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

Abstract

A detection device disclosed herein detects polarization fluctuations of signal light propagated through an optical transmission path. The detection device includes a detection unit that detects polarization fluctuations of the signal light on the basis of signals acquired from a plurality of detection signals detected by an optical delay interferometer from, respectively, a plurality of polarization components separated from the signal light and different from each other. The detection device includes a detection unit that detects polarization fluctuations of the signal light on the basis of signals acquired from a plurality of detection signals detected by an optical delay interferometer from, respectively, a plurality of branched light beams that have branched from the signal light and passed through wavelength plates each having a different birefringence axis direction.

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 polarization fluctuations of signal light that has propagated through an optical transmission line, and the detection device includes a detection unit that detects polarization fluctuations of the signal light based on a plurality of detection signals obtained by detecting light of a plurality of different polarization components separated from the signal light using an optical delay interferometer.

[0006] The detecting device may include a splitter that splits the signal light into the light of the plurality of polarization components, and a plurality of optical delay interferometers that detect each of the light of the plurality of polarization components that have been split from the signal light by the splitter.

[0007] Any of the above detection devices may include a first separator that separates the signal light into light of a first polarization component and light of a second polarization component. The detection device may include a first optical delay interferometer that detects light of the first polarization component and outputs a first detection signal. The detection device may include a second optical delay interferometer that detects light of the second polarization component and outputs a second detection signal. The detection unit may detect polarization fluctuations of the signal light based on signals obtained from the first detection signal and the second detection signal.

[0008] Any of the above detection devices may include a branching unit that branches the signal light into first branched light and second branched light. The detection device may include a first splitting unit that splits the first branched light into light of a first polarization component and light of a second polarization component. The detection device may include a second splitting unit that splits the second branched light into light of a third polarization component and light of a fourth polarization component. The detection device may include a first optical delay interferometer that detects the light of the first polarization component and outputs a first detection signal. The detection device may include a second optical delay interferometer that detects the light of the second polarization component and outputs a second detection signal. The detection device may include a third optical delay interferometer that detects the light of the third polarization component and outputs a third detection signal. The detection device may include a fourth optical delay interferometer that detects the light of the fourth polarization component and outputs a fourth detection signal. The detection unit may detect polarization fluctuation of the signal light based on signals acquired from the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal.

[0009] In a second aspect of the present invention, there is provided a detection device that detects polarization fluctuations of signal light that has propagated through an optical transmission line, and the detection device includes a detection unit that detects polarization fluctuations of the signal light based on a plurality of detection signals obtained by an optical delay interferometer detecting a plurality of branched light beams that have been branched from the signal light and passed through wave plates having different principal axis directions of birefringence.

[0010] In the detection device according to the second aspect, the wave plate may be a half wave plate.

[0011] Any of the detection devices in the second aspect may include a branching unit that splits the signal light into the plurality of branched lights. The detection device may include a plurality of wave plates through which each of the plurality of branched lights passes. The detection device may include a plurality of optical delay interferometers that detect each of the plurality of branched lights that have passed through the plurality of wave plates. The plurality of wave plates may have mutually different principal axis directions of birefringence.

[0012] Any of the detection devices in the second aspect above may include a branching unit that branches the signal light into first branched light and second branched light. The detection device may include a first wave plate through which the first branched light passes. The detection device may include a second wave plate through which the second branched light passes. The detection device may include a first optical delay interferometer that detects the first branched light that has passed through the first wave plate and outputs a first detection signal. The detection device may include a second optical delay interferometer that detects the second branched light that has passed through the second wave plate and outputs a second detection signal. The detection unit may detect polarization fluctuation of the signal light based on signals acquired from the first detection signal and the second detection signal.

[0013] Any of the detection devices in the second aspect may include a branching unit that branches the signal light into first branched light, second branched light, third branched light, and fourth branched light. The detection device may include a first wave plate through which the first branched light passes. The detection device may include a second wave plate through which the second branched light passes. The detection device may include a third wave plate through which the third branched light passes. The detection device may include a fourth wave plate through which the fourth branched light passes. The detection device may include a first optical delay interferometer that detects the first branched light that has passed through the first wave plate and outputs a first detection signal. The detection device may include a second optical delay interferometer that detects the second branched light that has passed through the second wave plate and outputs a second detection signal. The detection device may include a third optical delay interferometer that detects the third branched light that has passed through the third wave plate and outputs a third detection signal. The detection device may include a fourth optical delay interferometer that detects the fourth branched light that has passed through the fourth wave plate and outputs a fourth detection signal. The detector may detect polarization fluctuation of the signal light based on signals acquired from the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal.

[0014] In the detection device according to the first and / or second aspects, each of the optical delay interferometers included in the detection device 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 the first optical path and the second input light that has passed through the second optical path. ｃ However, n may be an integer and may be set to satisfy the relationship of the following formula 1: 2πf (Formula 1) ｃ τ=2nπ+π / 2

[0015] In the detection device according to any of the first and / or second aspects, the detection unit may preferentially select one or more detection signals having greater signal strength from the plurality of detection signals, and detect polarization fluctuations of the signal light based on the selected one or more detection signals.

[0016] In the detection device according to the first and / or second aspects, the detector may detect the polarization fluctuation based on a sum of squares of the plurality of detection signals.

[0017] In a third aspect of the present invention, there is provided an optical signal receiving device, comprising the detection device according to any one of the first and / or second aspects, and a demodulation unit that demodulates a received signal transmitted by the signal light to generate an information signal.

[0018] In a fourth 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.

[0019] In a fifth aspect of the present invention, a program is provided. The program is, for example, a program for causing a computer to function as any one of the detection devices in the first and / or second aspects. A computer-readable storage medium for storing the program may be provided. The storage medium may be a non-transitory computer-readable medium.

[0020] In a sixth aspect of the present invention, there is provided a detection method for detecting polarization fluctuations of signal light propagated through an optical transmission line, the detection method comprising the step of detecting polarization fluctuations of the signal light based on a signal obtained from a plurality of detection signals obtained by detecting, with an optical delay interferometer, light of a plurality of different polarization components separated from the signal light.

[0021] In a seventh aspect of the present invention, there is provided a detection method for detecting polarization fluctuations of signal light propagating through an optical transmission line, the detection method comprising the step of detecting polarization fluctuations of the signal light based on a plurality of detection signals obtained by an optical delay interferometer detecting a plurality of branched light beams that are branched from the signal light and passed through wave plates having different principal axis directions of birefringence.

[0022] 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.

[0023] 1 shows an example of a system configuration of a communication system 100. 1 shows an example of an internal configuration of an optical delay interferometer. 1 shows an example of a functional configuration of a detection device 120 in the first embodiment. 1 shows the relationship between the principal axis of the polarization plane of signal light and the principal axis of a separator. 1 shows an example of a functional configuration of a detection device 122. 1 shows the functional configuration of a detection unit 600 together with a first optical receiving unit 210. 1 shows the functional configuration of a detection unit 700 together with a first optical receiving unit 210. 1 shows the functional configuration of a detection unit 800 together with a first optical receiving unit 210. 1 shows the functional configuration of a detection unit 900 together with a first optical receiving unit 210. 1 shows the functional configuration of a detection device 1020 as a detection device in a second embodiment. 1 shows the relationship between the direction of the polarization plane of incident light and the direction in which lateral stress is applied. 1 shows the relationship between the principal axis angle of a half-wave plate and the peak-to-peak value of a detection signal detected by an optical delay interferometer. 1 is a flowchart showing a detection method executed by the detection device 120 in the first embodiment. 3 is a flowchart illustrating a detection method performed by the detection device 1020 in the second embodiment. FIG. 4 illustrates an example computer 3000 in which aspects of the present invention may be embodied in whole or in part.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] The optical transmission line 10 transmits light. An example of the optical transmission line 10 is an optical fiber.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] While the signal light propagates through the optical transmission line 10, phase fluctuations such as optical phase noise and polarization fluctuations may occur in the signal light. 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. Situations in which polarization fluctuations may occur include (i) fluctuations in the birefringence of the optical fiber due to, for example, the application of fluctuating lateral stress to the optical fiber, and (ii) rapid rotation of the polarization plane due to, for example, a sudden twist in the optical fiber. Therefore, in many cases, phase fluctuations in the signal light due to polarization fluctuations may occur suddenly.

[0035] The signal processing unit 70 includes a polarization separation processing circuit that performs polarization separation processing during the signal processing process. If polarization fluctuation occurs and the polarization fluctuation speed exceeds the processing speed of the polarization separation processing circuit, the polarization separation processing circuit may not be able to perform polarization separation processing normally, resulting in code errors.

[0036] The detecting device 120 analyzes the signal light input to the detecting 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 detecting device 120 analyzes, for example, the signal light to detect the presence or absence and / or degree of polarization fluctuation. In this way, the detecting device 120 detects the polarization fluctuation of the signal light propagated through the optical transmission line 10. The detecting device 120 outputs evaluation information regarding the detected polarization fluctuation.

[0037] Examples of the evaluation information regarding 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, which indicates 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 regarding polarization fluctuation may be output as an alarm or a flag. The evaluation information may be used in the demodulation process by the signal processing unit 70.

[0038] The optical signal receiving device 20 can evaluate the presence and / or degree of polarization fluctuation with relatively high accuracy, which can improve the operation efficiency and maintenance efficiency of the optical transmission system.

[0039] In this embodiment, the detection device 120 detects the polarization fluctuation of the signal light based on a plurality of detection signals obtained by optical delay interferometers, which detect light of a plurality of different polarization components separated from the signal light. First, with reference to FIG. 2 , a signal related to the polarization fluctuation detected by a single optical delay interferometer will be described.

[0040] 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.

[0041] 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.

[0042] In this embodiment, the optical delay interference unit 80 provides an optical path difference between the first input light and the second input light. If the delay time difference due to the optical path difference is τ, the time is t, and the phase at the time of output of the first signal light is φ(t), the phase at the time of output of the second signal light is φ(t-τ). The delay time difference τ is determined by the frequency f of the input light. ｃ , where n is an integer, and ｃ It is set so as to satisfy the relationship τ=2nπ+π / 2.

[0043] 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.

[0044] 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. This allows the operating point of the optical delay interference unit 80 to be adjusted. The delay time difference τ 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.

[0045] In this way, 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. ｃ and n is an integer, and 2πf ｃ It is set so as to satisfy the relationship τ=2nπ+π / 2.

[0046] 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.

[0047] 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 ｘ and 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:

[0048] 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.

[0049] 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:

[0050] In Equation 2, τ represents the delay time difference between the two paths of the optical delay interferometer, R represents the sensitivity of the optical receiver, and · represents the dot product.

[0051] 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)

[0052] In Equation 3, the constant is omitted and the delay time difference τ is 2πf ｃ When adjusted to τ=2nπ+π / 2 (n is a positive integer), the received current i is expressed by the following equation 4:

[0053] 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)

[0054] 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)

[0055] 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)

[0056] In Equation 7, the first term on the right side, Δφ ｎ (t) represents the current corresponding to the optical phase noise, and the second term on the right-hand side, −{E ｘ ２(t)-E ｙ ２ (t)}Δδ(t) / 2 is the current i corresponding to the polarization fluctuation ｐｏｌ Represents.

[0057] Here, E ｘ ２ (t): E ｙ ２ If (t) = α:1 - α (where 0≦α≦1), the current i corresponding to the polarization fluctuation in Equation 7 is ｐｏｌ is expressed by the following formula 8. (Formula 8)

[0058] From Equation 8, the current i corresponding to the polarization fluctuation ｐｏｌ The value of changes due to polarization fluctuation, and becomes ±Δδ(t) / 2 when α=0, 1, while it becomes 0 when α=1 / 2.

[0059] Here, the polarization fluctuation is E ｘ (t), E ｙ This occurs when either δ(t) or δ(t) changes, but E ｘ (t) = E ｙ When (t), the current i ｐｏｌ Since E becomes 0, polarization fluctuations due to fluctuations in δ(t) cannot be detected. A typical example of polarization fluctuations due to fluctuations in δ(t) is polarization fluctuations caused by lateral stress applied to the optical fiber and the stress fluctuating over time. Such polarization fluctuations often occur in optical fiber cables that are actually installed. Therefore, E ｘ (t) = E ｙ What is desired is a detection method that can substantially detect variations in δ(t) even when (t) is zero.

[0060] 3 shows an example of the functional configuration of the detection device 120 in the first embodiment. The detection device 120 includes a first optical receiving unit 210 and a detection unit 240. The first optical receiving unit 210 includes a first separating unit 201, a first optical delay interference unit 211, a first optical receiver 221, a second optical delay interference unit 212, and a second optical receiver 222.

[0061] The first separator 201 separates the signal light into a first polarization component and a second polarization component, and is, for example, a polarizing beam splitter.

[0062] The first optical delay interference unit 211 and the second optical delay interference unit 212 each have the same configuration as the optical delay interference unit 80 .

[0063] Light of the first polarization component is input to the first optical delay interference unit 211. The first optical receiver 221 detects the output light from the first optical delay interference unit 211 and outputs a first detection signal. The first optical delay interference unit 211 and the first optical receiver 221 function as a first optical delay interferometer that detects the light of the first polarization component and outputs a first detection signal.

[0064] Light of the second polarization component is input to the second optical delay line interferometer 212. The second optical receiver 222 detects the output light from the second optical delay line interferometer 212 and outputs a second detection signal. The second optical delay line interferometer 212 and the second optical receiver 222 function as a second optical delay line interferometer that detects light of the second polarization component and outputs a second detection signal.

[0065] The detector 240 detects the polarization fluctuation of the signal light based on the signals obtained from the first detection signal and the second detection signal.

[0066] 4 shows the relationship between the principal axis of the polarization plane of the signal light and the principal axis of the first separation unit 201. In FIG. 4, the principal axis of the polarization plane of the signal light is defined as xy, and the principal axis of the first separation unit 201, which is a polarizing beam splitter, is defined as XY. If the angle between the principal axis of the polarization plane of the signal light and the principal axis of the first separation unit 201 is defined as θ, the XY components of the electric field corresponding to Equation 1 are expressed by the following Equations 9 and 10. (Equation 9) (Formula 10)

[0067] The current i detected by the first optical receiver 221 is calculated in the same manner as in the calculation process of Equation 6. １ and the current i detected by the second optical receiver 222 ２ When the current i １ and i ２ are expressed by the following formulas 11 and 12, respectively. (Formula 11) (Formula 12)

[0068] Similar to the discussion of Equation 8, E ｘ ２(t): E ｙ ２ If (t) = α:1-α (where 0≦α≦1), the current i １ and i ２ are expressed by the following formulas 13 and 14, respectively. (Formula 13) (Formula 14)

[0069] The term i representing polarization fluctuation in Equation 13 ｐｏｌ１ is -(α-sin ２ (θ))Δδ(t) / 2, and the term i ｐｏｌ２ is -(α-cos ２ (θ))Δδ(t) / 2. Therefore, i ｐｏｌ１ and i ｐｏｌ２ are simultaneously 0 when α-sin ２ (θ) = α - cos ２ This is only true when (θ)=0, but this is only true when α=1 / 2 and θ=±π / 4. ｘ (t) = E ｙ (t), and the angle θ between the main axis of the polarization plane of the signal light and the main axis of the first separator 201 is ±π / 4. ｐｏｌ１ and i ｐｏｌ２ Both of these are never 0.

[0070] Therefore, the detection unit 240 selects the detection signal with the greater signal strength from the first detection signal by the first optical receiver 221 and the second detection signal by the second optical receiver 222, and detects δ(t) based on the selected detection signal. ｘ (t) = E ｙ (t), and except for the case where the angle θ is ±π / 4, δ(t) can be substantially detected.

[0071] Another signal processing method for detecting δ(t) by the detection device 120 will be described. The frequency of the polarization fluctuation component is at most about 1 MHz, which is a lower frequency than the frequency of the optical phase noise. The detection device 120 extracts low-frequency components from each of the first detection signal and the second detection signal, and converts the extracted low-frequency components into i ｐｏｌ１ and i in Equation 14 ｐｏｌ２Here, i ｐｏｌ１ and i ｐｏｌ２ The sum of squares of is calculated as in the following formula 15. (Formula 15)

[0072] The reason why Equation 15 becomes 0 is because E ｘ (t) = E ｙ (t) and θ is ±π / 4. Therefore, the detector 240 detects i extracted from the first detection signal by the first optical receiver 221 and the second detection signal by the second optical receiver 222. ｐｏｌ１ and i ｐｏｌ２ By calculating the sum of squares of δ(t), it is possible to practically detect δ(t). In the sum of squares, the contribution of signals with a large signal-to-noise ratio is large, so an output signal with a larger signal-to-noise ratio can be obtained compared to simple addition.

[0073] 5 shows an example of the functional configuration of the detection device 122. The detection device 122 is a modified example of the detection device 120. The detection device 122 includes a branching unit 300, a first optical receiving unit 310, a second optical receiving unit 320, and a detection unit 340.

[0074] The first optical receiving unit 310 has the same functional configuration as the first optical receiving unit 210. Therefore, a description of the first optical receiving unit 310 will be omitted. The second optical receiving unit 320 includes a second separating unit 202, a third optical delay interference unit 213, a third optical receiver 223, a fourth optical delay interference unit 214, and a fourth optical receiver 224.

[0075] The second separator 202 separates the signal light into a third polarization component and a fourth polarization component, and is, for example, a polarizing beam splitter.

[0076] The third optical delay interference unit 213 and the fourth optical delay interference unit 214 each have the same configuration as the optical delay interference unit 80 .

[0077] Light of the third polarization component is input to the third optical delay line interferometer 213. The third optical receiver 223 detects the output light from the third optical delay line interferometer 213 and outputs a third detection signal. The third optical delay line interferometer 213 and the third optical receiver 223 function as a third optical delay line interferometer that detects light of the third polarization component and outputs a third detection signal.

[0078] Light of the fourth polarization component is input to the fourth optical delay line interferometer 214. The fourth optical receiver 224 detects the output light from the fourth optical delay line interferometer 214 and outputs a fourth detection signal. The fourth optical delay line interferometer 214 and the fourth optical receiver 224 function as a fourth optical delay line interferometer that detects light of the fourth polarization component and outputs a fourth detection signal.

[0079] The detector 340 detects polarization fluctuation of the signal light based on signals acquired from the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal. For example, the detector 340 may preferentially select a detection signal having a greater signal strength from among the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal, and detect the polarization fluctuation of the signal light based on the selected detection signal. The detector 340 may detect the polarization fluctuation based on the sum of squares of the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal. For example, the detector 340 may detect the polarization fluctuation based on the sum of squares of a signal corresponding to the polarization fluctuation in the first detection signal, a signal corresponding to the polarization fluctuation in the second detection signal, a signal corresponding to the polarization fluctuation in the third detection signal, and a signal corresponding to the polarization fluctuation in the fourth detection signal.

[0080] In relation to Fig. 3, a configuration has been described in which polarization fluctuation is detected by using two optical delay interferometers to detect two polarization components of light separated from signal light. In Fig. 5, a configuration has been described in which polarization fluctuation is detected by using four optical delay interferometers to detect four polarization components of light separated from signal light. In general, a configuration can be adopted in which polarization fluctuation is detected by using multiple optical delay interferometers to detect multiple polarization components of light separated from signal light. That is, the detection device 120 may include a separator that separates the signal light into multiple polarization components, and multiple optical delay interferometers that respectively detect the multiple polarization components of light separated from the signal light by the separator.

[0081] 6 shows the functional configuration of the detection unit 600 together with the first optical receiving unit 210. The functional configuration of the detection unit 600 may be applied to the detection unit 240 or the detection unit 340 described above.

[0082] The detection unit 600 includes a selection unit 610, an AD converter 620, an intensity analysis unit 630, and a frequency analysis unit 640. The selection unit 610 preferentially selects a detection signal having a greater signal intensity from among a plurality of detection signals output from one or more optical receiving units including the first optical receiving unit 210. The AD converter 620 converts the detection signal selected by the selection unit 610 into a digital signal.

[0083] The intensity analysis unit 630 analyzes the intensity of the polarization fluctuation of the signal light based on the digital signal output from the AD converter 620. For example, the intensity analysis unit 630 calculates the magnitude of the signal output from the AD converter 620 to obtain, for example, −(α−sin ２ For example, the intensity analysis unit 630 may detect at least one of the peak-to-peak value, amplitude, peak value, and average value of the signal output from the AD converter 620.

[0084] The frequency analysis unit 640 performs frequency analysis on the polarization component of the polarization fluctuation of the signal light based on the digital signal output from the AD converter 620. For example, the frequency analysis unit 640 performs a Fourier transform (for example, a fast Fourier transform) on the signal output from the AD converter 620 to obtain, for example, −(α−sin ２ The variation frequency of Δδ(t) / 2 is calculated by Fourier transform. The spectrum component obtained by Fourier transform indicates the magnitude of the polarization variation at the corresponding variation frequency.

[0085] The evaluation information output from the detection unit 600 may include information indicating the magnitude of the polarization fluctuation calculated by the intensity analysis unit 630, and / or the fluctuation frequency of the polarization fluctuation and the magnitude of the polarization fluctuation at the fluctuation frequency calculated by the frequency analysis unit 640.

[0086] 7 shows the functional configuration of the detector 700 together with the first optical receiving unit 210. The functional configuration of the detector 700 may be applied to the detector 240 or the detector 340 described above.

[0087] The detection section 700 includes a plurality of AD converters including a first AD converter 721 and a second AD converter 722 , a selection section 710 , an intensity analysis section 630 , and a frequency analysis section 640 .

[0088] The first AD converter 721 converts the first detection signal output from the first optical receiving unit 210 into a digital signal. The second AD converter 722 converts the second detection signal output from the first optical receiving unit 210 into a digital signal. The multiple AD converters included in the detecting unit 700 are provided corresponding to the multiple detection signals output from one or more optical receiving units, and convert the corresponding detection signals into digital signals.

[0089] The selection section 710 preferentially selects a signal having a stronger signal strength from among the signals output from a plurality of AD converters including the first AD converter 721 and the second AD converter 722 .

[0090] The intensity analysis unit 630 analyzes the intensity of polarization fluctuation of the signal light based on the signal selected by the selection unit 710. The frequency analysis unit 640 performs frequency analysis of polarization fluctuation of the signal light based on the signal selected by the selection unit 710. The functions of the intensity analysis unit 630 and the frequency analysis unit 640 are similar to the functions of the intensity analysis unit 630 and the frequency analysis unit 640 described in relation to FIG. 6 and the like, and therefore description thereof will be omitted.

[0091] 8 shows the functional configuration of the detector 800 together with the first optical receiving unit 210. The functional configuration of the detector 800 may be applied to the detector 240 or the detector 340 described above.

[0092] The detection unit 800 includes a square adder 810, an AD converter 620, an intensity analysis unit 630, and a frequency analysis unit 640. The square adder 810 squares and adds signals corresponding to polarization components among a plurality of detection signals output from one or more optical receiving units including the first optical receiving unit 210. The AD converter 620 converts the signal output from the square adder 810 into a digital signal.

[0093] The intensity analysis unit 630 analyzes the intensity of polarization fluctuation of the signal light based on the digital signal output from the AD converter 620. The frequency analysis unit 640 performs frequency analysis on the polarization component of the polarization fluctuation of the signal light based on the digital signal output from the AD converter 620. The functions of the intensity analysis unit 630 and the frequency analysis unit 640 are similar to the functions of the intensity analysis unit 630 and the frequency analysis unit 640 described in relation to FIG. 6 and the like, and therefore description thereof will be omitted.

[0094] 9 shows the functional configuration of the detection unit 900 together with the first optical receiving unit 210. The functional configuration of the detection unit 900 may be applied to the detection unit 240 or the detection unit 340 described above.

[0095] The detection section 900 includes a plurality of AD converters including a first AD converter 721 and a second AD converter 722 , a square adder 910 , an intensity analysis section 630 , and a frequency analysis section 640 .

[0096] The first AD converter 721 converts the first detection signal output from the first optical receiving unit 210 into a digital signal. The second AD converter 722 converts the second detection signal output from the first optical receiving unit 210 into a digital signal. The multiple AD converters included in the detecting unit 900 are provided corresponding to the multiple detection signals output from one or more optical receiving units, and convert the corresponding detection signals into digital signals.

[0097] The square adder 910 squares and adds signals corresponding to the polarization components among the signals output from a plurality of AD converters including the first AD converter 721 and the second AD converter 722 .

[0098] The intensity analysis unit 630 analyzes the intensity of polarization fluctuation of the signal light based on the signal output from the square adder 910. The frequency analysis unit 640 performs frequency analysis of the polarization fluctuation of the signal light based on the signal output from the square adder 910. The functions of the intensity analysis unit 630 and the frequency analysis unit 640 are similar to the functions of the intensity analysis unit 630 and the frequency analysis unit 640 described in relation to FIG. 6 and the like, and therefore description thereof will be omitted.

[0099] 10 shows the functional configuration of a detection apparatus 1020 as a detection apparatus according to the second embodiment. The detection apparatus 1020 includes a branching unit 1000, a plurality of wave plates including a first wave plate 1201, a second wave plate 1202, a third wave plate 1203, and a fourth wave plate 1204, a plurality of optical delay interference units including a first optical delay interference unit 211, a second optical delay interference unit 212, a third optical delay interference unit 213, and a fourth optical delay interference unit 214, and a plurality of optical receivers including a first optical receiver 221, a second optical receiver 222, a third optical receiver 223, and a fourth optical receiver 224.

[0100] The first optical delay interference unit 211, the first optical receiver 221, the second optical delay interference unit 212, the second optical receiver 222, the third optical delay interference unit 213, the third optical receiver 223, the fourth optical delay interference unit 214, and the fourth optical receiver 224 are the same as the first optical delay interference unit 211, the first optical receiver 221, the second optical delay interference unit 212, the second optical receiver 222, the third optical delay interference unit 213, the third optical receiver 223, the fourth optical delay interference unit 214, and the fourth optical receiver 224 described in relation to Figures 3 and / or 5, etc., and therefore their descriptions may be omitted.

[0101] The branching unit 1000 branches the input signal light into a plurality of branched lights. In the example of Fig. 10, the branching unit 1000 branches the input signal light into a plurality of branched lights including a first branched light, a second branched light, a third branched light, and a fourth branched light. The branching unit 1000 branches the input signal light into at least N branched lights, where N is the number of wave plates included in the detection device 1020.

[0102] The plurality of branched beams branched by the branching unit 1000 pass through a corresponding one of a plurality of wave plates including a first wave plate 1201, a second wave plate 1202, a third wave plate 1203, and a fourth wave plate 1204. Specifically, the first branched beam passes through the first wave plate 1201, the second branched beam passes through the second wave plate 1202, the third branched beam passes through the third wave plate 1203, and the fourth branched beam passes through the fourth wave plate 1204.

[0103] The plurality of wave plates including the first wave plate 1201, the second wave plate 1202, the third wave plate 1203, and the fourth wave plate 1204 have mutually different principal axis directions of birefringence. The plurality of wave plates including the first wave plate 1201, the second wave plate 1202, the third wave plate 1203, and the fourth wave plate 1204 are preferably half wave plates.

[0104] The first branched light that has passed through the first wave plate 1201 is input to the first optical delay interference unit 211. The first optical receiver 221 detects the output light from the first optical delay interference unit 211 and outputs a first detection signal. The first optical delay interference unit 211 and the first optical receiver 221 function as a first optical delay interferometer that detects the first branched light that has passed through the first wave plate 1201 and outputs a first detection signal.

[0105] The second branched light that has passed through the second wave plate 1202 is input to the second optical delay interference unit 212. The second optical receiver 222 detects the output light from the second optical delay interference unit 212 and outputs a second detection signal. The second optical delay interference unit 212 and the second optical receiver 222 function as a second optical delay interferometer that detects the second branched light that has passed through the second wave plate 1202 and outputs a second detection signal.

[0106] The third branched light that has passed through the third wave plate 1203 is input to the third optical delay interference unit 213. The third optical receiver 223 detects the output light from the third optical delay interference unit 213 and outputs a third detection signal. The third optical delay interference unit 213 and the third optical receiver 223 function as a third optical delay interferometer that detects the third branched light that has passed through the third wave plate 1203 and outputs a third detection signal.

[0107] The fourth branched light that has passed through the fourth wave plate 1204 is input to the fourth optical delay interference unit 214. The fourth optical receiver 224 detects the output light from the fourth optical delay interference unit 214 and outputs a fourth detection signal. The fourth optical delay interference unit 214 and the fourth optical receiver 224 function as a fourth optical delay interferometer that detects the fourth branched light that has passed through the fourth wave plate 1204 and outputs a fourth detection signal.

[0108] The detection unit 340 detects polarization fluctuation of the signal light based on signals acquired from a plurality of detection signals including the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal. The detection unit 340 may detect polarization fluctuation of the signal light by performing processing similar to that of the detection unit 340 in the detection device 122. For example, the detection unit 340 may preferentially select a detection signal having a greater signal strength from among the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal, and detect the polarization fluctuation of the signal light based on the selected detection signal. The detection unit 340 may detect polarization fluctuation based on the sum of squares of the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal. For example, the detection unit 340 may detect polarization fluctuation based on the sum of squares of a signal corresponding to polarization fluctuation in the first detection signal, a signal corresponding to polarization fluctuation in the second detection signal, a signal corresponding to polarization fluctuation in the third detection signal, and a signal corresponding to polarization fluctuation in the fourth detection signal.

[0109] The detector 340 may have any of the configurations of the detector 600, the detector 700, the detector 800, and the detector 900 described in relation to FIGS. 6, 7, 8, and 9.

[0110] As described above, in the second embodiment, split light beams are passed through a plurality of wave plates having different principal axes, and each split light beam is detected. For example, if the principal axis of birefringence of a half-wave plate is tilted by θ with respect to the direction of the polarization plane of the incident light, the polarization plane is rotated by 2θ and emitted from the half-wave plate. In other words, if linearly polarized light is incident at an angle θ with respect to the principal axis of birefringence of the half-wave plate, the angle of the linearly polarized light beam emitted with respect to the principal axis of birefringence of the half-wave plate is −θ.

[0111] As an example, the direction of the principal axis of birefringence (sometimes referred to as the principal axis angle) relative to the direction of the polarization plane of the incident light of the first wave plate 1201 may be π / 12, the principal axis angle of the second wave plate 1202 may be π / 6, the principal axis angle of the third wave plate 1203 may be π / 4, and the principal axis angle of the fourth wave plate 1204 may be π / 3, in which case the polarization plane of the branched light will rotate by π / 6, π / 3, π / 2, and 2π / 3, respectively. In this way, by passing branched light through a plurality of wave plates having different principal axes and detecting each branched light with an optical delay interferometer, it is possible to prevent a signal corresponding to polarization fluctuation in the detection signal of at least any of the branched light from becoming 0.

[0112] 11 and 12 show the results of simulating the detection signal obtained when a periodically varying lateral stress is applied to an optical fiber by a fiber stretcher, and the signal light is passed through a half-wave plate and detected by an optical delay interferometer.

[0113] Figure 11 shows the relationship between the direction of the polarization plane of the incident light and the direction of the lateral stress application. The direction of the lateral stress application was set to π / 4 [rad] with respect to the principal axis of birefringence (x direction) of the optical fiber. This is because the polarization fluctuation due to the lateral stress is greatest when the polarization plane of the incident light is oriented along the principal axis of the optical fiber and the direction of the lateral stress application is π / 4 with respect to the principal axis of birefringence of the optical fiber. The frequency of the lateral stress was set to 142 [kHz], and the amplitude of the phase change due to the lateral stress was set to π / 2 [rad].

[0114] Fig. 12 shows the relationship between the angle of the principal axis of the half-wave plate and the peak-to-peak value of the detection signal detected by the optical delay interferometer. The horizontal axis of Fig. 12 represents the angle (sometimes referred to as the principal axis angle) that the principal axis of the half-wave plate makes with respect to the polarization plane of the signal light, and the vertical axis of Fig. 12 represents the peak-to-peak value of the detection signal detected by the optical delay interferometer from the signal light that has passed through the half-wave plate.

[0115] 12 shows that the detection signal detected by the optical delay interferometer varies depending on the principal axis angle of the half-wave plate and becomes zero when the principal axis angle of the half-wave plate is π / 8. This is because, when the principal axis angle of the half-wave plate is π / 8, the half-wave plate rotates the polarization plane of the signal light by π / 4, so that the polarization plane of the signal light coincides with the direction of application of the lateral stress, resulting in a situation where it is not affected by the lateral stress. Conversely, when the principal axis angle of the half-wave plate is other than π / 8, the detection signal by the optical delay interferometer does not become zero. In other words, when the direction of application of the lateral stress is θ, the principal axis angle of the half-wave plate is other than θ / 2, the detection signal by the optical delay interferometer does not become zero. Therefore, by passing branched light through multiple wave plates with different principal axes and detecting each branched light with an optical delay interferometer, it is possible to prevent the signal corresponding to polarization fluctuation in the detection signal of at least one branched light from becoming zero.

[0116] According to the detection device 1020 described in relation to FIG. 10 , a plurality of branched lights, including first branched light, second branched light, third branched light, and fourth branched light, branched from the signal light, pass through wave plates having different principal axis directions of birefringence, and are then detected by an optical delay interferometer. Based on signals obtained from the detected plurality of detection signals, polarization fluctuation of the signal light propagating through the optical transmission line 10 can be detected. The detection device 1020 described in relation to FIG. 10 has a configuration for branching the signal light into a plurality of branched lights, including first branched light, second branched light, third branched light, and fourth branched light, but the number of branched light signals is not limited to four or more. Any number of branched light signals, two or more, can be used. That is, the detection device can be configured to detect polarization fluctuation of the signal light based on signals obtained from a plurality of detection signals, each of which is detected by an optical delay interferometer, for a plurality of branched light signals branched from the signal light and passed through wave plates having different principal axis directions of birefringence.

[0117] Specifically, the detection device 1020 may be configured to include a branching section that separates the signal light into the plurality of branched lights, wave plates through which each of the plurality of branched lights passes and which have different principal axis directions of birefringence, a plurality of optical delay interferometers that detect each of the plurality of branched lights that have passed through the plurality of wave plates, and a detection section that detects polarization fluctuations of the signal light based on signals obtained from the plurality of detection signals.

[0118] As an example, when two is used as the number of branches for the signal light, the detection device 1020 may include a branching section that branches the signal light into first branched light and second branched light, a first wave plate through which the first branched light passes, a second wave plate through which the second branched light passes, a first optical delay interferometer that detects the first branched light that has passed through the first wave plate and outputs a first detection signal, a second optical delay interferometer that detects the second branched light that has passed through the second wave plate and outputs a second detection signal, and a detection section 340 that detects polarization fluctuations of the signal light based on signals obtained from the first detection signal and the second detection signal.

[0119] 13 is a flowchart showing a detection method executed by the detecting device 120 in the first embodiment. In S1402, the detecting device 120 separates light having a plurality of different polarization components from the input signal light. In S1404, the detecting device 120 detects each of the plurality of polarization components separated from the signal light in S1402 using an optical delay interferometer. In S1406, the detecting device 120 detects polarization fluctuations based on the detection signal obtained by detection using the optical delay interferometer in S1404. In S1406, the detecting device 120 outputs evaluation information based on the polarization fluctuations detected in S1406.

[0120] 14 is a flowchart showing a detection method executed by the detection device 1020 in the second embodiment. In S1502, the detection device 120 splits the input signal light into multiple different branched lights. In S1504, the detection device 120 passes each branched light split from the signal light in S1502 through wave plates with different principal axis directions, and in S1506, detects each of the signal components that have passed through the wave plates with an optical delay interferometer. In S1508, the detection device 120 detects polarization fluctuations based on the detection signal obtained by detection with the optical delay interferometer in S1506. In S1510, the detection device 120 outputs evaluation information based on the polarization fluctuations detected in S1508.

[0121] 15 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 122, and the detection device 1020, 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.

[0122] 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.

[0123] The CPU 2012 operates according to programs stored in the ROM 2026 and RAM 2014, thereby controlling each unit.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] A program installed in computer 2000 and causing computer 2000 to function as detection device 120, detection device 122, or detection device 1020 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 122, or detection device 1020. When the information processing described in these programs is loaded into computer 2000, it functions as each of detection device 120, detection device 122, or detection device 1020, 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 122, or detection device 1020 according to the intended use.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] REFERENCE SIGNS LIST 10 Optical transmission path 20 Optical signal receiving device 40 Splitter 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 Detector 122 Detector 201 First separator 202 Second separator 210 First optical receiving unit 211 First optical delay interference unit 212 Second optical delay interference unit 213 Third optical delay interference unit 214 Fourth optical delay interference unit 221 First optical receiver 222 Second optical receiver 223 Third optical receiver 224 Fourth optical receiver 240 Detector 300 Splitter 310 First optical receiving unit 320 Second optical receiving unit 340 Detector 600 Detector 610 Selector 620 AD converter 630 Intensity analyzer 640 Frequency analyzer 700 Detector 710 Selector 721 First AD converter 722 Second AD converter 800 Detector 810 Square adder 900 Detector 910 Square adder 1000 Branching section 1020 Detector 1201 First wave plate 1202 Second wave plate 1203 Third wave plate 1204 Fourth wave plate 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 3000 Computer

Claims

1. A detection device for detecting polarization fluctuations of signal light propagated through an optical transmission line, comprising a detection unit that detects polarization fluctuations of the signal light based on signals obtained from a plurality of detection signals obtained by an optical delay interferometer, each of which detects light of a plurality of different polarization components separated from the signal light.

2. The detection device according to claim 1, further comprising: a separator that separates the signal light into the plurality of polarization components; and a plurality of optical delay interferometers that detect each of the plurality of polarization components separated from the signal light by the separator.

3. The detection device according to claim 1, further comprising: a first separation unit that separates the signal light into light of a first polarization component and light of a second polarization component; a first optical delay interferometer that detects the light of the first polarization component and outputs a first detection signal; and a second optical delay interferometer that detects the light of the second polarization component and outputs a second detection signal, wherein the detection unit detects polarization fluctuations of the signal light based on signals obtained from the first detection signal and the second detection signal.

4. The detection device according to claim 1, further comprising: a branching unit that branches the signal light into first branched light and second branched light; a first separation unit that separates the first branched light into light of a first polarization component and light of a second polarization component; a second separation unit that separates the second branched light into light of a third polarization component and light of a fourth polarization component; a first optical delay interferometer that detects the light of the first polarization component and outputs a first detection signal; a second optical delay interferometer that detects the light of the second polarization component and outputs a second detection signal; a third optical delay interferometer that detects the light of the third polarization component and outputs a third detection signal; and a fourth optical delay interferometer that detects the light of the fourth polarization component and outputs a fourth detection signal, wherein the detection unit detects polarization fluctuations of the signal light based on signals obtained from the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal.

5. A detection device for detecting polarization fluctuations of signal light propagating through an optical transmission line, comprising a detection unit that detects polarization fluctuations of the signal light based on signals obtained from a plurality of detection signals obtained by an optical delay interferometer for a plurality of branched lights that are branched from the signal light and passed through wave plates with mutually different principal axis directions of birefringence.

6. The detection device according to claim 5, wherein the wave plate is a half wave plate.

7. The detection device according to claim 5 or 6, further comprising: a branching section that splits the signal light into the plurality of branched lights; a plurality of wave plates through which each of the plurality of branched lights passes; and a plurality of optical delay interferometers that detect each of the plurality of branched lights that have passed through the plurality of wave plates, wherein the plurality of wave plates have mutually different principal axis directions of birefringence.

8. The detection device according to claim 5 or 6, further comprising: a branching unit that branches the signal light into first branched light and second branched light; a first wave plate through which the first branched light passes; a second wave plate through which the second branched light passes; a first optical delay interferometer that detects the first branched light that has passed through the first wave plate and outputs a first detection signal; and a second optical delay interferometer that detects the second branched light that has passed through the second wave plate and outputs a second detection signal, wherein the detection unit detects polarization fluctuations of the signal light based on signals obtained from the first detection signal and the second detection signal.

9. The detection device according to claim 5 or 6, further comprising: a branching unit that branches the signal light into first branched light, second branched light, third branched light, and fourth branched light; a first wave plate through which the first branched light passes; a second wave plate through which the second branched light passes; a third wave plate through which the third branched light passes; a fourth wave plate through which the fourth branched light passes; a first optical delay interferometer that detects the first branched light that has passed through the first wave plate and outputs a first detection signal; a second optical delay interferometer that detects the second branched light that has passed through the second wave plate and outputs a second detection signal; a third optical delay interferometer that detects the third branched light that has passed through the third wave plate and outputs a third detection signal; and a fourth optical delay interferometer that detects the fourth branched light that has passed through the fourth wave plate and outputs a fourth detection signal, wherein the detection unit detects polarization fluctuations of the signal light based on signals acquired from the first detection signal, the second detection signal, the third detection signal, and the fourth detection signal.

10. The optical delay interferometers provided in the detection device each 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, 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 9.

11. A detection device according to any one of claims 1 to 10, wherein the detection unit preferentially selects one or more detection signals having greater signal strength from among the plurality of detection signals, and detects polarization fluctuations of the signal light based on the selected one or more detection signals.

12. The detection device according to any one of claims 1 to 10, wherein the detection unit detects the polarization fluctuation based on the sum of squares of the plurality of detection signals.

13. An optical signal receiving device comprising: a detection device according to any one of claims 1 to 12; and a demodulation section that demodulates a received signal transmitted by the signal light and generates an information signal.

14. An optical communication system comprising: an optical signal receiving device according to claim 13; and an optical signal transmitting device that transmits the signal light.

15. A program for causing a computer to function as the detection device according to any one of claims 1 to 12.

16. A detection method for detecting polarization fluctuations of signal light propagated through an optical transmission line, comprising a step of detecting polarization fluctuations of the signal light based on a signal obtained from a plurality of detection signals obtained by detecting, with an optical delay interferometer, light of a plurality of different polarization components separated from the signal light.

17. A detection method for detecting polarization fluctuations of signal light propagated through an optical transmission line, comprising a step of detecting polarization fluctuations of the signal light based on a plurality of detection signals obtained by an optical delay interferometer detecting a plurality of branched lights that are branched from the signal light and passed through wave plates having different principal axis directions of birefringence.

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