Relative phase difference compensation device, relative phase difference compensation method, and program

Abstract

To contribute to reducing the amount of calculation required for fitting processing performed on an interference signal of signal light and reference light in optical communication.SOLUTION: A relative phase difference compensation device according to the present disclosure separates signal light and reference light from an optical signal, generates inverse function data indicating the inverse function of a specific function indicated by an interference output signal generated by homodyne detection to modulate the separated reference light, combines the separated signal light and modulated reference light, separates a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light, performs homodyne detection on the separated polarization component of the signal light and the polarization component of the reference light to generate an interference output signal, estimates the relative phase difference between the signal light and the reference light based on the generated interference output signal, and generates offset value data that cancels out the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present disclosure relates to a relative phase difference compensation device, a relative phase difference compensation method, and a program.

Background Art

[0002] In the field of optical communication using coherent signal light such as a laser beam, homodyne detection is used to detect phase information and the like from signal light modulated using a reference light. Note that homodyne detection is generally also called homodyne demodulation (Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Note that the disclosure of the above prior art documents is incorporated herein by reference. The following analysis is made by the present inventors.

[0005] Here, fitting processing is performed to obtain the relative phase difference between the signal light and the reference light from an interference signal obtained by interfering the signal light and the reference light using a homodyne detection method. However, the amount of arithmetic processing for the fitting processing is large, and this arithmetic processing requires a lot of time. Therefore, in order to perform high-speed and wide-band data by optical communication, it is desirable to reduce the amount of arithmetic processing for the fitting processing and shorten the time required for the arithmetic processing.

[0006] An object of the present disclosure is to contribute to reducing the amount of calculation of fitting processing for obtaining the relative phase difference between the signal light and the reference light, which is performed on the interference signal between the signal light obtained by homodyne detection and the reference light in optical communication, and shortening the calculation time thereof.

Means for Solving the Problems

[0007] In a first aspect of the present disclosure, there is provided a relative phase difference compensation device including a polarization controller, a modulator, a balance receiver, a polarization beam splitter, a phase difference estimator, and an offset value controller, which compensates for the relative phase difference between signal light and reference light included in an optical signal and having polarizations orthogonal to each other. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by the balance receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. The balance receiver is configured to perform homodyne detection on the separated polarization component of the signal light and the polarization component of the reference light to generate the interference output signal. The phase difference estimator is configured to estimate the relative phase difference between the signal light and the reference light based on the generated interference output signal. The offset value controller is configured to generate offset value data for canceling the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light.

[0008] In a second aspect of the present disclosure, in a relative phase difference compensation apparatus including a polarization controller, a modulator, and a polarization beam splitter, which compensates for the relative phase difference between signal light and reference light included in an optical signal and having polarizations orthogonal to each other, a relative phase difference compensation method is provided, which includes an inverse function data generation step, an interference output signal generation step, a relative phase difference estimation step, and an offset value data generation step. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by a balanced receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and to separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. The inverse function data generation step generates inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection. The interference output signal generation step performs the homodyne detection on the polarization component of the separated signal light and the polarization component of the reference light to generate the interference output signal. The relative phase difference estimation step estimates the relative phase difference between the signal light and the reference light based on the generated interference output signal. The offset value data generation step generates offset value data that cancels out the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light.

[0009] In a third aspect of the present disclosure, a program executed in a relative phase difference compensation device that includes a polarization controller, a modulator, and a polarization beam splitter and compensates for the relative phase difference between signal light and reference light included in an optical signal and having polarizations orthogonal to each other. A relative phase difference compensation program including an inverse function generation process, an interference output signal generation process, a relative phase difference estimation process, and an offset value data generation process is provided. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by a balanced receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. The inverse function generation process generates inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection. The interference output signal generation process performs homodyne detection on the polarization component of the separated signal light and the polarization component of the reference light to generate the interference output signal. The relative phase difference estimation process estimates the relative phase difference between the signal light and the reference light based on the generated interference output signal. The offset value data generation process generates offset value data that cancels out the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light.

Advantages of the Invention

[0010] According to each aspect of the present disclosure, in optical communication, it is possible to contribute to reducing the amount of calculation of the fitting process for obtaining the relative phase difference between the signal light and the reference light performed on the interference signal between the signal light and the reference light obtained by homodyne detection and shortening the calculation time.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5A

Figure 5B

Figure 5C

Figure 6

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the embodiments described below. In each drawing, the same or corresponding elements and processes are appropriately assigned the same reference numerals. Furthermore, it should be noted that the drawings are schematic, and the dimensional relationships between elements, the ratios of elements, and the details of each process may be different from those in the present disclosure when implemented. Also, there may be portions where the dimensional relationships and process details are different between the drawings.

[0013] First, referring to FIG. 1, an optical communication system 1 used for continuous variable quantum key distribution (CV-QKD; Continuous Variable Quantum Key Distribution), which is used to enhance the security of optical communication in quantum cryptographic communication, will be described. FIG. 1 is a diagram illustrating a configuration example of an optical communication system 1 that transmits and receives data using coherent light such as a laser beam. As illustrated in FIG. 1, the optical communication system 1 includes a transmission device 2 and a reception device 3 connected via an optical transmission path 100 composed of an optical fiber cable.

[0014] The transmission device 2 includes a laser diode (LD; Laser Diode) 200, a non-polarization beam splitter (NPBS; Non-Polarization Beam Splitter) 202, a polarization beam splitter 206 (PBS; Polarization Beam Splitter), and a phase modulator (PM; Phase Modulator) 204, which are connected to each other by an optical fiber cable. The transmission device 2 generates a signal light S (signal light Signal) by phase-modulating coherent light such as a laser beam according to the voltage value of a transmission modulation signal input from the outside, and transmits it to the reception device 3 via the optical transmission path 100.

[0015] The reception device 3 includes a polarization controller (PC; Polarization Controller) 300, a non-polarization beam splitter 302, a phase modulator 304 (PM; Phase Modulator), variable optical attenuators (VOA; Variable Optical Attenuator) 306, 308, and a balance receiver (BR; Balance Receiver) 310. The transmission device 2 receives the signal light S from the transmission device 2, demodulates the transmission modulation signal by homodyne detection, and outputs it to the outside as the voltage value of an interference output signal through these components.

[0016] In the transmitting device 2, the laser diode 200 generates a laser beam as coherent light and outputs it to the non-polarizing beam splitter 202. The non-polarizing beam splitter 202 splits the laser beam input from the laser diode 200 into two components, for example, with intensities of 99% and 1%. It outputs the reference light LO with an intensity of 99% to the polarizing beam splitter 206 and outputs the signal light S with an intensity of 1% to the phase modulator 204.

[0017] The phase modulator 204 modulates the signal light S with an intensity of 1% of the laser beam input from the non-polarizing beam splitter 202 by the transmission modulation signal and outputs it to the polarizing beam splitter 206. The polarizing beam splitter 206 combines the signal light S modulated by the phase modulator 204 and the polarization of the reference light LO input from the polarizing beam splitter 206 so that their polarizations are orthogonal to each other and outputs it to the optical transmission path 100. Note that the optical transmission path 100 is a quantum channel transmitted by an optical fiber cable, and in the optical transmission path 100, the polarization of the optical signal output from the transmitting device 2 is not preserved.

[0018] When an optical signal is input from the optical transmission path 100 to the receiving device 3, the polarization controller 300 separates the optical signal into the polarization component of the signal light S and the polarization component of the reference light LO, outputs the polarization component of the signal light S to the non-polarizing beam splitter 302, and outputs the polarization component of the reference light LO to the phase modulator 304. A random reception modulation signal is input to the phase modulator 304, and the phase modulator 304 randomly phase-modulates the polarization component of the reference light LO input from the polarization controller 300 and outputs it to the non-polarizing beam splitter 302.

[0019] The non-polarizing beam splitter 302 is a half-mirror beam splitter that combines the polarization component of the signal light S input from the polarization controller 300 and the polarization component of the reference light LO modulated by the phase modulator 304. The non-polarizing beam splitter 302 divides the intensity of the signal light S obtained by this combination into two optical signals with 50% intensity that are orthogonal to each other, outputs an optical signal with one polarization of 50% intensity to the variable optical attenuator 306, and outputs an optical signal with the other polarization of 50% intensity to the variable optical attenuator 308.

[0020] The variable optical attenuators 306 and 308 equalize the intensities of the optical signals input to each of them and output them to the balanced receiver 310. The balanced receiver 310 performs homodyne detection on the optical signals input from the variable optical attenuators 306 and 308 to detect the amplitude and phase information of the signal light using the interference of the two lights, generates an interference output signal indicating the result of the homodyne detection as a voltage value, and outputs it externally. In FIG. 1 and other figures, the path of the signal light S component is shown by a thin dotted line, and the path of the reference light LO component is shown by a thick dotted line.

[0021] CV-QKD has advantages over standard quantum key distribution (QKD) in terms of practical application and implementation in the method of detecting weak signal light S, so it has become a research target attracting attention in quantum cryptographic communication technology. Note that the optical communication system 1 basically has the same configuration as a Mach-Zehnder interferometer. Therefore, the relative phase difference Φ ps caused by the difference in the optical path length (optical path length) of the signal light S from the laser diode 200 to the balanced receiver 310 and the optical path length of the reference light LO must be stable within the range of one wavelength of these lights.

[0022] Also, when realizing CV-QKD by the optical communication system 1, in order to suppress excess noise, the relative phase difference Φ ps between the signal light S and the reference light LO needs to be made very small. Also, the relative phase difference Φ psIn order to always accurately estimate and correct this relative phase difference Φ ps its detection is performed, for example, at about 0.1 seconds intervals within a 1 - second time interval, and compensation for the phase difference between the signal light S and the reference light LO based on the detected relative phase difference Φ ps must always be performed at 1 - second time intervals. Note that the estimation of the relative phase difference Φ ps between the signal light S and the reference light LO described below and the compensation of the phase difference based on the estimated relative phase difference Φ ps are methods for single - capture offline processing. When performing streaming data processing, another method for estimating and compensating the relative phase difference Φ ps is taken.

[0023] Next, using the optical communication system 1 shown in FIG. 1, the relative phase difference Φ ps between the signal light S and the reference light LO is estimated, and compensation using the estimated relative phase difference Φ ps is performed every 1 second while transmitting data from the transmitting device 2 side to the receiving device 3 side. FIG. 2 is a diagram illustrating one configuration of the optical communication system 4.

[0024] As shown in FIG. 2, the optical communication system 4 includes the optical communication system 1 shown in FIG. 1, a transmission modulation signal generator (TMSG; Transmission Modulation Signal Generator) 40, and an optical signal reception processing device 42. The optical signal reception processing device 42 includes a modulation data generator (MDG; Modulation Data Generator) 420, an adder 422, a selector (SEL; SELector) 424, a saw - tooth wave data generator (SWDG; Saw Wave Data Generator) 426, a reception modulation signal generator (RMSG; Reception Modulation Signal Generator) 428, a sine fitting processor (SFP; Sine Fitting Processor) 430, an offset value controller 432 (OVC; Offset Value Controller), and a post - selection processor (PSP; Post Selection Processor) 434.

[0025] These components enable the transmission modulation signal generator 40 and the transmission device 2 of the optical communication system 4 to modulate an optical signal with transmission data input from the outside to generate a signal light S, and transmit it to the receiving device 3 together with a reference light LO used to demodulate the transmission data from the signal light S. Further, the optical signal reception processing device 42 of the optical communication system 4 is based on the interference output signal output from the balanced receiver 310 of the receiving device 3, and the relative phase difference Φ ps between the signal light S and the reference light LO is calculated. Furthermore, the optical signal reception processing device 42 generates phase offset value data V ps indicating the relative phase difference Φ offset between the signal light S and the reference light LO, and compensates for the relative phase difference Φ ps between these two reference lights LO. In addition, the optical signal reception processing device 42 performs a hard decision on the interference output signal output from the balanced receiver 310, demodulates the transmission data, and outputs it to the outside as received data.

[0026] The transmission modulation signal generator 40 generates a transmission modulation signal indicating the transmission data input from the outside as a voltage value, and outputs it to the phase modulator 204 of the transmission device 2. The phase modulator 204 modulates the optical signal with the transmission modulation signal input from the transmission modulation signal generator 40 and outputs it to the polarization beam splitter 206.

[0027] FIG. 3 is a diagram illustrating the processing of the sine fitting processor 430 by the optical signal reception processing device 42 shown in FIG. 2. In the optical signal reception processing device 42, as shown in FIG. 3, the sine fitting processor 430 applies the voltage value of the cosine wave-shaped interference output signal represented by a trigonometric function, for example, a cosine function, to the voltage value represented by the cosine function by the non-linear least squares Marquardt-Levenberg algorithm. The non-linear least squares Marquardt-Levenberg algorithm applies the voltage value of the input interference output signal to a specific function, which is a cosine function in the optical communication system 4, and performs a process of estimating the relative phase difference Φ ps is an algorithm for performing. The sine fitting processor 430 uses the relative phase difference Φ obtained as a result of the estimation psOutput it to the offset value controller 432.

[0028] The offset value controller 432 is a phase offset value data V indicating a voltage value for compensating the relative phase difference Φ input from the sine fitting processor 430 ps Generate and output it to the adder 422. The modulation data generator 420 generates a random received modulation data indicating the generated random number and outputs it to the adder 422. offset Generate and output it to the adder 422. The modulation data generator 420 generates a random received modulation data indicating the generated random number and outputs it to the adder 422.

[0029] The adder 422 adds the phase offset value data V offset And the received modulation data to generate addition data and output it to the selector 424. Note that the adder 422 can add the phase offset value data V offset By subtracting the phase offset value data V multiplied by -1 from the received modulation data offset And the received modulation data. That is, whether addition or subtraction of these data is performed in the adder 422 is essentially the same.

[0030] The sawtooth wave data generator 426 generates sawtooth wave data whose voltage value waveform indicates a sawtooth wave and outputs it to the selector 424. The selector 424 selects the sawtooth wave data input from the sawtooth wave data generator 426 and outputs it to the received modulation signal generator 428 before the operation of the optical communication system 4 starts (the first period). On the other hand, the selector 424 selects the addition data input from the adder 422 after the operation of the optical communication system 4 starts (the second period). The selector 424 outputs the selected data to the received modulation signal generator 428.

[0031] The received modulation signal generator 428 generates a modulation signal indicating the sawtooth wave data or the addition data input from the selector 424 in terms of voltage value, and outputs it to the phase modulator 304 of the receiving apparatus 3. The post-selection processor 434 demodulates the transmission data by performing a hard decision on the voltage value of the interference output signal input from the balanced receiver 310 of the receiving apparatus 3, and outputs it as received data. When the received modulation signal generator 428 outputs a modulation signal indicating the sawtooth wave data in terms of voltage value to the phase modulator 304 of the receiving apparatus 3 and the phase modulator 304 modulates the reference light LO with this modulation signal, the waveform of the voltage value of the interference output signal output from the balanced receiver 310 becomes cosine wave-shaped as described below.

[0032] Let the optical signal received by the receiving apparatus 3 be E s and the reference light LO be E LO . Ignoring the amplitude, in the path shown in FIG. 1 etc., the phase shifts generated in these optical signals E s and the reference light E LO are respectively Φ ps1 , Φ ps2 . Then, these are represented by the following formula (1).

[0033]

Equation

[0034] When the phase modulator 304 performs phase modulation on the optical signal E s and the reference light E LO with a modulation signal Φ saw (t) indicating the sawtooth wave data in terms of voltage value, the optical signal E shown in the following formula (2) LO_PM is obtained.

[0035]

Equation

[0036] When the balanced receiver 310 interferes the optical signal E s and the optical signal E LO_PM , the optical signal E shown in the following formula (3)BR is obtained.

[0037]

Number

[0038] However, in the above equation (3), Φ ps = Φ ps1 - Φ ps2 is.

[0039] As described above, for the optical signal E s and the reference light E LO when modulation is performed by the modulation signal Φ saw (t) that indicates the sawtooth wave data by a voltage value, the waveform of the optical signal E BR obtained by the modulation becomes cosine wave-shaped.

[0040] In the optical communication system 4, before the operation of the receiving device 3 is started, the phase modulator 304 of the receiving device 3 modulates the reference light LO by a modulation signal that indicates the sawtooth wave data by a voltage value. As a result, in the optical signal receiving processing device 42, the relative phase difference Φ ps and the phase offset value data V offset are calculated. Note that the optical communication system 4 being in an operating state means that in the optical communication system 4, the transmitting device 2 transmits an optical signal indicating significant transmission data to the receiving device 3 via the optical transmission line 100, and the receiving device 3 and the optical signal receiving processing device 50 are in a state where they can demodulate a significant transmission signal from the optical signal.

[0041] On the other hand, after the operation of the receiving device 3 is started, the phase modulator 304 of the receiving device 3 modulates the reference light LO by the voltage value of the added value of the received modulation data and the phase offset value data V offset . As a result, the balanced receiver 310 generates an interference output signal in which the relative phase difference Φ ps between the signal light S and the reference light LO is compensated. The post-selection processor 434 demodulates the transmission data from the interference output signal in which the relative phase difference Φ ps is compensated, and further outputs it as received data.

[0042] As described above, the sine fitting processor 430 performs fitting processing on the optical signal E with a cosine wave-shaped waveform output from the balance receiver 310 BR using the non-linear least squares Marquardt-Levenberg algorithm. The amount of calculation for the fitting processing in such a sine fitting processor 430 is large. Therefore, a long calculation time is required for the fitting processing in the sine fitting processor 430. Therefore, when using the optical signal receiving processing apparatus 42, the relative phase difference Φ ps cannot be detected in about 0.1 seconds and compensated.

[0043] [Embodiment] Hereinafter, embodiments according to the present disclosure will be described. In an optical communication system 5 according to an embodiment of the present disclosure described below, on the receiving device side, when demodulating transmission data from the signal light received from the transmitting device side, an inverse trigonometric function, for example, an inverse cosine function, is input to the received modulation signal generator 428 as a modulation signal represented by voltage, and the reference light LO is modulated. As a result, an interference output signal with a triangular wave-shaped voltage value is output from the balance receiver 310 of the receiving device 3, and it is no longer necessary to perform fitting processing on the voltage value of the interference output signal to a cosine wave-shaped function, and it is only necessary to fit it to a triangular wave-shaped function.

[0044] As a result, the fitting processing using the non-linear least squares Marquardt-Levenberg algorithm becomes unnecessary, and it is only necessary to obtain the zero crossing time of the voltage value indicating a triangular wave-shaped function by linear zero crossing phase difference estimation. Therefore, the amount of calculation for the fitting processing can be reduced and the calculation time can also be shortened, so that the estimation of the relative phase difference between the signal light S and the reference light LO can be speeded up. Note that "estimation of relative phase difference" is generally also called "measurement of relative phase difference".

[0045] FIG. 4 is a diagram illustrating a configuration of an optical communication system 5 according to an embodiment of the present disclosure. As shown in FIG. 4, the optical communication system 5 has a configuration in which the optical signal reception processing device 42 of the optical communication system 4 shown in FIG. 2 is replaced with an optical signal reception processing device 50. The optical signal reception processing device 50 has a configuration in which the sawtooth wave data generator 426 of the optical signal reception processing device 42 is replaced with an arccosine wave data generator (ACWDG; ArcCosinWave Data Generator) 500, and the sine fitting processor 430 is replaced with a linear zero-crossing phase difference estimator (LZCPDE; Linear Zero-Crossing Phase Difference Estimator) 502. FIG. 5A is a diagram illustrating a waveform of a voltage value indicated by cosine wave data output from the arccosine wave data generator 500 of the optical signal reception processing device 50 shown in FIG. 4. As shown in FIG. 5A, the voltage value indicated by the arccosine wave data output from the arccosine wave data generator 500 has a waveform of an arccosine wave.

[0046] The arccosine wave data generator 500 generates modulation data whose voltage value indicates a waveform of an arccosine function, and outputs it to the selector 424. The linear zero-crossing phase difference estimator 502 performs linear zero-crossing phase difference estimation on the voltage value of the interference output signal input from the balanced receiver 310. Similar to the optical communication system 4, the optical communication system 5 transmits signal light S in which transmission data is modulated by a modulation signal indicated by a voltage value from the transmission device 2 to the reception device 3 by these components. Further, the optical communication system 5 detects the relative phase difference between the signal light S and the reference light LO, and corrects the relative phase difference between the signal light S and the reference light LO.

[0047] Hereinafter, the processing of the optical communication system 5 will be described in detail with reference to mathematical formulas as appropriate, centering on the processing of the optical signal reception processing device 50. The transmission device 2 and the reception device 3 perform the processing described with reference to FIG. 1. The estimation of the relative phase difference Φ ps between the signal light S and the reference light LO in the optical signal reception processing device 50 is performed before the operation of the optical communication system 5 is started. Also, before the operation is started, an unmodulated optical signal is transmitted from the transmission device 2 to the reception device 3.

[0048] Relative phase difference Φ ps While the estimation of the relative phase difference Φ between the signal light S and the reference light LO is being performed, the linear zero-crossing phase difference estimator 502 estimates the relative phase difference Φ between the signal light S and the reference light LO and outputs it to the offset value controller 432. ps The offset value controller 432 generates offset value data from the relative phase difference Φ between the signal light S and the reference light LO and outputs it to the adder 422. ps From the relative phase difference Φ between the signal light S and the reference light LO, offset value data is generated and output to the adder 422.

[0049] Before the operation of the optical communication system 5 is started, the selector 424 selects the addition value of the random modulation data input from the adder 422 and the offset value data and outputs it to the received modulation signal generator 428. Also, when the operation of the optical communication system 5 is started, the selector 424 selects the inverse cosine wave data input from the inverse cosine wave data generator 500 and outputs it to the received modulation signal generator 428. The received modulation signal generator 428 generates a received data modulation signal indicating, as a voltage value, the addition value of the random modulation data and the offset value selected by the selector 424, or the inverse cosine wave data generated by the inverse cosine wave data generator 500, and outputs it to the phase modulator 304.

[0050] Before the operation of the optical communication system 5 is started, the phase modulator 304 phase-modulates the reference light LO input from the polarization controller 300 and controls its phase as shown in the following equation (4).

[0051]

Equation

[0052] In the above equation (4), E in (t) is the reference light LO input from the polarization controller 300, E out (t) is the modulated reference light LO output from the phase modulator 304, V(t) is the voltage value of the received data modulation signal, and V pl is the change in the voltage value of the received data modulation signal required to change the phase of the reference light LO by π (radians; 180°).

[0053] V pl Substituting \( \pi \) into the above formula (4), the modulated reference light LO output from the balanced receiver 310 is represented by the following formula (5).

[0054]

Equation

[0055] The balanced receiver 310 receives the modulated reference light LO shown in formula (5) and the signal light S. When the balanced receiver 310 performs general homodyne detection, the balanced receiver 310 adds the reference light LO input from the variable optical attenuator 306 and the reference light LO input from the variable optical attenuator 308, and outputs an interference output signal I(t) whose voltage value is represented by the following formula (6).

[0056]

Equation

[0057] As described above, before the operation starts, no significant transmission data is input to the transmission device 2, and transmission data indicating a value of 0 or 1 is input. As a result, an unmodulated optical signal is input from the transmission device 2 to the reception device 3. If the difference in the optical path lengths between the signal light S and the reference light LO is equal, no phase difference occurs in these reference lights LO.

[0058] In this case, \( E \) indicating the reference light LO input from the polarization controller 300 in (t) is \( Ae \) jωt ( \( E \) in (t) = \( Ae \) jωt ) can be described. Furthermore, substituting \( E \) out (t) = \( E \) in (t)·\( e \) jV(t) and \( E \) in (t) = \( Ae \) jωt into the above formula (6), the voltage value of the interference output signal I(t) output from the balanced receiver 310 is represented by the following formula (7).

[0059]

Number

[0060] In the above formula (7), B is the amplitude of the voltage value of the interference output signal output from the balance receiver 310.

[0061] FIG. 5B is a diagram illustrating waveforms of interference output signals output by the balance receiver 310 of the optical signal receiving processing apparatus 50 shown in FIG. 4 under three different conditions. In the optical communication system 5, when a reception modulation signal indicating the voltage of the inverse cosine wave data V(t) shown in the following formula (8) is input from the inverse cosine wave data generator 500 to the phase modulator 304, the balance receiver 310 outputs an interference output signal whose voltage waveform is a triangular wave (period = 1) as illustrated in FIG. 5B.

[0062]

Number

[0063] Note that the period of the inverse cosine function is π (V PI = π), while the period of the cosine function is 2π. Therefore, when the period of the inverse cosine wave data V(t) is extended to 2π, the inverse cosine wave data V(t) is represented by a combination of two types of inverse cosine wave data V(t) with a phase difference of π, as shown in the following formula (9).

[0064]

Number

[0065] On the other hand, when the balance receiver 310 subtracts the reference light LO input from the variable optical attenuator 308 from the reference light LO input from the variable optical attenuator 306 and performs homodyne detection using the difference between these reference lights LO obtained as a result, the interference output signal I(t) is represented by the following formula (10) in the same manner as formula (9).

[0066]

Number

[0067] In this case, in the optical signal reception processing apparatus 50, instead of the inverse cosine wave data generator 500, an inverse sine wave data generator (not shown) that generates modulation data whose voltage value indicates a waveform of an inverse sine function and outputs it to the selector 424 is used. That is, when the balanced receiver 310 calculates the difference between the reference light LO passing through the variable optical attenuator 306 and the reference light LO passing through the variable optical attenuator 308 and performs homodyne detection, an inverse sine wave data generator is used instead of the inverse cosine wave data generator 500, and the received modulation signal of the voltage value indicated by the inverse sine wave data V(t) shown in the following formula (11) is input to the phase modulator 304.

[0068]

Equation

[0069] Note that when the optical communication system 5 is implemented, there may be a difference between the optical path length of the signal light S and the optical path length of the reference light LO. Due to this difference in the optical path length, a relative phase difference Φ ps is generated between the signal light S and the reference light LO. Stated as such, the voltage value of the interference output signal I(t) is represented by the following formula (12). That is, due to the above-described difference, the phase of the waveform indicated by the voltage value of the interference output signal I(t) is shifted by the relative phase difference Φ ps only. The estimation of the relative phase difference is to obtain the relative phase difference Φ ps from the interference output signal I(t) represented by the following formula (12).

[0070]

Equation

[0071] Next, the optical signal reception processing apparatus 50 uses the time (zero-crossing time) when the positive and negative values of the voltage value of the interference output signal output by the balanced receiver 310 are switched to obtain the relative phase difference Φ psThe estimation process will be described. FIG. 5C is a diagram illustrating sample points selected from each of the interference output signals output under three different conditions by the balance receiver 310 of the optical signal receiving processing apparatus 50 shown in FIG. 4. In FIG. 5C, two sample points selected at intervals less than half a cycle from the triangular wave waveform are marked with circles, and sample points (Interference Signal) that may be interference signals in the triangular wave waveform are marked with triangles.

[0072] Specifically, as shown in FIG. 5C, the linear zero-crossing phase difference estimator 502 selects two sample points included in this waveform at intervals less than half a cycle of the waveform of the voltage value of the interference output signal output by the balance receiver 310. By selecting two sample points at intervals less than half a cycle from the waveform of the interference output signal in this way and performing linear interpolation, as shown in FIG. 5C, the zero-crossing time T0 of the triangular wave waveform can be estimated.

[0073] Based on the estimated zero-crossing time T0 and the ideal zero-crossing time T when there is no difference between the optical path length of the signal light S and the optical path length of the reference light LO, the linear zero-crossing phase difference estimator 502 calculates the relative phase difference Φ ps efficiently by the following formula (13).

[0074]

Equation

[0075] Here, when there is a relationship of T - T0 > 0 between the zero-crossing times T and T0 shown in formula (13), the relative phase difference Φ ps with respect to the waveform of the ideal interference output signal in which no relative phase difference occurs at all is in the delay direction, and when there is a relationship of T - T0 < 0, the relative phase difference Φ ps similarly occurs in the advance direction.

[0076] As described above, the relative phase difference Φ psWhen estimating, since the waveform of the voltage value of the interference output signal output by the balance receiver 310 is a triangular wave, linear interpolation is very easy. Therefore, the relative phase difference Φ ps can be estimated with a small amount of calculation, and the calculation time can be short. Therefore, according to the relative phase difference Φ ps estimation method described above, phase correction can be easily realized to make the absolute value of the relative phase difference Φ ps as close to 0 as possible within 0.1 second.

[0077] As described above, the linear zero-crossing phase difference estimator 502 calculates the relative phase difference Φ ps and outputs the calculation result to the offset value controller 432. The offset value controller 432 calculates the voltage indicated by the phase offset value data V offset by the following formula (14).

[0078]

Equation

[0079] The offset value controller 432 outputs data indicating the voltage value of the phase offset value data V offset to the adder 422. The adder 422 adds the received modulation data input from the modulation data generator 420 and the phase offset value data V offset to perform correction to make the absolute value of the relative phase difference Φ ps as close to 0 as possible. The adder 422 outputs the corrected received modulation data to the selector 424. Note that adding the phase offset value data V offset to the received modulation data means advancing or delaying the phase of the received modulation signal generated by the received modulation signal generator 428 from the added value of these data. Therefore, by adding appropriate phase offset value data V offset to the received modulation data, the relative phase difference Φ ps can be made 0°.

[0080] The selector 424 selects the relative phase difference Φ before the operation of the optical communication system 5 is started.ps While the estimation of offset is being performed, the inverse cosine wave data input from the inverse cosine wave generator 500 is selected and output to the received modulation signal generator 428. On the other hand, after the operation of the optical communication system 5 is started, the adder 422 selects the added value of the received modulation data input from the adder 422 and the data indicating the voltage value of the phase offset value data V

[0081] and outputs it to the received modulation signal generator 428. The received modulation signal generator 428 generates a received modulation signal of the voltage value indicated by the data input from the selector 424 and outputs it to the phase modulator 304. After the operation of the optical communication system 5 is started, the post-selection processor 434 demodulates the transmission data by hard determination that maps the interference output signal input from the balance receiver 310 to the nearest reference signal, and outputs it to the outside as received data. When the interference output signal includes QPSK-modulated transmission data, the post-selection processor 434 demodulates the transmission data by performing quadrant determination.

[0082] According to the optical communication system 5 described above, the relative phase difference Φ ps can be estimated using the interference output signal whose waveform indicating the voltage value from the balance receiver 310 is a triangular wave. Therefore, the amount of calculation required for the estimation of the relative phase difference Φ ps is small. As a result, since the estimation of the relative phase difference Φ ps can be executed by hardware such as an FPGA, the estimation of the relative phase difference Φ ps can be performed within a short time, for example, within 0.1 second. Therefore, the time required for compensating the relative phase difference Φ ps can also be shortened.

[0083] [Information processing apparatus that can be used in the implementation of the embodiments of the present disclosure] Note that all or part of the functions of the transmission modulation signal generators 40 and the optical signal reception processing devices 42 and 50 of the optical communication systems 4 and 5 described above can be implemented by an information processing apparatus (computer) 6 described below. FIG. 6 is a diagram illustrating a configuration of an optical communication system 5 that can implement all or part of the functions of the components of the transmission modulation signal generator 40 and the optical signal reception processing devices 42 and 50 of the optical communication systems 4 and 5 shown in FIGS. 2 and 4.

[0084] As illustrated in FIG. 6, the information processing apparatus 6 includes a CPU (Central Processing Unit) 600, a main storage device 602, an auxiliary storage device 604, and an interface (IF; InterFace) 606 that are interconnected via a bus so as to be able to input and output information. However, the information processing apparatus 6 may include hardware components other than those shown in FIG. 6, and the components of the information processing apparatus 6 may be distributed over a plurality of devices. That is, the configuration of the information processing apparatus 6 is not limited to the configuration shown in FIG. 6.

[0085] The CPU 600 executes instruction commands included in a program that is executed by the information processing apparatus 6 and is required for processing each component of the transmission modulation signal generator 40 and the optical signal reception processing devices 42 and 50. The main storage device 602 includes storage elements such as, for example, a RAM (Random Access Memory) and a ROM (Read Only Memory), and temporarily stores data used by the CPU 600 to execute a program.

[0086] The auxiliary storage device 604 includes non-volatile storage devices such as HDD (Hard Disk Drive), SSD (Solid State Drive), and flash memory, and stores programs executed by the CPU 600 in the medium to long term. Note that the programs stored in the auxiliary storage device 604 can be provided and distributed as products stored in non-transitory computer-readable media such as magnetic storage media, CDs, and DVDs. The IF device 606 provides an interface for input / output of information between a drive control device and the like, and the transmission modulation signal generator 40 and the optical signal reception processing devices 42, 50.

[0087] As described above, each component of the transmission modulation signal generator 40 and the optical signal reception processing devices 42, 50 can be realized by a program executed in the information processing apparatus 6, except for components that must be configured by hardware due to their nature. On the other hand, these components may be realized by dedicated hardware, or may be realized by an appropriate combination of hardware and software (program) executed in the information processing apparatus 6. Also, as described above, since at least a part of each component of the transmission modulation signal generator 40 and the optical signal reception processing devices 42, 50 can be realized by a program, the content of the processing executed by such components can be grasped as a method.

[0088] The following forms are possible in the present application, but are not limited thereto. [Appendix 1] A relative phase difference compensation device comprising a polarization controller, a modulator, a balance receiver, a polarization beam splitter, a phase difference estimator, and an offset value controller, for compensating the relative phase difference between a signal light and a reference light included in an optical signal and having polarizations orthogonal to each other. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by the balance receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and to separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. The balance receiver is configured to perform homodyne detection on the separated polarization component of the signal light and the polarization component of the reference light to generate the interference output signal. The phase difference estimator is configured to estimate the relative phase difference between the signal light and the reference light based on the generated interference output signal. The offset value controller is configured to generate offset value data for canceling the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light. [Appendix 2] A relative phase difference compensation device further comprising an adder and a selector. The adder is configured to add modulation data and the offset value data to generate an added value. The selector is configured to select the inverse function data in at least a first period in which the offset value data is generated, and to select the generated added value in a second period different from the first period. When the selector selects the inverse function data, the modulator is configured to modulate the separated reference light with the received modulation signal indicated by the inverse function data, and when the selector selects the added value, the modulator is configured to modulate the separated reference light with the received modulation signal indicated by the added value to compensate the relative phase difference. [Appendix 3] The relative phase difference compensation device according to Supplementary Note 1 or 2, further comprising a post-selection processor configured to perform hard decision on the interference output signal so as to multiplex the data indicated by the signal light. [Supplementary Note 4] The relative phase difference compensation device according to any one of Supplementary Notes 1 to 3, wherein the specific function is a cosine function and the inverse function is an inverse cosine function. [Supplementary Note 5] The relative phase difference compensation device according to any one of Supplementary Notes 1 to 4, wherein the specific function is a sine function and the inverse function is an inverse sine function. [Supplementary Note 6] The relative phase difference compensation device according to any one of Supplementary Notes 1 to 5, wherein the waveform of the interference output signal is a triangular wave, and the phase difference estimator is configured to estimate the relative phase difference between the signal light and the reference light by performing linear zero-crossing phase difference estimation on the interference output signal. [Supplementary Note 7] A relative phase difference compensation method comprising an inverse function data generation step, an interference output signal generation step, a relative phase difference estimation step, and an offset value data generation step, in a relative phase difference compensation device including a polarization controller, a modulator, and a polarization beam splitter for compensating a relative phase difference between a signal light and a reference light included in an optical signal and having polarizations orthogonal to each other. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by a balanced receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and to separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. The inverse function data generation step generates inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection. The interference output signal generation step performs the homodyne detection on the polarization component of the separated signal light and the polarization component of the reference light to generate the interference output signal. The relative phase difference estimation step estimates a relative phase difference between the signal light and the reference light based on the generated interference output signal. The offset value data generation step generates offset value data for canceling the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light. [Appendix 8] A program executed in a relative phase difference compensation device that includes a polarization controller, a modulator, and a polarization beam splitter, and compensates for the relative phase difference between a signal light and a reference light that are included in an optical signal and have polarizations orthogonal to each other. The relative phase difference compensation program includes an inverse function generation process, an interference output signal generation process, a relative phase difference estimation process, and an offset value data generation process. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by a balanced receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. The inverse function generation process generates inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection. The interference output signal generation process performs homodyne detection on the polarization component of the separated signal light and the polarization component of the reference light to generate the interference output signal. The relative phase difference estimation process estimates the relative phase difference between the signal light and the reference light based on the generated interference output signal. The offset value data generation process generates offset value data that cancels out the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light. It goes without saying that any combination of the various aspects according to the appended claims of the present disclosure, or any arbitrary combination of the elements described in each aspect and embodiment (including non-selection of some elements) can be made by those skilled in the art at any time according to the basic concept of the present disclosure.

[0089] Note that the disclosures of the above-cited patent documents and the like are incorporated herein by reference. Within the scope of all the disclosures (including the claims), further modifications and adjustments of the embodiments or examples can be made based on their basic technical concepts. Also, within the scope of all the disclosures, various combinations or selections (including partial deletion) of various disclosure elements (including each element of each claim, each element of the embodiments or examples, each element of each drawing, etc.) are possible. That is, this disclosure naturally includes all the disclosures including the claims, as well as various modifications and corrections that could be made by those skilled in the art according to the technical concept. In particular, for the numerical ranges described in this document, any numerical value or small range included within the range should be construed as specifically described even in the absence of separate description. Furthermore, the disclosed matters of the above patent documents, as required and in accordance with the spirit of this disclosure, are regarded as being included in the disclosure of this application as a part of this disclosure, and can be used in combination with the matters described in this document, either in part or in whole.

Explanation of Reference Numerals

[0090] 1, 4, 5 Optical communication system 100 Optical transmission line 2 Transmitting device 200 Laser diode (LD; Laser Diode) 202, 302 Non-polarization beam splitter (NPBS; Non-Polarization Beam Splitter) 204 Phase modulator (PM; Phase Modulator) 206 Polarization beam splitter (PBS; Polarization Beam Splitter) 3 Receiving device 300 Polarization controller (PC; Polarization Controller) 304 Phase modulator (PM; Phase Modulator) 306, 308 Variable optical attenuator (VOA; Variable Optical Attenuator) 310 Balance receiver (BR; Balance Receiver) 40 Transmission Modulation Signal Generator (TMSG) 42, 50 Optical Signal Reception Processing Device 420 Modulation Data Generator (MDG) 422 Adder 424 SELector (SEL) 426 Saw Wave Data Generator (SWDG) 428 Reception Modulation Signal Generator (RMSG) 430 Sine Fitting Processor (SFP) 432 Offset Value Controller (OVC) 434 Post Selection Processor (PSP) 500 ArcCosinWave Data Generator (ACWDG) 502 Linear Zero-Crossing Phase Difference Estimator (LZCPDE) 6 Information Processing Device 600 Central Processing Unit (CPU) 602 Main Memory Device 604 Auxiliary Memory Device 606 InterFace (IF)

Claims

1. A relative phase difference compensation device comprising a polarization controller, a modulator, a balance receiver, a polarization beam splitter, a phase difference estimator, and an offset value controller, for compensating for the relative phase difference between a signal light and a reference light included in an optical signal and having polarizations orthogonal to each other, The polarization controller is configured to separate the signal light and the reference light from the optical signal, The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by the balance receiver, The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and to separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light, The balance receiver is configured to perform homodyne detection on the separated polarization component of the signal light and the polarization component of the reference light to generate the interference output signal, The phase difference estimator is configured to estimate the relative phase difference between the signal light and the reference light based on the generated interference output signal, The offset value controller is configured to generate offset value data that cancels out the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light Relative phase difference compensation device.

2. Further comprising an adder and a selector, The adder is configured to add modulation data and the offset value data to generate an added value, The selector is configured to select the inverse function data in a first period in which at least the offset value data is generated, and to select the generated added value in a second period different from the first period, The modulator is configured to modulate the separated reference light with the received modulation signal indicated by the inverse function data when the selector selects the inverse function data, and to modulate the separated reference light with the received modulation signal indicated by the added value when the selector selects the added value to compensate for the relative phase difference The relative phase difference compensation device according to Claim 1.

3. A post-selection processor configured to multiplex data indicated by the signal light by performing hard decision on the interference output signal The relative phase difference compensation device according to claim 1, further comprising.

4. The specific function is a cosine function, The inverse function is an inverse cosine function The relative phase difference compensation device according to claim 1.

5. The specific function is a sine function, The inverse function is an inverse sine function The relative phase difference compensation device according to claim 1.

6. The waveform of the interference output signal is a triangular wave, The phase difference estimator is configured to estimate the relative phase difference between the signal light and the reference light by performing linear zero-crossing phase difference estimation on the interference output signal. The relative phase difference compensation device according to claim 1.

7. A relative phase difference compensation method in a relative phase difference compensation device that includes a polarization controller, a modulator, and a polarization beam splitter and compensates for the relative phase difference between signal light and reference light having mutually orthogonal polarizations included in an optical signal. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by a balanced receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light and separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. An inverse function data generation step of generating inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection; An interference output signal generation step of performing homodyne detection on the polarization component of the separated signal light and the polarization component of the reference light to generate the interference output signal; A relative phase difference estimation step of estimating the relative phase difference between the signal light and the reference light based on the generated interference output signal; An offset value data generation step of generating offset value data that cancels the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light A relative phase difference compensation method comprising.

8. A program executed in a relative phase difference compensation device that includes a polarization controller, a modulator, and a polarization beam splitter, and compensates for the relative phase difference between a signal light and a reference light that are included in an optical signal and have polarizations orthogonal to each other. The polarization controller is configured to separate the signal light and the reference light from the optical signal. The modulator is configured to modulate the separated reference light with a received modulation signal indicated by inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection by a balanced receiver. The polarization beam splitter is configured to combine the signal light separated by the polarization controller and the modulated reference light, and separate a polarization component of the signal light and a polarization component of the reference light from the combined signal light and reference light. An inverse function generation process for generating inverse function data indicating an inverse function of a specific function indicated by an interference output signal generated by homodyne detection. An interference output signal generation process for performing homodyne detection on the polarization component of the separated signal light and the polarization component of the reference light to generate the interference output signal. A relative phase difference estimation process for estimating the relative phase difference between the signal light and the reference light based on the generated interference output signal. An offset value data generation process for generating offset value data that cancels the relative phase difference between the signal light and the reference light based on the estimated relative phase difference between the signal light and the reference light. A program comprising the above.

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