[0037] Crosstalk Improvement XT R To achieve 5 dB or more, r must be 0.18 or more for L=100 km, r must be 0.22 or more for L=80 km, r must be 0.5 or more for L=40 km, and r must be 0.68 or more for L=20 km.

[0038] Figure 6 shows the crosstalk improvement XT when the crosstalk characteristic difference ΔXT between high XT-MCF and low XT-MCF is 30 dB / km. R The crosstalk characteristics of the high XT-MCF are shown in Fig. -2 km -1 The crosstalk characteristic of low XT-MCF is 10 -5 km -1 is.

[0039] Crosstalk Improvement XT R To achieve 10 dB or more, r should be 0.3 or more when L = 100 km, r should be 0.41 or more when L = 80 km, r should be 0.78 or more when L = 40 km, and r should be 0.86 or more when L = 20 km.

[0040] Crosstalk Improvement XT R To achieve 5 dB or more, r should be 0.13 or more when L = 100 km, r should be 0.2 or more when L = 80 km, r should be 0.4 or more when L = 40 km, and r should be 0.6 or more when L = 20 km.

[0041] Figure 7 shows the crosstalk improvement XT when the crosstalk characteristic difference ΔXT between high XT-MCF and low XT-MCF is 40 dB / km. R The crosstalk characteristics of the high XT-MCF are shown in Fig. -2 km -1 The crosstalk characteristic of low XT-MCF is 10 -6 km -1 is.

[0042] Crosstalk Improvement XT RTo achieve 10 dB or more, r should be 0.3 or more when L = 100 km, r should be 0.4 or more when L = 80 km, r should be 0.75 or more when L = 40 km, and r should be 0.85 or more when L = 20 km.

[0043] Crosstalk Improvement XT R To achieve 5 dB or more, r should be 0.13 or more when L = 100 km, r should be 0.2 or more when L = 80 km, r should be 0.4 or more when L = 40 km, and r should be 0.6 or more when L = 20 km.

[0044] In this way, the crosstalk improvement amount XT R , the total transmission distance L, and the crosstalk characteristic difference ΔXT between the high XT-MCF and the low XT-MCF can be used to determine the length ratio r of the low XT-MCF.

[0045] Figures 8 to 10 show the relationship between the total transmission distance L and the length ratio r of the low XT-MCF for each achievable crosstalk improvement. The horizontal axis represents the total transmission distance L, and the vertical axis represents the length ratio r of the low XT-MCF. Figures 8 and 9 show graphs of the crosstalk characteristic difference ΔXT between high XT-MCF and low XT-MCF for 10, 20, 30, and 40 dB / km, and Figure 10 shows a graph of the crosstalk characteristic difference ΔXT between high XT-MCF and low XT-MCF for 20, 30, and 40 dB / km.

[0046] Figure 8 shows the crosstalk improvement amount XT R This figure shows the relationship between the total transmission distance L and the length ratio r of the low XT-MCF when the crosstalk characteristic difference ΔXT is 5 dB. The relationship between L and r when the crosstalk characteristic difference ΔXT is 10 dB / km is shown by the dashed line, and the relationship between L and r when the crosstalk characteristic difference ΔXT is 20, 30, or 40 dB / km is shown by the dashed lines.

[0047] Figure 9 shows the crosstalk improvement amount XT R This figure shows the relationship between the total transmission distance L and the length ratio r of the low XT-MCF when the crosstalk characteristic difference ΔXT is 10 dB / km. The dashed line shows the relationship between L and r when the crosstalk characteristic difference ΔXT is 20, 30, or 40 dB / km.

[0048] FIG. 10 shows the crosstalk improvement amount XT R1 shows the relationship between the total transmission distance L and the length ratio r of the low XT-MCF when the crosstalk characteristic difference ΔXT is 20, 30, and 40 dB / km.

[0049] Any crosstalk improvement ΔXT R Even in this case, there is almost no difference in the required r for crosstalk characteristic differences ΔXT = 20, 30, and 40 dB / km. R If r is set so that the relationship between r and L satisfies the following equation, the desired crosstalk improvement amount ΔXT R This can be achieved.

[0050]

[0051] 8 to 10, the approximate straight line expressed by the above formula is shown by a solid line. In the region above the approximate straight line, the desired crosstalk improvement amount XT R = 5, 10, 15 dB can be achieved. The difference in crosstalk characteristics ΔXT between high XT-MCF and low XT-MCF is the crosstalk improvement amount XT R It is desirable that this is the case.

[0052] [Modification of Optical Transmission Line] Fig. 11 is a diagram showing the configuration of a modification of an optical transmission line 10. In the optical transmission line 10 of Fig. 11, a multicore fiber 13 (medium XT-MCF) having crosstalk characteristics between the crosstalk characteristics of the multicore fibers 11 and 12 is arranged between the multicore fibers 11 and 12. A plurality of multicore fibers 12 and 13 having different crosstalk characteristics may be provided at both ends of the multicore fiber 11.

[0053] [Optical Transmission System] With reference to FIG. 12, an optical transmission system using the optical transmission line 10 of this embodiment will be described.

[0054] The optical transmission system shown in Fig. 12 includes an optical transmission line 10 in which multicore fibers 11 and 12, each having M+N cores, are connected. The optical transmission line 10 has a configuration in which multicore fibers (low XT-MCF) 12 are connected to both ends of a multicore fiber (high XT-MCF) 11. The lengths of the multicore fibers 12 at both ends of the optical transmission line 10 do not necessarily have to be the same.

[0055] The signal light propagates in the M cores u1 to uM and the N cores d1 to dN in different directions. The M cores u1 to uM are not adjacent to one another, and the N cores d1 to dN are not adjacent to one another either. In other words, the signal light propagates in adjacent cores in opposite directions.

[0056] The signal light output from the left transmitters 30-u1 to 30-uM is sent to each core u1 to uM of the left multicore fiber 12 via the optical coupling unit 50-1, and is received by the right receivers 40-u1 to 40-uM via the multicore fiber 11, the right multicore fiber 12, and the optical coupling unit 50-2.

[0057] The signal light output from the right-side transmitters 30-d1 to 30-dN is sent to each core d1 to dN of the right-side multicore fiber 12 via the optical coupling unit 50-2, and is received by the left-side receivers 40-d1 to 40-dN via the multicore fiber 11, the left-side multicore fiber 12, and the optical coupling unit 50-1.

[0058] The receiver 40 processes the received signal using a digital signal processor (DSP). If the receiver 40 is equipped with a multiple-input multiple-output digital signal processor (MIMO-DSP), the MIMO-DSP can remove noise caused by crosstalk, allowing the wiring distance of the optical transmission line 10 to be extended.

[0059] The optical coupling unit 50 can use a fan-in / fan-out device for a multi-core fiber.

[0060] The communication wavelength band of the signal light is approximately 1.3 to 1.7 μm. The signal light may also be in the mid-infrared region (2 μm or longer) or the visible light region, which has a longer wavelength.

[0061] An optical transmission system in which a plurality of optical transmission lines 10 are connected will be described with reference to FIG.

[0062] The optical transmission system shown in Figure 13 includes K optical transmission lines 10-1 to 10-K connected together. Each of the K optical transmission lines 10-1 to 10-K is an MCF in which a low XT-MCF is connected to both ends of a high XT-MCF. K-1 amplifier units 60-1 to 60-K-1 are disposed at the connection points of the K optical transmission lines 10-1 to 10-K. The amplifier units 60-1 to 60-K-1 are connected to the connection points of the optical transmission lines 10-1 to 10-K via optical coupling units 50-1 to 50-2K. The amplifier units 60 amplify the signal light propagating through each core of the optical transmission line 10.

[0063] 12, the optical transmission lines 10-1 to 10-K have M+N cores. The signal light propagates in directions different from the M cores u1 to uM and the N cores d1 to dN. The M cores u1 to uM are not adjacent to one another, and the N cores d1 to dN are not adjacent to one another either.

[0064] The signal light output from the transmitters 30-u1 to 30-uM on the left side is sent to each core u1 to uM of the optical transmission line 10-1 via the optical coupling unit 50-1, is amplified by the amplifiers 60-1 to 60-K-1, propagates through the K optical transmission lines 10-1 to 10-K, and is received by the receivers 40-u1 to 40-uM on the right side.

[0065] The signal light output from the transmitters 30-d1 to 30-dN on the right side is sent to each core d1 to dN of the optical transmission line 10-K via the optical coupling unit 50-2K, propagates through the K optical transmission lines 10-1 to 10-K while being amplified by the amplifiers 60-1 to 60-K-1, and is received by the receivers 40-d1 to 40-dN on the left side.

[0066] In this configuration, the loss caused by the propagation of the signal light through the MCF is compensated for by the amplifier 60, making it possible to accommodate long-distance transmission lines.

[0067] As described above, the optical transmission line 10 of this embodiment includes a multicore fiber 11 and multicore fibers 12 connected to both ends of the multicore fiber 11. The multicore fiber 12 has a characteristic of reducing crosstalk between cores more than the multicore fiber 11. By connecting low-crosstalk multicore fibers 12 to both ends of the optical transmission line 10, the effects of crosstalk can be mitigated, and longer distances can be expected. Furthermore, costs can be reduced compared to when all optical transmission lines are made of low-crosstalk multicore fibers.

[0068] 10 Optical transmission path 11, 12, 13 Multi-core fiber 30-d1 to 30-dN, 30-u1 to 30-uM Transmitter 40-d1 to 40-dN, 40-u1 to 40-uM Receiver 50-1 to 50-2K Optical coupling unit 60-1 to 60-K-1 Amplification unit

Claims

1. An optical transmission line comprising: a first multicore fiber; and a second multicore fiber connected to both ends of the first multicore fiber, wherein the second multicore fiber has a characteristic of reducing crosstalk between cores more than the first multicore fiber.

2. An optical transmission line according to claim 1, wherein a ratio of the length of said second multi-core fiber to a total transmission distance of said optical transmission line is determined according to an allowable crosstalk value.

3. The optical transmission line according to claim 1, wherein a desired crosstalk improvement amount is XT R a length ratio r of the second multicore fiber to a total transmission distance L of the optical transmission line satisfies formula (1).

4. An optical transmission system for transmitting signal light in both directions, comprising: an optical transmission line as defined in any one of claims 1 to 3; a first transmitter for sending a first signal light to a core of said optical transmission line; a first receiver for receiving said first signal light via said optical transmission line; a second transmitter for sending a second signal light to a core of said optical transmission line; and a second receiver for receiving said second signal light via said optical transmission line, wherein the first signal light and the second signal light have different transmission directions.

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