Crosstalk reduction system and crosstalk reduction method

Abstract

A crosstalk reduction system according to the present invention comprises: a multicore fiber that transmits an optical signal through each core; and at least one crosstalk reduction unit. The crosstalk reduction unit has a division unit that divides the spectrum of an inputted optical signal at a set division frequency along a frequency axis, a first inversion unit that inverts the spectrum in the frequency band below the division frequency along the frequency axis about an axis of symmetry that is the center frequency of the frequency band below the division frequency, a second inversion unit that inverts the spectrum in the frequency band at and above the division frequency along the frequency axis about an axis of symmetry that is the center frequency of the frequency band at and above the division frequency, and a merging unit that merges the spectrum inverted in the frequency band at and above the division frequency and the spectrum inverted in the frequency band below the division frequency.

Description

Crosstalk reduction system and crosstalk reduction method

[0001] The present invention relates to a system and method for reducing crosstalk.

[0002] In recent years, the spread of 5G (5th Generation) services, high-definition video services, the Internet of Things (IoT), big data, artificial intelligence (AI), and other technologies has led to an increase in communication traffic in optical communication systems. Currently, single-mode fiber is sometimes used as the foundation for high-capacity optical transmission networks. Single-mode fiber has a single core within the cladding, which serves as a pathway for optical signals. This enables optical transmission networks that can stably transmit large volumes of information over long distances.

[0003] Furthermore, optical transmission networks with speeds of over 100 gigabits per second per wavelength channel have been put into practical use through digital coherent transmission technology, which combines digital signal processing that compensates for optical signal distortion with coherent transmission and reception processing that utilizes the intensity and phase information of optical signals. Taking advantage of the property that two orthogonal light waves can be separated, these two light waves can carry different information. Digital coherent transmission can also be used for polarization multiplexed optical transmission, which utilizes this property, contributing to a dramatic improvement in information transmission efficiency.

[0004] However, even if the number of wavelength channels is increased by wavelength division multiplexing (WDM) in digital coherent transmission and a wideband optical signal is transmitted, the physical limit of the transmission capacity of a single-mode fiber that transmits the optical signal is approximately 100 terabits per second. Therefore, there are concerns that the transmission capacity will reach its physical limit in the future.

[0005] Therefore, in order to achieve a petabit-class transmission capacity that exceeds the physical limits mentioned above, active research is being conducted into expanding transmission capacity by introducing space division multiplexing (SDM).

[0006] Multicore fiber, one of the transmission media for space division multiplexing, is a transmission medium that realizes spatial multiplexing of optical signals using multiple cores within the fiber. In particular, uncoupled multicore fiber has the advantage that each core is used as an independent transmission path, making it easy to expand from single-mode fiber and increase the number of spatial multiplexes.

[0007] Optical signals propagating through each core in a fiber carry independent information for each core. This allows the transmission capacity per fiber to increase with the number of cores. Therefore, active research is being conducted to realize large-capacity optical transmission networks using multicore fibers (see Non-Patent Document 1).

[0008] However, in optical transmission lines using multicore fibers, crosstalk (inter-core crosstalk), in which part of the optical signal leaks from adjacent cores, can occur. Inter-core crosstalk is one of the causes of an increase in the bit error rate. Therefore, reducing inter-core crosstalk is an issue in order to realize large-capacity optical transmission networks.

[0009] In addition, in the long wavelength band, the refractive index of the optical signal is small and the mode field diameter is large. Therefore, the crosstalk also increases according to the mode field diameter. The amount of crosstalk increases approximately linearly in dB with respect to the wavelength of the optical signal. The linear slope with respect to the wavelength is, for example, about 0.1 dB / nm.

[0010] The penalty due to crosstalk (hereinafter referred to as "cumulative crosstalk") that accumulates in an optical signal depending on the transmission distance differs for each wavelength of the broadband optical signal. Therefore, wavelength-wise management of signal design parameters such as the signal modulation format and coding rate is required. This raises concerns about increased costs and more complex configurations of optical communication systems. In response to these issues, Non-Patent Document 2 proposes that an optical communication system reduces wavelength dependency by inverting the frequency spectrum of the optical signal midway along the optical transmission path.

[0011] RS Luis et al., “Crosstalk Impact on the Performance of Wideband Multicore-Fiber Transmission Systems”, IEEE Journal of selected topics in quantum electronics, 26(4), 1244-1245 (2020). M. Hoshi et al., “Mitigation of intercore crosstalk impact by PPLN-based optical spectrum inversion”, OECC2022, TuB2-3 (2022).

[0012] In the C-band (1530-1565 nm) used for commercial communication services, the wavelength dependence of inter-core crosstalk in a four-core multicore fiber has actually been reduced by spectral inversion using phase conjugation, a second-order nonlinear phenomenon. In the future, the transmission band may be extended to the L-band and extended L-band, which are longer wavelengths than the C-band. There is also a possibility that the transmission band may be extended to the S-band, which is shorter wavelengths than the C-band.

[0013] The difference between wavelengths of cumulative crosstalk increases as the transmission band expands. Therefore, when the transmission band of an optical signal propagating through a four-core multicore fiber is expanded, it is necessary to prevent a large difference between wavelengths of cumulative crosstalk from remaining in the frequency spectrum of the optical signal.

[0014] However, in the conventional method disclosed in Non-Patent Document 2, when the transmission band is expanded from the C band to 80 nm (multi-band of "C+L band"), the difference between wavelengths of cumulative crosstalk is about 1.6 dB over the entire transmission band. For example, when the transmission band is expanded from the C band to 120 nm, the difference between wavelengths of cumulative crosstalk is about 3.2 dB over the entire transmission band. Thus, there is a problem in that it is not possible to reduce the wavelength dependency of inter-core crosstalk.

[0015] In view of the above circumstances, an object of the present invention is to provide a crosstalk reduction system and a crosstalk reduction method that are capable of reducing the wavelength dependency of inter-core crosstalk.

[0016] One aspect of the present invention is a crosstalk reduction system comprising a multicore fiber that transmits an optical signal for each core, and one or more crosstalk reduction units, wherein the crosstalk reduction unit has a dividing unit that divides the spectrum of an input optical signal at a division frequency determined on a frequency axis, a first inversion unit that inverts the spectrum of a frequency band lower than the division frequency on the frequency axis, with the center frequency of the frequency band lower than the division frequency as an axis of symmetry, a second inversion unit that inverts the spectrum of a frequency band equal to or higher than the division frequency on the frequency axis, with the center frequency of the frequency band equal to or higher than the division frequency as an axis of symmetry, and a merging unit that merges the inverted spectrum in the frequency band equal to or higher than the division frequency and the inverted spectrum in the frequency band lower than the division frequency.

[0017] One aspect of the present invention is a crosstalk reduction method executed by a crosstalk reduction system including a multicore fiber that transmits an optical signal for each core and one or more crosstalk reduction units, wherein the crosstalk reduction unit divides the spectrum of an input optical signal at a division frequency determined on a frequency axis, inverts the spectrum of a frequency band lower than the division frequency on the frequency axis with the center frequency of the frequency band lower than the division frequency as an axis of symmetry, inverts the spectrum of a frequency band equal to or higher than the division frequency on the frequency axis with the center frequency of the frequency band equal to or higher than the division frequency as an axis of symmetry, and merges the inverted spectrum in the frequency band equal to or higher than the division frequency with the inverted spectrum in the frequency band lower than the division frequency.

[0018] According to the present invention, it is possible to reduce the wavelength dependency of inter-core crosstalk.

[0019] 1 is a diagram showing an example of the configuration of a crosstalk reduction system in a first embodiment; FIG. 2 is a diagram showing an example of division processing and inversion processing on the spectrum of an optical signal transmitted through one core included in a multicore fiber in the first embodiment; FIG. 3 is a diagram showing an example of the configuration of a crosstalk reduction system in a second embodiment; FIG. 4 is a diagram showing an example of the wavelength dependency of accumulated crosstalk at the end of each span from the first span to the fourth span in the second embodiment; FIG. 5 is an enlarged view showing an example of the wavelength dependency of accumulated crosstalk at the end of the fourth span in the second embodiment; FIG. 6 is a diagram showing an example of the configuration of a crosstalk reduction system in a third embodiment; FIG. 7 is a diagram showing an example of the wavelength dependency of accumulated crosstalk at the end of each span from the first span to the fourth span in the third embodiment; and FIG. 8 is a diagram showing an example of the relationship between the wavelength difference of accumulated crosstalk and bandwidth in each embodiment.

[0020] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) Fig. 1 is a diagram showing an example of the configuration of a crosstalk reduction system 1a in a first embodiment. The crosstalk reduction system 1a is a system that reduces the wavelength dependency of crosstalk between cores of a multicore fiber that transmits an optical signal. For example, the multicore fiber transmits an optical signal in an optical transmission path of multiple bands including the C band and the L band, or a band equal to or greater than the multiple bands (S band + C band + L band). In the following description, the number of cores in the multicore fiber is M (M is an integer equal to or greater than 2).

[0021] The crosstalk reduction system 1a includes M transmitters 2, an optical multiplexer 3, N (N is an integer equal to or greater than 1) division / inversion units 4, an optical demultiplexer 5, and M receivers 6. The m-th (m is an integer from 1 to M) core of the multicore fiber, the transmitter 2-m, and the receiver 6-m are associated in advance in the optical transmission path from the optical multiplexer 3 to the optical demultiplexer 5.

[0022] The transmitter 2 includes a light source (not shown), a transmission signal processing section (not shown), a digital-to-analog converter (not shown), an optical modulator (not shown), and an optical amplifier (not shown).

[0023] The splitting / inverting unit 4 has a demultiplexing unit 41, an inverting unit 42-1, an inverting unit 42-2, and a multiplexing unit 43. The pair of the inverting unit 42-1 and the inverting unit 42-2 is connected to the demultiplexing unit 41 using a single-mode fiber. Similarly, the pair of the inverting unit 42-1 and the inverting unit 42-2 is connected to the multiplexing unit 43 using a single-mode fiber. Note that if an optical signal input to the splitting / inverting unit 4 via a multicore fiber can be split and inverted without being split into a single-mode fiber, the splitting / inverting unit 4 does not need to have the splitting unit 41 and the multiplexing unit 43. Furthermore, because the effect of reducing wavelength dependency is independent for each core, the splitting / inverting unit 4 may have a pair of the inverting unit 42-1 and the inverting unit 42-2 only for the core for which wavelength dependency is desired to be reduced.

[0024] The receiver 6 includes a local light source (not shown), an opto-electric converter (not shown), an analog-to-digital converter (not shown), an optical demodulator (not shown), and a received signal processing section (not shown).

[0025] The transmitter 2-m executes a predetermined transmission process (for example, a process of generating transmission data, etc.). The transmitter 2-m generates an optical signal based on the result of the predetermined transmission process. The transmitter 2-m transmits the generated optical signal to the optical multiplexer 3 using a single mode fiber associated with the m-th core of the multicore fiber (a single mode fiber subsequent to the transmitter 2-m).

[0026] The optical multiplexer 3 is, for example, a fused optical fiber coupler, a bulk optical coupler, or a planar waveguide optical coupler. The optical multiplexer 3 inputs the optical signal transmitted from the transmitter 2-m to the m-th core of the multi-core fiber. That is, the optical multiplexer 3 transmits the optical signal transmitted from the transmitter 2-m to the division / inversion unit 4-1 using the m-th core of the multi-core fiber in the first span of the optical transmission line.

[0027] The division / inversion unit 4 (crosstalk reduction unit) demultiplexes an optical signal input via a multicore fiber into a single-mode fiber for each core of the multicore fiber. The division / inversion unit 4 divides the spectrum of the input optical signal at a division frequency determined on the frequency axis for each single-mode fiber. The division / inversion unit 4 inverts the spectrum of a frequency band lower than the division frequency on the frequency axis for each single-mode fiber, with the center frequency of the frequency band lower than the division frequency as the axis of symmetry. The division / inversion unit 4 inverts the spectrum of a frequency band equal to or higher than the division frequency on the frequency axis for each single-mode fiber, with the center frequency of the frequency band equal to or higher than the division frequency as the axis of symmetry. The division / inversion unit 4 combines, for each single-mode fiber, the spectrum inverted in the frequency band equal to or higher than the division frequency and the spectrum inverted in the frequency band lower than the division frequency. The division / inversion unit 4 combines the optical signal with the spectrum combined for each single-mode fiber into a subsequent multicore fiber.

[0028] More specifically, the demultiplexing unit 41 demultiplexes an optical signal input from a previous node via a multicore fiber into a single-mode fiber associated with each core of the multicore fiber. The single-mode fiber (splitting unit) on the output side of the demultiplexing unit 41 is, for example, an optical fiber wavelength division multiplexing coupler or an arrayed waveguide grating. The single-mode fiber (splitting unit) on the output side of the demultiplexing unit 41 splits the spectrum of the demultiplexed optical signal at a predetermined split frequency on the frequency axis. For example, the single-mode fiber on the output side of the demultiplexing unit 41-1 splits the spectrum of the optical signal input from the optical multiplexer 3 via the multicore fiber in the first span of the optical transmission line into two spectra at a predetermined split frequency on the frequency axis, for each core of the multicore fiber. For example, the single-mode fiber on the output side of the demultiplexing unit 41-N divides the spectrum of the optical signal input from the multiplexing unit 43-(N-1) via the multicore fiber in the Nth span of the optical transmission line into two spectra at a predetermined division frequency on the frequency axis, for each core of the multicore fiber.

[0029] The single mode fiber on the output side of the demultiplexing unit 41 outputs the optical signal of the divided spectrum in a frequency band (long wavelength band) lower than the split frequency to the inverting unit 42-1, and outputs the optical signal of the divided spectrum in a frequency band (short wavelength band) equal to or higher than the split frequency to the inverting unit 42-2.

[0030] The inversion unit 42 has, for example, a nonlinear medium (nonlinear optical medium). The inversion unit 42-1 (first inversion unit) is associated with a frequency band lower than the split frequency. The inversion unit 42-1 inverts the spectrum of the frequency band lower than the split frequency on the frequency axis, with the center frequency (aliasing frequency) of the frequency band lower than the split frequency as the axis of symmetry. The inversion unit 42-2 (second inversion unit) is associated with a frequency band equal to or higher than the split frequency. The inversion unit 42-2 inverts the spectrum of the frequency band equal to or higher than the split frequency on the frequency axis, with the center frequency (aliasing frequency) of the frequency band equal to or higher than the split frequency as the axis of symmetry.

[0031] The single mode fiber (combining unit) on the input side of the multiplexer 43 is, for example, an optical fiber wavelength division multiplexing coupler or an arrayed waveguide grating. The single mode fiber (combining unit) on the input side of the multiplexer 43 combines an inverted spectrum in a frequency band equal to or higher than the split frequency with an inverted spectrum in a frequency band lower than the split frequency. The multiplexer 43 combines the optical signals with the combined spectrum for each single mode fiber across multiple single mode fibers. The multiplexer 43 transmits the combined optical signal to a node downstream of the multiplexer 43 using a multicore fiber downstream of the multiplexer 43.

[0032] For example, the multiplexer 43-1 transmits the multiplexed optical signal to the demultiplexer 41-2 (not shown) of the division / inversion unit 4-2 (not shown) using the multicore fiber in the second span of the optical transmission line. For example, the multiplexer 43-N transmits the multiplexed optical signal to the optical demultiplexer 5 using the multicore fiber in the "N+1"th span of the optical transmission line.

[0033] The optical demultiplexer 5 is, for example, a fused optical fiber coupler, a free space (bulk) type optical coupler, a planar waveguide type optical coupler, etc. The optical demultiplexer 5 outputs an optical signal transmitted from the transmitter 2-m using the m-th core of the multi-core fiber in the "N+1"-th span of the optical transmission line to the receiver 6-m using a single mode fiber associated with the m-th core of the multi-core fiber.

[0034] The receiver 6-m uses a single mode fiber associated with the m-th core of the multi-core fiber (a single mode fiber upstream of the receiver 6-m) to receive the optical signal transmitted using the m-th core of the multi-core fiber from the optical demultiplexer 5. The receiver 6-m performs predetermined reception processing (for example, demodulation processing, etc.) on the received optical signal.

[0035] Next, the details of the division process and the inversion process will be described. Fig. 2 is a diagram showing an example of the division process and the inversion process for the spectrum of an optical signal transmitted through one core included in a multicore fiber in the first embodiment. That is, Fig. 2 is a diagram showing an example of the division process and the inversion process for the spectrum of an optical signal transmitted through the m-th core.

[0036] The mth single mode fiber on the output side of the demultiplexing unit 41 performs division processing on the spectrum of the optical signal transmitted through the mth core. Here, all single mode fibers on the output side of the demultiplexing unit 41 may perform division processing on the spectrum of each optical signal for all cores included in the multicore fiber. A single mode fiber selected from all single mode fibers on the output side of the demultiplexing unit 41 may perform division processing on the spectrum of each optical signal for each core selected from all cores included in the multicore fiber.

[0037] 2 , the single mode fiber on the output side of the demultiplexing unit 41 splits spectrum 101 of the optical signal split by the demultiplexing unit 41 into spectrum 102 and spectrum 103 at a predetermined split frequency on the frequency axis. That is, the single mode fiber on the output side of the demultiplexing unit 41 splits the optical signal with spectrum 101 into an optical signal with spectrum 102 and an optical signal with spectrum 103.

[0038] Spectrum 102 is the spectrum of the optical signal in a frequency band (long wavelength band) lower than the split frequency. Spectrum 103 is the spectrum of the optical signal in a frequency band (short wavelength band) equal to or higher than the split frequency. The demultiplexing unit 41 outputs the optical signal with spectrum 102 to the inversion unit 42-1. The demultiplexing unit 41 outputs the optical signal with spectrum 103 to the inversion unit 42-2.

[0039] The inversion unit 42 performs an inversion process on the spectrum of the optical signal input from the demultiplexing unit 41. In the graph illustrated in the middle of Fig. 2, the inversion unit 42-1 inverts the spectrum 102 on the frequency axis, with the center frequency of the spectrum 102 of the optical signal input from the demultiplexing unit 41 as the axis of symmetry. In the graph illustrated in the bottom of Fig. 2, the inversion unit 42-2 inverts the spectrum 103 on the frequency axis, with the center frequency of the spectrum 103 of the optical signal input from the demultiplexing unit 41 as the axis of symmetry.

[0040] The single mode fiber on the input side of the multiplexer 43 combines the spectrum 102 inverted on the frequency axis with the spectrum 103 inverted on the frequency axis. The multiplexer 43 combines the optical signals with the combined spectrum for each single mode fiber for multiple single mode fibers. The multiplexer 43 transmits the combined optical signal to a node downstream of the multiplexer 43 using a multicore fiber downstream of the multiplexer 43.

[0041] As described above, the crosstalk reduction system 1a includes, in an optical transmission path, a multicore fiber that transmits an optical signal for each core, and one or more splitting and inverting units 4. In the splitting and inverting unit 4 (crosstalk reduction unit), the single-mode fiber (splitting unit) on the output side of the demultiplexing unit 41 splits the spectrum 101 of the input optical signal at a split frequency determined on the frequency axis. The inverting unit 42-1 (first inverting unit) inverts the spectrum 102 of the frequency band lower than the split frequency on the frequency axis, with the center frequency of the frequency band lower than the split frequency as the axis of symmetry. The inverting unit 42-2 (second inverting unit) inverts the spectrum 103 of the frequency band equal to or higher than the split frequency on the frequency axis, with the center frequency of the frequency band equal to or higher than the split frequency as the axis of symmetry. The single-mode fiber (combining unit) on the input side of the multiplexing unit 43 combines the spectrum 102 inverted in the frequency band equal to or higher than the split frequency and the spectrum 103 inverted in the frequency band lower than the split frequency.

[0042] In this way, each division / inversion unit 4 (each node) in the multi-band, multi-core optical transmission line divides the spectrum of the input optical signal into a first spectrum and a second spectrum. Each division / inversion unit 4 inverts the first spectrum on the frequency axis. Each division / inversion unit 4 inverts the second spectrum on the frequency axis. This makes it possible to reduce the wavelength dependency of inter-core crosstalk.

[0043] Second Embodiment The second embodiment differs from the first embodiment mainly in that an inverting unit 42 is arranged at the end of one or more spans (between nodes) in the optical transmission line instead of the splitting and inverting unit 4. The second embodiment will be described focusing on the differences from the first embodiment.

[0044] 3 is a diagram showing an example of the configuration of a crosstalk reduction system 1b according to the second embodiment. The crosstalk reduction system 1b includes, as an example, M transmitters 2, four optical multiplexers 3, one splitter / inverter 4, two inverters 42, four optical demultiplexers 5, and M receivers 6.

[0045] In the optical transmission path from the optical multiplexer 3 to the optical demultiplexer 5, inter-core crosstalk accumulates in the optical signal according to the transmission distance. The optical power of the inter-core crosstalk accumulates in the optical signal linearly according to the transmission distance.

[0046] Therefore, in the second embodiment, the optical demultiplexer 5 outputs the entire band of the optical signal input from the stage preceding the optical demultiplexer 5 to an inversion unit 42 (third inversion unit) arranged on the optical transmission line outside the division and inversion unit 4. The inversion unit 42 arranged on the optical transmission line outside the division and inversion unit 4 performs inversion processing on the entire band of the optical signal input from the optical demultiplexer 5 preceding the inversion unit 42 (own node). In other words, the inversion unit 42 arranged on the optical transmission line outside the division and inversion unit 4 inverts the spectrum of the input optical signal on the frequency axis, with the center frequency of the frequency band of the optical signal input to the inversion unit 42 (own node) as the axis of symmetry.

[0047] In addition, if inversion processing is not required, the optical demultiplexer 5 may output the entire bandwidth of the optical signal input from the previous stage of the optical demultiplexer 5 to the optical multiplexer 3 subsequent to the inversion unit 42, rather than outputting it to the inversion unit 42 located outside the splitting and inversion unit 4.

[0048] FIG. 4 is a diagram showing an example of the wavelength dependence of cumulative crosstalk at the end of each span from the first span to the fourth span in the second embodiment. For example, the "first span" in FIG. 4 represents the input of the optical demultiplexer 5-1 (the end of the first span). FIG. 5 is an enlarged view showing an example of the wavelength dependence of cumulative crosstalk at the end of the fourth span in the second embodiment. For example, the "fourth span" in FIGS. 4 and 5 represents the input of the optical demultiplexer 5-4 (the end of the fourth span). The transmission bandwidth (maximum relative wavelength) is, for example, 80 nm. The cumulative crosstalk has a slope (0.1 dB / nm) with respect to the relative wavelength axis (frequency axis).

[0049] When the inversion process is not performed (the case of "no inversion" in the "fourth span" illustrated in FIG. 4), the difference between wavelengths of the cumulative crosstalk is about 8 dB across the entire transmission band. When the inversion process is performed using the method disclosed in Non-Patent Document 2 (the case of "conventional method" in the "fourth span" illustrated in FIG. 4), the difference between wavelengths of the cumulative crosstalk is about 1.6 dB across the entire transmission band.

[0050] In contrast, the "proposed method" of the crosstalk reduction system 1b combines multiple types of spectrum inversion units (the splitting and inversion unit 4 and the inversion unit 42). This effectively reduces the difference between wavelengths in the accumulated crosstalk. In the "proposed method" for the "fourth span" illustrated in FIG. 4, the difference between wavelengths in the accumulated crosstalk is approximately 0.45 dB across the entire transmission band.

[0051] Here, multiple types of spectrum inverting units are arranged with respect to a predetermined reference position on the optical transmission path from the optical multiplexer 3 to the optical demultiplexer 5 so that the cumulative crosstalk of the optical signal input to the receiver 6 is symmetrical on the frequency axis (on the axis of relative wavelength) (for example, the "proposed method" of the "fourth span" illustrated in FIG. 4). In FIG. 3, the inverting unit 42-1 is arranged before the division and inverting unit 4, and the inverting unit 42-2 is arranged after the division and inverting unit 4 so that the arrangement of the inverting unit 42-1 and the arrangement of the inverting unit 42-2 are symmetrical with respect to the position of the division and inverting unit 4. This more efficiently reduces the difference between wavelengths in the cumulative crosstalk.

[0052] As described above, the crosstalk reduction system 1b includes, in an optical transmission path, a multicore fiber that transmits an optical signal for each core and multiple types of spectrum inversion units. As an example, the crosstalk reduction system 1b includes, in the optical transmission path, a multicore fiber, one division / inversion unit 4, and two inversion units 42. Here, the two inversion units 42 may be arranged symmetrically with respect to the position of the division / inversion unit 4 in the optical transmission path so that the cumulative crosstalk of the optical signal input to the receiver 6 is symmetrical on the frequency axis. That is, the one division / inversion unit 4 (crosstalk reduction unit) and the two inversion units 42 (third inversion units) may be arranged symmetrically in the optical transmission path. This makes it possible to further reduce the wavelength dependency of inter-core crosstalk.

[0053] Third Embodiment In the third embodiment, the main difference from the second embodiment is the arrangement of the division / inversion unit 4 and the inversion unit 42 in the optical transmission line. In the third embodiment, the difference from the second embodiment will be mainly described.

[0054] 6 is a diagram showing an example of the configuration of a crosstalk reduction system 1c according to the third embodiment. As an example, the crosstalk reduction system 1c includes M transmitters 2, four optical multiplexers 3, one inverter 42, two division / inversion units 4, four optical demultiplexers 5, and M receivers 6. As such, the number of division / inversion units 4 and inverters 42 in the optical transmission path differs from that of the second embodiment.

[0055] 7 is a diagram showing an example of the wavelength dependency of the cumulative crosstalk at the end of each of the first to fourth spans in the third embodiment. When the inversion process is not performed (the case of "no inversion" in the "fourth span" illustrated in FIG. 7), the difference between wavelengths of the cumulative crosstalk is about 8 dB across the entire transmission band.

[0056] In contrast, the "proposed method" of the crosstalk reduction system 1c combines multiple types of spectrum inversion units (the splitting and inversion unit 4 and the inversion unit 42). This effectively reduces the difference between wavelengths in the accumulated crosstalk. In the "proposed method" for the "fourth span" illustrated in FIG. 7, the difference between wavelengths in the accumulated crosstalk is approximately 0.45 dB across the entire transmission band.

[0057] Here, multiple types of spectrum inversion units are arranged with respect to a predetermined reference position on the optical transmission path from the optical multiplexer 3 to the optical demultiplexer 5 so that the cumulative crosstalk of the optical signal input to the receiver 6 is symmetrical on the frequency axis (on the axis of relative wavelength) (for example, the "proposed method" of the "fourth span" illustrated in FIG. 7). In FIG. 6, the division and inversion unit 4-1 is arranged before the inversion unit 42, and the division and inversion unit 4-2 is arranged after the inversion unit 42 so that the arrangement of the division and inversion unit 4-1 and the arrangement of the division and inversion unit 4-2 are symmetrical with respect to the position of the inversion unit 42. This more efficiently reduces the difference between wavelengths in the cumulative crosstalk.

[0058] As described above, the crosstalk reduction system 1c includes a multicore fiber that transmits an optical signal for each core and multiple types of spectrum inversion units in an optical transmission path. As an example, the crosstalk reduction system 1c includes a multicore fiber, two division and inversion units 4, and one inversion unit 42 in the optical transmission path. Here, the two division and inversion units 4 may be arranged symmetrically with respect to the inversion unit 42 in the optical transmission path so that the cumulative crosstalk of the optical signal input to the receiver 6 is symmetric on the frequency axis. That is, the two division and inversion units 4 (crosstalk reduction units) and the one inversion unit 42 (third inversion unit) may be arranged symmetrically in the optical transmission path. This makes it possible to reduce the wavelength dependency of inter-core crosstalk.

[0059] When the bandwidth is expanded, the wavelength dependency of inter-core crosstalk can be further reduced by increasing the number of divisions of the bandwidth. The number of times the bandwidth is divided or the number of spans of the multicore fiber is determined in advance depending on the width of the bandwidth to be used, the flatness of the accumulated crosstalk that is required, the bandwidth that can be supported by a device used for spectrum inversion processing, etc.

[0060] (Effect) Fig. 8 is a diagram showing an example of the relationship between the difference between wavelengths of cumulative crosstalk and the bandwidth in each embodiment (example of simulation results). When inversion processing is not performed (in the case of "no inversion"), the difference between wavelengths of cumulative crosstalk increases linearly as the transmission band is expanded. Also, when inversion processing is performed using the method disclosed in Non-Patent Document 2 (in the case of "conventional method"), the difference between wavelengths of cumulative crosstalk is several dB over the entire transmission band.

[0061] In contrast to these, in the crosstalk reduction system "proposed method" in each embodiment, even when the transmission band is expanded from the C band to 160 nm (S+C+L band), the difference between wavelengths in cumulative crosstalk is less than 2 dB. Note that this difference can also be further reduced by increasing the number of band divisions (increasing the number of spans of the multicore fiber), thereby making it possible to further reduce the wavelength dependency of inter-core crosstalk.

[0062] (Hardware configurations of the control device (not shown), transmitter, and receiver) The control device (not shown), which controls, for example, the temperature of the division / inversion unit and the inversion unit, etc., the transmitter, and the receiver are each realized as software by a processor such as a CPU (Central Processing Unit) executing a program stored in a storage device having a non-volatile storage medium (non-transitory storage medium) and memory. The program may be recorded on a computer-readable storage medium. Examples of computer-readable storage media include portable media such as flexible disks, magneto-optical disks, ROMs (Read Only Memory), and CD-ROMs (Compact Disc Read Only Memory), and non-transitory storage media such as hard disks or solid state drives (SSDs) built into a computer system.

[0063] Each of the control device (not shown), the transmitter, and the receiver may be realized using hardware (accelerator) including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0064] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0065] The present invention is applicable to optical transmission systems.

[0066] 1a, 1b, 1c...crosstalk reduction system, 2...transmitter, 3...optical multiplexer, 4...division / inversion unit, 5...optical demultiplexer, 6...receiver, 41...demultiplexing unit, 42...inversion unit, 43...multiplexing unit, 101...spectrum, 102...spectrum, 103...spectrum

Claims

1. A crosstalk reduction system comprising a multicore fiber that transmits an optical signal for each core, and one or more crosstalk reduction units, the crosstalk reduction unit having: a division unit that divides the spectrum of an input optical signal at a division frequency determined on a frequency axis; a first inversion unit that inverts the spectrum of a frequency band lower than the division frequency on the frequency axis, with a center frequency of the frequency band lower than the division frequency as an axis of symmetry; a second inversion unit that inverts the spectrum of a frequency band equal to or higher than the division frequency on the frequency axis, with a center frequency of the frequency band equal to or higher than the division frequency as an axis of symmetry; and a merging unit that merges the spectrum inverted in the frequency band equal to or higher than the division frequency and the spectrum inverted in the frequency band lower than the division frequency.

2. The crosstalk reduction system of claim 1, further comprising a third inversion unit that inverts the spectrum of the optical signal input to the node on the frequency axis, with the center frequency of the frequency band of the optical signal input to the node as the axis of symmetry.

3. The crosstalk reduction system according to claim 2, wherein the crosstalk reduction section and the third inversion section are arranged symmetrically in the optical transmission path.

4. A crosstalk reduction method executed by a crosstalk reduction system having a multicore fiber that transmits an optical signal for each core and one or more crosstalk reduction units, wherein the crosstalk reduction unit: divides the spectrum of an input optical signal at a division frequency determined on a frequency axis; inverts the spectrum of a frequency band lower than the division frequency on the frequency axis with a center frequency of the frequency band lower than the division frequency as an axis of symmetry; inverts the spectrum of a frequency band equal to or higher than the division frequency on the frequency axis with a center frequency of the frequency band equal to or higher than the division frequency as an axis of symmetry; and merges the inverted spectrum in the frequency band equal to or higher than the division frequency with the inverted spectrum in the frequency band lower than the division frequency.

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