Polarization fluctuation monitoring device, communication system, polarization fluctuation monitoring method, and program

Abstract

To make it possible to estimate a polarization fluctuation frequency with high accuracy without requiring complicated calculations.SOLUTION: A light source 30 outputs an optical signal to a transmission path 13. A polarization characteristic measurement unit 21 measures the polarization characteristics of the optical signal transmitted through the transmission path 13. A Fourier transform unit 22 performs a Fourier transform on the polarization characteristics measured by the polarization characteristic measurement unit 21. A fluctuation estimation unit 23 estimates a polarization fluctuation frequency that occurs in the transmission path 13 on the basis of the polarization characteristics that have been Fourier transformed by the Fourier transform unit 22.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a polarization fluctuation monitoring device, a communication system, a polarization fluctuation monitoring method, and a program. [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. When 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 monitor system that monitors the polarization fluctuation and detects when a polarization fluctuation that can cause an error occurs is important.

[0003] As a related technique, Patent Document 1 discloses a method for estimating a polarization fluctuation location. In Patent Document 1, a light source sequentially outputs identifiably different signal lights to an optical fiber. An acquisition unit acquires the polarization state of each signal light from the scattered light in the optical fiber for the signal light. The acquisition unit acquires, for example, Stokes parameters S0, S1, S2, and S3 as the polarization state. An estimation unit converts the time-corresponding polarization state of each acquired signal light into a distance-corresponding polarization state that allows the reflection point of the scattered light in the optical fiber to be identified, and estimates the point where the polarization state changes from the converted distance-corresponding polarization state as the polarization fluctuation location.

[0004] As another related technique, Non-Patent Document 1 discloses an in-line polarization monitoring scheme. In Non-Patent Document 1, an Optical Supervisory Channel (OSC) is used to monitor the polarization state. The OSC is generally implemented with an optical signal having a wavelength of 1510 nm outside the band of the main signal. The OSC optical signal is inserted into an optical fiber for each span and extracted from the optical fiber. The OSC optical signal is split into two optical signals using a polarizing beam splitter, and the two split optical signals are detected using two photodetectors, respectively. A processing unit calculates the polarization rotation speed based on the difference between the detection signals of the two photodetectors. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-134161 A [Non-patent literature]

[0006] [Non-Patent Document 1] JE Simsarian and PJ Winzer, “Shake Before Break: Per Span Fiber Sensing with In Line Polarization Monitoring”, OFC2017, M2E.6. Summary of the Invention [Problem to be solved by the invention]

[0007] Non-Patent Document 1 discloses a result of comparing a polarization rotation velocity calculated based on the difference between the detection signals of two photodetectors, a polarization rotation velocity calculated from the Stokes parameters S1-S3, and a polarization rotation velocity calculated from two Stokes parameters. As described in Non-Patent Document 1, the polarization rotation velocity calculated based on the difference between the detection signals of two photodetectors is less accurate than the polarization rotation velocity calculated from the Stokes parameters S1-S3. In addition, the polarization rotation velocity calculated based on the two Stokes parameters is also less accurate than the polarization rotation velocity calculated from the Stokes parameters S1-S3. However, when the polarization rotation velocity is calculated from the Stokes parameters S1-S3, although the accuracy of the polarization rotation velocity is high, there is a problem that the calculation is complicated.

[0008] In view of the above circumstances, the present disclosure aims to provide a polarization fluctuation monitoring device, a communication system, a polarization fluctuation monitoring method, and a program that can accurately estimate a polarization fluctuation frequency without requiring complicated calculations. [Means for solving the problem]

[0009] The present disclosure provides a polarization fluctuation monitoring device including a polarization characteristic measurement unit that measures the polarization characteristics of an optical signal transmitted through a transmission path, a Fourier transform unit that performs a Fourier transform on the measured polarization characteristics, and a fluctuation estimation unit that estimates a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

[0010] The present disclosure provides a communication system including a transmitter that transmits a polarization multiplexed signal, a receiver that receives the polarization multiplexed signal transmitted from the transmitter via a transmission path, the polarization fluctuation monitoring device, and a light source that outputs the optical signal to the transmission path.

[0011] The present disclosure provides a polarization fluctuation monitoring method that includes measuring the polarization characteristics of an optical signal transmitted through a transmission path, performing a Fourier transform on the measured polarization characteristics, and estimating a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

[0012] The present disclosure provides a program for causing a computer to execute a process of acquiring polarization characteristics of an optical signal transmitted through a transmission path, performing a Fourier transform on the acquired polarization characteristics, and estimating a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics. Effect of the Invention

[0013] The polarization fluctuation monitoring device, communication system, polarization fluctuation monitoring method, and program according to the present disclosure can estimate the polarization fluctuation frequency with high accuracy without requiring complicated calculations. [Brief description of the drawings]

[0014] [Figure 1] 1 is a block diagram illustrating a schematic diagram of a communication system according to the present disclosure. [Diagram 2] 1 is a block diagram showing a configuration example of a first communication system according to the present disclosure. [Diagram 3] FIG. 1 is a block diagram showing a configuration example of an optical transmitter. [Figure 4] FIG. 1 is a block diagram showing a configuration example of an optical receiver. [Diagram 5] FIG. 2 is a block diagram showing a configuration example of a digital signal processing unit. [Figure 6] FIG. 1 is a block diagram showing a configuration example of a polarization fluctuation monitoring device. [Figure 7] FIG. 1 is a block diagram showing a system used in an experiment. [Figure 8] 1 is a graph showing an FFT waveform of the Stokes parameter S1. [Figure 9] 1 is a graph showing an FFT waveform of the Stokes parameter S2. [Figure 10] 13 is a graph showing an FFT waveform of the Stokes parameter S3. [Figure 11] 1 is a graph showing an FFT waveform of the Stokes parameter S1. [Figure 12] 1 is a graph showing an FFT waveform of the Stokes parameter S1. [Figure 13] 4 is a flowchart showing an operation procedure of the polarization fluctuation monitoring device. [Figure 14]FIG. 1 is a block diagram showing an example of the configuration of a Stokes parameter measurement device. [Figure 15] 11 is a block diagram showing a configuration example of a polarization fluctuation monitoring device used in a second communication system of the present disclosure. [Figure 16] 1 is a graph showing an FFT waveform of the Stokes parameter S1 obtained in an experiment. [Figure 17] FIG. 1 is a block diagram showing an example of the configuration of a computer device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Prior to describing the embodiments of the present disclosure, an overview of the present disclosure will be described. Fig. 1 shows a schematic diagram of a communication system according to the present disclosure. The communication system 10 includes a transmitter 11, a receiver 15, a polarization fluctuation monitoring device 20, and a light source 30. The transmitter 11 and the receiver 15 are connected to each other via a transmission path 13. The transmitter 11 outputs a polarization multiplexed signal to the transmission path 13. The receiver 15 receives the polarization multiplexed signal transmitted from the transmitter 11 via the transmission path 13.

[0016] The light source 30 outputs an optical signal used for monitoring the polarization state to the transmission line. The optical signal output from the light source 30 is, for example, added to the transmission line 13 using a multiplexer and branched from the transmission line 13 using a filter. The optical signal output from the light source 30 is transmitted through at least a part of the transmission line and input to the polarization fluctuation monitoring device 20.

[0017] The polarization fluctuation monitoring device 20 has a polarization characteristic measuring unit 21, a Fourier transform unit 22, and a fluctuation estimating unit 23. The polarization characteristic measuring unit 21 measures the polarization characteristics of an optical signal output from a light source 30 and transmitted through a transmission path. The Fourier transform unit 22 performs a Fourier transform on the measured polarization characteristics. The fluctuation estimating unit 23 estimates the polarization fluctuation frequency occurring in the transmission path 13 based on the Fourier transformed polarization characteristics.

[0018] In the present disclosure, the Fourier transform unit 22 performs a Fourier transform on the polarization characteristics measured by the polarization characteristic measurement unit 21. The fluctuation estimation unit 23 estimates the polarization fluctuation frequency based on the Fourier transformed polarization characteristics. In the present disclosure, the Fourier transform is used to estimate the polarization fluctuation frequency, and the polarization fluctuation monitoring device 20 according to the present disclosure can estimate the polarization fluctuation frequency from the waveform of the Fourier transformed polarization characteristics without calculating all of the Stokes parameters. Therefore, the polarization fluctuation monitoring device 20 can accurately estimate the polarization fluctuation frequency without requiring complicated calculations.

[0019] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the following description and drawings are omitted and simplified as appropriate for clarity of explanation. In addition, in each drawing, the same elements and similar elements are given the same reference numerals, and duplicate explanations are omitted as necessary.

[0020] A first embodiment of the present disclosure will be described. Fig. 2 shows a configuration example of a first communication system according to the present disclosure. In this embodiment, 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 in which optical signals of multiple wavelengths are multiplexed by a Wavelength Division Multiplexing (WDM) method. The multiplexing method is not limited to the WDM method, and multiple optical signals may be multiplexed by a spatial multiplexing method.

[0021] 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. The optical fiber communication system 100 corresponds to the communication system 10 shown in FIG.

[0022] The optical transmitter 110 converts a plurality of transmission data into a polarization multiplexed signal. The multiplexer 120 multiplexes a plurality of polarization multiplexed signals output from the plurality of 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 called Tx. The optical transmitter 110 corresponds to the transmitter 11 shown in FIG. 1.

[0023] The transmission line 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 the propagation loss in the optical fiber 132. The optical amplifier 133 is configured as, for example, an erbium doped fiber amplifier (EDFA). The transmission line 130 corresponds to the transmission line 13 shown in FIG. 1.

[0024] The demultiplexer 140 demultiplexes the polarization multiplexed signal multiplexed by WDM and converts the polarization multiplexed signal multiplexed by WDM into a plurality of polarization multiplexed signals. The demultiplexer 140 outputs the plurality of polarization multiplexed signals to a plurality of optical receivers 150. Each optical receiver 150 receives a polarization multiplexed signal transmitted from a corresponding optical transmitter 110. The optical receiver 150 is also referred to as Rx. The optical receiver 150 corresponds to the receiver 15 shown in FIG. 1.

[0025] 2 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 the optical transmitters 110 and the optical receivers 150 are not limited to three. Also, in FIG. 2, an example in which the transmission line 130 has three optical amplifiers 133, but the number of the optical amplifiers 133 in the transmission line 130 is not limited to three.

[0026] 3 shows a configuration example 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, for example, four series of signals including in-phase (I) components of X-polarized waves and Y-polarized waves, and quadrature (Q) components.

[0027] The pre-equalization unit 112 performs pre-equalization on the encoded four-sequence signal to compensate in advance for distortion of devices in 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 distortion occurring in the I and Q components in each polarization, and distortion occurring in the optical transmitter 110, such as crosstalk occurring between I and Q.

[0028] The DAC 113 converts each of the four series of signals that have been subjected to pre-equalization into an analog electric signal. The DAC 113 inputs the converted analog electric signal to the optical modulator 114. An electric amplifier is disposed between the DAC 113 and the optical modulator 114, and the analog electric signal whose amplitude has been amplified by the electric amplifier is input to the optical modulator 114.

[0029] The LD 115 outputs a continuous wave (CW) light. The optical modulator 114 is a modulator that modulates the CW light output from the LD 115 according to four series of analog electric 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.

[0030] 4 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 processor 154, and a decoder 155. The LD 151 outputs CW light that serves as local oscillator light. The coherent receiver 152 is configured as a polarization diversity type coherent receiver. The coherent receiver 152 performs coherent detection on the optical signal transmitted through the optical fiber 132 using the CW light output from the LD 151. 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.

[0031] The received signal output from the coherent receiver 152 is input to the ADC 153 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 signals. The digital signal processor 154 includes an equalization filter, which compensates for various distortions contained in the digital signal.

[0032] 5 shows a configuration example of the digital signal processing unit 154. The digital signal processing unit 154 has chromatic dispersion compensation filters 161X and 161Y, and carrier phase compensation filters 163X and 163Y, which are arranged for the X-polarized wave and the Y-polarized wave, respectively. The digital signal processing unit 154 also has a polarization fluctuation compensation filter 162.

[0033] The digital signal processing unit 154 receives the complex data of the X polarization and the complex data of the Y polarization, which are obtained by converting the IQ components of the X polarization and the IQ components of the Y polarization into complex signals, respectively, output from the ADC 153. In the digital signal processing unit 154, the chromatic dispersion compensation filters 161X and 161Y, the polarization fluctuation compensation filter 162, and the carrier phase compensation filters 163X and 163Y are connected in cascade with respect to the input signals. These filters constitute an adaptive multilayer filter.

[0034] The chromatic dispersion compensation filters 161X and 161Y compensate for signal distortion caused by chromatic dispersion during optical fiber transmission for each polarization. The chromatic dispersion compensation filters 161X and 161Y are also called Chromatic Dispersion Compensation (CDC) filters. The coefficients of the chromatic dispersion compensation filters 161X and 161Y are set based on a physical model of distortion caused by chromatic dispersion. The distortion caused by chromatic dispersion is fixed, and the chromatic dispersion compensation filters 161X and 161Y are treated as static filters.

[0035] The polarization fluctuation compensation filter 162 is configured as a complex coefficient MIMO filter with two complex signal inputs and two complex output. The polarization fluctuation compensation filter 162 compensates for signal distortion caused by polarization state fluctuation and polarization mode dispersion during optical fiber transmission. The carrier phase compensation filters 163X and 163Y compensate for signal distortion caused by frequency offset and phase offset between the carrier of the transmitted optical signal and the local oscillator light on the receiving side. For example, a SL 1×1 1-tap Finite Impulse Response (FIR) filter is used as the carrier phase compensation filters 163X and 163Y. The carrier phase compensation filters 163X and 163Y are also called Carrier Phase Recovery (CPR) filters.

[0036] The digital signal processing unit 154 outputs the signal with various distortions compensated for to the decoding unit 155. The decoding unit 155 decodes the input signal to restore the transmitted data. The decoding unit 155 is also called a decoder. In the optical receiver 150, circuits such as the digital signal processing unit 154 and the decoding unit 155 can be configured using devices such as a digital signal processor (DSP) or the like.

[0037] Returning to Fig. 2, the optical fiber communication system 100 includes a light source 171, a polarization fluctuation monitoring device 172, and a monitor light analyzer 173 in addition to the above-mentioned components. The optical fiber communication system 100 may include a plurality of sets of the light source 171, the polarization fluctuation monitoring device 172, and the monitor light analyzer 173. For example, the optical fiber communication system 100 includes a set of the light source 171, the polarization fluctuation monitoring device 172, and the monitor light analyzer 173 for each predetermined span in the transmission line 130. In the example of Fig. 2, the optical fiber communication system 100 includes one set of the light source 171, the polarization fluctuation monitoring device 172, and the monitor light analyzer 173 for each span defined by two adjacent optical amplifiers 131.

[0038] The light source 171 outputs an optical signal of a predetermined wavelength. The light source 171 outputs an optical signal of a wavelength different from the wavelength of the main signal, that is, the polarization multiplexed signal output by the multiple optical transmitters 110. In this embodiment, the light source 171 outputs an optical signal of a wavelength of 1510 nm. The optical signal output by the light source 171 is also called an OSC signal or a monitoring optical signal used for setting the operation of the transmission line and monitoring the state of the transmission line. In this embodiment, the optical signal output by the light source 171 is also used for monitoring the polarization state. The optical signal output by the light source 171 is inserted into the optical fiber 132 via a multiplexer 134 such as an optical coupler. The optical signal inserted from the light source 171 into the optical fiber 132 may experience polarization fluctuation during transmission through the optical fiber 132. The light source 171 corresponds to the light source 30 shown in FIG. 1.

[0039] The optical signal inserted from the light source 171 into the optical fiber 132 is selectively branched from the optical fiber 132 to the polarization fluctuation monitoring device 172 and the monitor light analyzing device 173 by using a branching filter 135 such as a wavelength demultiplexer or a wavelength selection switch. An optical signal having a wavelength of, for example, 1510 nm output from the light source 171 is input to the polarization fluctuation monitoring device 172 and the monitor light analyzing device 173. The monitor light analyzing device 173 monitors whether there is any change in loss in the transmission path. The monitor light analyzing device 173 also operates the optical amplifier 133 such as an EDFA. The polarization fluctuation monitoring device 172 monitors the polarization fluctuation in the optical fiber 132 by using the input optical signal.

[0040] 6 shows a configuration example of the polarization fluctuation monitoring device 172. The polarization fluctuation monitoring device 172 has a polarimeter 181, an ADC 182, a Fast Fourier Transformation (FFT) 183, and a fluctuation estimation unit 184. The polarization fluctuation monitoring device 172 may be physically configured as a device having one or more processors and one or more memories. At least a part of the function of each unit of the polarization fluctuation monitoring device 172 may be realized by the one or more processors operating in accordance with a program read from the one or more memories. The polarization fluctuation monitoring device 172 corresponds to the polarization fluctuation monitoring device 20 shown in FIG. 1.

[0041] The polarimeter 181 measures the polarization characteristics of the input optical signal. In this embodiment, the polarimeter 181 calculates the Stokes parameters S0-S3. The Stokes parameters S0-S3 are calculated based on the electric field information E of the X-polarized wave and the Y-polarized wave. 0x and E 0y is expressed by the following formula: TIFF2025006142000002.tif3038The Stokes parameter S0 corresponds to the total intensity. The Stokes parameter S1 corresponds to the difference between the horizontal and vertical components of the linearly polarized light. The Stokes parameter S2 corresponds to the 45 degree linearly polarized component. The Stokes parameter S3 corresponds to the circularly polarized component.

[0042] The polarimeter 181 outputs one or more of the Stokes parameters S1-S3 to the ADC 182. The polarimeter 181 outputs, for example, the Stokes parameter S1 to the ADC 182. Note that the polarimeter 181 does not necessarily need to calculate all of the Stokes parameters S0-S3. For example, the polarimeter 181 may calculate only one of the Stokes parameters S1-S3. The polarimeter 181 corresponds to the polarization characteristic measuring unit 21 shown in FIG. 1.

[0043] The ADC 182 converts the Stokes parameters output from the polarimeter 181 into digital signals. The ADC 182 converts the Stokes parameters output in time series into digital signals, and outputs the digital signals in time series to the FFT 183. The FFT 183 performs an FFT on one of the Stokes parameters converted into a digital signal, and converts the one of the Stokes parameters into a frequency domain signal. The FFT 183 corresponds to the Fourier transform unit 22 shown in FIG. 1.

[0044] The fluctuation estimation unit 184 estimates the characteristics of the polarization state based on one of the Stokes parameters converted into a frequency domain signal by the FFT 183. In this embodiment, the fluctuation estimation unit 184 estimates the polarization fluctuation frequency, i.e., the polarization fluctuation speed, as the characteristics of the polarization state. The fluctuation estimation unit 184 searches for a peak in, for example, one of the Stokes parameters converted into a frequency domain signal, i.e., the FFT waveform of one of the Stokes parameters. When a peak is found, the fluctuation estimation unit 184 estimates the frequency of the position of the peak as the polarization fluctuation speed. The fluctuation estimation unit 184 corresponds to the fluctuation estimation unit 23 shown in FIG. 1.

[0045] The inventors conducted an experiment to confirm that the polarization fluctuation speed can be estimated from the FFT waveform of the Stokes parameters. Figure 7 shows the system used in the experiment. The LD 510 emits a laser beam with a wavelength of 1510 nm. The attenuator 520 attenuates the laser beam emitted from the LD 510 in order to adjust the light receiving power at the polarimeter 550. The modulator 530 modulates the laser beam output from the LD 510 with 156 MHz on-off Non Return to Zero (NRZ) modulation.

[0046] The laser light modulated by the modulator 530, i.e., the optical signal, is input to a scrambler 540 via a single mode optical fiber (SMF). The scrambler 540 imparts polarization fluctuation to the optical signal at an arbitrary frequency. The optical signal to which polarization fluctuation has been imparted by the scrambler 540 is input to a polarimeter 550 via the SMF.

[0047] The polarimeter 550 calculates the Stokes parameters S1-S3 for the input optical signal. The ADC 560 converts each of the Stokes parameters S1-S3 from an analog signal to a digital signal. The FFT 570 performs a fast Fourier transform on one of the Stokes parameters S1-S3.

[0048] Fig. 8 shows the FFT waveform of the Stokes parameter S1. Fig. 9 shows the FFT waveform of the Stokes parameter S2. Fig. 10 shows the FFT waveform of the Stokes parameter S3. In the graphs shown in Figs. 8-10, the horizontal axis represents frequency [kHz] and the vertical axis represents intensity [dBm]. In the experiment, the scrambler 540 applied a polarization fluctuation of 100 kHz to the optical signal.

[0049] As a result of the experiment, it was confirmed that a peak was observed at 100 kHz in the FFT waveform for each of the Stokes parameters S1-S3, as shown in Figures 8-10. Therefore, it was confirmed that the fluctuation frequency of the polarization state, i.e., the polarization fluctuation speed, can be estimated by performing a fast Fourier transform on one of the Stokes parameters S1-S3.

[0050] In the optical fiber communication system 100 shown in Fig. 2, in order to suppress interference between the optical signal input from the light source 171 to the optical fiber 132 and the main signal, it is preferable that the power of the optical signal input to the optical fiber 132 is low. The inventors adjusted the attenuation of the attenuator 520 in the system shown in Fig. 7 and investigated what level of received light power is required for estimating the polarization fluctuation speed in the polarimeter 550.

[0051] Fig. 11 shows an FFT waveform of the Stokes parameter S1 when the received optical power at the polarimeter 550 is -5 dBm. Fig. 12 shows an FFT waveform of the Stokes parameter S1 when the received optical power at the polarimeter 550 is -40 dBm. In the graphs shown in Fig. 11 and Fig. 12, the horizontal axis represents frequency [kHz] and the vertical axis represents intensity [dBm]. In the experiment, the scrambler 540 applied a polarization fluctuation of 500 kHz to the optical signal.

[0052] As shown in Fig. 11, a peak was observed at 500 KHz in the FFT waveform of the Stokes parameter S1. Also, as shown in Fig. 12, when the received light power is -40 dBm, a peak was observed at 500 KHz in the FFT waveform of the Stokes parameter S1. Therefore, in the optical fiber communication system 100 shown in Fig. 3, if the received light power at the polarimeter 181 is -40 dBm or more, it is considered that the polarization fluctuation speed can be estimated. When the received light power is set low, the deterioration of the reception quality of the main signal caused by the optical signal inserted from the light source 171 into the optical fiber 132 can be suppressed.

[0053] Next, the operation procedure will be described. Fig. 13 shows the operation procedure of the polarization fluctuation monitoring device 172. The operation procedure of the polarization fluctuation monitoring device 172 corresponds to a polarization fluctuation monitoring method. The light source 171 inserts an optical signal with a wavelength of, for example, 1510 nm into the optical fiber 132. The polarimeter 181 measures the polarization characteristics of the optical signal with a wavelength of 1510 nm transmitted through the optical fiber 132 (step S1). In step S1, the polarimeter 181 measures one of the Stokes parameters S1-S3.

[0054] The ADC 182 converts the signal indicating the polarization characteristics measured in step S1 into a digital signal. The ADC 182 converts, for example, one of the Stokes parameters output from the polarimeter 181 into a digital signal, and outputs the converted digital signal to the FFT 183. The FFT 183 performs a Fourier transform on the digital signal indicating the measured polarization characteristics input from the ADC 182 (step S2). In step S2, the FFT 183 performs a fast Fourier transform on, for example, the Stokes parameter S1.

[0055] The fluctuation estimation unit 184 estimates the polarization fluctuation frequency, i.e., the polarization fluctuation speed, based on the Fourier transformed polarization characteristics (step S3). In step S3, the fluctuation estimation unit 184 estimates, for example, a frequency showing a peak in the FFT waveform of the Stokes parameter S1 as the polarization fluctuation frequency. The fluctuation estimation unit 184 may integrate the intensity of the peak frequency in the FFT waveform in the time direction, and estimate the amount of fluctuation of the polarization characteristics at the estimated polarization fluctuation frequency.

[0056] In this embodiment, the FFT 183 performs an FFT on one of the Stokes parameters measured by the polarimeter 181. The fluctuation estimation unit 184 estimates the polarization fluctuation frequency from the FFT waveform of one of the Stokes parameters. For example, the fluctuation estimation unit 184 estimates the frequency of a peak position in the FFT waveform as the polarization fluctuation frequency. In this embodiment, the polarization fluctuation monitoring device 172 can estimate the polarization fluctuation frequency from the FFT waveform of one of the Stokes parameters S1-S3 without using all of the Stokes parameters S1-S3.

[0057] In comparison with Non-Patent Document 1, in Non-Patent Document 1, the polarization fluctuation frequency is calculated based on the difference between the detection signals of the two photodetectors. In this case, the calculation can be simplified, but the accuracy of the calculated polarization fluctuation frequency is low. In Non-Patent Document 1, when the polarization fluctuation frequency is calculated from two Stokes parameters, the calculation can be simplified, but the accuracy of the calculated polarization fluctuation frequency is low. In contrast, the polarization fluctuation monitoring device 172 according to this embodiment uses an FFT waveform of one Stokes parameter to estimate the polarization fluctuation frequency. The polarization fluctuation monitoring device 172 according to this embodiment can estimate the polarization fluctuation frequency with high accuracy while simplifying the calculation by reducing the number of Stokes parameters used to estimate the polarization fluctuation frequency.

[0058] In this embodiment, the optical fiber communication system 100 has a pair of a light source 171 and a polarization fluctuation monitoring device 172 for each predetermined section in the transmission line 130. In this case, the presence or absence of polarization fluctuation and the polarization fluctuation frequency can be monitored for each section. Therefore, the user of the polarization fluctuation monitoring device 172 can know what frequency of polarization fluctuation has occurred in which section.

[0059] In this embodiment, the polarization fluctuation monitoring device 172 may control the coefficient of the polarization fluctuation compensation filter 162 (see FIG. 5) included in the digital signal processing unit 154 of the optical receiver 150 based on the estimated polarization fluctuation frequency. In this case, it is considered that the polarization fluctuation occurring in the transmission path 130 can be compensated for with high accuracy in the digital signal processing unit 154.

[0060] In the above description, the polarimeter 181 is used to measure the Stokes parameters. However, the present embodiment is not limited to this. The measurement of the Stokes parameters may be performed using a device having a polarizer, a photodetector, and a calculator.

[0061] 14 shows an example of the configuration of an apparatus used to measure the Stokes parameters. The Stokes parameter measurement apparatus 181a includes a splitter 301, polarizers 302-304, a quarter-wave plate 305, photodetectors 311-314, an ADC 320, and a computing unit 330. The splitter 301 splits the optical signal output from the light source 171, which is branched from the optical fiber 132, into four optical signals. The photodetector 311 detects the optical signal input from the splitter 301, and converts the detected optical signal into an electrical signal V0.

[0062] The polarizer 302 passes the component of the optical signal input from the splitter 301 that is in a direction perpendicular to the main axis to the photodetector 312. The photodetector 312 detects the optical signal that has passed through the polarizer 302 and converts the detected optical signal into an electrical signal V1. The polarizer 303 passes the component of the optical signal input from the splitter 301 that is inclined at 45 degrees to the main axis to the photodetector 313. The photodetector 313 detects the optical signal that has passed through the polarizer 303 and converts the detected optical signal into an electrical signal V2.

[0063] The quarter-wave plate 305 imparts a phase difference of 90 degrees (λ / 4) to two orthogonal polarization components of the optical signal input from the splitter 301. The polarizer 304 passes the component of the optical signal input via the quarter-wave plate 305 that is tilted 45 degrees with respect to the principal axis to the photodetector 314. The photodetector 314 detects the optical signal that has passed through the polarizer 304, and converts the detected optical signal into an electrical signal V3.

[0064] The ADC 320 converts the electrical signals V0-V3 output from the photodetectors 311-314 into digital signals. The calculator 330 calculates one of the Stokes parameters S1-S3 using V0-V3 converted into digital signals by the ADC 320. The calculator 330 calculates the Stokes parameter S1 by S1=2×V1-V0. The calculator 330 calculates the Stokes parameter S2 by S2=2×V2-V0. The calculator 330 calculates the Stokes parameter S3 by S3=2×V4-V0.

[0065] Next, a second embodiment of the present disclosure will be described. Fig. 15 shows a configuration example of a polarization fluctuation monitoring device used in a second communication system of the present disclosure. The polarization fluctuation monitoring device 172a has an amplifier 185 in addition to the configuration of the polarization fluctuation monitoring device 172 shown in Fig. 6. The operations of the polarimeter 181, the ADC 182, the FFT 183, and the fluctuation estimation unit 184 in the polarization fluctuation monitoring device 172a may be similar to those described in the first embodiment.

[0066] The amplifier 185 is an optical amplifier, and amplifies the optical signal output from the light source 171 and transmitted through the optical fiber 132. The polarimeter 181 measures the polarization characteristics of the optical signal amplified by the amplifier 185. In this embodiment, the optical signal is amplified by the amplifier 185. Therefore, the power of the optical signal input from the light source 171 to the optical fiber 132 can be reduced compared to the first embodiment.

[0067] The inventors conducted an experiment using a system having an amplifier between the scrambler 540 and the polarimeter 550 in the system shown in Fig. 7. The gain of the amplifier was set to 20 dB. Fig. 16 shows an FFT waveform of the Stokes parameter S1 obtained in the experiment. In the graph shown in Fig. 16, the horizontal axis represents frequency [kHz], and the vertical axis represents intensity [dBm].

[0068] In the experiment, the scrambler 540 applied a polarization fluctuation of 500 kHz to the optical signal. By adjusting the attenuation of the attenuator 520, the received light power at the polarimeter 550 was adjusted to -60 dBm. In this case, as shown in FIG. 16, a peak was observed at a position of 500 KHz in the FFT waveform of the Stokes parameter S1. As a result of the experiment, it was confirmed that it is possible to estimate the polarization fluctuation speed even when the power of the optical signal inserted into the optical fiber 132 is reduced compared to the first embodiment. Therefore, in this embodiment, it is possible to further suppress the deterioration of the reception quality of the main signal caused by the optical signal inserted from the light source 171 into the optical fiber 132.

[0069] 17 shows an example of the configuration of a computer device that can be used as the polarization fluctuation monitoring device 172. The computer device 400 has a central processing unit (CPU) 410, a storage unit 420, a read only memory (ROM) 430, a random access memory (RAM) 440, a communication interface (IF) 450, and a user interface 460.

[0070] The communication interface 450 is an interface used for communication with an external device. The communication interface 450 can be used to acquire four types of reception signals output by the ADC 153. The user interface 460 includes a display unit such as a display. The user interface 460 also includes an input unit such as a keyboard, a mouse, and a touch panel.

[0071] The storage unit 420 is an auxiliary storage device that can hold various data. The storage unit 420 does not necessarily need to be a part of the computer device 400, and may be an external storage device or a cloud storage connected to the computer device 400 via a network.

[0072] The ROM 430 is a non-volatile storage device. For example, a semiconductor storage device having a relatively small capacity, such as a flash memory, is used for the ROM 430. The programs executed by the CPU 410 may be stored in the storage unit 420 or the ROM 430. The storage unit 420 or the ROM 430 stores various programs for causing the CPU 410 to perform processes for performing distortion compensation and detection.

[0073] The program includes a set of instructions or software code for making the computer perform one or more functions described in the embodiments when the program is loaded into the computer. The program may be stored in a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, the computer-readable medium or tangible storage medium includes RAM, ROM, flash memory, solid-state drive (SSD) or other memory technology, Compact Disc (CD), digital versatile disc (DVD), Blu-ray (registered trademark) disk or other optical disk storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage device. The program may be transmitted on a transitory computer-readable medium or communication medium. By way of example and not limitation, the transitory computer-readable medium or communication medium includes electrical, optical, acoustic, or other form of propagated signal.

[0074] The RAM 440 is a volatile storage device. Various semiconductor memory devices such as Dynamic Random Access Memory (DRAM) or Static Random Access Memory (SRAM) are used for the RAM 440. The RAM 440 can be used as an internal buffer for temporarily storing data and the like. The CPU 410 loads a program stored in the storage unit 420 or the ROM 430 into the RAM 440 and executes it. The CPU 410 may have an internal buffer for temporarily storing data and the like.

[0075] In each of the above embodiments, the polarization fluctuation monitoring device 172 does not necessarily have to be configured as a single device. The polarization fluctuation monitoring device 172 may be configured using a plurality of devices that are physically separated. For example, the functions of the FFT 183 and the fluctuation estimation unit 184 may be realized by software processing in a single computer device. In that case, the computer device may acquire the Stokes parameters from the polarimeter 181 via the ADC 182.

[0076] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above-described embodiments, and changes and modifications to the above-described embodiments that do not deviate from the spirit of the present disclosure are also included in the present disclosure.

[0077] For example, some or all of the above embodiments may be described as, but are not limited to, the following supplementary notes.

[0078] [Appendix 1] a polarization characteristic measuring unit for measuring the polarization characteristic of an optical signal transmitted through a transmission line; a Fourier transform unit that performs a Fourier transform on the measured polarization characteristics; and a fluctuation estimating unit that estimates a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

[0079] [Appendix 2] 2. The polarization fluctuation monitoring device according to claim 1, wherein the fluctuation estimation unit searches for a peak in the Fourier transformed polarization characteristic, and estimates a frequency at a position of the peak found as the polarization fluctuation frequency.

[0080] [Appendix 3] 3. The polarization fluctuation monitoring device according to claim 1, further comprising an amplifier that amplifies the optical signal and outputs the amplified optical signal to the polarization characteristic measuring unit.

[0081] [Appendix 4] 4. The polarization fluctuation monitoring device according to claim 1, wherein the polarization characteristic measuring unit measures one of Stokes parameters as the polarization characteristic.

[0082] [Appendix 5] 5. The polarization fluctuation monitoring device according to claim 4, wherein the polarization characteristic measuring unit is a polarimeter.

[0083] [Appendix 6] The polarization fluctuation monitoring device described in Appendix 4, wherein the polarization characteristic measuring unit includes a polarizer that transmits a component of the optical signal in a predetermined polarization direction, a photodetector that detects the optical signal that has transmitted through the polarizer, and a calculator that calculates one of the Stokes parameters based on the optical signal detected using the photodetector.

[0084] [Appendix 7] A polarization fluctuation monitoring device as described in any one of appendix 1 to 6, wherein the optical signal is a signal having a wavelength different from the wavelength of a polarization multiplexed signal transmitted through the transmission path and transmitted between an optical transmitter and an optical receiver.

[0085] [Appendix 8] 8. The polarization fluctuation monitoring device according to claim 1, wherein the optical signal is added to the transmission line by using a multiplexer and is dropped from the transmission line by using a demultiplexer.

[0086] [Appendix 9] 9. The polarization fluctuation monitoring device according to claim 8, wherein the optical signal is inserted into the transmission line, transmitted through a predetermined span of the transmission line, and then branched off from the transmission line.

[0087] [Appendix 10] A transmitter for transmitting a polarization multiplexed signal; a receiver for receiving the polarization multiplexed signal transmitted from the transmitter via a transmission line; A polarization fluctuation monitoring device according to any one of appendix 1 to 9, a light source that outputs the optical signal to the transmission path.

[0088] [Appendix 11] A communication system as described in claim 10, comprising a pair of the polarization fluctuation monitoring device and the light source for each predetermined span in the transmission path.

[0089] [Appendix 12] the receiver has a polarization fluctuation compensation filter that compensates for polarization fluctuation in the transmission path; 12. The communication system according to claim 10, wherein the polarization fluctuation monitoring device controls a coefficient of the polarization fluctuation compensation filter based on the estimated polarization fluctuation frequency.

[0090] [Appendix 13] Measure the polarization characteristics of the optical signal transmitted through the transmission line; Fourier transforming the measured polarization characteristics; A polarization fluctuation monitoring method for estimating a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

[0091] [Appendix 14] Obtaining the polarization characteristics of the optical signal transmitted through the transmission line; Fourier transforming the acquired polarization characteristics; A program for causing a computer to execute a process of estimating a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics. [Explanation of symbols]

[0092] 10:Communication Systems 11: Transmitter 15: Receiver 13: Transmission line 20: Polarization fluctuation monitor 21: Polarization characteristic measurement section 22: Fourier transform section 23: Fluctuation estimation unit 30:Light source 100: Optical fiber communication systems 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: Decoding section 161X, 161Y: chromatic dispersion compensation filters 162: Polarization fluctuation compensation filter 163X, 163Y: Carrier phase compensation filter 171: Light source 172: Polarization fluctuation monitor 173: Monitoring light analysis device 181: Polarimeter 182: ADC 183:FFT 184: Fluctuation estimation unit 185: Amplifier

Claims

1. a polarization characteristic measuring unit for measuring the polarization characteristic of an optical signal transmitted through a transmission line; a Fourier transform unit that performs a Fourier transform on the measured polarization characteristics; and a fluctuation estimating unit that estimates a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

2. 2. The polarization fluctuation monitoring device according to claim 1, wherein the fluctuation estimating section searches for a peak in the Fourier transformed polarization characteristic, and estimates a frequency at the position of the searched peak as the polarization fluctuation frequency.

3. 3. The polarization fluctuation monitoring device according to claim 1, further comprising an amplifier that amplifies the optical signal and outputs the amplified optical signal to the polarization characteristic measuring section.

4. 3. The polarization fluctuation monitoring device according to claim 1, wherein the polarization characteristic measuring section measures one or more Stokes parameters as the polarization characteristics.

5. 5. The polarization fluctuation monitoring device according to claim 4, wherein the polarization characteristic measuring section is a polarimeter.

6. 5. The polarization fluctuation monitoring device according to claim 4, wherein the polarization characteristic measuring unit includes a polarizer that transmits a component of the optical signal in a predetermined polarization direction, a photodetector that detects the optical signal that has transmitted through the polarizer, and a calculator that calculates one or more of the Stokes parameters based on the optical signal detected using the photodetector.

7. 3. The polarization fluctuation monitoring device according to claim 1, wherein the optical signal is a signal having a wavelength different from that of a polarization multiplexed signal transmitted through the transmission path and transmitted between an optical transmitter and an optical receiver.

8. A transmitter for transmitting a polarization multiplexed signal; a receiver for receiving the polarization multiplexed signal transmitted from the transmitter via a transmission line; A polarization fluctuation monitoring device according to claim 1 or 2, a light source that outputs the optical signal to the transmission path.

9. Measure the polarization characteristics of the optical signal transmitted through the transmission line; Fourier transforming the measured polarization characteristics; A polarization fluctuation monitoring method for estimating a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

10. Obtaining the polarization characteristics of the optical signal transmitted through the transmission line; Fourier transforming the acquired polarization characteristics; A program for causing a computer to execute a process of estimating a polarization fluctuation frequency occurring in the transmission path based on the Fourier transformed polarization characteristics.

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