Optical Transmission System and Optical Transmission Method

Abstract

This optical transmission system is provided with: a first transmission line for transmitting, using a first multimode fiber, a first optical signal that has been mode-division multiplexed; a spectrum inversion unit for generating a second optical signal by inverting the spectrum of the first optical signal; and, a second transmission line for transmitting the second optical signal using a second multimode fiber. The spectrum inversion unit inverts the spectrum of the first optical signal by using, as an axis of symmetry, a frequency at which the inter-mode delay difference in at least one of the first transmission line and the second transmission line is equal to or less than a threshold.

Description

Technical Field

[0001] The present invention relates to an optical transmission system and an optical transmission method.

Background Art

[0002] With the start of operation of the fifth-generation mobile communication system and the spread of rich content such as high-definition moving images, communication traffic has been increasing exponentially. Further, with the increase in communication traffic, continuous capacity expansion of optical networks has been demanded.

[0003] In an optical fiber transmission network that forms the backbone of an optical network, a single-mode fiber (SMF) is used as a transmission line. In a single-mode fiber, an optical signal of a single spatial mode (single-mode component) is transmitted. On the other hand, in a multi-mode fiber (MMF), optical signals of a plurality of spatial modes are transmitted.

[0004] Each spatial mode of an optical signal can be geometrically distinguished based on the difference in the angles of the optical signals that are totally reflected at the interface between the core and the cladding of the optical fiber. Therefore, the longitudinal propagation constant of the optical fiber is different for each spatial mode. Due to this difference in the propagation constant, a difference in transmission delay occurs between the spatial mode components. The difference in transmission delay that occurs between the spatial mode components is called differential modal delay (DMD).

[0005] When a single optical signal is transmitted through a multi-mode fiber, the pulse of the optical signal spreads due to the cumulative mode dispersion according to the differential modal delay. For this reason, when a single optical signal is transmitted through a multi-mode fiber, the transmission distance of the optical signal is limited. For such reasons, when a single optical signal is transmitted over a long distance, a single-mode fiber capable of stably transmitting an optical signal of a single spatial mode is used.

[0006] In addition, digital coherent optical transmission technology was put into practical use in the 2010s. Digital coherent optical transmission technology is a technology that combines coherent transmission and reception technology with digital signal processing (DSP). In coherent transmission and reception technology, based on the interference pattern between an optical signal and local light emission, the optical intensity information and complex amplitude information of the optical signal are detected.

[0007] Digital signal processing compensates for the relative fluctuations between the optical signal and local light emission with respect to phase and frequency. As a result, stable detection of complex amplitude information and equalization of linear signal distortions such as wavelength dispersion and polarization mode dispersion are possible. In addition, narrowing of the spectral components of the optical signal is possible using Nyquist filtering.

[0008] In a high-order signal modulation scheme, symbols are arranged in the amplitude direction and phase direction of the optical signal. High-order signal modulation schemes have become adoptable due to the practical implementation of digital coherent technology. In addition, dense wavelength division multiplexing has become possible due to the narrowing of the spectral components of the optical signal. As a result, the transmission capacity has improved dramatically.

[0009] On the other hand, the transmission capacity per single-mode fiber is approaching the theoretical limit. According to Shannon's theorem, in order to increase the transmission capacity, it is necessary to improve the transmission bandwidth and the signal-to-noise ratio (SNR). In order to improve the signal-to-noise ratio, it is necessary to increase the transmission power of the optical signal. However, the greater the transmission power of the optical signal, the more non-linear optical effects are induced in the optical fiber. Non-linear optical effects cause signal distortion in the optical signal due to self-phase modulation and cross-phase modulation, and degrade the signal-to-noise ratio of the optical signal. Therefore, the transmission power of the optical signal cannot be increased indefinitely. Therefore, there exists an optimal transmission power.

[0010] In addition, the bandwidth of an optical signal transmitted over a long distance through an optical fiber is limited by the amplification bandwidth (about 4 THz) of an erbium-doped fiber amplifier (EDFA) used for optical signal amplification and relay. Therefore, research and development of optical amplifiers having an amplification bandwidth exceeding that of erbium-doped fiber amplifiers are underway.

[0011] However, when an optical signal with a certain amount of power or more is input into an optical fiber, a combustion phenomenon called fiber fuse may occur in the optical fiber. For this reason, even if the amplification bandwidth of the optical amplifier is improved, the transmission capacity is limited by the bandwidth determined by the fiber fuse threshold and the signal-to-noise ratio.

[0012] In order to break through the limit of the transmission capacity of an optical signal transmitted through a single-mode fiber, mode-division multiplexing (MDM) of an optical signal transmitted through a multi-mode fiber has been studied. Hereinafter, the mode-division multiplexed optical signal is referred to as a "mode multiplexed signal".

[0013] In mode-division multiplexing transmission, the transmission capacity is improved in proportion to the number of spatial modes. In mode-division multiplexing transmission, each spatial mode of an optical signal transmitted through a multi-mode fiber is treated as an independent transmission channel. Further, in mode-division multiplexing transmission, the cross-phase modulation effect appears strongly between channels where the inter-mode delay difference and the chromatic dispersion cancel each other out (see Non-Patent Document 1).

[0014] As a display of spatial modes in mode-division multiplexing transmission, spatial modes of linearly polarized (LP) waves may be displayed. In the display of spatial modes of linearly polarized waves, eigenmode groups (degenerate eigenmode groups) with close propagation constants are grouped together as one spatial mode. Ideally, since each spatial mode of linearly polarized waves has a spatially complex amplitude distribution that is orthogonal to each other, spatial modes of linearly polarized waves do not couple with each other during transmission. For this reason, a multimode fiber can be used like a transmission line in which single-mode fibers are arranged in parallel. In this way, in a multimode fiber, it is possible to increase the capacity in proportion to the number of spatial modes.

[0015] However, due to the generation of stress caused by bending, non-uniformity, and fusion of multimode fibers, the incompleteness of mode conversion in mode multiplexers and mode demultiplexers, and the misalignment of connector connection parts, spatial modes of linearly polarized waves may couple with spatial modes of other linearly polarized waves. When spatial modes of linearly polarized waves couple with each other, modal cross-talk (MXT), which is interference between signals modulated into different spatial modes, occurs in the mode multiplexed signal. In order to remove such modal cross-talk, the digital signal processor (digital signal processing) of a communication device that receives a mode multiplexed signal performs MIMO (multiple-input multiple-output) signal processing on the received mode multiplexed signal.

Prior Art Documents

Non-Patent Documents

[0016]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0017] In mode division multiplex transmission, MIMO signal processing is performed, a finite impulse response (FIR) filter removes the inter-mode crosstalk component, and the signal is restored. On the other hand, inter-mode crosstalk spreads on the time axis due to mode dispersion according to the inter-mode delay difference. Therefore, a digital signal processing circuit functioning as an FIR filter requires a time buffer for acquiring samples on the time axis according to the distribution of the inter-mode crosstalk component, and a multiplier for the number of acquired samples.

[0018] Since mode dispersion accumulates according to the transmission distance of the mode multiplex signal, the longer the transmission distance, the wider each sample needs to be acquired on the time axis. For this reason, a large number of time buffers and multipliers are required. Also, the greater the difference in the order of the spatial modes, the greater the inter-mode delay difference. Therefore, the larger the number of spatial modes of the optical signal transmitted in the optical transmission system, the more time buffers and multipliers are required. Thus, the increase in the scale and power consumption of the digital signal processing circuit due to mode dispersion has become one of the problems in the realization of mode division multiplex transmission and the expansion of the number of spatial modes.

[0019] Also, in mode division multiplex transmission, the non-linear optical effect between linearly polarized mode components may cause signal distortion. In transmission using a single-mode fiber, the non-linear optical effect (cross-phase modulation) occurring between different wavelength channels can be suppressed by the walk-off effect due to wavelength dispersion. On the other hand, it has been reported as described above that the cross-phase modulation effect appears strongly between channels where the inter-mode delay difference and wavelength dispersion cancel each other out in mode division multiplex transmission.

[0020] Thus, in mode division multiplexing transmission using a multimode fiber, in addition to signal distortion corresponding to the nonlinear distortion that occurs in transmission using a single mode fiber, signal distortion due to the nonlinear action between spatial modes occurs. As described above, there is a problem that the nonlinear distortion and mode dispersion generated in the mode multiplexed signal cannot be compensated for.

[0021] In view of the above circumstances, an object of the present invention is to provide an optical transmission system and an optical transmission method capable of compensating for nonlinear distortion and mode dispersion generated in a mode multiplexed signal.

Means for Solving the Problems

[0022] One aspect of the present invention is an optical transmission system including: a first transmission path that transmits a first optical signal, which is a mode division multiplexed optical signal, using a first multimode fiber; a spectrum inversion unit that generates a second optical signal by inverting the spectrum of the first optical signal; and a second transmission path that transmits the second optical signal using a second multimode fiber.

[0023] One aspect of the present invention is an optical transmission method executed by an optical transmission system, the method including: a first transmission step of transmitting a first optical signal, which is a mode division multiplexed optical signal, using a first multimode fiber; a spectrum inversion step of generating a second optical signal by inverting the spectrum of the first optical signal; and a second transmission step of transmitting the second optical signal using a second multimode fiber.

Effects of the Invention

[0024] According to the present invention, it is possible to compensate for nonlinear distortion and mode dispersion generated in a mode multiplexed signal.

Brief Description of the Drawings

[0025]

Figure 1

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Figure 9

Embodiments for Carrying Out the Invention

[0026] (Overview) The spectral inversion section (amplification relay section) of the optical transmission system inverts the spectrum of the optical signal (mode multiplexed signal) to generate the phase conjugate light of the optical signal. Thereby, the optical transmission system compensates for the non-linear distortion generated in the mode multiplexed signal due to the non-linear optical effect during transmission. Further, the optical transmission system compensates for the mode dispersion generated in the mode multiplexed signal due to the inter-mode delay difference.

[0027] The method by which the spectrum inversion section inverts the spectrum is not limited to a specific method. For example, the spectrum inversion section may use optical parametric amplification (OPA) using a highly nonlinear medium (Reference 1: T. Umeki et al., “Simultaneous nonlinearity mitigation in 92 × 180-Gbit / s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation,” Optics Express, 24, 15, 16945-16951 (2016).) to invert the spectrum of the mode multiplexed signal. For example, the spectrum inversion section may utilize the mutual gain modulation effect in a semiconductor optical amplifier to invert the spectrum of the mode multiplexed signal.

[0028] Optical parametric amplification is one of the nonlinear optical processes. In optical parametric amplification, both the pump light and the optical signal are input into the nonlinear medium, and the input optical signal is amplified. The nonlinear medium is, for example, a third-order nonlinear medium such as a highly nonlinear optical fiber. The nonlinear medium may also be a second-order nonlinear medium such as lithium niobate.

[0029] In the process of optical parametric amplification, conjugate light is generated as the optical intensity of the mode multiplexed signal is amplified. This conjugate light is called idler light. The conjugate light is generated with the center frequency (center wavelength) of the amplification band of optical parametric amplification as the axis of symmetry. That is, the spectrum of the conjugate light has a shape in which the spectrum of the input optical signal is inverted with the center frequency of the amplification band of optical parametric amplification as the boundary.

[0030] The conjugate light is a replica of the input mode multiplexed signal, except that it is the phase conjugate of the input mode multiplexed signal. Therefore, in order for the input mode multiplexed signal to be relayed, one of the input mode multiplexed signal and the generated conjugate light may be extracted.

[0031] By the spectral inversion section performing spectral inversion and extraction of phase-conjugated light, it is possible to compensate for phase noise due to non-linear optical effects such as self-phase modulation and cross-phase modulation through processes (1) to (4) below, for example.

[0032] (1) In the transmission path of the mode multiplexed signal, phase rotation occurs in the mode multiplexed signal due to the non-linear optical effect of the transmission medium (optical fiber). (2) The spectral inversion section converts (phase-conjugate converts) the mode multiplexed signal input from the transmission path in the previous stage of the spectral inversion section into phase-conjugated light. The sign (positive or negative) of the phase rotation of the input mode multiplexed signal is different from the sign of the phase rotation of the converted phase-conjugated light. (3) The transmission path in the subsequent stage of the spectral inversion section transmits the phase-conjugated light. In the transmission path of the phase-conjugated light, phase rotation occurs in the phase-conjugated light due to the non-linear optical effect, similar to the transmission path of the mode multiplexed signal input to the spectral inversion section. (4) The sign of the phase rotation that occurs in the transmission path in the previous stage (before phase-conjugate conversion) of the spectral inversion section is different from the sign of the phase rotation that occurs in the transmission path in the subsequent stage (after phase-conjugate conversion) of the spectral inversion section. Therefore, in the transmission path in the subsequent stage of the spectral inversion section, these phase rotations cancel each other out.

[0033] In this way, by the optical transmission system utilizing the spectral inversion effect of optical parametric amplification, it is possible to compensate for the non-linear phase noise generated in the mode multiplexed signal. This non-linear phase noise also includes noise due to the cross-phase modulation effect occurring between spatial modes. Since the non-linear phase noise is compensated, it becomes possible to increase the transmission power of the mode multiplexed signal. Also, the signal-to-noise ratio of the mode multiplexed signal is improved.

[0034] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) FIG. 1 is a diagram showing a configuration example of an optical transmission system 1. The optical transmission system 1 is a system that transmits an optical signal (mode multiplexed signal). The optical transmission system 1 includes a transmission unit 2, a plurality of optical relay units 3, a plurality of optical transmission paths 4, one or more spectral inversion units 5a, and a reception unit 6. For example, the optical transmission path 4-1, the spectral inversion unit 5a-1, and the optical transmission path 4-2 constitute one set. The optical transmission system 1 may include a plurality of such sets. Such a plurality of sets are connected in series in the optical transmission system 1.

[0035] In FIG. 1, the optical transmission path 4-1 (first transmission path) includes one or more optical relay units 3-1 that amplify and relay a mode multiplexed signal (first optical signal). The optical transmission path 4-2 (second transmission path) includes one or more optical relay units 3-2 that amplify and relay a new mode multiplexed signal (second optical signal) that is phase conjugate light. In order to improve the signal-to-noise ratio, amplification and relaying of the optical signal a predetermined number of times are performed in each transmission path before and after the spectral inversion unit 5a. For example, the number of optical relay units 3-1 in the optical transmission path 4-1 before the spectral inversion unit 5a-1 is equal to the number of optical relay units 3-2 in the optical transmission path 4-2 after the spectral inversion unit 5a-1.

[0036] In FIG. 1, the process of generating phase conjugate light of the mode multiplexed signal is executed, for example, every two spans (for example, two optical transmission paths 4). In the optical transmission system 1 where the optical phase conjugation (OPC: Optical Phase Conjugation) process is executed every two spans, the span length before the optical phase conjugation process and the span length after the optical phase conjugation process are equal, and the number of amplification and relaying times before the optical phase conjugation process and the number of amplification and relaying times after the optical phase conjugation process are equal. Therefore, the number of times the optical phase conjugation process (the process of generating phase conjugate light) is executed is not limited to a specific number.

[0037] For example, when six times of optical amplification relay is performed in the optical transmission system 1, the optical phase conjugation process may be performed every three spans. Even when it is performed every three spans, it is possible to make the distance (span length) of the transmission line in the front stage of the optical phase conjugation process equal to the distance (span length) of the transmission line in the rear stage of the optical phase conjugation process.

[0038] On the other hand, when the optical transmission system 1 is applied to an optical network where the merging (add) and separation (drop) of mode multiplex signals are performed, each mode multiplex signal passing through various paths in the optical network is merged and separated. For this reason, it is desirable that many spectral inversion parts 5a are provided in the optical transmission system 1 and mode dispersion is compensated in a short span.

[0039] Also, in FIG. 1, since the gain of the optical parametric amplification performed by the spectral inversion part 5a is sufficiently high, the amplification function of the spectral inversion part 5a also serves as the amplification function of the optical relay part 3 (signal amplification part). If the gain of the optical parametric amplification performed by the spectral inversion part 5a is not sufficient, the spectral inversion part 5a may be provided with the optical relay part 3.

[0040] The optical relay part 3, the optical transmission line 4, and the spectral inversion part 5a are provided in the section from the transmission part 2 to the reception part 6. In FIG. 1, the optical relay part 3-1 is connected to the output end of the transmission part 2. The optical transmission line 4-1 is connected to the output end of the optical relay part 3-1 and the input end of the spectral inversion part 5a-1. The optical transmission line 4-2 is connected to the output end of the spectral inversion part 5a-1 and the input end of the optical relay part 3-2. The optical transmission line 4-3 is connected to the output end of the optical relay part 3-2 and the input end of the spectral inversion part 5a-2.

[0041] The optical transmission path 4-4 is connected to the output end of the spectral inversion section 5a-2 and the input end of the optical relay section 3-3. The optical transmission path 4-5 is connected to the output end of the optical relay section 3-3 and the input end of the spectral inversion section 5a-3. The optical transmission path 4-6 is connected to the output end of the spectral inversion section 5a-3 and the input end of the optical relay section 3-4. The optical relay section 3-4 is connected to the input end of the receiving section 6.

[0042] The transmitting section 2 is a communication device on the transmitting side. The transmitting section 2 generates a mode multiplexed signal. In the mode multiplexed signal, optical signals of a plurality of spatial modes (wavelengths) are multiplexed. The transmitting section 2 transmits the mode multiplexed signal to the optical relay section 3-1.

[0043] The optical relay section 3 is a functional section that relays optical signals. The optical relay section 3 compensates for the loss of the mode multiplexed signal transmitted through the multimode fiber by performing an amplification process on the mode multiplexed signal. The optical relay section 3 compensates for the loss of the phase conjugate light transmitted through the multimode fiber by performing an amplification process on the phase conjugate light of the mode multiplexed signal.

[0044] The optical transmission path 4 has a multimode fiber. The optical transmission path 4 in the front stage of the spectral inversion section 5a transmits the mode multiplexed signal using the multimode fiber. The optical transmission path 4 in the rear stage of the spectral inversion section 5a transmits the phase conjugate light generated by the spectral inversion section 5a using the multimode fiber.

[0045] The spectral inversion section 5a performs spectral inversion processing on the mode multiplexed signal transmitted through the optical transmission path 4 in the front stage of the spectral inversion section 5a, with a frequency of half of the excitation light frequency as the axis of symmetry (boundary). The spectral inversion section 5a performs spectral inversion processing by utilizing, for example, the spectral inversion effect of optical parametric amplification. Thereby, the spectral inversion section 5a generates the phase conjugate light of the mode multiplexed signal.

[0046] The receiving unit 6 is a communication device on the receiving side. The receiving unit 6 receives the phase-conjugated light of the mode multiplexed signal from the optical relay unit 3-4. The receiving unit 6 performs predetermined reception processing on the phase-conjugated light of the mode multiplexed signal.

[0047] FIG. 2 is a diagram showing an example of the frequency arrangement of the optical signal before spectral inversion and the optical signal after spectral inversion in the first embodiment. Each spectral component 10 is an optical signal (original optical signal) before spectral inversion. Each spectral component 11 is an optical signal after spectral inversion.

[0048] The wavelength-multiplexed optical signal input to the spectral inversion unit 5a-1 is a mode multiplexed signal. The mode multiplexed signal output from the optical relay unit 3-1 is transmitted through the optical transmission line 4-1 (first transmission line). The center frequency of the mode multiplexed signal transmitted through the optical transmission line 4-1 is, for example, the frequency “f0 - f1” of the spectral component 10-3. Also, the center frequency “f0” of the spectral inversion is half of the frequency of the pump light used for optical parametric amplification.

[0049] In the optical transmission line 4-1, the mode multiplexed signal is transmitted through the optical transmission line 4-1 while receiving phase noise due to a non-linear optical effect including the interaction between spatial modes. The mode multiplexed signal transmitted through the optical transmission line 4-1 is input to the spectral inversion unit 5a-1. The spectral inversion unit 5a-1 performs optical parametric amplification on the mode multiplexed signal. As a result, the optical intensity of the mode multiplexed signal is amplified. Also, the spectrum of the mode multiplexed signal is inverted with the center frequency “f0” as the axis of symmetry.

[0050] When a second-order non-linear medium is used in optical parametric amplification, the frequency of the pump light used for optical parametric amplification is “2f0”. The center frequency of the mode multiplexed signal (wavelength-multiplexed signal) whose spectrum has been inverted by the spectral inversion unit 5a-1 is the frequency “f0 + f1” of the spectral component 11-3. The mode multiplexed signal (phase-conjugated light) whose spectrum has been inverted by the spectral inversion unit 5a-1 is transmitted through the optical transmission line 4-2 (second transmission line).

[0051] In the optical transmission line 4-2, the sign of the phase noise generated in the mode multiplexed signal due to the non-linear optical effect is opposite to the sign of the phase noise of the mode multiplexed signal whose spectrum is inverted by the spectrum inversion section 5a-1. Therefore, the phase noise of the mode multiplexed signal whose spectrum is inverted by the spectrum inversion section 5a-1 and the phase noise generated in the mode multiplexed signal in the optical transmission line 4-2 cancel each other out. In such a state where the phase noise is compensated, the mode multiplexed signal is input to the optical relay section 3-2. These processes (the processes from (1) to (4) above) form one set.

[0052] In this way, by performing spectrum inversion by optical parametric amplification on the mode multiplexed signal, it is possible to compensate for the phase noise generated in the mode multiplexed signal due to the non-linear optical effect.

[0053] In FIG. 1, the processes from (1) to (4) above (one set) are executed in the section from one optical relay section 3 before the spectrum inversion section 5a to one optical relay section 3 after the spectrum inversion section 5a. That is, two optical relay sections 3 are used as an example in the processes from (1) to (4) above. The number of optical relay sections 3 does not have to be limited to a specific number, and more optical relay sections 3 may be used in the processes from (1) to (4) above.

[0054] FIG. 3 is a diagram showing a configuration example of the spectrum inversion section 5a in the first embodiment. The spectrum inversion section 5a includes a mode demultiplexing section 50, N polarization demultiplexing sections 51, 2N optical amplification sections 52, N polarization multiplexing sections 53, N band-pass filters 54, and a mode multiplexing section 55. The optical amplification section 52 has a non-linear medium (non-linear optical medium). "N" is an integer of 2 or more and is the number of spatial modes handled by the spectrum inversion section 5a. In FIG. 3, "N" is 3 as an example.

[0055] The efficiency of optical parametric amplification increases when the phase matching condition is satisfied in a nonlinear medium. Therefore, single-mode excitation is often performed in the nonlinear medium so that the guided light can be represented using a single propagation constant.

[0056] In FIG. 3, the mode demultiplexer 50 demultiplexes (separates) the components of each spatial mode of the optical signal from the mode multiplexed signal. The mode demultiplexer 50 converts the components of each spatial mode of the optical signal into each optical signal of the fundamental mode. Also, generally, nonlinear optical effects including the process of optical parametric amplification have polarization dependence. Therefore, when the optical amplifier 52 performs optical parametric amplification on all polarization components in the input optical signal, the polarization demultiplexer 51 demultiplexes the input optical signal into a first polarization component and a second polarization component. Here, the first polarization component and the second polarization component are orthogonal to each other. The polarization demultiplexer 51 demultiplexes the optical signal into a first polarization component and a second polarization component using, for example, a polarization beam splitter.

[0057] An excitation light is input to the nonlinear medium of the optical amplifier 52. In the spectral inversion unit 5a, the optical amplifier 52 performs spectral inversion for each demultiplexed polarization component. The optical amplifier 52 performs spectral inversion using, for example, the spectral inversion effect of optical parametric amplification.

[0058] The first polarization component is input from the polarization demultiplexer 51 to the nonlinear medium of the optical amplifier 52-1-n (n is an integer of 1 or more). The second polarization component is input from the polarization demultiplexer 51 to the nonlinear medium of the optical amplifier 52-1-(n + 1). The same applies to the optical amplifier 52-2 and the optical amplifier 52-3.

[0059] The optical amplifier 52 multiplexes the input polarization component and the excitation light using, for example, a dichroic mirror. The optical amplifier 52 may multiplex the input polarization component and the excitation light using, for example, a wavelength division multiplexer.

[0060] The polarization multiplexing section 53 multiplexes each polarization component using a polarization beam combiner or the like. The band-pass filter 54 passes the optical signal in the spectrally inverted band among the optical signals of the multiplexed polarization components. In this way, the band-pass filter 54 removes the optical signals in the band that are not spectrally inverted (components other than the idler light) from the multiplexed polarization components. That is, the band-pass filter 54 extracts the optical signal (phase-conjugated light) in the band whose spectrum is inverted from the multiplexed polarization components.

[0061] In this way, the spatial modes of the mode multiplexed signal are demultiplexed before polarization splitting and before recombining. Also, the spatial modes of the mode multiplexed signal are multiplexed after polarization splitting and after recombining. As a result, it is possible to collectively apply spectral inversion to all spatial mode components and polarization components of the input mode multiplexed signal.

[0062] Next, an operation example of the optical transmission system 1 will be described. FIG. 4 is a flowchart showing an operation example of the optical transmission system in the first embodiment. The optical transmission path 4 in front of the spectral inversion section 5a transmits a mode multiplexed signal (first optical signal) to the spectral inversion section 5a using a first multimode fiber (step S101). The spectral inversion section 5a generates a new mode multiplexed signal (second optical signal) that is phase-conjugated light by inverting the spectrum of the mode multiplexed signal (step S102). The optical transmission path 4 behind the spectral inversion section 5a transmits the new mode multiplexed signal (second optical signal) that is phase-conjugated light using a second multimode fiber (step S103).

[0063] Note that the characteristics of the first multimode fiber and the characteristics of the second multimode fiber may be substantially the same. The length of the first multimode fiber and the length of the second multimode fiber may be substantially the same.

[0064] As described above, the transmission line (first transmission line) before the spectrum inversion section 5a transmits the mode multiplexed signal (first optical signal) using the first multimode fiber. The spectrum inversion section 5a generates a new mode multiplexed signal (second optical signal) which is phase conjugate light by inverting the spectrum of the mode multiplexed signal. The transmission line (second transmission line) after the spectrum inversion section 5a transmits the phase conjugate light of the mode multiplexed signal using the second multimode fiber. A plurality of sets including the first transmission line, the spectrum inversion section, and the second transmission line may be connected in series.

[0065] Thereby, it is possible to compensate for the nonlinear distortion and mode dispersion generated in the mode multiplexed signal. By inverting the spectrum of the mode multiplexed signal by the spectrum inversion section, it is possible to compensate for the nonlinear distortion and mode dispersion caused by the nonlinear optical effect generated in the mode multiplexed signal during transmission. It is possible to improve the signal-to-noise ratio at the time of signal reception and reduce the circuit scale and power consumption required for the digital signal processing section of the receiving section. The nonlinear distortion generated in the mode multiplexed signal is compensated. Since the nonlinear distortion is compensated, the signal-to-noise ratio of the optical signal is improved.

[0066] (Second Embodiment) The point that optical parametric amplification is performed in which the frequency at which the inter-mode delay difference becomes near 0 becomes the center frequency (symmetry axis) of the amplification band is the difference from the first embodiment. In the second embodiment, the description will be centered on the difference from the first embodiment.

[0067] The inter-mode delay difference has a dependence that is nearly linear with respect to frequency (wavelength). In the second embodiment, the spectrum inversion section 5a inverts the spectrum using optical parametric amplification in which the frequency at which the inter-mode delay difference becomes near 0 becomes the center frequency (symmetry axis) of the amplification band. Around the frequency at which the inter-mode delay difference becomes 0, the sign of the inter-mode delay difference is inverted.

[0068] The mode dispersion that occurs in the mode multiplexed signal in the transmission line before the spectrum is inverted cancels out the mode dispersion that occurs in the mode multiplexed signal in the transmission line after the spectrum is inverted. By inverting the spectrum of the mode multiplexed signal in this way at the spectrum inversion section, the mode dispersion is reduced. Also, it is possible to reduce the scale and power consumption of the digital signal processing circuit required for MIMO signal processing.

[0069] FIG. 5 is a diagram (mode dispersion map) showing an example of changes in the amount of mode dispersion in the second embodiment. "L0" represents the distance from the transmission unit 2 to the optical relay unit 3-1. In other words, "L0" represents the position of the optical relay unit 3-1. "L1" represents the distance from the transmission unit 2 to the spectrum inversion unit 5a-1. In other words, "L1" represents the position of the spectrum inversion unit 5a-1.

[0070] "L2" represents the distance from the transmission unit 2 to the optical relay unit 3-2. In other words, "L2" represents the position of the optical relay unit 3-2. "L3" represents the distance from the transmission unit 2 to the spectrum inversion unit 5a-2. In other words, "L3" represents the position of the spectrum inversion unit 5a-2. "L4" represents the distance from the transmission unit 2 to the optical relay unit 3-3. In other words, "L4" represents the position of the optical relay unit 3-3.

[0071] "L5" represents the distance from the transmission unit 2 to the spectrum inversion unit 5a-3. In other words, "L5" represents the position of the spectrum inversion unit 5a-3. "L6" represents the distance from the transmission unit 2 to the optical relay unit 3-4. In other words, "L6" represents the position of the optical relay unit 3-4.

[0072] Optical transmission paths 4 (transmission paths having multimode fibers) with the same length and the same characteristics are respectively arranged in front of and behind the spectral inversion section 5a that performs optical parametric amplification. That is, the difference between the distance "L1" and the distance "L0" is equal to the difference between the distance "L2" and the distance "L1". The difference between the distance "L3" and the distance "L2" is equal to the difference between the distance "L4" and the distance "L3". Also, the difference between the distance "L5" and the distance "L4" is equal to the difference between the distance "L6" and the distance "L5". Note that the criterion for determining whether they have the same length is predetermined. Also, the criterion for determining whether they have the same characteristics is predetermined.

[0073] As shown in FIG. 5, when the mode multiplexed signal is input to the spectral inversion section 5a, the mode dispersion amount of the mode multiplexed signal is the largest. When the mode multiplexed signal is input to the optical relay section 3, the mode dispersion amount of the mode multiplexed signal is the smallest.

[0074] FIG. 6 is a diagram showing an example of the inter-mode delay difference according to the frequency of the optical signal in the second embodiment. As illustrated in FIG. 6, the inter-mode delay difference has a dependency that is nearly linear with respect to the frequency (wavelength). The optical transmission path 4 having a multimode fiber transmits a mode multiplexed signal with a center frequency "f0 - f1". The optical transmission path 4-1 (first transmission path) transmits the mode multiplexed signal affected by the inter-mode delay difference and the inter-mode crosstalk to the spectral inversion section 5a-1.

[0075] The spectral inversion section 5a-1 amplifies the mode multiplexed signal by optical parametric amplification and inverts the spectrum of the mode multiplexed signal. The frequency of the pump light is determined to be twice the center frequency "f0" of the spectral inversion (= 2f0).

[0076] The center frequency “f0” is the frequency at which the inter-mode delay difference in at least one of the transmission lines before and after the spectrum inversion section 5a is near 0 (below the threshold value). For example, the spectrum inversion section 5a executes spectrum inversion processing on the mode multiplexed signal transmitted through the optical transmission line 4 before the spectrum inversion section 5a, with the frequency “f0” at which the inter-mode delay difference is near 0 as the axis of symmetry (boundary). The spectrum inversion section 5a executes spectrum inversion processing by utilizing the spectrum inversion effect of optical parametric amplification.

[0077] The center frequency of the mode multiplexed signal (wavelength multiplexed signal) whose spectrum has been inverted by the spectrum inversion section 5a is the frequency “f0 + f1”. The mode multiplexed signal (phase conjugate light) whose spectrum has been inverted by the spectrum inversion section 5a is transmitted through the optical transmission line 4 (second transmission line) after the spectrum inversion section 5a.

[0078] The sign of the inter-mode delay difference that occurs in the optical transmission line 4 after the spectrum inversion section 5a is inverted with respect to the sign of the inter-mode delay difference that occurs in the optical transmission line 4 before the spectrum inversion section 5a. Therefore, when the phase conjugate light is transmitted through the optical transmission line 4 after the spectrum inversion section 5a, mode dispersion is compensated.

[0079] As described above, the characteristics of the first transmission line and the second transmission line are substantially the same. The lengths of the first transmission line and the second transmission line are substantially the same. The spectrum inversion section 5a inverts the spectrum of the first optical signal input to the spectrum inversion section 5a, with the frequency at which the inter-mode delay difference in at least one of the first transmission line and the second transmission line is below the threshold value (near 0) as the axis of symmetry.

[0080] Thereby, it is possible to compensate for the non-linear distortion generated in the mode multiplexed signal. Also, it is possible to improve the efficiency of compensating for the mode dispersion generated in the mode multiplexed signal.

[0081] (Third Embodiment) In the third embodiment, the difference from the first embodiment is that the optical relay section includes a spectral inversion section. In the third embodiment, the description will focus on the differences from the first embodiment.

[0082] FIG. 7 is a diagram showing a configuration example of the spectral inversion section 5b in the third embodiment. The spectral inversion section 5b includes a mode demultiplexing section 50, N polarization demultiplexing sections 51, 2N optical amplification sections 52, N polarization multiplexing sections 53, N band-pass filters 54, a mode multiplexing section 55, and N optical amplification relay sections 56. The amplification function of the N optical amplification relay sections 56 corresponds to the amplification function of the optical relay section 3.

[0083] In mode division multiplex transmission, amplification relay may not be performed on the mode multiplexed signal (a signal in a multimode state). For example, the mode demultiplexing section 50 demultiplexes (separates) each spatial mode component of the mode multiplexed signal into a single mode component that is a component of the fundamental mode (an optical signal of a single spatial mode). The optical amplification relay section 56 performs an amplification process on the single mode component. The mode multiplexing section 55 multiplexes the amplified single mode components.

[0084] In such a case, re-executing the spatial mode demultiplexing and multiplexing to perform spectral inversion causes an increase in the number of components and excessive optical loss. Therefore, the optical relay section 3 and the spectral inversion section 5b may be integrated. That is, as shown in FIG. 7, the spectral inversion section 5b may include N polarization multiplexing sections 53. The spectral inversion section 5b performs spectral inversion processing in one-time mode demultiplexing and mode multiplexing.

[0085] As described above, the mode demultiplexer 50 separates the mode multiplexed signal (first optical signal) into components of spatial modes. The mode demultiplexer 50 converts the separated components of spatial modes into components of the fundamental mode. The optical amplifier 52 inverts the spectrum of the converted components of the fundamental mode. The band-pass filter 54 converts the components of the fundamental mode with the inverted spectrum into components of the separated spatial modes. The mode multiplexer 55 multiplexes the converted components of spatial modes as phase conjugate light into a new mode multiplexed signal (second optical signal).

[0086] By integrating the optical relay section 3 (amplifying relay section) and the spectrum inversion section 5b, it is possible to suppress an increase in the number of components and compensate for the non-linear distortion and mode dispersion generated in the mode multiplexed signal. Also, it is possible to suppress optical loss and compensate for the non-linear distortion and mode dispersion generated in the mode multiplexed signal.

[0087] (Fourth Embodiment) In the fourth embodiment, the difference from the first embodiment is that the optical transmission system includes a complementary spectrum inversion section. In the fourth embodiment, the description will focus on the differences from the first embodiment.

[0088] Since an optical signal cannot be arranged in the band where phase conjugate light (idler light) is generated in spectrum inversion, the frequency utilization efficiency is halved. Therefore, in the fourth embodiment, for the purpose of not reducing the frequency utilization efficiency, the optical transmission system 1 includes a complementary spectrum inversion section.

[0089] FIG. 8 is a diagram showing a configuration example of the spectrum inversion section 5c (complementary spectrum inversion section) per component of a single spatial mode in the fourth embodiment. The spectrum inversion section 5c per component of a single spatial mode includes two polarization demultiplexers 51, four optical amplifiers 52, two polarization multiplexers 53, two band-pass filters 54, a mode multiplexer 55, a band demultiplexer 57, and a band multiplexer 58.

[0090] A single spatial mode component is input to the band demultiplexer 57, for example, from the mode demultiplexer 50. The band demultiplexer 57 divides the band of the single spatial mode component into a first band and a second band with respect to the center frequency "f0" of the optical parametric amplification by the optical amplifier 52 as a boundary.

[0091] The optical amplifier 52-1 performs spectral inversion for the first band. The optical amplifier 52-2 performs spectral inversion for the second band. The band-pass filter 54 extracts the optical signal (phase conjugate light) in the band whose spectrum has been inverted from the multiplexed polarization components. The band multiplexer 58 multiplexes the single spatial mode component in the first band and the single spatial mode component in the second band. The band multiplexer 58 outputs the multiplexed single spatial mode component to the mode multiplexer 55.

[0092] Note that the spectral inversion unit 5c may have a configuration similar to the configuration of the complementary spectral inversion unit shown in the above reference 1.

[0093] As described above, the mode demultiplexer 50 separates the mode multiplexed signal (first optical signal) into the components of each spatial mode. The mode demultiplexer 50 converts the separated spatial mode components into the fundamental mode components (optical signals of a single spatial mode). The band demultiplexer 57 divides the band of the fundamental mode components into a first band and a second band with respect to the center frequency "f0" of the optical parametric amplification by the optical amplifier 52 as a boundary. The optical amplifier 52 inverts the spectrum for the first band of the converted fundamental mode components. The optical amplifier 52 inverts the spectrum for the second band of the converted fundamental mode components. The band multiplexer 58 multiplexes the fundamental mode components in the first band and the fundamental mode components in the second band. The mode multiplexer 55 converts the multiplexed fundamental mode components (each single mode component) into the spatial mode components. The mode multiplexer 55 multiplexes the converted spatial mode components as a new mode multiplexed signal (second optical signal) which is phase conjugate light.

[0094] This makes it possible to suppress a decrease in the utilization efficiency of the frequency and perform spectral inversion of all signal bands of the mode multiplexed signal.

[0095] (Hardware configuration example) FIG. 9 is a diagram showing a hardware configuration example of a communication device (transmission unit) (reception unit) in each embodiment. The communication device 100 corresponds to at least one of the transmission unit and the reception unit in each embodiment. The communication device 100 generates or processes data transmitted using an optical signal. Some or all of the functional units of the communication device 100 are realized as software by a processor 101 such as a CPU (Central Processing Unit) executing a program stored in a storage device 102 having a non-volatile recording medium (non-temporary recording medium) and a memory 103. The program may be recorded on a computer-readable non-temporary recording medium. A computer-readable non-temporary recording medium is, for example, a non-temporary recording medium such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), a CD-ROM (Compact Disc Read Only Memory) and other portable media, and a storage device such as a hard disk built into a computer system. The communication unit 104 executes predetermined communication processing. The communication unit 104 may acquire data and programs.

[0096] Some or all of the functional units of the communication device 100 may be realized using hardware 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).

[0097] As described above in detail with reference to the drawings for the embodiments of the present invention, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Industrial Applicability

[0098] The present invention is applicable to an optical transmission system (optical communication system).

Explanation of Signs

[0099] 1... optical transmission system, 2... transmission unit, 3... optical relay unit, 4... optical transmission path, 5a, 5b, 5c... spectral inversion unit, 6... reception unit, 10... spectral component, 11... spectral component, 50... mode demultiplexer, 51... polarization demultiplexer, 52... optical amplifier, 53... polarization multiplexer, 54... bandpass filter, 55... mode multiplexer, 56... optical amplification relay unit, 57... band demultiplexer, 58... band multiplexer, 100... communication device, 101... processor, 102... storage device, 103... memory, 104... communication unit

Claims

1. A first transmission path for transmitting a first optical signal, which is a mode-division multiplexed optical signal, using a first multimode fiber, A spectrum inversion unit that generates a second optical signal by inverting the spectrum of the first optical signal after transmission through the first transmission path, A second transmission path for transmitting the second optical signal using a second multimode fiber and comprising: The spectrum inversion unit separates the first optical signal into components of spatial modes, converts the separated components of the spatial modes into components of the fundamental mode, inverts the spectrum of the converted components of the fundamental mode, converts the components of the fundamental mode with the inverted spectrum into components of the spatial modes, and multiplexes the converted components of the spatial modes as the second optical signal. An optical transmission system.

2. A first transmission path for transmitting a first optical signal, which is a mode-division multiplexed optical signal, using a first multimode fiber, A spectrum inversion unit that generates a second optical signal by inverting the spectrum of the first optical signal after transmission through the first transmission path, A second transmission path for transmitting the second optical signal using a second multimode fiber and comprising: The spectrum inversion unit separates the first optical signal into components of spatial modes, converts the separated components of the spatial modes into components of the fundamental mode, inverts the spectrum for the first band of the converted components of the fundamental mode, inverts the spectrum for the second band of the converted components of the fundamental mode, multiplexes the components of the fundamental mode in the first band and the components of the fundamental mode in the second band, converts the multiplexed components of the fundamental mode into components of the spatial modes, and multiplexes the converted components of the spatial modes as the second optical signal. An optical transmission system.

3. The spectrum inversion unit inverts the spectrum of the first optical signal with a frequency at which the inter-mode delay difference in at least one of the first transmission path and the second transmission path is equal to or less than a threshold value as the axis of symmetry. The optical transmission system according to Claim 1 or Claim 2.

4. The first transmission path includes one or more first optical relay units that amplify and relay the first optical signal. The second transmission path includes one or more second optical relay units that amplify and relay the second optical signal. The number of the first optical relay units is equal to the number of the second optical relay units. The optical transmission system according to any one of Claims 1 to 3.

5. An optical transmission method executed by an optical transmission system, a first transmission step of transmitting a first optical signal, which is a mode-division multiplexed optical signal, using a first multimode fiber; a spectrum inversion step of generating a second optical signal by inverting the spectrum of the first optical signal after transmission in the first transmission step; a second transmission step of transmitting the second optical signal using a second multimode fiber and including: in the spectrum inversion step, separating the first optical signal into components of spatial modes, converting the separated components of the spatial modes into components of a fundamental mode, inverting the spectrum of the converted components of the fundamental mode, converting the components of the fundamental mode with the inverted spectrum into components of the spatial modes, and multiplexing the converted components of the spatial modes as the second optical signal; An optical transmission method.

6. An optical transmission method executed by an optical transmission system, a first transmission step of transmitting a first optical signal, which is a mode-division multiplexed optical signal, using a first multimode fiber; a spectrum inversion step of generating a second optical signal by inverting the spectrum of the first optical signal after transmission in the first transmission step; a second transmission step of transmitting the second optical signal using a second multimode fiber and including: in the spectrum inversion step, separating the first optical signal into components of spatial modes, converting the separated components of the spatial modes into components of a fundamental mode, inverting the spectrum for a first band of the converted components of the fundamental mode, inverting the spectrum for a second band of the converted components of the fundamental mode, multiplexing the components of the fundamental mode in the first band and the components of the fundamental mode in the second band, converting the multiplexed components of the fundamental mode into components of the spatial modes, and multiplexing the converted components of the spatial modes as the second optical signal; An optical transmission method.

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