Polarization fluctuation detection method, polarization fluctuation detection device, and polarization fluctuation detection system

Abstract

To estimate the position where polarization fluctuation has occurred even when polarization fluctuation has occurred at a plurality of points on a transmission path.SOLUTION: A polarization fluctuation detection device receives a plurality of optical signals output from a transmitter via a transmission path that transmits the plurality of optical signals in a predetermined direction at different propagation speeds. The polarization fluctuation detection device includes a fluctuation detection unit that detects polarization fluctuations in the received plurality of optical signals, and an estimation unit that estimates a position in the transmission path where polarization fluctuations occur on the basis of a time difference at which polarization fluctuations between the plurality of optical signals are detected.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a polarization fluctuation detection method, a polarization fluctuation detection device, and a polarization fluctuation detection system. [Background technology]

[0002] Polarization-multiplexed digital coherent communication has been introduced to increase the capacity of communication systems. In polarization-multiplexed digital coherent communication, fluctuations in the state of polarization (SOP) of the transmitted signal can occur. Factors that cause SOP fluctuations include construction vibrations, lightning, vehicle traffic, earthquakes, and wind. If a rapid SOP fluctuation occurs, the polarization compensation in the equalization process cannot keep up with the polarization fluctuation, which can result in an error. Therefore, a monitoring system that monitors polarization fluctuations and detects any polarization fluctuations that may cause errors is important. In particular, when polarization fluctuations occur, it is important to identify or estimate the location where the polarization fluctuations occur.

[0003] As a related technique, Patent Document 1 discloses a distributed waveguide sensor that detects a physical phenomenon acting at an arbitrary position in a waveguide such as an optical fiber. The distributed waveguide sensor described in Patent Document 1 uses two sets of electromagnetic waves transmitted in opposite directions within the waveguide to estimate the location where the physical phenomenon occurs. In the distributed waveguide sensor, a signal recognition unit analyzes the time relationship between the state changes of the two sets of electromagnetic waves transmitted in opposite directions that occur when a physical phenomenon acts on the waveguide, and estimates the location where the physical phenomenon occurs. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-307896 Summary of the Invention [Problem to be solved by the invention]

[0005] In the distributed waveguide sensor described in Patent Document 1, state changes of two sets of electromagnetic waves transmitted in opposite directions are detected at both ends of the waveguide. Generally, the distance from the location where a physical phenomenon occurs to one end of the waveguide is different from the distance from the location where the physical phenomenon occurs to the other end of the waveguide. In other words, the distance from the location where the physical phenomenon occurs to the receiving end where one electromagnetic wave is detected is different from the distance from the location where the physical phenomenon occurs to the receiving end where the other electromagnetic wave is detected. In this case, if physical phenomena occur at multiple locations in the waveguide, it may be difficult to associate the state changes detected at one end with the state changes detected at the other end. If the state changes detected in both electromagnetic waves cannot be correctly associated, the location where the physical phenomenon occurred cannot be accurately estimated.

[0006] One of the objectives of the present disclosure is to provide a polarization fluctuation detection method, a polarization fluctuation detection device, and a polarization fluctuation detection system that can estimate the location where polarization fluctuation occurs even if polarization fluctuation occurs at multiple points on a transmission path. [Means for solving the problem]

[0007] A polarization fluctuation detection method according to a first aspect of the present disclosure includes detecting polarization fluctuations of multiple optical signals that are transmitted in a predetermined direction through a transmission path at different propagation speeds, and estimating a position in the transmission path where the polarization fluctuations occur based on the time difference between the detection times of the polarization fluctuations between the multiple optical signals.

[0008] A polarization fluctuation detection device according to a second aspect of the present disclosure includes a fluctuation detection unit that detects polarization fluctuations of multiple optical signals that are transmitted in a predetermined direction through a transmission path at different propagation speeds, and an estimation unit that estimates the position at which the polarization fluctuations occur in the transmission path based on the time difference at which the polarization fluctuations between the multiple optical signals are detected.

[0009] A polarization fluctuation detection system according to a third aspect of the present disclosure comprises a transmitter that outputs a plurality of optical signals, and a polarization fluctuation detection device that receives the plurality of optical signals output from the transmitter via a transmission path that transmits the plurality of optical signals in a predetermined direction at different propagation speeds, wherein the polarization fluctuation detection device includes a fluctuation detection unit that detects polarization fluctuations in the received plurality of optical signals, and an estimation unit that estimates the position at which the polarization fluctuation occurs in the transmission path based on the time difference at which the polarization fluctuations between the plurality of optical signals are detected. [Effects of the Invention]

[0010] The polarization fluctuation detection method, polarization fluctuation detection device, and polarization fluctuation detection system according to the present disclosure can estimate the position where polarization fluctuation has occurred even when polarization fluctuation has occurred at multiple points on a transmission path. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a block diagram illustrating an example configuration of a polarization fluctuation detection system according to the present disclosure. [Figure 2] 1 is a block diagram showing an example of the configuration of a polarization fluctuation detection device; [Figure 3] FIG. 10 is a waveform diagram showing a specific example of detection of polarization fluctuation. [Figure 4] FIG. 10 is a schematic diagram showing a specific example of estimating the position where polarization fluctuation occurs. [Figure 5] 4 is a flowchart showing an operation procedure of the polarization fluctuation detection device. [Figure 6] 1 is a block diagram showing an example of the configuration of an optical fiber communication system. [Figure 7] 1 shows an example of the configuration of an optical transmitter. [Figure 8] 1 shows an example of the configuration of an optical receiver. [Figure 9] 1 is a block diagram showing an example of the configuration of a signal processing circuit that can be used as a polarization fluctuation detection device 15. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the following description and drawings have been omitted and simplified as appropriate for clarity of explanation. In addition, in each drawing, the same or similar elements are designated by the same reference numerals, and duplicate explanations are omitted as necessary.

[0013] Fig. 1 is a block diagram showing an example configuration of a polarization fluctuation detection system according to the present disclosure. One embodiment of the present disclosure will be described using Fig. 1. As shown in Fig. 1, a polarization fluctuation detection system 10 includes a transmitter 11 and a polarization fluctuation detection device 15. In the polarization fluctuation detection system 10, the transmitter 11 is disposed at one end of a transmission line 13, i.e., a first end. The polarization fluctuation detection device 15 is disposed at the other end of the transmission line 13, i.e., a second end that is the end opposite the first end.

[0014] The transmitter 11 outputs a plurality of optical signals. The transmitter 11 has, for example, a plurality of light sources corresponding to the plurality of optical signals, and outputs the optical signals output from the plurality of light sources to a transmission path 13. The transmitter 11 outputs, for example, two optical signals, i.e., a first optical signal and a second optical signal, to the transmission path 13. The plurality of optical signals output from the transmitter 11 are multiplexed in the transmission path 13 and received by a polarization fluctuation detection device 15, which is a receiver.

[0015] A group delay velocity difference occurs among the multiple optical signals transmitted through the transmission line 13. In other words, the transmission line 13 transmits the multiple optical signals from the transmitter 11 side to the polarization fluctuation detection device 15 side at mutually different propagation velocities.

[0016] In the transmission path 13, the multiple optical signals may be multiplexed using fiber multiplexing. For example, the transmission path 13 includes multiple optical fibers. The multiple optical fibers have different refractive index profiles. In the transmission path 13, the multiple optical fibers are inserted, for example, into the sheath of an optical fiber cable that constitutes the transmission path 13. The multiple optical signals output from the transmitter 11 are input, for example, to different optical fibers in the optical fiber cable and transmitted to the polarization fluctuation detection device 15. In the transmission path 13, the multiple optical signals are transmitted at different propagation speeds depending on the differences in the refractive index profiles of the optical fibers through which they are transmitted.

[0017] Specifically, the transmission path 13 may include a first optical fiber that is a single-mode fiber conforming to ITU-T (International Telecommunication Union-Telecommunication Sector) G.652 and a second optical fiber that is a cutoff shifted fiber (CSF) conforming to ITU-T G.654. ​​The first optical signal is transmitted using the first optical fiber, and the second optical signal is transmitted using the second optical fiber. The group delay time per unit length of the first optical fiber is, for example, 4.901 (μs / km). The group delay time per unit length of the second optical fiber is, for example, 4.883 (μs / km). In this case, the time required for the first optical signal and the second optical signal to reach the receiving end from the transmitting end varies depending on the difference in group delay time between the two types of optical fibers.

[0018] In the transmission path 13, the multiple optical signals may be multiplexed by core multiplexing using a multicore fiber. In this case, the transmission path 13 includes a multicore fiber having multiple cores. In the multicore fiber, the multiple cores have different refractive index profiles. The multiple optical signals output from the transmitter 11 are input to different cores in the multicore fiber and transmitted to the polarization fluctuation detection device 15. For example, the multicore fiber has a first core and a second core with different refractive index profiles. In the multicore fiber, the first optical signal is transmitted using the first core, and the second optical signal is transmitted using the second core. In this case, as in the case where multiple optical fibers are used as described above, the multiple optical signals are transmitted at different propagation speeds depending on the difference in the refractive index profiles of the cores through which they are transmitted.

[0019] In the transmission path 13, the multiple optical signals may be mode-multiplexed in multiple modes. In this case, the transmission path 13 includes a multimode fiber. In the multimode fiber, the multiple optical signals are multiplexed in, for example, mutually different LP (Linearly Polarized) modes. In the multimode fiber, the first optical signal is transmitted in, for example, an LP01 mode, and the second optical signal is transmitted in an LP11 mode. The group delay time per unit length of the fiber between the modes is, for example, approximately 8.7 (ns / km). In this case, too, the multiple optical signals are transmitted at different propagation speeds depending on the transmission mode.

[0020] In the transmission path 13, the optical signals may be multiband multiplexed in a plurality of bands having mutually different wavelength bands. In this case, the transmitter 11 outputs optical signals of a plurality of wavelengths having mutually different frequency bands to the transmission path 13. The plurality of bands include an O-band (original band) of 18 THz, an E-band (extended band) of 15 THz, an S-band (short-wavelength band) of 9 THz, a C-band (conventional band) of 4.5 THz, an L-band (long-wavelength band) of 7 THz, and a U-band (ultralong-wavelength band) of 5.5 THz. For example, an optical signal having a wavelength of 1260 nm, which is the shortest wavelength in the O-band, is used as the first optical signal, and an optical signal having a wavelength of 1675 nm, which is the longest wavelength in the U-band, is used as the second optical signal. If the chromatic dispersion is 20 ps / nm / km, the group delay time per unit distance for an optical signal with a wavelength of 1260 nm and an optical signal with a wavelength of 1675 nm will be approximately 8 ns / km. Even when multiple optical signals are multiplexed using multiband multiplexing, the multiple optical signals are transmitted through transmission path 130 at different propagation speeds.

[0021] The above-described methods for multiplexing multiple optical signals may be used alone or in appropriate combination. As an example, fiber multiplexing and multiband multiplexing may be used in combination. In this case, the transmission path 13 may use, for example, a first optical fiber that is a single-mode fiber to transmit an optical signal having a wavelength of 1260 nm as a first optical signal, and a cutoff shift fiber to transmit an optical signal having a wavelength of 1675 nm as a second optical signal.

[0022] The polarization fluctuation detection device 15 receives the multiple optical signals transmitted from the transmitter 11 via the transmission path 13. If a disturbance such as an impact or vibration is applied to the optical fiber cable at a specific position on the transmission path 13, a polarization fluctuation occurs in each of the multiple optical signals transmitted through the transmission path 13. The polarization fluctuation detection device 15 detects the polarization fluctuation occurring in the multiple optical signals received. Based on the detected polarization fluctuation, the polarization fluctuation detection device 15 estimates the position on the transmission path 13 where the disturbance was applied, i.e., the point where the polarization fluctuation occurred.

[0023] 2 is a block diagram showing an example configuration of the polarization fluctuation detection device 15. The polarization fluctuation detection device 15 has a fluctuation detection unit 21 and an estimation unit 22. The polarization fluctuation detection device 15 can be physically configured as a device including one or more memories and one or more processors. At least a part of the function of each unit in the polarization fluctuation detection device 15 can be realized by one or more processors executing processing in accordance with instructions read from one or more memories.

[0024] The fluctuation detection unit 21 detects polarization fluctuations of multiple optical signals transmitted in a predetermined direction through the transmission path 13. The fluctuation detection unit 21 includes, for example, a polarizer, a photodetector, and a timer. The fluctuation detection unit 21 monitors the polarization state of each of the multiple optical signals. In monitoring the polarization state, the fluctuation detection unit 21 acquires the amount of polarization fluctuation for each of the multiple optical signals. If the amount of polarization fluctuation exceeds a predetermined threshold, the fluctuation detection unit 21 detects polarization fluctuation. The fluctuation detection unit 21 measures the time difference at which polarization fluctuations are detected between the multiple optical signals.

[0025] 3 is a waveform diagram showing a specific example of polarization fluctuation detection. For example, the fluctuation detection unit 21 acquires the amount of polarization fluctuation, which may change from moment to moment, for each of the first optical signal and the second optical signal. The fluctuation detection unit 21 compares the amount of polarization fluctuation with a predetermined threshold. The fluctuation detection unit 21 detects the time t1 at which the amount of polarization fluctuation for the first optical signal exceeds the threshold. The fluctuation detection unit 21 also detects the time t2 at which the amount of polarization fluctuation for the second optical signal exceeds the threshold.

[0026] The fluctuation detection unit 21 measures the time difference between time t1 and time t2. For example, the fluctuation detection unit 21 starts the operation of a timer at time t1 and stops the operation of the timer at time t2, thereby measuring the time difference between time t1 and time t2. Time t1 is also called the first time, and time t2 is also called the second time.

[0027] The estimation unit 22 estimates the position where polarization fluctuation occurs in the transmission path 13 based on the time difference at which polarization fluctuation between the multiple optical signals is detected. For example, when polarization fluctuation occurs at a certain position in the transmission path 13, the time at which the polarization fluctuation is observed at the end where the polarization fluctuation detection device 15, which is the receiving end, is located varies depending on the propagation speed of the optical signal. The estimation unit 22 estimates the position where polarization fluctuation occurs in the transmission path 13 using the time difference at which polarization fluctuation between the multiple optical signals is detected and the propagation speeds of the multiple optical signals in the transmission path 13.

[0028] 4 is a schematic diagram showing a specific example of estimating the position where polarization fluctuation occurs. In this example, the total length of transmission line 13 is assumed to be L [m]. In transmission line 13, polarization fluctuation occurs at point P, which is xm [m] away from transmitting end Tx where transmitter 11 is located. In this case, the polarization fluctuations occurring in the first optical signal and the second optical signal are detected at receiving end Rx, which is a distance Lx [m] away from point P.

[0029] Let v1 be the propagation velocity of the first optical signal, and v2 be the propagation velocity of the second optical signal. Furthermore, let v1 be faster than v2, i.e., v1 > v2. Let t0 be the time when polarization fluctuation occurs. In this case, the time t1 when the polarization fluctuation of the first optical signal reaches the receiving end Rx, and the time t2 when the polarization fluctuation of the second optical signal reaches the receiving end Rx are expressed by the following equations: t1=(LX) / v1+t0 (1) t2=(LX) / v2+t0 (2)

[0030] If v1>v2, time t2 is later than time t1. The difference in time t2-t1 when the polarization fluctuation reaches the receiving end Rx is expressed by the following equation. t2-t1=(LX) / v2+t0-{(LX) / v1+t0} =(LX) / v2-(LX) / v1 =(LX)(1 / v2-1 / v1) =(LX)(v1-v2) / (v1×v2) (3) By modifying the above formula 3, the following formula 4 is obtained. Lx=v1×v2 / (v1-v2)×(t2-t1) (4)

[0031] Let v1 be the propagation velocity of the first optical signal and v2 be the propagation velocity of the second optical signal. The propagation velocity of each optical signal is the group velocity of the respective light waves, which can be calculated or measured from the wavelength and the refractive index distribution of the core. In this case, the distance Lx from point P to the receiving end can be calculated from the difference in time (t2-t1) at which the polarization fluctuation arrives at the receiving end using Equation 4. Even if polarization fluctuation occurs at multiple points in the transmission path 13, the order in which the polarization fluctuations arrive at the receiving end is the same for the first optical signal and the second optical signal. Therefore, in this embodiment, even if polarization fluctuations occur at multiple points in the transmission path, the point at which the polarization fluctuation occurred can be accurately estimated.

[0032] Next, the operation procedure will be explained. Fig. 5 is a flowchart showing the operation procedure of the polarization fluctuation detection device 15. The operation procedure of the polarization fluctuation detection device 15 corresponds to a polarization fluctuation detection method. The transmitter 11 transmits a plurality of optical signals having different propagation speeds to the polarization fluctuation detection device 15 via the transmission path 13. In the polarization fluctuation detection device 15, the fluctuation detection unit 21 monitors the polarization state of each of the plurality of optical signals (step S1).

[0033] The fluctuation detection unit 21 detects polarization fluctuation in each of the multiple optical signals (step S2). For example, in step S2, the fluctuation detection unit 21 detects the time when the amount of polarization fluctuation in each optical signal exceeds a predetermined threshold as the time when polarization fluctuation is detected. The estimation unit 22 estimates the position where polarization fluctuation occurs based on the time difference when polarization fluctuation is detected between the multiple optical signals (step S3). The estimation unit 22 estimates the position where polarization fluctuation occurs in the transmission path 13 from, for example, the time difference between the time when polarization fluctuation is detected in the first optical signal and the time when polarization fluctuation is detected in the second optical signal, the propagation speed of the first optical signal, and the propagation speed of the second optical signal.

[0034] In this embodiment, the transmitter 11 outputs a plurality of optical signals to the transmission path 13. In the transmission path 13, the plurality of optical signals are transmitted at mutually different propagation speeds. The polarization fluctuation detection device 15 receives the plurality of optical signals via the transmission path 13. In this case, when polarization fluctuation occurs in the transmission path 13, the time required for the polarization fluctuation to be received by the polarization fluctuation detection device 15 at the receiving end varies depending on the propagation speed of the optical signal. In the polarization fluctuation detection device 15, the fluctuation detection unit 21 detects polarization fluctuation in each of the plurality of optical signals. The estimation unit 22 estimates the point at which polarization fluctuation occurs in the transmission path 13 based on the time difference at which polarization fluctuation is detected in the plurality of optical signals and the propagation speed of each optical signal.

[0035] In comparison with Patent Document 1, Patent Document 1 uses two optical signals transmitted in opposite directions. In Patent Document 1, the distance in the transmission path from the point where polarization fluctuation occurs to the point where the optical signal is received differs between the two signals. In this case, if polarization fluctuation occurs at multiple points, the order in which polarization fluctuation is detected at both ends may change depending on the position where the polarization fluctuation occurs and the timing at which the polarization fluctuation occurs.

[0036] For example, consider a case where a first polarization fluctuation occurs in a transmission line, and then a second polarization fluctuation occurs in the transmission line. In this case, in Patent Document 1, the order in which the first polarization fluctuation and the second polarization fluctuation are detected at one end and the other end of the transmission line may change depending on the position and timing at which each polarization fluctuation occurs. If the polarization fluctuations detected in both optical signals cannot be correctly associated with each other, the point at which the polarization fluctuation occurred cannot be correctly estimated.

[0037] In contrast, in this embodiment, the polarization fluctuation detection device 15 detects polarization fluctuations using multiple optical signals received at one end of the transmission line 13. The polarization fluctuation detection device 15 utilizes the time difference in polarization fluctuation detection that corresponds to the difference in propagation speed between the multiple optical signals. In this case, the order in which the first polarization fluctuation and the second polarization fluctuation are detected in the first optical signal is the same as the order in which the first polarization fluctuation and the second polarization fluctuation are detected in the second optical signal. Therefore, even if polarization fluctuations occur at multiple locations in the transmission line 13, the polarization fluctuation detection device 15 can correctly estimate the position at which the polarization fluctuation occurred.

[0038] The polarization fluctuation detection system 10 can be applied to a communication system such as an optical fiber communication system. Fig. 6 is a block diagram showing an example of the configuration of an optical fiber communication system. In the following description, it is assumed that the communication system is an optical fiber communication system that employs a polarization multiplexing multilevel modulation method and performs coherent reception. It is also assumed that the communication system is a communication system that multiplexes optical signals of multiple wavelengths using a Wavelength Division Multiplexing (WDM) method. The multiplexing method is not limited to the WDM method, and the multiple optical signals may also be multiplexed using a spatial multiplexing method.

[0039] The optical fiber communication system 100 includes a plurality of optical transmitters 110, a multiplexer 120, a transmission line 130, a demultiplexer 140, and a plurality of optical receivers 150. The optical fiber communication system 100 constitutes, for example, a land metro communication system or an optical submarine cable system.

[0040] The optical transmitter 110 converts multiple transmission data into a polarization multiplexed signal. The multiplexer 120 multiplexes the multiple polarization multiplexed signals output from the multiple optical transmitters 110. The transmission path 130 transmits the optical signal output from the multiplexer 120 to the optical receiver 150. The optical transmitter 110 is also referred to as Tx.

[0041] The transmission path 130 includes an optical fiber 132 and an optical amplifier 133. The optical fiber 132 guides the optical signal transmitted from the optical transmitter 110. The optical amplifier 133 amplifies the optical signal and compensates for propagation loss in the optical fiber 132. The optical amplifier 133 is configured as, for example, an erbium-doped fiber amplifier (EDFA).

[0042] The demultiplexer 140 demultiplexes the polarization multiplexed signal multiplexed by WDM and converts the polarization multiplexed signal multiplexed by WDM into multiple polarization multiplexed signals. The demultiplexer 140 outputs the multiple polarization multiplexed signals to multiple optical receivers 150. Each optical receiver 150 receives the polarization multiplexed signal transmitted from the corresponding optical transmitter 110. The optical receiver 150 is also referred to as Rx.

[0043] 6 shows an example in which the optical fiber communication system 100 has three optical transmitters 110 and three optical receivers 150, but the numbers of optical transmitters 110 and optical receivers 150 are not limited to three. Also, while Fig. 6 shows an example in which the transmission line 130 has three optical amplifiers 133, the number of optical amplifiers 133 in the transmission line 130 is not limited to three.

[0044] 7 shows an example of the configuration of the optical transmitter 110. The optical transmitter 110 includes an encoding unit 111, a pre-equalization unit 112, a digital-to-analog converter (DAC) 113, an optical modulator 114, and a laser diode (LD) 115. The encoding unit 111 encodes data. The encoding unit 111 outputs four series of signals, for example, in-phase (I) components of X polarization and Y polarization, and quadrature (Q) components.

[0045] The pre-equalization unit 112 performs pre-equalization on the coded four-sequence signal to compensate in advance for distortions of devices within the optical transmitter. The pre-equalization unit 112 has, for example, a multiple-input and multiple-output (MIMO) filter for each polarization, with I and Q components as inputs and outputs. The MIMO filter compensates for distortions occurring within the optical transmitter 110, such as distortions occurring in the I and Q components in each polarization and crosstalk occurring between I and Q.

[0046] The DAC 113 converts each of the four series of signals that have been pre-equalized into an analog electrical signal. The DAC 113 inputs the converted analog electrical signals to the optical modulator 114. An electrical amplifier is disposed between the DAC 113 and the optical modulator 114, and the analog electrical signals whose amplitudes have been amplified by the electrical amplifier are input to the optical modulator 114.

[0047] The LD 115 outputs continuous wave (CW) light. The optical modulator 114 is a modulator that modulates the CW light output from the LD 115 in accordance with four-series analog electrical signals input from the DAC 113 to generate a polarization multiplexed optical signal such as a polarization multiplexed quadrature-amplitude modulation (QAM) signal. The optical modulator 114 includes, for example, a Mach-Zehnder (MZ) modulator. The optical modulator 114 outputs the generated polarization multiplexed signal to the multiplexer 120.

[0048] 8 shows a configuration example of the optical receiver 150. The optical receiver 150 includes an LD 151, a coherent receiver 152, an analog-to-digital converter (ADC) 153, a digital signal processing unit 154, and a decoding unit 155. The LD 151 outputs CW light that serves as local oscillator light. The coherent receiver 152 is configured as a polarization diversity coherent receiver. The coherent receiver 152 uses the CW light output from the LD 151 to perform coherent detection on the optical signal transmitted through the optical fiber 132. The coherent receiver 152 outputs four series of received signals (electrical signals) corresponding to the I and Q components of the coherently detected X and Y polarizations.

[0049] The ADC 153 receives the received signal output from the coherent receiver 152 via an electric amplifier. The ADC 153 samples the received signal output from the coherent receiver 152 and converts the received signal into a digital signal. The ADC 153 outputs the converted digital signal to the digital signal processor 154. The digital signal processor 154 performs digital signal processing on the four series of received signals sampled by the ADC 153 and demodulates the received signal. The digital signal processor 154 includes an equalization filter, which compensates for various distortions contained in the digital signal. Specifically, the digital signal processor 154 performs, for example, chromatic dispersion compensation, carrier phase compensation, and polarization fluctuation compensation.

[0050] In the above-described optical fiber communication system 100, at least two optical transmitters 110 may be used as the transmitter 11 shown in Fig. 1. In this case, optical signals output from the at least two optical transmitters 110 are multiplexed in a transmission path 130 by fiber multiplexing, core multiplexing, mode multiplexing, wavelength multiplexing, or a combination of two or more of these. In the optical receiver 150, the optical signal transmitted through the transmission path 130 may be branched to a polarization fluctuation detection device 15 shown in Fig. 1, and the polarization fluctuation detection device 15 may estimate the position of polarization fluctuation occurring in the transmission path 130.

[0051] 1 in addition to the optical transmitter 110 and the optical receiver 150. In this case, the transmitter 11 may output a plurality of optical signals having wavelengths different from the wavelength of the main signal, i.e., the polarization multiplexed signals output by the plurality of optical transmitters 110. At least some of the plurality of optical signals output by the transmitter 11 may be used as an OSC (Optical Supervisory Channel) signal or a supervisory optical signal used for setting up the operation of the transmission line 130 and for monitoring the state of the transmission line 130.

[0052] In the optical fiber communication system 100, a plurality of optical signals output from the transmitter 11 are branched from the transmission path 130 to the polarization fluctuation detection device 15. For example, the plurality of optical signals output from the transmitter 11 may be selectively branched from the optical fiber to the polarization fluctuation detection device 15 at the receiving end using a branching filter such as a wavelength demultiplexer or a wavelength selective switch.

[0053] In the above-described optical fiber communication system 100, the number of pairs of transmitters 11 and polarization fluctuation detection devices 15 is not limited to one. The optical fiber communication system 100 may have a plurality of pairs of transmitters 11 and polarization fluctuation detection devices 15. For example, the optical fiber communication system 100 may have a pair of transmitters 11 and polarization fluctuation detection devices for each predetermined span in the transmission line 130.

[0054] Next, the physical configuration of the polarization fluctuation detection device 15 will be described. Fig. 9 is a block diagram showing an example configuration of a signal processing circuit that can be used as the polarization fluctuation detection device 15. A digital signal processing circuit 500 has one or more processors 510 and one or more memories 520. In the digital signal processing circuit 500, the one or more processors 510 read out programs stored in the one or more memories 520 and execute processing on the read out programs. This allows at least part of the functions of the fluctuation detection unit 21 and the estimation unit 22 to be realized.

[0055] The program includes instructions (or software code) that, when loaded into a computer or processor, cause the computer to perform one or more functions described in the embodiments. The program may be stored in a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, computer-readable media or tangible storage media include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technologies, compact discs (CDs), digital versatile discs (DVDs), Blu-ray discs or other optical disk storage, magnetic cassettes, magnetic tapes, magnetic disk storage or other magnetic storage devices. The program may also be transmitted on a transitory computer-readable medium or communication medium. By way of example and not limitation, transitory computer-readable media or communication media include electrical, optical, acoustic, or other forms of propagated signals.

[0056] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0057] Each drawing is merely an example for describing one or more embodiments. Each drawing may relate not only to one particular embodiment, but also to one or more other embodiments. As will be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0058] A part or all of the above-described embodiments can be described as, but not limited to, the following supplementary notes.

[0059] [Appendix 1] each detecting polarization fluctuation of a plurality of optical signals transmitted in a predetermined direction through a transmission line at different propagation speeds; a time difference between the detection times of the polarization fluctuations among the plurality of optical signals, the time difference being used to estimate a position where the polarization fluctuation occurs in the transmission path;

[0060] [Appendix 2] 2. A polarization fluctuation detection method as described in claim 1, wherein the plurality of optical signals are transmitted from a first end of the transmission line to a second end of the transmission line, and the polarization fluctuation of the plurality of optical signals is detected at the second end.

[0061] [Appendix 3] 3. The polarization fluctuation detection method according to claim 1, wherein the transmission path includes a plurality of optical fibers having different refractive index distributions, and the plurality of optical signals are transmitted using the plurality of optical fibers.

[0062] [Appendix 4] 4. The polarization fluctuation detection method according to claim 1, wherein the transmission path includes a multicore fiber including a plurality of cores having mutually different refractive index distributions, and the plurality of optical signals are transmitted using the plurality of cores of the multicore fiber.

[0063] [Appendix 5] 5. The polarization fluctuation detection method according to claim 1, wherein the transmission path includes a multimode fiber, and the plurality of optical signals are transmitted in different transmission modes in the multimode fiber.

[0064] [Appendix 6] 6. The polarization fluctuation detection method according to claim 1, wherein the plurality of optical signals include optical signals of a plurality of wavelengths having mutually different frequency bands.

[0065] [Appendix 7] the plurality of optical signals include a first optical signal and a second optical signal; A polarization fluctuation detection method according to any one of appendices 1 to 6, wherein a position where the polarization fluctuation occurs in the transmission path is estimated based on the time difference between a first time when the polarization fluctuation in the first optical signal exceeds a threshold value and a second time when the polarization fluctuation in the second optical signal exceeds a threshold value, the propagation speed of the first optical signal in the transmission path, and the propagation speed of the second optical signal in the transmission path.

[0066] [Appendix 8] a fluctuation detection unit that detects polarization fluctuations of a plurality of optical signals transmitted in a predetermined direction through a transmission line at different propagation speeds; and an estimation unit that estimates a position where the polarization fluctuation occurs in the transmission path based on a time difference at which the polarization fluctuation between the plurality of optical signals is detected.

[0067] [Appendix 9] 9. The polarization fluctuation detection device described in Appendix 8, wherein the plurality of optical signals are transmitted from a first end of the transmission path to a second end of the transmission path, and the fluctuation detection unit detects the polarization fluctuation of the plurality of optical signals at the second end.

[0068] [Appendix 10] a transmitter that outputs a plurality of optical signals; a polarization fluctuation detection device that receives the plurality of optical signals output from the transmitter via a transmission path that transmits the plurality of optical signals in a predetermined direction at different propagation speeds, The polarization fluctuation detection device a fluctuation detection unit that detects polarization fluctuations of the received optical signals; an estimation unit that estimates a position where the polarization fluctuation occurs in the transmission path based on a time difference at which the polarization fluctuation between the plurality of optical signals is detected.

[0069] Some or all of the elements (e.g., configurations and functions) described in Supplementary Notes 2 to 7 that are dependent on Supplementary Note 1 may also be dependent on Supplementary Notes 8 and 10 in the same dependency relationship as Supplementary Notes 2 to 7. Some or all of the elements described in any Supplementary Note may be applied to various hardware, software, recording means for recording software, systems, and methods. [Explanation of symbols]

[0070] 10: Polarization fluctuation detection system 11:Transmitter 13: Transmission path 15: Polarization fluctuation detector 21: Fluctuation detection unit 22: Estimation part 100: Optical fiber communication system 110: Optical transmitter 111: Encoding section 112: Pre-equalization section 113:DAC 114: Optical modulator 115:LD 120: Multiplexer 130: Transmission line 132: Optical fiber 133: Optical amplifier 134: Multiplexer 135: Duplexer 140: Demultiplexer 150: Optical receiver 151:LD 152: Coherent receiver 153:ADC 154: Digital signal processing unit 155: Decryption unit

Claims

1. each detecting polarization fluctuation of a plurality of optical signals transmitted in a predetermined direction through a transmission line at different propagation speeds; a time difference between the detection times of the polarization fluctuations among the plurality of optical signals, the time difference being used to estimate a position where the polarization fluctuation occurs in the transmission path;

2. 2. The polarization fluctuation detection method according to claim 1, wherein the plurality of optical signals are transmitted from a first end of the transmission line to a second end of the transmission line, and the polarization fluctuation of the plurality of optical signals is detected at the second end.

3. 3. The polarization fluctuation detection method according to claim 1, wherein the transmission path includes a plurality of optical fibers having different refractive index profiles, and the plurality of optical signals are transmitted using the plurality of optical fibers.

4. 3. The polarization fluctuation detection method according to claim 1, wherein the transmission path includes a multicore fiber including a plurality of cores having mutually different refractive index distributions, and the plurality of optical signals are transmitted using the plurality of cores of the multicore fiber.

5. 3. The polarization fluctuation detection method according to claim 1, wherein the transmission path includes a multimode fiber, and the plurality of optical signals are transmitted in different transmission modes in the multimode fiber.

6. 3. The polarization fluctuation detection method according to claim 1, wherein the plurality of optical signals include optical signals of a plurality of wavelengths having mutually different frequency bands.

7. the plurality of optical signals include a first optical signal and a second optical signal; 3. The polarization fluctuation detection method according to claim 1, further comprising estimating a position where the polarization fluctuation occurs in the transmission path based on a time difference between a first time when the polarization fluctuation in the first optical signal exceeds a threshold value and a second time when the polarization fluctuation in the second optical signal exceeds a threshold value, a propagation speed of the first optical signal in the transmission path, and a propagation speed of the second optical signal in the transmission path.

8. a fluctuation detection unit that detects polarization fluctuations of a plurality of optical signals transmitted in a predetermined direction through a transmission line at different propagation speeds; and an estimation unit that estimates a position where the polarization fluctuation occurs in the transmission path based on a time difference at which the polarization fluctuation between the plurality of optical signals is detected.

9. 9. The polarization fluctuation detection device according to claim 8, wherein the plurality of optical signals are transmitted from a first end of the transmission line to a second end of the transmission line, and the fluctuation detection unit detects the polarization fluctuation of the plurality of optical signals at the second end.

10. a transmitter that outputs a plurality of optical signals; a polarization fluctuation detection device that receives the plurality of optical signals output from the transmitter via a transmission path that transmits the plurality of optical signals in a predetermined direction at different propagation speeds, The polarization fluctuation detection device a fluctuation detection unit that detects polarization fluctuations of the received optical signals; an estimation unit that estimates a position where the polarization fluctuation occurs in the transmission path based on a time difference at which the polarization fluctuation between the plurality of optical signals is detected.

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