Detector, optical-signal reception device, optical communication system, program, and detection method

Abstract

This detector for detecting optical phase noise of signal light propagated through an optical transmission path comprises a detection unit for detecting optical phase noise of the signal light on the basis of a difference signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delay signal obtained by delaying the detection signal. This detection method for detecting optical phase noise of signal light propagated through an optical transmission path comprises a step for detecting optical phase noise of the signal light on the basis of a difference signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delay signal obtained by delaying the detection signal.

Description

Detection device, optical signal receiving device, optical communication system, program, and detection method

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

[0002] Non-Patent Documents 1 to 5 disclose that optical transmission characteristics deteriorate due to the generation of phase noise in signal light, for example, via the Kerr effect. Non-Patent Document 6 discloses that optical phase noise from which 1 / f noise components have been removed, as disclosed in Non-Patent Document 7, can be derived by subtracting the moving average value of the optical phase from the phase of the measured received signal. Non-Patent Documents 8 and 9 disclose demodulating DPSK signals using a delay interferometer. 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 fluctuations of about 10 kHz or more can occur in coherent optical communication systems. Non-Patent Document 15 discloses that there is a proportional relationship between the value of the voltage applied to a fiber stretcher and the values ​​of the lateral pressure and birefringence that are generated.

[0004] (Prior art document) (Non-approved document) (Non-approved document 1) JP Gordon and LF Mullenauer, "Phase noise in photonic communications systems using linear amplifiers," Optics Letters, Vol. 15, No. 23, pp. 1351-1353, 1990. (Non-approved document 2) S. Ryu, "Signal linewidth broadening due to nonlinear Kerr effect in long-haul coherent systems using cascaded optical amplifiers," IEEE Journal of Lightwave Technology, Vol. 10, No. 10, pp. 1450-1457, 1992. (Unauthorized 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. (Non-licensed 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. (Non-licensed 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. (Non-patent document 6) M. Nakazawa et al., Editor, High spectral density optical communication technologies, Springer-Verlag, 2010. (Non-patent document 7) K. Kikuchi, "Effect of 1 / f-type FM noise on semiconductor-laser linewidth residual in high-power limit," IEEE Journal of Quantum Electronics, Vol. 25, No. 4, pp. 684-688, 1989. 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. (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. (Non-Patent Document 10) T. Kominato et al., "Ring resonators composed of GeO2-doped silica waveguides," IEEE Journal of Lightwave Technology, Vol. 10, No. 12, pp. 1781-1788, 1992.(Non-patent document 11) S. Suzuki et al., "Integrated-optic double-ring resonators with a wide free spectral range of 100 GHz," IEEE Journal of Lightwave Technology, Vol. 13, No. 8, pp. 1766-1771, 1995. (Non-patent document 12) W. Bogaerts et al., "Silicon microring resonators," Laser and Photonics Reviews, Vol. 6, No. 1, pp. 47-73, 2012. (Non-patent document 13) PM Krummrich, E.-D. Schmidt, W. Weiershausen, and A. Mattheus, "Field trial results on statistics of fast polarization changes in long haul WDM transmission systems," OFC2005, paper OThT6, March 2005. (Non-patent document 14) M. Boroditsky, M. Brodsky, NJ Frigo, P. Magill, and H. Rosenfeldt, "Polarization dynamics in installed fiberoptic systems," 2005 IEEE LEOS Annual Meeting, paper TuCC1, October 2005. General disclosure

[0005] In a first aspect of the present invention, there is provided a detection device that detects optical phase noise of signal light that has propagated through an optical transmission line, and the detection device includes a detection unit that detects the optical phase noise of the signal light based on a differential signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delayed signal obtained by delaying the detection signal.

[0006] In the above detection device, the detection section may detect optical phase noise of the signal light based on a distribution of the differential signals detected at a plurality of points in time.

[0007] In any of the above detection devices, the detection unit may include a histogram generation unit that generates a histogram of the differential signals detected at the multiple time points, and a calculation unit that calculates optical phase noise of the signal light based on a standard deviation of the histogram.

[0008] In any of the above detection devices, the delayed signal may be a signal delayed from the detection signal by a predetermined time, and the predetermined time may be 1 / (2B) or more or longer than 1 / (2B), where B is a detection band of the detection signal.

[0009] Any of the above detection devices may include a delay section that generates the delayed signal by delaying the detection signal.

[0010] In any of the detection devices described above, the delay section may include one of a delay line and a low-pass filter.

[0011] Any of the above detection devices may include a subtraction section that subtracts the detection signal from the delayed signal to generate the difference signal.

[0012] In any of the above detection devices, the optical delay interferometer may include a delay interference unit that splits input light into first input light and second input light, and multiplexes and interferes with the first input light that has passed through a first optical path and the second input light that has passed through a second optical path. ｄ and the frequency f of the input light ｃHowever, n may be an integer and may be set to satisfy the relationship of the following formula 1: 2πf (Formula 1) ｃ τ ｄ = 2nπ + π / 2

[0013] In a second aspect of the present invention, there is provided an optical signal receiving device, comprising any one of the detecting devices described above, and a demodulation unit that demodulates a received signal transmitted by the signal light and generates an information signal.

[0014] In a third aspect of the present invention, there is provided an optical communication system, comprising: the optical signal receiving device described above; and an optical signal transmitting device for transmitting the signal light.

[0015] In a fourth aspect of the present invention, a program is provided. The program is, for example, a program for causing a computer to function as any of the detection devices described above. A computer-readable storage medium storing the program may be provided. The storage medium may be a non-transitory computer-readable medium.

[0016] In a fifth aspect of the present invention, there is provided a detection method for detecting optical phase noise of signal light propagated through an optical transmission line, the detection method comprising the step of detecting the optical phase noise of the signal light based on a differential signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delayed signal obtained by delaying the detection signal.

[0017] The above summary of the invention does not list all of the necessary features of the present invention, and subcombinations of these features may also constitute inventions.

[0018] 1 shows an example of the system configuration of a communication system 100.

[0033] FIG. 1 shows an example of the internal configuration of an optical delay interferometer.

[0034] FIG. 1 shows an example of the internal configuration of an optical delay interferometer.

[0035] FIG. 1 shows an example of the functional configuration of a detection device 120.

[0036] FIG. 2 shows a graph illustrating the characteristics of an autocorrelation function r(τ).

[0037] FIG. 2 shows an example of the functional configuration of a detection unit 250.

[0038] FIG. 3 shows a histogram of a calculation target signal corresponding to Δi(t), obtained under the same experimental conditions as the first experiment described with reference to FIGS. 3 to 5 .

[0039] FIG. 4 shows a histogram of an optical phase fluctuation when polarization fluctuation is generated.

[0039] FIG. 5 shows a signal of polarization fluctuation in the signal light.

[0039] FIG. 6 shows a histogram of an optical phase fluctuation corresponding to optical phase noise when polarization fluctuation is generated.

[0039] FIG. 7 shows a histogram of an optical phase fluctuation when polarization fluctuation is generated.

[0039] FIG. 8 shows a signal of polarization fluctuation in the signal light.

[0039] FIG. 9 shows a histogram of an optical phase fluctuation corresponding to optical phase noise when polarization fluctuation is generated.

[0039] FIG. 10 shows a histogram of an optical phase fluctuation when polarization fluctuation is generated. 20 shows a functional configuration of a detection device 1620 as a variation of the detection device 120. FIG. 21 shows a functional configuration of a detection device 1720 as a variation of the detection device 1620. FIG. 22 shows a flowchart illustrating a detection method performed by the detection device 120. FIG. 23 shows an example of a computer 2000 in which multiple embodiments of the present invention may be embodied in whole or in part.

[0019] The present invention will be described below through embodiments of the invention, but the following embodiments do not limit the scope of the invention. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention. In the drawings, the same reference numerals are used to designate the same or similar parts, and redundant explanations may be omitted.

[0020] 1 schematically illustrates an example of the system configuration of a communication system 100. The communication system 100 includes an optical signal transmitting device 110 and an optical signal receiving device 20. The optical signal receiving device 20 includes a branching device 40, an optical receiver 50, an AD converter 60, a signal processing unit 70, and a detecting device 120.

[0021] In this embodiment, the details of the communication system 100 are described using as an example a case in which an information signal is transmitted by the signal light output by the optical signal transmitting device 110 propagating through the optical transmission path 10 and reaching the optical signal receiving device 20.

[0022] The optical signal transmitter 110 generates an optical signal. The optical signal transmitter 110 generates a polarization-multiplexed optical signal, for example, by modulating an optical carrier with an information signal to be transmitted. The optical signal may be a signal on which a symbol time series is superimposed. The optical signal transmitter 110 outputs light (sometimes referred to as signal light) that constitutes the optical signal. The signal light is transmitted to the optical signal receiver 20 via the optical transmission path 10.

[0023] The optical transmission line 10 transmits light. An example of the optical transmission line 10 is an optical fiber.

[0024] The optical signal receiving device 20 receives the signal light that has propagated through the optical transmission line 10. The optical signal receiving device 20 demodulates the received optical signal to generate an information signal.

[0025] The splitter 40 splits the input signal light into a signal light (sometimes referred to as a communication optical signal) that is input to the optical receiver 50 and a signal light (sometimes referred to as a polarization fluctuation detection signal) that is input to the detection device 120. The communication optical signal is used to detect an information signal. The polarization fluctuation detection signal is used to detect polarization fluctuation.

[0026] The optical receiver 50 converts an optical signal into an electrical signal. The optical receiver 50 converts, for example, an optical signal for communication into an electrical signal (sometimes referred to as an "electrical signal corresponding to the optical signal for communication"). The optical receiver 50 outputs the electrical signal corresponding to the optical signal for communication to the AD converter 60.

[0027] The AD converter 60 converts the electrical signal from an analog signal to a digital signal. The AD converter 60 generates one or more digital signals corresponding to each of a plurality of sampling points by, for example, sampling the electrical signal. The AD converter 60 converts, for example, an electrical signal corresponding to an optical signal for communication into a digital signal (sometimes referred to as a "digital signal corresponding to the optical signal for communication"). The AD converter 60 outputs the digital signal corresponding to the optical signal for communication to the signal processing unit 70.

[0028] The signal processing unit 70 demodulates the received signal transmitted by the signal light to generate an information signal. The method for generating the information signal from the received signal is not particularly limited. The signal processing unit 70 may perform various digital signal processing to demodulate and compensate the received signal. The signal processing unit 70 may perform error correction processing, decoding processing, etc. The signal processing unit 70 is an example of a demodulation unit.

[0029] Phase fluctuations such as optical phase noise and polarization fluctuations may occur in the signal light while the signal light propagates through the optical transmission line 10. Causes of phase fluctuations in the signal light include nonlinear optical effects, vibrations of the optical fiber, twisting of the optical fiber, fluctuations in stress applied to the optical fiber, laser linewidth or laser phase fluctuations, and phase fluctuations in the optical field caused by lightning strikes.

[0030] Depending on the state of optical phase fluctuation, bit errors may occur in an optical transmission system. For example, if the optical phase noise in the signal light is greater than a predetermined threshold determined for each modulation method, bit errors may occur when the demodulated signal is subjected to threshold determination. The predetermined threshold has a one-to-one relationship with the modulation method.

[0031] Situations in which polarization fluctuations occur include (i) when fluctuating lateral stress is applied to the optical fiber, causing fluctuations in the birefringence of the optical fiber, and (ii) when a sudden twist occurs in the optical fiber, causing the polarization plane to rotate at high speed. Therefore, in many cases, phase fluctuations in signal light due to polarization fluctuations can occur suddenly. The signal processing unit 70 includes a polarization separation processing circuit that performs polarization separation processing during the signal processing process. When polarization fluctuations occur and the polarization fluctuation speed exceeds the processing speed of the polarization separation processing circuit, the polarization separation processing circuit cannot perform polarization separation processing normally, and code errors may occur.

[0032] The detection device 120 analyzes the signal light input to the detection device 120 to detect the state of the optical transmission line 10 and / or the state of the signal light propagated through the optical transmission line 10. Specifically, the detection device 120, for example, analyzes the signal light to evaluate (i) the presence and / or the degree of optical phase noise. In this manner, the detection device 120 detects the optical phase noise of the signal light propagated through the optical transmission line 10. The detection device 120 outputs evaluation information related to the detected optical phase noise. The detection device 120 may further analyze the signal light to evaluate (ii) the presence and / or the degree of polarization fluctuation. In this manner, the detection device 120 can detect polarization fluctuation. The detection device 120 may output evaluation information related to the detected polarization fluctuation.

[0033] Examples of the evaluation information related to optical phase noise include information indicating the magnitude of the optical phase noise and information indicating that the magnitude of the optical phase noise has exceeded a predetermined value. Examples of the evaluation information related to polarization fluctuation include information indicating that polarization fluctuation has occurred in the signal light, information indicating that the angular velocity of movement on the Poincaré sphere indicating the polarization state of light has exceeded a specified angular velocity, information indicating that the movement angle has exceeded a specified angle, and information indicating that the fluctuation frequency has exceeded a specified frequency. The evaluation information related to optical phase noise and / or the evaluation information related to polarization fluctuation may be output as an alarm or a flag. The evaluation information may be used in demodulation processing by the signal processing unit 70.

[0034] The optical signal receiving device 20 can evaluate the presence and / or extent of optical phase noise with relatively high accuracy, which can improve the operation efficiency and maintenance efficiency of the optical communication system.

[0035] In this embodiment, the detection device 120 includes an optical delay interferometer, and detects at least the optical phase noise of the signal light based on a detection signal detected by the optical delay interferometer. First, with reference to FIG. 2 , the optical phase noise and the signal related to polarization fluctuation detected by the optical delay interferometer will be described.

[0036] 2 is a schematic diagram showing an example of the internal configuration of an optical delay interference unit 80 included in the optical delay interferometer. The optical delay interference unit 80 includes a semi-transmitting mirror 82, a semi-transmitting mirror 84, a total reflecting mirror 86, and a total reflecting mirror 88.

[0037] The signal light as input light input to the optical delay interference unit 80 is split into a first input light that passes through the semi-transparent mirror 82 and a second input light that is reflected by the semi-transparent mirror 82. The first input light passes through the semi-transparent mirror 84 and is output from the optical delay interference unit 80. On the other hand, the second input light is reflected by the total reflecting mirror 86, the total reflecting mirror 88, and the semi-transparent mirror 84, and then undergoes multiplexing interference with the first signal light that passed through the semi-transparent mirror 84, and is output as output light from the optical delay interference unit 80.

[0038] In this embodiment, the optical delay interference unit 80 provides an optical path difference between the first input light and the second input light. The delay time difference due to the optical path difference is τ ｄ Let time be t and the phase of the first signal light when it is output be φ(t). Then, the phase of the second signal light when it is output is φ(t-τ ｄ ) The delay time difference τ ｄ is the frequency of the input light f ｃ , where n is an integer, and ｃ τ ｄ =2nπ+π / 2.

[0039] An optical phase adjuster may be disposed in a region of the optical path of the second signal light (sometimes referred to as the second optical path) that does not overlap with the optical path of the first signal light (sometimes referred to as the first optical path) (for example, between the total reflection mirror 88 and the semi-transparent mirror 84). The optical phase adjuster adjusts the phase of light passing through the optical phase adjuster. For example, the optical phase adjuster adjusts the phase of light passing through the optical phase adjuster in accordance with a voltage applied to the optical phase adjuster. Examples of optical phase adjusters include a liquid-phase element and a lithium niobate crystal.

[0040] The delay time difference τ ｄ is adjusted. This allows the operating point of the optical delay interference unit 80 to be adjusted. ｄ may be adjusted during calibration or in real time. In another embodiment, the optical phase adjuster may be disposed in a region of the first optical path that does not overlap with the second optical path.

[0041] Thus, the optical delay interferometer includes a delay interference unit that splits input light into first input light and second input light, and multiplexes and interferes with the first input light that has passed through the first optical path and the second input light that has passed through the second optical path. ｄ and the frequency of the input light f ｃ and n is an integer, and 2πf ｃ τ ｄ = 2nπ + π / 2.

[0042] The optical delay interference unit included in the optical delay interferometer is not limited to the optical delay interference unit 80. In another example of the optical delay interference unit, a configuration in which at least one of the first optical path and the second optical path is realized using a waveguide may be employed. In yet another example of the optical delay interference unit, a configuration in which a ring resonator is provided in the second optical path may be employed.

[0043] Next, the signal detected by the optical delay interferometer will be described. The orthogonal polarization mode of the signal light propagating in the z direction is defined as E ｘ (t) and E ｙ (t), and the unit vectors in the x and y directions are k ｘand k ｙ Then, the electric field E(t) of the light propagated through the optical transmission line and received (sometimes referred to as the received optical field) is expressed by the following equation 1:

[0044] In Equation 1, δ(t) is E ｘ (t) and E ｙ (t) represents the phase difference between ｎ (t) represents the optical phase noise. ｃ represents the frequency of the signal light. ｘ and k ｙ is a vector. In equation (1), E ｘ (t) and E ｙ It is assumed that the E(t) appears in the opposite direction by δ(t) / 2. For example, when a mechanical stress fluctuation is applied to an optical fiber, the stress causes E ｘ (t) and E ｙ This is based on the fact that (t) is affected by the refractive index fluctuations in the opposite directions.

[0045] Assuming that the optical delay interferometer has no polarization dependency, the output current i of the optical receiver connected to the optical delay interferometer is expressed by the following equation 2:

[0046] In Equation 2, τ ｄ represents the delay time difference between the two paths of the optical delay interferometer. R represents the sensitivity of the optical receiver. · represents the dot product.

[0047] Considering that the photodiode used to detect light in the optical receiver does not respond to the frequency of light, and ignoring the DC component in Equation 2, the received current i output from the optical receiver is expressed by the following Equation 3: (Equation 3)

[0048] In Equation 3, the constant is omitted and the delay time difference τ ｄ is 2πf ｃ τ ｄ = 2nπ + π / 2 (n is a positive integer), the received current i is expressed by the following equation 4: (Equation 4)

[0049] In Formula 4, E ｘ (t), E ｙ (t), φ ｎ Let (t) and δ(t) be expressed as in the following Equation 5. (Equation 5)

[0050] Here, ΔE ｘ / E ｘ <<1, ΔE ｙ / Ey<<1, Δφ ｎ <<1 and Δδ<<1, Equation 4 can be expressed by the following Equation 6: (Equation 6)

[0051] Here, E ｘ ２ (t) + E ｙ ２ When the electric field components are normalized so that (t)=1, Equation 6 is expressed by the following Equation 7: (Equation 7)

[0052] In Equation 7, the first term on the right side, Δφ ｎ (t) is the current i corresponding to the optical phase noise ｐｎ The second term on the right side is −{E ｘ ２ (t)-E ｙ ２ (t)}Δδ(t) / 2 is the current i corresponding to the polarization fluctuation ｐｏｌ That is, i ｐｎ and i ｐｏｌ is expressed by the following formula 8. (Formula 8)

[0053] Optical phase noise is constantly present because it is constantly output from a semiconductor laser. On the other hand, polarization fluctuations often occur only under specific conditions, such as when an external force is applied to an optical fiber. Therefore, one method for detecting optical phase noise is to measure it when no polarization fluctuations are occurring. Polarization fluctuations can be measured by applying a low-pass filter to i(t), taking advantage of the fact that the highest fluctuation frequency is below a certain level.

[0054] 3 to 5 show an example of experimental results in which known polarization fluctuations were generated and optical phase noise and polarization fluctuations were detected using the above-mentioned method. Light with a wavelength of 1550 nm output from a semiconductor laser was used as signal light and input to an optical fiber. The signal light was then input to a fiber stretcher. The fiber stretcher had a structure in which an optical fiber was wound around a piezoelectric element. Applying a varying voltage to the piezoelectric element stretched and contracted the piezoelectric element, thereby stretching and contracting the optical fiber wound around the piezoelectric element, thereby varying the birefringence of the optical fiber. A function generator generated a 142 kHz, 10 Vpp sinusoidal voltage to drive the piezoelectric element, varying the birefringence of the optical fiber wound around the piezoelectric element. The light passing through the fiber stretcher was detected by an optical delay interferometer consisting of an optical delay interference unit and a balanced receiver. The detected signal was A / D converted to obtain the signal i expressed by Equation 8. ｐｎ and i ｐｏｌ A signal (sometimes referred to as signal i) corresponding to the sum of the signals i and i is output.

[0055] A histogram is generated to analyze the optical phase noise of signal i. Signal i contains frequency components up to the bandwidth of the balanced receiver. In this experiment, the bandwidth of the balanced receiver is approximately 150 MHz. On the other hand, the polarization fluctuation caused by the fiber stretcher is mainly 142 kHz and its harmonic components. Because the polarization fluctuation is caused by mechanical vibration, the main frequency components of the fluctuation are thought to be up to about 1 MHz, even when considering harmonics of the fluctuation frequency. Therefore, the polarization fluctuation components can be obtained by applying a low-pass filter to signal i.

[0056] Fig. 3 shows a histogram of optical phase fluctuations when no polarization fluctuations are generated. The horizontal axis of Fig. 3 represents signal i, and the vertical axis represents probability density (frequency). The standard deviation, which indicates optical phase noise, is calculated by fitting a Gaussian distribution function 130 to the histogram. In this experiment, the standard deviation calculated from the fitted Gaussian distribution function 130 was 0.0141 (rad).

[0057] Fig. 4 shows a histogram of optical phase fluctuations when polarization fluctuations are generated. The horizontal axis of Fig. 4 represents signal i, and the vertical axis represents probability density (frequency). As mentioned above, this histogram includes the effects of phase fluctuations, including optical phase noise and polarization fluctuations.

[0058] 5 shows the polarization fluctuation signal in the signal light. The horizontal axis of FIG. 5 represents time, and the vertical axis represents i ｐｏｌ It is. ｐｏｌ is obtained by applying a low-pass filter with a cutoff frequency of 1 MHz to signal i. As shown in Fig. 4, signal i contains the effects of polarization fluctuations in addition to optical phase noise, so it is difficult to accurately calculate the standard deviation of the optical phase noise from the histogram in Fig. 4 using the method shown in Fig. 3.

[0059] As described above, since it is difficult to detect only the optical phase noise component from the output signal of an optical delay interferometer when polarization fluctuations are present, a method of measuring the optical phase noise by capturing a state in which there is no polarization fluctuation is adopted. However, since polarization fluctuations can always occur in an actually installed optical fiber, it is not easy to capture a state in which there is no polarization fluctuation. The detection device 120 according to this embodiment has a configuration for appropriately detecting the optical phase noise in the signal light even when polarization fluctuations are present.

[0060] 6 shows an example of the functional configuration of the detection device 120. The detection device 120 includes an optical delay interference unit 200, an optical receiver 210, an AD converter 220, a delay unit 230, a subtraction unit 240, a detection unit 250, a low-pass filter 270, and a detection unit 280.

[0061] The optical delay interference unit 200 has the same configuration as the optical delay interference unit 80. That is, the optical delay interference unit 200 splits input light into first input light and second input light, and multiplexes and interferes 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. The delay time difference τ between the first input light that has passed through the first optical path and the second input light that has passed through the second optical path is ｄ and the frequency fc of the input light is 2πf, where n is an integer. ｃ τ ｄ = 2nπ + π / 2.

[0062] The optical receiver 210 detects the output light from the optical delay line interferometer 200. The optical receiver 210 detects the output light from the optical delay line interferometer 200 and generates a detection signal. The optical delay line interferometer 200 and the optical receiver 210 function as an optical delay line interferometer. The optical receiver 210 may be a balanced receiver.

[0063] The AD converter 220 converts the analog detection signal generated by the optical receiver 210 into a digital signal. The digital detection signal generated by the AD converter 220 is branched into three signals, which are input to the delay unit 230, the subtraction unit 240, and the low-pass filter 270, respectively.

[0064] The delay unit 230 delays the detection signal, which is a digital signal generated by the AD converter 220, by a predetermined delay time. The delay unit 230 may delay the detection signal by digital signal processing. The delay unit 230 may be realized by a low-pass filter using digital signal processing. In this way, the delay unit 230 generates the delayed signal by delaying the detection signal.

[0065] The delayed signal may be a signal obtained by delaying the detection signal by a predetermined delay time. The delay time may be 1 / (2B) or more, where B is the detection band of the detection signal. The delay time may be longer than 1 / (2B). The detection band B may be the detection band of the optical receiver 210.

[0066] The subtraction section 240 subtracts the detection signal from the delay signal to generate a differential signal. The subtraction section 240 may perform subtraction processing based on the detection signal as a digital signal and the delay signal as a digital signal through digital signal processing to generate the differential signal. The detection section 250 detects optical phase noise of the signal light based on the differential signal. The detection section 250 generates evaluation information based on the detected optical phase noise.

[0067] The detecting unit 250 detects the optical phase noise of the signal light based on the differential signal, thereby enabling the detecting unit 250 to appropriately detect the optical phase noise of the signal light even when polarization fluctuations occur. The effects obtained by detecting the optical phase noise of the signal light based on the differential signal will be described later.

[0068] In this way, the detecting unit 250 detects the optical phase noise of the signal light based on a differential signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delayed signal obtained by delaying the detection signal. Specifically, the detecting unit 250 detects the optical phase noise of the signal light based on a distribution of differential signals detected at multiple points in time. As will be described later, the detecting unit 250 generates a histogram of the differential signals detected at multiple points in time and detects the optical phase noise of the signal light based on the generated histogram. More specifically, the detecting unit 250 detects the optical phase noise based on the standard deviation of the generated histogram.

[0069] The low-pass filter 270 reduces frequency components higher than the cutoff frequency in the detection signal output from the AD converter 220 and substantially passes frequency components lower than the cutoff frequency. The cutoff frequency of the low-pass filter 270 may be approximately 1 MHz. The detector 280 detects polarization fluctuations in the signal light based on the frequency components of the detection signal that have passed through the low-pass filter 270. The detector 280 outputs evaluation information based on the detected polarization fluctuations.

[0070] Here, the differential signal generated by the subtractor 240 will be described. Here, the signal light will be described as being generated based on the light output from the semiconductor laser. The phase noise of the light output from the semiconductor laser is white noise, and its bandwidth is said to be about several GHz (see, for example, Non-Patent Document 7). As mentioned above, polarization fluctuations occurring in optical fibers are mainly caused by mechanical vibrations, and their maximum frequency is at most about 1 MHz. Therefore, it is considered that the optical signal does not contain fluctuation components caused by polarization fluctuations in bandwidths of about 1 MHz or higher.

[0071] If the bandwidth of delayed white noise is infinite, it will be completely uncorrelated with the original white noise. This property can be expressed by the autocorrelation function. In general, the autocorrelation function r(τ) of a stochastic process x(t) is defined as follows (see, for example, S. Stein and JJ Jones, Modern communication principles with application to digital signaling, McGraw Hill, 1967): (Equation 9)

[0072] where E represents the ensemble average. r(τ) represents the degree of correlation between x(t) at time t and x(t+τ) at time t+τ as a function of τ. When x(t) is a stochastic process with the statistical properties of white noise and the band is infinite, that is, when the power spectral density S(f) of x(t) is n ０ is a constant, (Equation 10) When expressed as above, r(τ) becomes a δ function as shown in the following equation 11. (Equation 11)

[0073] In other words, equation 11 indicates that delayed white noise will be completely uncorrelated with the original white noise if the band is infinite. However, in reality, the band is limited by the circuit including the optical receiver, so equation (10) does not hold true, and S(f) is limited to the limited band B. In other words, in reality, the following equation 12 holds true: (Equation 12)

[0074] In this case, the autocorrelation function r(τ) is expressed by the following equation 13 (see, for example, S. Stein and JJ Jones, Modern communication principles with application to digital signaling, McGraw Hill, 1967). (Equation 13)

[0075] r(τ) reaches a value of 0 at τ = 1 / (2B), then as τ increases, it reaches 0 again at τ = 1 / B and decays oscillatorily with increasing τ.

[0076] Here, taking the experiment described above in connection with FIGS. 3 to 5 as an example, the bandwidth of the balanced receiver as an optical receiver is 150 MHz. Therefore, B in Equations 12 and 13 is 150 MHz. Furthermore, in the above experiment, the output voltage of the balanced receiver is sampled at a sample rate of 3.125 (GS / s). Therefore, the time difference τ between sampled data samples is ｓ is 3.2 x 10 －１０ (s).

[0077] 7 is a graph showing the characteristics of the autocorrelation function r(τ). The horizontal axis of FIG. 7 represents the number of samples, and the vertical axis represents the value of the autocorrelation function. In FIG. 7, the autocorrelation function in Equation 13 is expressed as n ０ Normalize τ as B=1 and set τ ｓ Normalized by (τ / τ ｓ , i.e., the number of samples), the autocorrelation function is expressed.

[0078] 7, r(τ) becomes 0 at a sample count of 10.4, and again becomes 0 at a sample count of 20.8, demonstrating oscillatory decay as the sample count increases. Therefore, it can be seen that the correlation of white noise limited to a 150 MHz band becomes low when the sample count is separated by 10.4 or more (corresponding to 1 / (2B)) or more, or when the sample count is separated by 20.8 or more (corresponding to 1 / (B)).

[0079] On the other hand, as mentioned above, the polarization fluctuation in the signal light is mainly low frequency components of about 1 MHz or less, so it is about 10 to 20 samples (3.2 × 10 －９ ~6.4 x 10 －９ Even with a time delay of about 1 / 2 second, there can be a strong correlation between the original and delayed signals.

[0080] Therefore, i expressed by Equation 8 ｐｎ (t) and i ｐｏｌRegarding (t), the following can be said to be true: ｐｎ (t) and i ｐｎ (t-τ) can be considered as having almost no correlation when τ=1 / (2B) or 1 / B or greater. ｐｏｌ (t) and i ｐｏｌ (t-τ) is a strong correlation when τ=1 / (2B) or 1 / B or more, and i ｐｏｌ (t)≒i ｐｏｌ It can be regarded as (t-τ).

[0081] Therefore, by calculating the difference Δi(τ) between i(t) and i(t−τ) as shown in the following equation 14, it can be said that a signal containing a phase noise component and from which the polarization fluctuation component has been removed can be obtained. (Equation 14)

[0082] Next, we consider the statistical properties of Δi(t). Δi(t) is the difference between two random variables, and optical phase noise is white noise that follows a Gaussian process. Therefore, i ｐｎ If the standard deviation of (t) is σ, then i ｐｎ The variance of (t) is σ ２ The variance of Δi(t) is 2σ ２ (That is, the standard deviation of Δi(t) is 2 １／２ σ). Therefore, the standard deviation of Δi(t) is 1 / 2 １／２ By multiplying this, the standard deviation of i(t) is calculated.

[0083] Therefore, the detection unit 250 calculates the difference signal corresponding to Δi(t) output from the subtraction unit 240 by 2 １／２ A histogram of samples of the signal to be calculated is generated by dividing by , and the standard deviation of the signal to be calculated is calculated by fitting a Gaussian distribution function to the histogram, and the calculated standard deviation is set as the standard deviation of i(t).

[0084] 8 shows an example of the functional configuration of the detection unit 250. The detection unit 250 includes a histogram generation unit 810 and a calculation unit 820.

[0085] The detection section 250 includes a histogram generation section 810 and a calculation section 820. The histogram generation section 810 generates a histogram of the difference signals detected at a plurality of time points.

[0086] The histogram generating unit 810 calculates the difference signal corresponding to Δi(t) output from the subtracting unit 240 by 2 １／２ A histogram of the samples of the signal to be operated on is generated by dividing by .

[0087] The calculation unit 820 calculates the optical phase noise of the signal light based on the standard deviation of the histogram generated by the histogram generation unit 810. The calculation unit 820 fits a Gaussian distribution function to the histogram generated by the histogram generation unit 810. The calculation unit 820 calculates the standard deviation of the calculation target signal based on the fitted Gaussian distribution function, and sets the calculated standard deviation as the standard deviation of i(t).

[0088] 9 shows a histogram of the signal to be calculated corresponding to Δi(t), obtained under the same experimental conditions as the first experiment described in relation to FIGS. 3 to 5. The horizontal axis of FIG. 9 represents Δi(t) / 2 １／２ 9, the vertical axis represents the probability density (frequency). Unlike FIG. 3, FIG. 9 is a histogram detected when polarization fluctuations are generated. However, the delay time t ｄ As 21τ ｓ is applied, and the delay time in this case is 6.72 × 10 －９ is.

[0089] By fitting a Gaussian distribution function 900 to the histogram shown in Fig. 9, a standard deviation of 0.0142 (rad) was obtained. This standard deviation value agrees well with the standard deviation detected when no polarization fluctuation is occurring, as shown in Fig. 3. Thus, even when polarization fluctuation is occurring, the method of detecting optical phase noise based on the histogram of the signal to be calculated corresponding to Δi(t) (sometimes referred to as a "detection method based on a differential signal") successfully detected optical phase noise with the influence of polarization fluctuation suppressed.

[0090] 10 to 12 show the results of a second experiment based on the differential signal-based detection technique. The experimental conditions in the second experiment differ from those in the first experiment in that the drive voltage of the fiber stretcher was 19 Vpp.

[0091] Fig. 10 shows a histogram of optical phase fluctuations when polarization fluctuations are occurring. The horizontal axis of Fig. 10 represents signal i, and the vertical axis of Fig. 10 represents probability density (frequency). As mentioned above, this histogram includes the effects of phase fluctuations, including optical phase noise and polarization fluctuations.

[0092] 11 shows the polarization fluctuation signal in the signal light. The horizontal axis of FIG. 11 is time, and the vertical axis of FIG. 11 is i ｐｏｌ It is. ｐｏｌ is obtained by applying a low-pass filter with a cutoff frequency of 1 MHz to the signal i.

[0093] 12 shows a histogram of optical phase fluctuations corresponding to optical phase noise when polarization fluctuations are generated. The horizontal axis of FIG. 12 represents Δi(t) / 2 １／２ 12, the vertical axis represents the probability density (frequency). ｄ is 21τ ｓ A standard deviation of 0.0142 (rad) was obtained from a Gaussian distribution function 1200 fitted to the histogram shown in Fig. 12. As described above, even when larger polarization fluctuations occurred than in the first experiment, optical phase noise was successfully detected with the influence of polarization fluctuations suppressed.

[0094] 13 to 15 show the results of a third experiment based on a differential signal-based detection technique. The experimental conditions in the third experiment differed from those in the first experiment in that a polarization scrambler was used to impart polarization fluctuations to the signal light instead of the fiber stretcher. The polarization scrambler rotated the polarization of the signal light at 150 kHz by rotating a half-wave plate at a frequency of 75 kHz.

[0095] Fig. 13 shows a histogram of optical phase fluctuations when polarization fluctuations are occurring. The horizontal axis of Fig. 13 represents signal i, and the vertical axis of Fig. 13 represents probability density (frequency). As mentioned above, this histogram includes the effects of phase fluctuations including optical phase noise and polarization fluctuations.

[0096] 14 shows the polarization fluctuation signal in the signal light. The horizontal axis of FIG. 14 represents time, and the vertical axis of FIG. 14 represents i ｐｏｌ It is. ｐｏｌ is obtained by applying a low-pass filter with a cutoff frequency of 1 MHz to the signal i.

[0097] 15 shows a histogram of optical phase fluctuations corresponding to optical phase noise when polarization fluctuations are generated. The horizontal axis of FIG. 15 represents Δi(t) / 2 １／２ 15, the vertical axis represents the probability density (frequency). ｄ is 21τ ｓ A standard deviation of 0.0141 (rad) was obtained from a Gaussian distribution function 1500 fitted to the histogram shown in Fig. 15. In this way, even for signal light to which polarization fluctuations have been imparted by the polarization scrambler, optical phase noise was successfully detected with the influence of polarization fluctuations suppressed.

[0098] 16 shows the functional configuration of a detection device 1620 as a modified example of the detection device 120. The detection device 1620 includes a delay line 232 instead of the delay unit 230, and a subtraction circuit 242 as an example of the subtraction unit 240. In the detection device 1620, an AD converter 222 is provided after the subtraction circuit 242 and before the detection unit 250, and further includes an AD converter 260. In these respects, the detection device 1620 differs from the detection device 120. Here, the differences between the detection device 120 and the detection device 1620 will be mainly described.

[0099] The delay line 232 is applied to a signal line between the optical receiver 210 and the subtraction circuit 242. The delay line 232 delays the detection signal, which is an analog signal output from the optical receiver 210, and inputs the delayed signal to the subtraction circuit 242. The delay line 232 delays the detection signal to generate a delayed signal, which is an analog signal, and inputs the generated delayed signal to the subtraction circuit 242. The subtraction circuit 242 subtracts the delayed signal generated by the delay line 232 from the detection signal detected by the optical receiver 210, thereby generating a differential signal, which is an analog signal.

[0100] The AD converter 222 converts the differential signal output from the subtraction circuit 242 into a digital signal. The detection unit 250 processes the digital differential signal output from the AD converter 222 to detect the optical phase signal.

[0101] The AD converter 260 converts the detection signal output as an analog signal from the optical receiver 210 into a digital signal. The low-pass filter 270 reduces frequency components higher than a cutoff frequency in the detection signal output as a digital signal from the AD converter 260, and substantially passes frequency components lower than the cutoff frequency.

[0102] 17 shows the functional configuration of a detection device 1720, which is a modified example of the detection device 1620. The detection device 1720 differs from the detection device 1620 in that it includes a low-pass filter 234 instead of the delay line 232. Here, the differences between the detection device 1620 and the detection device 1720 will be mainly described.

[0103] The low-pass filter 234 reduces frequency components higher than the cutoff frequency in the detection signal, which is an analog signal output from the optical receiver 210, and substantially passes frequency components lower than the cutoff frequency. When passing through the low-pass filter, the signal is delayed by a certain amount of time. The detection signal delayed by the low-pass filter 234 is input to the subtraction circuit 242 as a delayed signal.

[0104] 18 is a flowchart showing a detection method executed by the detection device 120. In S1802, the detection device 120 generates a detection signal by detecting signal light using the optical delay interference unit 200 and the optical receiver 210. In S1804, the detection device 120 splits the detection signal and delays one of the split detection signals to generate a delayed signal. In S1806, the detection device 120 subtracts the delayed signal from the other of the split detection signals to generate a differential signal.

[0105] In S1808, the detecting device 120 detects optical phase noise based on the differential signal. In S1810, the detecting device 120 detects polarization fluctuation in the signal light based on the low-frequency component in the detection signal. In S1812, the detecting device 120 outputs evaluation information based on the optical phase noise detected in S1808 and evaluation information based on the polarization fluctuation detected in S1810.

[0106] The detection method described in relation to FIG. 18 is also applicable to the detection method using the detection device 1620 and the detection device 1720.

[0107] 19 shows an example of a computer 2000 in which multiple embodiments of the present invention may be embodied in whole or in part. A program installed on the computer 2000 may cause the computer 2000 to function as the communication system 100 according to an embodiment or each part of the system, the optical signal receiving device 20 or each part of the device, or an apparatus or each part of the apparatus, such as the detection device 120, the detection device 1620, and the detection device 1720, or each part of the apparatus, perform operations associated with the system or each part of the system, or the apparatus or each part of the apparatus, and / or perform a process or steps of the process according to an embodiment. Such a program may be executed by the CPU 2012 to cause the computer 2000 to perform specific operations associated with some or all of the processing procedures and blocks of the block diagrams described herein.

[0108] The computer 2000 according to this embodiment includes a CPU 2012 and a RAM 2014, which are interconnected by a host controller 2010. The computer 2000 also includes a ROM 2026, a flash memory 2024, a communication interface 2022, and an input / output chip 2040. The ROM 2026, the flash memory 2024, the communication interface 2022, and the input / output chip 2040 are connected to the host controller 2010 via the input / output controller 2020.

[0109] The CPU 2012 operates according to programs stored in the ROM 2026 and RAM 2014, thereby controlling each unit.

[0110] The communication interface 2022 communicates with other electronic devices via a network. The flash memory 2024 stores programs and data used by the CPU 2012 in the computer 2000. The ROM 2026 stores a boot program or the like executed by the computer 2000 upon activation and / or programs dependent on the hardware of the computer 2000. The input / output chip 2040 may also connect various input / output units such as a keyboard, mouse, and monitor to the input / output controller 2020 via input / output ports such as a serial port, a parallel port, a keyboard port, a mouse port, a monitor port, a USB port, an HDMI port, etc.

[0111] The programs are provided via a computer-readable storage medium such as a CD-ROM, DVD-ROM, or memory card, or via a network. RAM 2014, ROM 2026, and flash memory 2024 are examples of computer-readable storage media. The programs are installed in flash memory 2024, RAM 2014, or ROM 2026 and executed by CPU 2012. Information processing described in these programs is read by computer 2000, and causes cooperation between the programs and the various types of hardware resources described above. An apparatus or method may be configured by implementing operations or processing of information in accordance with the use of computer 2000.

[0112] For example, when communication is performed between the computer 2000 and an external device, the CPU 2012 may execute a communication program loaded into the RAM 2014 and instruct the communication interface 2022 to perform communication processing based on the processing described in the communication program. Under the control of the CPU 2012, the communication interface 2022 reads transmission data stored in a transmission buffer processing area provided in a recording medium such as the RAM 2014 or flash memory 2024, transmits the read transmission data to a network, and writes received data received from the network to a reception buffer processing area or the like provided on the recording medium.

[0113] The CPU 2012 may also cause all or a necessary portion of a file or database stored on a recording medium such as the flash memory 2024 to be read into the RAM 2014, and may perform various types of processing on the data on the RAM 2014. The CPU 2012 then writes the processed data back to the recording medium.

[0114] Various types of information, such as various types of programs, data, tables, and databases, may be stored on the recording medium and subjected to information processing. The CPU 2012 may perform various types of processing on data read from the RAM 2014, including various types of operations, information processing, conditional judgment, conditional branching, unconditional branching, information search / replacement, etc., as described herein and specified by the instruction sequences of the programs, and write the results back to the RAM 2014. The CPU 2012 may also search for information in a file, database, etc. on the recording medium. For example, if multiple entries each having an attribute value of a first attribute associated with an attribute value of a second attribute are stored on the recording medium, the CPU 2012 may search for an entry that matches a condition and specifies an attribute value of the first attribute from among the multiple 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 satisfies a predetermined condition.

[0115] The above-described programs or software modules may be stored in a computer-readable storage medium on or near the computer 2000. A recording medium such as a hard disk or RAM provided in a server system connected to a dedicated communication network or the Internet can be used as the computer-readable storage medium. The programs stored in the computer-readable storage medium may be provided to the computer 2000 via a network.

[0116] A program installed in computer 2000 and causing computer 2000 to function as detection device 120, detection device 1620, or detection device 1720 may, when executed by the computer, act on CPU 2012 or the like to cause computer 2000 to function as each of detection device 120, detection device 1620, or detection device 1720. When the information processing described in these programs is loaded into computer 2000, it functions as each of detection device 120, detection device 1620, or detection device 1720, which is a specific means formed by the software and the various hardware resources described above working together. These specific means then perform calculations or processing of information according to the intended use of computer 2000 in this embodiment, thereby constructing a specific detection device 120, detection device 1620, or detection device 1720 according to the intended use.

[0117] Various embodiments have been described with reference to block diagrams. In the block diagrams, each block may represent (1) a stage of a process in which an operation is performed or (2) a portion of an apparatus responsible for performing the operation. Particular stages and portions may be implemented by dedicated circuitry, programmable circuitry provided with computer-readable instructions stored on a computer-readable storage medium, and / or a processor provided with computer-readable instructions stored on a computer-readable storage medium. Dedicated circuitry may include digital and / or analog hardware circuitry, and may include integrated circuits (ICs) and / or discrete circuits. Programmable circuitry may include reconfigurable hardware circuitry, including logical AND, OR, XOR, NAND, NOR, and other logic operations, flip-flops, registers, memory elements such as field programmable gate arrays (FPGAs), programmable logic arrays (PLAs), and the like.

[0118] A computer-readable storage medium may include any tangible device capable of storing instructions that are executed by an appropriate device, such that the computer-readable storage medium having instructions stored thereon constitutes at least a portion of an article of manufacture containing instructions that can be executed to provide means for performing the operations specified in a process or block diagram. Examples of computer-readable storage media may include electronic storage media, magnetic storage media, optical storage media, electromagnetic storage media, semiconductor storage media, etc. More specific examples of computer-readable storage media may include floppy disks, diskettes, hard disks, random access memories (RAMs), read-only memories (ROMs), erasable programmable read-only memories (EPROMs or flash memories), electrically erasable programmable read-only memories (EEPROMs), static random access memories (SRAMs), compact disc read-only memories (CD-ROMs), digital versatile discs (DVDs), Blu-ray discs, memory sticks, integrated circuit cards, etc.

[0119] The computer readable instructions may include either assembler instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state-setting data, or source or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk®, JAVA®, C++, etc., and conventional procedural programming languages ​​such as the “C” programming language or similar programming languages.

[0120] The computer-readable instructions may be provided to a processor or programmable circuit of a programmable data processing apparatus locally or over a local area network (LAN), a wide area network (WAN) such as the Internet, etc., and the computer-readable instructions may be executed to provide means for performing the operations specified in the described procedures or block diagrams.

[0121] Here, the computer may be a computer such as a PC (personal computer), a tablet computer, a smartphone, a workstation, a server computer, or a general-purpose computer, or may be a computer system in which multiple computers are connected. Such a computer system in which multiple computers are connected is also called a distributed computing system, which is a computer in the broad sense. In a distributed computing system, the multiple computers collectively execute a program by each executing a part of the program and transferring data between the computers as needed during program execution.

[0122] Examples of processors include computer processors, central processing units (CPUs), processing units, microprocessors, digital signal processors, controllers, microcontrollers, etc. A computer may have one processor or multiple processors. In a multiprocessor system with multiple processors, each processor executes a portion of a program and passes data between processors as needed during program execution, allowing the multiple processors to collectively execute the program. For example, in multitasking, each of the multiple processors may execute a portion of each task in small chunks by switching tasks at each time slice. In this case, which portion of a program each processor executes changes dynamically. Alternatively, which portion of a program each of the multiple processors executes may be statically determined by multiprocessor-aware programming.

[0123] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications and improvements can be made to the above embodiments. Furthermore, to the extent that they are not technically inconsistent, the details described for a particular embodiment can be applied to other embodiments. It is apparent from the claims that such modifications and improvements can also be included within the technical scope of the present invention.

[0124] It should be noted that the order of execution of each process, such as operations, procedures, steps, and stages, in the devices, systems, programs, and methods shown in the claims, specifications, and drawings is not specifically stated as "before," "prior to," etc., and that the processes can be performed in any order unless the output of a previous process is used in a subsequent process. Even if the operational flow in the claims, specifications, and drawings is described using "first," "next," etc. for convenience, this does not mean that the processes must be performed in this order.

[0125] REFERENCE SIGNS LIST 10 optical transmission path 20 optical signal receiving device 40 branching device 50 optical receiver 60 AD converter 70 signal processing unit 100 communication system 110 optical signal transmitting device 80 optical delay interference unit 82 semi-transparent mirror 84 semi-transparent mirror 86 total reflection mirror 88 total reflection mirror 120 detection device 130 Gaussian distribution function 200 optical delay interference unit 210 optical receiver 220 AD converter 222 AD converter 230 delay unit 232 delay line 234 low-pass filter 240 subtraction unit 242 subtraction circuit 250 detection unit 260 AD converter 270 low-pass filter 280 detection unit 810 histogram generation unit 820 calculation unit 1200 Gaussian distribution function 1500 Gaussian distribution function 1620 Detector 1720 Detector 2000 Computer 2010 Host controller 2012 CPU 2014 RAM 2020 Input / output controller 2022 Communication interface 2024 Flash memory 2026 ROM 2040 Input / output chip

Claims

1. A detection device for detecting optical phase noise of signal light propagated through an optical transmission line, comprising a detection unit that detects the optical phase noise of the signal light based on a differential signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delayed signal obtained by delaying the detection signal.

2. The detection device according to claim 1, wherein the detection section detects the optical phase noise of the signal light based on a distribution of the differential signals detected at multiple points in time.

3. The detection device according to claim 2, wherein the detection unit comprises: a histogram generation unit that generates a histogram of the differential signal detected at the multiple time points; and a calculation unit that calculates the optical phase noise of the signal light based on the standard deviation of the histogram.

4. A detection device according to any one of claims 1 to 3, wherein the delayed signal is a signal delayed by a predetermined time from the detection signal, and the predetermined time is equal to or greater than 1 / (2B), where B is the detection band of the detection signal.

5. The detection device according to any one of claims 1 to 4, further comprising a delay unit that generates the delayed signal by delaying the detection signal.

6. The detection device according to claim 5, wherein the delay section includes one of a delay line and a low-pass filter.

7. The detection device according to any one of claims 1 to 6, further comprising a subtraction unit that subtracts the detection signal from the delayed signal to generate the difference signal.

8. The optical delay interferometer comprises a delay interference unit that splits input light into first input light and second input light, and multiplexes and interferes with the first input light that has passed through a first optical path and the second input light that has passed through a second optical path, and calculates a delay time difference τ between the first input light that has passed through the first optical path and the second input light that has passed through the second optical path, and a frequency f of the input light. ｃ The n is an integer and is set to satisfy the relationship of the following formula 1: 2πf (Formula 1) ｃ τ=2nπ+π / 2 Detector device according to any one of claims 1 to 7.

9. An optical signal receiving device comprising: a detection device according to any one of claims 1 to 8; and a demodulation section for demodulating a received signal transmitted by said signal light and generating an information signal.

10. An optical communication system comprising: an optical signal receiving device according to claim 9; and an optical signal transmitting device that transmits the signal light.

11. A program for causing a computer to function as the detection device according to any one of claims 1 to 8.

12. A detection method for detecting optical phase noise of signal light propagated through an optical transmission line, comprising a step of detecting the optical phase noise of the signal light based on a differential signal between a detection signal obtained by detecting the signal light with an optical delay interferometer and a delayed signal obtained by delaying the detection signal.

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