Signal detection device, signal detection method, and program

Abstract

One embodiment of the present invention is a signal detection device that comprises: a mode splitter that converts N-number (N is a natural number) of spatial mode signals including a plurality of high-order modes and being excited in a transmission path fiber, into N-number of base modes, and outputs the resultant signals; a frequency conversion unit that converts the N-number of optical signals so as to have frequencies different from each other; a multiplexer that multiplexes the N-number of optical signals having converted frequencies; and a reception unit that subjects the multiplexed optical signal to photoelectric conversion while maintaining modulation information for phase and polarization, and to digital signal conversion and signal processing, thereby demodulating the optical signal to an original information sequence.

Description

[Technical field]

[0001] The present invention relates to a signal detection device, a signal detection method, and a program technology. [Background technology]

[0002] In recent years, with the launch of 5th Generation (5G) mobile communications systems, the distribution of high-definition video services, and the development of Internet of Things (IoT) services, the amount of communication traffic flowing through optical networks has been steadily increasing year by year.

[0003] The optical fibers that form the basis of current high-capacity optical networks are single-mode fibers, except for short-distance local networks such as LANs (Local Area Networks). Single-mode fibers have a single core in the cladding that serves as a path for optical signals. Single-mode fibers allow only a single mode of propagation in wavelength bands such as the C-band (1528-1565 nm) and L-band (1570-1605 nm) used in high-capacity long-distance optical networks. As a result, single-mode fibers have realized high-capacity long-distance optical networks that can stably transfer information of several terabits per second over long distances.

[0004] In such optical networks, as a measure for dealing with the increasing demand for communication traffic, for example, digital coherent transmission technology, which uses digital signal processing technology and coherent transmission and reception technology, has been introduced into 100 gigabit per second-class optical transmission equipment. Digital coherent transmission technology is a technology that combines a coherent reception method with ultra-high-speed digital signal processing. The coherent reception method is a reception method that detects the interference light between light and local oscillator light on the receiving side. Ultra-high-speed digital signal processing is a process that removes noise from phase components caused by frequency and phase fluctuations in the transmitting side light source that generates the signal light and the receiving side light source that generates the local oscillator light after the signal is digitized.

[0005] An example of a transmission method using digital coherent transmission technology in an optical transmission system is polarization multiplexing optical transmission using two orthogonal polarization modes for a single mode fiber. In polarization multiplexing optical transmission, different information can be carried on each of the orthogonal polarizations. Another example of a transmission method using digital coherent transmission technology is mode multiplexing optical transmission using multiple spatial modes (hereinafter also referred to as "modes") in a multimode optical fiber. In mode multiplexing optical transmission, for example, a fiber with a wider core diameter than a single mode fiber is used as a transmission medium. As a result, in mode multiplexing optical transmission, multiple modes can be excited even in existing wavelength bands such as the C band, and different information can be carried and transmitted in each mode. In the case of mode multiplexing optical transmission, as in the case of polarization multiplexing optical transmission, the mode multiplexed optical signals are mixed in a complex manner while propagating through a multimode optical fiber. A receiving device compatible with a mode diversity structure receives the mixed mode multiplexed optical signals, converts the received mode multiplexed optical signals into digital signals, and separates them using MIMO digital signal processing of a scale according to the number of excited modes.

[0006] For example, in a few-mode fiber that excites two LP (Linearly Polarized) modes, the fundamental mode LP01 mode and the higher-order mode LP11 mode are excited. Furthermore, by utilizing the two degenerate modes of the LP11 mode (referred to as LP11a and LP11b, respectively) and the polarization modes of each mode (referred to as X-polarized and Y-polarized, respectively), a few-mode fiber for 2LP modes can carry different information in a total of six spatial modes: LP01X, LP01Y, LP11aX, LP11aY, LP11bX, and LP11bY. As a result, if the nonlinear optical effect of the optical fiber is ignored, a few-mode fiber for 2LP modes can theoretically achieve a transmission capacity three times that of existing single-mode fibers. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] F. Hamaoka, S. Okamoto, K. Horikoshi, K. Yonenaga, A. Hirano and Y. Miyamoto, "Mode and polarization division multiplexed signal detection with single coherent receiver using mode-selective coherent detection technique," 2016 Optical Fiber Communications Conference and Exhibition (OFC), Th3A.6 (2016) Summary of the Invention [Problem to be solved by the invention]

[0008] However, in a transmission system that applies the conventional space division multiplexing transmission technology, there is a problem that the number of components in the transceiver device increases according to the number of spatial modes used. For example, in a few-mode fiber for 2LP modes, in order to use three LP modes, the number of required components and elements, such as a digital-to-analog converter that converts a digital signal to an analog signal, an analog-to-digital converter that converts an analog signal to a digital signal, a driver amplifier, an optical modulator, an optical front end for reception, a semiconductor optical amplifier, a photodetector, and a feedback monitor, in the optical transceiver is at least three times as many as in a system that uses a single-mode fiber as a transmission medium. Thus, in a transmission system that applies the space division multiplexing transmission technology, the footprint and power consumption of the optical transceiver included in the optical transceiver increase, and the manufacturing cost of the device increases due to changes in the manufacturing process of the integrated components.

[0009] As a means for reducing the number of devices and elements constituting the receiver in a mode multiplexing transmission system, for example, the mode selective coherent detection technology described in Non-Patent Document 1 has been proposed. The technology described in Non-Patent Document 1 requires the provision of a mode multiplexer for local light and a mechanism for generating synchronous light in an additional manner for each wavelength channel, and therefore has a problem in that the effect of reducing the number of devices and elements constituting the entire communication system is limited.

[0010] In view of the above circumstances, an object of the present invention is to provide a technique capable of reducing the number of components in a receiving device in a transmission system to which space division multiplexing transmission technology is applied. [Means for solving the problem]

[0011] One aspect of the present invention is a signal detection device that includes a mode splitter that converts N spatial mode signals (N is a natural number) including multiple higher-order modes excited in a transmission fiber into N fundamental modes and outputs the converted signals; a frequency conversion unit that converts the frequency of each of the N optical signals into frequencies different from each other; a combining unit that combines the N optical signals whose frequencies have been converted; and a receiving unit that performs opto-electrical conversion on the combined optical signal while retaining modulation information for phase and polarization, and performs digital signal conversion and signal processing to demodulate the combined optical signal back to the original information sequence.

[0012] One aspect of the present invention is a signal detection method in which a signal detection device converts N (N is a natural number) spatial mode signals including a plurality of higher-order modes excited in a transmission fiber into N fundamental modes and outputs the converted signals, converts the frequencies of the N optical signals to frequencies different from each other, combines the N optical signals whose frequencies have been converted, performs opto-electrical conversion on the combined optical signal while retaining modulation information for phase and polarization, and performs digital signal conversion and signal processing to demodulate the signal back to the original information sequence.

[0013] One aspect of the present invention is a program for causing a computer to function as the above-mentioned signal detection device. Effect of the Invention

[0014] According to the present invention, it is possible to reduce the number of components in a receiving device in a transmission system to which space division multiplexing transmission technology is applied. [Brief description of the drawings]

[0015] [Figure 1] 1 is a diagram illustrating an example of the configuration of a signal detection device according to a first embodiment. [Diagram 2] 4A and 4B are diagrams illustrating optical spectra at an input portion and an output portion of an optical signal of the N×1 optical coupler according to the first embodiment. [Diagram 3] 4 is a flowchart of an example of a processing procedure of the signal detection device according to the first embodiment. [Figure 4] FIG. 11 is a diagram illustrating an example of the configuration of a signal detection system according to a second embodiment. [Diagram 5] 10 is a flowchart of an example of a processing procedure of a signal detection device according to a second embodiment. [Figure 6] FIG. 11 is a diagram illustrating an example of the configuration of a signal detection system according to a third embodiment. [Figure 7] 13 is a flowchart of an example of a processing procedure of a signal detection device according to a third embodiment. [Figure 8] FIG. 13 is a diagram illustrating an example of the configuration of a signal detection system according to a fourth embodiment. [Figure 9] 13 is a diagram illustrating a connection relationship between a WDM demultiplexer and K N×1 optical couplers in a spatial frequency conversion unit according to the fourth embodiment. FIG. [Figure 10] 13 is a flowchart of an example of a processing procedure of a signal detection device according to a fourth embodiment. [Figure 11] FIG. 13 is a diagram illustrating an example of the configuration of a signal detection system according to a fifth embodiment. [Figure 12] FIG. 13 is a diagram showing transitions of optical spectra before and after frequency conversion in the first frequency conversion unit to the Nth frequency conversion unit in the spatial frequency conversion unit according to the fifth embodiment. [Figure 13] FIG. 13 is a diagram illustrating an example of the configuration of a spatial frequency conversion unit according to the fifth embodiment. [Figure 14] 13 is a flowchart of an example of a processing procedure of a signal detection device according to the fifth embodiment. [Figure 15] FIG. 13 is a diagram illustrating an example of the configuration of a signal detection system according to a sixth embodiment. [Figure 16] 13 is a flowchart of an example of a processing procedure of a signal detection device according to the sixth embodiment. [Figure 17] FIG. 13 is a diagram illustrating an example of the configuration of a signal detection system according to a seventh embodiment. [Figure 18] 13 is a flowchart of an example of a processing procedure of a signal detection device according to the eighth embodiment. [Figure 19] 3 is a diagram showing an example of the spectrum of a received signal in the digital domain obtained by the signal detection device of the first embodiment; FIG. [Figure 20] FIG. 1 is a diagram showing an example of a QPSK constellation obtained according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings.

[0017] First Embodiment 1 is a diagram showing an example of the configuration of a signal detection system according to the present embodiment. As shown in FIG. 1, the signal detection system 1 includes a signal detection device 10 and an optical transmission fiber 2. The signal detection device 10 includes a mode demultiplexer 3, a space-frequency conversion section 4 (frequency conversion section), an N×1 optical coupler 5 (combiner) (multiplexing section), and a receiving section 6. The spatial frequency conversion unit 4 includes, for example, N (N is an integer greater than or equal to 1) clock units 41 (first clock unit 41-1, second clock unit 41-2, ..., Nth clock unit 41-N) and N frequency shifters 42 (first frequency shifter 42-1, second frequency shifter 42-2, ..., Nth frequency shifter 42-N). The receiving unit 6 includes, for example, a local light receiving unit 61, a coherent receiver 62, an AD converter 64, and a signal processing unit 65. The configuration shown in FIG. 1 is just an example, and the present invention is not limited to this.

[0018] The signal detection device 10 converts the optical signal propagating through the transmission fiber 2 into a digital signal via photoelectric conversion, and then performs various signal processing to demodulate the signal series sent from the transmitting side without any information errors.

[0019] The transmission fiber 2 is, for example, a spatially multiplexed fiber such as a multimode fiber that can excite a plurality of spatial modes in a predetermined optical communication band.

[0020] The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes and outputs the converted signals.

[0021] The spatial frequency converter 4 converts the frequency of each optical signal. The spatial frequency converter 4 converts the frequency of each of the N optical signals into frequencies different from each other. In this embodiment, the spatial frequency converter 4 converts the frequency of each optical signal by frequency shifting.

[0022] Each of the N clock units 41 generates and outputs a clock signal with a frequency f. From the viewpoint of reducing the load on the signal processing unit, it is desirable that the N clock units are synchronized by means of an external clock for synchronization or the like, but they do not necessarily have to be synchronized.

[0023] The N frequency shifters 42 receive as input each optical signal output from the mode demultiplexer 3 and a clock signal from the clock unit 41. The n-th (n is one of 1 to N) frequency shifter 42-n imparts a frequency shift amount of nΔf to the optical signal. That is, the first frequency shifter 42-1 imparts a frequency shift amount of Δf to the optical signal, and the second frequency shifter 42-2 imparts a frequency shift amount of 2Δf to the optical signal. The frequency shift in the frequency shifter 42 can be realized by, for example, a Mach-Zehnder type optical modulator made of a lithium niobate-based waveguide using the electro-optic effect (for example, Reference 1), but is not limited to this as long as it has a configuration having a predetermined frequency shift function.

[0024] Reference 1; M. Izutsu, S. Shikama, and T. Sueta, “Integrated optical SSB modulator / frequency shifter,” IEEE J. Quantum Electron. 17(11), 2225-2227 (1981).

[0025] The N×1 optical coupler 5 (multiplexing unit) multiplexes and outputs the optical signals output from the frequency shifters. Note that the N×1 optical coupler may be, for example, a combiner that combines a plurality of laser beams into one fiber.

[0026] The receiver 6 mixes the received signal light with the local light to decode the signal, and outputs the signal to, for example, an external device.

[0027] The local light receiving unit 61 includes, for example, a laser diode as a light source, and emits and outputs local light.

[0028] The coherent receiver 62 has, for example, a phase and polarization diversity configuration. The coherent receiver 62 converts the received and combined received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for the phase and polarization.

[0029] The AD converter 64 converts the analog electrical signal into a digital signal by a sampling process.

[0030] The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, and the like, and demodulates the signal to the original correct information sequence (symbol sequence or binary sequence).

[0031] Next, the optical spectrum of the optical signal at the input and output portions of the N×1 optical coupler 5 will be described. 2 is a diagram showing the optical spectrum of the input and output parts of the optical signal of the N×1 optical coupler according to the present embodiment. As shown in FIG. 2, the signal input to the N×1 optical coupler 5 is shifted by each frequency shifter 42 to the original carrier frequency f c The N×1 optical coupler 5 multiplexes n optical signals (g11 to g13), thereby obtaining optical signals spaced on the frequency axis at intervals of Δf, as in optical spectrum g14.

[0032] Next, an example of a processing procedure of the signal detection device 10 will be described. FIG. 3 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0033] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0034] (Step S2) The n-th frequency shifter 42-n converts the frequency of the optical signal by imparting a frequency shift amount of nΔf to the optical signal.

[0035] (Step S3) The N×1 optical coupler 5 multiplexes the optical signals output from the frequency shifters .

[0036] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0037] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0038] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0039] In the signal detection device 10 configured in this manner, the output signal of the N×1 optical coupler 5 is mixed with local light and photoelectric conversion is performed by intradyne coherent detection or heterodyne coherent detection in a photodetector. This allows N spatial modes in the transmission fiber to be received by a single receiving unit 6, making it possible to reduce the number of components in the signal detection device 10.

[0040] In the conventional configuration, N receiving units were required, but in this embodiment, this can be realized with one receiving unit 6. Even in the digital domain converted by the AD converter 64, the received signals are arranged on the frequency axis with the frequency interval of Δf maintained under the condition that the sampling theorem that no alias occurs is satisfied. Furthermore, in this embodiment, the obtained n-th signal is frequency shifted by -nΔf by digital signal processing and converted into a baseband signal, so that a received signal sequence equivalent to the signal received by N receiving units in the conventional configuration can be obtained by one receiving unit 6 under the condition that the sampling speed limit of the AD converter 64 and the band limit of each device can be ignored.

[0041] In the above example, the case of coherent reception using local light has been described as an example, but the receiving method may be a direct reception method that does not use local light. In the above example, the configuration in which N spatial modes are received by a single receiving function unit has been described, but the effect of this embodiment is not limited to this. For example, the configuration of the signal detection device 10 may be such that N spatial modes are divided into p groups (p is an integer between 1 and N) and p configurations in which the N spatial modes are received by the receiving function unit after a predetermined frequency shift and multiplexing are arranged in parallel. Note that p is a number determined by the bandwidth and required cost of the components of the receiving unit 6, and the reduction effect of the receiving unit 6 in this case is equivalent to reducing the number of all receiving units to 1 / p compared to the conventional configuration.

[0042] <Second embodiment> 4 is a diagram showing an example of the configuration of a signal detection system according to this embodiment. As shown in FIG. 4, a signal detection system 1A includes a signal detection device 10A and an optical transmission fiber 2. The signal detection device 10A includes a mode separator 3, a spatial frequency conversion section 4 (frequency conversion section), an N×1 optical coupler 5 (multiplexing section), and a receiving section 6A. The receiving unit 6A includes, for example, a local light receiving unit 61, a coherent receiver 62, a band dividing unit 63, an AD converter 64, and a signal processing unit 65. The configuration shown in FIG. 4 is just an example, and the present invention is not limited to this.

[0043] The spatial frequency converter 4 converts the frequency of each optical signal. In this embodiment, similar to the first embodiment, the spatial frequency converter 4 converts the frequency of each optical signal by shifting the frequency of the optical signal.

[0044] The band splitting unit 63 outputs the analog electric signal as a plurality of parallelized signals to the AD converter 64. The band splitting function of the band splitting unit 63 means, for example, splitting the analog electric signal into high-frequency and low-frequency sidebands and converting the center of each sideband to a frequency corresponding to a DC component and outputting it, which can be realized by a wideband circuit implementation technology such as that shown in Reference 2. The analog electric signals photoelectrically converted by the coherent receiver 62 are arranged on the frequency axis with a frequency interval of Δf by each frequency shifter.

[0045] Reference 2; F. Hamoaka et al., “Ultra-wideband Optical Receiver Using Electrical Spectrum Decomposition Technique,” ​​We1C-3, ECOC2020.

[0046] Next, an example of a processing procedure of the signal detection device 10A will be described. FIG. 5 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0047] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0048] (Step S2) The n-th frequency shifter 42-n converts the frequency of the optical signal by imparting a frequency shift amount of nΔf to the optical signal.

[0049] (Step S3) The N×1 optical coupler 5 multiplexes the optical signals output from the frequency shifters .

[0050] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0051] (Step S11) The band dividing unit 63 outputs the analog electrical signal to the AD converter 64 as a plurality of parallelized signals.

[0052] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0053] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0054] As described above, in this embodiment, the receiving unit 6A further includes the band splitting unit 63, so that the maximum frequency contained in the analog electrical signal is reduced, and it is expected that the requirements for band limitation and operating frequency limitation on the downstream electrical circuit and digital circuit will be relaxed.

[0055] In this embodiment, an example in which the band splitting unit splits and outputs two sidebands has been shown, but the present invention is not limited to this, and the band splitting unit may have a function of converting a single analog electrical signal input into q (q is an integer between 1 and N) low-speed output signals. In other words, the band splitting unit 63 may perform digital signal processing and signal processing after splitting into parallel signals consisting of multiple signal bands.

[0056] <Third embodiment> 6 is a diagram showing an example of the configuration of a signal detection system according to this embodiment. As shown in FIG. 6, a signal detection system 1B includes a signal detection device 10B and an optical transmission fiber 2. The signal detection device 10B includes a mode separator 3, a spatial frequency conversion section 4B (frequency conversion section), an N×1 optical coupler 5 (multiplexing section), and a receiving section 6. The spatial frequency conversion unit 4B includes, for example, N clock units 41 (first clock unit 41-1, second clock unit 41-2, ..., Nth clock unit 41-N), 2N frequency shifters 42 (first frequency shifter 42-1-1, first frequency shifter 42-1-2, second frequency shifter 42-2-1, second frequency shifter 42-2-2, ..., Nth frequency shifter 42-N-1, Nth frequency shifter 42-N-2), polarizing beam splitters 43 (43-1, 43-2, ..., 43-N), and polarizing beam combiners 44 (44-1, 44-2, ..., 44-N). The configuration shown in FIG. 6 is just an example, and the present invention is not limited to this.

[0057] The spatial frequency converter 4B converts the frequency of each optical signal. In this embodiment, the spatial frequency converter 4 converts the frequency of each optical signal by frequency shifting for each change, and outputs the converted signal.

[0058] The spatial frequency conversion unit 4B has two frequency shifters 42-n-1, 2 for each optical signal sequence. A polarization beam splitter 43 is connected to each of the inputs of the two frequency shifters 42-n-1, and a polarization beam combiner 44 is connected to each of the outputs of the two frequency shifters.

[0059] Optical modulators that realize frequency shifting are generally of the waveguide type and therefore have polarization dependency on the input light. For this reason, in this embodiment, a polarization beam splitter 43 or a polarization beam combiner 44 is provided before and after the input of the frequency shifter 42 to achieve polarization diversity and enable compatibility with polarization multiplexed signals.

[0060] The polarizing beam splitter 43 is an optical element that splits each optical signal output from the mode demultiplexer 3 into S-polarized light and P-polarized light. The n-th frequency shifter 42-n-1 imparts a frequency shift of nΔf to the S-polarized light. The n-th frequency shifter 42-n-2 converts the frequency of the optical signal by imparting a frequency shift of nΔf to the P-polarized light. The polarization beam combiner 44 is an optical element that combines the optical signal output from the n-th frequency shifter 42-n-1 and the optical signal output from the n-th frequency shifter 42-n-2, and outputs the combined signal to the N×1 optical coupler 5.

[0061] Next, an example of a processing procedure of the signal detection device 10B will be described. FIG. 7 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0062] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0063] (Step S21) The polarizing beam splitter 43 splits each optical signal output from the mode demultiplexer 3 into S-polarized light and P-polarized light.

[0064] (Step S22) The frequency shifter 42 converts the frequency of the optical signal by imparting a frequency shift amount of Δf to the S-polarized light or the P-polarized light.

[0065] (Step S23) The polarization beam combiner 44 is an optical element, and combines the optical signal output from the n-th frequency shifter 42-n-1 and the optical signal output from the n-th frequency shifter 42-n-2.

[0066] (Step S3) The N×1 optical coupler 5 multiplexes the optical signals output from the frequency shifters .

[0067] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0068] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0069] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0070] As described above, in this embodiment, the polarization beam splitter 43 or the polarization beam combiner 44 is provided before and after the input of the frequency shifter 42, so that a frequency shift amount is applied to each polarized light. As a result, according to this embodiment, polarization diversity is achieved, making it possible to handle polarization multiplexed signals.

[0071] The receiving section 6 may be the receiving section 6B of the second embodiment that includes the band splitting section 63.

[0072] <Fourth embodiment> In this embodiment, an example is described in which optical signals of each spatial mode propagating through the optical transmission fiber 2 are WDM (Wavelength division multiplexing) signals each having K wavelength multiplexed signals (K is an integer equal to or greater than 1) processed collectively in a spatial frequency conversion unit.

[0073] 8 is a diagram showing an example of the configuration of a signal detection system according to this embodiment. As shown in FIG. 8, a signal detection system 1C includes a signal detection device 10C and an optical transmission fiber 2. The signal detection device 10C includes a mode demultiplexer 3, a spatial frequency conversion unit 4C (frequency conversion unit), K N×1 optical couplers 5 (multiplexing units) (N×1 optical couplers 5-1, ..., N×1 optical couplers 5-K), and K receiving units 6 (receiving units 6-1, ..., receiving units 6-K). The spatial frequency conversion unit 4C includes, for example, N (N is an integer greater than or equal to 1) clock units 41 (first clock unit 41-1, second clock unit 41-2, ..., Nth clock unit 41-N), N frequency shifters 42 (first frequency shifter 42-1, second frequency shifter 42-2, ..., Nth frequency shifter 42-N), and a WDM demultiplexer 45 (WDM demultiplexer 45-1, WDM demultiplexer 45-2, ..., WDM demultiplexer 45-N). The configuration shown in FIG. 8 is just an example, and the present invention is not limited to this.

[0074] The spatial frequency converter 4C converts the frequency of each optical signal. In this embodiment, the spatial frequency converter 4C converts the frequency of each optical signal by frequency shifting, and the WDM demultiplexer 45 further demultiplexes the optical signals for each channel.

[0075] The n-th frequency shifter 42-n converts the frequency of the optical signal by collectively imparting a frequency shift amount of nΔf to K WDM wavelength channels. WDM demultiplexer 45 demultiplexes the K WDM wavelength channels for each channel. The N×1 optical coupler 5 multiplexes and outputs the output signals for each wavelength channel. The K receiving units 6 perform reception processing and demodulation processing for each wavelength channel by the same means as in the first embodiment.

[0076] Fig. 9 is a diagram showing the connection relationship between the WDM demultiplexers of the spatial frequency conversion unit according to this embodiment and K N×1 optical couplers. As shown in Fig. 9, the first channel signal of each of the WDM demultiplexers 45-1 to 45-N is input to the N×1 optical coupler 5-1, the second channel signal of each of the WDM demultiplexers 45-1 to 45-N is input to the N×1 optical coupler 5-2, ..., the Kth channel signal of each of the WDM demultiplexers 45-1 to 45-N is input to the N×1 optical coupler 5-K.

[0077] Next, an example of a processing procedure of the signal detection device 10C will be described. FIG. 10 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0078] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0079] (Step S31) The n-th frequency shifter 42-n converts the frequency of the optical signal by collectively imparting a frequency shift amount of nΔf to K WDM wavelength channels.

[0080] (Step S32) WDM demultiplexer 45 demultiplexes the K WDM wavelength channels for each channel.

[0081] (Step S33) The N×1 optical coupler 5 multiplexes the output signals for each wavelength channel in the optical signal to which the frequency shift amount has been added.

[0082] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0083] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0084] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0085] As described above, in this embodiment, the spatial frequency conversion unit 4C includes the WDM demultiplexer 45 at the rear of each frequency shifter 42, and the output signals for each wavelength channel are multiplexed at the rear N×1 optical coupler 5. In this embodiment, the wavelength channels are subjected to reception processing and demodulation processing by K receivers 6 using the same means as in the first embodiment.

[0086] As a result, according to this embodiment, it is possible to frequency-shift K WDM wavelength channels all at once, and the optical signals of each spatial mode propagating through the transmission fiber can correspond to a WDM configuration having K wavelength-multiplexed signals. When a WDM signal having K wavelength multiplexed signals is handled in the configurations of the first to third embodiments, it is necessary to provide K spatial frequency conversion units 4 (or 4A, 4B). In contrast, according to this embodiment, there is an advantage that the number of spatial frequency conversion sections can be reduced by 1 / K compared to the first to third embodiments in which each WDM wavelength channel is frequency shifted.

[0087] Each receiving section 6 may be the receiving section 6B of the second embodiment that includes a band dividing section 63.

[0088] <Fifth embodiment> This embodiment is an example of a configuration in which the frequency shifter 42 in the spatial frequency conversion section 4C of the fourth embodiment is replaced with a frequency conversion section 46.

[0089] 11 is a diagram showing an example of the configuration of a signal detection system according to this embodiment. As shown in FIG. 11, a signal detection system 1D includes a signal detection device 10D and an optical transmission fiber 2. The signal detection device 10D includes a mode demultiplexer 3, a spatial frequency conversion section 4D (frequency conversion section), K N×1 optical couplers 5 (multiplexing section), and K receiving sections 6 (receiving section 6-1, receiving section 6-2, . . . , receiving section 6-K). The spatial frequency conversion unit 4D includes, for example, N (N is an integer greater than or equal to 1) frequency conversion units 46 (first frequency conversion unit 46-1, second frequency conversion unit 46-2, ..., Nth frequency conversion unit 46-N) and a WDM demultiplexer 45 (WDM demultiplexer 45-1, WDM demultiplexer 45-2, ..., WDM demultiplexer 45-N). The configuration shown in FIG. 11 is just an example, and the present invention is not limited to this.

[0090] The spatial frequency converter 4D converts the frequency of each optical signal. In this embodiment, the frequency converter 46 converts the wavelength of each optical signal to convert the frequency of the optical signal, and the WDM demultiplexer 45 further demultiplexes the optical signals into individual channels.

[0091] The frequency converter 46 converts the frequency of the optical signal by performing wavelength conversion on K WDM wavelength channels. c The wavelength conversion can be achieved by using nonlinear optical effects such as highly nonlinear fiber, periodically poled lithium niobate (PPLN), semiconductor optical amplifiers, and silicon waveguides.

[0092] An example of frequency conversion using a PPLN wavelength conversion element will be described below. 12 is a diagram showing the transition of optical spectra before and after frequency conversion in the first frequency conversion unit to the Nth frequency conversion unit in the spatial frequency conversion unit according to this embodiment. In the optical spectrum transition diagrams g201 to g203, the horizontal axis represents frequency. K WDM signals are input as input light to the spatial frequency conversion unit 4D.

[0093] The nth frequency conversion unit 46-n is 2f c When pump light of +nΔf is also input, the original K WDM signals are transformed into f c It produces output light (idler light) that is folded back with the frequency of +nΔ / 2 (hereafter referred to as the center frequency) as the symmetric axis. By shifting the frequency of the pump light to each frequency conversion section 46 by Δf, it is possible to obtain output light with a shift of Δf from the WDM signal generated in the spatial frequency conversion section 4D. Each wavelength channel to the receiver 6 has a phase conjugate relationship with the optical signal originally sent from the transmitter. To return the phase conjugate light to the original signal, for example, adjustment of the optical path length in the phase diversity configuration inside the coherent receiver or phase conjugate inversion processing in the digital signal processor can be used.

[0094] Fig. 13 is a diagram showing an example of the configuration of a spatial frequency conversion unit according to the present embodiment. As shown in Fig. 13, the spatial frequency conversion unit 4D includes, for example, a light source 401, a frequency comb generation unit 402, a 1xN optical splitter 403, N optical filters 404 (404-1, 404-2, ..., 404-N), N optical amplifiers 405 (405-1, 405-2, ..., 405-N), N optical filters 406 (406-1, 406-2, ..., 406-N), N PPLNs 407 (407-1, 407-2, ..., 407-N), and N frequency conversion units 46 (first frequency conversion unit 46-1, ..., Nth frequency conversion unit 46-N).

[0095] The light source 401 outputs continuous light, which serves as seed light for the excitation light, to the frequency comb generation unit 402 .

[0096] The frequency comb generator 402 generates frequency combs arranged at frequency intervals of Δf using continuous light, and outputs the generated frequency combs to the 1×N optical splitter 403. Methods for generating a frequency comb include, but are not limited to, a configuration using multiple intensity modulators or a configuration using a mode-locked laser.

[0097] Next, 1×N optical splitter 403 splits the frequency comb into N portions and outputs each portion to optical filters 404 (404-1, 404-2, . . . , 404-N).

[0098] The optical filters 404 (404-1, 404-2, . . . , 404-N) perform filtering on the input optical signals and output the filtered signals to the optical amplifiers 405 (405-1, 405-2, . . . , 405-N) corresponding to the optical filters 404.

[0099] The optical amplifiers 405 (405-1, 405-2, . . . , 405-N) amplify the filtered optical signals and output them to optical filters 406 (406-1, 406-2, . . . , 406-N) corresponding to the optical amplifiers 405.

[0100] The optical filters 406 (406-1, 406-2, . . . , 406-N) perform filtering on the input optical signals and output the filtered signals to the PPLNs 407 (407-1, 407-2, . . . , 407-N) corresponding to the optical filters 406.

[0101] The PPLN 407 (407-1, 407-2, ..., 407-N) generates a second harmonic wave from the input optical signal and outputs the second harmonic wave to the frequency converter 46 (the first frequency converter 46-1, ..., the N-th frequency converter 46-N) corresponding to the PPLN 407. For example, the PPLN 407-1 generates a second harmonic wave of 2f c The PPLN 407-N outputs an optical signal of +Δf to the first frequency converter 46-1, . . . , the PPLN 407-N outputs an optical signal of 2fc+NΔf to the N-th frequency converter 46-N. As a result, the spatial frequency conversion unit 4D c +Δf to 2f c N pump lights are obtained, spaced at frequency intervals of Δf up to +NΔf.

[0102] The spatial frequency conversion unit 4D inputs these as excitation light to the frequency conversion unit 46 for difference frequency generation, thereby obtaining output light that is folded around the center frequency as an axis. Note that the configuration shown in Fig. 13 is an example, and is not limited to this. For example, in Fig. 13, for the sake of simplicity of configuration, a method of generating N excitation light beams is illustrated as a method of generating them collectively via frequency comb generation, but this is not limiting. For example, N excitation light beams may be generated independently using N individual light sources.

[0103] Next, an example of the processing procedure of the signal detection device 10D will be described. FIG. 14 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0104] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0105] (Step S41) The frequency conversion unit 46 converts the frequency of the optical signal by performing wavelength conversion on the K WDM wavelength channels.

[0106] (Step S42) WDM demultiplexer 45 demultiplexes the K WDM wavelength channels for each channel.

[0107] (Step S43) The N×1 optical coupler 5 multiplexes output signals for each wavelength channel in the frequency-converted and demultiplexed optical signal.

[0108] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0109] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0110] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0111] As described above, in this embodiment, the frequency shifter in the spatial frequency conversion section of the fourth embodiment is replaced with the frequency conversion section . As a result, according to this embodiment, it is possible to avoid electrical band limitations on the amount of frequency shift in the frequency shifter section and to improve the shift conversion efficiency (extinction ratio).

[0112] Each receiving section 6 may be the receiving section 6B of the second embodiment that includes a band dividing section 63.

[0113] Sixth Embodiment This embodiment is different from the fifth embodiment in that the operating band of the device in the transmission band is not changed.

[0114] 15 is a diagram showing an example of the configuration of a signal detection system according to this embodiment. As shown in FIG. 15, a signal detection system 1E includes a signal detection device 10E and an optical transmission fiber 2. The signal detection device 10E includes a mode demultiplexer 3, a spatial frequency conversion section 4E (frequency conversion section), K N×1 optical couplers 5 (multiplexing section), and K receiving sections 6 (receiving section 6-1, receiving section 6-2, . . . , receiving section 6-K). The spatial frequency conversion unit 4E includes, for example, N (N is an integer greater than or equal to 1) frequency conversion units 46 (first frequency conversion unit 46-1-1, second frequency conversion unit 46-2-1, ..., Nth frequency conversion unit 46-N-1), N frequency conversion units 47 (first frequency conversion unit 47-1-1, second frequency conversion unit 47-2-1, ..., Nth frequency conversion unit 47-N-1), and a WDM splitter 45 (WDM splitter 45-1, WDM splitter 45-2, ..., WDM splitter 45-N). The configuration shown in FIG. 15 is just an example, and the present invention is not limited to this.

[0115] In this embodiment, the nth frequency converter 47-n-1 is connected in series after the nth frequency converter 46-n-1, and the output of the nth frequency converter 47-n-1 is connected to the input of the WDM demultiplexer 45-n.

[0116] The spatial frequency converter 4E converts the frequency of each optical signal. In this embodiment, after the frequency converter 46 converts the frequency of each optical signal, the frequency converter 47 converts the frequency again, and the WDM demultiplexer 45 demultiplexes the signals into individual channels.

[0117] The frequency converter 46 converts the frequency of the optical signal by performing wavelength conversion on the K WDM wavelength channels, and outputs the converted optical signal to a frequency converter 47 that is connected to the frequency converter 46 in series. The frequency converter 47 converts the frequency of the input optical signal by performing wavelength conversion on the optical signal, and outputs the converted optical signal to the WDM demultiplexer 45 connected to the frequency converter 47 .

[0118] As explained with reference to Fig. 12, the output light from the frequency conversion unit 46 becomes phase conjugate light folded back around the center frequency. This qualitatively means that the farther the output light is from the center frequency on the frequency axis and the larger the number K of WDM signal channels is, the farther the output light is generated on the frequency axis compared to the input light. Therefore, there is a concern that the new transmission band in which the output light is located will be outside the operating band of each element inside the receiving unit.

[0119] Therefore, in this embodiment, in order to avoid the occurrence of an event in which frequency conversion falls outside the operating band of the receiving element, the frequency conversion units are arranged in series and frequency conversion is performed continuously, making it possible to handle the input light and output light of the frequency conversion unit in the same transmission band. The frequency of the excitation light in each of the frequency converters 46 and 47 can be set arbitrarily. For example, c The n-th subsequent frequency converter 47 (first frequency converter 47-1-1, second frequency converter 47-2-1, ..., N-th frequency converter 47-N-1) converts the input light into an output light of f c The frequency of each excitation light is set so as to satisfy the condition of +nΔf.

[0120] Next, an example of a processing procedure of the signal detection device 10E will be described. FIG. 16 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0121] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0122] (Step S41) The frequency conversion unit 46 performs wavelength conversion on the K WDM wavelength channels.

[0123] (Step S51) The frequency converter 47 converts the frequency of the wavelength-converted optical signal by performing wavelength conversion on the wavelength-converted optical signal.

[0124] (Step S42) WDM demultiplexer 45 demultiplexes the K WDM wavelength channels for each channel.

[0125] (Step S43) The N×1 optical coupler 5 multiplexes output signals for each wavelength channel in the frequency-converted and demultiplexed optical signal.

[0126] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0127] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0128] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0129] As described above, in this embodiment, in each frequency conversion section inside the spatial frequency conversion unit 4E, two frequency conversion sections are arranged in series. As a result, according to this embodiment, processing can be performed without changing the operating band of the device in the transmission band.

[0130] Each receiving section 6 may be the receiving section 6B of the second embodiment that includes a band dividing section 63.

[0131] Seventh embodiment This embodiment is an example in which the operating band of a device in a transmission band is not changed.

[0132] 17 is a diagram showing an example of the configuration of a signal detection system according to this embodiment. As shown in FIG. 17, a signal detection system 1F includes a signal detection device 10F and an optical transmission fiber 2. The signal detection device 10F includes a mode demultiplexer 3, a spatial frequency conversion section 4F (frequency conversion section), K N×1 optical couplers 5 (multiplexing section), and K receiving sections 6 (receiving section 6-1, receiving section 6-2, . . . , receiving section 6-K). The spatial frequency conversion unit 4F includes, for example, N (N is an integer greater than or equal to 1) complementary frequency conversion units 48 (first complementary frequency conversion unit 48-1-1, second complementary frequency conversion unit 48-2-1, ..., Nth complementary frequency conversion unit 48-N-1) and a WDM splitter 45 (WDM splitter 45-1, WDM splitter 45-2, ..., WDM splitter 45-N). The configuration shown in FIG. 17 is just an example, and the present invention is not limited to this.

[0133] The complementary frequency converter 48 separates the WDM signal into a low frequency side and a high frequency side by means of an optical filter or the like, performs frequency conversion on each branch by an optical parametric process, and then combines the signals again (see, for example, Reference 3).

[0134] Reference 3; Patent No. 6096831

[0135] Next, an example of the processing procedure of the signal detection device 10F will be described. FIG. 18 is a flowchart of an example of a processing procedure of the signal detection device according to this embodiment.

[0136] (Step S1) The mode demultiplexer 3 converts N spatial mode signals including a plurality of higher-order modes excited in the transmission fiber 2 into N fundamental modes.

[0137] (Step S61) The complementary frequency converter 48 separates the optical signals into a low frequency side and a high frequency side, converts the frequencies of the optical signals by performing frequency conversion on each branch using an optical parametric process, and then combines the optical signals again after conversion.

[0138] (Step S62) The WDM demultiplexer 45 demultiplexes the signal for each channel.

[0139] (Step S63) The N×1 optical coupler 5 multiplexes the output signals for each wavelength channel.

[0140] (Step S4) The coherent receiver 62 converts the received signal and the local light input into an analog electric signal by opto-electrical conversion while maintaining modulation information for phase and polarization.

[0141] (Step S5) The AD converter 64 converts the analog electrical signal into a digital signal by sampling.

[0142] (Step S6) The signal processing unit 65 performs predetermined signal processing including, for example, waveform distortion removal, MIMO type signal processing, frequency offset compensation, phase noise compensation, error correction decoding, etc., to demodulate the signal to the original correct information sequence.

[0143] As described above, in this embodiment, the spatial frequency conversion section 4F includes the complementary frequency conversion section . As a result, according to this embodiment, it becomes possible to handle the input light and output light of the wavelength conversion unit in the same transmission band.

[0144] Each receiving section 6 may be the receiving section 6B of the second embodiment that includes a band dividing section 63.

[0145] In each of the above-mentioned embodiments, all or part of the functions of the spatial frequency conversion unit (frequency conversion unit) and the receiving unit may be configured using a processor such as a CPU (Central Processing Unit) and a memory. All or part of the functions of the spatial frequency conversion unit and the receiving unit function as, for example, a frequency shifter unit, a coherent receiver, an AD converter, and a signal processing unit by the processor executing a program. All or part of the functions of the spatial frequency conversion unit and the receiving unit may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. The computer-readable recording medium is, for example, a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, a semiconductor storage device (for example, an SSD: Solid State Drive), or a storage device such as a hard disk or a semiconductor storage device built into a computer system. The above program may be transmitted via an electric communication line or an optical communication line, or may be supplied from the cloud.

[0146] <Explanation of effect> Below, an example of the results of an evaluation performed to confirm the effects of the above-described embodiment will be described. In order to confirm the effect of the first embodiment, a transmission experiment was carried out using a few-mode fiber with three modes. First, a 6 GBaud QPSK (Quadrature Phase Shift Keying) signal was generated by an optical modulator, and three input signals from a delay system were input to a mode multiplexer to obtain a three-mode mode multiplexed optical signal. Next, the optical signal was input to a pair of mode splitters connected by a patch cord of a few-mode fiber, and was converted again to three fundamental modes (LP01). At this time, the three fundamental mode signals were mixed with each other. This is because, although the transmission path in the transmission evaluation system was short, a non-negligible level of coupling occurred between the three modes due to the imperfection of the multiplexer / splitter. Of the three fundamental mode signals, the signal that was originally in LP11a mode was given a frequency shift of +12 (GHz) by the frequency shifter, and the signal that was originally in 11b mode was given a frequency shift of -12 (GHz) by the frequency shifter 42. The LP11a mode and 11b mode are propagation modes. The three fundamental mode signals were then combined by a 3×1 coupler and received by a single receiver 6 .

[0147] Fig. 19 is a diagram showing an example of the spectrum of a received signal in the digital domain obtained by the signal detection device 10 of the first embodiment. In Fig. 19, the horizontal axis is frequency (GHz) and the vertical axis is intensity (10 dB / div). As shown in Fig. 19, the evaluation result shows that signals of mode 3 (LP11b) (g301), mode 1 (LP01) (g302), and mode 2 (LP11a) (g303) are frequency-multiplexed at intervals of 12 (GHz) from the low frequency side.

[0148] Next, the signal processing unit 65 performs frequency shifting by digital signal processing on the mode 2 and mode 3 signals of this signal to convert them into baseband signals, and then demodulates them into the original three QPSK signal sequences by MIMO equalization processing. Fig. 20 is a diagram showing an example of a QPSK constellation obtained by the first embodiment. As shown in Fig. 20, in the evaluation, it was confirmed that the three-mode signals were received with high quality by the single receiving unit 6 in the signal detection device 10 of the first embodiment.

[0149] As described above, the signal detection device 10 (or 10A, 10B, 10C, 10D, 10E, 10F) of each embodiment includes a mode splitter 3 that converts N spatial mode signals including a plurality of higher-order modes excited in the transmission line fiber 2 into N fundamental modes and outputs the converted signals, a spatial frequency conversion unit 4 (or 4B, 4C, 4D, 4E, 4F) (frequency conversion unit) that converts the frequency of each optical signal, an N×1 optical coupler 5 (combiner) that combines the N optical signals, and a receiving unit 6 (or 6A) that performs opto-electrical conversion on the combined optical signal while retaining modulation information for phase and polarization, performs digital signal conversion and signal processing, and demodulates the combined optical signal to the original information sequence.

[0150] As a result, according to each embodiment, in a transmission system applying space division multiplexing transmission technology, signals of multiple modes can be received by a number of receiving units that is fewer than the number of modes, thereby reducing the configuration of the receiving device, thereby enabling reductions in device size, power consumption, and manufacturing costs.

[0151] In recent years, digital coherent transmission technology has enabled the realization of small, inexpensive, and low-power optical transmission modules and optical transceivers using such modules, without the use of complex phase-locked loop circuits, etc. The advent of digital coherent transmission technology has made it possible not only to improve the receiving sensitivity during optical transmission in large-capacity optical networks, but also to dramatically improve information transmission efficiency by encoding information onto the amplitude, phase, and polarization of optical carrier waves. By applying each embodiment to digital coherent transmission, the configuration of a receiving device can be reduced.

[0152] (Modification) In each of the above-described embodiments, the signal detection device 10 (or 10A, 10B, 10C, 10D, 10E, or 10F) may have a functional unit other than the configuration described above. In the above evaluation, an example using a QPSK signal has been described, but the present invention is not limited to this. The signal input to the signal detection device 10 (or 10A, 10B, 10C, 10D, 10E, or 10F) may be, for example, BPSK (Binary Phase Shift Keying), DPSK (Differential Phase Shift Keying), or the like.

[0153] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0154] The present invention is applicable to transmission systems. [Explanation of symbols]

[0155] 1, 1A, 1B, 1C...signal detection system, 10, 10A, 10B, 10C...signal detection device, 2...optical transmission fiber, 3...mode splitter, 4, 4B, 4C, 4D, 4E, 4F...spatial frequency conversion unit, 5, 5-1, ..., 5-K...N×1 optical coupler, 6, 6-1, ..., 6-K, 6A...receiving unit, 41...clock unit (first clock unit 41-1, second clock unit 41-2, ..., Nth clock unit 41-N), 42... Frequency shifters (first frequency shifter 42-1, second frequency shifter 42-2, ..., Nth frequency shifter 42-N, first frequency shifter 42-1-1, first frequency shifter 42-1-2, second frequency shifter 42-2-1, second frequency shifter 42-2-2, ..., Nth frequency shifter 42-N-1, Nth frequency shifter 42-N-2), 43, 43-1, 43-2, ..., 43-N ... polarizing beam splitters, 44, 44- 1, 44-2, ..., 44-N... polarized beam combiner, 45, 45-1, 45-2, ..., 45-N... WDM splitter, 46... frequency conversion unit (first frequency conversion unit 46-1, second frequency conversion unit 46-2, ..., Nth frequency conversion unit 46-N), 47... frequency conversion unit (first frequency conversion unit 47-1, second frequency conversion unit 47-2, ..., Nth frequency conversion unit 47-N), 61... local light receiving unit, 62... coherent receiver, 63...band splitting unit, 64...AD converter, 65...signal processing unit, 401...light source, 402...frequency comb generation unit, 403...1×N optical splitter, 404, 404-1, 404-2, ..., 404-N...optical filters, 405, 405-1, 405-2, ..., 405-N...optical amplifiers, 406, 406-1, 406-2, ..., 406-N...optical filters, 407, 407-1, 407-2, ..., 407-N...PPLN

Claims

1. a mode splitter that converts N spatial mode signals (N is a natural number) including a plurality of higher-order modes excited in a transmission fiber into N fundamental modes and outputs the converted signals; a frequency conversion unit that converts the frequencies of the N optical signals into frequencies different from each other; a multiplexing unit that multiplexes the N frequency-converted optical signals; a receiver that performs opto-electrical conversion on the combined optical signal while maintaining modulation information for phase and polarization, and demodulates the signal to the original information sequence by performing digital signal conversion and signal processing; A signal detection device comprising:

2. The receiving unit performs digital signal processing and signal processing on the photoelectrically converted signal after dividing the signal into parallel signals consisting of a plurality of signal bands.

2. The signal detection device according to claim 1.

3. The frequency conversion unit is After performing polarization processing on the input light, frequency conversion is performed for each polarization, and the signals subjected to the frequency conversion are multiplexed.

2. The signal detection device according to claim 1.

4. The frequency conversion unit is After frequency conversion, the wavelength division multiplexed signal is demultiplexed.

2. The signal detection device according to claim 1.

5. The frequency conversion unit is After the frequency conversion, the optical signal is frequency converted again, and then the wavelength multiplexed signal is demultiplexed.

2. The signal detection device according to claim 1.

6. The frequency conversion unit is The signal is split into a low-frequency branch and a high-frequency branch, and frequency conversion is performed for each branch using an optical parametric process. The wavelength-multiplexed signal is then demultiplexed.

2. The signal detection device according to claim 1.

7. The signal detection device converting N spatial mode signals (N is a natural number) including a plurality of higher-order modes excited in a transmission fiber into N fundamental modes and outputting the converted signals; converting each of the N optical signals to a different frequency; multiplexing the N frequency-converted optical signals; The combined optical signal is photoelectrically converted while maintaining the modulation information for phase and polarization, and then demodulated to the original information sequence through digital signal conversion and signal processing. Signal detection methods.

8. A program for causing a computer to function as the signal detection device according to any one of claims 1 to 6.

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