Optical transmission system, optical receiving device, and optical transmission method

Abstract

A light transmission device according to the present invention comprises light output units, branching parts, and light modulation parts. The light output units output light having a single wavelength. The branching parts branch the outputted light into light for signal generation and locally emitted light. The light modulation parts modulate the light for signal generation into signal light that has been polarized and multiplexed. A light reception device according to the present invention comprises light detection units, signal processing units, and polarization control units. The light detection units each coherently detect signal light transmitted through a light transmission path by using locally emitted light transmitted through a light transmission path that differs from the light transmission path through which the signal light has been transmitted. On the basis of the signals obtained through the coherent detection, the signal processing units generate locally emitted light polarization control information for compensating for variations in polarization of locally emitted light during the transmission. The polarization control units use the locally emitted light polarization control information to control the polarization of the locally emitted light used by the respective light detection units in the coherent detection.

Description

[Technical Field]

[0001] The present invention relates to an optical transmission system, an optical receiving device, and an optical transmission method. [Background technology]

[0002] There is an optical transmission system that uses a self-homodyne method (see, for example, Non-Patent Document 1). The self-homodyne method is a method in which an optical transmitter transmits signal light and local light output from the same laser to an optical receiver via different optical transmission paths (for example, optical fibers), and performs coherent detection using the signal light and local light received by the optical receiver.

[0003] FIG. 6 is a diagram showing an example of a self-homodyne optical transmission system. In an optical transmitting device, light generated by an optical output unit is branched into two, one of which is output to an optical modulation unit and the other to an optical transmission line as local light. The optical modulation unit generates polarization division multiplexed signal light by modulating the light generated by the optical output unit using an electrical signal input from a digital signal processing unit. The optical modulation unit outputs the generated signal light to an optical transmission line different from the local light. An optical receiving device receives the signal light and the local light. The optical detection unit performs homodyne detection of the signal light using the local light whose polarization is controlled by the polarization control unit. The digital signal processing unit demodulates the homodyne-detected signal light.

[0004] The polarization control unit performs polarization control of the local oscillator signal as follows: The optoelectric conversion unit receives a portion of the local oscillator light that has been polarization-controlled by the polarization control device unit and converts the received local oscillator light into an electrical signal. The ADC unit converts the converted electrical signal from an analog signal to a digital signal. The digital signal processing unit generates local oscillator light polarization control information based on the converted digital signal. The local oscillator light polarization control information is information used to compensate for polarization rotation and polarization mode dispersion that the local oscillator light experiences when transmitting through the optical transmission line. The ADC unit converts the local oscillator light polarization control information from a digital signal to an analog signal. The electrical signal amplifier unit amplifies the local oscillator light polarization control information. The polarization control device unit uses the amplified local oscillator light polarization control information to perform polarization control on the local oscillator signal received from the optical transmitting device and outputs the control result to the optical detection unit.

[0005] In the self-homodyne system, the signal light and local light have the same center frequency and reference phase, which makes it possible to suppress the effects of laser phase noise and eliminate frequency offset. Therefore, compared to methods that perform coherent detection using local light installed in an optical receiving device, the self-homodyne system offers the following advantages. One is that it is possible to use a laser with a wide linewidth and low frequency accuracy as the optical output section of the optical receiving device. The other is that it simplifies the signal processing performed by the digital signal processing section for phase noise compensation and frequency offset compensation after coherent detection is performed in the optical detection section of the optical receiving device. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] T Gui, et al., “Real-Time Demonstration of Homodyne Coherent Bidirectional Transmission for Next-Generation Data Center Interconnects,” Journal of Lightwave Technology, Vol.39, No.4, pp.1231-1238, 2021. Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, in order to perform coherent detection, it is necessary to control the polarization of the local light transmitted from the transmitting side in the optical receiving device. The self-homodyne method shown in Non-Patent Document 1 requires an opto-electrical conversion unit, an ADC unit, a digital signal processing unit, a DAC unit, an electric signal amplifier unit, and a polarization control device unit to control the polarization of the local light transmitted from the optical transmitting device. This imposes restrictions on reducing power consumption and miniaturization.

[0008] In view of the above circumstances, an object of the present invention is to provide an optical transmission system, an optical receiving device, and an optical transmission method that can reduce power consumption and downsize the device in self-homodyne optical transmission. [Means for solving the problem]

[0009] One aspect of the present invention is an optical transmission system having an optical transmitting device and an optical receiving device, wherein the optical transmitting device comprises an optical output unit that outputs light of a single wavelength, a branching unit that branches the light output by the optical output unit into light for signal generation and local light, and an optical modulation unit that modulates the light for signal generation into polarization-multiplexed signal light, and the optical receiving device comprises an optical detection unit that coherently detects the signal light that has transmitted through an optical transmission path using local light that has transmitted through an optical transmission path different from the optical transmission path that the signal light has transmitted through, a signal processing unit that generates local light polarization control information based on the signal obtained by coherent detection to compensate for fluctuations in the polarization of the local light that has occurred during transmission, and a polarization control unit that performs polarization control of the local light that the optical detection unit uses for coherent detection based on the local light polarization control information.

[0010] One aspect of the present invention is an optical receiving device that includes: an optical detection unit that receives the signal light and the local light transmitted via different optical transmission paths from an optical transmitting device that splits light of a single wavelength into light for signal generation and local light and modulates the light for signal generation to generate polarization-multiplexed signal light; an optical detection unit that coherently detects the signal light using the local light, a signal processing unit that generates local light polarization control information based on the signal obtained by coherent detection to compensate for fluctuations in the polarization of the local light during transmission; and a polarization control unit that performs polarization control of the local light used by the optical detection unit for coherent detection based on the local light polarization control information.

[0011] One aspect of the present invention is an optical transmission method in an optical transmission system having an optical transmitting device and an optical receiving device, the method comprising: an optical output step in which the optical transmitting device outputs light of a single wavelength; a branching step in which the optical transmitting device branches the light output in the optical output step into light for signal generation and local light; an optical modulation step in which the optical transmitting device modulates the light for signal generation into polarization-multiplexed signal light; an optical detection step in which the optical receiving device coherently detects the signal light that has transmitted through an optical transmission path using local light that has transmitted through an optical transmission path different from the optical transmission path through which the signal light has transmitted; a signal processing step in which the optical receiving device generates local light polarization control information based on a signal obtained by coherent detection to compensate for fluctuations in the polarization of the local light that has occurred during transmission; and a polarization control step in which the optical receiving device performs polarization control of the local light used for the coherent detection based on the local light polarization control information. [Effects of the Invention]

[0012] The present invention makes it possible to reduce power consumption and downsize the device in self-homodyne optical transmission. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram illustrating an example of the configuration of an optical transmission system according to a first embodiment of the present invention. [Figure 2]1 is a diagram illustrating an example of the configuration of an optical transmission system according to a first embodiment. [Figure 3] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmission system according to a second embodiment. [Figure 4] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmission system according to a second embodiment. [Figure 5] FIG. 10 is a diagram illustrating an example of the configuration of an optical transmission system according to a third embodiment. [Figure 6] FIG. 1 is a diagram illustrating an example of the configuration of a conventional optical transmission system. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the same parts in the drawings are designated by the same reference numerals, and the description thereof will be omitted.

[0015] This embodiment relates to an optical transmission system using a self-homodyne optical transmission method. An optical transmitter transmits signal light and local light having the same wavelength output from the same laser to an optical receiver via different optical fibers. The optical receiver performs coherent detection of the received signal light using the received local light. The optical receiver includes a digital signal processor that generates local light polarization control information based on the output signal of the coherent detection to compensate for fluctuations in the polarization of the local light during transmission through the optical fiber, and a polarization controller that controls the polarization of the local light using the local light polarization control information. This reduces the number of functional blocks for polarization control of the local light transmitted from the optical transmitter, and enables efficient polarization control.

[0016] (First embodiment) The optical transmission system of the first embodiment uses a self-homodyne optical communication system that utilizes wavelength division multiplexing and polarization division multiplexing. FIG. 1 is a block diagram showing the configuration of an optical transmission system 1 according to the first embodiment. Among the connecting lines in FIG. 1, thin solid arrows indicate paths for electrical data signals, thick solid arrows indicate paths for optical data signals, and thin dashed arrows indicate paths for electrical control signals. The same applies to other figures unless otherwise defined.

[0017] The optical transmission system 1 includes an optical transmitting device 10 and an optical receiving device 20. The optical transmitting device 10 and the optical receiving device 20 are connected by a plurality of optical transmission paths 30. M (M is an integer of 2 or more) optical transmission paths 30 are referred to as optical transmission paths 30-1 to 30-M, respectively. The internal configurations of the optical transmitting device 10 and the optical receiving device 20 will be described below.

[0018] The optical transmitting device 10 includes N (N is an integer equal to or greater than 2 and N≧M) optical output units 11, N branching units 12, N digital signal processing units 13, N optical modulation units 14, and M wavelength multiplexing units 15. The optical output units 11, branching units 12, and digital signal processing units 13 may be the optical output units, branching units, and digital signal processing units included in the conventional optical transmitting device shown in FIG. 6. The N optical output units 11 are referred to as optical output units 11-1 to 11-N, respectively, the N branching units 12 are referred to as branching units 12-1 to 12-N, respectively, the N digital signal processing units 13 are referred to as digital signal processing units 13-1 to 13-N, respectively, the N optical modulation units 14 are referred to as optical modulation units 14-1 to 14-N, respectively, and the M wavelength multiplexing units 15 are referred to as wavelength multiplexing units 15-1 to 15-M, respectively. FIG. 1 shows an example where N=M=2. Furthermore, signal light of wavelength λi (i is an integer between 1 and N) is also referred to as signal light λi, and local light of wavelength λi is also referred to as local light λi.

[0019] The optical output unit 11 can be configured using, for example, a DFB (Distributed Feedback) laser, an ECL (External Cavity Laser), or a VCSEL (Vertical Cavity Surface Emitting Laser). The optical output unit 11-i generates light of a single wavelength λi. The branching unit 12-i branches the light generated by the optical output unit 11-i and outputs the branched light to the optical modulation unit 14-i and the wavelength multiplexing unit 15-j (j is an integer between 1 and N, and i≠j).

[0020] The digital signal processing unit 13 can be configured using, for example, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-programmable Gate Array), etc. The digital signal processing unit 13-i generates an electric signal and outputs the generated electric signal to the optical modulation unit 14-i.

[0021] The optical modulation unit 14 can be configured using, for example, a Mach-Zehnder optical modulator. The optical modulation unit 14-i receives the light of wavelength λi output from the optical output unit 11-i and branched by the branching unit 12-i. The optical modulation unit 14-i modulates the input light of wavelength λi using an electrical signal input from the digital signal processing unit 13-i to generate signal light with a power ratio of 1:1 between x-polarized wave and y-polarized wave. The optical modulation unit 14-i outputs the generated signal light of wavelength λi to a wavelength multiplexing unit 15-m (m is an integer between 1 and M). Here, i=m.

[0022] The wavelength multiplexing unit 15 can be configured using, for example, a PLC (Planer Lightwave Circuit) or a MEMS (Micro Electro Mechanical Systems). The wavelength multiplexing unit 15 multiplexes input light beams of different wavelengths by wavelength division multiplexing. The wavelength multiplexing unit 15-m receives the signal light beam of wavelength λi modulated by the optical modulation unit 14-i and the local light beam of wavelength λj output by the optical output unit 11-j and branched by the branching unit 12-j. The wavelength multiplexing unit 15-m multiplexes the signal light beam of wavelength λi with the local light beam of wavelength λj, and outputs the combined light to the optical transmission line 30-m.

[0023] The optical receiving device 20 includes M wavelength demultiplexers 21, N polarization control units 22, N optical detection units 23, and N digital signal processing units 24. The M wavelength demultiplexers 21 are respectively referred to as wavelength demultiplexers 21-1 to 21-M, the N polarization control units 22 are respectively referred to as polarization control units 22-1 to 22-N, the N optical detection units 23 are respectively referred to as optical detection units 23-1 to 23-N, and the N digital signal processing units 24 are respectively referred to as digital signal processing units 24-1 to 24-N.

[0024] The polarization control unit 22 includes a polarization control device unit 221 and an electric signal amplifier unit 222. The polarization control device unit 221 and the electric signal amplifier unit 222 included in the polarization control unit 22-i will be referred to as the polarization control device unit 221-i and the electric signal amplifier unit 222-i, respectively. The digital signal processing unit 24 includes an adaptive equalization unit 241, an information conversion unit 242, and a DAC (digital-to-analog conversion) unit 243. The adaptive equalization unit 241, the information conversion unit 242, and the DAC unit 243 included in the digital signal processing unit 24-i will be referred to as the adaptive equalization unit 241-i, the information conversion unit 242-i, and the DAC unit 243-i, respectively.

[0025] The wavelength separator 21 can be configured using, for example, a PLC, a MEMS, or the like. The wavelength separator 21-m separates the multiplexed light by wavelength and outputs the separated light of each wavelength to a different path. The wavelength separator 21-m separates the light input from the optical transmission path 30-m by wavelength. The wavelength separator 21-m outputs the separated signal light of wavelength λi to the optical detector 23-i, and outputs the separated local light of wavelength λj to the polarization control device unit 221-j of the polarization controller 22-j.

[0026] The polarization control device unit 221 can be configured using, for example, an optical waveguide. The polarization control device unit 221 performs polarization control of the local light using local light polarization control information. The polarization control of the local light in the polarization control device unit 221 may be, for example, control so that the power ratio between x polarization and y polarization is 1:1, or control so that the local light has a single polarization direction such as x polarization or y polarization, and control so that the power ratio between x polarization and y polarization of the local light in the optical detection unit 23 is 1:1. The polarization control device unit 221-j performs polarization control related to polarization rotation and polarization mode dispersion on the local light of wavelength λj input from the wavelength demultiplexer 21-m using local light polarization control information input from the electric signal amplifier 222-j. The local light polarization control information represents fluctuations such as polarization rotation and polarization mode dispersion that the polarization of the local light of wavelength λj undergoes when it transmits through the optical transmission line 30. The polarization control device section 221-j outputs the polarization-controlled local light with wavelength λj to the optical detection section 23-j.

[0027] The electric signal amplifier 222 can be configured using, for example, a driver. The electric signal amplifier 222-j receives local light polarization control information from the digital signal processor 24-i, which has processed the results of coherent detection of the signal light of wavelength λi that was multiplexed with the local light of wavelength λj. The electric signal amplifier 222-j amplifies the received local light polarization control information and outputs the amplified local light polarization control information to the polarization control device unit 221-j.

[0028] The optical detection unit 23 can be configured using, for example, a 90-degree hybrid and a BPD (Balanced Photo Diode). The optical detection unit 23 performs coherent detection of signal light using local light and converts the signal light into an electrical signal. The optical detection unit 23-i performs coherent detection of signal light with wavelength λi input from the wavelength demultiplexer 21-i using local light with wavelength λi input from the polarization control device unit 221-i, and generates an electrical signal. The optical detection unit 23-i outputs the generated electrical signal to the adaptive equalization unit 241-i of the digital signal processing unit 24-i.

[0029] The adaptive equalizer 241 can be configured using, for example, an ASIC, an FPGA, etc. The adaptive equalizer 241-i compensates for polarization rotation and polarization mode dispersion of the electrical signal input from the optical detector 23-i using, for example, a blind equalization algorithm (CMA: Constant Modulus Algorithm). The adaptive equalizer 241-i outputs tap coefficient information generated when compensating for polarization rotation and polarization mode dispersion of the electrical signal input from the optical detector 23-i as signal light polarization control information to the information converter 242-i.

[0030] The information converter 242 can be configured using, for example, an ASIC, an FPGA, or the like. The information converter 242-i generates Jones matrix information, which is local light polarization control information, by Fourier transforming tap coefficient information, which is signal light polarization control information, input from the adaptive equalizer 241-i. The Jones matrix information represents the polarization rotation and polarization mode dispersion that the light has undergone during transmission through the optical transmission line 30-m. The information converter 242-i outputs the local light polarization control information to the DAC unit 243-i.

[0031] The DAC unit 243-i converts the local light polarization control information generated by the information converter 242-i into an analog signal, and outputs the converted analog signal to the electrical signal amplifier 222-j of the polarization controller 22-j, which corresponds to the local light with wavelength λj that has been multiplexed with the signal light with wavelength λi and transmitted.

[0032] The optical transmission paths 30-1 to 30-M can be configured using, for example, M separate SMFs (Single Mode Fibers). Alternatively, the optical transmission paths 30-1 to 30-M can be configured using a single MCF (Multi Core Fiber). Light input from the wavelength multiplexing unit 15-m of the optical transmitting device 10 is transmitted through the optical transmission path 30-m. The light transmitted through the optical transmission path 30-m is output to the wavelength demultiplexing unit 21-m of the optical receiving device 20.

[0033] With the above configuration, the optical transmission system 1 shown in Figure 1 operates as follows: The optical output unit 11-1 of the optical transmitter 10 generates light of a single wavelength λ1. The branching unit 12-1 branches the light of wavelength λ1 output from the optical output unit 11-1 into two. The branching unit 12-1 outputs one of the branched light beams of wavelength λ1 to the optical modulation unit 14-1. The branching unit 12-1 outputs the other branched light beam of wavelength λ1 to the wavelength multiplexing unit 15-2 as local light.

[0034] The optical output unit 11-2 generates light of a single wavelength λ2. The branching unit 12-2 branches the light of wavelength λ2 output from the optical output unit 11-2 into two. The branching unit 12-2 outputs one of the branched light beams of wavelength λ2 to the optical modulation unit 14-2. The branching unit 12-2 outputs the other branched light beam of wavelength λ2 to the wavelength multiplexing unit 15-1 as local light.

[0035] The optical modulation unit 14-1 generates polarization division multiplexed signal light by modulating light of wavelength λ1 using the electrical signal generated by the digital signal processing unit 13-1, and outputs the generated signal light to the wavelength multiplexing unit 15-1. The wavelength multiplexing unit 15-1 multiplexes the signal light of wavelength λ1 input from the optical modulation unit 14-1 with the local light of wavelength λ2 input from the optical output unit 11-2, and outputs the multiplexed light to the optical transmission line 30-1. The light input from the wavelength multiplexing unit 15-1 is output to the wavelength demultiplexing unit 21-1 of the optical receiving device 20 after transmitting through the optical transmission line 30-1.

[0036] The optical modulation unit 14-2 generates polarization division multiplexed signal light by modulating light of wavelength λ2 using the electrical signal generated by the digital signal processing unit 13-2, and outputs the generated signal light to the wavelength multiplexing unit 15-2. The wavelength multiplexing unit 15-2 multiplexes the signal light of wavelength λ2 input from the optical modulation unit 14-2 with the local light of wavelength λ1 input from the optical output unit 11-1, and outputs the combined light to the optical transmission line 30-2. The light input from the wavelength multiplexing unit 15-2 is output to the wavelength demultiplexing unit 21-2 of the optical receiving device 20 after transmitting through the optical transmission line 30-2.

[0037] The wavelength separation unit 21-1 separates the light input from the optical transmission path 30-1 by wavelength. The wavelength separation unit 21-1 outputs the separated signal light of wavelength λ1 to the optical detection unit 23-1, and outputs the separated local light of wavelength λ2 to the polarization control device unit 221-2. The polarization control device unit 221-2 performs polarization control related to polarization rotation and polarization mode dispersion on the local light of wavelength λ2 input from the wavelength separation unit 21-1, using the local light polarization control information input from the electrical signal amplification unit 222-2. The polarization control device unit 221-2 outputs the polarization-controlled local light of wavelength λ2 to the optical detection unit 23-2.

[0038] The wavelength separation unit 21-2 separates the light input from the optical transmission path 30-2 by wavelength. The wavelength separation unit 21-2 outputs the separated signal light of wavelength λ2 to the optical detection unit 23-2, and outputs the separated local light of wavelength λ1 to the polarization control device unit 221-1. The polarization control device unit 221-1 performs polarization control related to polarization rotation and polarization mode dispersion on the local light of wavelength λ1 input from the wavelength separation unit 21-2, using the local light polarization control information input from the electrical signal amplification unit 222-1. The polarization control device unit 221-1 outputs the polarization-controlled local light of wavelength λ1 to the optical detection unit 23-1.

[0039] Immediately after optical communication is started, the polarization control device units 221-1 and 221-2 control the polarization without using the local light polarization control information obtained by converting the signal light polarization control information, and after the signal light polarization control information is generated, they control the polarization using the local light polarization control information obtained by converting the signal light polarization control information.

[0040] The optical detector 23-1 performs coherent detection on the signal light of wavelength λ1 input from the wavelength separator 21-1 using the local oscillation light of wavelength λ1 input from the polarization control device 221-1 to generate an electrical signal. The optical detector 23-1 outputs the generated electrical signal to the adaptive equalizer 241-1.

[0041] The adaptive equalizer 241-1 compensates for polarization rotation and polarization mode dispersion in the electrical signal input from the optical detector 23-1. The adaptive equalizer 241-1 outputs tap coefficient information generated when compensating for polarization rotation and polarization mode dispersion to the information converter 242-1 as signal light polarization control information. The information converter 242-1 generates local light polarization compensation information based on the signal light polarization control information input from the adaptive equalizer 241-1. The information converter 242-1 outputs the generated local light polarization compensation information to the DAC unit 243-1. The DAC unit 243-1 converts the local light polarization control information into an analog signal and outputs the local light polarization control information in the analog signal to the electrical signal amplifier 222-2. The electrical signal amplifier 222-2 amplifies the local light polarization control information input from the DAC unit 243-1, and outputs the amplified local light polarization control information to the polarization control device unit 221-2.

[0042] The optical detector 23-2 performs coherent detection on the signal light of wavelength λ2 input from the wavelength separator 21-2 using the local oscillation light of wavelength λ2 input from the polarization control device 221-2 to generate an electrical signal, and outputs the generated electrical signal to the adaptive equalizer 241-2.

[0043] The adaptive equalizer 241-2 compensates for polarization rotation and polarization mode dispersion in the electrical signal input from the optical detector 23-2. The adaptive equalizer 241-2 outputs tap coefficient information generated when compensating for polarization rotation and polarization mode dispersion to the information converter 242-2 as signal light polarization control information. The information converter 242-2 generates local light polarization compensation information based on the signal light polarization control information input from the adaptive equalizer 241-2. The information converter 242-2 outputs the generated local light polarization compensation information to the DAC unit 243-2. ​​The DAC unit 243-2 converts the local light polarization control information into an analog signal and outputs the local light polarization control information in the analog signal to the electrical signal amplifier 222-1. The electrical signal amplifier 222-1 amplifies the local light polarization control information input from the DAC unit 243-2, and outputs the amplified local light polarization control information to the polarization control device unit 221-1.

[0044] When the optical transmitter 10 generates light with three or more wavelengths, each wavelength multiplexer 15 wavelength-division multiplexes the signal light and local light to include at least one signal light and one local light of different wavelengths, and outputs the multiplexed signal light and local light to the optical transmission line 30. The optical receiver 20 also includes polarization controllers 22, optical detectors 23, and digital signal processors 24, each corresponding to the number N of wavelengths. The local light output from the optical transmission line 30 is polarization-controlled by the polarization controllers 22 corresponding to the wavelength of the local light. The signal light and local light are coherently detected by the optical detectors 23 for each wavelength, and the electrical signal obtained by the coherent detection is output to the digital signal processors 24 corresponding to the wavelength. The digital signal processors 24 generate local light polarization control information based on the input electrical signal. The polarization controllers 22 control the polarization of the local light using at least one of the local light polarization control information generated by the signal light that has traveled through the same optical transmission line 30 as the local light whose polarization is to be controlled. A specific example is shown below.

[0045] 2 is a block diagram showing an example of the configuration of the optical transmission system 1 when N=3 and M=2. The operation of the optical transmission system 1 when N=M=2 differs from the operation of the optical transmission system 1 when N=3 and M=2 in the following respects.

[0046] The optical output unit 11-3 of the optical transmitting device 10 generates light of a single wavelength λ3. The branching unit 12-3 branches the light of wavelength λ3 output from the optical output unit 11-3 into two. The branching unit 12-3 outputs one of the branched light beams of wavelength λ3 to the optical modulation unit 14-3 and outputs the other light beam as local light to the wavelength multiplexing unit 15-1. The optical modulation unit 14-3 modulates the light of wavelength λ3 using an electrical signal generated by the digital signal processing unit 13-3 to generate polarization division multiplexed signal light and outputs the generated signal light to the wavelength multiplexing unit 15-2. The wavelength multiplexing unit 15-1 multiplexes the signal light of wavelength λ1 input from the optical modulation unit 14-1, the local light beam of wavelength λ2 input from the optical output unit 11-2, and the local light beam of wavelength λ3 input from the optical output unit 11-3, and outputs the combined light to the optical transmission line 30-1. The wavelength multiplexing unit 15-2 multiplexes the local light of wavelength λ1 input from the optical output unit 11-1, the signal light of wavelength λ2 input from the optical modulation unit 14-2, and the signal light of wavelength λ3 input from the optical modulation unit 14-3, and outputs the combined light to the optical transmission path 30-2.

[0047] The wavelength separation unit 21-1 of the optical receiving device 20 separates the light input from the optical transmission line 30-1 by wavelength. The wavelength separation unit 21-1 outputs the signal light of wavelength λ1 to the optical detection unit 23-1, the local light of wavelength λ2 to the polarization control unit 22-2, and the local light of wavelength λ3 to the polarization control unit 22-3. The wavelength separation unit 21-2 separates the light input from the optical transmission line 30-2 by wavelength. The wavelength separation unit 21-2 outputs the signal light of wavelength λ2 to the optical detection unit 23-2, the signal light of wavelength λ3 to the optical detection unit 23-3, and the local light of wavelength λ1 to the polarization control unit 22-1.

[0048] The polarization control device unit 221-3 of the polarization control unit 22-3 performs polarization control related to polarization rotation and polarization mode dispersion on the local light of wavelength λ3 input from the wavelength separation unit 21-1, using the local light polarization control information input from the electrical signal amplifier unit 222-3. The polarization control device unit 221-3 outputs the polarization-controlled local light of wavelength λ3 to the optical detection unit 23-3. The optical detection unit 23-3 performs coherent detection on the signal light of wavelength λ3 input from the wavelength separation unit 21-2, using the local light of wavelength λ3 input from the polarization control device unit 221-3, to generate an electrical signal. The optical detection unit 23-3 outputs the generated electrical signal to the digital signal processing unit 24-3.

[0049] The digital signal processing unit 24-3 outputs local light polarization control information generated using the electrical signal input from the optical detection unit 23-3 to the polarization control unit 22-1. The electrical signal amplifier 222-1 of the polarization control unit 22-1 amplifies at least one of the local light polarization control information input from the digital signal processing unit 24-2 and the local light polarization control information input from the digital signal processing unit 24-3, and outputs the amplified information to the polarization control device unit 221-1.

[0050] Meanwhile, the digital signal processing unit 24-1 generates local light polarization control information using the electrical signal input from the optical detection unit 23-1 and outputs it to the polarization control unit 22-2 and the polarization control unit 22-3. The electrical signal amplifier 222-3 of the polarization control unit 22-3 amplifies the local light polarization control information input from the digital signal processing unit 24-1 and outputs it to the polarization control device unit 221-3.

[0051] 2 , the operation of the optical transmission system 1 differs, for example, as follows: The wavelength multiplexing unit 15-1 of the optical transmitting device 10 multiplexes the signal light of wavelength λ1 with the local light of wavelength λ3 and outputs the result to the optical transmission line 30-1, the wavelength multiplexing unit 15-2 multiplexes the signal light of wavelength λ2 with the local light of wavelength λ1 and outputs the result to the optical transmission line 30-2, and the wavelength multiplexing unit 15-3 (not shown) multiplexes the signal light of wavelength λ3 with the local light of wavelength λ2 and outputs the result to the optical transmission line 30-3 (not shown). The wavelength demultiplexing unit 21-1 of the optical receiving device 20 demultiplexes the light input from the optical transmission line 30-1, and outputs the signal light of wavelength λ1 to the optical detection unit 23-1 and the local light of wavelength λ3 to the polarization control unit 22-3. The wavelength demultiplexer 21-2 demultiplexes the light input from the optical transmission line 30-2, outputs the signal light of wavelength λ2 to the optical detector 23-2, and outputs the local light of wavelength λ1 to the polarization controller 22-1. The wavelength demultiplexer 21-3 (not shown) demultiplexes the light input from the optical transmission line 30-3 (not shown), outputs the signal light of wavelength λ3 to the optical detector 23-3, and outputs the local light of wavelength λ2 to the polarization controller 22-2. The digital signal processor 24-1 outputs local light polarization control information generated using the electrical signal input from the optical detector 23-1 to the polarization controller 22-3. The digital signal processor 24-2 outputs local light polarization control information generated using the electrical signal input from the optical detector 23-2 to the polarization controller 22-1. The digital signal processing unit 24-3 generates local light polarization control information using the electrical signal input from the optical detection unit 23-3 and outputs the information to the polarization control unit 22-1.

[0052] As described above, in optical communication using wavelength division multiplexing and polarization division multiplexing in a self-homodyne system, the optical transmission system 1 of the first embodiment performs polarization control of local light by utilizing the signal light polarization control information output from the adaptive equalizer 241. This makes it possible to eliminate the opto-electrical converter and ADC unit that were conventionally required for polarization control.

[0053] (Second embodiment) The optical transmission system of the second embodiment uses a self-homodyne optical communication method using wavelength division multiplexing. Fig. 3 is a block diagram showing the configuration of an optical transmission system 2 according to the second embodiment. The optical transmission system 2 includes an optical transmitting device 40 and an optical receiving device 50. The optical transmitting device 40 may be the conventional optical transmitting device shown in Fig. 6. The optical transmitting device 40 and the optical receiving device 50 are connected by an optical transmission path 30-1 and an optical transmission path 30-2.

[0054] The optical transmitting device 40 includes an optical output unit 11, a branching unit 12, a digital signal processing unit 13, and an optical modulation unit 14. The optical output unit 11 generates light of a single wavelength λ1. The branching unit 12 branches the light generated by the optical output unit 11 into two, outputs one of the branches to the optical modulation unit 14, and outputs the other branched light to the optical transmission line 30-2 as local light. The digital signal processing unit 13 outputs the generated electrical signal to the optical modulation unit 14. The optical modulation unit 14 receives the light of wavelength λ1 output by the optical output unit 11 and branched by the branching unit 12. The optical modulation unit 14 modulates the light of wavelength λ1 using the electrical signal input from the digital signal processing unit 13, thereby generating polarization division multiplexed signal light with a 1:1 power ratio between the x polarization and the y polarization. The optical modulation unit 14 outputs the polarization division multiplexed signal light of wavelength λ1 to the optical transmission line 30-1.

[0055] The optical receiving device 50 includes a polarization control unit 22, an optical detection unit 53, and a digital signal processing unit 54. The polarization control unit 22 receives local light output from the optical transmission line 30-2. The polarization control device unit 221 of the polarization control unit 22 controls the polarization of the local light input from the optical transmission line 30-2 using local light polarization control information. The polarization control of the local light in the polarization control device unit 221 may be, for example, control so that the power ratio between x polarization and y polarization is 1:1, or control so that the local light has a single polarization direction such as x polarization or y polarization, and control so that the power ratio between x polarization and y polarization of the local light in the optical detection unit 53 is 1:1. The polarization control device unit 221 outputs the polarization-controlled local light to the optical detection unit 53.

[0056] The optical detection unit 53 performs homodyne detection of the signal light having a wavelength of λ1 input from the optical transmission line 30-1 using the local light having a wavelength of λ1 input from the polarization control device unit 221, and converts the signal light into an electrical signal. The optical detection unit 53 inputs electrical signal output power information to the digital signal processing unit 54. The electrical signal output power information indicates the output power of the x-polarized and y-polarized electrical signals output after homodyne detection. The signal light is a polarization multiplexed signal, and the power ratio of the x-polarized and y-polarized waves of the signal light is output at the transmitting side at 1:1. Therefore, the power ratio of the x-polarized and y-polarized waves of the received signal light is also ideally 1:1. Therefore, the difference between the output power of the x-polarized wave and the output power of the y-polarized wave indicated by the electrical signal output power information is equal to the difference between the output power of the x-polarized wave and the y-polarized wave of the local light.

[0057] The digital signal processing unit 54 includes an ADC unit 541, an information conversion unit 542, and a DAC unit 543. The ADC unit 541 converts the electrical signal output power information input from the optical detection unit 53 from an analog signal to a digital signal, and outputs the converted electrical signal output power information to the information conversion unit 542.

[0058] The information conversion unit 542 can be configured using, for example, an ASIC, an FPGA, etc. The information conversion unit 542 generates local light polarization control information using, for example, a steepest descent method for the electrical signal output power information input from the ADC unit 541. The information conversion unit 542 outputs the generated local light polarization control information to the DAC unit 543.

[0059] The DAC unit 543 converts the local light polarization control information input from the information conversion unit 542 from a digital signal to an analog signal, and outputs the converted local light polarization control information to the polarization control unit 22. The electrical signal amplifier 222 of the polarization control unit 22 amplifies the local light polarization control information input from the DAC unit 543 and outputs it to the polarization control device unit 221.

[0060] The optical transmitting device may generate light having two or more wavelengths. In this case, the optical transmitting device further includes the wavelength multiplexing unit 15 of the first embodiment. The wavelength multiplexing unit 15 wavelength-division multiplexes signal light and local light having different wavelengths and outputs the multiplexed signal light to the optical transmission line 30. The optical transmitting device further includes the wavelength demultiplexing unit 21 of the first embodiment. The optical receiving device further includes a polarization control unit 22, an optical detection unit 53, and a digital signal processing unit 54, each of which corresponds to the number of wavelengths. The wavelength demultiplexing unit 21 demultiplexes the signal light and local light. The local light output from the optical transmission line 30 is polarization-controlled by the polarization control unit 22 according to the wavelength. The signal light and local light are coherently detected by the optical detection unit 53 for each wavelength. The optical detection unit 53 outputs electrical signal output power information obtained as a result of the coherent detection to the digital signal processing unit 54. The digital signal processing unit 54 generates local light polarization control information for each wavelength. The polarization control unit 22 performs polarization control by utilizing signal light having the same wavelength as the local light to be controlled and the output power of an electrical signal generated based on the local light. A specific example is shown below.

[0061] Fig. 4 is a block diagram showing an example of the configuration of an optical transmission system 2a that generates light having a wavelength number N (N is an integer equal to or greater than 2). Fig. 4 shows an example where N=2. The optical transmission system 2a includes an optical transmitter 40a and an optical receiver 50a. The optical transmitter 40a and the optical receiver 50a are connected by an optical transmission path 30-1 and an optical transmission path 30-2.

[0062] The optical transmitting device 40a shown in Fig. 4 has the same configuration as the optical transmitting device 10 shown in Fig. 1 when N = M = 2. The optical receiving device 50a has a configuration in which the optical detecting unit 23 and the digital signal processing unit 24 in the optical receiving device 20 shown in Fig. 1 when N = M = 2 are replaced with an optical detecting unit 53 and a digital signal processing unit 54, respectively. The N optical detecting units 53 are respectively referred to as optical detecting units 53-1 to 53-N, and the N digital signal processing units 54 are respectively referred to as digital signal processing units 54-1 to 54-N. The ADC unit 541, the information converting unit 542, and the DAC unit 543 included in the digital signal processing unit 54-i (i is an integer between 1 and N) are respectively referred to as the ADC unit 541-i, the information converting unit 542-i, and the DAC unit 543-i.

[0063] The optical transmitting device 40a operates in the same manner as the optical transmitting device 10 of the first embodiment shown in FIG. 1. The wavelength separating unit 21-1 of the optical receiving device 50a separates light input from the optical transmission path 30-1 by wavelength. The wavelength separating unit 21-1 outputs signal light of wavelength λ1 to the optical detecting unit 53-1 and outputs local light of wavelength λ2 to the polarization control device unit 221-2. The polarization control device unit 221-2 performs polarization control on the local light of wavelength λ2 using local light polarization control information input from the electrical signal amplifying unit 222-2. The polarization control device unit 221-2 outputs the polarization-controlled local light of wavelength λ2 to the optical detecting unit 53-2.

[0064] The wavelength separation unit 21-2 separates the light input from the optical transmission path 30-2 by wavelength. The wavelength separation unit 21-2 outputs the signal light of wavelength λ2 to the optical detection unit 53-2 and outputs the local light of wavelength λ1 to the polarization control device unit 221-1. The polarization control device unit 221-1 performs polarization control on the local light of wavelength λ1 using the local light polarization control information input from the electrical signal amplification unit 222-1. The polarization control device unit 221-1 outputs the polarization-controlled local light of wavelength λ1 to the optical detection unit 53-1.

[0065] The optical detection unit 53-1 performs coherent detection on the signal light of wavelength λ1 input from the wavelength separation unit 21-1 using the local light of λ1 input from the polarization control device unit 221-1 to generate an electrical signal. The optical detection unit 53-1 outputs electrical signal output power information to the digital signal processing unit 54-1. The digital signal processing unit 54-1 outputs local light polarization control information generated using the electrical signal output power information input from the optical detection unit 53-1 to the electrical signal amplification unit 222-1 of the polarization control unit 22-1.

[0066] The optical detection unit 53-2 performs coherent detection on the signal light of wavelength λ2 input from the wavelength separation unit 21-2 using the local light of λ2 input from the polarization control device unit 221-2 to generate an electrical signal. The optical detection unit 53-2 outputs electrical signal output power information to the digital signal processing unit 54-2. The digital signal processing unit 54-2 outputs local light polarization control information generated using the electrical signal output power information input from the optical detection unit 53-2 to the electrical signal amplification unit 222-2 of the polarization control unit 22-2.

[0067] As described above, the optical transmission systems 2 and 2a of the second embodiment perform polarization control of the local light using the electrical signal output power information of the x-polarized and y-polarized waves output after self-homodyne detection of the signal light and the local light in optical communication using polarization division multiplexing. This makes it possible to eliminate the optoelectric conversion unit that was previously required for polarization control of the local light.

[0068] (Third embodiment) The optical transmission system of the third embodiment uses a self-homodyne optical communication method that utilizes wavelength division multiplexing and polarization division multiplexing. In the optical transmission system of the third embodiment, the optical receiving device has the functions of the digital signal processing unit of the first embodiment and the functions of the digital signal processing unit of the second embodiment.

[0069] Fig. 5 is a block diagram showing the configuration of an optical transmission system 3 according to the third embodiment. Thin dashed arrows and thin dashed-dotted arrows indicate paths of electrical control signals. The optical transmission system 3 shown in Fig. 5 differs from the optical transmission system 1 of the first embodiment shown in Fig. 1 in that it includes an optical receiving device 60 instead of the optical receiving device 20.

[0070] 1 in that it has N optical detection units 63 instead of the N (N is an integer of 2 or more) optical detection units 23, and N digital signal processing units 64 instead of the N digital signal processing units 24. The N optical detection units 63 are respectively referred to as optical detection units 63-1 to 63-N, and the N digital signal processing units 64 are respectively referred to as digital signal processing units 64-1 to 64-N.

[0071] The optical detection unit 63-i (i is an integer between 1 and N) performs coherent detection of the signal light of wavelength λi using the local light of wavelength λi output from the polarization control device unit 221-i of the polarization control unit 22-i, and converts the signal light into an electrical signal. The optical detection unit 63-i outputs the generated electrical signal and electrical signal output power information, which is information on the output power of the x-polarized wave and the y-polarized wave of the generated electrical signal, to the digital signal processing unit 64-i.

[0072] The digital signal processing unit 64 includes an adaptive equalization unit 241, an information conversion unit 242, an ADC unit 541, an information conversion unit 542, and a DAC unit 641. The adaptive equalization unit 241, the information conversion unit 242, the ADC unit 541, the information conversion unit 542, and the DAC unit 641 included in the digital signal processing unit 64-i will be referred to as the adaptive equalization unit 241-i, the information conversion unit 242-i, the ADC unit 541-i, the information conversion unit 542-i, and the DAC unit 243-i, respectively.

[0073] The adaptive equalizer 241-i and information converter 242-i of the digital signal processor 64-i perform the same processing as in the first embodiment. The information converter 242-i outputs the generated local light polarization control information to the DAC unit 641-i. The DAC unit 641-i converts the local light polarization control information input from the information converter 242-i from a digital signal to an analog signal. The DAC unit 641-i outputs the local light polarization control information converted into an analog signal to the electrical signal amplifier 222-j of the polarization controller 22-j, which corresponds to the local light with wavelength λj (j is an integer between 1 and N, and i≠j), which is multiplexed with the signal light with wavelength λi and transmitted.

[0074] The ADC unit 541-i and the information conversion unit 542-i of the digital signal processing unit 64-i perform the same processing as in the second embodiment. The information conversion unit 542-i outputs the generated local light polarization control information to the DAC unit 641-i. The DAC unit 641-i converts the local light polarization control information input from the information conversion unit 542-i from a digital signal to an analog signal. The DAC unit 641-i outputs the local light polarization control information converted into an analog signal to the electrical signal amplifier unit 222-i of the polarization control unit 22-i.

[0075] The electrical signal amplifier 222-i of the polarization controller 22-i amplifies either the local light polarization control information input from the digital signal processor 64-i or the local light polarization control information input from the digital signal processor 64-j, and outputs the amplified information to the polarization control device 221-i.

[0076] As described above, the polarization control unit 22 may control the polarization of a local light having a different wavelength that is transmitted by wavelength division multiplexing through the same optical transmission line 30 as the signal light, using local light polarization control information obtained by converting, by the information converter 242, the signal light polarization control information output by the adaptive equalizer 241. Alternatively, the polarization control unit 22 may control the polarization of a local light that is input through an optical transmission line 30 different from the signal light and has the same wavelength as the signal light, using local light polarization control information obtained by converting, by the information converter 542, the electrical signal output power information output by the optical detection unit 63.

[0077] As described above, in optical communication using wavelength division multiplexing and polarization division multiplexing in a self-homodyne system, the optical transmission system 3 of the third embodiment converts the signal light polarization control information of the adaptive equalizer 241 into local light polarization control information to perform polarization control of the local light, or converts the electrical signal output power information of the x polarization and y polarization output after self-homodyne detection of the signal light and the local light into local light polarization control information to perform polarization control of the local light. This makes it possible to eliminate the optoelectric conversion unit that was conventionally required for polarization control of the local light.

[0078] At least some of the functions of the digital signal processing units 24, 54, and 64 included in the optical receiving device of the above-described embodiment may be implemented by a computer. In this case, a program for implementing the functions of the digital signal processing units 24, 54, and 64 may be recorded on a computer-readable recording medium, and the program may be read and executed by a computer system. The computer system may include hardware such as a processor, an OS, and peripheral devices. The program for the signal processing unit may be recorded on a computer-readable recording medium. Examples of the computer-readable recording medium include portable media such as a flexible disk, a magneto-optical disk, a ROM, and a CD-ROM, and storage devices such as a hard disk built into a computer system. The program for the signal processing unit may be transmitted via a telecommunications line.

[0079] According to the embodiment described above, the optical receiving device can reduce the number of functional blocks for controlling the polarization of the local light transmitted from the optical transmitting device, and can perform efficient polarization control.

[0080] According to the above-described embodiment, the optical transmission system includes an optical transmitting device and an optical receiving device. The optical transmitting device includes an optical output unit, a branching unit, and an optical modulation unit. The optical output unit outputs light of a single wavelength. The branching unit branches the light output by the optical output unit into light for signal generation and local light. The optical modulation unit modulates the light for signal generation into polarization-multiplexed signal light. The optical receiving device includes an optical detection unit, a signal processing unit, and a polarization control unit. The optical detection unit coherently detects the signal light transmitted through the optical transmission line using local light transmitted through an optical transmission line different from the optical transmission line through which the signal light transmitted. The signal processing unit generates local light polarization control information based on the signal obtained by coherent detection to compensate for fluctuations in the polarization of the local light during transmission. The polarization control unit controls the polarization of the local light used by the optical detection unit for coherent detection based on the local light polarization control information.

[0081] The optical output unit may output light of different wavelengths generated by each of the multiple lasers. The branching unit branches the light of each of the multiple wavelengths output by the optical output unit into light for signal generation and local light. The optical modulation unit modulates the light for signal generation of each of the multiple wavelengths into polarization-multiplexed signal light. The optical transmitting device further includes a wavelength multiplexing unit. The wavelength multiplexing unit generates light by multiplexing at least one signal light and at least one local light having a wavelength different from that of the signal light, and transmits each of the generated multiple light beams to the optical receiving device via different optical transmission paths. The optical receiving device further includes a wavelength separation unit. The wavelength separation unit separates the light transmitted through each of the different optical transmission paths into signal light and local light beams according to wavelength. The optical detection unit coherently detects the signal light using local light beams having the same wavelength as the signal light. The signal processing unit performs one or both of the following processes: compensates for polarization rotation and polarization mode dispersion of the signal obtained by coherent detection, and generates, based on the compensation performed on the signal, local light polarization control information for the local light that is multiplexed with the signal light used for coherent detection and transmitted; and generates, based on information on the electrical signal output power of two different polarizations obtained by coherent detection, local light polarization control information for the local light having the same wavelength as the signal light used for coherent detection.

[0082] The signal processing unit may generate local light polarization control information for compensating for polarization rotation and polarization mode dispersion of the signal light based on information on the electrical signal output power of two different polarized waves obtained by coherent detection.

[0083] The signal processing unit may generate, as the local light polarization control information, Jones matrix information obtained by Fourier transforming tap coefficient information generated when the signal obtained by coherent detection is compensated for polarization rotation and polarization mode dispersion. Alternatively, the signal processing unit may generate the local light polarization control information by using the steepest descent method with respect to information on the electrical signal output power of two different polarized waves obtained by coherent detection.

[0084] Although the embodiments of the present invention have been described in detail above with reference to the drawings, the specific configurations are not limited to these embodiments, and include designs within the scope of the present invention that do not deviate from the gist of the present invention. [Explanation of symbols]

[0085] 1, 2, 2a, 3 Optical transmission system 10 Optical transmitter 11, 11-1 to 11-3 Optical output section 12, 12-1 to 12-3 Branch 13, 13-1 to 13-3 Digital signal processing section 14, 14-1 to 14-3 Optical modulation section 15, 15-1, 15-2 Wavelength multiplexing section 20 Optical receiving device 21-1, 21-2 Wavelength separation section 22, 22-1 to 22-3 Polarization control section 23-1 to 23-3 Optical detection section 24-1~24-3 Digital signal processing section 30-1, 30-2 Optical transmission line 40, 40a Optical transmitter 50, 50a Optical receiving device 53, 53-1, 53-2 Optical detection section 54, 54-1, 54-2 Digital signal processing section 60 Optical receiving device 63-1, 63-2 Optical detection section 64-1, 64-2 Digital signal processing section 221, 221-1 to 221-3 Polarization control device section 222, 222-1 to 222-3 Electric signal amplifier 241-1 to 241-3 Adaptive equalization section 242-1~242-3 Information conversion section 243-1~243-3 DAC section 541, 541-1, 541-2 ADC section 542, 542-1, 542-2 Information conversion unit 543, 543-1, 543-2 DAC section 641-1, 641-2 DAC section

Claims

1. An optical transmission system having an optical transmitting device and an optical receiving device, The optical transmitter comprises: an optical output unit that outputs light of a single wavelength; a branching unit that branches the light output from the optical output unit into light for signal generation and local light; an optical modulation unit that modulates the signal generation light into polarization multiplexed signal light, The optical receiving device an optical detection unit that coherently detects the signal light transmitted through an optical transmission line using the local light transmitted through an optical transmission line different from the optical transmission line through which the signal light is transmitted; a signal processing unit that generates local light polarization control information for compensating for fluctuations in the polarization of the local light during transmission, based on the signal obtained by coherent detection; a polarization control unit that controls polarization of the local light used by the optical detection unit for coherent detection based on the local light polarization control information, the optical output unit outputs light beams of different wavelengths generated by a plurality of lasers, the branching unit branches the light of each of the plurality of wavelengths output by the optical output unit into light for signal generation and local light; the optical modulation unit modulates the signal generation light of each of a plurality of wavelengths into polarization-multiplexed signal light; the optical transmission device further includes a wavelength multiplexing unit that generates light by multiplexing at least one of the signal light and the local light having a wavelength different from that of the signal light, and outputs each of the generated multiple lights to a different optical transmission path; the optical receiving device further includes a wavelength separation unit that separates the light transmitted through each of the different optical transmission paths into the signal light and the local light based on wavelengths; the optical detection unit coherently detects the signal light using the local light having the same wavelength as the signal light, the signal processing unit compensates for polarization rotation and polarization mode dispersion of the signal obtained by coherent detection, and generates local light polarization control information of the local light that is multiplexed with the signal light used in the coherent detection and transmitted, based on the compensation performed on the signal. Optical transmission system.

2. the signal processing unit performs a process of generating local light polarization control information of the local light having the same wavelength as the signal light used for the coherent detection based on information on electrical signal output power of two different polarizations obtained by coherent detection, instead of or in addition to a process of compensating for polarization rotation and polarization mode dispersion of the signal obtained by coherent detection and generating local light polarization control information of the local light multiplexed with the signal light used for the coherent detection and transmitted based on the compensation performed on the signal.

2. The optical transmission system according to claim 1.

3. the signal processing unit generates Jones matrix information obtained by Fourier transforming tap coefficient information generated when the signal obtained by coherent detection is compensated for polarization rotation and polarization mode dispersion, as local light polarization control information; 2. The optical transmission system according to claim 1.

4. the signal processing unit generates the local light polarization control information based on information on electrical signal output power of two different polarized waves obtained by coherent detection for each wavelength.

2. The optical transmission system according to claim 1.

5. An optical transmission system having an optical transmitting device and an optical receiving device, The optical transmitter comprises: an optical output unit that outputs light of a single wavelength; a branching unit that branches the light output from the optical output unit into light for signal generation and local light; an optical modulation unit that modulates the signal generation light into polarization multiplexed signal light, The optical receiving device an optical detection unit that coherently detects the signal light transmitted through an optical transmission line using the local light transmitted through an optical transmission line different from the optical transmission line through which the signal light is transmitted; a signal processing unit that generates local light polarization control information for compensating for fluctuations in the polarization of the local light during transmission, based on the signal obtained by coherent detection; a polarization control unit that controls polarization of the local light used by the optical detection unit for coherent detection based on the local light polarization control information, the signal processing unit generates the local light polarization control information using a steepest descent method for information on the electrical signal output power of two different polarized waves obtained by coherent detection. Optical transmission system.

6. an optical detection unit that receives the signal light and the local light transmitted via different optical transmission paths from an optical transmitter that splits light of a single wavelength into light for signal generation and local light and modulates the light for signal generation to generate polarization-multiplexed signal light, and that coherently detects the signal light using the local light; a signal processing unit that generates local light polarization control information for compensating for fluctuations in the polarization of the local light during transmission, based on the signal obtained by coherent detection; a polarization control unit that controls polarization of the local light used for coherent detection by the optical detection unit based on the local light polarization control information; Equipped with the optical transmitting device further comprises a wavelength separating unit that separates, by wavelength, the light beams of different wavelengths generated by the plurality of lasers into light beams for signal generation and local light beams, modulates the light beams for signal generation of the plurality of wavelengths into polarization-multiplexed signal light beams, and separates the light beams that have been transmitted through the different optical transmission paths into the signal light beams and the local light beams by wavelength, the light beams being a plurality of light beams generated by combining at least one signal light beam and at least one local light beam having a wavelength different from that of the signal light beam; the optical detection unit coherently detects the signal light using the local light having the same wavelength as the signal light, the signal processing unit compensates for polarization rotation and polarization mode dispersion of the signal obtained by coherent detection, and generates local light polarization control information of the local light that is multiplexed with the signal light used in the coherent detection and transmitted, based on the compensation performed on the signal. Optical receiving device.

7. an optical detection unit that receives the signal light and the local light transmitted via different optical transmission paths from an optical transmitter that splits light of a single wavelength into light for signal generation and local light and modulates the light for signal generation to generate polarization-multiplexed signal light, and that coherently detects the signal light using the local light; a signal processing unit that generates local light polarization control information for compensating for fluctuations in the polarization of the local light during transmission, based on the signal obtained by coherent detection; a polarization control unit that controls polarization of the local light used for coherent detection by the optical detection unit based on the local light polarization control information; Equipped with the signal processing unit generates the local light polarization control information using a steepest descent method for information on the electrical signal output power of two different polarized waves obtained by coherent detection. Optical receiving device.

8. An optical transmission method in an optical transmission system having an optical transmitting device and an optical receiving device, an optical output step in which the optical transmitter outputs light of a single wavelength; a branching step in which the optical transmitting device branches the light output in the optical output step into light for signal generation and local light; an optical modulation step in which the optical transmitter modulates the signal generation light into polarization-multiplexed signal light; an optical detection step in which the optical receiving device coherently detects the signal light transmitted through an optical transmission line using the local light transmitted through an optical transmission line different from the optical transmission line through which the signal light is transmitted; a signal processing step in which the optical receiving device generates local light polarization control information for compensating for fluctuations in the polarization of the local light during transmission, based on the signal obtained by coherent detection; a polarization control step in which the optical receiving device performs polarization control of the local light used for the coherent detection based on the local light polarization control information; and In the optical output step, the optical transmitter outputs light beams having different wavelengths generated by a plurality of lasers, In the branching step, the optical transmitter branches the light of each of the plurality of wavelengths output in the optical output step into light for signal generation and local light; In the optical modulation step, the optical transmitter modulates the signal generation light of each of a plurality of wavelengths into polarization-multiplexed signal light; The optical transmission method includes: a wavelength multiplexing step in which the optical transmitter generates light by multiplexing at least one of the signal light and the local light having a wavelength different from that of the signal light, and outputs each of the generated lights to a different optical transmission line; the optical receiving device further comprises a wavelength separation step of separating the light transmitted through the different optical transmission paths into the signal light and the local light based on wavelengths, In the optical detection step, the optical receiving device coherently detects the signal light using the local light having the same wavelength as the signal light, In the signal processing step, the optical receiving device compensates for polarization rotation and polarization mode dispersion of the signal obtained by coherent detection, and generates local light polarization control information of the local light multiplexed with the signal light used in the coherent detection and transmitted, based on the compensation performed on the signal. Optical transmission method.

9. An optical transmission method in an optical transmission system having an optical transmitting device and an optical receiving device, an optical output step in which the optical transmitter outputs light of a single wavelength; a branching step in which the optical transmitting device branches the light output in the optical output step into light for signal generation and local light; an optical modulation step in which the optical transmitter modulates the signal generation light into polarization-multiplexed signal light; an optical detection step in which the optical receiving device coherently detects the signal light transmitted through an optical transmission line using the local light transmitted through an optical transmission line different from the optical transmission line through which the signal light is transmitted; a signal processing step in which the optical receiving device generates local light polarization control information for compensating for fluctuations in the polarization of the local light during transmission, based on the signal obtained by coherent detection; a polarization control step in which the optical receiving device performs polarization control of the local light used for the coherent detection based on the local light polarization control information; and In the signal processing step, the optical transmitting device generates the local light polarization control information using a steepest descent method with respect to information on the electrical signal output power of two different polarized waves obtained by coherent detection. Optical transmission method.

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