Control device, compensation device, program, and control method

Abstract

A control device controls operation of a compensation device that compensates for birefringence and / or polarization mode dispersion experienced by signal light propagating in an optical transmission path by digital signal processing using a finite impulse response filter, the control device having: a detection signal receiving section that receives a detection signal that is a signal representing a detection result of a detection device that optically detects a polarization fluctuation in the optical transmission path; and a setting section that decides a setting related to an update frequency or update interval of a tap number of the finite impulse response filter or a setting related to an update frequency or update interval of a tap coefficient of the finite impulse response filter based on the detection result of the detection device.

Description

Technical Field

[0001] This invention relates to control devices, compensation devices, programs, and control methods. Background Technology

[0002] Non-Patent Documents 1-5 disclose methods for generating phase noise in signal light, such as through the Kerr effect, thereby degrading optical transmission characteristics. Non-Patent Document 6 discloses a method for deriving optical phase noise, as disclosed in Non-Patent Document 7, by subtracting the shifted average of the optical phase from the measured phase of the received signal. Non-Patent Documents 8-9 disclose the use of a delay interferometer to demodulate DPSK signals. Non-Patent Documents 10-12 disclose that the input-output characteristics of an optical ring resonator are steeper than those of an optical delay interferometer.

[0003] Non-patent documents 13-14 disclose the possibility of high-speed polarization variations of around 10 kHz or higher in coherent optical communication. Non-patent document 15 discloses a proportional relationship between the applied voltage of the fiber stretcher and the resulting lateral pressure and birefringence.

[0004] Non-patent documents 16-18 disclose coherent optical communication methods using FIR filters. Non-patent document 19 discloses that the maximum polarization variation rate generated in a practical optical fiber transmission path, as the movement angle of the trajectory on the Poincaré sphere, is approximately 45,000 revolutions / s. Non-patent documents 20-21 disclose that the delay time of a transponder for optical reception, identification regeneration, and optical transmission in an intensity modulation-direct detection method is 4-30 ns.

[0005] Existing technical documents

[0006] Non-patent literature

[0007] Non-patent document 1: JPGordon and LFMullenauer, "Phase noise in photoniccommunications systems using linear amplifiers," Optics Letters, Vol. 15, No. 23, pp. 1351-1353, 1990.

[0008] Non-Patent Document 2: S. Ryu, “Signal linewidth broadening due to nonlinear Kerr effect in long-haul coherent systems using cascaded optical amplifiers,” IEEE Journal of Lightwave Technology, Vol. 10, No. 10, pp. 1450 - 1457, 1992.

[0009] Non-Patent Document 3: J. Cheng et al., “Relative phase noise induced impairment in M-ary phase shift-keying coherent optical communication system using distributed fiber Raman amplifier,” Optics Letters, Vol. 38, No. 7, pp. 1055 - 1057, 2013.

[0010] Non-Patent Document 4: S. Zhang et al., “Bit-error rate performance of coherent optical M-ary PSK / QAM using decision-aided maximum likelihood phase estimation,” Optics Express, Vol. 18, No. 12, pp. 12088 - 12103, 2010.

[0011] Non-Patent Document 5: T. Pfau et al., “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” IEEE Journal of Lightwave Technology, Vol. 27, No. 8, pp. 989 - 999, 2009.

[0012] Non-patent literature 6: M. Nakazawa et al., Editor, High spectral density optical communication technologies, Springer-Verlag, 2010.

[0013] Non-patent document 7: K. Kikuchi, "Effect of 1 / f-type FM noise on semiconductor-laser linewidth residual in high-power limit," IEEE Journal of Quantum Electronics, Vol. 25, No. 4, pp. 684-688, 1989.

[0014] Non-patent document 8: J. Gamet and G. Pandraud, "C-and L-band planar delay interferometer for DPSK decoders," IEEE Photonics Technology Letters, Vol. 17, No. 6, pp. 1217-1219, 2005.

[0015] Non-patent literature 9: K. Voigt et al., “Performance of 40-Gb / s DPSK demodulator in SOI-technology,” IEEE Photonics Technology Letters, Vol.20, No.8, pp.614-616, 2008.

[0016] Non-patent document 10: T.Kominato at al., "Ring resonators composed of GeO2-doped silica waveguides," IEEE Journal of Lightwave Technology, Vol. 10, No. 12, pp. 1781-1788, 1992.

[0017] Non-patent document 11: S. Suzuki et al., "Integrated-optic double-ring resonators with a wide free spectral range of 100GHz," IEEE Journal of Lightwave Technology, Vol. 13, No. 8, pp. 1766-1771, 1995.

[0018] Non-patent literature 12: W. Bogaerts et al., “Silicon microring resonators,” Laser and Photonics Reviews, Vol. 6, No. 1, pp. 47-73, 2012.

[0019] Non-patent document 13: PMKrummrich, E.-D. Schmidt, W. Weiershausen, and A. Mattheus, "Field trial results on statistics of fast polarization changes in long haul WDM transmission systems," OFC2005, paper OThT6, March 2005.

[0020] Non-patent document 14: M.Boroditsky, M.Brodsky, NJFrigo, P.Magill, andH.Rosenfeldt, "Polarization dynamics in installed fiberoptic systems," 2005 IEEELEOS Annual Meeting, paper TuCC1, October 2005.

[0021] Non-patent literature 15: R. Ulrich and A. Simon, “Polarization optics of twisted single-mode fibers,” Applied Optics, Vol. 18, No. 13, pp. 2241-2251, July 1979.

[0022] Non-patent literature 16: K. Kikuchi, “Digital coherent optical communication systems: fundamentals and future prospects,” IEICE Electronics Express, Vol. 8, No. 20, 1642-1662, 2011.

[0023] Non-patent literature 17: C. Fougstedt, P. Johannisson, L. Svensson, and P. Larsson-Edefors, “Dynamic equalizer power dissipation optimization,” OFC2016, paper W4A.2, 2016.

[0024] Non-patent document 18: D. Cardenas, D. Lavery, P. Watts and SJ Savory, "Reducing the power consumption of the CMA equalizer update for a digital coherent receiver," OFC2014, paper Th4D.5, 2014.

[0025] Non-patent literature 19: PM Krummrich and K. Kotten, “Extremely fast (microsecond timescale) polarization changes in high speed long haul WDM transmission systems,” OFC2004, paper FI3, 2004.

[0026] Non-patent literature 20: M. Freiberger, D. Templeton, and E. Mercado, “Low latency optical services,” OFC / NFOEC 2012, paper NTu2E.1, 2012.

[0027] Non-patent literature 21: V. Bobrovs, S. Spolitis, and G. Ivanovs, “Latency causes and reduction in optical metro networks,” Proc. of SPIE Vol. 9008, 9008-11, 2014. Summary of the Invention

[0028] A first aspect of the present invention provides a control device. This control device, for example, controls the operation of a compensation device for signal light propagating in an optical transmission path. In this control device, the compensation device, for example, compensates for birefringence and / or polarization mode dispersion experienced by the signal light propagating in the optical transmission path using digital signal processing of a finite impulse response (FIR) filter. The control device, for example, includes a detection signal receiving unit that receives a detection signal, which is a signal representing the detection result of a detection device that optically detects polarization variations in the optical transmission path. The control device, for example, includes a setting unit that determines a setting related to the update frequency or update interval of the tap count of the FIR filter, or a setting related to the update frequency or update interval of the tap coefficients of the FIR filter, based on the detection result of the detection device.

[0029] In any of the control devices described above, the setting unit can determine whether the absolute value of the polarization variation is greater than a predetermined threshold based on the detection result of the detection device. When it is determined that the absolute value of the polarization variation is greater than the predetermined threshold, the setting unit can decide to (i) set the update frequency of the number of taps and / or tap coefficients to a value greater than the current setting value, (ii) set the update frequency of the number of taps and / or tap coefficients to a predetermined first value, (iii) set the update interval of the number of taps and / or tap coefficients to a value less than the current setting value, or (iv) set the update interval of the number of taps and / or tap coefficients to a predetermined second value.

[0030] In any of the control devices described above, the setting unit can determine whether the absolute value of the polarization variation is greater than a predetermined threshold based on the detection result of the detection device. If it is not determined that the absolute value of the polarization variation is greater than the threshold, the setting unit can decide to (i) set the update frequency of the number of taps and / or tap coefficients to be less than the current setting value, (ii) set the update frequency of the number of taps and / or tap coefficients to a predetermined third value, (iii) set the update interval of the number of taps and / or tap coefficients to be greater than the current setting value, or (iv) set the update interval of the number of taps and / or tap coefficients to a predetermined fourth value.

[0031] Any of the above control devices may include an update unit that updates the number of taps or the tap coefficient at an update frequency or update interval determined by a setting unit.

[0032] In any of the above control devices, the compensation device can compensate for the birefringence and / or polarization mode dispersion experienced by the first signal light propagating in the optical transmission path. In any of the above control devices, the detection device can optically detect the polarization variation of the second signal light propagating in the optical transmission path. The wavelengths of the first signal light and the second signal light can be different from each other. In any of the above control devices, the detection device can be equipped with an optical delay interferometer, a polarimeter, a polarization state measuring instrument, or a Stokes parameter measuring instrument for optically detecting the polarization variation of the second signal light.

[0033] Any of the above-described control devices can control the operation of multiple compensation devices. Each of the multiple compensation devices compensates for birefringence and / or polarization mode dispersion experienced by multiple signal lights propagating in the optical transmission path by using digital signal processing of finite impulse response filters. The wavelengths of the multiple signal lights can be different. In any of the above-described control devices, the setting unit determines the settings related to the multiple finite impulse response filters disposed in the multiple compensation devices based on the detection results of the detection device.

[0034] A second aspect of the present invention provides a compensation device. This compensation device, for example, compensates for birefringence and / or polarization mode dispersion experienced by signal light propagating in an optical transmission path. The compensation device, for example, includes a finite impulse response filter for compensating for birefringence and / or polarization mode dispersion experienced by the signal light. The compensation device, for example, includes any of the control devices involved in the first aspect described above.

[0035] A third aspect of the present invention provides a compensation device. This compensation device, for example, compensates for birefringence and / or polarization mode dispersion experienced by signal light propagating in an optical transmission path. The compensation device includes, for example, a finite impulse response (FIR) filter for compensating for birefringence and / or polarization mode dispersion experienced by the signal light. The compensation device includes, for example, a control signal input unit that receives a control signal for controlling the update frequency or interval of the tap count of the FIR filter, or the update frequency or interval of the tap coefficients of the FIR filter.

[0036] Any of the above-mentioned compensation devices may include a setting unit that determines, during the operation of the compensation device, a setting related to the update frequency or update interval of the number of taps of the finite impulse response filter, or a setting related to the update frequency or update interval of the tap coefficients of the finite impulse response filter, based on a control signal input to the control signal input unit during the operation of the compensation device.

[0037] A fourth aspect of the present invention provides an optical receiving device. This optical receiving device, for example, includes any of the compensation devices involved in the second or third aspects described above. The optical receiving device, for example, includes a demodulation unit that demodulates the received signal transmitted via signal light based on the output from the compensation device and generates an information signal.

[0038] A fifth aspect of the present invention provides an optical communication system. This optical communication system, for example, includes an optical transmitting device for transmitting signal light. The optical communication system, for example, includes any of the optical receiving devices described in the fourth aspect.

[0039] A sixth aspect of the present invention provides a control method. This control method is, for example, a method for controlling the operation of a compensation device for signal light propagating in an optical transmission path. In this control method, the compensation device, for example, compensates for birefringence and / or polarization mode dispersion experienced by the signal light propagating in the optical transmission path by using digital signal processing of a finite impulse response (FIR) filter. The control method includes, for example, a detection signal receiving stage, whereby the detection signal is a signal representing the detection result of a detection device that optically detects polarization variations in the optical transmission path. The control method also includes, for example, a setting stage, in which a setting related to the update frequency or update interval of the tap count of the FIR filter, or a setting related to the update frequency or update interval of the tap coefficients of the FIR filter, is determined based on the detection result of the detection device.

[0040] A seventh aspect of the present invention provides a program. This program is, for example, used to enable a computer to function as any of the control devices involved in the first aspect. The program is, for example, used to enable a computer to execute the control method involved in the sixth aspect. A computer-readable storage medium storing the program may also be provided. This storage medium is a non-transitory computer-readable medium.

[0041] Furthermore, the above summary of the invention does not list all the essential features of the invention. Additionally, sub-combinations of these feature groups can also constitute inventions. Attached Figure Description

[0042] Figure 1 This is a simplified representation of the system configuration of the communication system 100.

[0043] Figure 2 Here is a simplified representation of the internal structure of the signal processing unit 170.

[0044] Figure 3 This is a simplified representation of an example of the system configuration of the phase noise evaluation device 320.

[0045] Figure 4This is a simplified representation of an example of the internal structure of an optical delay interferometer 340.

[0046] Figure 5 This is a simplified representation of an example of the internal structure of an optical delay interferometer 540.

[0047] Figure 6 This is a simplified representation of an example of the internal structure of an optical delay interferometer 640.

[0048] Figure 7 Here is a simplified representation of the internal structure of the signal processing unit 370.

[0049] Figure 8 This is a simplified representation of an example of the system configuration of the phase noise evaluation device 820.

[0050] Figure 9 Here is a simplified representation of the internal structure of the signal processing unit 870.

[0051] Figure 10 This is a simplified representation of an example of the internal structure of an optical delay interferometer 1040.

[0052] Figure 11 This is a simplified representation of an example of the internal structure of an optical delay interferometer 1140.

[0053] Figure 12 This is a simplified representation of an example of the internal structure of an optical delay interferometer 1240.

[0054] Figure 13 This is a simplified representation of an example of the internal structure of an optical delay interferometer 1340.

[0055] Figure 14 This is a simplified illustration of an example of the circuit configuration of the balanced light receiver 1350.

[0056] Figure 15 This is a simplified representation of an example of the internal structure of an optical delay interferometer 1540.

[0057] Figure 16 This is a simplified representation of the system configuration of the communication system 1600.

[0058] Figure 17 Here is a simplified representation of the internal structure of the optical signal transmitting device 110.

[0059] Figure 18 Here is a simplified representation of the internal structure of the status monitoring device 1680.

[0060] Figure 19 Here is a simplified representation of the internal structure of the signal processing unit 1870.

[0061] Figure 20This is a simplified illustration of an example of information processing in the status monitoring device 1680.

[0062] Figure 21 This is a rough representation of an example of data table 2100.

[0063] Figure 22 This is a simplified example of the system configuration of a computer 3000.

[0064] Figure 23 This represents the measurement results of the Stokes parameters in Experiment Example 1.

[0065] Figure 24 The histogram represents the differential phase in Experiment Example 1.

[0066] Figure 25 This represents the time variation of the differential phase in Experiment Example 1.

[0067] Figure 26 This represents the spectrum of the differential phase in Experiment Example 1.

[0068] Figure 27 This indicates the results of the Stokes parameter measurements in Comparative Experiment Example 1.

[0069] Figure 28 The histogram represents the differential phase in comparative experiment example 1.

[0070] Figure 29 This indicates the time variation of the differential phase in comparative experiment example 1.

[0071] Figure 30 This represents the spectrum of the differential phase in Comparative Experiment Example 1.

[0072] Figure 31 This represents the measurement results of the Stokes parameters in Experiment Example 2.

[0073] Figure 32 The histogram represents the differential phase in Experiment Example 2.

[0074] Figure 33 This represents the time variation of the differential phase in Experiment Example 2.

[0075] Figure 34 This represents the spectrum of the differential phase in Experiment Example 2.

[0076] Figure 35 This is a simplified representation of the system configuration of the communication system 3500.

[0077] Figure 36 Here is a simplified representation of the internal structure of the digital signal processor 3560.

[0078] Figure 37This is a brief illustration of an example of the internal structure of the compensation unit 3664.

[0079] Figure 38 This is a brief illustration of an example of information processing in the compensation department 3664.

[0080] Figure 39 Other examples that roughly illustrate the internal structure of the compensation unit 3664.

[0081] Figure 40 Here is a simplified representation of the internal structure of the status monitoring device 4080. Detailed Implementation

[0082] The present invention will now be described through embodiments thereof, but these embodiments do not limit the invention as defined in the claims. Furthermore, not all combinations of features described in the embodiments are necessary for the solution of the invention. Additionally, in the accompanying drawings, the same or similar parts are labeled with the same reference numerals, and sometimes repeated descriptions are omitted.

[0083] In optical transmission systems, phase variations such as optical phase noise and polarization shifts may occur in the received signal. Examples of causes for phase variations in the received signal include nonlinear optical effects, fiber vibration, fiber twisting, variations in stress applied to the fiber, laser linewidth or laser phase fluctuations, and phase variations in the photoelectric field caused by lightning strikes. Depending on the nature of these phase variations, the optical transmission system may generate code errors. An example of an optical transmission system using coherent optical communication is provided.

[0084] According to this embodiment, an example will be described, for instance, an apparatus or method for evaluating the presence and / or degree of optical phase noise, an apparatus or method for evaluating the presence and / or degree of polarization variation, and an apparatus or method for evaluating both optical phase noise and polarization variation. According to this embodiment, it is possible to evaluate the presence and / or degree of at least one of optical phase noise and polarization variation with relatively high precision. This improves the operational efficiency and maintenance efficiency of the optical transmission system.

[0085] (I. Principle of Optical Phase Noise Measurement)

[0086] First, the principle of optical phase noise measurement is explained. Details of the measurement device and method for optical phase noise based on the above principle are then provided. Figures 1 to 15 The details will be provided later.

[0087] Previously, optical phase noise (sometimes simply called phase noise) was inferred by making a linear approximation of the change in optical phase with respect to time t. For example, when measuring the phase of signal light propagating in an optical transmission path, the measured value of the phase of the signal light at the i-th (i is an integer greater than 1 and less than N) of N (N is a positive integer) specimen points (sometimes called samples) was approximated using the following Equation 1.

[0088] (Equation 1)

[0089]

Mathematical Formula 1

[0090]

[0091] in, It is the measured value of the phase of the signal light at the i-th specimen point.

[0092] θ(i) is the phase noise of the i-th sample point.

[0093] a and b are constants.

[0094] For example, by pre-determining the constants a and b using the least squares method, the phase noise θ(i) can be inferred from the measured value of the phase of the signal light. However, in reality, there are variations in low-frequency components (sometimes called 1 / f noise components) in the frequency variation of the laser, so the value of the phase noise θ(i) cannot be accurately inferred using the above approximation Equation 1.

[0095] As described in Non-Patent Document 6, optical phase noise, after removing the 1 / f noise component, can be derived by subtracting the moving average of the optical phase from the measured phase of the received signal. Specifically, the phase noise θ(i) is derived using Equation 2 below. Furthermore, in Equation 2, m and l represent integers. The value of l is set appropriately.

[0096] (Equation 2)

[0097]

Mathematical Formula 2

[0098]

[0099] However, it is known that phase noise is a Wiener process, and phase noise follows a Gaussian distribution whose variance diverges proportionally to time t. Therefore, according to the method described in Non-Patent Document 6, there are problems such as the variance becoming negative when time t is short and the inability to accurately determine the approximate curve due to the influence of measurement errors.

[0100] According to one example of this embodiment, by evaluating the statistical distribution of the differential phase, the impact of nonlinear phase noise on the transmission of optical signals can be evaluated more accurately. Examples of nonlinear phase noise include (i) self-phase modulation, (ii) mutual phase modulation, and (iii) in fiber Raman amplifiers, the phenomenon where intensity noise of the pump light causes phase noise in the signal light via the Kerr effect.

[0101] Differential phase refers to the phase difference between points before and after a sample (sometimes called a specimen). The differential phase of the i-th specimen point... As shown in Equation 3 below.

[0102] (Equation 3)

[0103]

Mathematical Expression 3

[0104]

[0105] in, It is the measured value of the phase of the signal light at the (i+1)th specimen point.

[0106] It is the measured value of the phase of the signal light at the i-th specimen point.

[0107] i is an integer greater than 1 and less than (N-1).

[0108] For example, by setting the sampling frequency to the same value as the symbol rate and measuring the differential phase at time intervals equal to the symbol time, the variance or standard deviation of the phase noise within a symbol time can be derived. Furthermore, the setting of the sampling frequency is not limited to the above implementation. For example, the sampling frequency can be set to a value greater than the symbol rate. In this case, by using samples corresponding to appropriate time intervals to calculate the differential phase, the differential phase can be measured at time intervals equal to the symbol time. Sometimes the time interval between temporally adjacent samples (e.g., the time interval between the (i+1)th sample and the ith sample) is referred to as the sample time interval Δt.

[0109] For example, in coherent heterodyne detection, the phase noise θ(t) is used to measure the phase of the signal light. Phase with the local oscillator light (sometimes called the local oscillator light) Represented as Because the phase noise generated in the optical transmission path includes Therefore, according to the above formula, θ(t) also includes the phase noise generated in the optical transmission path.

[0110] Without applying modulation for communication to the optical signal, the phase component of the i-th sample point is derived by Equation 4 below.

[0111] (Equation 4)

[0112]

Mathematical Expression 4

[0113]

[0114] in, It is the phase component of the i-th specimen point.

[0115] i I (i) is the input current of the photoreceiving element at the i-th sample point that has been input with the I signal component.

[0116] i Q (i) is the output current of the photoreceiving element at the i-th sample point that has been input with the Q signal component.

[0117] On the other hand, when the optical signal is modulated for communication, the phase component of the i-th sample point can be calculated by eliminating the modulation component. For example, in the case of M-phase modulation, the modulation component can be eliminated by calculating the received signal raised to the power of M. M is a positive integer.

[0118] For example, in the case of an optical signal that has been QPSK modulated, the received signal I represented by a complex number... QPSK (t) is represented by the following equation 5.

[0119] (Equation 5)

[0120] I QPSK (t)=i I,QPSK (t)+ji Q,QPSK (t)

[0121] In the above formula, i I,QPSK (t) represents the output current corresponding to the I signal component obtained by coherent heterodyne detection of the QPSK modulated optical signal. Q,QPSK (t) represents the output current corresponding to the Q signal component obtained by coherent heterodyne detection of the QPSK modulated optical signal.

[0122] Here, since QPSK modulation is a 4-phase modulation method, therefore, if the received signal I... QPSK If (t) is raised to the fourth power, then the received signal I... QPSK The following equation 6 holds true between the fourth power of the deflection angle of (t) and the phase noise θ(t).

[0123] (Equation 6)

[0124] arg{I QPSK (t)} 4 =4(2πf) c t+θ(t))

[0125] In the above formula, f c f represents the frequency of the signal light s With the frequency f of the local oscillator light L The difference (f) s -f L ), and is referred to as beat frequency.

[0126] By dividing both sides of Equation 6 by 4 (equivalent to M above), Equation 7 is derived. Thus, the phase noise θ(t) from which the modulation component is removed is derived.

[0127] (Equation 7)

[0128] arg{I QPSK (t)} 4 / 4=2πf c t+θ(t)

[0129] If we consider equations 1 and 7, then the differential phase of the i-th specimen point The relationship between the phase noise θ(i+1) of the (i+1)th sample point, the phase noise θ(i) of the ith sample point, and the sample time interval Δt is expressed by the following Equation 8.

[0130] (Equation 8)

[0131]

Mathematical Expression 5

[0132]

[0133] As mentioned above, the phase noise θ(i) follows a Gaussian distribution, and therefore the differential phase, as its difference, is... It also follows a Gaussian distribution. Furthermore, the mean of θ(i) is 0. Therefore, the differential phase... The average value is 2πfcΔt. Furthermore, if the standard deviation of the phase noise θ(i) of the signal light is set as σ... sig Then the differential phase variance σ M 2 2×σ sig 2 .

[0134] Therefore, when deriving the differential phase The standard deviation σ of the measurement results M Then, the standard deviation σ of the phase noise of the signal light propagating in the optical transmission path is derived by the following equation 9. sig .

[0135] (Equation 9)

[0136] σ sig =σ M / √2

[0137] Therefore, according to this embodiment, the phase noise of the signal light can be derived more accurately. For example, according to this embodiment, the possibility of uncertainty factors caused by the length of the moving average time influencing the measured value is significantly reduced, as in the method of subtracting the moving average of the light phase from the measured phase of the received signal.

[0138] Furthermore, when considering transmission effects, it is best to also consider the effect of the spectral linewidth of the local oscillator. In this case, the standard deviation σ of the phase noise of the optical signal received in the optical receiving device (sometimes referred to as the received signal) RX The standard deviation σ of the phase noise based on the spectral linewidth of the local oscillator light LO and the standard deviation σ of the phase noise of the signal light sig The relationship is represented by the following equation 10.

[0139] (Equation 10)

[0140] σ RX 2 =σ sig 2 +σ LO 2

[0141] As described above, using the differential phase of the signal light The standard deviation σ of the measurement results M The standard deviation σ of the phase noise of the derived signal light sig The standard deviation σ of the phase noise based on the spectral linewidth of the local oscillator light. LO By the standard deviation σ of the phase noise of the signal light sig The same procedure was followed, and the standard deviation σ of the differential phase measurement results of the local oscillator was used. ML It is derived from Equation 11 below.

[0142] (Equation 11)

[0143] σ LO =σ ML / √2

[0144] Therefore, the standard deviation σ of the phase noise of the received signal is derived using equations 9 to 11. RX The standard deviation σ of the phase noise of the received signal RX Used for evaluating and monitoring the impact of transmission.

[0145] (Methods for determining differential phase)

[0146] In one implementation, the differential phase of each of the N specimen points It is derived through signal processing of digital data obtained by sampling and quantizing the electrical signal after the light being measured (sometimes called the object light) is converted into an electrical signal. For example, time-series data of the phase of the object light is generated at time intervals that are the same as or approximately the same as the symbol time. The differential phase is derived by calculating the difference between two temporally adjacent data points in the time-series data.

[0147] In other implementations, firstly, the target light is branched into a first light and a second light. Next, the time delay τ between the first and second lights is adjusted. Specifically, the time delay τ is adjusted to 2πfτ = 2nπ + π / 2. Here, f is the frequency of the target light, and n is a positive integer. Then, the first and second lights with the adjusted time delay τ are subjected to wave-combining interference. Finally, the differential phase is derived by photoelectric conversion of the combined light, followed by sampling and quantization.

[0148] As described above, the differential phase is derived through a relatively simple process. Furthermore, the variance or standard deviation of the differential phase is also derived through relatively simple calculations. Therefore, according to this embodiment, the computer load can be reduced.

[0149] (Example of evaluating phase noise of signal light)

[0150] When evaluating the phase noise of signal light used in practical communications, the optical signal has modulation-based spectral components. Therefore, modulation components can also interfere with the measurement results, affecting the measurement of optical phase noise.

[0151] Therefore, in one embodiment, the communication system 100 uses the optical signal receiving device 120 to evaluate phase noise, for example, during trial operation testing after the construction of the communication system 100. In other embodiments, the communication system 100 is provided with a dedicated wavelength for measuring phase noise during the operation of the communication system 100, and the phase noise of the light at that wavelength is always measured. For example, in an embodiment where differential phase is derived through signal processing of digital data, phase noise is evaluated by measuring the differential phase of the I and Q components of the light at the aforementioned wavelength.

[0152] (II. Detection principle of phase change caused by polarization variation)

[0153] Next, the detection principle of phase variation caused by polarization variation will be explained. Details regarding the detection device and method for polarization variation based on the above detection principle will be provided later. Figures 16 to 34 Describe it.

[0154] The inventors have discovered that, in addition to optical phase noise, the differential phase (especially the differential phase measured using an optical delay interferometer) can also include phase variations caused by polarization variations. For example, in the coherent optical communication method described above, the amount of information transmitted can be increased by carrying independent data signals in two orthogonal polarization modes, HE11x and HE11y. In the above communication method, since the two modes couple during transmission in the optical fiber, high-speed digital signal processing is performed at the receiver side to separate the received signal into the original orthogonal polarization modes.

[0155] Previously, polarization variations were considered sufficiently slow compared to the signal processing speed at the receiver side. However, recent studies indicate that high-speed polarization variations exceeding approximately 10 kHz can occur. Such high-speed polarization variations, along with the aforementioned optical phase noise, are accompanied by phase shifts in the light. Therefore, depending on the polarization variation, code errors may occur in the optical transmission system.

[0156] In optical transmission systems, situations that generate relatively high-speed polarization changes can be envisioned, such as (i) applying varying lateral stress to the optical fiber, causing changes in the fiber's birefringence, or (ii) causing the optical fiber to undergo rapid twisting, resulting in high-speed rotation of the polarization plane. Therefore, in many cases, a sudden phase change based on polarization variation occurs.

[0157] In the above-described principle for measuring optical phase noise, statistical processing is performed during the derivation of the phase noise of the signal light. Therefore, sudden phase variations are difficult to reflect in the measurement results of the signal light's phase noise. Thus, not only can statistical quantities such as the standard deviation be obtained, but phase variations caused by sudden polarization changes can also be detected based on real-time measurement results, thereby enabling a more accurate evaluation of the state of the optical transmission system and / or the optical signal.

[0158] When the orthogonal polarization mode of the signal light propagating in the z-direction is set to E x (t) and E y (t), where the unit vectors in the x and y directions are respectively set as k. x and k y When the light propagates in the optical transmission path and is received, the electric field (sometimes called the received photoelectric field) E(t) is represented by the following equation B-1.

[0159] (Formula B-1)

[0160]

[0161] In equation B-1, δ(t) represents E x (t) and E y The phase difference of (t). Represents optical phase noise. f represents the frequency of the signal light. E(t), k x and k y It is a vector.

[0162] Assuming that the optical delay interferometer is not polarization dependent, the output current i of the optical receiver connected to the optical delay interferometer is represented by the following equation B-2.

[0163] (Formula B-2)

[0164] i=R{E(t)+E(t-τ)}·{E(t)+E(t-τ)}

[0165] In Equation B-2, τ represents the time difference between the two paths of the optical delay interferometer. R represents the sensitivity of the optical receiver. · represents the inner product.

[0166] Considering that the optical receiver does not respond to the frequency of light, if the DC component of equation B-2 is ignored, the received current i output from the optical receiver is represented by the following equation B-3.

[0167] (Formula B-3)

[0168]

Mathematical Expression 6

[0169] i = R[E x (t)E x (t-τ)cos{φ n (t)-φ n (t-τ)+2πfτ}+E y (t)E y (t-τ)cos{φ n (t-φ n (t-τ)+δ(t)-δ(t-τ)+2πfτ}]

[0170] In equation B-3, when the time delay difference τ is adjusted to 2πfτ=2nπ+π / 2 (n is a positive integer), the above-mentioned receiving current i is represented by the following equation B-4.

[0171] (Formula B-4)

[0172]

Mathematical Expression 7

[0173] i = -R[E x (t)E x (t-τ)sin{φ n (t)-φ n (t-τ)}+E y (t)E y (t-τ)sin{φ n (t)-φ n (t-τ)+δ(t)-δ(t-τ)}]

[0174] When using the output light of a semiconductor laser as the signal light, the AM noise of the signal light is very low due to the properties of the semiconductor laser output light. In this case, the aforementioned receiving current i is represented by the following equation B-5.

[0175] (Formula B-5)

[0176]

Mathematical Expression 8

[0177]

[0178] Here, when τ is very short, we can assume And |δ(t)-δ(t-τ)| << 1. For example, when τ is a symbol time, we can assume And |δ(t)-δ(t-τ)|<<1. In this case, the above-mentioned receiving current i is approximated by the following equation B-6.

[0179] (Formula B-6)

[0180]

Mathematical Expression 9

[0181]

[0182] As shown in Equation B-6, the above-mentioned received current i includes, in addition to the term caused by the phase noise of the signal light propagating in the optical transmission path, the term ipol caused by the polarization variation represented by Equation B-7 below.

[0183] (Formula B-7)

[0184] i pol =-RE y 2 (t)sin{δ(t)-δ(t-τ)}

[0185] According to Equation B-7, as the aforementioned received current i, a current variation caused by polarization variation was observed in addition to phase noise. That is, phase variation caused by sudden polarization variation can be detected based on the real-time measurement result of the aforementioned received current i. Thus, the inventors discovered that by observing the output current i of the optical receiver connected to an optical delay interferometer whose delay time difference τ is adjusted to 2πfτ=2nπ+π / 2 (n is a positive integer), the occurrence of a sudden polarization variation in the signal light can be detected.

[0186] (When A.δ(t) changes slightly)

[0187] As mentioned above, in equation B-6, τ is a very short time, and the absolute value of the difference between δ(t) and δ(t-τ) is much less than 1. At this time, the term i caused by polarization variation...pol An approximation is made using the following equation B-8.

[0188] (Formula B-8)

[0189]

Mathematical Formula 10

[0190]

[0191] In this practical device, when measuring the output current i of the optical receiver connected to the optical delay interferometer, the output current i is amplified, for example, by an amplifier immediately following the optical receiver. At this time, based on the low-pass filter characteristics of the amplifier, i is expressed by Equation B-8. pol The integral is performed relative to time t. Under ideal integration, the amplifier's output current i... pol_LPF This is represented by the following equation B-9. An amplifier can be an example of an integrating circuit.

[0192] (Formula B-9)

[0193] i pol_LPF =-RE y 2 (t)δ(t)

[0194] Equation B-9 holds true within the range where the aforementioned low-pass filter operates as an integrator circuit. That is, Equation B-9 holds true at frequencies above the cutoff frequency of the low-pass filter. In this case, if the contribution of R in Equation B-9 is normalized using the power ratio α (0 ≤ α ≤ 1) of the orthogonally polarized signals defined by Equations B-10 and B-11 below, then the normalized amplifier output current i pol_LPF_norm It is represented by the following equation B-12. Therefore, δ(t) can be directly measured by using an optical delay interferometer.

[0195] (Formula B-10)

[0196] Ex 2 (t)=αE 2

[0197] (Formula B-11)

[0198] Ey 2 (t)=(1-α)E 2

[0199] (Formula B-12)

[0200]

Mathematical Expression 11

[0201]

[0202] Furthermore, the phase noise-induced term in Equation B-6 is a random process, and therefore, the phase noise-induced term is not phase continuous. Consequently, the integration effect in the amplifier does not affect the phase noise-induced term. As a result, the phase noise-induced term in Equation B-6 is output from the amplifier as is in Equation B-6.

[0203] As shown in Equation B-6, since the received current i is measured under a mixed condition of terms caused by phase noise (sometimes called phase noise component) and terms caused by polarization variation (sometimes called polarization variation component), the measurement of the phase noise component is preferably performed during a period when the polarization variation component is small. Therefore, for example, the measurement and / or analysis of phase noise can be effectively performed by outputting an alarm when a polarization variation is detected, assigning a flag indicating that a polarization variation has been detected to the measured data of the received current i, or recording the time when the polarization variation was detected.

[0204] (When B.δ(t) changes significantly)

[0205] In this case, the condition that the absolute value of the difference between δ(t) and δ(t-τ) is much less than 1 may not hold. Therefore, equation B-12 above may not hold. However, even in this case, equation B-7 holds. Therefore, when the output current i of the optical receiver connected to the optical delay interferometer is amplified, for example, by an amplifier immediately following the optical receiver, δ(t) is integrated while being constrained by a sine function.

[0206] Here, since δ(t) varies between -π / 2 ≤ δ(t) ≤ π / 2, the measurement results of the optical delay interferometer (i as expressed by Equation B-7) are... pol The value of sin{δ(t)-δ(t-τ)} in the equation () varies most with respect to δ(t) within the range of -1 ≤ sin{δ(t)-δ(t-τ)} ≤ 1. Furthermore, for i represented by equation B-7... pol The spectral components obtained by performing a Fourier transform on the measurement results become the varying amplitude values.

[0207] (The trajectory on the Poincaré ball)

[0208] Given that point P(S1, S2, S3) on a Poincaré sphere, expressed using Stokes parameters S0, S1, S2, and S3, represents the polarization state of light. E x (t) and E y The relationship between (t), δ(t) and the Stokes parameters S0, S1, S2 and S3 is expressed by the following equations B-13, B-14, B-15 and B-16.

[0209] (Formula B-13)

[0210]

Mathematical Expression 12

[0211]

[0212] (Formula B-14)

[0213]

Mathematical Expression 13

[0214]

[0215] (Formula B-15)

[0216]

Mathematical Expression 14

[0217] S2 = 2E x (t)E y (t)cosδ(t)

[0218] =S0cos 2χsin 2ψ

[0219] (Formula B-16)

[0220]

Mathematical Expression 15

[0221] S3 = 2E x (t)E y (t)sinδ(t)=S0sin 2χ

[0222] In equations B-13, B-14, B-15, and B-16, χ represents the ellipticity. ψ represents the orientation of elliptically polarized light. Equations B-13, B-14, B-15, and B-16 express the relationship between rectangular coordinates (S1, S2, S3) and polar coordinates (S0, 2χ, 2ψ).

[0223] A sphere with radius S0 in rectangular coordinates (S1, S2, S3) is called a Poincaré sphere. The longitude of a Poincaré sphere represents twice the azimuth ψ, and the latitude represents twice the ellipticity χ. For example, the angle UOQ formed by the origin O (0, 0, 0) of the rectangular coordinates, a point U (S1, S2, 0) on the Poincaré sphere, and a point Q (S1, 0, 0) on the Poincaré sphere is 2ψ. Similarly, the angle POQ formed by the origin O (0, 0, 0) of the rectangular coordinates, a point U (S1, S2, 0) on the Poincaré sphere, and a point P (S1, S2, S3) on the Poincaré sphere is 2χ.

[0224] Here, as shown in equations B-15 and B-16, point P (S1, S2, S3) on the Poincaré sphere moves along the circle R centered at point Q (S1, 0, 0) as δ(t) changes. Therefore, consider the case where point P on the Poincaré sphere moves slightly to point P' on the circle R as δ(t) changes. If we define the angle of POP' formed by point P, the origin O, and point P' as dθ, and the slight shift of δ(t) as dδ, then the relationship between dθ and dδ is expressed by equation B-17. Furthermore, dδ represents the angle of PQP' formed by point P, point Q, and point P'.

[0225] (Formula B-17)

[0226]

Mathematical Expression 16

[0227]

[0228] As described above, by measuring the output current i of the optical receiver connected to the optical delay interferometer, the phase variation approximated by Equation B-8 is observed. If the above phase variation is converted to the above dθ, let dθ be... DL Then dθ DL Using the power ratio α described above, it is expressed as shown in Equation B-18 below.

[0229] (Formula B-18)

[0230]

Mathematical Expression 17

[0231]

[0232] Therefore, according to the method using an optical delay interferometer, the measured phase shift on the Poincaré sphere is c(t) times the angle of movement on the Poincaré sphere. c(t) is represented by B-19 below.

[0233] (Formula B-19)

[0234]

Mathematical Expression 18

[0235]

[0236] The relationship between c(t) and α is expressed by equation B-20 below. In this case, α can be greater than 0 and less than 1. Furthermore, when α is 0 or 1, the electric field is concentrated at HE11x or HE11y, becoming completely linearly polarized. Therefore, when α is 0 or 1, the concept of polarization variation does not need to be considered.

[0237] (Formula B-20)

[0238]

Mathematical Expression 19

[0239]

[0240] As described above, based on this principle, the phase variation amplitude measured by an optical delay interferometer represents an approximate value of the angle of movement (i.e., the amount of phase change) of the trajectory on the Poincaré sphere. This phase variation amplitude is obtained as the amplitude value of the frequency component that gives the interpeak value in the Fourier transform result of the output current of the optical receiver.

[0241] (Overview of Communication System 100)

[0242] Figure 1 This is a simplified example of the system configuration of the communication system 100. In this embodiment, the details of the communication system 100 will be described using the case where the signal light output by the optical signal transmitting device 110 propagates in the optical transmission path 10 and reaches the optical signal receiving device 120 to transmit an information signal.

[0243] In this embodiment, the optical transmission path 10 transmits light. An optical fiber is exemplified as the optical transmission path 10.

[0244] In this embodiment, the communication system 100 includes an optical signal transmitting device 110 and an optical signal receiving device 120. In this embodiment, the optical signal receiving device 120 includes a local oscillator 130, an optical 90-degree mixer 140, an optical receiver 152, an optical receiver 154, an AD converter 162, an AD converter 164, and a signal processing unit 170.

[0245] In this embodiment, the optical signal transmitting device 110 generates an optical signal. The optical signal transmitting device 110 generates a polarization-multiplexed optical signal, for example, by modulating an optical carrier using an information signal to be transmitted. The optical signal may be a signal superimposed with a signed time sequence. The optical signal transmitting device 110 outputs light constituting the optical signal (sometimes referred to as signal light). The signal light is transmitted to the optical signal receiving device 120 via the optical transmission path 10.

[0246] In this embodiment, the optical signal receiving device 120 receives the signal light propagating in the optical transmission path 10. The optical signal receiving device 120 demodulates the received optical signal to generate an information signal.

[0247] During the propagation of signal light in the optical transmission path 10, the signal light is subjected to various linear and nonlinear effects. When the signal light is subjected to nonlinear effects, the phase of the signal light is affected. Therefore, the signal light propagating in the optical transmission path 10 includes a phase noise component. In this embodiment, the optical signal receiving device 120 derives an index for evaluating the phase noise of the signal light. Thus, the optical signal receiving device 120 can evaluate the phase noise of the signal light propagating in the optical transmission path 10. The aforementioned index can be an indicator representing the degree of phase noise of the signal light. As described above, according to this embodiment, the degree of deviation of the differential phase of the signal light is used as the aforementioned index. Variance or standard deviation can be exemplified as an indicator representing the degree of deviation.

[0248] In this embodiment, the local oscillator 130 outputs local oscillating light (as described above, sometimes referred to as local oscillator light). The local oscillator light output from the local oscillator 130 is input to the light 90-degree mixer 140.

[0249] In this embodiment, the optical 90-degree mixer 140 interferes with the signal light propagating in the optical transmission path 10 and the local oscillator light from the local oscillator 130, outputting multiple optical signals that separate the received signal transmitted through the signal light into multiple signal components. In this embodiment, the optical 90-degree mixer 140 mixes the signal light and the local oscillator light, and outputs two optical signals with a 90-degree phase difference. These two signals can be referred to as the I signal and the Q signal, respectively.

[0250] In this embodiment, the optical receiver 152 converts the optical signal into an electrical signal. The optical receiver 152 converts the I signal of the received signal transmitted via signal light into an electrical signal. Thus, the I signal is converted from an optical signal into an electrical signal. The optical receiver 152 outputs the electrical signal corresponding to the I signal to the AD converter 162.

[0251] In this embodiment, the optical receiver 154 converts the optical signal into an electrical signal. The optical receiver 154 converts the Q signal of the received signal transmitted via the signal light into an electrical signal. Thus, the Q signal is converted from an optical signal into an electrical signal. The optical receiver 154 outputs the electrical signal corresponding to the Q signal to the AD converter 164.

[0252] In this embodiment, the AD converter 162 converts the electrical signal from an analog signal to a digital signal. The AD converter 162 converts the electrical signal corresponding to the I signal into a digital signal. The AD converter 162 outputs multiple digital signals (sometimes called sampled I signals) corresponding to multiple time points during the measurement period (sometimes called the evaluation period) for deriving the evaluation index of phase noise to the signal processing unit 170. The time interval between the multiple time points can be approximately constant.

[0253] The sampling rate of the AD converter 162 can be higher than or equal to the symbol rate of the received signal. Therefore, the sample time interval is set to be less than or equal to the length of the symbol time. Alternatively, the sampling rate of the AD converter 162 can be the same as the symbol rate of the received signal. Therefore, the sample time interval is set to be equal to the length of the symbol time.

[0254] In this embodiment, the AD converter 164 converts the electrical signal from an analog signal to a digital signal. The AD converter 164 converts the electrical signal corresponding to the Q signal into a digital signal. The AD converter 164 outputs multiple digital signals (sometimes referred to as sampled Q signals) corresponding to multiple time points during the evaluation period to the signal processing unit 170. The time interval between the multiple time points can be approximately constant.

[0255] The sampling rate of the AD converter 164 can be greater than or equal to the symbol rate of the received signal. Therefore, the sample time interval is set to be less than or equal to the length of the symbol time. Alternatively, the sampling rate of the AD converter 164 can be the same as the symbol rate of the received signal. Therefore, the sample time interval is set to be equal to the length of the symbol time.

[0256] In this embodiment, the signal processing unit 170 demodulates the received signal transmitted via the signal light to generate an information signal. Furthermore, in this embodiment, the signal processing unit 170 evaluates the phase noise of the signal light propagating in the optical transmission path 10. The signal processing unit 170 can also monitor the phase noise of the signal light propagating in the optical transmission path 10. The signal processing unit 170 will be described in detail later.

[0257] (Specific composition of each part of the communication system 100)

[0258] The components of the communication system 100 can be implemented in hardware, software, or both. At least a portion of the components of the communication system 100 can be implemented using analog circuits or digital circuits. At least a portion of the components of the communication system 100 can be implemented using a single server or multiple servers. At least a portion of the components of the communication system 100 can also be implemented on a virtual machine or a cloud system. At least a portion of the components of the communication system 100 can also be implemented using a personal computer or a portable terminal. Examples of portable terminals include mobile phones, smartphones, PDAs (registered trademarks), tablets, laptops or laptop computers, wearable computers, etc. The components of the communication system 100 can also utilize distributed ledger technologies such as blockchain or distributed networks to store information.

[0259] When at least a portion of the constituent elements constituting the communication system 100 are implemented by software, the constituent elements implemented by software can be implemented by activating a program that specifies the actions related to the constituent elements in a generally configured information processing device. The aforementioned information processing device includes, for example: (i) a data processing device having a processor such as a CPU or GPU, ROM, RAM, communication interface, etc.; (ii) an input device such as a keyboard, touch panel, camera, microphone, various sensors, GPS receiver, etc.; (iii) an output device such as a display device, speaker, vibration device, etc.; and (iv) a storage device (including external storage devices) such as a memory or HDD.

[0260] In the aforementioned information processing apparatus, the data processing device or storage device can store a program. The program can be stored on a non-transitory computer-readable recording medium. The program, when executed by a processor, causes the information processing apparatus to perform actions specified by the program.

[0261] The program can be stored on computer-readable media such as CD-ROM, DVD-ROM, memory, and hard disk, or on a network-connected storage device. The program can be installed on the computer constituting at least a part of the communication system 100 from either a computer-readable medium or a network-connected storage device. By executing the program, the computer can also function as at least a part of the communication system 100.

[0262] The program that enables the computer to function as at least a part of the communication system 100 may include modules that define the operation of each part of the communication system 100. These programs or modules activate data processing devices, input devices, output devices, storage devices, etc., to enable the computer to function as a part of the communication system 100, or to enable the computer to execute information processing methods in each part of the communication system 100.

[0263] The information processing described in the program functions as a specific unit cooperating with various hardware resources of the associated software and communication system 100 by reading the program into the computer. Furthermore, the aforementioned specific unit constructs a communication system 100 corresponding to the intended use of the computer in this embodiment by performing calculations or processing of information.

[0264] The aforementioned program can be a program for enabling the computer to function as an optical signal receiving device 120 or a part thereof. The aforementioned program can also be a program for enabling the computer to execute information processing methods within the optical signal receiving device 120 or a part thereof. Examples of components of the optical signal receiving device 120 include an AD converter 162, an AD converter 164, and a signal processing unit 170.

[0265] In one embodiment, the information processing method described above can be an evaluation method for evaluating the phase noise of signal light propagating in an optical transmission path. The evaluation method, for example, includes a differential phase information acquisition stage that acquires information representing the differential phase of the input light at each of a plurality of time points included in the evaluation period. In the evaluation method, there is an index derivation stage that derives the degree of deviation of the differential phase at each of the plurality of time points as an index for evaluating the phase noise. In the evaluation method, the differential phase, for example, represents the phase difference between two temporally adjacent input light points among the plurality of time points. In the evaluation method, the time interval between the plurality of time points is, for example, approximately constant. The time interval between the plurality of time points is, for example, less than or equal to the symbol time of the signal transmitted by the input light.

[0266] Communication system 100 can be an example of an optical communication system. Optical signal transmitting device 110 can be an example of an optical transmitter. Optical signal receiving device 120 can be an example of an evaluation device or an optical receiver. Local oscillator 130 can be an example of a local light source. Optical receiver 152 can be an example of a photoelectric conversion unit. Optical receiver 154 can be an example of a photoelectric conversion unit. AD converter 162 can be an example of an analog-to-digital converter. AD converter 164 can be an example of an analog-to-digital converter. Signal processing unit 170 can be an example of an evaluation device. I signal can be an example of an optical signal with I signal components. Sampled I signal can be an example of a first digital signal. Q signal can be an example of an optical signal with Q signal components. Sampled Q signal can be an example of a second digital signal. Target light can be an example of input light. Light constituting various optical signals can be an example of input light.

[0267] (Another example of an implementation method)

[0268] In this embodiment, for the purpose of understanding the communication system 100, the details of the communication system 100 are described using the example of signal light output from the optical signal transmitting device 110 propagating in the optical transmission path 10 and reaching the optical signal receiving device 120 to transmit information signals. However, the communication system 100 is not limited to this embodiment. For example, various multiplexing techniques are known in the field of optical communication. Therefore, various multiplexing techniques can be applied in the above-described communication system 100 according to other embodiments.

[0269] Examples of multiplexing technologies include optical wavelength multiplexing (sometimes called wavelength division multiplexing) and space division multiplexing. Optical wavelength multiplexing allows the simultaneous transmission of multiple optical signals with different wavelengths using a single optical fiber. Space division multiplexing technologies include those using multi-core optical fibers with multiple cores configured in a single cladding, and those using multimode optical fibers designed to propagate multiple modes of light.

[0270] Figure 2 Here is a simplified representation of an example of the internal configuration of the signal processing unit 170. In this embodiment, the signal processing unit 170 includes a digital signal processing circuit 210, a decoding circuit 220, an amplitude noise evaluation unit 230, and a phase noise evaluation unit 240. In this embodiment, the phase noise evaluation unit 240 includes a differential phase signal generation unit 242, a histogram generation unit 244, and a standard deviation calculation unit 246.

[0271] In this embodiment, the digital signal processing circuit 210 receives a sampled I signal from the AD converter 162. The digital signal processing circuit 210 also receives a sampled Q signal from the AD converter 164. The digital signal processing circuit 210 uses the I and Q signals to perform various digital signal processing operations for demodulating or compensating the received signal. In this embodiment, the decoding circuit 220 performs error correction processing, decoding processing, etc. Thus, an information signal is extracted from the received signal.

[0272] In this embodiment, the amplitude noise evaluation unit 230 evaluates the amplitude noise of the signal light propagating in the optical transmission path 10. The amplitude noise evaluation unit 230 derives various indicators for evaluating the amplitude noise of the signal light propagating in the optical transmission path 10. Examples of these indicators include the optical signal-to-noise ratio (SNR). The SNR is measured using at least one of a sampled I signal and a sampled Q signal. For example, when measuring the SNR of the signal light using the sampled I signal, this I signal is input to the amplitude noise evaluation unit 230. Various known methods can be used to measure the SNR. For example, the amplitude noise evaluation unit 230 includes an average value calculation unit, a histogram generation unit, and a standard deviation calculation unit. Thus, the amplitude noise of the signal light is measured.

[0273] In this embodiment, the phase noise evaluation unit 240 evaluates the phase noise of the signal light propagating in the optical transmission path 10. The phase noise evaluation unit 240 derives various indicators for evaluating the phase noise of the signal light propagating in the optical transmission path 10. As described above, the degree of deviation of the differential phase can be exemplified as such indicators. Variance, standard deviation, etc., can be exemplified as indicators representing the degree of deviation.

[0274] Furthermore, in this embodiment, an example of the phase noise evaluation unit 240 is described using the case where the standard deviation of the differential phase during the evaluation period is derived by the phase noise evaluation unit 240 as an indicator of the degree of deviation of the measured differential phase. However, the phase noise evaluation unit 240 is not limited to this embodiment. In other embodiments, the phase noise evaluation unit 240 may also derive the variance of the differential phase during the evaluation period as an indicator of the degree of deviation of the differential phase.

[0275] In this embodiment, the differential phase signal generation unit 242 acquires, for example, I signal data and Q signal data (sometimes referred to as input signals) of each of N (N is a positive integer) sample points during the evaluation period from the digital signal processing circuit 210. The N sample points correspond to each of the multiple time points included in the evaluation period.

[0276] The differential phase signal generation unit 242 determines the phase of the signal light measured at each of the N sample points based on the data of the I signal and the data of the Q signal at each of the N sample points. The phase of the signal light at the i-th sample point (i is an integer greater than or equal to 1 and less than or equal to (N-1)) is determined, for example, according to Equation 4 above.

[0277] The differential phase signal generation unit 242 calculates the differential phase value of each of the N sample points based on the measured phase values ​​of the signal light at each of the N sample points. The differential phase value of the i-th sample point... For example, calculate according to Equation 3 above.

[0278] According to Equation 3, the phase difference between two temporally adjacent time points among multiple time points is calculated. The differential phase signal generation unit 242 can acquire information representing the differential phase of the signal light at each of the multiple time points included in the evaluation period.

[0279] The differential phase signal generation unit 242 can remove the modulation component and generate information representing the differential phase of each of the N sample points. For example, the differential phase signal generation unit 242 removes the modulation component included in the received signal based on multiple sampled I and Q signals. As described above, in the case of M-phase modulation, the modulation component can be eliminated by calculating the M-th power of the received signal.

[0280] In this embodiment, the histogram generation unit 244 acquires information representing (N-1) differential phases (sometimes referred to as differential phase signals) from the differential phase signal generation unit 242. The histogram generation unit 244 generates a histogram of (N-1) differential phases. The histogram generation unit 244 outputs information representing the generated histogram to the standard deviation calculation unit 246.

[0281] In this embodiment, the standard deviation calculation unit 246 calculates the standard deviation of (N-1) differential phases. For example, the standard deviation calculation unit 246 performs fitting to the histogram generated by the histogram generation unit 244 towards a normal distribution. Furthermore, the standard deviation calculation unit 246 calculates the standard deviation of the aforementioned normal distribution.

[0282] The standard deviation calculation unit 246 can derive the calculated standard deviation as an indicator for evaluating the phase noise of the signal light. As shown in Equation 6, the square of the standard deviation of the phase noise of the signal light is expressed as 1 / 2 times the square of the standard deviation of the differential phase. The standard deviation calculation unit 246 can derive the standard deviation of the phase noise of the signal light based on the standard deviations of (N-1) differential phases and according to Equation 9.

[0283] Furthermore, as shown in Equation 7, the square of the standard deviation of the phase noise of the received signal is expressed as the square root of the sum of the square of the standard deviation of the phase noise of the signal light and the square of the standard deviation of the phase noise based on the spectral linewidth of the local oscillation light. The standard deviation calculation unit 246 can derive the standard deviation of the phase noise of the received signal based on the standard deviations of N differential phases and according to Equations 9 to 11.

[0284] The standard deviation calculation unit 246 can generate information (sometimes called evaluation information) for evaluating the phase noise of the signal light. Examples of this information for evaluating the phase noise of the signal light include the standard deviation of the N differential phases, the standard deviation of the phase noise of the signal light, and / or the standard deviation of the phase noise of the received signal.

[0285] The phase noise evaluation unit 240 can be an example of an evaluation device. The differential phase signal generation unit 242 can be an example of a differential phase information acquisition unit. The histogram generation unit 244 can be an example of a differential phase information acquisition unit. The standard deviation calculation unit 246 can be an example of an index derivation unit.

[0286] Figure 3 This is a simplified illustration of the system configuration of the phase noise evaluation device 320. In this embodiment, the phase noise evaluation device 320 includes an optical delay interferometer 340, an optical receiver 350, an AD converter 360, and a signal processing unit 370.

[0287] In Figure 1 and Figure 2 In the communication system 100 described in connection with this, the standard deviation of the differential phase in the evaluation mechanism is calculated by performing high-speed A / D conversion and digital signal processing by the optical signal receiving device 120. In this embodiment, the high-speed computational processing relative to the electrical signal in the optical signal receiving device 120 is performed at the optical level, and... Figure 1 and Figure 2 The optical signal receiving device 120, which is described in connection with this, is different.

[0288] In this embodiment, the phase noise evaluation device 320 outputs the degree of deviation of the differential phase of the light (sometimes referred to as the input light) input to the phase noise evaluation device 320 as an index for evaluating the phase noise of the input light. Thus, the phase noise evaluation device 320 is able to evaluate the phase noise of the input light.

[0289] The input light can be either the signal light propagating in the optical transmission path 10 or the output light of the optical 90-degree mixer 140. The phase noise evaluation device 320 is installed, for example, in the optical signal receiving device 120, and a portion of the signal light propagating in the optical transmission path 10 is input to the phase noise evaluation device 320.

[0290] In the signal light used in actual communication, the optical signal has modulation-based spectral components. Therefore, the modulation components sometimes interfere with the measurement results, making the measurement of optical phase noise difficult.

[0291] Therefore, in one embodiment, the phase noise evaluation device 320 is installed, for example, in the communication system 100 to evaluate phase noise during trial operation testing after the communication system 100 has been constructed. In other embodiments, the phase noise evaluation device 320 is installed in the communication system 100 to set a dedicated wavelength for measuring phase noise during the operation of the communication system 100, and to always measure the phase noise of light at that wavelength. For example, in an embodiment where differential phase is derived through signal processing of digital data, phase noise is evaluated by measuring the differential phase of the I and Q components of light at the aforementioned wavelength.

[0292] In this embodiment, light (sometimes referred to as input light, object light, etc.) is input to the optical delay interferometer 340. The optical delay interferometer 340, for example, has a first optical path and a second optical path. The optical delay interferometer 340, for example, branches the input light into a first input light and a second input light. The optical delay interferometer 340, for example, causes the first input light passing through the first optical path to undergo wave combination interference with the second input light passing through the second optical path.

[0293] In this embodiment, the first optical path and the second optical path of the optical delay interferometer 340 are set such that the time difference τ between the first input light passing through the first optical path and the second input light passing through the second optical path satisfies the following Equation 12 relationship with the frequency f of the input light.

[0294] (Equation 12)

[0295] 2πfτ=2nπ+π / 2 (where n is an integer)

[0296] When the relationship in Equation 12 above holds, the output current i of the optical receiver 350, which is fed with the output light of the optical delay interferometer 340, is approximated by Equation 13 below.

[0297] (Equation 13)

[0298]

[0299] In Equation 12 above, R is the sensitivity of the optical receiver 350. E is the electric field of the first input light and the second input light. It is the phase noise of the input light at time t.

[0300] In this embodiment, the optical receiver 350 converts the output light of the optical delay interferometer 340 into an electrical signal. As described above, the output current of the optical receiver 350 represents the differential phase of the input light at each of the multiple time points included in the evaluation period. Therefore, the optical receiver 350 can acquire information representing the differential phase of the input light at each of the multiple time points included in the evaluation period. As described above, the differential phase represents the phase difference between two temporally adjacent input light points among the multiple time points.

[0301] In this embodiment, the AD converter 360 converts the electrical signal output from the optical receiver 350 from an analog signal to a digital signal. The AD converter 360 outputs multiple digital signals corresponding to the differential phase of each of the multiple time points included in the evaluation period to the signal processing unit 370. The time interval between the multiple time points can be approximately constant.

[0302] According to this embodiment, the sampling rate of AD converter 360 can be significantly reduced compared to the sampling rate of AD converter 162 or AD converter 164, which samples the signal light. For example, when the sampling rate of the communication light is 100 GS / s, the sampling rate of AD converter 360 can be reduced to about 1 GS / s.

[0303] As shown in Equation 13 above, when using the optical delay interferometer 340, the value of the differential phase converted into intensity is measured. That is, the differential phase noise is converted into amplitude noise by the optical delay interferometer 340. Furthermore, the frequency components of the differential phase noise are approximately white noise. Considering the above, a narrow-bandwidth optical receiver can be used as the optical receiver 350. Specifically, an optical receiver with a bandwidth of approximately 100MHz is used as the optical receiver 350. This also allows for a reduction in the sampling rate of the AD converter 360 that samples the output current of the optical receiver 350.

[0304] In this embodiment, the signal processing unit 370 receives the aforementioned plurality of digital signals from the AD converter 360. The signal processing unit 370 derives the degree of deviation of the differential phase at each of the plurality of time points as an indicator for evaluating the phase noise of the input light. As described above, standard deviation, variance, etc., can be cited as indicators representing the degree of deviation. Furthermore, as shown in Equation 6, the square of the standard deviation of the phase noise of the input light is expressed as half a times the square of the standard deviation of the differential phase. The signal processing unit 370 can output information representing the evaluation of the phase noise of the input light (sometimes referred to as evaluation information). Details of the signal processing unit 370 will be described later.

[0305] In this embodiment, high-speed computation processing relative to the electrical signal in the optical signal receiving device 120 is performed at the optical level. This enables power saving and cost reduction. Furthermore, in this embodiment, the reciprocal of the delay time is equivalent to the sampling rate in the AD converter. Therefore, power used for sampling can be saved.

[0306] Phase noise evaluation device 320 can be an example of an evaluation device. Optical delay interferometer 340 can be an example of a delay interferometer. Optical receiver 350 can be an example of a differential phase information acquisition unit or a photoelectric conversion unit. AD converter 360 can be an example of a differential phase information acquisition unit or an analog-to-digital conversion unit. Signal processing unit 370 can be an example of a differential phase information acquisition unit or an index derivation unit.

[0307] (Another example of an implementation method)

[0308] In this embodiment, an example of the phase noise evaluation device 320 is described, taking the case where the phase noise evaluation device 320 includes an AD converter 360 and a signal processing unit 370 as an example. However, the phase noise evaluation device 320 is not limited to this embodiment. In other embodiments, the signal processing unit 370 has the function of the AD converter 360, and the phase noise evaluation device 320 may not include the AD converter 360.

[0309] Figure 4 Here is a simplified illustration of the internal configuration of the optical delay interferometer 340. In this embodiment, the optical delay interferometer 340 includes a semi-transparent mirror 422, a semi-transparent mirror 424, a total reflection mirror 432, a total reflection mirror 434, and an optical phase adjuster 436.

[0310] The input light is split into a first signal light that passes through the semi-transparent mirror 422 and a second signal light that is reflected by the semi-transparent mirror 422. The first signal light passes through the semi-transparent mirror 424 and is output from the optical delay interferometer 340. On the other hand, the second signal light, after being reflected by the total reflection mirror 432, the total reflection mirror 434, and the semi-transparent mirror 424, mixes with the first signal light that passes through the semi-transparent mirror 424 and is output from the optical delay interferometer 340.

[0311] In this embodiment, the optical delay interferometer 340 imparts an optical path difference to the first signal light and the second signal light. If we define the time difference caused by the optical path difference as τ, the time as t, and the phase when the first signal light is input to the optical receiver 350 as... The phase of the second signal light input to the optical receiver 350 is then... The time delay difference τ is adjusted to satisfy the relationship in Equation 8 above.

[0312] Furthermore, in this embodiment, a phase adjuster 436 is disposed in a region of the optical path of the second signal light (the aforementioned second optical path) that does not overlap with the optical path of the first signal light (the aforementioned first optical path). The phase adjuster 436 adjusts the phase of the light passing through it. For example, the phase adjuster 436 adjusts the phase of the light passing through it based on the voltage applied to it. Examples of the phase adjuster 436 include liquid crystal elements and lithium niobate crystals.

[0313] According to this embodiment, the aforementioned delay time difference τ is adjusted by adjusting the distance between the first and second optical paths and / or the voltage applied to the optical phase adjuster 436. This allows adjustment of the operating point of the optical delay interferometer 340. The delay time difference τ can be adjusted during calibration or in real time.

[0314] Furthermore, in other embodiments, the optical phase adjuster 436 may also be configured in a region of the first optical path that does not overlap with the second optical path. Alternatively, the optical delay interferometer 340 may not have the optical phase adjuster 436.

[0315] Figure 5 This is a simplified illustration of the internal structure of the optical delay interferometer 540. The optical delay interferometer 540 is another example of the optical delay interferometer 340 and can be installed in the phase noise evaluation device 320 instead of the optical delay interferometer 340.

[0316] In this embodiment, the optical delay interferometer 540 includes a substrate 510 and waveguides 520 and 530 formed on the substrate 510. The substrate 510 may be a semiconductor substrate such as a silicon substrate.

[0317] The input light is branched at the branch point to waveguides 520 and 530. The first signal light propagating in waveguide 520 and the second signal light propagating in waveguide 530 are combined and output from the optical delay interferometer 540. The time delay difference τ between waveguides 520 and 530 is adjusted to satisfy the relationship in Equation 8 above.

[0318] In this embodiment, an electrode 526 is disposed on a portion of the waveguide 520. By adjusting the voltage applied to the electrode 526, the phase of the light passing through the waveguide 520 is finely adjusted.

[0319] According to this embodiment, the aforementioned time delay difference τ is adjusted by adjusting the distance between waveguides 520 and 530 and / or the voltage applied to electrode 526. This allows adjustment of the operating point of the optical delay interferometer 540. The time delay difference τ can be adjusted during calibration or in real time.

[0320] In other embodiments, electrode 526 may also be disposed on waveguide 530. Alternatively, optical delay interferometer 540 may not have electrode 526.

[0321] Figure 6 This is a simplified illustration of an example of the internal structure of an optical delay interferometer 640. In this embodiment, the optical delay interferometer 640 includes a substrate 610, a waveguide 620 for inputting input light, a ring resonator 630 disposed adjacent to the waveguide 620, and electrodes 636 disposed on a portion of the ring resonator 630. The substrate 610 may be a semiconductor substrate such as a silicon substrate.

[0322] Input light incident from the input terminal propagates in waveguide 620 and reaches ring resonator 630. A portion of the input light is then guided into ring resonator 630. The input-output characteristics of ring resonator 630 are configured such that the length of the ring is an integer multiple of the wavelength of the input light in the ring medium.

[0323] By using a ring resonator 630, the output current becomes steeper near the operating point relative to the optical frequency. Light is output from the optical delay interferometer 640 through the waveguide 620 and input to the optical receiver 350. The phase of the light in the ring resonator 630 is adjusted by applying a voltage to the electrode 636. Thus, the operating point of the optical delay interferometer 640 can be adjusted.

[0324] Figure 7 Here is a simplified representation of an example of the internal structure of the signal processing unit 370. In this embodiment, the signal processing unit 370 includes a histogram generation unit 244, a standard deviation calculation unit 246, and a correction unit 710.

[0325] In this embodiment, the calibration unit 710 performs various calibration processes. As described above, the output current i of the optical receiver 350 is approximated by Equation 13. Therefore, the calibration unit 710 performs the calibration process of the phase noise evaluation device 320 using signal light with known phase noise. As a result, the receiving sensitivity of the optical receiver 350 and the noise of the circuitry of the optical receiver 350 are corrected.

[0326] For example, the variance of the differential phase noise can be derived by subtracting the variance of the circuit noise from the variance of the received signal. Furthermore, the intensity noise measured by the above derivation method corresponds to the statistical distribution of the phase noise. Therefore, the measured results of various variances and the standard deviation derived from these variances can be pre-corrected against a known optical phase noise standard. This is also true when using an optical ring resonator.

[0327] Figure 8 This section outlines an example of the system configuration of the phase noise evaluation device 820. In this embodiment, the phase noise evaluation device 820 differs from the phase noise evaluation device 320 in that it includes a light receiver 850, an AD converter 860, and a light phase control unit 880. Furthermore, according to this embodiment, the input light is branched at a branch point into a first light incident on the optical delay interferometer 340 and a second light incident on the light receiver 850. Moreover, in this embodiment, the phase noise evaluation device 820 differs from the phase noise evaluation device 320 in that it includes a signal processing unit 870 instead of a signal processing unit 370. Aside from the differences mentioned above, the phase noise evaluation device 820 may have the same configuration as the phase noise evaluation device 320.

[0328] In this embodiment, the optical receiver 850 converts the input second light into an electrical signal. In this embodiment, the analog-to-digital converter 860 converts the electrical signal output by the optical receiver 850 from an analog signal into a digital signal. Thus, information representing a measured value of the optical power of the input light is obtained.

[0329] In this embodiment, the signal processing unit 870 obtains information representing the measured value of the optical power of the input light from the AD converter 860. The signal processing unit 870 uses the measured value of the optical power of the input light to normalize the differential phase of the input light. In addition, the signal processing unit 870 uses the normalized differential phase to derive the degree of deviation of the differential phase.

[0330] In this embodiment, the optical phase control unit 880 acquires an electrical signal corresponding to the output light of the optical delay interferometer 340 from the optical receiver 350. The optical phase control unit 880 adjusts the operating point of the optical delay interferometer 340 based on the aforementioned electrical signal.

[0331] The optical phase control unit 880 adjusts the operating point of the optical delay interferometer 340 so that the point where the ratio of the change in output current to the change in optical frequency is greater than a predetermined value becomes the operating point of the optical delay interferometer 340. The optical phase control unit 880 can also adjust the operating point of the optical delay interferometer 340 so that the point where the ratio of the change in output current to the change in optical frequency is approximately the largest becomes the operating point of the optical delay interferometer 340.

[0332] The optical frequency versus output current characteristic of the optical delay interferometer 340 is sinusoidal relative to the optical frequency, with a period of 1 / τ. Therefore, it can be seen that in the above graph of optical frequency versus output current characteristic, when the point where the sine wave intersects with 0 is set as the operating point, the ratio of the change in output current to the change in optical frequency is approximately the largest.

[0333] Therefore, the optical phase control unit 880 can determine the position where the average value of the output current is 0 as the operating point of the optical delay interferometer 340. At this time, since the optical frequency of the output current characteristic (that is, a graph where the vertical axis is the output current and the horizontal axis is the optical frequency) can be replaced by the differential phase, the change in the differential phase directly becomes the change in the output current.

[0334] The AD converter 860 can be an example of a photoelectric information acquisition unit. The signal processing unit 870 can be an example of a photoelectric information acquisition unit. The optical phase control unit 880 can be an example of an adjustment unit.

[0335] (Another example of an implementation method)

[0336] In this embodiment, an example of the optical phase control unit 880 is described, taking the case where the optical phase control unit 880 detects the average value of the output current of the optical receiver 350 and sets the operating point of the optical delay interferometer 340 such that the average value becomes 0. However, the optical phase control unit 880 is not limited to this embodiment.

[0337] In other embodiments, when the operating point of the optical delay interferometer 340 is optimized, the peak-to-peak value or amplitude of the output current of the optical receiver 350 becomes maximum. Therefore, the optical phase control unit 880 can detect the peak-to-peak value of the output current of the optical receiver 350 and set the operating point of the optical delay interferometer 340 in such a way that this value is maximized. Similarly, the optical phase control unit 880 can detect the amplitude of the output current of the optical receiver 350 and set the operating point of the optical delay interferometer 340 in such a way that this value is maximized.

[0338] Figure 9 Here is a simplified representation of an example of the internal structure of the signal processing unit 870. In this embodiment, the signal processing unit 870 includes a histogram generation unit 244, a standard deviation calculation unit 246, a correction unit 710, and a standardization unit 930.

[0339] In this embodiment, the standardization unit 930 obtains information representing the measured value of the optical power of the input light from the AD converter 860. The standardization unit 930 uses the measured value of the optical power of the input light to standardize the differential phase of the input light. The standardization unit 930 outputs information representing the standardized differential phase of the input light to the histogram generation unit 244.

[0340] In this embodiment, the histogram generation unit 244 generates a histogram of the differential phase using the standardized differential phase. In this embodiment, the standard deviation calculation unit 246 derives the standard deviation of the differential phase using the histogram generated based on the standardized differential phase.

[0341] Figure 10 This is a simplified illustration of an example of the internal configuration of the optical delay interferometer 1040. In this embodiment, the optical delay interferometer 1040 differs from the optical delay interferometer 340 in that it includes a semi-transparent mirror 1060 for branching the input light and directing a portion of the input light to the light receiver 850. Furthermore, in this embodiment, the optical delay interferometer 1040 differs from the optical delay interferometer 340 in that the voltage applied to the optical phase adjuster 436 is controlled by the optical phase control unit 880.

[0342] Figure 11 This is a simplified illustration of an example of the internal configuration of the optical delay interferometer 1140. In this embodiment, the optical delay interferometer 1140 differs from the optical delay interferometer 540 in that it includes a waveguide 1160 for branching the input light and directing a portion of the input light to the optical receiver 850. Furthermore, in this embodiment, the optical delay interferometer 1140 differs from the optical delay interferometer 540 in that the voltage applied to the electrode 526 is controlled by the optical phase control unit 880.

[0343] Figure 12 This is a simplified illustration of an example of the internal configuration of the optical delay interferometer 1240. In this embodiment, the optical delay interferometer 1240 differs from the optical delay interferometer 640 in that it includes a waveguide 1260 for branching the input light and directing a portion of the input light to the optical receiver 850. Furthermore, in this embodiment, the optical delay interferometer 1240 differs from the optical delay interferometer 640 in that the voltage applied to the electrode 636 is controlled by the optical phase control unit 880.

[0344] Figure 13 This is a simplified illustration of an example of the internal configuration of the optical delay interferometer 1340. In this embodiment, the optical delay interferometer 1340 differs from the optical delay interferometer 340 in that a balanced optical receiver 1350 is used instead of the optical receiver 350. This allows for the suppression of intensity noise included in the signal light.

[0345] The balanced optical receiver 1350 includes an optical receiver 1352, an optical receiver 1354, and a differential processing unit 1356. The optical receiver 1352 receives a first output light, which is formed by combining a first signal light transmitted through a semi-transparent mirror 424 and a second signal light reflected by the semi-transparent mirror 424. The optical receiver 1354 receives a second output light, which is formed by combining a first signal light reflected by the semi-transparent mirror 424 and a second signal light transmitted through the semi-transparent mirror 424. The differential processing unit 1356 outputs an electrical signal corresponding to the difference between the outputs of the optical receivers 1352 and 1354 to the AD converter 360.

[0346] Figure 14 This is a simplified illustration of an example of the circuit configuration of the balanced light receiver 1350. In this embodiment, the balanced light receiver 1350 includes photodiodes 1452 and 1454 connected in series. The balanced light receiver 1350 outputs an electrical signal from the connection point 1456 of photodiodes 1452 and 1454.

[0347] Figure 15 This is a simplified illustration of an example of the internal configuration of the optical delay interferometer 1540. In this embodiment, the optical delay interferometer 1540 differs from the optical delay interferometer 540 in that it uses a balanced light receiver 1350 instead of a light receiver 350. This allows for the suppression of intensity noise included in the signal light.

[0348] (Overview of Communication System 1600)

[0349] Figure 16 This section provides a simplified example of the system configuration of the communication system 1600. In this embodiment, the details of the communication system 1600 will be explained using the example of a signal light output from the optical signal transmitting device 110 propagating in the optical transmission path 10 and reaching the optical signal receiving device 1620 to transmit an information signal.

[0350] In this embodiment, the communication system 1600 may have the same configuration as the communication system 100, except that it includes a configuration for detecting polarization variations. Details regarding the communication system 1600 are sometimes omitted in the description of its configuration being the same as that of the communication system 100.

[0351] In this embodiment, the communication system 1600 includes, for example, an optical signal transmitting device 110 and an optical signal receiving device 1620. In this embodiment, the optical signal receiving device 1620 includes a wavelength division multiplexer 1640, an optical receiver 1650, an analog-to-digital converter 1660, a signal processing unit 1670, and a status monitoring device 1680.

[0352] In this embodiment, the optical signal receiving device 1620 receives the signal light propagating in the optical transmission path 10. The optical signal receiving device 120 demodulates the received optical signal to generate an information signal.

[0353] As described above, for example, when a sudden anomaly occurs in the optical transmission line 10 during the propagation of the signal light in the optical transmission line 10, a sudden polarization change occurs in the signal light. In the communication system 100, the phase noise evaluation unit 240, the phase noise evaluation device 320, the phase noise evaluation device 820, or variations thereof, evaluate the phase noise generated in the communication system 100 by statistically processing the measurement results of the phase change.

[0354] However, compared to phase noise, phase variations caused by polarization variations tend to occur abruptly. Since abruptly occurring phase variations are difficult to reflect in the results of the above statistical processing, it is difficult to detect phase variations caused by polarization variations using the phase noise evaluation unit 240, the phase noise evaluation device 320, the phase noise evaluation device 820, or their modifications.

[0355] Therefore, according to this embodiment, the optical signal receiving device 1620 continuously measures the phase variation generated in the communication system 1600. The optical signal receiving device 1620 detects phase variations caused by sudden polarization changes based on the real-time measurement results of this phase noise. For example, the optical signal receiving device 1620 detects an increase in real-time phase variation and issues an alarm. Information processing in the optical signal receiving device 1620 will be described in detail later.

[0356] In this embodiment, when the signal light comprises multiple optical signals with different wavelengths, the wavelength divider 1640 divides the multiple optical signals according to their wavelengths. The optical signal received in the optical signal receiving device 1620 (sometimes referred to as the received signal) includes, for example, light of a first wavelength used for transmitting information signals (sometimes referred to as a communication optical signal) and light of a second wavelength used for detecting polarization variations (sometimes referred to as a monitoring optical signal). The values ​​of the first wavelength and the second wavelength are different. The monitoring optical signal can be unmodulated. The wavelength divider 1640 outputs the communication optical signal to the optical receiver 1650. The wavelength divider 1640 outputs the monitoring optical signal to the status monitoring device 1680.

[0357] In this embodiment, the optical receiver 1650 converts an optical signal into an electrical signal. For example, the optical receiver 1650 converts a communication optical signal into an electrical signal (sometimes referred to as an electrical signal corresponding to the communication optical signal). The optical receiver 1650 outputs the electrical signal corresponding to the communication optical signal to the AD converter 1660.

[0358] In this embodiment, the AD converter 1660 converts an electrical signal from an analog signal to a digital signal. For example, the AD converter 1660 generates one or more digital signals corresponding to one or more sample points (sometimes called samples) by sampling the electrical signal. The AD converter 1660 also converts an electrical signal corresponding to a communication optical signal into a digital signal (sometimes called a digital signal corresponding to a communication optical signal). The AD converter 1660 outputs the digital signal corresponding to the communication optical signal to the signal processing unit 1670.

[0359] In this embodiment, the signal processing unit 1670 demodulates the received signal transmitted via signal light and generates an information signal. The method for generating the information signal based on the received signal is not particularly limited. The signal processing unit 1670 can perform various digital signal processing operations for demodulating or compensating the received signal. The signal processing unit 1670 can perform error correction processing, decoding processing, etc.

[0360] In this embodiment, the state monitoring device 1680 analyzes the light (sometimes referred to as input light) input to the state monitoring device 1680 to monitor the state of the optical transmission path 10 and / or the state of the signal light propagating in the optical transmission path 10. For example, the state monitoring device 1680 analyzes the phase variation of the input light, evaluating at least one of (i) the presence and / or degree of phase noise, and (ii) the presence and / or degree of polarization variation. Thus, the state monitoring device 1680 can, for example, detect polarization variations of the signal light propagating in the optical transmission path 10.

[0361] The status monitoring device 1680, for example, acquires information representing the differential phase of the input light being evaluated at one or more time points. The differential phase represents the phase difference between two temporally adjacent input light points within one or more time points. At one or more time points, the time interval between two temporally adjacent time points can be approximately constant.

[0362] The length of the aforementioned time interval is not particularly limited, but it can be the length of the symbol time of the signal transmitted via the input light, or it can be less than or greater than the symbol time. In one embodiment, the time interval is set such that the difference between it and the symbol time is a predetermined value. In other embodiments, the time interval is set such that the difference between it and the symbol time is less than or less than a predetermined value.

[0363] The status monitoring device 1680, for example, determines whether the differential phase at at least a portion of the time points at more than one time point meets a predetermined condition. When it is determined that the differential phase at at least a portion of the time points at more than one time point meets the predetermined condition, the status monitoring device 1680, for example, outputs information indicating that the signal light has undergone a polarization change. Details regarding the status monitoring device 1680 will be described later.

[0364] (Specific composition of each part of the communication system 1600)

[0365] Similar to the components of communication system 100, the components of communication system 1600 can be implemented in hardware, software, or both. When at least a portion of the constituent elements of communication system 1600 are implemented in software, those software-implemented constituent elements can be implemented by activating a program in a generally configured information processing apparatus that specifies actions related to those constituent elements.

[0366] In the aforementioned information processing apparatus, the data processing device or storage device can store a program. The program can be stored on a non-transitory computer-readable recording medium. The program, when executed by a processor, causes the information processing apparatus to perform actions specified by the program.

[0367] The aforementioned program can be a program for enabling the computer to function as an optical signal receiving device 1620 or a part thereof. The aforementioned program can also be a program for enabling the computer to execute information processing methods within the optical signal receiving device 1620 or a part thereof. Examples of components that can be included as part of the optical signal receiving device 1620 include a signal processing unit 1670 and a status monitoring device 1680.

[0368] The aforementioned information processing method can be a detection method for detecting polarization variations of signal light propagating in an optical transmission path. For example, the detection method includes a differential phase information acquisition stage that acquires information representing the differential phase of the input light as an evaluation object at each of more than one time points. The detection method also includes a determination stage that determines whether the differential phase at at least a portion of the more than one time points satisfies a predetermined condition. The detection method may also include an output stage that outputs information indicating that the signal light has undergone polarization variations when it is determined that the differential phase at at least a portion of the more than one time points satisfies the predetermined condition.

[0369] The predetermined conditions may include the following first condition: the magnitude of the differential phase at least one time point of more than one time point is equal to or greater than a predetermined first threshold. The predetermined conditions may include the following second condition: within an evaluation period of at least a portion of time points of more than one time points and a predetermined length, the number of time points whose differential phase magnitude is equal to or greater than the first threshold is equal to or greater than a predetermined second threshold. The predetermined conditions may include the following third condition: the ratio of the number of time points whose differential phase magnitude is equal to or greater than the first threshold to the total number of time points included in the evaluation period is equal to or greater than a predetermined third threshold. The predetermined conditions may include the following fourth condition: the angle of movement at the variation frequency of the trajectory on the Poincaré sphere, derived from the magnitude of the differential phase at each of the more than one time points, is equal to or greater than a predetermined fourth threshold. The predetermined conditions may include the following fifth condition: the angular velocity of movement at the variation frequency of the trajectory on the Poincaré sphere is equal to or greater than a predetermined fifth threshold.

[0370] The aforementioned predetermined conditions may include a combination of at least two conditions selected from the group consisting of the first, second, third, fourth, and fifth conditions. The first, second, third, fourth, and fifth thresholds may be determined independently. The first, second, third, fourth, and fifth thresholds may be distinct values ​​or at least two may be the same. A summary of the steps for determining each threshold is described later.

[0371] Communication system 1600 can be an example of a detection device, an optical receiving device, or an optical communication system. Optical signal receiving device 1620 can be an example of a detection device or an optical receiving device. Wavelength divider 1640 can be an example of a wavelength division unit. Optical receiver 1650 can be an example of a photoelectric conversion unit. AD converter 1660 can be an example of an analog-to-digital conversion unit. Signal processing unit 1670 can be an example of a demodulation unit. Status monitoring device 1680 can be an example of a detection device. Optical signal transmitting device 110 can be an example of an optical transmitting device. Optical signal can be any kind of light. More than one sample point can be an example of more than one time point. Monitoring optical signal can be an example of light of a second wavelength or input light.

[0372] (Another example of an implementation method)

[0373] In this embodiment, for the purpose of understanding the communication system 1600, the details of the communication system 1600 are described using the example of signal light output from the optical signal transmitting device 110 propagating in the optical transmission path 10 and reaching the optical signal receiving device 1620 to transmit information signals. However, the communication system 1600 is not limited to this embodiment. For example, various multiplexing techniques are known in the field of optical communication. Therefore, various multiplexing techniques can be applied in the above-described communication system 1600 according to other embodiments.

[0374] Examples of multiplexing technologies include optical wavelength multiplexing (sometimes called wavelength division multiplexing) and space division multiplexing. Optical wavelength multiplexing allows the simultaneous transmission of multiple optical signals with different wavelengths using a single optical fiber. Space division multiplexing technologies include those using multi-core optical fibers with multiple cores configured in a single cladding, and those using multimode optical fibers designed to propagate multiple modes of light.

[0375] In this embodiment, the communication system 1600 is described in detail as having a configuration for detecting polarization variations in addition to the same configuration as the communication system 100. However, the communication system 1600 is not limited to this embodiment. In other embodiments, the communication system 1600 may not include at least a portion of the configuration of the communication system 100. For example, the communication system 1600 may not include at least one of the amplitude noise evaluation unit 230 and the phase noise evaluation unit 240.

[0376] In this embodiment, the communication system 1600 is described in detail using the case where the signal light includes a monitoring light signal used to detect polarization variations. However, the communication system 1600 is not limited to this embodiment. In other embodiments, the signal light may include various monitoring light signals with different wavelengths. Examples of objects to be monitored, inspected, or evaluated using the monitoring light signal include the state of the optical transmission path 10 and the state of the signal light propagating in the optical transmission path 10.

[0377] Examples of the states of the optical transmission path 10 include the state of the electromagnetic environment surrounding the optical transmission path 10, the state of the electric field inside or around the optical transmission path 10, the vibration state of the optical transmission path 10, the bending state of the optical transmission path 10, and the connection state of the optical transmission path 10 (e.g., the offset of the connection between the two fiber cores constituting the optical fiber). Examples of the state of the electromagnetic environment surrounding the optical transmission path 10 include variations in that electromagnetic environment. Examples of the state of the electric field inside or around the optical transmission path 10 include variations in that electric field. Examples of the state of the optical signal include variations in intensity, amplitude, phase, and polarization.

[0378] In this embodiment, the communication system 1600 is described in detail using the case where the optical signal transmitting device 110 transmits signal light and the optical signal receiving device 1620 receives signal light as an example. However, the communication system 1600 is not limited to this embodiment. In other embodiments, at least one of the optical signal transmitting device 110 and the optical signal receiving device 1620 may be a transceiver device that has the functions of transmitting and receiving signal light. For example, the optical signal transmitting device 110 may have at least one component that is identical to the components of the optical signal receiving device 1620. For example, the optical signal receiving device 1620 may have at least one component that is identical to the components of the optical signal transmitting device 110.

[0379] Figure 17 Here is a simplified illustration of an example of the internal configuration of the optical signal transmitting device 110. In this embodiment, the optical signal transmitting device 110 includes, for example, a communication optical signal output unit 1722, a monitoring optical signal output unit 1724, and a multiplexer 1730.

[0380] In this embodiment, the communication optical signal output unit 1722 modulates light of a first wavelength according to the information signal and outputs a communication optical signal. In this embodiment, the monitoring optical signal output unit 1724 outputs one or more monitoring optical signals. Each of the one or more monitoring optical signals is generated using light of a different wavelength than the communication optical signal. Each of the one or more monitoring optical signals can be generated using light of mutually different wavelengths. In this embodiment, the combiner 1730 combines the communication optical signal output by the communication optical signal output unit 1722 and the monitoring optical signal output by the monitoring optical signal output unit 1724 to generate signal light.

[0381] (Another example of an implementation method)

[0382] In this embodiment, for the purpose of understanding the communication system 1600, the details of the optical signal transmitting device 110 will be described using the case where the optical signal transmitting device 110 outputs a single signal light as an example. However, as mentioned above, various multiplexing techniques are known in the field of optical communication. Therefore, according to other embodiments, various multiplexing techniques can be applied in the above-described communication system 1600. When optical wavelength multiplexing technology is applied in the communication system 1600, the communication optical signal output unit 1722 can output multiple communication optical signals with different wavelengths.

[0383] Figure 18This is a simplified illustration of an example of the internal configuration of the status monitoring device 1680. In this embodiment, the status monitoring device 1680 includes, for example, an optical delay interferometer 340, a light receiver 350, an AD converter 360, and a signal processing unit 1870. In this embodiment, the light receiver 350 includes, for example, a photoelectric conversion element 1852 and an integrating circuit 1854.

[0384] In this embodiment, the photoelectric conversion element 1852 converts the optical signal into an electrical signal. More specifically, the photoelectric conversion element 1852 converts the output light of the optical delay interferometer 340 into an electrical signal. The photoelectric conversion element 1852, for example, uses a load resistor to convert the photocurrent after photoelectric conversion into a voltage, generating the aforementioned electrical signal. The photoelectric conversion element 1852 outputs the aforementioned electrical signal to the integrating circuit 1854.

[0385] and with Figure 3 Similarly, the output current of the photodetector 350, which has been described in connection with this, represents the differential phase of the input light. As described above, the input light in this embodiment is the monitoring light signal of the input status monitoring device 1680.

[0386] In this embodiment, the integrator circuit 1854 outputs a voltage waveform whose time integral is equal to that of the input voltage waveform. The output voltage of the integrator circuit 1854 is proportional to the integral value of the input voltage. More specifically, the electrical signal output by the photoelectric conversion element 1852 is input to the integrator circuit 1854, and the integrator circuit 1854 outputs a voltage waveform whose time integral is equal to that of the electrical signal. This generates an electrical signal corresponding to the output light of the optical delay interferometer 340. The integrator circuit 1854 outputs this electrical signal to the AD converter 360.

[0387] The time constant of the integrator circuit 1854 is determined, for example, based on the maximum rate of phase change accompanying the envisioned polarization variation. The time constant of the integrator circuit 1854 can be a time constant comparable to the aforementioned maximum rate. For example, if a polarization variation of approximately 10 kHz is envisioned, the time constant of the integrator circuit 1854 is set to approximately 100 μs. Similarly, the cutoff frequency of the integrator circuit 1854 is set to approximately 1.59 kHz.

[0388] The time constant of the integrator circuit 1854 can be 10–1000 μs, 50–500 μs, 75–150 μs, or 80–120 μs. The cutoff frequency of the integrator circuit 1854 can be 0.1–100 kHz, 0.5–50 kHz, 1–20 kHz, or 1–10 kHz.

[0389] The integrating circuit 1854 can be an amplifier or a low-pass filter that processes the electrical signal output by the photoelectric conversion element 1852, or it can be part of the components constituting the amplifier or the low-pass filter. It can be embedded in the photoelectric conversion element 1852 or disposed in the stage following the photoelectric conversion element 1852.

[0390] In this embodiment, the AD converter 360 converts the electrical signal output from the photodetector 350 from an analog signal into a digital signal. This generates one or more digital signals corresponding to the differential phase of each of the one or more sample points. The AD converter 360 outputs these digital signals to the signal processing unit 1870.

[0391] In this embodiment, the sampling rate of the AD converter 360 is set to a value sufficient to sample the signal in the frequency band of the optical receiver 350. For example, the sampling rate of the AD converter 360 is set to approximately 5 to 10 times the bandwidth of the optical receiver 350.

[0392] In this embodiment, the signal processing unit 1870 acquires one or more sampled differential phase signals from the AD converter 360. The signal processing unit 1870 evaluates the state of the optical transmission line 10 and / or the state of the signal light propagating in the optical transmission line 10 based on the differential phase of each of the more than one differential phase signals at one or more time points. For example, the signal processing unit 1870 evaluates at least one of (i) the presence and / or degree of phase noise, and (ii) the presence and / or degree of polarization variation. Thus, the signal processing unit 1870 can, for example, detect polarization variation of the signal light propagating in the optical transmission path 10.

[0393] The signal processing unit 1870 determines, for example, whether the differential phase at at least a portion of the time points at more than one time point satisfies a predetermined condition. When it is determined that the differential phase at at least a portion of the time points at more than one time point satisfies the predetermined condition, the signal processing unit 1870 outputs, for example, information indicating that a polarization change has occurred in the signal light.

[0394] There are no particular limitations on how the signal processing unit 1870 is provided. The signal processing unit 1870 may be an integrated circuit (IC), a large-scale integrated circuit (LSI), a system LSI, a system-on-a-chip, or a microprocessor, or a device constructed from a combination of these. Details regarding the signal processing unit 1870 will be described later.

[0395] The photoelectric conversion element 1852 can be an example of a photoelectric conversion unit. The integrating circuit 1854 can be an example of an integrating unit. The signal processing unit 1870 can be an example of a detection device. The sampled differential phase signal can be an example of information representing the differential phase of the input light at each of the more than one time points. The sample point can be an example of more than one time point.

[0396] (Another example of an implementation method)

[0397] In this embodiment, the state monitoring device 1680 is described in detail using the case where it includes an optical delay interferometer 340 as an example. However, the state monitoring device 1680 is not limited to this embodiment. In other embodiments, the state monitoring device 1680 may replace the optical delay interferometer 340 with an optical delay interferometer having any configuration. For example, the state monitoring device 1680 may include an optical delay interferometer 540, an optical delay interferometer 640, an optical delay interferometer 1040, an optical delay interferometer 1140, an optical delay interferometer 1240, or an optical delay interferometer 1340.

[0398] In this embodiment, the status monitoring device 1680 is described in detail using the case where the status monitoring device 1680 includes an optical receiver 350 as an example. However, the status monitoring device 1680 is not limited to this embodiment. In other embodiments, the status monitoring device 1680 may replace the optical receiver 350 with an optical receiver of arbitrary configuration. For example, the status monitoring device 1680 may include a balanced optical receiver 1350.

[0399] In this embodiment, the status monitoring device 1680 is described in detail using the case where the optical receiver 350 includes the integrating circuit 1854 as an example. However, the status monitoring device 1680 is not limited to this embodiment. In other embodiments, the integrating circuit 1854 may be disposed outside the optical receiver 350. For example, the integrating circuit 1854 may be disposed after the optical receiver 350. The integrating circuit 1854 may also be disposed inside a device disposed after the optical receiver 350. Examples of such devices include integrating circuits and devices with integrating characteristics. Examples of devices with integrating characteristics include amplifiers and low-pass filters.

[0400] Figure 19This is a simplified representation of an example of the internal structure of the signal processing unit 1870. In this embodiment, the signal processing unit 1870 includes a histogram generation unit 244, a standard deviation calculation unit 246, and a correction unit 710. In this embodiment, the signal processing unit 1870 includes an increase detection unit 1922, a frequency resolution unit 1924, a movement angle derivation unit 1926, a movement angular velocity derivation unit 1928, a polarization variation detection unit 1930, and an information output unit 1940.

[0401] In this embodiment, the additional detection unit 1922 acquires one or more sampled differential phase signals from the AD converter 360. As described above, each of the one or more differential phase signals represents a differential phase at one or more time points. The additional detection unit 1922 analyzes the one or more differential phase signals in the time domain.

[0402] For example, the addition detection unit 1922 compares the magnitude of the differential phase at one or more time points with a predetermined first threshold. Thus, the addition detection unit 1922 can determine whether the magnitude of the differential phase at one or more time points is equal to or greater than the first threshold. The addition detection unit 1922 outputs information indicating the determination result to the polarization variation detection unit 1930.

[0403] In this embodiment, the frequency analysis unit 1924 acquires one or more sampled differential phase signals from the AD converter 360. The frequency analysis unit 1924 analyzes the one or more differential phase signals in the frequency domain (sometimes referred to as frequency analysis).

[0404] For example, the frequency analysis unit 1924 performs a Fourier transform (e.g., a fast Fourier transform) on one or more differential phase signals. From this, the variation frequency of the phase difference δ(t) between the orthogonal polarization modes Ex(t) and Ey(t) of the signal light propagating in the z-direction is derived. Furthermore, the spectral components obtained through the Fourier transform represent the variation amplitude values.

[0405] Specifically, the results of the Fourier transform are analyzed, and the frequencies representing prominent peaks and other prominent components are determined as the variation frequencies. For example, the amplitude values ​​are observed to change while altering the frequencies of the measured data obtained through the Fourier transform. The frequencies at which the amplitude values ​​change sharply are determined as the variation frequencies.

[0406] In this embodiment, the frequency analysis unit 1924 outputs information representing the analysis result to the polarization variation detection unit 1930. The frequency analysis unit 1924 can output information representing the analysis result to the movement angle derivation unit 1926 and / or the movement angular velocity derivation unit 1928 as needed. Examples of information representing the analysis result include the variation frequency and the variation amplitude value. The variation amplitude value is a parameter related to the movement angle, and the movement angular velocity is derived from the variation amplitude value and the variation frequency.

[0407] In this embodiment, the motion angle derivation unit 1926 acquires one or more sampled differential phase signals from the AD converter 360. The motion angle derivation unit 1926 derives the motion angle at the variation frequency of the trajectory on the Poincaré sphere, derived from the magnitude of the differential phase at each of the more than one time points. Specifically, the variation frequency of the trajectory on the Poincaré sphere is derived based on the results of the Fourier transform described above. Furthermore, the interpeak value (pp value) is calculated based on the vibration amplitude value (peak value) of the Fourier transform, thereby deriving the motion angle. The motion angle derivation unit 1926 outputs information representing the derivation result to the polarization variation detection unit 1930.

[0408] In this embodiment, the angular velocity derivation unit 1928 acquires one or more sampled differential phase signals from the AD converter 360. The angular velocity derivation unit 1928 derives the angular velocity at the variation frequency of the trajectory on the Poincaré sphere, derived from the magnitude of the differential phase at each of the more than one time points. The angular velocity is derived based on the angular velocity angle and variation frequency of the trajectory on the Poincaré sphere. For example, the angular velocity is derived as angular velocity angle (pp value) × 2 × variation frequency. The angular velocity derivation unit 1928 outputs information representing the derivation result to the polarization variation detection unit 1930.

[0409] In this embodiment, the polarization variation detection unit 1930 evaluates the presence and / or degree of polarization variation. For example, the polarization variation detection unit 1930 detects polarization variation. If polarization variation is detected, it can be evaluated that polarization variation has occurred and / or the degree of polarization variation is relatively large.

[0410] For example, the polarization variation detection unit 1930 determines whether the differential phase at at least a portion of more than one time point satisfies a predetermined condition. When it is determined that the differential phase at at least a portion of more than one time point satisfies the predetermined condition, the polarization variation detection unit 1930 outputs information indicating that a polarization variation has been detected to the information output unit 1940.

[0411] The predetermined conditions may include the following first condition: the magnitude of the differential phase at least one time point of one or more time points is equal to or greater than a predetermined first threshold. The predetermined conditions may include the following second condition: within an evaluation period of at least a portion of time points of one or more time points and a predetermined length, the number of time points whose differential phase magnitude is equal to or greater than the first threshold is equal to or greater than a predetermined second threshold. The predetermined conditions may include the following third condition: the ratio of the number of time points whose differential phase magnitude is equal to or greater than the first threshold to the total number of time points included in the evaluation period is equal to or greater than a predetermined third threshold.

[0412] The aforementioned predetermined conditions may include the following fourth condition: the angle of movement at the variation frequency of the trajectory on the Poincaré sphere, derived from the magnitude of the differential phase at more than one time point, is equal to or greater than a predetermined fourth threshold. The aforementioned predetermined conditions may also include the following fifth condition: the angular velocity of movement at the variation frequency of the trajectory on the Poincaré sphere is equal to or greater than a predetermined fifth threshold.

[0413] The aforementioned predetermined conditions may include a combination of at least two conditions selected from the group consisting of the first, second, third, fourth, and fifth conditions. The first, second, third, fourth, and fifth thresholds may be determined independently. The first, second, third, fourth, and fifth thresholds may be distinct values ​​or at least two may be the same.

[0414] At least one of the first, second, third, fourth, and fifth thresholds can be determined based on the polarization resistance of the communication system 1600 or the optical signal receiving device 1620. For example, at least one of the first, fourth, and fifth thresholds is set to ka times the polarization resistance specified by the specifications of the device mounted on the communication system 1600 or the optical signal receiving device 1620 (ka is a positive number. ka can be less than 1 or less than 1). As mentioned above, the first, fourth, and fifth thresholds can also be different from each other. At least one of the second and third thresholds can be determined through pre-testing, commissioning tests, etc.

[0415] In one embodiment, the polarization variation detection unit 1930 acquires information from the addition detection unit 1922 representing determination results associated with one or more specimen points. For example, the polarization variation detection unit 1930 determines that the first condition is met when at least one of the determination results associated with one or more specimen points indicates that the magnitude of the differential phase of that specimen point is equal to or greater than a first threshold. When the first condition is met, the polarization variation detection unit 1930 can determine that a polarization variation has been detected.

[0416] According to the above implementation, the first condition may still hold even when the actual impact on transmission characteristics is minimal. For example, if the magnitude of the differential phase at a specific sample point is a statistically specific value, the first condition holds, and polarization variation can be detected. In this case, the impact of the detected polarization variation on transmission characteristics may become very small.

[0417] Therefore, the polarization variation detection unit 1930 can, for example, use the determination results related to multiple specimen points included in a predetermined period (sometimes called the evaluation period) from the determination results associated with one or more specimen points respectively to detect polarization variation. As a result, the detection accuracy of polarization variation is improved.

[0418] For example, the polarization variation detection unit 1930 determines that the second condition is met when the number of sample points whose differential phase magnitude is equal to or greater than the first threshold among the multiple sample points included in the evaluation period is equal to or greater than the second threshold. When the second condition is met, the polarization variation detection unit 1930 can determine that a polarization variation has been detected.

[0419] For example, the polarization variation detection unit 1930 determines that the third condition is met when the ratio of the number of sample points whose differential phase magnitude is equal to or greater than the first threshold to the number of multiple sample points included in the evaluation period is equal to or greater than a predetermined third threshold. When the third condition is met, the polarization variation detection unit 1930 can determine that a polarization variation has been detected.

[0420] In other embodiments, the polarization variation detection unit 1930 obtains information from the movement angle derivation unit 1926 about the derived result of the movement angle at the frequency of the change in the trajectory on the Poincaré sphere. When the aforementioned movement angle is equal to or greater than the fourth threshold, the polarization variation detection unit 1930 determines that the aforementioned fourth condition is met. When the fourth condition is met, the polarization variation detection unit 1930 can determine that a polarization variation has been detected.

[0421] Furthermore, in other embodiments, the polarization variation detection unit 1930 obtains information from the angular velocity derivation unit 1928 regarding the derived angular velocity at the frequency of the change in the trajectory on the Poincaré sphere. When the aforementioned angular velocity is equal to or greater than the fifth threshold, the polarization variation detection unit 1930 determines that the aforementioned fifth condition is met. When the fifth condition is met, the polarization variation detection unit 1930 can determine that a polarization variation has been detected.

[0422] In this embodiment, the information output unit 1940 outputs evaluation information representing various evaluation results from the signal processing unit 1870. In one embodiment, the evaluation information includes evaluation information for evaluating and... Figure 2 Information regarding the phase noise of the signal light was described in connection with the description. In other embodiments, the evaluation information includes evaluation information related to polarization variation. Examples of evaluation information related to polarization variation include information indicating that polarization variation has occurred in the signal light, information indicating that the angular velocity of movement exceeds a specified angular velocity, information indicating that the angle of movement exceeds a specified angle, and information indicating that the frequency of variation exceeds a specified frequency. Evaluation information related to polarization variation can also be output as an alarm or flag.

[0423] As described above, information indicating a polarization change in the signal light can be an example of information indicating a large degree of polarization change. Other examples of evaluation information related to polarization change include information indicating the amount of polarization change, information indicating the absolute value of the amount of polarization change, etc. As the amount of polarization change, the change in the angle of movement of the trajectory on the Poincaré sphere can be used. As described above, the phase shift amplitude measured by an optical delay interferometer represents the approximate value of the angle of movement of the trajectory on the Poincaré sphere.

[0424] The addition of a detection unit 1922 can be an example of a differential phase information acquisition unit. The frequency resolution unit 1924 can be an example of a differential phase information acquisition unit. The movement angle derivation unit 1926 can be an example of a differential phase information acquisition unit. The movement angular velocity derivation unit 1928 can be an example of a differential phase information acquisition unit. The polarization variation detection unit 1930 can be an example of a determination unit. The information output unit 1940 can be an example of an output unit. The information indicating the detection of a polarization variation can be an example of information indicating that a polarization variation has occurred in the signal light.

[0425] (Another example of an implementation method)

[0426] In this embodiment, the signal processing unit 1870 will be described in detail as having the same histogram generation unit 244, standard deviation calculation unit 246, and correction unit 710 as the signal processing unit 370. However, the signal processing unit 1870 is not limited to this embodiment.

[0427] In other embodiments, the signal processing unit 1870 may not include at least one of the histogram generation unit 244, the standard deviation calculation unit 246, and the correction unit 710. Furthermore, in other embodiments, the signal processing unit 1870 may also include a standardization unit 930, similar to the signal processing unit 870.

[0428] Figure 20 This is a simplified illustration of an example of information processing in the status monitoring device 1680. According to this embodiment, firstly, in step 2022 (sometimes omitted as step S), the optical receiver 350, equipped with the integrating circuit 1854, receives light output from the optical delay interferometer 340. The optical receiver 350 generates an electrical signal corresponding to the light output from the optical delay interferometer 340.

[0429] Next, in S2024, the AD converter 360 samples the electrical signal output by the optical receiver 350. This generates one or more digital signals corresponding to the differential phase of each of the one or more sample points.

[0430] Additionally, in step S2026, the digital signal output from the AD converter 360 is stored in a memory (not shown), which is configured in the status monitoring device 1680 or the signal processing unit 1870. The digital signal output from the AD converter 360 is stored in the memory, for example, corresponding to the identification information of each specimen point. In the memory, for example, the latest N (N being an integer greater than or equal to 1) data points are stored sequentially.

[0431] When the processing in S2026 ends, in S2032, the signal processing unit 1870 determines whether the first condition, the fourth condition, and / or the fifth condition are met. This evaluates the presence and / or degree of polarization variation. The signal processing unit 1870 outputs information indicating the determination result to the information output unit 1940.

[0432] In S2032, the signal processing unit 1870 can use the data stored in the memory in S2026 to determine whether the first condition, the fourth condition, and / or the fifth condition are met. For example, after performing a moving average processing on the data stored in the memory in S2026, it determines whether the first condition, the fourth condition, and / or the fifth condition are met. This reduces the impact of noise. Furthermore, the signal processing unit 1870 can also sequentially determine whether the first condition, the fourth condition, and / or the fifth condition are met for the sampled data. In this case, the processing in S2032 begins after the processing in S2024 ends and before the processing in S2026 ends.

[0433] When the processing in S2026 ends, in S2034, the signal processing unit 1870 uses the data stored in the memory in S2026 to determine whether the second condition and / or the third condition are met. This evaluates the presence and / or degree of polarization variation. The signal processing unit 1870 outputs information indicating the determination result to the information output unit 1940.

[0434] When the processing in S2026 ends, in S2036, the histogram generation unit 244 and the standard deviation calculation unit 246 use the data stored in the memory in S2026 to generate information for evaluating the phase noise of the signal light. Thus, the presence and / or degree of phase noise are evaluated. The standard deviation calculation unit 246 outputs the information for evaluating the phase noise of the signal light to the information output unit 1940.

[0435] In step S2042, the information output unit 1940 outputs evaluation information related to polarization variation. Furthermore, in step S2044, the information output unit 1940 outputs evaluation information related to phase noise. The processing then concludes.

[0436] Figure 21 This is a simplified representation of a data table 2100. In this embodiment, data table 2100 stores evaluation information related to polarization variations. In this embodiment, data table 2100 has one or more records associated with one or more specimen points. Each of these records can be an example of evaluation information for a specific time point or specimen point.

[0437] In this embodiment, the data table 2100 stores, for one or more specimen points, the specimen point number 2120, the time corresponding to the specimen point 2122, the output value 2124 of the light receiver 350 of the specimen point, the variation frequency 2132 derived by the frequency resolution unit 1924, the movement angle 2134 derived by the movement angle derivation unit 1926, the movement angular velocity 2136 derived by the movement angular velocity derivation unit 1928, the fulfillment or non-fulfillment of the first to fifth conditions 2140, and the detection result 2150 of polarization variation. Furthermore, the data items in the data table 2100 are not limited to this embodiment. For example, in other embodiments, the data table 2100 stores, for one or more specimen points, the specimen point number 2120 or the time 2122 and the detection result 2150 of polarization variation in correspondence.

[0438] (Another example of an implementation method)

[0439] In this embodiment, the evaluation information related to polarization variation is described in detail using the example where the evaluation information is stored in data table 2100. However, the evaluation information related to polarization variation is not limited to this embodiment. In other embodiments, the evaluation information related to polarization variation may also be a list of more than one time period at which polarization variation was detected.

[0440] Figure 22 An example of a computer 3000, which can be embodied in whole or in part in various ways of the present invention, is shown. At least a portion of the optical signal receiving device 120 can be implemented by the computer 3000. At least a portion of the phase noise evaluation device 320 can be implemented by the computer 3000. At least a portion of the optical signal receiving device 1620 can be implemented by the computer 3000. At least a portion of the status monitoring device 1680 can be implemented by the computer 3000.

[0441] A program installed on computer 3000 can cause computer 3000 to function as an operation associated with an apparatus according to embodiments of the present invention, or to perform one or more "parts" of that apparatus, or to execute that operation or the one or more "parts," and / or to cause computer 3000 to perform a process or a stage of a process according to embodiments of the present invention. Such a program can be executed by CPU 3012 to cause computer 3000 to perform specific operations associated with several or all of the blocks in the flowcharts and block diagrams described in this specification.

[0442] The computer 3000 involved in this embodiment includes a CPU 3012, RAM 3014, GPU 3016, and display device 3018, which are interconnected via a host controller 3010. The computer 3000 also includes input / output units such as a communication interface 3022, a hard disk drive 3024, a DVD-ROM drive 3026, and an IC card driver, which are connected to the host controller 3010 via an input / output controller 3020. The computer also includes conventional input / output units such as a ROM 3030 and a keyboard 3042, which are connected to the input / output controller 3020 via an input / output chip 3040.

[0443] CPU 3012 operates according to the program stored in ROM 3030 and RAM 3014, thereby controlling each unit. GPU 3016 obtains the image data generated by CPU 3012 from the frame buffer provided in RAM 3014 or from itself, and displays the image data on display device 3018.

[0444] Communication interface 3022 communicates with other electronic devices via a network. Hard disk drive 3024 stores programs and data used by CPU 3012 in computer 3000. DVD-ROM drive 3026 reads programs or data from DVD-ROM 3001 and provides programs or data to hard disk drive 3024 via RAM 3014. IC card drive reads programs and data from IC card and / or writes programs and data to IC card.

[0445] ROM 3030 stores the boot program executed by computer 3000 during activation, and / or programs that depend on the hardware of computer 3000. Input / output chip 3040 can also connect various input / output units to input / output controller 3020 via parallel port, serial port, keyboard port, mouse port, etc.

[0446] The program is provided by a computer-readable storage medium such as a DVD-ROM 3001 or an IC card. The program is read from the computer-readable storage medium and installed in a hard disk drive 3024, RAM 3014, or ROM 3030, which are also examples of computer-readable storage media, and executed by the CPU 3012. The information processing described within these programs is read by the computer 3000, resulting in cooperation between the program and the aforementioned various types of hardware resources. The apparatus or method can be configured to perform the manipulation or processing of information as the computer 3000 is used.

[0447] For example, when communication is performed between computer 3000 and an external device, CPU 3012 can execute a communication program loaded into RAM 3014 and issue communication processing commands to communication interface 3022 according to the processing described in the communication program. Under the control of CPU 3012, communication interface 3022 reads transmission data stored in the transmission buffer area provided in recording media such as RAM 3014, hard disk drive 3024, DVD-ROM 3001, or IC card, and sends the read transmission data to the network, or writes received data received from the network into the receive buffer area provided on the recording medium, etc.

[0448] In addition, CPU 3012 can read all or necessary portions of files or databases stored on external recording media such as hard disk drive 3024, DVD-ROM drive 3026 (DVD-ROM 3001), and IC cards into RAM 3014, and perform various types of processing on the data in RAM 3014. CPU 3012 can then write the processed data back to the external recording media.

[0449] Various types of information, such as programs, data, tables, and databases, can be stored in the recording medium and processed. The CPU 3012 can perform various types of processing on data read from the RAM 3014 and write the results back to the RAM 3014. These various types of processing include operations specified by a sequence of program commands, as described in various parts of this disclosure, such as information processing, conditional judgment, conditional branching, unconditional branching, and information retrieval / replacement. Furthermore, the CPU 3012 can retrieve information from files, databases, etc., within the recording medium. For example, if the recording medium stores multiple entries, each with an attribute value of a first attribute associated with a second attribute value, the CPU 3012 can retrieve from these multiple entries an entry whose first attribute value matches a specified condition, read the attribute value of the second attribute stored in that entry, and thereby obtain the attribute value of the second attribute associated with the first attribute that satisfies a predefined condition.

[0450] The programs or software modules described above can be stored on the computer 3000 or on a computer-readable storage medium near the computer 3000. Alternatively, a recording medium such as a hard disk or RAM provided in a server system connected to a dedicated communication network or the Internet can be used as the computer-readable storage medium, thereby providing the above programs to the computer 3000 via the network.

[0451] (Experimental example and comparative experimental example)

[0452] The present invention will now be described in detail using experimental examples and comparative examples. However, the present invention is not limited to the experimental examples and comparative examples described below. Figure 23 , Figure 24 , Figure 25 as well as Figure 26 This represents the various measurement results in Experiment Example 1. Figure 27 , Figure 28 , Figure 29 as well as Figure 30 This indicates the various measurement results in comparative experiment example 1. Figure 31 , Figure 32 , Figure 33 as well as Figure 34 This represents the various measurement results in Experiment Example 2.

[0453] (Experimental Example 1)

[0454] (Measurement using a polarization state measuring instrument)

[0455] First, prepare an optical fiber stretcher (OPTIPHASE PZ1-SMF4-APC-E) with four layers of 12.3m long optical fiber (Corning, SMF28e+ fiber) wound on a piezoelectric element. Connect one end of the optical fiber to a laser oscillation device (Pure Photonics PPCL550). Connect the other end of the optical fiber to a polarization state measuring instrument (Novoptel PM1000 Polarimeter).

[0456] A 1550nm laser beam is emitted from a laser oscillator, and a voltage is applied to an optical fiber stretcher, resulting in a 140kHz lateral voltage variation in the optical fiber. The Stokes parameters of the output light from the fiber stretcher are measured using a polarization state analyzer. Furthermore, the angle of motion of the trajectory on the Poincaré sphere is derived using the Stokes parameter measurements.

[0457] The above experiment was conducted by varying the voltage applied to the fiber optic stretcher. The voltage applied to the fiber optic stretcher (peak-to-peak value) was 500mV, 1V, and 2V.

[0458] Figure 23 The results of Stokes parameter measurements are shown when a voltage of 1V is applied to the fiber optic stretcher. Figure 23 As shown, a minute polarization variation was confirmed at a frequency of 140 kHz. Furthermore, the polarization variation on the Poincaré sphere was confirmed using Stokes parameter measurements, revealing a minute polarization variation on the sphere.

[0459] (Measurement using an optical delay interferometer)

[0460] Next, disconnect the other end of the optical fiber from the polarization state measuring instrument and connect the other end of the optical fiber to one end of an optical delay interferometer (Optoplex, DI-C1EFAM512). Connect the other end of the optical delay interferometer to a balanced optical receiver (Optoplex, BR-C0200B1DC). The balanced optical receiver is equipped with a low-pass filter and has a cutoff frequency of 150MHz. The output of the balanced optical receiver is sampled using an AD converter (Tektronix, oscilloscope model MSO64).

[0461] A 1550 nm laser beam is emitted from a laser oscillator, and a voltage is applied to an optical fiber stretcher, resulting in a 140 kHz lateral voltage variation in the optical fiber. Using measurements of the output of a sampled balanced optical receiver, histograms (a) relating to the magnitude of the differential phase and (b) graphs showing the time variation of the differential phase magnitude are constructed. A fast Fourier transform is performed on the measurements of the output of the sampled balanced optical receiver to derive (c) the spectrum of the differential phase.

[0462] The above experiment was conducted by varying the voltage applied to the fiber optic stretcher. The voltage applied to the fiber optic stretcher (peak-to-peak value) was 500mV, 1V, and 2V.

[0463] Figure 24 The histogram of the output of the balanced optical receiver is shown when a voltage of 1V is applied to the fiber stretcher. Figure 25 The time variation of the output of the balanced optical receiver is shown when a voltage of 1V is applied to the fiber stretcher. Figure 26 The spectrum of the output of the balanced optical receiver is shown when a voltage of 1V is applied to the fiber stretcher.

[0464] like Figure 24 As shown, the histogram is divided into two parts, indicating that a phase shift has occurred. Figure 25 As shown, the rise and fall of the fluctuations are steep, indicating the integration effect of the low-pass filter in the balanced optical receiver. As mentioned above, it is speculated that the phase shift is directly measured when the integration effect occurs. Furthermore, the cause of the abrupt changes is unclear, but it is speculated that mechanical vibrations cannot follow the sinusoidal input voltage, thus causing localized abrupt changes.

[0465] As mentioned above, in Figure 26 In this experiment, the magnitude of the variation was derived by measuring the frequency component at 140 kHz. Furthermore, the phase shift was derived by measuring the peak-to-peak value of the frequency component at 140 kHz. As a result, the applied voltage of the fiber optic stretcher was approximately proportional to the measured phase shift. As shown in Non-Patent Document 15, since there is a proportional relationship between the applied voltage of the fiber optic stretcher and the values ​​of lateral pressure and birefringence caused by the operation of the fiber optic stretcher, the results of Experimental Example 1 demonstrate that the above-described measurement principle is functioning.

[0466] (evaluate)

[0467] The results obtained using a polarization state measuring instrument (sometimes called a polarimeter) were compared with those obtained using an optical delay interferometer. The results showed a very high degree of agreement between the two methods. For example, with a voltage (peak-to-peak value) of 2V applied to the fiber stretcher, the coordinates of the two ends of the movement of the trajectory on the Poincaré sphere, measured by the polarization measuring instrument, were (0.31, 0.94, -0.15) and (0.17, 0.98, -0.06). In this case, the pp value of the phase change was 0.17 rad. On the other hand, according to the measurements using the optical delay interferometer, the pp value of the 140kHz component in the spectral measurement was 0.16 rad.

[0468] Therefore, it can be concluded that an approximate value of the shift angle of the trajectory on the Poincaré sphere can be determined using an optical delay interferometer. Furthermore, the effectiveness of the detection principle for the shift caused by the aforementioned polarization variation has been confirmed.

[0469] (Comparative Experiment Example 1)

[0470] (Measurement using a polarization state measuring instrument)

[0471] Except that the voltage applied to the fiber stretcher (interpeak value) was set to 200 mV, the Stokes parameters of the output light from the fiber stretcher were measured using the same procedure as in Experimental Example 1. Furthermore, the angle of movement of the trajectory on the Poincaré sphere was derived using the results of the Stokes parameter measurements.

[0472] Figure 27 The results of Stokes parameter measurements are shown when a voltage of 200 mV is applied to the fiber optic stretcher. Figure 27 As shown, a minute polarization variation was confirmed at a frequency of 140 kHz. Furthermore, the polarization variation on the Poincaré sphere was confirmed using Stokes parameter measurements, revealing a minute polarization variation on the sphere.

[0473] (Measurement using an optical delay interferometer)

[0474] Except that the voltage applied to the fiber stretcher (peak-to-peak value) was set to 200mV, a histogram (a) showing the relationship between the magnitude of the differential phase and a graph (b) showing the time variation of the magnitude of the differential phase were prepared using the same procedure as in Experimental Example 1. Furthermore, the spectrum of the differential phase (c) was derived.

[0475] Figure 28 The histogram of the output of the balanced optical receiver is shown when a voltage of 200mV is applied to the fiber stretcher. Figure 29The time variation of the output of the balanced optical receiver is shown when a voltage of 200mV is applied to the fiber stretcher. Figure 30 The spectrum of the output of the balanced optical receiver is shown when a voltage of 200mV is applied to the fiber stretcher.

[0476] like Figure 28 As shown, only phase noise was observed in the histogram. On the other hand, according to Figure 29 and Figure 30 It can be seen that this resulted from Figure 28 The minute 140kHz phase shift was unreadable. This confirms that, through statistical processing, actual high-speed shifts become difficult to read.

[0477] (Experimental Example 2)

[0478] (Measurement using a polarization state measuring instrument)

[0479] First, connect one end of the optical fiber to the laser oscillation device (Pure Photonics PPCL550). Connect the other end of the optical fiber to the polarization scrambler (Luna Innovations NRT-2500). Connect the output of the polarization scrambler to the polarization state meter (Novoptel PM1000 Polarimeter).

[0480] A 1550nm wavelength laser is emitted from a laser oscillator, and a high-speed polarization variation is generated using a polarization scrambler in Spinner Mode. The polarization variation frequency of the polarization scrambler is set to 75kHz. This corresponds to a moving angular velocity of approximately 470krad / s on a Poincaré sphere.

[0481] The Stokes parameters of the polarization-shifted light output from the polarization scrambler were measured using a polarization state measuring instrument. Furthermore, the angle of shift of the trajectory on the Poincaré sphere was derived using the Stokes parameter measurements.

[0482] Figure 31 The results of the Stokes parameter measurements in Experiment Example 2 are shown. Figure 31 As shown, it can be confirmed that very large and high-speed polarization variations are generated, as per the specifications of the polarization scrambler. Furthermore, the polarization variation on the Poincaré sphere was confirmed using Stokes parameter measurements, and the results show that the sphere traces an orbit close to the largest great circle in radius.

[0483] (Measurement using an optical delay interferometer)

[0484] Next, the output of the polarization scrambler was disconnected from the polarization state measuring instrument, and the output of the polarization scrambler was connected to one end of an optical delay interferometer (Optoplex, DI-C1EFAM512). The other end of the optical delay interferometer was connected to a balanced optical receiver (Optoplex, BR-C0200B1DC) equipped with a low-pass filter. The output of the balanced optical receiver was sampled using an AD converter (Tektronix, MSO64 oscilloscope).

[0485] A 1550nm wavelength laser is emitted from a laser oscillator, and a high-speed polarization variation is generated using a polarization scrambler in Spinner Mode. The polarization variation frequency of the polarization scrambler is set to 75kHz. Using measurements of the output of the sampled balanced light receiver, histograms (a) showing the magnitude of the differential phase and (b) graphs representing the time variation of the differential phase magnitude are constructed. A Fast Fourier Transform (FFT) is performed on the measurements of the output of the sampled balanced light receiver to derive (c) the spectrum of the differential phase.

[0486] Figure 32 The histogram represents the output of the balanced light receiver in Experiment Example 2. Figure 33 This indicates the time variation of the output of the balanced light receiver in Experiment Example 2. Figure 34 This represents the spectrum of the output of the balanced optical receiver in Experiment Example 2. Figure 32 , Figure 33 as well as Figure 34 The results all indicate that significant polarization changes occurred.

[0487] like Figure 33 As shown, the rise and fall of the phase shift are steep, indicating the integration effect of the low-pass filter in the balanced optical receiver. As mentioned above, it is speculated that the phase shift is directly measured when the integration effect occurs. Furthermore, the polarization scrambler NRT-2500 generates polarization shift by rapidly rotating a half-wavelength plate. At the instant the inherent axis of the half-wavelength plate crosses the inherent axis of the orthogonal polarization mode, a phase shift caused by the half-wavelength plate occurs, resulting in what is speculated to be a sharp shift.

[0488] like Figure 34 As shown, the 150kHz component was measured as the dominant frequency component. Regarding this, the nominal value of 75kHz for the polarization scrambler NRT-2500 is the frequency of polarization rotation. The phase change resulting from the rotation of the half-wavelength plate is a frequency change at twice the polarization rotation frequency. Therefore, it is inferred that frequency components of 75kHz and 150kHz were observed.

[0489] Therefore, it can be confirmed that even with large polarization variations, the approximate angle of movement of the trajectory on the Poincaré sphere can be determined using an optical delay interferometer. This confirms the effectiveness of the aforementioned principle for detecting movement variations caused by polarization variations.

[0490] (Application of evaluation information related to polarization variation in the control of communication systems)

[0491] In Figures 16 to 21 In the embodiments described in connection with this example, the details of the communication system 1600 are explained using the case where the status monitoring device 1680 outputs evaluation information related to polarization variation as an alarm or indicator. However, the utilization of the evaluation information related to polarization variation is not limited to the embodiments described above. In other embodiments, the evaluation information related to polarization variation can be used for the control of the communication system. Figure 35 , Figure 36 , Figure 37 , Figure 38 , Figure 39 as well as Figure 40 An example of an implementation method that utilizes evaluation information related to polarization variation in the control of a communication system will be described.

[0492] The inventors, focusing on the state monitoring device 1680's detection of abrupt polarization changes in the optical region, conceived of utilizing evaluation information related to polarization changes in the control of signal processing in the electrical region (sometimes referred to as the electrical signal region). For example, optical signal receiving devices in digital / coherent optical communication methods use digital signal processors (sometimes called DSPs) to perform various signal processing tasks, such as (i) processing for wavelength dispersion equalization, (ii) processing for compensating for polarization separation and / or polarization mode dispersion, (iii) processing for inferring carrier phase, and (iv) processing for decoding information signals.

[0493] The characteristics of the optical transmission path 10 are determined by wavelength dispersion, birefringence, polarization mode dispersion, etc., within the optical transmission path 10. Furthermore, the characteristics of the optical transmission path 10 can be described using the propagation matrix H. The optical signal receiving device for digital / coherent optical communication can substantially completely extract information related to the complex amplitude of the optical signal. Therefore, the optical signal receiving device for digital / coherent optical communication can compensate for the group velocity dispersion of the optical transmission path. Specifically, in the aforementioned (i) processing for equalizing wavelength dispersion and (ii) processing for compensating for polarization separation and / or polarization mode dispersion, the inverse matrix H of the propagation matrix H is used. -1 This is to compensate for the various effects that occur during the propagation of the signal light in the optical transmission path 10.

[0494] In one embodiment, the propagation matrix H is represented as the product of a first matrix representing the effect of wavelength-based dispersion and a second matrix representing the effect of birefringence and / or polarization mode dispersion. In other embodiments, the propagation matrix H is represented as the product of a first matrix representing the effect of wavelength-based dispersion, a second matrix representing the effect of birefringence, and a third matrix representing the effect of polarization mode dispersion. In yet another embodiment, the propagation matrix H is represented as a single matrix that summarizes the effects of wavelength-based dispersion and the effects of birefringence and / or polarization mode dispersion.

[0495] Using the inverse matrix H -1 The compensation process is implemented in the electrical region, for example, using a finite impulse response filter (sometimes called an FIR filter). Any known FIR filter can be used as the FIR filter. The FIR filter disclosed in Non-Patent Document 16 can also be used. In one embodiment, the FIR filter includes a first FIR filter corresponding to the first matrix and a second FIR filter corresponding to the second matrix. In other embodiments, the FIR filter includes a first FIR filter corresponding to the first matrix, a second FIR filter corresponding to the second matrix, and a third FIR filter corresponding to the third matrix. In other embodiments, the FIR filter includes a single FIR filter corresponding to a single matrix that summarizes the effects of wavelength dispersion and birefringence / polarization mode dispersion.

[0496] Because the polarization state of the optical transmission path 10 changes constantly, in the optical signal receiving device of digital / coherent optical communication, the tap coefficients of the FIR filter are updated at predetermined time intervals based on the oversampling ratio of the AD conversion. In most cases, the oversampling ratio is set to 2. Therefore, since the tap coefficients are updated at twice the symbol rate, data can be demodulated stably even in the event of drastic polarization changes.

[0497] Various algorithms have been proposed for updating the aforementioned tap coefficients. One example of such an algorithm is the CMA (constant-modulus algorithm). These algorithms involve performing complex multiplication, thus requiring significant power to update the tap coefficients.

[0498] Non-Patent Document 17 discloses the following: downsampling is performed to reduce power consumption in a DSP, and the tap coefficients are updated at the same frequency as the symbol rate. According to the technology disclosed in Non-Patent Document 17, the update frequency of the tap coefficients is set to be the same as the symbol rate, taking advantage of the fact that the phase rotation generated in one symbol time is extremely small.

[0499] Furthermore, in existing FIR filters, once the update frequency of the tap coefficients is determined, it cannot be changed. Therefore, in existing DSPs, the tap coefficients are always updated at a high frequency to cope with abrupt polarization changes. As mentioned above, although the tap coefficient update process consumes a lot of power, abrupt polarization changes mostly occur only a few times a day.

[0500] The inventors, focusing on the need to detect abrupt polarization changes in the optical region before they occur in the electrical region, conceived of reducing unnecessary power consumption in the control of FIR filters. The aforementioned FIR filter control method, for example, includes a detection signal receiving stage and a setting stage. In the detection signal receiving stage, a detection signal is received, which is a signal representing the detection result of a detection device that optically detects polarization changes in the optical transmission path. In the setting step, based on the detection result of the detection device, a setting related to the update frequency or update interval of the FIR filter's tap count, or a setting related to the update frequency or update interval of the FIR filter's tap coefficients, is determined.

[0501] More specifically, the inventors conceived of setting the update frequency of the FIR filter tap coefficients to a small value, for example, when no abrupt polarization changes are detected in the optical region, and increasing the update frequency of the FIR filter tap coefficients before abrupt polarization changes occur in the electrical region, when abrupt polarization changes are detected in the optical region. Thus, for example, when no abrupt polarization changes are detected in the optical region, the update frequency of the FIR filter tap coefficients can be set to a value, for example, smaller than the symbol rate. This update frequency can be less than 1 / 2 of the symbol rate, less than 1 / 5 of the symbol rate, less than 1 / 10 of the symbol rate, less than 1 / 20 of the symbol rate, less than 1 / 50 of the symbol rate, and less than 1 / 100 of the symbol rate.

[0502] As described above, since sharp polarization changes occur several times a day, the power consumption in the DSP is significantly reduced through the above processing. Furthermore, during periods of sharp polarization changes in the electrical region, the tap count or tap coefficients of the FIR filter are updated at a high frequency. This enables stable data demodulation.

[0503] Similarly, the inventors conceived of setting the update frequency of the FIR filter taps to a small value, for example, when no abrupt polarization changes are detected in the optical region, and increasing the update frequency of the FIR filter taps before abrupt polarization changes occur in the electrical region, when abrupt polarization changes are detected in the optical region. Thus, for example, when no abrupt polarization changes are detected in the optical region, the update frequency of the FIR filter taps can be set to a value, for example, smaller than the symbol rate. This update frequency can be less than 1 / 2 of the symbol rate, less than 1 / 5 of the symbol rate, less than 1 / 10 of the symbol rate, less than 1 / 20 of the symbol rate, less than 1 / 50 of the symbol rate, and less than 1 / 100 of the symbol rate.

[0504] According to one embodiment, the tap count update process includes steps of increasing or decreasing the tap count itself. According to other embodiments, the tap count update process includes steps of substantially decreasing the tap count by setting a portion of the tap coefficient to 0. The tap count update process may also include steps of substantially increasing the tap count by changing a portion of the tap coefficient from 0 to a positive number other than 0.

[0505] In existing FIR filter control, the number of taps is fixed. However, according to the above processing, for example, when no sharp polarization change is detected in the optical region, the number of taps in the FIR filter is set to a small value. Furthermore, the number of taps is updated at a small update frequency. On the other hand, when a sharp polarization change is detected in the optical region, the number of taps in the FIR filter is set to a large value before a sharp polarization change occurs in the electrical region. Furthermore, the number of taps is updated at a larger update frequency. As a result, the power consumption in the DSP is significantly reduced. Additionally, stable data demodulation is achieved.

[0506] (Overview of Communication System 3500)

[0507] Figure 35 This is a simplified example of the system configuration of communication system 3500. To the extent that there is no technical inconsistency, communication system 3500 may have the same configuration as communication system 100 and / or communication system 1600. Furthermore, descriptions of configurations identical to those of communication system 100 and / or communication system 1600 are sometimes omitted.

[0508] In this embodiment, the communication system 3500 includes, for example, an optical signal transmitting device 110 and an optical signal receiving device 3520. In this embodiment, the details of the communication system 3500 will be described as follows: the signal light output by the optical signal transmitting device 110 propagates in the optical transmission path 10 and reaches the optical signal receiving device 3520 to transmit an information signal.

[0509] In this embodiment, the optical signal receiving device 3520 includes a wavelength division multiplexer 1640, an optical receiver 3550, a local oscillator 3552, a digital signal processor 3560, and a status monitoring device 1680. In this embodiment, the digital signal processor 3560 has a signal input terminal 3562, a signal output terminal 3564, and a control input terminal 3566.

[0510] In this embodiment, when the signal light includes multiple optical signals with different wavelengths, the wavelength divider 1640 divides the multiple optical signals according to their wavelengths. The optical signal received in the optical signal receiving device 1620 (sometimes called the received signal) includes, for example, light of a first wavelength used for transmitting information signals (sometimes called a communication optical signal) and light of a second wavelength used for detecting polarization variations (sometimes called a monitoring optical signal). The values ​​of the first wavelength and the second wavelength are different. The monitoring optical signal can be unmodulated. In this embodiment, the wavelength divider 1640 outputs the communication optical signal to the optical receiving unit 3550. The wavelength divider 1640 outputs the monitoring optical signal to the status monitoring device 1680.

[0511] In this embodiment, the state monitoring device 1680 analyzes the light (sometimes referred to as input light) input to the state monitoring device 1680 to monitor the state of the optical transmission path 10 and / or the state of the signal light propagating in the optical transmission path 10. For example, the state monitoring device 1680 analyzes the phase variation of the input light, evaluating at least one of (i) the presence and / or degree of phase noise, and (ii) the presence and / or degree of polarization variation. Thus, the state monitoring device 1680 can, for example, detect polarization variations of the signal light propagating in the optical transmission path 10.

[0512] In this embodiment, the status monitoring device 1680 generates a signal (sometimes called a control signal) for controlling the operation of the digital signal processor 3560 based on the evaluation or detection results related to the polarization variation described above. The control signal includes, for example, information indicating the presence and / or degree of polarization variation. The degree of polarization variation is sometimes referred to as the amount of polarization variation.

[0513] As described above, the state monitoring device 1680 outputs evaluation information related to polarization variation. This evaluation information, for example, represents the detection result of the state monitoring device 1680. Examples of evaluation information related to polarization variation include information indicating that a polarization variation has been detected, information indicating that a polarization variation has occurred, information indicating that the angular velocity of movement exceeds a predetermined angular velocity, information indicating that the angle of movement exceeds a predetermined angle, information indicating that the frequency of variation exceeds a predetermined frequency, information indicating the amount of polarization variation, and information indicating the absolute value of the amount of polarization variation.

[0514] Information indicating the detection of a polarization variation can indicate that a polarization variation exceeding a predetermined degree has been detected. Information indicating that a polarization variation has occurred can indicate that a polarization variation exceeding a predetermined degree has occurred. As described above, the amount of polarization variation can be the amount of change in the angle of movement of the trajectory on the Poincaré sphere.

[0515] In this embodiment, the light receiver 3550 converts the optical signal into an electrical signal. In this embodiment, the light receiver 3550 includes, for example, a light 90-degree mixer and a light receiver. The light receiver 3550 will be described in detail later.

[0516] In this embodiment, the local oscillator 3552 outputs local oscillator light. The local oscillator light output from the local oscillator 3552 is input to the light receiver 3550's 90-degree light mixer. The local oscillator 3552 may have the same configuration as the local oscillator 130.

[0517] In this embodiment, the digital signal processor 3560 performs various signal processing operations in the electrical region, demodulating the received signal transmitted via signal light. This generates an information signal.

[0518] For example, the digital signal processor 3560 compensates for the polarization mode dispersion of the signal light propagating in the optical transmission path 10. The digital signal processor 3560 compensates for this polarization mode dispersion, for example, by using digital signal processing with a finite impulse response (FIR) filter. The digital signal processor 3560 can control the operation of the FIR filter. The digital signal processor 3560 will be described in detail later.

[0519] In this embodiment, the signal input terminal 3562 receives a signal that is the object of signal processing in the digital signal processor 3560. Specifically, the signal input terminal 3562 receives an electrical signal output from the optical receiver 3550. In this embodiment, the signal output terminal 3564 outputs an information signal generated by the digital signal processor 3560. In this embodiment, the control input terminal 3566 receives evaluation information output from the status monitoring device 1680. The evaluation information functions, for example, as a control signal for controlling the update frequency or update interval of the tap number of the finite impulse response filter, or the update frequency or update interval of the tap coefficients of the finite impulse response filter.

[0520] Optical signal transmitting device 110 can be an example of an optical transmitting device. Status monitoring device 1680 can be an example of a detection device. The evaluation information of status monitoring device 1680 can be an example of a detection signal. Communication system 3500 can be an example of an optical communication system. Optical signal receiving device 3520 can be an example of an optical receiving device, a compensation device, or a control device. Digital signal processor 3560 can be an example of a compensation device or a control device. Control input terminal 3566 can be an example of a control signal input terminal.

[0521] (Another example of an implementation method)

[0522] In this embodiment, for the purpose of facilitating understanding of the communication system 3500, details of the communication system 3500 will be described using the case where the received signal includes a single communication optical signal and a single monitoring optical signal as an example. However, the communication system 3500 is not limited to this embodiment. For example, various multiplexing techniques are known in the field of optical communication. Therefore, various multiplexing techniques can be applied in the above-described communication system 3500 according to other embodiments.

[0523] For example, in the application of optical wavelength multiplexing technology to communication system 3500, the received signal includes one or more (sometimes referred to as more than one) communication optical signals. These multiple communication optical signals propagate through signal light of different wavelengths. Similarly, the received signal may also include more than one surveillance optical signal. These multiple surveillance optical signals propagate through signal light of different wavelengths.

[0524] When the received signal includes multiple communication optical signals, the communication system 3500 may include an optical receiver 3550 and a digital signal processor 3560, the same number as the number of communication optical signals. When the received signal includes multiple monitoring optical signals, the communication system 3500 may include a status monitoring device 1680, the same number as the number of monitoring optical signals.

[0525] For example, when the received signal includes multiple communication optical signals and a single monitoring optical signal, the evaluation information output by the single status monitoring device 1680 is used as a control signal for controlling the operation of multiple digital signal processors 3560. Thus, the operation of multiple digital signal processors 3560 can be controlled by a single control signal.

[0526] Figure 36This section schematically illustrates an example of the internal configuration of the optical receiver 3550 and the digital signal processor 3560. In this embodiment, the optical receiver 3550 includes an optical 90-degree mixer 3654 and an optical receiver 3656. The optical receiver 3656 may have multiple optical receivers (e.g., four optical receiving elements). In this embodiment, the digital signal processor 3560 includes an analog-to-digital converter 3662, a compensation unit 3664, a carrier phase estimation unit 3666, and a decoding unit 3668. The analog-to-digital converter 3662 may have multiple analog-to-digital converters (e.g., four analog-to-digital conversion elements).

[0527] The optical 90-degree mixer 3654 can have the same configuration as the optical 90-degree mixer 140. The optical receiver 3656 can have the same configuration as the optical receiver 152 or optical receiver 154. The AD converter 3662 can have the same configuration as the AD converter 162 or AD converter 164. The compensation unit 3664, the carrier phase estimation unit 3666, and the decoding unit 3668 can have the same configuration as the signal processing unit 170 or signal processing unit 1670. The compensation unit 3664 and the carrier phase estimation unit 3666 can have the same configuration as the digital signal processing circuit 210. The decoding unit 3668 can have the same configuration as the decoding circuit 220.

[0528] In this embodiment, the optical 90-degree mixer 3654 interferes with the local oscillator light from the local oscillator 130, outputting multiple optical signals that separate the received signal transmitted through the signal light into multiple signal components. In this embodiment, the optical receiver 3656 converts the multiple optical signals output by the optical 90-degree mixer 3654 into electrical signals. The optical receiver 3656 outputs electrical signals corresponding to the multiple optical signals output by the optical 90-degree mixer 3654.

[0529] Thus, the electrical signal E corresponding to the in-phase component (sometimes called the I component) of the x-polarization component of the communication optical signal is obtained. xI (t) The electrical signal E corresponding to the orthogonal component (sometimes called the 90-degree phase component, Q component, etc.) of the x-polarization component of the optical signal used for communication. xQ (t) The electrical signal E corresponding to the in-phase component (sometimes called the I component) of the y-polarization component. yI (t), and the electrical signal E corresponding to the orthogonal component to the y-polarization component (sometimes called the 90-degree phase component, Q component). yQ (t). Each electrical signal is input to the AD converter 3662 via the signal input terminal 3562 of the digital signal processor 3560.

[0530] In this embodiment, the AD converter 3662 converts an electrical signal from an analog signal to a digital signal. The AD converter 3662 samples the analog signal corresponding to each of the aforementioned components at a predetermined sampling frequency. Thus, the analog signal corresponding to each of the aforementioned components is converted into a complex digital signal. Figure 36 In the above, the x and y components of the nth signal, which has been converted into a digital signal, are expressed as E. x (n) and E y (n).

[0531] In this embodiment, the compensation unit 3664 compensates for the birefringence and / or polarization mode dispersion (sometimes referred to as the birefringence and / or polarization mode dispersion of the signal light propagating in the optical transmission path 10. The compensation unit 3664 compensates for the birefringence and / or polarization mode dispersion of the signal light and performs polarization separation. The compensation unit 3664 compensates for the birefringence and / or polarization mode dispersion of the signal light and performs polarization separation, for example, by using digital signal processing with a finite impulse response filter. Thus, orthogonal polarization is restored.

[0532] In this embodiment, the compensation unit 3664 controls the operation of the finite impulse response filter based on the evaluation information from the input control input terminal 3566. This significantly reduces the power consumption of the compensation unit 3664. In this embodiment, the complex digital signal E output from the AD converter 3662 is input to the compensation unit 3664. x (n) and E y (n), the compensation unit 3664 outputs the restored signal E after birefringence and / or polarization mode dispersion have been compensated. X (n) and E Y Taking the case of (n) as an example, the details of compensation part 3664 are illustrated. E X (n) is the x component of the nth restored signal obtained through polarization separation, E Y (n) is the y component of the nth restored signal obtained through polarization separation. The compensation unit 3664 will be described in detail later.

[0533] In this embodiment, the carrier phase estimation unit 3666 has the function of estimating the phase of the optical carrier. As a result, the phase of the optical carrier required by the decoding unit 3668 is restored.

[0534] In this embodiment, the decoding unit 3668 performs error correction processing, decoding processing, etc. As a result, an information signal is extracted from the received signal. The decoding unit 3668 outputs the decoded information signal to the signal output terminal 3564.

[0535] (Another example of an implementation method)

[0536] In this embodiment, the details of the digital signal processor 3560 are described using the case where the digital signal processor 3560 includes an AD converter 3662 as an example. However, the digital signal processor 3560 is not limited to this embodiment. In other embodiments, the AD converter 3662 may also be configured externally to the digital signal processor 3560.

[0537] Figure 37 This is a simplified illustration of an example of the internal structure of the compensation unit 3664. In this embodiment, the compensation unit 3664 includes a wavelength dispersion equalization unit 3720 and a polarization separation / polarization mode dispersion compensation unit 3740. In this embodiment, the polarization separation / polarization mode dispersion compensation unit 3740 includes a control signal receiving unit 3750, an FIR filter 3760, and a filter control unit 3770. In this embodiment, the filter control unit 3770 includes a setting unit 3772 and an updating unit 3774.

[0538] In this embodiment, the wavelength dispersion equalization unit 3720 adjusts the complex digital signal E x (n) and E y The group delay characteristic of (n) is compensated. Therefore, a complex digital signal E with its group delay characteristic compensated can be obtained. xc (n) and E yc (n).

[0539] In this embodiment, the polarization separation / polarization mode dispersion compensation unit 3740 compensates for the effects of birefringence and / or polarization mode dispersion on the signal light, restoring the original orthogonal polarization signal E. X (n) and E Y (n). Specifically, the polarization separation / polarization mode dispersion compensation unit 3740 performs the operation using the aforementioned inverse matrix H. -1 The compensation process. For example, the polarization separation / polarization mode dispersion compensation unit 3740 performs the compensation processing on the aforementioned complex digital signal E. x (n) and E y (n) Input and inverse matrix H -1 The corresponding finite impulse response filter (sometimes called an FIR filter). From this, the restored signal E can be obtained. X (n) and E Y (n).

[0540] The above inverse matrix H -1 Using E x (n) includes E X (n) The coefficient h of the component xx (ω) represents E y (n) includes E X (n) The coefficient h of the component xy (ω) represents E x(n) includes E Y (n) The coefficient h of the component yx (ω) represents E y (n) includes E Y (n) The coefficient h of the component yy (ω), is represented by the following equation C-1. In equation C-1, ω represents the angular frequency of the optical signal.

[0541] (Equation C-1)

[0542]

Mathematical Expression 20

[0543]

[0544] In equation C-1, h xx (ω), h xy (ω), h yx (ω) and h yy (ω) is denoted as hp (p is xx, xy, yx or yy), and will be compared with the inverse matrix H. -1 When the number of taps of the corresponding FIR filter is set to k, the x-component of the input vector of the FIR filter is represented by Equation C-2 below. Similarly, the y-component of the input vector of the above FIR filter is represented by Equation C-3 below. In addition, the tap coefficient vector of the FIR filter is represented by Equation C-4 below. In Equations C-2, C-3 and C-4, the subscript T indicates transpose.

[0545] In this case, the x-component E of the restored signal X (n) is derived from the following equation C-5. Similarly, the y-component E of the above-mentioned restored signal... Y (n) is derived from the following formula C-6.

[0546] Furthermore, with the symbol interval set to Ts and the oversampling ratio of the AD converter 3662 set to m, the delay time interval of the aforementioned FIR filter is Ts / m. Additionally, in the aforementioned CMA, E... X The sum of the squares of the absolute values ​​of (n) E Y The tap coefficient is controlled by making the square of the absolute value of (n) close to 1.

[0547] (Equation C-2)

[0548]

Mathematical Expression 21

[0549]

[0550] (Formula C-3)

[0551]

Mathematical Expression 22

[0552]

[0553] (Equation C-4)

[0554]

Mathematical Expression 23

[0555]

[0556] (Formula C-5)

[0557]

Mathematical Expression 24

[0558]

[0559] (Formula C-6)

[0560]

Mathematical Expression 25

[0561]

[0562] In this embodiment, the control signal receiving unit 3750 receives evaluation information output by the status monitoring device 1680. As described above, the status monitoring device 1680 optically detects the polarization variation of the monitoring light signal propagating in the optical transmission path 10. Furthermore, the status monitoring device 1680 outputs evaluation information (sometimes referred to as polarization variation-related evaluation information) indicating the evaluation result or detection result related to the aforementioned polarization variation. The control signal receiving unit 3750 can output the aforementioned evaluation information to the setting unit 3772.

[0563] In this embodiment, the FIR filter 3760 compensates for the effects of birefringence and / or polarization mode dispersion on the signal light, restoring the original orthogonally polarized signal E. X (n) and E Y (n). Specifically, the FIR filter 3760 is input to the aforementioned complex digital signal E. x (n) and E y (n), and output the above-mentioned restored signal E. X (n) and E Y (n).

[0564] According to this embodiment, the number of taps or tap coefficients of the FIR filter 3760 are dynamically changed (this change is sometimes referred to as an update). For example, the number of taps or tap coefficients of the FIR filter 3760 are changed during the operation of the polarization separation / polarization mode dispersion compensation unit 3740. According to this embodiment, the update frequency or update interval of the number of taps or tap coefficients of the FIR filter 3760 is dynamically changed. For example, the update frequency or update interval of the number of taps or tap coefficients of the FIR filter 3760 is changed during the operation of the polarization separation / polarization mode dispersion compensation unit 3740.

[0565] In this embodiment, the filter control unit 3770 controls the operation of the polarization separation / polarization mode dispersion compensation unit 3740. Specifically, the filter control unit 3770 controls the operation of the FIR filter 3760. More specifically, the filter control unit 3770, for example, changes the number of taps in the FIR filter 3760. For example, the filter control unit 3770 changes the tap coefficients of the FIR filter 3760. For example, the filter control unit 3770 changes the update frequency or update interval of the number of taps in the FIR filter 3760. For example, the filter control unit 3770 changes the update frequency or update interval of the tap coefficients of the FIR filter 3760. The filter control unit 3770 can substantially increase or decrease the number of taps in the FIR filter 3760 by adjusting the tap coefficients of the FIR filter 3760.

[0566] In this embodiment, the setting unit 3772 determines settings related to the update frequency or update interval of the tap number of the FIR filter 3760, and / or settings related to the update frequency or update interval of the tap coefficients of the FIR filter 3760, based on evaluation information related to polarization variation. The setting unit 3772 can determine the above settings during the operation of the polarization separation / polarization mode dispersion compensation unit 3740 based on evaluation information acquired during operation of the polarization separation / polarization mode dispersion compensation unit 3740.

[0567] As described above, the evaluation information related to polarization variation includes information indicating that a polarization variation exceeding a predetermined level has been detected, information indicating that a polarization variation exceeding a predetermined level has occurred, information indicating the amount of polarization variation variation, and information indicating the absolute value of the amount of polarization variation variation. Therefore, the setting unit 3772 can determine whether the absolute value of the amount of polarization variation variation exceeds a predetermined threshold based on the evaluation information related to polarization variation. The setting unit 3772 determines the above setting, for example, based on the determination result.

[0568] In one embodiment, when the absolute value of the polarization variation is determined to be greater than a threshold, the setting unit 3772 decides to (i) set the update frequency of the number of taps and / or tap coefficients to a greater than the current setting value, (ii) set the update frequency of the number of taps and / or tap coefficients to a predetermined first value, (iii) set the update interval of the number of taps and / or tap coefficients to a less than the current setting value, or (iv) set the update interval of the number of taps and / or tap coefficients to a predetermined second value. The first value and the second value may be the same or different.

[0569] In other embodiments, if the absolute value of the amount of polarization variation is not determined to be greater than a threshold, the setting unit 3772 decides to (i) set the update frequency of the number of taps and / or tap coefficients to a lower than the current setting value, (ii) set the update frequency of the number of taps and / or tap coefficients to a predetermined third value, (iii) set the update interval of the number of taps and / or tap coefficients to a greater than the current setting value, or (iv) set the update interval of the number of taps and / or tap coefficients to a predetermined fourth value. The third value and the fourth value can be the same or different. The third value can be a value different from the first value, and the fourth value can be a value different from the second value.

[0570] According to these embodiments, for example, if no sharp polarization change is detected in the optical region, the update frequency of the FIR filter tap number and / or tap coefficients can be set to a small value initially. If a sharp polarization change is detected in the optical region, the update frequency of the FIR filter tap number and / or tap coefficients can be increased before a sharp polarization change occurs in the electrical region. As a result, the power consumption in the digital signal processor 3560 is significantly reduced. Furthermore, stable data demodulation is possible.

[0571] In this embodiment, the updating unit 3774 updates the number of taps and / or tap coefficients of the FIR filter 3760. The updating unit 3774 can update the number of taps and / or tap coefficients at an updating frequency or updating interval determined by the setting unit 3772. The updating unit 3774 can update the number of taps and / or tap coefficients during the operation of the polarization separation / polarization mode dispersion compensation unit 3740.

[0572] (The time difference between detecting polarization variations in the optical region and detecting polarization variations in the electrical region)

[0573] Because the DSP performs massive computational processing, a significant latency is generated. For example, according to Non-Patent Documents 20 and 21, a latency of approximately 1 μs is disclosed in digital coherent optical communication. Although the individual latency of the wavelength dispersion equalization unit 3720 and the polarization separation / polarization mode dispersion compensation unit 3740 is not clearly defined, considering that the wavelength dispersion equalization unit 3720 also uses an FIR filter with a large number of taps, it is speculated that a latency of several hundred ns is generated in the wavelength dispersion equalization unit 3720.

[0574] On the other hand, as described above, the delay time in the optical delay interferometer 340 of the state monitoring device 1680 is 35.7 ps at a symbol rate of 28 GSymbol / s. Furthermore, Non-Patent Documents 20 and 21 disclose transponders with delay times of 4 to 30 ns for optical reception, identification regeneration, and optical transmission. Assuming that the delay time of the electronic circuitry configured in the state monitoring device 1680 is similar to that of the transponders disclosed in Non-Patent Documents 20 and 21, it is speculated that the overall delay time of the state monitoring device 1680 is tens of ns.

[0575] Considering the above speculation and the structure of the DSP, it is estimated that the length of the period from the detection of a polarization change in the optical region to the detection of a polarization change in the electrical region is approximately several hundred ns. Therefore, according to this embodiment, after the state monitoring device 1680 detects a sharp polarization change in the optical region, the filter control unit 3770 can change the setting of the FIR filter 3760 before the electrical signal input to the FIR filter 3760 generates such a sharp polarization change.

[0576] The polarization separation / polarization mode dispersion compensation unit 3740 can be an example of a compensation device or a control device. The control signal receiving unit 3750 can be an example of a detection signal receiving unit. The FIR filter 3760 can be an example of a compensation device or a finite impulse response filter. The filter control unit 3770 can be an example of a control device.

[0577] (Another example of an implementation method)

[0578] In this embodiment, the details of the polarization separation / polarization mode dispersion compensation unit 3740 will be described using the case where the polarization separation / polarization mode dispersion compensation unit 3740 includes a control signal receiving unit 3750 and a filter control unit 3770 as an example. However, the polarization separation / polarization mode dispersion compensation unit 3740 is not limited to this embodiment.

[0579] In other embodiments, the control signal receiving unit 3750 and the filter control unit 3770 may also be disposed outside the polarization separation / polarization mode dispersion compensation unit 3740. Thus, for example, when the optical signal receiving device 3520 has multiple digital signal processors 3560, a single filter control unit 3770 can control the FIR filters 3760 of multiple digital signal processors 3560.

[0580] In another embodiment, when the optical signal receiving device 3520 includes multiple digital signal processors 3560, some of the digital signal processors 3560 may have a control signal receiving unit 3750 and a filter control unit 3770, while the remaining digital signal processors 3560 may not have a control signal receiving unit 3750 and a filter control unit 3770. In this case, the filter control unit 3770 disposed in some of the digital signal processors 3560 can control the FIR filters 3760 of the other digital signal processors 3560.

[0581] Figure 38 This is a brief illustration of an example of information processing in the compensation department 3664. Figure 38 For example, this roughly illustrates an example of information processing by the filter control unit 3770 for controlling the operation of the compensation unit 3664.

[0582] According to this embodiment, firstly, in step 3820 (sometimes step S is omitted), the control signal receiving unit 3750 receives evaluation information related to polarization variation output by the state monitoring device 1680. Next, the setting unit 3772 analyzes the evaluation information related to polarization variation and determines whether the absolute value of the change in polarization state is greater than a predetermined threshold.

[0583] The change in polarization state can be represented, for example, by the approximate change in the angle of movement of the trajectory on the Poincaré sphere, which represents the change in polarization state. The setting unit 3772 performs the above-mentioned determination process by comparing, for example, the approximate change in the angle of movement of the trajectory on the Poincaré sphere with a preset reference value related to the change.

[0584] When the absolute value of the change in polarization state is determined to be greater than a predetermined threshold in S3820 (when "Yes" is selected in S3820), in S3842, the setting unit 3772, for example, decides to change the setting value of the tap coefficient update frequency to a value greater than the current value, or a predetermined first setting value. Furthermore, the setting unit 3772 can perform operations related to... Figure 37 The various processes described in connection determine the settings related to the update frequency or update interval of the number of taps and / or tap coefficients.

[0585] On the other hand, when the absolute value of a change in polarization state not determined in S3820 is greater than a predetermined threshold (when "No" is defined in S3820), in S3844, the setting unit 3772 decides, for example, to change the setting value of the tap coefficient update frequency to a value less than the current value, or a predetermined second setting value. The second setting value can be less than the first setting value. Furthermore, the setting unit 3772 can perform operations related to... Figure 37 The various processes described in connection determine the settings related to the update frequency or update interval of the number of taps and / or tap coefficients.

[0586] Then, in S3860, the updating unit 3774 updates the tap coefficients of the FIR filter 3760. Specifically, the updating unit 3774 updates the tap coefficients of the FIR filter 3760 at an update frequency shown by the set value determined in S3842 or S3844. Furthermore, as with... Figure 37 As explained in the related section, the update unit 3774 can also update the number of taps in the FIR filter 3760.

[0587] Figure 39 Other examples of the internal structure of the compensation unit 3664 are shown in a simplified manner. Besides the delay unit 3810 being positioned in front of the wavelength dispersion equalization unit 3720, the compensation unit 3664 according to this embodiment has and... Figure 37 The compensation unit 3664, which is described in connection with this, has the same structure.

[0588] In this embodiment, the delay unit 3810 is input to the output of the AD converter 3662, and after a predetermined time, the output of the AD converter 3662 is output to the wavelength dispersion equalization unit 3720. Therefore, after the status monitoring device 1680 detects a sharp polarization change in the optical region, and before that sharp polarization change is generated in the electrical signal input to the FIR filter 3760, the filter control unit 3770 can reliably change the settings of the FIR filter 3760.

[0589] Figure 40 This is a simplified illustration of the internal configuration of the status monitoring device 4080. The status monitoring device 4080 can be another example of the status monitoring device 1680. In this embodiment, the status monitoring device 4080 includes a polarimeter 4082 and a signal processing unit 4084. Except that the polarimeter 4082 is used instead of the optical delay interferometer 340 to obtain information representing the time differential of the phase difference, the status monitoring device 4080 can have the same configuration as the status monitoring device 1680.

[0590] If the change in the phase difference between the x-polarized component and the y-polarized component of the electric field of the monitoring optical signal is greater than a predetermined value, it can be determined that a polarization variation has occurred. Therefore, as long as the change in the phase difference between the x-polarized component and the y-polarized component of the electric field of the monitoring optical signal can be measured, the setting unit 3772 can use information representing this change to adjust the number of taps of the FIR filter and / or the settings related to the update frequency or update interval of the tap coefficients, instead of the aforementioned evaluation information related to polarization variation.

[0591] In this embodiment, the polarimeter 4082 measures the state of polarization (SOP) and / or degree of polarization (DOP) of the light (e.g., the monitoring light signal described above) input to the polarimeter 4082. The polarimeter 4082 outputs the sampled Stokes parameters. The Stokes parameters are represented by the above-described formulas B-13, B-14, B-15, and B-16.

[0592] The measurement principle and structure of the polarimeter 4082 are not particularly limited. Any known polarimeter can be used as the polarimeter 4082. The polarimeter 4082 may include, for example, four photodetectors, three polarizing mirrors, a quarter-wave plate, and a data processing device.

[0593] In this embodiment, the signal processing unit 4084 first acquires the sampled Stokes parameters. The signal processing unit 4084 then derives the phase difference δ(t) between the x-polarization component of the electric field of the monitoring optical signal and the y-polarization component of the electric field of the monitoring optical signal based on the sampled Stokes parameters. The phase difference δ(t) is calculated using Stokes parameters S2 and S3 and by the following equation C-7.

[0594] (Formula C-7)

[0595] δ(t)=tan -1 (S3 / S2)

[0596] When the i-th sampled δ(t) is represented as δ(i) (where i is an integer greater than or equal to 1), the signal processing unit 4084 can derive δ(i) based on the sampled Stokes parameters and Equation C-7. The signal processing unit 4084 calculates, for example, a quantity corresponding to the time derivative of the phase difference δ(t) by calculating δ(i+1) - δ(i). The quantity corresponding to the time derivative of the phase difference δ(t) represents the change in the phase difference. The signal processing unit 4084 generates information representing the change in the phase difference. The signal processing unit 4084 outputs the information representing the change in the phase difference as a control signal to the control input terminal 3566 of the digital signal processor 3560.

[0597] The polarimeter 4082 can be an example of a polarimeter, a polarization state measuring instrument, or a Stokes parameter measuring instrument. Information representing the amount of change in phase difference can be an example of a detection signal.

[0598] The present invention has been described above using embodiments, but the technical scope of the present invention is not limited to the scope described in the above embodiments. Those skilled in the art will understand that various modifications or improvements can be made to the above embodiments. Furthermore, to the extent that there is no technical contradiction, the matters described for a particular embodiment can be applied to other embodiments. As clearly understood from the claims, methods implementing the above modifications or improvements are also included within the technical scope of the present invention.

[0599] The execution order of actions, processes, steps, and stages in the apparatus, systems, programs, and methods shown in the claims, specification, and drawings is not specifically indicated as "before" or "first." Furthermore, it should be noted that any order is permissible as long as the output of a previous process is not used in a later process. Even if terms such as "firstly" or "next" are used for convenience in describing the flow of actions in the claims, specification, and drawings, it does not imply that the actions must be performed in that order.

[0600] For example, the following matters are disclosed in this application specification.

[0601] (Project A-1) An evaluation device for evaluating the phase noise of signal light propagating in an optical transmission path, comprising:

[0602] The differential phase information acquisition unit acquires information representing the differential phase of the input light at each of the multiple time points included during the evaluation period; and

[0603] The index derivation unit derives the degree of deviation of the differential phase at each of the multiple time points mentioned above as an index for evaluating the phase noise.

[0604] The aforementioned differential phase represents the phase difference between two temporally adjacent input light points among the plurality of time points.

[0605] The time intervals between multiple time points are approximately constant and are less than or equal to the length of the symbol time of the signal transmitted through the aforementioned input light.

[0606] (Project A-2) An optical receiver, comprising:

[0607] The evaluation device described in Project A-1; and

[0608] The demodulation unit demodulates the received signal transmitted through the aforementioned signal light and generates an information signal.

[0609] (Project A-3) An optical communication system, comprising:

[0610] The optical transmitter sends the aforementioned signal light; and

[0611] The optical receiver described in Project A-2.

[0612] (Item A-4) A program for enabling a computer to function as the evaluation device described in Item A-1.

[0613] (Project A-5) An evaluation method for evaluating the phase noise of signal light propagating in an optical transmission path, having the following characteristics:

[0614] In the differential phase information acquisition stage, information representing the differential phase of the input light at each of the multiple time points included in the evaluation period is acquired; and

[0615] In the index derivation stage, the degree of deviation of the differential phase at each of the multiple time points is derived as an index for evaluating the phase noise.

[0616] The aforementioned differential phase represents the phase difference between two temporally adjacent input light points among the plurality of time points.

[0617] The time intervals between multiple time points are approximately constant and are less than or equal to the length of the symbol time of the signal transmitted through the aforementioned input light.

[0618] For example, the following matters are disclosed in this application specification.

[0619] (Item B-1) A detection device for detecting polarization variations of signal light propagating in an optical transmission path, the detection device comprising:

[0620] The differential phase information acquisition unit acquires information representing the differential phase of the input light being evaluated at one or more time points; and

[0621] The determination unit determines whether the differential phase at at least a portion of the aforementioned time points at one or more time points satisfies a predetermined condition.

[0622] The aforementioned predetermined conditions include at least one of the first, second, third, fourth, and fifth conditions.

[0623] The first condition is that the magnitude of the differential phase at at least one of the aforementioned time points is equal to or greater than a predetermined first threshold.

[0624] The second condition is that, among the multiple time points included in the evaluation period of one or more time points and a period of predetermined length, the number of time points whose differential phase magnitude is equal to or greater than the first threshold is equal to or greater than a predetermined second threshold.

[0625] The third condition is that the ratio of the number of time points at which the magnitude of the differential phase at that time point is equal to or greater than the first threshold to the number of time points included in the evaluation period is equal to or greater than a predetermined third threshold.

[0626] The fourth condition mentioned above is derived from the magnitude of the differential phase at each of the above-mentioned time points, and the angle of movement at the variation frequency of the trajectory on the Poincaré sphere is equal to or greater than a predetermined fourth threshold.

[0627] The fifth condition is that the angular velocity of the trajectory on the Poincaré sphere at the aforementioned variation frequency is equal to or greater than the predetermined fifth threshold.

[0628] (Project B-2) An optical receiving device, comprising:

[0629] The detection device described in Item B-1; and

[0630] The demodulation unit demodulates the received signal transmitted through the aforementioned signal light and generates an information signal.

[0631] (Project B-3) An optical communication system, comprising:

[0632] The optical transmitting device transmits the aforementioned signal light; and

[0633] The optical receiving device described in Project B-2.

[0634] (Item B-4) A program for enabling a computer to function as the detection device described in Item B-1.

[0635] (Project B-5) A detection method for detecting polarization variations of signal light propagating in an optical transmission path, the detection method comprising:

[0636] In the differential phase information acquisition stage, information representing the differential phase of the input light used as the evaluation object at more than one time point is acquired; and

[0637] In the determination phase, it is determined whether the differential phase at at least a portion of the aforementioned time points meets predetermined conditions.

[0638] The aforementioned predetermined conditions include at least one of the first, second, third, fourth, and fifth conditions.

[0639] The first condition is that the magnitude of the differential phase at at least one of the aforementioned time points is equal to or greater than a predetermined first threshold.

[0640] The second condition is that, among the multiple time points included in the evaluation period of one or more time points and a period of predetermined length, the number of time points whose differential phase magnitude is equal to or greater than the first threshold is equal to or greater than a predetermined second threshold.

[0641] The third condition is that the ratio of the number of time points at which the magnitude of the differential phase at that time point is equal to or greater than the first threshold to the number of time points included in the evaluation period is equal to or greater than a predetermined third threshold.

[0642] The fourth condition mentioned above is derived from the magnitude of the differential phase at each of the above-mentioned time points, and the angle of movement at the variation frequency of the trajectory on the Poincaré sphere is equal to or greater than a predetermined fourth threshold.

[0643] The fifth condition is that the angular velocity of the trajectory on the Poincaré sphere at the aforementioned variation frequency is equal to or greater than the predetermined fifth threshold.

[0644] Explanation of reference numerals in the attached figures

[0645] 10: Optical transmission path; 100: Communication system; 110: Optical signal transmitting device; 120: Optical signal receiving device; 130: Local oscillator; 140: Optical 90-degree mixer; 152: Optical receiver; 154: Optical receiver; 162: AD converter; 164: AD converter; 170: Signal processing unit; 210: Digital signal processing circuit; 220: Decoding circuit; 230: Amplitude noise evaluation unit; 240: Phase noise evaluation unit; 242: Differential phase signal generation unit; 244: Histogram generation unit; 246: Standard deviation calculation unit; 320: Phase noise evaluation device; 340: Optical delay interferometer; 350: Optical receiver; 360: AD converter; 370: Signal processing unit; 422 424: Semi-transparent mirror, 432: Total reflection mirror, 434: Total reflection mirror, 436: Optical phase adjuster, 510: Substrate, 520: Waveguide, 526: Electrode, 530: Waveguide, 540: Optical delay interferometer, 610: Substrate, 620: Waveguide, 630: Ring resonator, 636: Electrode, 640: Optical delay interferometer, 710: Correction unit, 820: Phase noise evaluation device, 850: Optical receiver, 860: AD converter, 870: Signal processing unit, 880: Optical phase control unit, 930: Normalization unit, 1040: Optical delay interferometer, 1060: Semi-transparent mirror, 1140: Optical delay interferometer, 1160: Waveguide, 1240: Optical delay interferometer, 1260 1340: Waveguide; 1350: Optical Delay Interferometer; 1352: Optical Receiver; 1354: Optical Receiver; 1356: Differential Processing Unit; 1452: Photodiode; 1454: Photodiode; 1456: Connector; 1540: Optical Delay Interferometer; 1600: Communication System; 1620: Optical Signal Receiver; 1640: Wavelength Demultiplexer; 1650: Optical Receiver; 1660: AD Converter; 1670: Signal Processing Unit; 1680: Status Monitoring Device; 1722: Optical Signal Output Unit for Communication; 1724: Optical Signal Output Unit for Monitoring; 1730: Wavelength Combiner; 1852: Photoelectric Conversion Element; 1854: Integrating Circuit; 1870: Signal Processing Unit. 1922: Added detection unit; 1924: Frequency analysis unit; 1926: Movement angle derivation unit; 1928: Movement angular velocity derivation unit; 1930: Polarization variation detection unit; 1940: Information output unit; 2100: Data table; 2120: Number; 2122: Time; 2124: Output value; 2132: Variation frequency; 2134: Movement angle; 2136: Movement angular velocity; 2140: Success or failure; 2150: Detection result; 3000: Computer; 3001: DVD-ROM; 3010: Host controller; 3012: CPU; 3014: RAM; 3016: GPU; 3018: Display device; 3020: Input / output controller; 3022: Communication interface.3024: Hard disk drive; 3026: DVD-ROM drive; 3030: ROM; 3040: Input / output chip; 3042: Keyboard; 3500: Communication system; 3520: Optical signal receiver; 3550: Optical receiver unit; 3552: Local oscillator; 3560: Digital signal processor; 3562: Signal input terminal; 3564: Signal output terminal; 3566: Control input terminal; 3654: Optical 90-degree mixer; 3656: Optical receiver. 3662: AD converter; 3664: Compensation unit; 3666: Carrier phase estimation unit; 3668: Decoding unit; 3720: Wavelength dispersion equalization unit; 3740: Polarization separation / polarization mode dispersion compensation unit; 3750: Control signal receiving unit; 3760: FIR filter; 3770: Filter control unit; 3772: Setting unit; 3774: Update unit; 3810: Delay unit; 4080: Status monitoring device; 4082: Polarimeter; 4084: Signal processing unit.

Claims

1. A control device for controlling the operation of a compensation device, said compensation device compensating for birefringence and / or polarization mode dispersion of signal light propagating in an optical transmission path by using digital signal processing of a finite impulse response filter, said control device comprising: A detection signal receiving unit receives a detection signal, which is a signal representing the detection result of the detection device, wherein the detection device optically detects polarization variations in the optical transmission path; and The setting unit determines, based on the detection results from the detection device, a setting related to the update frequency or update interval of the number of taps of the finite impulse response filter, or a setting related to the update frequency or update interval of the tap coefficients of the finite impulse response filter.

2. The control device according to claim 1, wherein, The setting unit determines whether the absolute value of the polarization variation is greater than a predetermined threshold based on the detection result of the detection device. When it is determined that the absolute value of the change in polarization variation is greater than the threshold, the setting unit decides to (i) set the update frequency of the number of taps and / or the tap coefficient to a value greater than the current setting, (ii) set the update frequency of the number of taps and / or the tap coefficient to a predetermined first value, (iii) set the update interval of the number of taps and / or the tap coefficient to a value less than the current setting, or (iv) set the update interval of the number of taps and / or the tap coefficient to a predetermined second value.

3. The control device according to claim 1, wherein, The setting unit determines whether the absolute value of the polarization variation is greater than a predetermined threshold based on the detection result of the detection device. When it is not determined that the absolute value of the polarization variation is greater than the threshold, the setting unit decides to (i) set the update frequency of the number of taps and / or the tap coefficient to be less than the current setting value, (ii) set the update frequency of the number of taps and / or the tap coefficient to a predetermined third value, (iii) set the update interval of the number of taps and / or the tap coefficient to be greater than the current setting value, or (iv) set the update interval of the number of taps and / or the tap coefficient to a predetermined fourth value.

4. The control device according to claim 1, wherein, The control device further includes an update unit that updates the number of taps or the tap coefficient at an update frequency or update interval determined by the setting determined by the setting unit.

5. The control device according to claim 1, wherein, The compensation device compensates for the birefringence and / or polarization mode dispersion experienced by the first signal light propagating in the optical transmission path. The detection device optically detects the polarization variation of the second signal light propagating in the optical transmission path. The wavelengths of the first signal light and the second signal light are different.

6. The control device according to claim 5, wherein, The detection device includes an optical delay interferometer, a polarimeter, a polarization state measuring instrument, or a Stokes parameter measuring instrument for optically detecting polarization variations of the second signal light.

7. The control device according to claim 1, wherein, The control device controls the operation of the plurality of compensation devices, which compensate for the birefringence and / or polarization mode dispersion of the plurality of signal lights propagating in the optical transmission path by using digital signal processing with a finite impulse response filter. The setting unit determines the settings related to the plurality of finite impulse response filters configured in the plurality of compensation devices based on the detection results of the detection device. The multiple signal lights each have a different wavelength.

8. A compensation device for compensating for birefringence and / or polarization mode dispersion of signal light propagating in an optical transmission path, said compensation device comprising: A finite impulse response filter is used to compensate for birefringence and / or polarization mode dispersion of the signal light; and The control device according to claim 1.

9. A storage medium storing a program for enabling a computer to function as a control device according to any one of claims 1 to 7.

10. A control method for controlling the operation of a compensation device, said compensation device compensating for birefringence and / or polarization mode dispersion of signal light propagating in an optical transmission path by using digital signal processing of a finite impulse response filter, the control method comprising: In the signal reception stage, a detection signal is received, which is a signal representing the detection result of the detection device. The detection device optically detects polarization variations in the optical transmission path; and During the setting phase, based on the detection results from the detection device, a setting related to the update frequency or update interval of the number of taps of the finite impulse response filter, or a setting related to the update frequency or update interval of the tap coefficients of the finite impulse response filter, is determined.

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