Detection device, light receiving device, optical communication system, storage medium and detection method

Abstract

A detection device for detecting polarization variation of signal light propagating in an optical transmission path comprises: a differential phase information acquisition unit for acquiring information representing the differential phase of input light as an evaluation object at each of more than one time points; and a determination unit for determining whether the differential phase of at least a portion of the more than one time points satisfies a predetermined condition. The predetermined condition includes, for example, the following condition: the magnitude of the differential phase of at least one of the more than one time points is equal to or greater than a predetermined first threshold value. The predetermined condition includes, for example, the following condition: the number of time points at which the magnitude of the differential phase of at least a portion of the more than one time points is equal to or greater than a first threshold value among a plurality of time points included in an evaluation period having a predetermined length is equal to or greater than a predetermined second threshold value.

Description

Technical Field

[0001] The present invention relates to a detection device, a light receiving device, an optical communication system, a program and a detection method. Background Art

[0002] Non-patent documents 1 to 5 disclose that, for example, phase noise is generated in the signal light via the Kerr effect, thereby degrading the optical transmission characteristics. Non-patent document 6 discloses that, by subtracting the moving average of the optical phase from the measured phase of the received signal, an optical phase noise with the 1 / f noise component removed is derived as disclosed in non-patent document 7. Non-patent documents 8 to 9 disclose the use of a delay interferometer to demodulate a DPSK signal. Non-patent documents 10 to 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 and 14 disclose that high-speed polarization variation of about 10 kHz or more may occur in coherent optical communication. Non-patent document 15 discloses that there is a proportional relationship between the value of applied voltage of an optical fiber stretcher and the values ​​of lateral pressure and birefringence generated thereby.

[0004] Prior art literature

[0005] Non-patent literature

[0006] 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.

[0007] Non-patent document 2: S.Ryu, "Signal linewidth broadening due to nonlinear Kerreffect in long-haul coherent systems using cascaded optical amplifiers," IEEE Journal of Lightwave Technology, Vol.10, No.10, pp.1450-1457, 1992.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] Non-Patent Document 6: M. Nakazawa et al., Editor, High spectral density optical communication technologies, Springer-Verlag, 2010.

[0012] Non-Patent Document 7: K. Kikuchi, “Effect of 1 / f-type FM noise on semiconductor-laser linewidth residual in high-power limit,” IEEE Joumal of Quantum Electronics, Vol. 25, No. 4, pp. 684-688, 1989.

[0013] 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.

[0014] Non-patent document 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.

[0015] Non-patent document 10: T.Kominato at al., "Ring resonators composed of GeO2-doped silica waveguides," IEEE Joumal of Lightwave Technology, Vol.10, No.12, pp.1781-1788, 1992.

[0016] Non-patent literature 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.

[0017] Non-patent document 12: W. Bogaerts et al., “Silicon microring resonators,” Laser and Photonics Reviews, Vol. 6, NO. 1, pp. 47-73, 2012.

[0018] Non-patent literature 13: PMKrummrich, E.-D.Schmidt, W.Weiershausen, andA.Mattheus, "Field trial results on statistics of fast polarization changes in long haul WDM transmission systems," OFC2005, paper OThT6, March 2005.

[0019] Non-patent literature 14: M. Boroditsky, M. Brodsky, NJ Frigo, P Magill, and H. Rosenfeldt, "Polarization dynamics in installed fiberoptic systems," 2005 IEEELEOS Annual Meeting, paper TuCC1, October 2005.

[0020] Non-patent document 15: R. Ulrich and A. Simon, "Polarization optics of twisted single-mode fibers," Applied Optics, Vol. 18, No. 13, pp. 2241-2251, July 1979. Summary of the invention

[0021] A first aspect of the present invention provides a detection device. The detection device is used, for example, to detect polarization changes of signal light propagating in an optical transmission path. The detection device is provided, for example, with a differential phase information acquisition unit that acquires information indicating the differential phase of input light as an evaluation object at each of more than one time point. The detection device is provided, for example, with a determination unit that determines whether the differential phase of at least a part of the more than one time point satisfies a predetermined condition.

[0022] In the above detection device, the predetermined conditions include, for example, the following first condition: the magnitude of the differential phase of at least one of the one or more time points is equal to or greater than the first threshold. In the above detection device, the predetermined conditions include, for example, the following second condition: the number of time points at which the magnitude of the differential phase of at least one of the one or more time points is equal to or greater than the first threshold among the multiple time points included in the evaluation period having a predetermined length, which is at least a part of the one or more time points, is equal to or greater than the second threshold. In the above detection device, the predetermined conditions include, for example, the following third condition: the ratio of the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold to the number of the multiple time points included in the evaluation period is equal to or greater than the third threshold. In the above detection device, the predetermined conditions include, for example, the following fourth condition: the moving angle of the trajectory on the Poincare sphere at the variation frequency derived from the magnitude of the differential phase of each of the one or more time points is equal to or greater than the fourth threshold. In the above detection device, the predetermined condition includes, for example, the following fifth condition: the moving angular velocity at the variation frequency of the track on the Poincare sphere is equal to or greater than a predetermined fifth threshold value. In the above detection device, the predetermined condition includes, for example, a combination of at least two of the above first condition, second condition, third condition, fourth condition, and fifth condition.

[0023] Any of the above detection devices may include an output unit, and when the differential phase of at least a part of the one or more time points is determined to satisfy a predetermined condition, the output unit outputs information indicating that the signal light has undergone a polarization change. In any of the above detection devices, the differential phase may represent the difference in phase between the input light at two temporally adjacent time points in the one or more time points. The time interval between the one or more time points may be substantially constant.

[0024] In any of the above detection devices, the signal light may include light of a first wavelength for transmitting an information signal and light of a second wavelength for detecting polarization change. The value of the first wavelength may be different from the value of the second wavelength. The input light may be light of the second wavelength. Any of the above detection devices further includes a demultiplexer that demultiplexes the light of the second wavelength from the signal light.

[0025] In any of the above detection devices, the differential phase information acquisition unit may include a delayed interferometer, and the input light is input into the delayed interferometer. The differential phase information acquisition unit may include a photoelectric conversion unit, and the photoelectric conversion unit converts the output light of the delayed interferometer into an electrical signal. The differential phase information acquisition unit may include an integration unit, and the electrical signal output by the photoelectric conversion unit is input into the integration unit, and the integration unit outputs a voltage after the waveform of the input voltage is time-integrated.

[0026] In any of the above detection devices, the delay interference unit can branch the input light into a first input light and a second input light. The delay interference unit can cause the first input light passing through the first optical path and the second input light passing through the second optical path to combine and interfere. The delay 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 and the frequency f of the input light satisfy the relationship of the following mathematical formula 1,

[0027] (Mathematical formula 1)

[0028] 2πfτ=2nπ+π / 2 (where n is an integer).

[0029] A second aspect of the present invention provides an optical receiving device. The optical receiving device includes, for example, any detection device according to the first aspect. The optical receiving device includes, for example, a demodulator that demodulates a reception signal transmitted by signal light to generate an information signal.

[0030] A third aspect of the present invention provides an optical communication system. The optical receiving device includes, for example, an optical transmitting device that transmits signal light. The optical receiving device includes, for example, any of the optical receiving devices according to the second aspect.

[0031] A fourth aspect of the present invention provides a detection method. The detection method is used, for example, to detect polarization variation of signal light propagating in an optical transmission path. The detection method can be executed by a computer.

[0032] The detection method includes, for example, a differential phase information acquisition stage for acquiring information indicating the differential phase of input light as an evaluation object at each of more than one time point. The detection method includes, for example, a determination stage for determining whether the differential phase of at least some of the more than one time points satisfies a predetermined condition.

[0033] In the above detection method, the predetermined conditions include, for example, the following first condition: the magnitude of the differential phase of at least one of the more than one time points is equal to or greater than the predetermined first threshold value. In the above detection method, the predetermined conditions include, for example, the following second condition: the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold value among the multiple time points included in the evaluation period of at least a part of the more than one time points and a period of predetermined length is equal to or greater than the second threshold value. In the above detection method, the predetermined conditions include, for example, the following third condition: the ratio of the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold value to the number of the multiple time points included in the evaluation period is equal to or greater than the third threshold value. In the above detection method, the predetermined conditions include, for example, the following fourth condition: the moving angle of the trajectory on the Poincare sphere at the variation frequency derived from the magnitude of the differential phase of each of the more than one time points is equal to or greater than the fourth threshold value. In the above detection method, the predetermined condition includes, for example, the following fifth condition: the moving angular velocity of the trajectory on the Poincare sphere at the change frequency is equal to or greater than the predetermined fifth threshold value. In the above detection method, the predetermined condition includes, for example, a combination of at least two of the above first condition, second condition, third condition, fourth condition, and fifth condition.

[0034] The fifth aspect of the present invention provides a program. The above-mentioned program is, for example, a program for causing a computer to function as any detection device involved in the above-mentioned first aspect. The above-mentioned program is, for example, a program for causing a computer to execute any detection method involved in the above-mentioned fourth aspect. A computer-readable storage medium storing the above-mentioned program can also be provided. The above-mentioned storage medium is a non-temporary computer-readable medium.

[0035] In addition, the above summary of the invention does not list all the essential features of the present invention. In addition, sub-combinations of these feature groups can also constitute inventions. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 An example of the system configuration of the communication system 100 is schematically shown.

[0037] Figure 2 An example of the internal configuration of the signal processing unit 170 is schematically shown.

[0038] Figure 3 An example of the system configuration of the phase noise evaluation device 320 is schematically shown.

[0039] Figure 4 An example of the internal structure of the optical delay interferometer 340 is schematically shown.

[0040] Figure 5 An example of the internal structure of the optical delay interferometer 540 is schematically shown.

[0041] Figure 6 An example of the internal structure of the optical delay interferometer 640 is schematically shown.

[0042] Figure 7 An example of the internal configuration of the signal processing unit 370 is schematically shown.

[0043] Figure 8 An example of the system configuration of the phase noise evaluation device 820 is schematically shown.

[0044] Fig. 9 An example of the internal structure of the signal processing unit 870 is schematically shown.

[0045] Fig.10 An example of the internal structure of the optical delay interferometer 1040 is schematically shown.

[0046] Fig.11 An example of the internal structure of the optical delay interferometer 1140 is schematically shown.

[0047] Fig.12 An example of the internal structure of the optical delay interferometer 1240 is schematically shown.

[0048] Fig.13 An example of the internal structure of the optical delay interferometer 1340 is schematically shown.

[0049] Fig.14 An example of the circuit configuration of the balanced optical receiver 1350 is schematically shown.

[0050] Fig.15 An example of the internal structure of the optical delay interferometer 1540 is schematically shown.

[0051] Fig.16 An example of the system configuration of the communication system 1600 is schematically shown.

[0052] Fig.17 An example of the internal structure of the optical signal transmission device 110 is schematically shown.

[0053] Fig.18 An example of the internal structure of the status monitoring device 1680 is schematically shown.

[0054] Fig.19 An example of the internal structure of the signal processing unit 1870 is schematically shown.

[0055] Fig. 20An example of information processing in the state monitoring device 1680 is schematically shown.

[0056] Fig.21 An example of the data table 2100 is schematically shown.

[0057] Fig. 22 An example of the system configuration of the computer 3000 is schematically shown.

[0058] Fig.23 The measurement results of the Stokes parameters in Experimental Example 1 are shown.

[0059] Fig.24 A bar graph showing the differential phase in Experimental Example 1.

[0060] Fig.25 FIG. 1 shows the time variation of the differential phase in Experimental Example 1. FIG.

[0061] Fig.26 The frequency spectrum of the differential phase in Experimental Example 1 is shown.

[0062] Fig. 27 The measurement results of the Stokes parameters in Comparative Experimental Example 1 are shown.

[0063] Fig.28 A bar graph showing the differential phase in Comparative Experimental Example 1.

[0064] Fig.29 FIG. 1 shows the time variation of the differential phase in Comparative Experiment Example 1. FIG.

[0065] Fig.30 The frequency spectrum of the differential phase in Comparative Experimental Example 1 is shown.

[0066] Fig.31 The measurement results of the Stokes parameters in Experimental Example 2 are shown.

[0067] Fig.32 A bar graph showing the differential phase in Experimental Example 2.

[0068] Fig.33 FIG. 2 shows the time variation of the differential phase in Experimental Example 2. FIG.

[0069] Fig.34 The frequency spectrum of the differential phase in Experimental Example 2 is shown. DETAILED DESCRIPTION

[0070] The present invention is described below by way of the embodiments of the invention, but the following embodiments do not limit the invention involved in the claims. In addition, not all combinations of features described in the embodiments are necessary for the solution of the invention. In addition, in the drawings, the same reference numerals are given to the same or similar parts, and repeated descriptions are sometimes omitted.

[0071] In an optical transmission system, phase variations such as optical phase noise and polarization variation may occur in a received signal. Examples of causes of phase variations in a received signal include nonlinear optical effects, optical fiber vibration, optical fiber twisting, variations in stress applied to an optical fiber, laser line width or laser phase fluctuations, and phase variations of an optical electric field caused by a lightning strike. Depending on the state of the optical phase variation, a code error may occur in an optical transmission system. As an optical transmission system, an optical transmission system using a coherent optical communication method is exemplified.

[0072] According to this embodiment, for example, (i) an apparatus or method for evaluating the presence and / or degree of optical phase noise, (ii) an apparatus or method for evaluating the presence and / or degree of polarization variation, and (iii) an apparatus or method for evaluating the presence and / or degree of optical phase noise and polarization variation are described. According to this embodiment, the presence and / or degree of at least one of optical phase noise and polarization variation can be evaluated with relatively high accuracy. As a result, the operation efficiency and maintenance efficiency of the optical transmission system can be improved.

[0073] (I. Principles of Optical Phase Noise Measurement)

[0074] First, the principle of measuring optical phase noise is described. The details of the optical phase noise measuring device and the measuring method based on the above-mentioned measurement principle are described. Figures 1 to 15 This will be described later.

[0075] Conventionally, optical phase noise (sometimes simply referred to as phase noise) is estimated by linearly approximating the variation of the optical phase with respect to time t. For example, when the phase of a signal light propagating in an optical transmission path is measured, the measured value of the phase of the signal light at the i-th (i is an integer greater than or equal to 1 and less than or equal to N) sample point among N (N is a positive integer) sample points (sometimes referred to as samples) is approximated using the following equation 1.

[0076] (Formula 1)

[0077]

Mathematical formula 1

[0078]

[0079] in, is the measured value of the phase of the signal light at the i-th specimen point,

[0080] θ(i) is the phase noise of the i-th sample point,

[0081] a and b are constants.

[0082] For example, by predetermining the constants a and b using the least squares method, the phase noise θ(i) can be estimated based on the measured value of the phase of the signal light. However, in reality, there are low-frequency component fluctuations (sometimes referred to as 1 / f noise components) in the frequency fluctuations of the laser, so the value of the phase noise θ(i) cannot be accurately estimated using the above approximate formula 1.

[0083] As described in non-patent document 6, by subtracting the moving average of the optical phase from the measured phase of the received signal, the optical phase noise with the 1 / f noise component removed can be derived. Specifically, the phase noise θ(i) is derived using the following formula 2. In addition, in formula 2, m and 1 represent integers. The value of l is appropriately set.

[0084] (Formula 2)

[0085]

Mathematical formula 2

[0086]

[0087] However, it is known that phase noise is a Wiener process and follows a Gaussian distribution in which the variance diverges in proportion 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.

[0088] According to an example of this embodiment, by evaluating the statistical distribution of the differential phase, the influence of nonlinear phase noise on the transmission of the optical signal can be more accurately evaluated. Examples of nonlinear phase noise include (i) self-phase modulation, (ii) mutual phase modulation, and (iii) in a fiber Raman amplifier, the intensity noise of the pump light causes the signal light to generate phase noise via the Kerr effect.

[0089] The differential phase refers to the phase difference before and after the sample point (sometimes called a sample). The differential phase of the i-th sample point As shown in the following formula 3.

[0090] (Formula 3)

[0091]

Mathematical formula 3

[0092]

[0093] in,

[0094] is the measured value of the phase of the signal light at the i+1th specimen point,

[0095] is the measured value of the phase of the signal light at the i-th specimen point,

[0096] i is an integer greater than or equal to 1 and less than or equal to (N-1).

[0097] For example, by setting the sampling frequency to the same value as the symbol rate and measuring the differential phase at the same time interval as the symbol time, the variance or standard deviation of the phase noise in one symbol time is derived. In addition, the setting of the sampling frequency is not limited to the above-mentioned embodiment. For example, the sampling frequency is set to be greater than the symbol rate. In this case, by calculating the differential phase using samples corresponding to appropriate time intervals, the differential phase can be measured at the same time interval as the symbol time. Sometimes the time interval between samples adjacent in time (for example, the time interval between the i+1th sample and the ith sample) is called the sample time interval Δt.

[0098] For example, in coherent heterodyne detection, the phase noise θ(t) is calculated by using the phase of the signal light. and the phase of the local oscillator light (sometimes called the local oscillator light) Expressed as 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.

[0099] When modulation for communication is not applied to the optical signal, the phase component of the i-th sample point is derived by the following equation 4.

[0100] (Formula 4)

[0101]

Mathematical formula 4

[0102]

[0103] in, is the phase component of the i-th specimen point,

[0104] i I (i) is the input current of the light receiving element at the i-th sample point to which the I signal component is input,

[0105] i Q (i) is the output current of the light receiving element at the i-th sample point to which the Q signal component is input.

[0106] On the other hand, when modulation for communication is applied to the optical signal, the phase component of the i-th sample point is calculated by eliminating the modulation component. For example, in the case of M-phase phase modulation, the modulation component can be eliminated by calculating the Mth power of the received signal. M is a positive integer.

[0107] For example, in the case of an optical signal subjected to QPSK modulation, the complex received signal I QPSK (t) is represented by the following formula 5.

[0108] (Formula 5)

[0109] I QPSK (t) = i I，QPSK (t)+ji Q，QPSK (t)

[0110] 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 optical signal subjected to QPSK modulation. Q，QPSK (t) represents an output current corresponding to a Q signal component obtained by coherent heterodyne detection of an optical signal subjected to QPSK modulation.

[0111] Here, since QPSK modulation is a 4-phase modulation method, if the received signal I QPSK (t) is raised to the fourth power, then when receiving the signal I QPSK The following equation 6 holds true between the fourth power of the deviation angle (t) and the phase noise θ(t).

[0112] (Formula 6)

[0113] arg{I QPSK (t)} 4 =4(2πf c t+θ(t))

[0114] In the above formula, f c Indicates the angular frequency f of the signal light s The angular frequency f of the local oscillator light L The difference (f s -f L ), and is called the beat frequency.

[0115] By dividing both sides of the equation 6 by 4 (corresponding to the above-mentioned M), the following equation 7 is derived. Thus, the phase noise θ(t) with the modulation component removed is derived.

[0116] (Formula 7)

[0117] arg{I QPSK (t)} 4 / 4=2πf c t+θ(t)

[0118] If equations 1 and 7 are considered, the differential phase of the i-th sample point is The relationship between the phase noise θ(i+1) at the i+1th sample point, the phase noise θ(i) at the i-th sample point, and the sample time interval Δt is expressed by the following equation 8.

[0119] (Formula 8)

[0120]

Mathematical formula 5

[0121]

[0122] As mentioned above, the phase noise θ(i) follows a Gaussian distribution, so the differential phase Also follows the Gaussian distribution. In addition, the average value of θ(i) is 0. Therefore, the differential phase The average value of the signal light phase noise θ(i) is 2πfcΔt. Furthermore, if the standard deviation of the signal light phase noise θ(i) is σ sig , then the differential phase The variance σ M 2 =2×σ sig 2 .

[0123] Therefore, when deriving the differential phase The standard deviation of the measurement results is M When , the standard deviation σ of the phase noise of the signal light propagating in the optical transmission path is derived by the following formula 9: sig .

[0124] (Formula 9)

[0125] σ sig =σ M / √2

[0126] Therefore, it can be seen that according to this embodiment, the phase noise of the signal light can be derived more accurately. For example, according to this embodiment, as in the method of subtracting the moving average of the optical phase from the measured phase of the received signal, the possibility of the influence of the uncertainty factor caused by the length of the moving average time being mixed into the measured value is significantly reduced.

[0127] Furthermore, when considering the transmission effect, it is best to also consider the effect of the spectral line width of the local oscillator light. In this case, the standard deviation σ of the phase noise of the optical signal (sometimes referred to as the received signal) received by the optical receiving device is RX , the standard deviation of the phase noise based on the spectral line width of the local oscillator light σ Lo And the standard deviation of the phase noise of the signal light σ sig The relationship is expressed by the following formula 10.

[0128] (Formula 10)

[0129] σ RX 2 =σ sig 2 +σ LO 2

[0130] As mentioned above, using the differential phase of the signal light The standard deviation of the measurement results is M The standard deviation σ of the phase noise of the signal light is derived sig The standard deviation of the phase noise based on the spectral line width of the local oscillator light is σ LO The standard deviation of the phase noise of the signal light is σ sig The same procedure is used to measure the standard deviation σ of the differential phase of the local oscillation light. ML It is derived by the following formula 11.

[0131] (Formula 11)

[0132] σ LO =σ ML / √2

[0133] Therefore, the standard deviation σ of the phase noise of the received signal is derived using equations 9 to 11: RX The standard deviation of the phase noise of the received signal is σ RX Used for transmission impact evaluation and monitoring, etc.

[0134] (Method for measuring differential phase)

[0135] In one embodiment, the differential phase of each of the N sample points The phase difference is derived by converting the light to be measured (sometimes referred to as the object light) into an electrical signal, sampling the electrical signal, and performing signal processing on the digital data obtained by quantizing the electrical signal. For example, the time series data of the phase of the object light is generated at a time interval that is the same as or approximately the same as the symbol time. The differential phase is derived by calculating the difference between two data that are adjacent in time in the time series data.

[0136] In other embodiments, first, the object light is branched into a first light and a second light. Then, the delay time difference τ between the first light and the second light is adjusted. Specifically, the delay time difference τ is adjusted to 2πfτ=2nπ+π / 2. Here, f is the frequency of the object light, and n is a positive integer. Next, the first light and the second light with the adjusted delay time difference τ are combined and interfered. Then, the differential phase is derived by photoelectric conversion of the combined light, sampling and quantization.

[0137] As described above, the differential phase is derived through a relatively simple process. In addition, the variance or standard deviation of the differential phase is also derived through a relatively simple calculation process. Therefore, according to this embodiment, the load on the computer can be reduced.

[0138] (Evaluation Example of Phase Noise of Signal Light)

[0139] When evaluating the phase noise of signal light used in actual communications, the optical signal has a spectral component due to modulation. Therefore, there is a possibility that the modulation component is mixed into the measurement result and affects the measurement of the optical phase noise.

[0140] Therefore, in one embodiment, the communication system 100 uses the optical signal receiving device 120 to evaluate the phase noise, for example, in a test run after the communication system 100 is constructed. In other embodiments, the communication system 100 is provided with a dedicated wavelength for measuring the phase noise during operation of the communication system 100, and the phase noise of the light of the wavelength is always measured. For example, in an embodiment in which the differential phase is derived by signal processing of digital data, the phase noise is evaluated by measuring the differential phase of the I component and the Q component of the light of the wavelength.

[0141] (II. Principle of detecting phase change caused by polarization change)

[0142] Next, the detection principle of phase change caused by polarization change is explained. The details of the detection device and detection method of polarization change based on the above detection principle will be described later. Figures 16 to 34 Give a description.

[0143] The inventors have found that, in addition to optical phase noise, the above differential phase (especially the differential phase measured using an optical delay interferometer) may also include phase variations caused by polarization variations. For example, in the above coherent optical communication method, 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 are coupled during the transmission process of the optical fiber, high-speed digital signal processing is performed on the receiver side to separate the received signal into the original orthogonal polarization modes.

[0144] In the past, it was believed that polarization variation was sufficiently slow compared to the speed of signal processing on the receiver side. However, recent studies have shown that high-speed polarization variation of about 10kHz or more can be produced. Such high-speed polarization variation is accompanied by optical phase variation, just like the above-mentioned optical phase noise. Therefore, depending on the state of polarization variation, code errors may occur in optical transmission systems.

[0145] In an optical transmission system, situations in which relatively high-speed polarization fluctuations occur include (i) a situation in which a variable lateral stress is applied to an optical fiber, causing the birefringence of the optical fiber to fluctuate, and (ii) a situation in which a sharp twist is applied to the optical fiber, causing the polarization plane to rotate at high speed. Therefore, in many cases, a phase fluctuation due to polarization fluctuations occurs suddenly.

[0146] In the above-mentioned measurement principle of optical phase noise, statistical processing is performed in the process of deriving the phase noise of the signal light. Therefore, sudden phase changes are difficult to be reflected in the measurement results of the phase noise of the signal light. Therefore, not only can statistical quantities such as standard deviation be grasped, but also phase changes caused by sudden polarization changes can be detected based on real-time measurement results, thereby being able to more accurately evaluate the state of the optical transmission system and / or optical signal.

[0147] When the orthogonal polarization mode of the signal light propagating in the z direction is set to E x (t) and E y (t), let the unit vectors in the x-direction and y-direction be k x and k y When , the electric field of the light propagating in the optical transmission path and received (sometimes referred to as the received optical electric field) E(t) is expressed by the following formula B-1.

[0148] (Formula B-1)

[0149]

[0150] In formula B-1, δ(t) represents E x (t) and E y (t) phase difference. represents the optical phase noise. f represents the frequency of the signal light. E(t), k x and k y is a vector.

[0151] Assuming that the optical delay interferometer has no polarization dependence, the output current i of the optical receiver connected to the optical delay interferometer is expressed by the following formula B-2.

[0152] (Formula B-2)

[0153] i=R{E(t)+E(t-τ)}·{E(t)+E(t-τ)}

[0154] In formula B-2, τ represents the delay time difference between the two paths of the optical delay interferometer. R represents the sensitivity of the optical receiver. · represents the inner product.

[0155] Considering that the optical receiver does not respond to the frequency of light, if the DC component of equation B-2 is ignored, the reception current i output from the optical receiver is expressed by the following equation B-3.

[0156] (Formula B-3)

[0157]

Mathematical formula 6

[0158] 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}]

[0159] In Formula B-3, when the delay time difference τ is adjusted to 2πfτ=2nπ+π / 2 (n is a positive integer), the above-mentioned reception current i is expressed by the following Formula B-4.

[0160] (Formula B-4)

[0161]

Mathematical formula 7

[0162] 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)}]

[0163] When semiconductor laser output light is used as signal light, AM noise of the signal light is very small due to the properties of semiconductor laser output light. In this case, the above-mentioned reception current i is expressed by the following formula B-5.

[0164] (Formula B-5)

[0165]

Mathematical formula 8

[0166]

[0167] Here, when τ is very short, it can be assumed that 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 reception current i is approximated by the following formula B-6.

[0168] (Formula B-6)

[0169]

Mathematical formula 9

[0170]

[0171] As shown in equation B-6, the above-mentioned reception 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 i caused by the polarization change expressed by the following equation B-7: pol .

[0172] (Formula B-7)

[0173] i pol =-RE y 2 (t)sin{δ(t)-δ(t-τ)}

[0174] According to formula B-7, as the above-mentioned receiving current i, a current with a change caused by polarization change is observed in addition to phase noise. That is, the phase change caused by the sudden polarization change can be detected based on the real-time measurement result of the above-mentioned receiving current i. In this way, the inventors found that by observing the output current i of the optical receiver connected to the optical delay interferometer whose delay time difference τ is adjusted to 2πfτ=2nπ+π / 2 (n is a positive integer), when a sudden polarization change occurs in the signal light, the occurrence of the polarization change can be detected.

[0175] (A. When δ(t) changes slightly)

[0176] 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 the polarization change is pol Approximation was performed by the following formula B-8.

[0177] (Formula B-8)

[0178]

Mathematical formula 10

[0179]

[0180] Here, in an actual device, when measuring the output current i of the optical receiver connected to the optical delay interferometer, the output current i is amplified by, for example, an amplifier immediately after the optical receiver. At this time, according to the low-pass filter characteristics of the amplifier, i represented by equation B-8 is pol is integrated with respect to time t. In the case of ideal integration, the output current i pol_LPF It is represented by the following formula B-9. The amplifier can be an example of an integrating circuit.

[0181] (Formula B-9)

[0182] i pol_LPF =-RE y 2 (t)δ(t)

[0183] Equation B-9 holds true within the range where the low-pass filter operates as an integrating circuit. That is, Equation B-9 holds true for frequencies above the cutoff frequency of the low-pass filter. In this case, if the contribution of R is normalized in Equation B-9 using the power ratio α (0≤α≤1) of the orthogonal polarization signal defined by the following Equations B-10 and B-11, the output current i of the amplifier after normalization is pol_LPF_norm It is represented by the following formula B-12. Thus, by using an optical delay interferometer, δ(t) can be directly measured.

[0184] (Formula B-10)

[0185] Ex 2 (t) = αE 2

[0186] (Formula B-11)

[0187] Ey 2 (t) = (1-α)E 2

[0188] (Formula B-12)

[0189]

Mathematical formula 11

[0190]

[0191] In addition, the term caused by phase noise in equation B-6 is a random process, so the term caused by phase noise is not phase continuous. Therefore, the integration effect in the amplifier does not act on the term caused by phase noise. As a result, the term caused by phase noise in equation B-6 is output from the amplifier as it is in equation B-6.

[0192] As shown in Formula B-6, since the above-mentioned reception current i is measured in a state where the term caused by phase noise (sometimes referred to as phase noise component) and the term caused by polarization variation (sometimes referred to as polarization variation component) are mixed, the measurement of the phase noise component is preferably performed during a period when the polarization variation component is small. Therefore, for example, by outputting an alarm when a polarization variation is detected, or by giving a flag indicating that a polarization variation is detected to the measurement data of the reception current i, or by recording the time when the polarization variation is detected, the measurement and / or analysis of the phase noise can be effectively performed.

[0193] (B. When δ(t) changes greatly)

[0194] In this case, the condition that the absolute value of the difference between δ(t) and δ(t-τ) is much smaller than 1 may not be satisfied. Therefore, the above formula B-12 may not be satisfied. However, even in this case, formula B-7 is satisfied. Therefore, when the output current i of the optical receiver connected to the optical delay interferometer is amplified by, for example, an amplifier immediately after the optical receiver, δ(t) is integrated while being limited by the sine function.

[0195] Here, since δ(t) varies within the range of -π / 2≤δ(t)≤π / 2, the measurement result of the optical delay interferometer (i represented by equation B-7) is pol The value of sin{δ(t)-δ(t-τ)} in the equation B-7 varies with respect to the maximum change of δ(t) within the range -1≤sin{δ(t)-δ(t-τ)}≤1. pol When the measurement result is Fourier transformed, the spectral components become the varying amplitude values.

[0196] (Trajectory on the Poincare sphere)

[0197] It is known that a point P (S1, S2, S3) on the Poincare sphere represented by Stokes parameters S0, S1, S2, and S3 represents the polarization state of light. x (t) and E y The relationship between (t), δ(t) and Stokes parameters S0, S1, S2 and S3 is expressed by the following equations B-13, B-14, B-15 and B-16.

[0198] (Formula B-13)

[0199]

Mathematical formula 12

[0200]

[0201] (Formula B-14)

[0202]

Mathematical formula 13

[0203]

[0204] (Formula B-15)

[0205]

Mathematical formula 14

[0206] S2=2E x (t)E y (t)cosδ(t)

[0207] =s o cos2χsin2ψ

[0208] (Formula B-16)

[0209]

Mathematical formula 15

[0210] s3=2E x (t)E y (t)sinδ(t)=S0 sin 2χ

[0211] In formulas B-13, B-14, B-15 and B-16, χ represents ellipticity. ψ represents the orientation of elliptically polarized light. Formulas B-13, B-14, B-15 and B-16 represent the relationship between rectangular coordinates (S1, S2, S3) and polar coordinates (S0, 2χ, 2ψ).

[0212] The sphere of radius S0 in rectangular coordinates (S1, S2, S3) is called the Poincare sphere. The longitude of the Poincare sphere represents twice the azimuth ψ, and the latitude of the Poincare sphere represents twice the ellipticity χ. For example, the angle UOQ formed by the origin O (0, 0, 0) of the rectangular coordinates, the point U (S1, S2, 0) on the Poincare sphere, and the point Q (S1, 0, 0) on the Poincare sphere is 2ψ. Similarly, the angle POQ formed by the origin O (0, 0, 0) of the rectangular coordinates, the point U (S1, S2, 0) on the Poincare sphere, and the point P (S1, S2, S3) on the Poincare sphere is 2χ.

[0213] Here, as shown in equations B-15 and B-16, point P (S1, S2, S3) on the Poincare sphere moves on the circle R centered at point Q (S1, 0, 0) as δ(t) changes. Therefore, consider the case where point P on the Poincare sphere moves slightly to point P' on the circle R as δ(t) changes. If the angle POP' formed by point P, origin O, and point P' is dθ, and the slight deviation of δ(t) is dδ, the relationship between dθ and dδ is expressed by the following equation B-17. In addition, dδ represents the angle PQP' formed by point P, point Q, and point P'.

[0214] (Formula B-17)

[0215]

Mathematical formula 16

[0216]

[0217] 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 into the above dθ, the result is dθ DL , then dθ DL Using the above power ratio α, it is expressed as shown in the following formula B-18.

[0218] (Formula B-18)

[0219]

Mathematical formula 17

[0220]

[0221] Therefore, according to the method using the optical delay interferometer, the phase change amount on the Poincare sphere measured is c(t) times the shift angle on the Poincare sphere. c(t) is represented by B-19 below.

[0222] (Formula B-19)

[0223]

Mathematical formula 18

[0224]

[0225] The relationship between c(t) and the above α is expressed by the following formula B-20. In this case, α can be greater than 0 and less than 1. In addition, when α is 0 or 1, the electric field is concentrated on HE11x or HE11y, and becomes completely linearly polarized. Therefore, when α is 0 or 1, there is no need to consider the concept of polarization change.

[0226] (Formula B-20)

[0227]

Mathematical formula 19

[0228]

[0229] As described above, according to the present principle, the phase change amplitude measured by the optical delay interferometer represents the approximate value of the moving angle (i.e., the phase change amount) of the trajectory on the Poincare sphere. The above phase change amplitude is obtained as the amplitude value of the frequency component that gives the peak-to-peak value in the Fourier transform result of the optical receiver output current.

[0230] (Overview of Communication System 100)

[0231] Figure 1 This schematically shows an example of the system configuration of the communication system 100. In this embodiment, the details of the communication system 100 are described by taking as an example a case where a signal light outputted from the optical signal transmitting device 110 propagates through the optical transmission path 10 and reaches the optical signal receiving device 120 to transmit an information signal.

[0232] In this embodiment, light is transmitted through the optical transmission path 10. As the optical transmission path 10, an optical fiber is exemplified.

[0233] In this embodiment, communication system 100 includes optical signal transmitter 110 and optical signal receiver 120. In this embodiment, optical signal receiver 120 includes local oscillator 130, optical 90-degree hybrid 140, optical receiver 152, optical receiver 154, AD converter 162, AD converter 164, and signal processor 170.

[0234] 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 by, for example, modulating an optical carrier with an information signal to be transmitted. The optical signal may be a signal with a symbol time sequence superimposed. The optical signal transmitting device 110 outputs light constituting an 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.

[0235] 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.

[0236] During the propagation of the signal light in the optical transmission path 10, the signal light is affected by various linear effects and nonlinear effects. When the signal light is affected by the nonlinear effect, 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 above-mentioned index can be an index 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 above-mentioned index. As an index representing the degree of deviation, variance or standard deviation can be exemplified.

[0237] In the present embodiment, the local oscillator 130 outputs local oscillation light (as described above, sometimes referred to as local oscillation light). The local oscillation light output from the local oscillator 130 is input to the optical 90-degree hybrid 140 .

[0238] In this embodiment, the optical 90-degree hybrid 140 causes interference between the signal light propagating in the optical transmission path 10 and the local oscillator light from the local oscillator 130, and outputs a plurality of optical signals that separate the received signal transmitted by the signal light into a plurality of signal components. In this embodiment, the optical 90-degree hybrid 140 mixes the signal light and the local oscillator light, and outputs two optical signals with a phase difference of 90 degrees. The above two signals can be respectively referred to as an I signal and a Q signal.

[0239] 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 by the signal light into an electrical signal. Thus, the I signal is converted from the optical signal into an electrical signal. The optical receiver 152 outputs the electrical signal corresponding to the I signal to the AD converter 162.

[0240] 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 by the signal light into an electrical signal. Thus, the Q signal is converted from the optical signal into an electrical signal. The optical receiver 154 outputs the electrical signal corresponding to the Q signal to the AD converter 164.

[0241] 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 a plurality of digital signals (sometimes referred to as sampled I signals) corresponding to a plurality of time points during a measurement period (sometimes referred to as an evaluation period) for deriving an evaluation index of phase noise to the signal processing unit 170. The time intervals between the plurality of time points may be substantially constant.

[0242] The sampling rate of the AD converter 162 may be greater than the symbol rate of the received signal. Thus, the sample time interval is set to be less than the length of the symbol time. The sampling rate of the AD converter 162 may be the same as the symbol rate of the received signal. Thus, the sample time interval is set to be the length of the symbol time.

[0243] 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 a plurality of digital signals (sometimes referred to as sampled Q signals) corresponding to a plurality of time points during the evaluation period to the signal processing unit 170. The time intervals between the plurality of time points may be substantially constant.

[0244] The sampling rate of the AD converter 164 may be greater than the symbol rate of the received signal. Thus, the sample time interval is set to be less than the length of the symbol time. The sampling rate of the AD converter 164 may be the same as the symbol rate of the received signal. Thus, the sample time interval is set to be the length of the symbol time.

[0245] In this embodiment, the signal processing unit 170 demodulates the received signal transmitted by the signal light to generate an information signal. In addition, 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 may 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.

[0246] (Specific Configuration of Each Component of Communication System 100)

[0247] Each part of the communication system 100 may be implemented by hardware, software, or both. At least a portion of each part of the communication system 100 may be implemented by an analog circuit or a digital circuit. At least a portion of each part of the communication system 100 may be implemented by a single server or by multiple servers. At least a portion of each part of the communication system 100 may also be implemented on a virtual machine or a cloud system. At least a portion of each part of the communication system 100 may also be implemented by a personal computer or a portable terminal. Examples of portable terminals include mobile phones, smart phones, PDAs, tablet computers, notebook computers or laptop computers, wearable computers, and the like. Each part of the communication system 100 may also use distributed ledger technologies such as blockchain or distributed networks to store information.

[0248] When at least a part of the components constituting the communication system 100 is implemented by software, the components implemented by the software can be implemented by starting a program that specifies actions related to the components in a generally configured information processing device. The above-mentioned information processing device includes, for example: (i) a data processing device having a processor such as a CPU and a GPU, a ROM, a RAM, a communication interface, etc.; (ii) an input device such as a keyboard, a touch panel, a camera, a microphone, various sensors, a GPS receiver, etc.; (iii) an output device such as a display device, a speaker, a vibration device, etc.; and (iv) a storage device such as a memory and a HDD (including an external storage device).

[0249] In the above-mentioned information processing device, the above-mentioned data processing device or storage device may store a program. The above-mentioned program may be stored in a non-transitory computer-readable recording medium. The above-mentioned program is executed by the processor to make the above-mentioned information processing device perform the action specified by the program.

[0250] The program may be stored in a computer-readable medium such as a CD-ROM, a DVD-ROM, a memory, a hard disk, or a storage device connected to a network. The program may be installed from a computer-readable medium or a storage device connected to a network to a computer constituting at least a part of the communication system 100. By executing the program, the computer may also function as at least a part of each part of the communication system 100.

[0251] The program that causes the computer to function as at least a part of each part of the communication system 100 may include a module that specifies the operation of each part of the communication system 100. These programs or modules activate the data processing device, input device, output device, storage device, etc., and cause the computer to function as each part of the communication system 100, or cause the computer to execute the information processing method in each part of the communication system 100.

[0252] The information processing described in the program is read into the computer, so that the software associated with the program and various hardware resources of the communication system 100 function as specific units that cooperate. In addition, the above-mentioned specific units construct the communication system 100 corresponding to the purpose of use by realizing the calculation or processing of information corresponding to the purpose of use of the computer in this embodiment.

[0253] The program may be a program for causing a computer to function as the optical signal receiving device 120 or a part thereof. The program may also be a program for causing a computer to execute an information processing method in the optical signal receiving device 120 or a part thereof. As a part of the optical signal receiving device 120, an AD converter 162, an AD converter 164, a signal processing unit 170, etc. are exemplified.

[0254] In one embodiment, the information processing method may be an evaluation method for evaluating the phase noise of a signal light propagating in an optical transmission path. The evaluation method may include, for example, a differential phase information acquisition stage for acquiring 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 for deriving 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 represents, for example, the difference in phase between the input light at two temporally adjacent time points among the plurality of time points. In the evaluation method, the time intervals between the plurality of time points are, for example, approximately constant. The time intervals between the plurality of time points are, for example, less than the length of the symbol time of the signal transmitted by the input light.

[0255] The communication system 100 may be an example of an optical communication system. The optical signal transmitting device 110 may be an example of an optical transmitter. The optical signal receiving device 120 may be an example of an evaluation device or an optical receiver. The local oscillator 130 may be an example of a local light source. The optical receiver 152 may be an example of a photoelectric conversion unit. The optical receiver 154 may be an example of a photoelectric conversion unit. The AD converter 162 may be an example of an analog-to-digital conversion unit. The AD converter 164 may be an example of an analog-to-digital conversion unit. The signal processing unit 170 may be an example of an evaluation device. The I signal may be an example of an optical signal of an I signal component. The sampled I signal may be an example of a first digital signal. The Q signal may be an example of an optical signal of a Q signal component. The sampled Q signal may be an example of a second digital signal. The object light may be an example of input light. The light constituting various optical signals may be an example of input light.

[0256] Figure 2An example of the internal structure of the signal processing unit 170 is schematically shown. In the present 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 the present 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.

[0257] In this embodiment, the digital signal processing circuit 210 receives the sampled I signal from the AD converter 162. The digital signal processing circuit 210 receives the sampled Q signal from the AD converter 164. The digital signal processing circuit 210 uses the I signal and the Q signal to perform various digital signal processing for demodulating or compensating the received signal. In this embodiment, the decoding circuit 220 performs error correction processing, decoding processing, etc. Thus, the information signal is extracted from the received signal.

[0258] In the present 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 indices for evaluating the amplitude noise of the signal light propagating in the optical transmission path 10. As the above-mentioned indices, an optical signal-to-noise ratio and the like are exemplified. The optical signal-to-noise ratio is measured using at least one of a sampled I signal and a sampled Q signal. For example, when the optical signal-to-noise ratio of the signal light is measured using a sampled I signal, the I signal is input to the amplitude noise evaluation unit 230. The optical signal-to-noise ratio can be measured by various known methods. For example, the amplitude noise evaluation unit 230 includes an average value calculation unit, a bar graph generation unit, and a standard deviation calculation unit. Thus, the amplitude noise of the signal light is measured.

[0259] 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 indices for evaluating the phase noise of the signal light propagating in the optical transmission path 10. As described above, as the above-mentioned index, the degree of deviation of the differential phase can be exemplified. As an index indicating the degree of deviation, the variance, the standard deviation, etc. can be exemplified.

[0260] In addition, in this embodiment, the phase noise evaluation unit 240 is described by taking the case where the phase noise evaluation unit 240 derives the standard deviation of the differential phase during the evaluation period as an index representing the degree of deviation of the measured differential phase as an example. 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 index representing the degree of deviation of the differential phase.

[0261] In this embodiment, the differential phase signal generating unit 242 obtains, for example, I signal data and Q signal data (sometimes referred to as input signals) at 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 plurality of time points included in the evaluation period.

[0262] The differential phase signal generating 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 Q signal at each of the N sample points. The phase of the signal light at the i-th (i is an integer greater than or equal to 1 and less than or equal to (N-1)) sample point is determined, for example, according to the above-mentioned formula 4.

[0263] The differential phase signal generating unit 242 calculates the differential phase value of each of the N sample points based on the measured value of the phase of the signal light at each of the N sample points. The differential phase value of the i-th sample point is For example, calculation is performed according to the above-mentioned formula 3.

[0264] The phase difference between the signal light at two temporally adjacent time points among the plurality of time points is calculated according to Formula 3. The differential phase signal generation unit 242 can obtain information indicating the differential phase of the signal light at each of the plurality of time points included in the evaluation period.

[0265] The differential phase signal generating unit 242 can remove the modulation component and generate information indicating the differential phase of each of the N sample points. The differential phase signal generating unit 242 removes the modulation component included in the received signal based on a plurality of sampled I signals and Q signals, for example. As described above, in the case of M-phase phase modulation, the modulation component can be eliminated by calculating the Mth power of the received signal.

[0266] In the present embodiment, the histogram generator 244 obtains information indicating (N-1) differential phases (sometimes referred to as differential phase signals) from the differential phase signal generator 242. The histogram generator 244 generates a histogram of the (N-1) differential phases. The histogram generator 244 outputs information indicating the generated histogram to the standard deviation calculator 246.

[0267] 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 a normal distribution on the histogram generated by the histogram generation unit 244. In addition, the standard deviation calculation unit 246 calculates the standard deviation of the normal distribution.

[0268] 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 Formula 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 according to Formula 9 based on the standard deviations of the (N-1) differential phases.

[0269] In addition, as shown in Formula 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 line width of the local oscillator light. The standard deviation calculation unit 246 can derive the standard deviation of the phase noise of the received signal according to Formulas 9 to 11 based on the standard deviations of the N differential phases.

[0270] The standard deviation calculation unit 246 can generate information for evaluating the phase noise of the signal light (sometimes referred to as evaluation information). As information for evaluating the phase noise of the signal light, information indicating the standard deviation of 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 can be exemplified.

[0271] The phase noise evaluation unit 240 may be an example of an evaluation device. The differential phase signal generation unit 242 may be an example of a differential phase information acquisition unit. The histogram generation unit 244 may be an example of a differential phase information acquisition unit. The standard deviation calculation unit 246 may be an example of an index derivation unit.

[0272] Figure 3 An example of the system configuration of the phase noise evaluation device 320 is schematically shown. In the present 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 .

[0273] In with Figure 1 and Figure 2 In the communication system 100 described above, the optical signal receiving device 120 performs high-speed A / D conversion and digital signal processing, thereby calculating the standard deviation of the differential phase in the evaluation mechanism. In this embodiment, high-speed calculation processing is performed on the electrical signal in the optical signal receiving device 120 at an optical level, which is different from the above. Figure 1 and Figure 2 The optical signal receiving device 120 described in the related description is different.

[0274] In this embodiment, phase noise evaluation device 320 outputs the degree of deviation of the differential phase of light (sometimes referred to as input light) input to phase noise evaluation device 320 as an index for evaluating the phase noise of the input light. Thus, phase noise evaluation device 320 can evaluate the phase noise of the input light.

[0275] The input light may be signal light propagating in the optical transmission path 10 or output light of the optical 90-degree hybrid 140. The phase noise evaluation device 320 is installed in the optical signal receiving device 120, for example, and a part of the signal light propagating in the optical transmission path 10 is input to the phase noise evaluation device 320.

[0276] In the signal light used in actual communication, the optical signal has a spectral component due to modulation. Therefore, the modulation component may be mixed into the measurement result, making the measurement of optical phase noise difficult.

[0277] Therefore, in one embodiment, the phase noise evaluation device 320 is installed in the communication system 100, for example, to evaluate the phase noise in a test run after the communication system 100 is constructed. In other embodiments, the phase noise evaluation device 320 is installed in the communication system 100 so as to set a dedicated wavelength for measuring the phase noise during the operation of the communication system 100 and always measure the phase noise of the light of the wavelength. For example, in an embodiment in which the differential phase is derived by signal processing of digital data, the phase noise is evaluated by measuring the differential phase of the I component and the Q component of the light of the wavelength.

[0278] 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 has, for example, a first optical path and a second optical path. The optical delay interferometer 340, for example, branches the input light into the first input light and the second input light. The optical delay interferometer 340, for example, combines the first input light that has passed through the first optical path with the second input light that has passed through the second optical path to cause interference.

[0279] In this embodiment, the first optical path and the second optical path of the optical delay interferometer 340 are set so that the delay 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 relationship of the frequency f of the input light as shown in the following equation 12.

[0280] (Formula 12)

[0281] 2πfτ=2nπ+π / 2 (where n is an integer)

[0282] When the relationship of the above-mentioned Expression 12 holds true, the output current i of the optical receiver 350 to which the output light of the optical delay interferometer 340 is input is approximated by the following Expression 13.

[0283] (Formula 13)

[0284]

[0285] In the above-mentioned Equation 12, R is the sensitivity of the optical receiver 350. E is the electric field of the first input light and the second input light. is the phase noise of the input light at time t.

[0286] 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. Thus, the optical receiver 350 can obtain 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 difference between the phases of the input light at two time points that are temporally adjacent among the multiple time points.

[0287] 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 a plurality of digital signals corresponding to the differential phase of each of the plurality of time points included in the evaluation period to the signal processing unit 370. The time intervals between the plurality of time points may be substantially constant.

[0288] 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 that 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.

[0289] As shown in the above formula 13, when the optical delay interferometer 340 is used, the value of the differential phase converted into the intensity is measured. That is, the differential phase noise is converted into amplitude noise by the optical delay interferometer 340. In addition, the frequency component of the differential phase noise is approximately white noise. In view of the above situation, as the optical receiver 350, an optical receiver with a narrow frequency band can be used. Specifically, an optical receiver with a frequency band of about 100 MHz is used as the optical receiver 350. As a result, the sampling rate of the AD converter 360 that samples the output current of the optical receiver 350 can also be reduced.

[0290] In the present embodiment, the signal processing unit 370 receives the above-mentioned multiple 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 multiple time points as an indicator for evaluating the phase noise of the input light. As described above, as an indicator representing the degree of deviation, a standard deviation, a variance, etc. can be exemplified. In addition, as shown in Formula 6, the square of the standard deviation of the phase noise of the input light is expressed as 1 / 2 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). The details of the signal processing unit 370 will be described later.

[0291] In this embodiment, high-speed computation processing is performed on the electrical signal in the optical signal receiving device 120 at an optical level. This can achieve power saving and cost reduction. In addition, in this embodiment, the reciprocal of the delay time is equivalent to the sampling rate in the AD converter. Therefore, the power used for sampling can be saved.

[0292] The phase noise evaluation device 320 may be an example of an evaluation device. The optical delay interferometer 340 may be an example of a delay interferometer. The optical receiver 350 may be an example of a differential phase information acquisition unit or a photoelectric conversion unit. The AD converter 360 may be an example of a differential phase information acquisition unit or an analog-to-digital conversion unit. The signal processing unit 370 may be an example of a differential phase information acquisition unit or an index derivation unit.

[0293] (An example of another embodiment)

[0294] In this embodiment, an example of the phase noise evaluation device 320 is described by taking the case where the phase noise evaluation device 320 includes the AD converter 360 and the 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.

[0295] Figure 4 An example of the internal structure of the optical delay interferometer 340 is schematically shown. In the present embodiment, the optical delay interferometer 340 includes a half mirror 422 , a half mirror 424 , a total reflection mirror 432 , a total reflection mirror 434 , and an optical phase adjuster 436 .

[0296] The input light is branched into a first signal light transmitted through the semi-mirror 422 and a second signal light reflected by the semi-mirror 422. The first signal light is transmitted through the semi-mirror 424 and output from the optical delay interferometer 340. On the other hand, the second signal light is reflected by the total reflection mirror 432, the total reflection mirror 434, and the semi-mirror 424, and then mixed with the first signal light transmitted through the semi-mirror 424 and output from the optical delay interferometer 340.

[0297] In this embodiment, the optical delay interferometer 340 provides an optical path difference between the first signal light and the second signal light. If the delay time difference caused by the optical path difference is τ, the time is t, and the phase when the first signal light is input to the optical receiver 350 is Then the phase when the second signal light is input to the optical receiver 350 is The delay time difference τ is adjusted so as to satisfy the relationship of the above-mentioned equation 8.

[0298] In addition, in the present embodiment, an optical phase adjuster 436 is arranged in a region of the optical path of the second signal light (the second optical path) that does not overlap with the optical path of the first signal light (the first optical path). The optical phase adjuster 436 adjusts the phase of the light passing through the optical phase adjuster 436. For example, the optical phase adjuster 436 adjusts the phase of the light passing through the optical phase adjuster 436 according to a voltage applied to the optical phase adjuster 436. As examples of the optical phase adjuster 436, a liquid phase element, a lithium niobate crystal, etc. can be exemplified.

[0299] According to this embodiment, the delay time difference τ is adjusted by adjusting the distance between the first optical path and the second optical path and / or the voltage applied to the optical phase adjuster 436. Thus, the operating point of the optical delay interferometer 340 can be adjusted. The delay time difference τ can be adjusted during calibration or in real time.

[0300] In other embodiments, the optical phase adjuster 436 may be disposed in a region of the first optical path that does not overlap with the second optical path. In addition, the optical delay interferometer 340 may not include the optical phase adjuster 436 .

[0301] Figure 5 An example of the internal structure of the optical delay interferometer 540 is schematically shown. The optical delay interferometer 540 is another example of the optical delay interferometer 340 , and can be mounted in the phase noise evaluation device 320 instead of the optical delay interferometer 340 .

[0302] In this embodiment, the optical delay interferometer 540 includes a substrate 510, and a waveguide 520 and a waveguide 530 formed on the substrate 510. The substrate 510 may be a semiconductor substrate such as a silicon substrate.

[0303] The input light is branched into the waveguide 520 and the waveguide 530 at the branch point. The first signal light propagating in the waveguide 520 and the second signal light propagating in the waveguide 530 are combined and output from the optical delay interferometer 540. The delay time difference τ between the waveguide 520 and the waveguide 530 is adjusted to satisfy the relationship of the above equation 8.

[0304] 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 light passing through the waveguide 520 is finely adjusted.

[0305] According to this embodiment, the delay time difference τ is adjusted by adjusting the distance between the waveguide 520 and the waveguide 530 and / or the voltage applied to the electrode 526. Thus, the operating point of the optical delay interferometer 540 can be adjusted. The delay time difference τ can be adjusted during calibration or in real time.

[0306] In other embodiments, the electrode 526 may be disposed on the waveguide 530. In addition, the optical delay interferometer 540 may not include the electrode 526.

[0307] Figure 6 An example of the internal structure of the optical delay interferometer 640 is schematically shown. In this embodiment, the optical delay interferometer 640 includes a substrate 610, a waveguide 620 for inputting input light, a ring resonator 630 arranged adjacent to the waveguide 620, and an electrode 636 arranged on a part of the ring resonator 630. The substrate 610 may be a semiconductor substrate such as a silicon substrate.

[0308] Input light incident from the input terminal propagates in the waveguide 620 and reaches the ring resonator 630. Then, a part of the input light is introduced into the ring resonator 630. The input-output characteristic of the ring resonator 630 is set so that the length of the ring is an integral multiple of the wavelength of the input light in the ring medium.

[0309] By using the ring resonator 630, the output current relative to the optical frequency becomes steeper particularly near the operating point. The light passing through the waveguide 620 is output from the optical delay interferometer 640 and input to the optical receiver 350. The optical phase in the ring resonator 630 is adjusted by the voltage applied to the electrode 636. Thus, the operating point of the optical delay interferometer 640 can be adjusted.

[0310] Figure 7 An example of the internal configuration of the signal processing unit 370 is schematically shown. In the present embodiment, the signal processing unit 370 includes a histogram generating unit 244 , a standard deviation calculating unit 246 , and a correcting unit 710 .

[0311] 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 uses the signal light with known phase noise to perform the calibration process of the phase noise evaluation device 320. Thus, the receiving sensitivity of the optical receiver 350 and the noise of the circuit of the optical receiver 350 are corrected.

[0312] For example, the variance of the differential phase noise is derived by subtracting the variance of the circuit noise from the variance of the received signal. In addition, the intensity noise corresponding to the statistical distribution of the phase noise is measured by the above-mentioned derivation method. Therefore, the measurement results such as the above-mentioned various variances and the standard deviation derived from the variances can be calibrated in advance with the known optical phase noise as the reference. This is also the case when using an optical ring resonator.

[0313] Figure 8 An example of the system configuration of the phase noise evaluation device 820 is schematically shown. In the present embodiment, the phase noise evaluation device 820 is different from the phase noise evaluation device 320 in that it includes an optical receiver 850, an AD converter 860, and an optical phase control unit 880. In addition, according to the present 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 optical receiver 850. Furthermore, in the present embodiment, the phase noise evaluation device 820 is different from the phase noise evaluation device 320 in that it includes a signal processing unit 870 instead of the signal processing unit 370. With respect to features other than the above-mentioned differences, the phase noise evaluation device 820 may have the same configuration as the phase noise evaluation device 320.

[0314] In this embodiment, the optical receiver 850 converts the input second light into an electric signal. In this embodiment, the AD converter 860 converts the electric signal output by the optical receiver 850 from an analog signal into a digital signal. Thus, information indicating the measured value of the optical power of the input light is obtained.

[0315] In this embodiment, the signal processing unit 870 obtains information indicating 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.

[0316] 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 electrical signal.

[0317] 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 variation of the output current to the variation of the optical frequency is greater than a predetermined value becomes the operating point of the optical delay interferometer 340. The optical phase control unit 880 may also adjust the operating point of the optical delay interferometer 340 so that the point where the ratio of the variation of the output current to the variation of the optical frequency is substantially the largest becomes the operating point of the optical delay interferometer 340.

[0318] The optical frequency to output current characteristic of the optical delay interferometer 340 is sinusoidal relative to the optical frequency, and its period is 1 / τ. Therefore, it can be seen that in the above-mentioned graph of the optical frequency to output current characteristic, when the point where the above-mentioned sine wave intersects with 0 is set as the operating point, the ratio of the change amount of the output current to the change amount of the optical frequency is approximately the largest.

[0319] 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 optical frequency to output current characteristic (that is, the graph in which the vertical axis is the output current and the horizontal axis is the optical frequency) can be replaced by the differential phase, the change of the differential phase directly becomes the change of the output current.

[0320] The AD converter 860 may be an example of an optoelectronic information acquisition unit. The signal processing unit 870 may be an example of an optoelectronic information acquisition unit. The optical phase control unit 880 may be an example of an adjustment unit.

[0321] (An example of another embodiment)

[0322] In this embodiment, an example of the optical phase control unit 880 is described by 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 so that the average value becomes 0. However, the optical phase control unit 880 is not limited to this embodiment.

[0323] 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 so that the value becomes maximum. Similarly, the optical phase control unit 880 can detect the value of the amplitude of the output current of the optical receiver 350 and set the operating point of the optical delay interferometer 340 so that the value becomes maximum.

[0324] Fig. 9 An example of the internal configuration of the signal processing unit 870 is schematically shown. In the present embodiment, the signal processing unit 870 includes a histogram generating unit 244 , a standard deviation calculating unit 246 , a correcting unit 710 , and a normalizing unit 930 .

[0325] In this embodiment, the normalization unit 930 obtains information indicating the measured value of the optical power of the input light from the AD converter 860. The normalization unit 930 normalizes the differential phase of the input light using the measured value of the optical power of the input light. The normalization unit 930 outputs information indicating the normalized differential phase of the input light to the histogram generation unit 244.

[0326] In the present embodiment, the histogram generation unit 244 generates a histogram of the differential phase using the normalized differential phase. In the present embodiment, the standard deviation calculation unit 246 derives the standard deviation of the differential phase using the histogram generated based on the normalized differential phase.

[0327] Fig.10 An example of the internal structure of the optical delay interferometer 1040 is schematically shown. In this embodiment, the optical delay interferometer 1040 is different from the optical delay interferometer 340 in that a half mirror 1060 is provided for branching the input light and inputting a part of the input light into the optical receiver 850. In addition, in this embodiment, the optical delay interferometer 1040 is different 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.

[0328] Fig.11 An example of the internal structure of the optical delay interferometer 1140 is schematically shown. In this embodiment, the optical delay interferometer 1140 is different from the optical delay interferometer 540 in that a waveguide 1160 is provided for branching input light and inputting a part of the input light into the optical receiver 850. In addition, in this embodiment, the optical delay interferometer 1140 is different from the optical delay interferometer 540 in that the voltage applied to the electrode 526 is controlled by the optical phase control unit 880.

[0329] Fig.12 An example of the internal structure of the optical delay interferometer 1240 is schematically shown. In this embodiment, the optical delay interferometer 1240 is different from the optical delay interferometer 640 in that a waveguide 1260 is provided for branching input light and inputting a part of the input light into the optical receiver 850. In addition, in this embodiment, the optical delay interferometer 1240 is different from the optical delay interferometer 640 in that the voltage applied to the electrode 636 is controlled by the optical phase control unit 880.

[0330] Fig.13 An example of the internal structure of the optical delay interferometer 1340 is schematically shown. In this embodiment, the optical delay interferometer 1340 is different from the optical delay interferometer 340 in that a balanced optical receiver 1350 is used instead of the optical receiver 350. This can suppress the intensity noise included in the signal light.

[0331] 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 a combination of a first signal light transmitted through the half mirror 424 and a second signal light reflected by the half mirror 424. The optical receiver 1354 receives a second output light, which is a combination of a first signal light reflected through the half mirror 424 and a second signal light transmitted through the half mirror 424. The differential processing unit 1356 outputs an electrical signal corresponding to the difference between the output of the optical receiver 1352 and the output of the optical receiver 1354 to the AD converter 360.

[0332] Fig.14 An example of the circuit configuration of the balanced optical receiver 1350 is schematically shown. In this embodiment, the balanced optical receiver 1350 includes a photodiode 1452 and a photodiode 1454 connected in series. The balanced optical receiver 1350 outputs an electrical signal from a connection point 1456 between the photodiode 1452 and the photodiode 1454.

[0333] Fig.15 An example of the internal structure of the optical delay interferometer 1540 is schematically shown. In this embodiment, the optical delay interferometer 1540 is different from the optical delay interferometer 540 in that a balanced optical receiver 1350 is used instead of the optical receiver 350. This can suppress the intensity noise included in the signal light.

[0334] (Overview of Communication System 1600)

[0335] Fig.16 An example of the system configuration of the communication system 1600 is schematically shown. In this embodiment, the details of the communication system 100 are described by taking as an example the case where the signal light output by the optical signal transmitting device 110 propagates through the optical transmission path 10 and reaches the optical signal receiving device 1620 to transmit the information signal.

[0336] In this embodiment, the communication system 1600 may have the same configuration as the communication system 100 except that the communication system 1600 has a configuration for detecting polarization variation. Regarding the details of the communication system 1600, description of the configurations that are the same as those of the communication system 100 may be omitted.

[0337] In this embodiment, the communication system 1600 includes, for example, the optical signal transmission device 110 and the optical signal reception device 1620. In this embodiment, the optical signal reception device 1620 includes a demultiplexer 1640, an optical receiver 1650, an AD converter 1660, a signal processing unit 1670, and a state monitoring device 1680.

[0338] 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.

[0339] As described above, for example, when a sudden abnormality occurs in the optical transmission line 10 while the signal light propagates 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 a modified example thereof, performs statistical processing on the measurement results of the phase change, thereby evaluating the phase noise generated in the communication system 100.

[0340] However, compared with phase noise, phase variations caused by polarization variations tend to occur suddenly. Since sudden phase variations are difficult to reflect in the above statistical processing results, 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 variations.

[0341] Therefore, according to the present embodiment, the optical signal receiving device 1620 always measures the phase variation generated in the communication system 1600. The optical signal receiving device 1620 detects the phase variation caused by the sudden polarization variation based on the real-time measurement result of the phase noise. For example, the optical signal receiving device 1620 detects the increase of the real-time phase variation and issues an alarm. The information processing in the optical signal receiving device 1620 will be described in detail later.

[0342] In this embodiment, when the signal light includes a plurality of optical signals of different wavelengths, the demultiplexer 1640 demultiplexes the plurality of optical signals by wavelength. The optical signal received by the optical signal receiving device 1620 (sometimes referred to as a received signal) includes, for example, a light of a first wavelength for transmitting an information signal (sometimes referred to as a communication optical signal) and a light of a second wavelength for detecting polarization variation (sometimes referred to as a monitoring optical signal). The value of the first wavelength is different from the value of the second wavelength. The demultiplexer 1640 outputs the communication optical signal to the optical receiver 1650. The demultiplexer 1640 outputs the monitoring optical signal to the status monitoring device 1680.

[0343] 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 .

[0344] In this embodiment, the AD converter 1660 converts the electrical signal from an analog signal to a digital signal. The AD converter 1660 generates one or more digital signals corresponding to one or more sample points (sometimes referred to as samples) by, for example, sampling the electrical signal. The AD converter 1660 converts the electrical signal corresponding to the communication optical signal into a digital signal (sometimes referred to as a digital signal corresponding to the communication optical signal). The AD converter 1660 outputs the digital signal corresponding to the communication optical signal to the signal processing unit 1670.

[0345] In this embodiment, the signal processing unit 1670 demodulates the received signal transmitted by the signal light and generates an information signal. The method for generating the information signal from the received signal is not particularly limited. The signal processing unit 1670 can perform various digital signal processing for demodulating or compensating the received signal. The signal processing unit 1670 can perform error correction processing, decoding processing, etc.

[0346] In this embodiment, the state monitoring device 1680 analyzes the light input to the state monitoring device 1680 (sometimes referred to as input light) 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. The state monitoring device 1680 analyzes the phase variation of the input light, for example, and 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 state monitoring device 1680 can detect the polarization variation of the signal light propagating in the optical transmission path 10, for example.

[0347] The state monitoring device 1680 obtains information indicating the differential phase of the input light to be evaluated at each of one or more time points, for example. The differential phase indicates the difference in phase between the input light at two temporally adjacent time points among the one or more time points. At one or more time points, the time interval between the two temporally adjacent time points may be substantially constant.

[0348] The length of the time interval is not particularly limited, but the time interval may be the length of the symbol time of the signal transmitted by the input light, may be less than the length of the symbol time, or may be greater than the length of the symbol time. In one embodiment, the time interval is set so that the difference with the symbol time becomes a predetermined value. In other embodiments, the time interval is set so that the difference with the symbol time becomes less than or smaller than a predetermined value.

[0349] The state monitoring device 1680 determines, for example, whether the differential phase of at least a part of the one or more time points satisfies a predetermined condition. When it is determined that the differential phase of at least a part of the one or more time points satisfies the predetermined condition, the state monitoring device 1680 outputs, for example, information indicating that the signal light has undergone a polarization change. The state monitoring device 1680 will be described in detail later.

[0350] (Specific Configuration of Each Component of Communication System 1600)

[0351] As with the components of the communication system 100, the components of the communication system 1600 may be implemented by hardware, software, or both. When at least a portion of the components constituting the communication system 1600 is implemented by software, the components implemented by the software may be implemented by activating a program that specifies operations related to the components in an information processing device of a general configuration.

[0352] In the above-mentioned information processing device, the above-mentioned data processing device or storage device may store a program. The above-mentioned program may be stored in a non-transitory computer-readable recording medium. The above-mentioned program is executed by the processor to make the above-mentioned information processing device perform the action specified by the program.

[0353] The program may be a program for causing a computer to function as the optical signal receiving device 1620 or a part thereof. The program may also be a program for causing a computer to execute an information processing method in the optical signal receiving device 1620 or a part thereof. As a part of the optical signal receiving device 1620, a signal processing unit 1670, a state monitoring device 1680, etc. may be exemplified.

[0354] The information processing method may be a detection method for detecting polarization variation of signal light propagating in an optical transmission path. The detection method may include, for example, a differential phase information acquisition stage for acquiring information indicating differential phases of input light as an evaluation object at each of more than one time points. The detection method may include, for example, a determination stage for determining whether the differential phases of at least a portion of the more than one time points satisfy a predetermined condition. The detection method may also include an output stage for outputting information indicating that polarization variation has occurred in the signal light when it is determined that the differential phases of at least a portion of the more than one time points satisfy a predetermined condition.

[0355] The predetermined conditions may include the following first condition: the magnitude of the differential phase of at least one of the more than one time points is equal to or greater than the predetermined first threshold value. The predetermined conditions may include the following second condition: the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold value among the multiple time points included in the evaluation period having a predetermined length and at least a part of the more than one time points is equal to or greater than the first threshold value is equal to or greater than the second threshold value. The predetermined conditions may include the following third condition: the ratio of the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold value to the number of the multiple time points included in the evaluation period is equal to or greater than the third threshold value. The predetermined conditions may include the following fourth condition: the moving angle of the trajectory on the Poincare sphere at the variation frequency derived from the magnitude of the differential phase of each of the more than one time points is equal to or greater than the fourth threshold value. The predetermined conditions may include the following fifth condition: the moving angular velocity of the trajectory on the Poincare sphere at the variation frequency is equal to or greater than the fifth threshold value.

[0356] The predetermined condition may include a combination of at least two conditions selected from the group consisting of a first condition, a second condition, a third condition, a fourth condition, and a fifth condition. The first threshold, the second threshold, the third threshold, the fourth threshold, and the fifth threshold may be determined independently. The first threshold, the second threshold, the third threshold, the fourth threshold, and the fifth threshold may be different values ​​from each other, or at least two of them may be the same. An overview of the steps for determining each threshold is described later.

[0357] The communication system 1600 may be an example of a detection device, an optical receiving device, or an optical communication system. The optical signal receiving device 1620 may be an example of a detection device or an optical receiving device. The demultiplexer 1640 may be an example of a demultiplexer. The optical receiver 1650 may be an example of a photoelectric converter. The AD converter 1660 may be an example of an analog-to-digital converter. The signal processing unit 1670 may be an example of a demodulator. The state monitoring device 1680 may be an example of a detection device. The optical signal transmitting device 110 may be an example of an optical transmitting device. The optical signal may be an example of light. One or more sample points may be an example of one or more time points. The monitoring optical signal may be an example of light of a second wavelength or an example of input light.

[0358] (An example of another embodiment)

[0359] In this embodiment, the communication system 1600 is described in detail by taking the case where the communication system 1600 has a configuration for detecting polarization variation in addition to the same configuration as the communication system 100 as an example. However, the communication system 1600 is not limited to this embodiment. In other embodiments, the communication system 1600 may not have at least a part of the configuration of the communication system 100. For example, the communication system 1600 may not have at least one of the amplitude noise evaluation unit 230 and the phase noise evaluation unit 240.

[0360] In this embodiment, the communication system 1600 is described in detail by taking the case where the signal light includes an optical signal for detecting polarization variation as a monitoring optical signal as an example. However, the communication system 1600 is not limited to this embodiment. In other embodiments, the signal light may include various monitoring optical signals with different wavelengths. As an object to be monitored, inspected, or evaluated using the monitoring optical signal, the state of the optical transmission path 10, the state of the signal light propagating in the optical transmission path 10, etc. can be exemplified.

[0361] As the state of the optical transmission path 10, the state of the electromagnetic environment around 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, the connection state of the optical transmission path 10 (for example, the offset of the connection between the two optical fiber cores constituting the optical fiber), etc. can be exemplified. As the state of the electromagnetic environment around the optical transmission path 10, the change state of the electromagnetic environment can be exemplified. As the state of the electric field inside or around the optical transmission path 10, the change state of the electric field can be exemplified. As the state of the optical signal, the intensity change, amplitude change, phase change, polarization change, etc. of the optical signal can be exemplified.

[0362] In this embodiment, the communication system 1600 is described in detail by taking 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 also be a transceiver having the function of transmitting signal light and the function of receiving signal light. For example, the optical signal transmitting device 110 has at least one component that is the same as the component of the optical signal receiving device 1620. For example, the optical signal receiving device 1620 has at least one component that is the same as the component of the optical signal transmitting device 110.

[0363] Fig.17The following schematically shows an example of the internal configuration of the optical signal transmission device 110. In the present embodiment, the optical signal transmission device 110 includes, for example, a communication optical signal output unit 1722, a monitoring optical signal output unit 1724, and a multiplexer 1730.

[0364] In this embodiment, the communication optical signal output unit 1722 modulates the light of the first wavelength according to the information signal and outputs the communication optical signal. In this embodiment, the monitoring optical signal output unit 1724 outputs one or more monitoring optical signals. The one or more monitoring optical signals are generated using light of wavelengths different from the communication optical signal. The one or more monitoring optical signals can be generated using light of wavelengths different from each other. 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.

[0365] Fig.18 The following schematically shows an example of the internal structure of the state monitoring device 1680. In this embodiment, the state monitoring device 1680 includes, for example, an optical delay interferometer 340, an optical receiver 350, an AD converter 360, and a signal processing unit 1870. In this embodiment, the optical receiver 350 includes, for example, a photoelectric conversion element 1852 and an integration circuit 1854.

[0366] 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 converts the photocurrent after photoelectric conversion into a voltage using a load resistor, for example, to generate the electrical signal. The photoelectric conversion element 1852 outputs the electrical signal to the integration circuit 1854.

[0367] and with Figure 3 The output current of the photoelectric conversion element 1852 represents the differential phase of the input light, similarly to the output current of the optical receiver 350 described above. As described above, the input light in this embodiment is a monitoring optical signal input to the state monitoring device 1680.

[0368] In this embodiment, the integration circuit 1854 outputs a voltage of a waveform equal to the time integral of the waveform of the input voltage. The output voltage of the integration circuit 1854 is proportional to the integral value of the input voltage of the integration circuit 1854. More specifically, the electrical signal output by the photoelectric conversion element 1852 is input to the integration circuit 1854, and the integration circuit 1854 outputs a voltage of a waveform equal to the time integral of the waveform of the electrical signal. Thus, an electrical signal corresponding to the output light of the optical delay interferometer 340 is generated. The integration circuit 1854 outputs the above-mentioned electrical signal to the AD converter 360.

[0369] The time constant of the integration circuit 1854 is determined, for example, based on the maximum speed of phase variation accompanying the assumed polarization variation. The time constant of the integration circuit 1854 may be a time constant corresponding to the above-mentioned maximum speed. For example, in the case where a polarization variation of about 10 kHz is assumed to occur, the time constant of the integration circuit 1854 is set to about 100 μs. Similarly, the cutoff frequency of the integration circuit 1854 is set to about 1.59 kHz.

[0370] The time constant of the integration circuit 1854 may be 10 to 1000 μs, 50 to 500 μs, 75 to 150 μs, or 80 to 120 μs. The cutoff frequency of the integration circuit 1854 may be 0.1 to 100 kHz, 0.5 to 50 kHz, 1 to 20 kHz, or 1 to 10 kHz.

[0371] The integrator circuit 1854 may be an amplifier or a low-pass filter that processes the electrical signal output from the photoelectric converter 1852 , or may be a part of a component constituting the amplifier or the low-pass filter. The integrator circuit 1854 may be embedded in the photoelectric converter 1852 or may be disposed at a subsequent stage of the photoelectric converter 1852 .

[0372] In this embodiment, the AD converter 360 converts the electrical signal outputted from the optical receiver 350 from an analog signal to a digital signal, thereby generating one or more digital signals corresponding to the differential phases of the one or more sample points, and outputs the digital signals to the signal processing unit 1870 .

[0373] 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 about 5 to 10 times the bandwidth of the optical receiver 350.

[0374] In this embodiment, the signal processing unit 1870 obtains 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 phases at one or more time points indicated by the one or more differential phase signals. The signal processing unit 1870, for example, 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 detect, for example, the polarization variation of the signal light propagating in the optical transmission path 10.

[0375] The signal processing unit 1870 determines, for example, whether the differential phase of at least a part of the one or more time points satisfies a predetermined condition. When it is determined that the differential phase of at least a part of the one or more time points satisfies the predetermined condition, the signal processing unit 1870 outputs information indicating, for example, that polarization change has occurred in the signal light.

[0376] The signal processing unit 1870 is not particularly limited in the manner in which it 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 may be a device constructed by a combination of these. The signal processing unit 1870 will be described in detail later.

[0377] The photoelectric conversion element 1852 may be an example of a photoelectric conversion unit. The integration circuit 1854 may be an example of an integration unit. The signal processing unit 1870 may be an example of a detection device. The sampled one or more differential phase signals may be an example of information indicating the differential phase of the input light at one or more time points. The one or more sample points may be an example of one or more time points.

[0378] (An example of another embodiment)

[0379] In this embodiment, the state monitoring device 1680 is described in detail by taking the case where the state monitoring device 1680 includes the 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 include an optical delay interferometer having any structure instead of the optical delay interferometer 340. For example, the state monitoring device 1680 includes the optical delay interferometer 540, the optical delay interferometer 640, the optical delay interferometer 1040, the optical delay interferometer 1140, the optical delay interferometer 1240, or the optical delay interferometer 1340.

[0380] In this embodiment, the state monitoring device 1680 is described in detail by taking the case where the state monitoring device 1680 includes the optical receiver 350 as an example. However, the state monitoring device 1680 is not limited to this embodiment. In other embodiments, the state monitoring device 1680 may include an optical receiver having any structure instead of the optical receiver 350. For example, the state monitoring device 1680 includes the balanced optical receiver 1350.

[0381] In this embodiment, the state monitoring device 1680 is described in detail by taking the case where the optical receiver 350 has the integration circuit 1854 as an example. However, the state monitoring device 1680 is not limited to this embodiment. In other embodiments, the integration circuit 1854 may be configured outside the optical receiver 350. For example, the integration circuit 1854 is configured at the subsequent stage of the optical receiver 350. The integration circuit 1854 may also be configured inside a device configured at the subsequent stage of the optical receiver 350. As the above-mentioned device, an integration circuit, a device having an integration characteristic, etc. can be exemplified. As the device having an integration characteristic, an amplifier, a low-pass filter, etc. can be exemplified.

[0382] Fig.19 An example of the internal structure of the signal processing unit 1870 is schematically shown. In the present embodiment, the signal processing unit 1870 includes a histogram generating unit 244, a standard deviation calculating unit 246, and a correcting unit 710. In the present embodiment, the signal processing unit 1870 includes an increase detecting unit 1922, a frequency analyzing unit 1924, a moving angle deriving unit 1926, a moving angular velocity deriving unit 1928, a polarization change detecting unit 1930, and an information output unit 1940.

[0383] In this embodiment, the increase detection unit 1922 obtains one or more sampled differential phase signals from the AD converter 360. As described above, the one or more differential phase signals respectively represent differential phases at one or more time points. The increase detection unit 1922 analyzes the one or more differential phase signals in the time domain.

[0384] For example, the increase detection unit 1922 compares the magnitude of the differential phase at one or more time points with a predetermined first threshold value. Thus, the increase detection unit 1922 can determine whether the magnitude of the differential phase at one or more time points is equal to the first threshold value or is greater than the first threshold value. The increase detection unit 1922 outputs information indicating the determination result to the polarization change detection unit 1930.

[0385] In the present 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).

[0386] For example, the frequency analysis unit 1924 performs Fourier transform (e.g., fast Fourier transform) on one or more differential phase signals. Thus, the frequency variation 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. In addition, the spectral component obtained by Fourier transform represents the amplitude value of the variation.

[0387] Specifically, the Fourier transform result is analyzed, and the frequency representing the prominent peak and the frequencies of other prominent components are determined as the variation frequency. For example, the variation of the amplitude value is observed while changing the frequency of the measurement data obtained by Fourier transform. The frequency at which the amplitude value changes sharply is determined as the variation frequency.

[0388] In this embodiment, the frequency analysis unit 1924 outputs information indicating the analysis result to the polarization change detection unit 1930. The frequency analysis unit 1924 can output information indicating the analysis result to the movement angle derivation unit 1926 and / or the movement angular velocity derivation unit 1928 as needed. As information indicating the analysis result, a change frequency, a change amplitude value, etc. can be exemplified. The change amplitude value is a parameter related to the movement angle, and the movement angular velocity is derived from the change amplitude value and the change frequency.

[0389] In the present embodiment, the movement angle derivation unit 1926 obtains one or more sampled differential phase signals from the AD converter 360. The movement angle derivation unit 1926 derives the movement angle at the variation frequency of the track on the Poincare sphere derived from the magnitude of the differential phase at one or more time points. Specifically, the variation frequency of the track on the Poincare sphere is derived based on the result of the Fourier transform. In addition, the movement angle is derived by calculating the peak-to-peak value (pp value) based on the vibration amplitude value (peak value) of the Fourier transform. The movement angle derivation unit 1926 outputs information indicating the derivation result to the polarization change detection unit 1930.

[0390] In this embodiment, the moving angular velocity deriving unit 1928 obtains one or more sampled differential phase signals from the AD converter 360. The moving angular velocity deriving unit 1928 derives the moving angular velocity at the variation frequency of the track on the Poincare sphere derived from the magnitude of the differential phase at one or more time points. The moving angular velocity is derived from the moving angle and variation frequency of the track on the Poincare sphere. The moving angular velocity is derived, for example, as the moving angle (pp value) × 2 × variation frequency. The moving angular velocity deriving unit 1928 outputs information indicating the derivation result to the polarization variation detection unit 1930.

[0391] In this embodiment, the polarization change detection unit 1930 evaluates the presence and / or degree of polarization change. For example, the polarization change detection unit 1930 detects polarization change. When polarization change is detected, it can be evaluated that polarization change has occurred and / or the degree of polarization change is relatively large.

[0392] For example, the polarization change detection unit 1930 determines whether the differential phase at at least a part of one or more time points satisfies a predetermined condition. When it is determined that the differential phase at at least a part of one or more time points satisfies the predetermined condition, the polarization change detection unit 1930 outputs information indicating that the polarization change is detected to the information output unit 1940.

[0393] The above-mentioned predetermined conditions may include the following first condition: the magnitude of the differential phase of at least one of the more than one time points is equal to or greater than the first threshold value. The above-mentioned predetermined conditions may include the following second condition: the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold value among the multiple time points included in the evaluation period of at least a part of the more than one time points and having a predetermined length is equal to or greater than the second threshold value. The above-mentioned predetermined conditions may include the following third condition: the ratio of the number of time points at which the magnitude of the differential phase of the time point is equal to or greater than the first threshold value to the number of the multiple time points included in the evaluation period is equal to or greater than the third threshold value.

[0394] The predetermined conditions may include the following fourth condition: the moving angle at the variation frequency of the track on the Poincare sphere derived from the magnitude of the differential phase at one or more time points is equal to or greater than a predetermined fourth threshold value. The predetermined conditions may include the following fifth condition: the moving angular velocity at the variation frequency of the track on the Poincare sphere is equal to or greater than a predetermined fifth threshold value.

[0395] The predetermined condition may include a combination of at least two conditions selected from the group consisting of a first condition, a second condition, a third condition, a fourth condition, and a fifth condition. The first threshold, the second threshold, the third threshold, the fourth threshold, and the fifth threshold may be determined independently. The first threshold, the second threshold, the third threshold, the fourth threshold, and the fifth threshold may be different values, or at least two of them may be the same.

[0396] At least one of the first threshold, the second threshold, the third threshold, the fourth threshold, and the fifth threshold may be determined based on the polarization variability resistance of the communication system 1600 or the optical signal receiving device 1620. For example, at least one of the first threshold, the fourth threshold, and the fifth threshold is set to ka times (ka is a positive number. ka may be less than 1 or less than 1) the polarization variability resistance specified by the specifications of the device mounted on the communication system 1600 or the optical signal receiving device 1620. As described above, the first threshold, the fourth threshold, and the fifth threshold may also be different from each other. At least one of the second threshold and the third threshold may be determined by a pre-test, a test during a trial run, or the like.

[0397] In one embodiment, the polarization change detection unit 1930 obtains information indicating the determination results respectively related to one or more sample points from the increase detection unit 1922. For example, when at least one determination result among the determination results respectively related to one or more sample points indicates that the magnitude of the differential phase of the sample point is equal to or greater than the first threshold value, the polarization change detection unit 1930 determines that the first condition is satisfied. When the first condition is satisfied, the polarization change detection unit 1930 may determine that the polarization change is detected.

[0398] According to the above embodiment, even when the actual influence on the transmission characteristics is small, the first condition may be satisfied. For example, when the magnitude of the differential phase at a specific sample point is a statistically specific value, the above first condition is satisfied and the polarization change can be detected. In this case, the influence of the detected polarization change on the transmission characteristics may become very small.

[0399] Therefore, the polarization variation detection unit 1930 may detect polarization variation using, for example, determination results related to a plurality of sample points included in a period having a predetermined length (sometimes referred to as an evaluation period) among determination results related to one or more sample points. This improves the detection accuracy of polarization variation.

[0400] For example, the polarization variation detecting unit 1930 determines that the second condition is satisfied when the number of sample points whose differential phase magnitude is equal to or greater than the first threshold value among the plurality of sample points included in the evaluation period is equal to or greater than the second threshold value. When the second condition is satisfied, the polarization variation detecting unit 1930 may determine that the polarization variation is detected.

[0401] For example, the polarization variation detection unit 1930 determines that the third condition is satisfied when the ratio of the number of sample points whose differential phase magnitude is equal to or greater than the first threshold value to the number of the plurality of sample points included in the evaluation period is equal to or greater than a predetermined third threshold value. When the third condition is satisfied, the polarization variation detection unit 1930 may determine that the polarization variation is detected.

[0402] In other embodiments, the polarization change detection unit 1930 obtains information indicating the derivation result of the movement angle at the change frequency of the trajectory on the Poincare sphere from the movement angle derivation unit 1926. When the movement angle is equal to or greater than the fourth threshold, the polarization change detection unit 1930 determines that the fourth condition is satisfied. When the fourth condition is satisfied, the polarization change detection unit 1930 may determine that the polarization change is detected.

[0403] In another embodiment, the polarization change detection unit 1930 obtains information indicating the derivation result of the movement angular velocity at the change frequency of the trajectory on the Poincare sphere from the movement angular velocity derivation unit 1928. When the movement angle is equal to or greater than the fifth threshold, the polarization change detection unit 1930 determines that the fifth condition is satisfied. When the fifth condition is satisfied, the polarization change detection unit 1930 may determine that the polarization change is detected.

[0404] In this embodiment, the information output unit 1940 outputs evaluation information indicating various evaluation results in the signal processing unit 1870. In one embodiment, the evaluation information includes information for evaluating the Figure 2 In other embodiments, the evaluation information includes evaluation information related to polarization change. As evaluation information related to polarization change, information indicating that polarization change has occurred in the signal light, information indicating that the moving angular velocity exceeds a specified angular velocity, information indicating that the moving angle exceeds a specified angle, and information indicating that the change frequency exceeds a specified frequency can be exemplified. The evaluation information related to polarization change can also be output as an alarm or a flag.

[0405] The increase detection unit 1922 may be an example of a differential phase information acquisition unit. The frequency analysis unit 1924 may be an example of a differential phase information acquisition unit. The movement angle derivation unit 1926 may be an example of a differential phase information acquisition unit. The movement angular velocity derivation unit 1928 may be an example of a differential phase information acquisition unit. The polarization change detection unit 1930 may be an example of a determination unit. The information output unit 1940 may be an example of an output unit. The information indicating that a polarization change is detected may be an example of information indicating that a polarization change has occurred in the signal light.

[0406] (An example of another embodiment)

[0407] In this embodiment, the signal processing unit 1870 is described in detail by taking as an example a case where the signal processing unit 1870 includes the histogram generating unit 244, the standard deviation calculating unit 246, and the correcting unit 710 similarly to the signal processing unit 370. However, the signal processing unit 1870 is not limited to this embodiment.

[0408] In other embodiments, the signal processing unit 1870 may not include at least one of the histogram generator 244, the standard deviation calculator 246, and the corrector 710. In still other embodiments, the signal processing unit 1870 may include the normalizer 930 similarly to the signal processing unit 870.

[0409] Fig. 20An example of information processing in the state monitoring device 1680 is schematically shown. According to the present embodiment, first, in step 2022 (step is sometimes abbreviated as S), the optical receiver 350 having the integration circuit 1854 receives the light output by the optical delay interferometer 340. The optical receiver 350 generates an electrical signal corresponding to the light output by the optical delay interferometer 340.

[0410] Next, in S2024, the AD converter 360 samples the electrical signal outputted from the optical receiver 350. Thus, one or more digital signals corresponding to the differential phases of the one or more sample points are generated.

[0411] In step S2026, the digital signal output from the AD converter 360 is stored in a memory (not shown) provided in the state monitoring device 1680 or the signal processing unit 1870. The digital signal output from the AD converter 360 is stored in the memory in correspondence with the identification information of each sample point, for example. In the memory, for example, the latest N (N is an integer greater than or equal to 1) data are sequentially stored.

[0412] When the processing of S2026 is completed, in S2032, the signal processing unit 1870 determines whether the first condition, the fourth condition, and / or the fifth condition are satisfied. Thus, the presence and / or degree of polarization change are evaluated. The signal processing unit 1870 outputs information indicating the determination result to the information output unit 1940.

[0413] In S2032, the signal processing unit 1870 can use the data accumulated in the memory in S2026 to determine whether the first condition, the fourth condition and / or the fifth condition are satisfied. For example, after performing moving average processing on the data accumulated in the memory in S2026, it is determined whether the first condition, the fourth condition and / or the fifth condition are satisfied. Thus, the influence of noise is reduced. In addition, the signal processing unit 1870 can also determine whether the first condition, the fourth condition and / or the fifth condition are satisfied in sequence with respect to the sampled data. In this case, after the processing of S2024 is completed and before the processing of S2026 is completed, the processing of S2032 is started.

[0414] When the processing of S2026 is completed, 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 satisfied. Thus, the presence and / or degree of polarization change are evaluated. The signal processing unit 1870 outputs information indicating the determination result to the information output unit 1940.

[0415] When the processing of S2026 is completed, in S2036, the histogram generation unit 244 and the standard deviation calculation unit 246 generate information for evaluating the phase noise of the signal light using the data stored in the memory in S2026. 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.

[0416] In step S2042, the information output unit 1940 outputs the evaluation information related to the polarization variation. In step S2044, the information output unit 1940 outputs the evaluation information related to the phase noise. The process ends here.

[0417] Fig.21 An example of a data table 2100 is schematically shown. In this embodiment, the data table 2100 stores evaluation information related to polarization variation. In this embodiment, the data table 2100 has one or more records respectively related to one or more sample points. The one or more records can each be an example of evaluation information of a specific time point or sample point.

[0418] In the present embodiment, the data table 2100 stores, for one or more sample points, the number 2120 of the sample point, the time 2122 corresponding to the sample point, the output value 2124 of the optical receiver 350 of the sample point, the frequency of change 2132 derived by the frequency analysis unit 1924, the angle of change 2134 derived by the angle of change deriving unit 1926, the angular velocity of change 2136 derived by the angular velocity of change deriving unit 1928, whether the first to fifth conditions are satisfied or not 2140, and the detection result 2150 of the polarization change, respectively. In addition, the data items of the data table 2100 are not limited to the present embodiment. For example, in other embodiments, the data table 2100 stores, for one or more sample points, the number 2120 or the time 2122 of the sample point and the detection result 2150 of the polarization change, respectively.

[0419] (An example of another embodiment)

[0420] In this embodiment, the evaluation information related to polarization change is described in detail by taking the case where the evaluation information related to polarization change is stored in the data table 2100 as an example. However, the evaluation information related to polarization change is not limited to this embodiment. In other embodiments, the evaluation information related to polarization change may also be a list of more than one time when polarization change is detected.

[0421] Fig. 22An example of a computer 3000 that can embody various aspects of the present invention in whole or in part 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 state monitoring device 1680 can be implemented by the computer 3000.

[0422] The program installed in the computer 3000 can make the computer 3000 function as an operation associated with the device involved in the embodiment of the present invention or one or more "parts" of the device, or execute the operation or the one or more "parts", and / or can make the computer 3000 execute the process involved in the embodiment of the present invention or the stage of the process. Such a program can be executed by the CPU 3012 to make the computer 3000 execute specific operations associated with some or all of the blocks in the flowcharts and block diagrams described in this specification.

[0423] The computer 3000 involved in this embodiment includes a CPU 3012, a RAM 3014, a GPU 3016, and a display device 3018, which are connected to each other through a host controller 3010. The computer 3000 also includes input and output units such as a communication interface 3022, a hard disk drive 3024, a DVD-ROM drive 3026, and an IC card drive, which are connected to the host controller 3010 via an input and output controller 3020. The computer also includes conventional input and output units such as a ROM 3030 and a keyboard 3042, which are connected to the input and output controller 3020 via an input and output chip 3040.

[0424] The CPU 3012 controls each unit by operating according to the program stored in the ROM 3030 and the RAM 3014. The GPU 3016 acquires the image data generated by the CPU 3012 in a frame buffer or the like provided in the RAM 3014 or in itself, and displays the image data on the display device 3018.

[0425] The communication interface 3022 communicates with other electronic devices via a network. The hard disk drive 3024 stores programs and data used by the CPU 3012 in the computer 3000. The DVD-ROM drive 3026 reads programs or data from the DVD-ROM 3001 and provides the programs or data to the hard disk drive 3024 via the RAM 3014. The IC card drive reads programs and data from an IC card and / or writes programs and data to an IC card.

[0426] The ROM 3030 stores a boot program executed by the computer 3000 when activated, and / or a program that depends on the hardware of the computer 3000. The input / output chip 3040 can also connect various input / output units to the input / output controller 3020 via a parallel port, a serial port, a keyboard port, a mouse port, and the like.

[0427] The program is provided by a computer-readable storage medium such as DVD-ROM 3001 or IC card. The program is read from the computer-readable storage medium, installed in the hard disk drive 3024, RAM 3014 or ROM 3030, which is also an example of a computer-readable storage medium, and executed by the CPU 3012. The information processing described in these programs is read by the computer 3000, resulting in cooperation between the program and the various types of hardware resources described above. The device or method can be constructed by implementing the operation or processing of information with the use of the computer 3000.

[0428] For example, when communication is performed between the computer 3000 and an external device, the CPU 3012 can execute the communication program loaded into the RAM 3014, and issue a communication processing command to the communication interface 3022 according to the processing described in the communication program. The communication interface 3022 reads the transmission data stored in the transmission buffer area provided in the RAM 3014, the hard disk drive 3024, the DVD-ROM 3001, or the recording medium such as the IC card under the control of the CPU 3012, and transmits the read transmission data to the network, or writes the reception data received from the network into the reception buffer area provided on the recording medium, etc.

[0429] In addition, the CPU 3012 can read all or a necessary part of a file or database stored in an external recording medium such as the hard disk drive 3024, the DVD-ROM drive 3026 (DVD-ROM 3001), or an IC card into the RAM 3014, and perform various types of processing on the data on the RAM 3014. The CPU 3012 can then write the processed data back to the external recording medium.

[0430] Various types of information such as various types of programs, data, tables, and databases can be stored in the recording medium and receive information processing. CPU3012 can perform various types of processing on the data read from RAM3014 and write the results back to RAM3014. The various types of processing include various types of operations, information processing, conditional judgment, conditional branching, unconditional branching, information retrieval / replacement, etc., which are recorded in various parts of this disclosure and specified by the command sequence of the program. In addition, CPU3012 can retrieve information from files, databases, etc. in the recording medium. For example, in the case where a plurality of entries having attribute values ​​of the first attribute associated with the attribute value of the second attribute are stored in the recording medium, CPU3012 can retrieve an entry whose attribute value of the first attribute is consistent with the specified condition from the plurality of entries, read the attribute value of the second attribute stored in the entry, and thereby obtain the attribute value of the second attribute associated with the first attribute that meets the predetermined condition.

[0431] The program or software module described above may be stored in a computer-readable storage medium on or near the computer 3000. In addition, a recording medium such as a hard disk or RAM provided in a server system connected to a dedicated communication network or the Internet may be used as a computer-readable storage medium, thereby providing the program to the computer 3000 via the network.

[0432] (Experimental Examples and Comparative Experimental Examples)

[0433] Hereinafter, the present invention will be specifically described using experimental examples and comparative experimental examples. In addition, the present invention is not limited to the following experimental examples and comparative experimental examples. Fig.23 , Fig.24 , Fig.25 as well as Fig.26 Various measurement results in Experimental Example 1 are shown. Fig. 27 , Fig.28 , Fig.29 as well as Fig.30 Various measurement results in Comparative Experiment Example 1 are shown. Fig.31 , Fig.32 , Fig.33 as well as Fig.34 Various measurement results in Experimental Example 2 are shown.

[0434] (Experimental Example 1)

[0435] (Measurement using a polarization state measuring instrument)

[0436] First, prepare an optical fiber stretcher (OPTIPHASE, PZ1-SMF4-APC-E) with four layers of 12.3 m long optical fiber (Coming, SMF28e+ optical fiber) wound on a piezoelectric element. Connect one end of the optical fiber to a laser oscillator (Pure Photonics, PPCL550). Connect the other end of the optical fiber to a polarization state meter (Novoptel, PM1000 Polarimeter).

[0437] A laser with a wavelength of 1550nm was emitted from a laser oscillator, a voltage was applied to the fiber stretcher, and a lateral pressure change of 140kHz was applied to the optical fiber. The Stokes parameters of the output light of the fiber stretcher were measured using a polarization state measuring instrument. In addition, the movement angle of the trajectory on the Poincare sphere was derived using the measurement results of the Stokes parameters.

[0438] The above experiment was conducted by changing the magnitude of the voltage applied to the optical fiber stretcher. The magnitude (peak value) of the voltage applied to the optical fiber stretcher was 500 mV, 1 V, and 2 V.

[0439] Fig.23 The results of measuring the Stokes parameters when the voltage applied to the optical fiber stretcher is 1V are shown. Fig.23 As shown, it can be confirmed that a small polarization change occurs at a frequency of 140 kHz. In addition, the state of the polarization change on the Poincare sphere was confirmed using the measurement results of the Stokes parameters, and as a result, a small polarization change was confirmed on the Poincare sphere.

[0440] (Measurement using optical delay interferometer)

[0441] Next, the other end of the optical fiber was disconnected from the polarization state measuring instrument, and the other end of the optical fiber was connected to one end of an optical delay interferometer (manufactured by Optoplex, DI-C1EFAM512). The other end of the optical delay interferometer was connected to a balanced optical receiver (manufactured by Optoplex, BR-C0200B1DC). The balanced optical receiver was equipped with a low-pass filter, and the cutoff frequency of the balanced optical receiver was 150 MHz. The output of the balanced optical receiver was sampled using an AD converter (manufactured by Tektronix, oscilloscope model MSO64).

[0442] A laser with a wavelength of 1550nm was emitted from a laser oscillator, a voltage was applied to the fiber stretcher, and a 140kHz lateral pressure change was applied to the optical fiber. Using the measurement results of the sampled balanced optical receiver output, (a) a bar graph related to the magnitude of the differential phase and (b) a graph showing the time variation of the magnitude of the differential phase were prepared. The measurement results of the sampled balanced optical receiver output were fast Fourier transformed to derive (c) a spectrum of the differential phase.

[0443] The above experiment was conducted by changing the magnitude of the voltage applied to the optical fiber stretcher. The magnitude (peak value) of the voltage applied to the optical fiber stretcher was 500 mV, 1 V, and 2 V.

[0444] Fig.24 A bar graph showing the output of a balanced optical receiver when the voltage applied to the fiber stretcher is 1V. Fig.25 The graph shows the time variation of the output of the balanced optical receiver when the voltage applied to the optical fiber stretcher is 1 V. Fig.26 The frequency spectrum of the output of the balanced optical receiver when the voltage applied to the optical fiber stretcher is 1V is shown.

[0445] like Fig.24 As shown in the figure, the bar graph is divided into two, which shows that a phase shift has occurred. Fig.25 As shown, it can be seen that the rise and fall of the change are steep, and the integration effect of the low-pass filter of the balanced optical receiver appears. As mentioned above, it is speculated that when the integration effect appears, the phase change is directly measured. In addition, the cause of the rapid change is not clear, but it is speculated that the mechanical vibration cannot follow the sine wave input voltage, resulting in local rapid changes.

[0446] As mentioned above, in Fig.26 In the experiment, the magnitude of the change was derived by measuring the frequency component at 140 kHz. In addition, 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 optical fiber stretcher is roughly proportional to the measured result of the phase shift. As shown in non-patent document 15, since there is a proportional relationship between the applied voltage of the optical fiber stretcher and the values ​​of the lateral pressure and birefringence caused by the operation of the optical fiber stretcher, the results of Experimental Example 1 show that the above-mentioned measurement principle works.

[0447] (evaluate)

[0448] The measurement results using a polarization state meter (sometimes called a polarimeter) are compared with the measurement results using an optical delay interferometer. As a result, the measurement results using the optical delay interferometer are very consistent with the measurement results using the polarization state meter. For example, when the magnitude (peak-to-peak value) of the voltage applied to the optical fiber stretcher is 2V, the coordinates of the moving ends of the trajectory on the Poincare sphere measured by the polarimeter are (0.31, 0.94, -0.15) and (0.17, 0.98, -0.06). In this case, the pp value of the phase change is 0.17 rad. On the other hand, according to the measurement results using the optical delay interferometer, the pp value of the 140kHz component in the spectrum measurement results is 0.16 rad.

[0449] This shows that the approximate value of the movement angle of the trajectory on the Poincare sphere can be measured using the optical delay interferometer. In addition, the effectiveness of the detection principle of the movement change caused by the above-mentioned polarization change was confirmed.

[0450] (Comparative Experimental Example 1)

[0451] (Measurement using a polarization state measuring instrument)

[0452] The Stokes parameters of the output light of the optical fiber stretcher were measured by the same procedure as in Experimental Example 1, except that the magnitude (peak value) of the voltage applied to the optical fiber stretcher was set to 200 mV. Furthermore, the movement angle of the trajectory on the Poincare sphere was derived using the measurement results of the Stokes parameters.

[0453] Fig. 27 The results of measuring the Stokes parameters when the voltage applied to the optical fiber stretcher is 200 mV are shown. Fig. 27 As shown, it can be confirmed that a small polarization change occurs at a frequency of 140 kHz. In addition, the state of the polarization change on the Poincare sphere was confirmed using the measurement results of the Stokes parameters, and as a result, a small polarization change was confirmed on the Poincare sphere.

[0454] (Measurement using optical delay interferometer)

[0455] Except that the magnitude (peak-to-peak value) of the voltage applied to the optical fiber stretcher was set to 200 mV, (a) a bar graph related to the magnitude of the differential phase and (b) a graph showing the temporal variation of the magnitude of the differential phase were prepared by the same process as in Experimental Example 1. In addition, (c) a frequency spectrum of the differential phase was derived.

[0456] Fig.28 A bar graph showing the output of a balanced optical receiver when the magnitude of the voltage applied to the fiber stretcher is 200 mV. Fig.29The graph shows the temporal variation of the output of the balanced optical receiver when the voltage applied to the optical fiber stretcher is 200 mV. Fig.30 The frequency spectrum of the output of the balanced optical receiver when the voltage applied to the optical fiber stretcher is 200 mV is shown.

[0457] like Fig.28 As shown in the bar graph, only phase noise is observed. Fig.29 and Fig.30 It can be seen that the Fig.28 The tiny 140kHz phase change cannot be read. This confirms that the actual high-speed changes become difficult to read by implementing statistical processing.

[0458] (Experimental Example 2)

[0459] (Measurement using a polarization state measuring instrument)

[0460] First, one end of the optical fiber was connected to a laser oscillator (Pure Photonics, model PPCL550). The other end of the optical fiber was connected to a polarization scrambler (Luna Innovations, NRT-2500). The output end of the polarization scrambler was connected to a polarization state meter (Novoptel, PM1000 Polarimeter).

[0461] The laser oscillator emits a laser with a wavelength of 1550nm, and uses the spinner mode of the polarization scrambler to generate high-speed polarization changes. The polarization change frequency of the polarization scrambler is set to 75kHz. This is equivalent to a moving angular velocity of about 470 krad / s on the Poincare sphere.

[0462] The Stokes parameter of the polarization-modulated light output from the polarization scrambler is measured using a polarization state measuring instrument, and the movement angle of the trajectory on the Poincare sphere is derived using the measurement result of the Stokes parameter.

[0463] Fig.31 The results of the determination of the Stokes parameters in Experimental Example 2 are shown. Fig.31 As shown, it can be confirmed that a very large and high-speed polarization change occurs as specified by the polarization scrambler. In addition, the state of polarization change on the Poincare sphere was confirmed using the measurement results of the Stokes parameters, and it was confirmed that an orbit close to a great circle with the largest radius was drawn on the Poincare sphere.

[0464] (Measurement using optical delay interferometer)

[0465] Next, the output end of the polarization scrambler was disconnected from the polarization state measuring instrument, and the output end of the polarization scrambler was connected to one end of an optical delay interferometer (manufactured by Optoplex, DI-C1EFAM512). The other end of the optical delay interferometer was connected to a balanced optical receiver equipped with a low-pass filter function (manufactured by Optoplex, BR-C0200B 1DC). The output of the balanced optical receiver was sampled using an AD converter (manufactured by Tektronix, oscilloscope MSO64).

[0466] A laser beam with a wavelength of 1550 nm was emitted from a laser oscillator, and a high-speed polarization change was generated by using the spinner mode of the polarization scrambler. The polarization change frequency of the polarization scrambler was set to 75 kHz. Using the measurement results of the sampled balanced optical receiver output, (a) a bar graph related to the magnitude of the differential phase and (b) a graph showing the time variation of the magnitude of the differential phase were prepared. The measurement results of the sampled balanced optical receiver output were fast Fourier transformed to derive (c) a spectrum of the differential phase.

[0467] Fig.32 A bar graph showing the output of the balanced optical receiver in Experimental Example 2. Fig.33 FIG. 1 shows the temporal variation of the output of the balanced optical receiver in Experimental Example 2. FIG. Fig.34 The frequency spectrum of the output of the balanced optical receiver in Experimental Example 2 is shown. Fig.32 , Fig.33 as well as Fig.34 The results all indicate that a large polarization change occurs.

[0468] like Fig.33 As shown, it can be seen that the rise and fall of the change are steep, and the integration effect of the low-pass filter of the balanced optical receiver appears. As mentioned above, it is estimated that when the integration effect appears, the phase change is directly measured. In addition, the polarization scrambler NRT-2500 generates polarization change by rotating the 1 / 2 wavelength plate at high speed. At the moment when the natural axis of the 1 / 2 wavelength plate crosses the natural axis of the orthogonal polarization mode, the phase change caused by the 1 / 2 wavelength plate occurs, and as a result, it is estimated that a sharp change occurs.

[0469] like Fig.34 As shown in the figure, the 150kHz component is measured as the main frequency component. In this regard, the nominal value of 75kHz of the polarization scrambler NRT-2500 is the frequency of polarization rotation. The phase change caused by the rotation of the 1 / 2 wavelength plate changes at a frequency twice that of the polarization rotation. Therefore, it is estimated that the frequency components of 75kHz and 150kHz are observed.

[0470] This confirmed that even when a large polarization variation occurs, the approximate value of the movement angle of the trajectory on the Poincare sphere can be measured using the optical delay interferometer. The effectiveness of the detection principle of the movement variation caused by the polarization variation described above was confirmed.

[0471] The present invention has been described above using the embodiments, but the technical scope of the present invention is not limited to the scope described in the above embodiments. It should be clear to those skilled in the art that various changes or improvements can be made to the above embodiments. In addition, within the scope of technical non-contradiction, the matters described in a specific embodiment can be applied to other embodiments. It is clear from the description of the claims that the method of implementing the above changes or improvements is also included in the technical scope of the present invention.

[0472] The execution order of each process such as actions, processes, steps and stages in the devices, systems, programs and methods shown in the claims, specifications and drawings is not specifically indicated as "before", "first", etc. In addition, it should be noted that as long as the output of the previous process is not used in the subsequent process, it can be implemented in any order. Even if the action flow in the claims, specifications and drawings is described using "first", "next", etc. for convenience, it does not mean that it must be implemented in this order.

[0473] For example, the following matters are disclosed in the present application specification.

[0474] [Item A-1] An evaluation device for evaluating phase noise of signal light propagating in an optical transmission path, comprising:

[0475] a differential phase information acquisition unit that acquires information indicating a differential phase of input light at each of a plurality of time points included in an evaluation period; and

[0476] an index deriving unit for deriving a degree of deviation of the differential phase at each of the plurality of time points as an index for evaluating the phase noise;

[0477] The differential phase indicates the difference in phase between the input light at two temporally adjacent time points among the plurality of time points.

[0478] The time intervals between the plurality of time points are substantially constant and are equal to or shorter than the length of the symbol time of the signal transmitted by the input light.

[0479] [Item A-2] The evaluation device according to Item A-1, wherein:

[0480] The index deriving unit derives a variance or a standard deviation of the differential phase of the input light as the index.

[0481] [Item A-3] The evaluation device according to Item A-1 or Item A-2, further comprising:

[0482] a photoelectric conversion unit that converts a received signal transmitted by the signal light from an optical signal to an electrical signal; and

[0483] The analog-to-digital conversion unit converts the electrical signal from an analog signal to a digital signal.

[0484] The analog-to-digital conversion unit outputs a plurality of digital signals corresponding to each of the plurality of time points.

[0485] The differential phase information acquisition unit generates the differential phase representing each of the plurality of time points based on the plurality of digital signals.

[0486] The index deriving unit derives the degree of deviation of the differential phase corresponding to each of the plurality of time points as an index for evaluating the phase noise of the received signal.

[0487] The symbol time series of the above received signals overlap.

[0488] The sampling rate of the analog-to-digital conversion unit is greater than or equal to a symbol rate of a symbol time series of the received signal.

[0489] [Item A-4] The evaluation device according to Item A-3, further comprising:

[0490] A local light source outputting local oscillator light; and

[0491] An optical 90-degree hybrid causes the signal light to interfere with the local oscillator light from the local light source, and outputs a plurality of optical signals that separate the received signal into a plurality of signal components.

[0492] The photoelectric converter converts the optical signal of the I signal component output by the optical 90-degree hybrid into an electrical signal, and converts the optical signal of the Q signal component output by the optical 90-degree hybrid into an electrical signal.

[0493] The analog-to-digital conversion unit outputs a plurality of first digital signals corresponding to the optical signal of the I signal component at each of the plurality of time points, and outputs a plurality of second digital signals corresponding to the optical signal of the Q signal component at each of the plurality of time points.

[0494] The differential phase information acquisition unit generates information indicating the differential phase at each of the plurality of time points by removing a modulation component included in the received signal based on the plurality of first digital signals and the plurality of second digital signals.

[0495] [Item A-5] The evaluation device according to Item A-4, wherein:

[0496] 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 line width of the local oscillator light.

[0497] [Item A-6] The evaluation device according to Item A-1 or Item A-2, wherein:

[0498] The differential phase information acquisition unit includes:

[0499] a delayed interferometer section, into which the input light is input; and

[0500] A photoelectric conversion unit converts the output light of the delayed interferometer unit into an electrical signal.

[0501] The delayed interference unit branches the input light into a first input light and a second input light, and causes the first input light passing through the first optical path and the second input light passing through the second optical path to perform combined interference.

[0502] The delay interferometer is set such that the delay 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 and the frequency f of the input light satisfy the relationship of the following mathematical formula FA1:

[0503] (Mathematical formula FA1)

[0504] 2πfτ=2nπ+π / 2 (where n is an integer).

[0505] [Item A-7] The evaluation device according to Item A-6, wherein:

[0506] The evaluation device further includes an adjustment unit configured to adjust an operating point of the delayed interferometer unit based on the electrical signal outputted from the photoelectric conversion unit.

[0507] [Item A-8] The evaluation device according to Item A-1 or Item A-2, wherein:

[0508] The differential phase information acquisition unit includes:

[0509] a waveguide into which the input light is input;

[0510] a ring resonator disposed adjacent to the waveguide; and

[0511] The photoelectric conversion unit converts the output light of the waveguide into an electrical signal.

[0512] The input-output characteristics of the ring resonator are set so that the length of the ring is an integral multiple of the wavelength of input light in the ring medium.

[0513] [Item A-9] The evaluation device according to any one of Items A-6 to A-8, wherein

[0514] The differential phase information acquisition unit further includes an analog-to-digital conversion unit.

[0515] The analog-to-digital converter converts the electrical signal output by the photoelectric converter from an analog signal to a digital signal.

[0516] The analog-to-digital converter outputs a plurality of digital signals corresponding to the differential phase at each of the plurality of time points.

[0517] [Item A-10] The evaluation device according to any one of Items A-6 to A-9, wherein

[0518] The square of the standard deviation of the phase noise of the input light is expressed as 1 / 2 times the square of the standard deviation of the differential phase.

[0519] [Item A-11] The evaluation device according to any one of Items A-1 to A-10, wherein

[0520] The evaluation device further includes an optical power information acquisition unit that acquires information indicating a measured value of optical power of input light.

[0521] The index deriving unit normalizes the differential phase of the input light using the measured value of the optical power of the input light, and derives the degree of deviation of the differential phase using the normalized differential phase.

[0522] [Item A-12] An optical receiver comprising:

[0523] The evaluation device according to any one of items A-1 to A-11; and

[0524] The demodulation unit demodulates the reception signal transmitted by the signal light to generate an information signal.

[0525] [Item A-13] An optical communication system comprising:

[0526] an optical transmitter for transmitting the above-mentioned signal light; and

[0527] An optical receiver as described in item A-12.

[0528] [Item A-14] A program for causing a computer to function as the evaluation device according to any one of Items A-1 to A-11.

[0529] [Item A-14] A method for evaluating phase noise of signal light propagating in an optical transmission path, comprising:

[0530] a differential phase information acquisition stage of acquiring information indicating the differential phase of input light at each of a plurality of time points included in the evaluation period; and

[0531] In the index derivation stage, the degree of deviation of the differential phase at each of the plurality of time points is derived as an index for evaluating the phase noise.

[0532] The differential phase indicates the difference in phase between the input light at two temporally adjacent time points among the plurality of time points.

[0533] The time intervals between the plurality of time points are substantially constant and are equal to or shorter than the length of the symbol time of the signal transmitted by the input light.

[0534] For example, the following matters are disclosed in the present application specification.

[0535] [Item B-1] A detection device for detecting polarization variation of signal light propagating in an optical transmission path, the detection device comprising:

[0536] a differential phase information acquisition unit that acquires information indicating a differential phase of input light to be evaluated at each of one or more time points; and

[0537] a determination unit for determining whether the differential phase at at least a part of the one or more time points satisfies a predetermined condition,

[0538] The predetermined condition includes at least one of the first condition, the second condition, the third condition, the fourth condition and the fifth condition.

[0539] The first condition is that the magnitude of the differential phase at at least one of the one or more time points is equal to or greater than a predetermined first threshold value,

[0540] The second condition is that the number of time points at which the magnitude of the differential phase at the time point is equal to or greater than the first threshold value among the plurality of time points included in the evaluation period as a period having a predetermined length, which is at least a part of the one or more time points, is equal to or greater than a predetermined second threshold value,

[0541] The third condition is that the ratio of the number of time points at which the magnitude of the differential phase at the time point is equal to or greater than the first threshold value among the multiple time points to the number of the multiple time points included in the evaluation period is equal to or greater than a predetermined third threshold value,

[0542] The fourth condition is that the moving angle at the variation frequency of the trajectory on the Poincare sphere derived from the magnitude of the differential phase at each of the one or more time points is equal to or greater than a predetermined fourth threshold value,

[0543] The fifth condition is that the angular velocity of movement of the trajectory on the Poincare sphere at the variation frequency is equal to or greater than a predetermined fifth threshold.

[0544] [Item B-2] The detection device according to Item B-1, wherein:

[0545] The detection device further includes an output unit configured to output information indicating that polarization change has occurred in the signal light when the differential phase at at least a part of the one or more time points is determined to satisfy the predetermined condition.

[0546] [Item B-3] The detection device according to Item B-1 or Item B-2, wherein:

[0547] The differential phase indicates the difference in phase between the input light at two temporally adjacent time points among the one or more time points.

[0548] The time intervals between the one or more time points are substantially constant.

[0549] [Item B-4] The detection device according to any one of Items B-1 to B-3, wherein

[0550] The above-mentioned signal light includes:

[0551] Light of a first wavelength is used to transmit information signals; and

[0552] A second wavelength of light is used to detect polarization changes,

[0553] The value of the first wavelength is different from the value of the second wavelength,

[0554] The input light is the light of the second wavelength.

[0555] [Item B-5] The detection device according to Item B-4, wherein:

[0556] The detection device further includes a demultiplexer configured to demultiplex the light having the second wavelength from the signal light.

[0557] [Item B-6] The detection device according to any one of Items B-1 to B-5, wherein

[0558] The differential phase information acquisition unit includes:

[0559] A delayed interferometer section, into which the input light is input;

[0560] a photoelectric conversion unit that converts the output light of the delayed interferometer unit into an electrical signal; and

[0561] an integrating section, wherein the electrical signal outputted from the photoelectric conversion section is inputted into the integrating section, and the integrating section outputs a voltage obtained by integrating the waveform of the input voltage by time,

[0562] The delay interference section

[0563] The input light is branched into a first input light and a second input light,

[0564] The first input light passing through the first optical path and the second input light passing through the second optical path are combined to interfere with each other.

[0565] The delay interferometer is set such that the delay 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 and the frequency f of the input light satisfy the following mathematical formula FB1:

[0566] (Mathematical formula FB1)

[0567] 2πfτ=2nπ+π / 2 (where n is an integer).

[0568] [Item B-7] A light receiving device comprising:

[0569] The detection device according to any one of items B-1 to B-6; and

[0570] The demodulation unit demodulates the reception signal transmitted by the signal light and generates an information signal.

[0571] [Item B-8] An optical communication system comprising:

[0572] An optical transmitting device for transmitting the above-mentioned signal light; and

[0573] The light receiving device described in item B-7.

[0574] [Item B-9] A program for causing a computer to function as the detection device according to any one of Items B-1 to B-6.

[0575] [Item B-10] A detection method for detecting polarization variation of signal light propagating in an optical transmission path, the detection method comprising:

[0576] A differential phase information acquisition stage of acquiring information indicating the differential phase of input light to be evaluated at each of one or more time points; and

[0577] In the determination stage, it is determined whether the differential phase of at least a part of the one or more time points satisfies a predetermined condition.

[0578] The predetermined condition includes at least one of the first condition, the second condition, the third condition, the fourth condition and the fifth condition.

[0579] The first condition is that the magnitude of the differential phase at at least one of the one or more time points is equal to or greater than a predetermined first threshold value,

[0580] The second condition is that the number of time points at which the magnitude of the differential phase at the time point is equal to or greater than the first threshold value among the plurality of time points included in the evaluation period as a period having a predetermined length, which is at least a part of the one or more time points, is equal to or greater than a predetermined second threshold value,

[0581] The third condition is that the ratio of the number of time points at which the magnitude of the differential phase at the time point is equal to or greater than the first threshold value among the multiple time points to the number of the multiple time points included in the evaluation period is equal to or greater than a predetermined third threshold value,

[0582] The fourth condition is that the moving angle at the variation frequency of the trajectory on the Poincare sphere derived from the magnitude of the differential phase at each of the one or more time points is equal to or greater than a predetermined fourth threshold value,

[0583] The fifth condition is that the angular velocity of movement of the trajectory on the Poincare sphere at the variation frequency is equal to or greater than a predetermined fifth threshold.

[0584] Description of Reference Numerals

[0585] 10: optical transmission path, 100: communication system, 110: optical signal transmission device, 120: optical signal receiving device, 130: local oscillator, 140: optical 90 degree hybrid, 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 unit, 340: optical delay interferometer, 350: optical receiver, 360: AD converter, 370: signal processing unit, 422 : half mirror, 424: half 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: standardization unit, 1040: optical delay interferometer, 1060: half mirror, 1140: optical delay interferometer, 1160: waveguide, 1240: optical delay interferometer, 1260 : waveguide, 1340: optical delay interferometer, 1350: balanced optical receiver, 1352: optical receiver, 1354: optical receiver, 1356: differential processing unit, 1452: photodiode, 1454: photodiode, 1456: connection point, 1540: optical delay interferometer, 1600: communication system, 1620: optical signal receiving device, 1640: splitter, 1650: optical receiver, 1660: AD converter, 1670: signal processing unit, 1680: state monitoring device, 1722: communication optical signal output unit, 1724: monitoring optical signal output unit, 1730: combiner, 1852: photoelectric conversion element, 1854: integration circuit, 1870: signal processing unit, 1922: increase detection unit, 1924: frequency analysis unit, 1926: movement angle derivation unit, 1928: movement angular velocity derivation unit, 1930: polarization change detection unit, 1940: information output unit, 2100: data table, 2120: number, 2122: time, 2124: output value, 2132: change frequency, 2134: movement angle, 2136: movement angular velocity, 2140: establishment or not, 2150: detection result, 3000: computer, 3001: DVD-ROM, 3010: host controller, 3012: CPU, 3014: RAM, 3016: GPU, 3018: display device, 3020: input and output controller, 3022: communication interface,3024: Hard disk drive, 3026: DVD-ROM drive, 3030: ROM, 3040: Input / output chip, 3042: Keyboard.

Claims

1. A detection device for detecting polarization variation of signal light propagating in an optical transmission path, the detection device comprising: a differential phase information acquisition unit that acquires information indicating a differential phase of input light to be evaluated at each of one or more time points; and a determination unit for determining whether the differential phase at at least a part of the one or more time points satisfies a predetermined condition, The differential phase information acquisition unit comprises: a delayed interferometer that branches the input light into a first input light and a second input light, causes the first input light to pass through a first optical path and the second input light to pass through a second optical path so that a delay time difference is given between the first input light and the second input light, and outputs output light that is obtained by combining and interfering the first input light and the second input light that are given the delay time difference; and a photoelectric conversion unit that converts the output light of the delayed interferometer unit into an electric signal, obtains information indicating the differential phase of the input light, and outputs an output current indicating the differential phase, The predetermined condition includes at least one of a first condition, a second condition, a third condition, a fourth condition, and a fifth condition, The first condition is that the magnitude of the differential phase at at least one of the more than one time points is equal to a predetermined first threshold or greater than the first threshold, The second condition is that the number of time points at which the magnitude of the differential phase at the time point is equal to or greater than the first threshold value among the plurality of time points included in the evaluation period having a predetermined length, which is at least a part of the one or more time points, is equal to or greater than a predetermined second threshold value, The third condition is that the ratio of the number of time points at which the magnitude of the differential phase at the time point is equal to the first threshold or greater than the first threshold to the number of the multiple time points included in the evaluation period is equal to a predetermined third threshold or greater than the third threshold, The fourth condition is that the moving angle at the variation frequency of the trajectory on the Poincare sphere derived from the magnitude of the differential phase at each of the one or more time points is equal to or greater than a predetermined fourth threshold value, The fifth condition is that the moving angular velocity of the trajectory on the Poincare sphere at the variation frequency is equal to a predetermined fifth threshold or is greater than the fifth threshold.

2. The detection device according to claim 1, wherein: The detection device further includes an output unit configured to output information indicating that polarization change has occurred in the signal light when the differential phase at at least a part of the one or more time points is determined to satisfy the predetermined condition.

3. The detection device according to claim 1, wherein: The differential phase represents a difference in phase between the input light at two temporally adjacent time points among the one or more time points. The time intervals of the one or more time points are substantially constant.

4. The detection device according to claim 1, wherein: The signal light includes: Light of a first wavelength is used to transmit information signals; and A second wavelength of light is used to detect polarization changes, The value of the first wavelength is different from the value of the second wavelength, The input light is light of the second wavelength.

5. The detection device according to claim 4, wherein: The detection device further includes a demultiplexer configured to demultiplex the light having the second wavelength from the signal light.

6. The detection device according to claim 1, wherein: The differential phase information acquisition unit further includes an integration unit, the output current output by the photoelectric conversion unit is input to the integration unit, and the integration unit outputs a voltage obtained by time-integrating the waveform of the input voltage. In the delayed interference section, The delay 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 and the frequency f of the input light are set to satisfy the relationship of the following mathematical formula 1: Mathematical formula 1 2πfτ=2nπ+π / 2, where n is an integer.

7. A light receiving device comprising: The detection device according to any one of claims 1 to 6; and The demodulation unit demodulates the reception signal transmitted by the signal light to generate an information signal.

8. An optical communication system comprising: an optical transmitting device that transmits signal light; and The light receiving device as claimed in claim 7. 9 . A computer-readable storage medium storing a program for causing a computer to function as the detection device according to claim 1 .

10. A detection method for detecting polarization variation of signal light propagating in an optical transmission path, the detection method comprising: A differential phase information acquisition stage of acquiring information indicating the differential phase of input light to be evaluated at each of one or more time points; and In a determination stage, it is determined whether the differential phase of at least a part of the one or more time points satisfies a predetermined condition. The differential phase information acquisition stage includes a stage of branching the input light into a first input light and a second input light, a stage of causing the first input light to pass through a first optical path and the second input light to pass through a second optical path so that a delay time difference is given between the first input light and the second input light, a stage of outputting output light after the first input light and the second input light given the delay time difference are subjected to multiplexing interference, a stage of converting the output light into an electrical signal and acquiring information indicating the differential phase of the input light, and a stage of outputting an output current indicating the differential phase. The predetermined condition includes at least one of a first condition, a second condition, a third condition, a fourth condition, and a fifth condition, The first condition is that the magnitude of the differential phase at at least one of the more than one time points is equal to a predetermined first threshold or greater than the first threshold, The second condition is that the number of time points at which the magnitude of the differential phase at the time point is equal to or greater than the first threshold value among the plurality of time points included in the evaluation period having a predetermined length, which is at least a part of the one or more time points, is equal to or greater than a predetermined second threshold value, The third condition is that the ratio of the number of time points at which the magnitude of the differential phase at the time point is equal to the first threshold or greater than the first threshold to the number of the multiple time points included in the evaluation period is equal to a predetermined third threshold or greater than the third threshold, The fourth condition is that the moving angle at the variation frequency of the trajectory on the Poincare sphere derived from the magnitude of the differential phase at each of the one or more time points is equal to or greater than a predetermined fourth threshold value, The fifth condition is that the moving angular velocity of the trajectory on the Poincare sphere at the variation frequency is equal to a predetermined fifth threshold or is greater than the fifth threshold.

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