Modulation timing adjustment method and device in optical communication system

Abstract

Provided are new modulation timing adjustment method and device with which the modulation timing of a modulator provided in an optical communication system can be optimized. A device for adjusting the modulation timing of a modulator 11 for modulating an optical pulse train PIN having a predetermined period comprises: a variance calculation unit 14 that calculates a variance V of a detection signal DS of an output optical pulse POUT of the modulator 11; and a control unit 15 that acquires the variance V of the detection signal DS obtained by detecting an output optical pulse train at each modulation timing while shifting the modulation timing of the modulator 11, and adjusts the modulation timing on the basis of the variance V. The control unit 15 uses, as a reference for determining an optimum modulation timing, a modulation timing at which the variance V becomes maximum.

Description

[Technical Field]

[0001] The present invention relates to optical communication systems, and more particularly to a technique for adjusting the modulation timing of optical pulses. [Background technology]

[0002] When modulating an optical pulse with a period T using an optical modulator, the optical modulator must be driven at the timing when the optical pulse passes. If the modulation timing is off, the desired modulation amount cannot be obtained, resulting in an increase in the error rate of the demodulated data. Therefore, several modulation timing adjustment methods have been proposed to drive the optical modulator in accordance with the timing when the optical pulse passes.

[0003] For example, Patent Document 1 describes a method in which the receiving side monitors the error rate of received data and adjusts the modulation timing on the transmitting side to minimize the error rate. Also, Patent Document 2 describes a method in which the transmitter combines light modulated by multiple optical modulators and adjusts the modulation timing based on the intensity of the combined light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-020011 [Patent Document 2] International Publication No. 2018 / 061303 Brochure Summary of the Invention [Problem to be solved by the invention]

[0005] However, the error rate of received data can deteriorate due to various factors. For example, changes in environmental temperature can cause delays in optical transmission paths and electrical circuits to fluctuate, which can cause the timing of optical pulses passing through the modulator to be off, resulting in an increase in the error rate. The error rate can also increase due to failures in the stabilization control of the optical system. For example, in quantum key distribution (QKD) systems, an interferometer is configured between the transmitter and receiver, so malfunctions in the stabilization control can be a major cause of an increase in the error rate. Therefore, simply monitoring the error rate makes it difficult to determine whether the problem is due to modulation timing or an optical system problem.

[0006] Furthermore, while monitoring the intensity of the combined light allows for adjustment of the modulation timing at the transmitter, it does not allow for adjustment of the modulation timing at the receiver. For example, in a QKD system, a modulator may be installed not only at the transmitter but also at the receiver, and therefore modulation timing adjustment is also required at the receiver modulator.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a new modulation timing adjustment method and device, and a receiver, that can optimize the modulation timing of a modulator provided in an optical communication system. [Means for solving the problem]

[0008] According to a first aspect of the present invention, there is provided an apparatus for adjusting the modulation timing of a modulator that modulates an optical pulse train with a predetermined period, comprising: a variance calculation unit that calculates the variance of a detection signal of an output optical pulse from the modulator; and a control unit that acquires the variance of a detection signal that detects the output optical pulse train at each modulation timing while shifting the modulation timing of the modulator, and adjusts the modulation timing based on the variance, wherein the control unit uses the modulation timing at which the variance is maximum as a criterion for determining an optimal modulation timing. According to a second aspect of the present invention, there is provided a method for adjusting the modulation timing of a modulator that modulates an optical pulse train with a predetermined period by a data processing unit, characterized in that the method calculates the variance of a detection signal obtained by detecting the output optical pulse train of the modulator while shifting the modulation timing of the modulator, and optimizes the modulation timing based on the modulation timing at which the variance is maximized. According to a third aspect of the present invention, there is provided a program for causing a computer to function as a device for adjusting the modulation timing of a modulator that modulates an optical pulse train with a predetermined period, characterized in that the program causes the computer to realize a function of calculating the variance of a detection signal obtained by detecting the output optical pulse train of the modulator while shifting the modulation timing of the modulator, and a function of optimizing the modulation timing based on the modulation timing at which the variance is maximized. According to a fourth aspect of the present invention, there is provided a receiver connected to a transmitter via an optical transmission line in an optical communication system, the receiver comprising: a modulator of the transmitter modulates an optical pulse train of a predetermined period; a receiving unit that receives an output optical pulse train of the modulator via the optical transmission line, detects the output optical pulse train, and outputs a detection signal; a variance calculation unit that calculates the variance of the detection signal; and a control unit that acquires the variance of the detection signal by detecting the output optical pulse train at each modulation timing while shifting the modulation timing of the modulator of the transmitter, and optimizes the modulation timing based on the modulation timing at which the variance is maximum. According to a fifth aspect of the present invention, there is provided a receiver connected to a transmitter via an optical transmission line in an optical communication system, the receiver comprising: a first modulator of the transmitter modulating an optical pulse train of a predetermined period; a receiving unit receiving a first output optical pulse train of the first modulator via the optical transmission line; a second modulator modulating the first output optical pulse train received from the transmitter via the optical transmission line; a dispersion calculating unit calculating dispersion of a detection signal that detects the second output optical pulse train of the second modulator; and a control unit adjusting modulation timings of the first modulator and the second modulator based on the dispersion, With the modulation operation stopped, the modulation timing of the first modulator is shifted while obtaining the variance of the detection signal of the second output optical pulse train detected at each modulation timing, and the modulation timing of the first modulator is optimized based on the modulation timing at which the variance is maximized. After optimizing the modulation timing of the first modulator, the modulation timing of the second modulator is shifted while obtaining the variance of the detection signal of the second output optical pulse train detected at each modulation timing, and the modulation timing of the second modulator is optimized based on the modulation timing at which the variance is maximized. [Effects of the Invention]

[0009] As described above, according to the present invention, by using the variance of the detection signal, it is possible to optimize the modulation timing without using the error rate or the intensity of the combined light, and independently of malfunctions in the stabilization control of the optical system. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a block diagram showing a schematic configuration of a modulation timing adjustment device according to an embodiment of the present invention; [Figure 2] FIG. 2 is a diagram for explaining an example of quadrature modulation in this embodiment. [Figure 3] FIG. 3 is a schematic diagram for explaining a change in the variance of the detection signal due to a change in the modulation timing of the quadrature modulator in this embodiment. [Figure 4]FIG. 4 is a graph showing an example of a change in dispersion to explain a method for setting the optimum modulation timing according to this embodiment. [Figure 5] FIG. 5 is a schematic diagram showing an example of a modulation voltage for explaining a change in variance of a detection signal in this embodiment. [Figure 6] FIG. 6 is a schematic diagram showing an example of the change in dispersion due to the positional relationship between the modulation voltage and the optical pulse train in this embodiment. [Figure 7] FIG. 7 is a schematic diagram showing an example of a change in modulation voltage used for adjusting the modulation timing according to this embodiment. [Figure 8] FIG. 8 is a flowchart showing a first example of the operation of the modulation timing adjustment device according to this embodiment. [Figure 9] FIG. 9 is a flowchart showing a second example of the operation of the modulation timing adjustment device according to this embodiment. [Figure 10] FIG. 10 is a block diagram showing a schematic configuration of a continuous quantity QKD system to which a modulation timing adjustment device according to an embodiment of the present invention is applied. [Figure 11] FIG. 11 is a flowchart showing an example of the operation of the modulation timing adjustment device in FIG. [Figure 12] FIG. 12 is a diagram showing an example of a transmission bit sequence and mapping to the IQ space of the bases in the transmitter (Alice) shown in FIG. [Figure 13] FIG. 13 is a schematic diagram for explaining basis matching in the receiver (Bob) shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Outline of the embodiment> According to an embodiment of the present invention, the variance of the detection signal of the modulated output optical pulse train is calculated while shifting the modulation timing of the modulator, and the modulation timing at which the variance is maximum is used as a criterion for determining the optimal modulation timing. By using the variance of the detection signal in this way, it is possible to optimize the modulation timing without using the error rate or the intensity of the combined light, and independently of malfunctions of the stabilization control of the optical system.

[0012] For example, using two adjacent modulation timings at which dispersion is maximized as a reference, the midpoint between them can be set as the optimal modulation timing.Alternatively, using one modulation timing at which dispersion is maximized as a reference, a point shifted by half a period from that point can be set as the optimal modulation timing.Embodiments and examples of the present invention will now be described in detail with reference to the drawings.

[0013] 1. One embodiment 1.1) Configuration 1, the modulation timing adjustment system 1 includes a modulation timing adjustment device 10 according to an embodiment of the present invention, a modulator 11, a modulation control unit 12, and a detection unit 13. The modulator 11 receives an optical pulse train P IN and modulates each optical pulse of the modulated optical pulses to output an optical pulse train P OUT Output.

[0014] The modulator 11 receives the modulation voltage V MOD According to the input optical pulse train P IN The modulation control unit 12 applies a modulation voltage according to the data series to the modulator 11 in accordance with the timing of the passage of the input optical pulse. The timing at which this modulation voltage is applied (modulation timing) is adjusted by a modulation timing adjustment device 10, which will be described later.

[0015] The modulator 11 may be a phase modulator or a quadrature modulator (IQ modulator). The quadrature modulator splits each input optical pulse into two, one through an in-phase (I-phase) path and the other through a quadrature (Q-phase) path, modulates the split optical pulses according to the transmission bits, and then combines them to generate a single output optical pulse. The output optical pulse is a multilevel phase / amplitude modulated optical pulse. Such a quadrature modulator may be configured with a Mach-Zehnder (MZ) modulator provided in each of the I-phase and Q-phase paths. The quadrature modulation in this embodiment includes at least phase modulation, and includes QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), etc., with each symbol represented as a different signal point on the IQ plane.

[0016] The output optical pulse train P of the modulator 11 OUT is detected by a detector 13 through an optical transmission path such as an optical fiber or free space. The detector 13 has an opto-electrical converter and a demodulator corresponding to the modulator 11. The detector 13 detects the output optical pulse train P OUT A detection signal DS consisting of an I component and a Q component is generated from each optical pulse and output to the modulation timing adjustment device 10.

[0017] The modulation timing adjustment device 10 includes a variance calculation unit 14, a control unit 15, a storage unit 16, and a modulation timing control unit 17. The variance calculation unit 14 calculates a numerical value (hereinafter referred to as variance V) indicating the spread of the signal on the IQ plane from the detection signal DS input from the detection unit 13. This variance V may be any numerical value that indicates the degree of signal variation, and a variance calculation formula used in statistics may also be used.

[0018] When the control unit 15 receives the variance V from the variance calculation unit 14, it calculates the modulation timing T M The control unit 15 stores the modulation timing T MWhile shifting the modulation timing T M The modulation control unit 12 outputs an instruction to shift the modulated voltage V according to the data sequence in accordance with each modulation timing controlled by the modulation timing control unit 17. MOD is output to the modulator 11.

[0019] The control unit 15 calculates the variance V from the detection signal DS at each modulation timing and outputs it to the modulation timing T M The control unit 15 stores the modulation timing T M From the correspondence data between V and variance V, the modulation timing showing the maximum variance is detected, and this modulation timing is used as a reference for determining the optimum modulation timing.

[0020] 1.2) Modulation timing adjustment The modulation timing adjustment function realized by the modulation timing adjustment device 10 will be described in detail below with reference to Figures 2 to 6. For simplicity, however, QPSK modulation will be described here as an example.

[0021] Figure 2 shows the signal point arrangement on the IQ plane when the quadrature modulation is QPSK. As shown in the figure, the four-value modulation voltage V MOD are determined by 2-bit data (1,0), (0,0), (1,1), and (0,1), respectively, and the modulation voltages correspond to the phase modulation depths of 0, π / 2, π, and 3π / 2. The modulator 11 applies the modulation voltage V MOD The input optical pulses are phase-modulated according to the following equation, whereby the data sequences are mapped to corresponding signal points on the IQ plane.

[0022] <Distribution-based optimization> As shown in Fig. 3, the modulation voltage V MOD rises at time t1 and the predetermined voltage V MODAfter maintaining a steady state of , it falls at time t2. Ideally, it will have a rectangular pulse waveform as shown in waveform 20. However, in reality, the voltage near times t1 and t2 has a slope before it reaches a steady state, and at the rising point, an overshoot occurs and it is not stable (waveform 21). Therefore, the modulation voltage V MOD It is desirable to adjust the timing so that the input optical pulse is modulated in the central portion R where the input optical pulse is stable, avoiding the rising and falling transition portions.

[0023] Such delicate timing adjustments become more difficult as optical communication speeds increase and the optical pulse period and optical pulse width become shorter, which necessitates delicate timing control. According to an embodiment of the present invention, delicate modulation timing adjustments are possible by monitoring the dispersion of the detection signal, as described below.

[0024] In FIG. 3, the input optical pulse 22 is assumed to be phase-modulated with the correct modulation timing. That is, the modulation voltage V MOD The relative position of the input optical pulse 22 with respect to the input optical pulse 22 is within the central portion R, and the predetermined modulation voltage is stably applied to perform the predetermined phase modulation. When the predetermined stable phase modulation is performed in this manner, the detection signal DS of the phase-modulated output optical pulse is distributed around the correct signal point on the IQ plane with a relatively small variance (detection signal distribution 31 during stable modulation). For example, if an appropriate threshold is used, the variance V of the detection signal at that time will be smaller than the threshold.

[0025] The input optical pulse 23 is modulated by a voltage V MOD In this case, the modulation voltage V MOD Input optical pulse for 23 The relative position of is at the edge, and the modulation voltage V MODSince the phase modulation depth changes rapidly, it is impossible to determine the level of phase modulation that has occurred. Therefore, the detected signal DS has a wide variation in the depth of phase modulation and is distributed with a large variance across multiple signal points on the IQ plane (detected signal distribution 32 during unstable modulation). Furthermore, in the positional relationship between the edge and center R of the modulated voltage pulse, as in input optical pulse 24, the variance V becomes relatively large when there is significant distortion, as in waveform 21.

[0026] By sequentially shifting the modulation timing in this way, the variance V of the detected signal increases or decreases, making it possible to search for the modulation timing that shows the minimum variance. However, there are cases where the optimal modulation timing cannot be found using the modulation timing search method that shows the minimum variance V, as follows:

[0027] As shown in Fig. 4, when the modulation timing is sequentially shifted, the variance V of the detection signal repeatedly increases and decreases every period T. Therefore, if there is a modulation timing at which the variance V is smallest as described above, that can be set as a good modulation timing.

[0028] However, the modulation voltage V MOD is close to the ideal waveform 20, there may be multiple modulation timings at which the variance V is minimum. In this case, even if one is selected from multiple modulation timings, it will not be the modulation voltage V MOD Even if the center timing of multiple modulation timings is selected, the modulation voltage V MOD It is not necessarily located in the central part R of the image.

[0029] Therefore, in this embodiment, the relative position of the input light pulse 22 is determined by the modulation voltage V MOD It is noted that the variance of the detection signal DS is much larger at the edge, i.e., the transition portion, of the modulation voltage V MOD Since the edge of always exists as a rising or falling edge, the modulation timing at which the variance is maximum within the period T is the modulation voltage V MOD It can be determined that this corresponds to the edge portion of the

[0030] Once the modulation timing at which the variance V is maximized is identified, the modulation voltage V is calculated based on that timing. MOD As shown in Fig. 4, the central part R of the modulation timing T M1 and T M2 At the midpoint T MO can be determined as the optimum modulation timing within the central part R of the modulation voltage. Alternatively, the modulation timing T at which the variance V is maximized can be determined as the optimum modulation timing within the central part R of the modulation voltage. M1 If the modulation timing T M1 Timing T shifted by half a period (T / 2) from MO can be determined as the optimum modulation timing within the central part R of the modulation voltage.

[0031] As shown in Figure 5, the modulation voltage V MOD Even when V changes between multiple voltages, an edge always exists as a rising or falling edge. Therefore, even in this case, if the modulation timing showing the maximum dispersion is extracted, that timing can be used as the modulation voltage V MOD The edge portion of the image can be identified as the edge portion of the image.

[0032] As shown in Figure 6, the modulation voltage V MOD is assumed to vary with a period T according to the data sequence. In this case, as a first optimum modulation timing setting method, the variance V of the detection signal DS is calculated while shifting the modulation timing for each input pulse in increments of Δt. This allows for the calculation of at least two modulation timings T at which the variance V reaches its maximum value. M1 , T M2 , T M3 Next, the adjacent modulation timing T M1 and T M2 (or T M2 and T M3 ) and so on, and at the intermediate time T MO is determined as the optimum modulation timing.

[0033] Alternatively, as a second optimum modulation timing setting method, the variance V of the detection signal DS is calculated while sequentially shifting the modulation timing for each input pulse by Δt within the range of one period T. This allows for the calculation of one modulation timing T at which the variance V reaches its maximum value. M1 Once the modulation timing T M1 The timing shifted by half a period (T / 2) from the center R of the modulation voltage can be determined as the optimum modulation timing.

[0034] When adjusting the modulation timing, the modulation voltage V MOD Therefore, as shown in FIG. 7, the modulation voltage V is set to a value that is as large as possible. MOD between the maximum value V(3π / 2) and the minimum value V(0). This allows the maximum value of the variance V to be detected more clearly, and the optimum modulation timing T MO can be set with higher reliability.

[0035] 1.3) Control Flow The functions of the modulation timing adjustment device 10 can be realized by a computer. In particular, the functions of the variance calculation unit 14, control unit 15, and modulation timing control unit 17 can be realized by executing a program on a processor such as a CPU (Central Processing Unit), or can also be realized by hardware such as an FPGA (Field-Programmable Gate Array) or ASIC (Application-Specific Integrated Circuit). Hereinafter, assuming that the data processing unit includes a processor, FPGA, or ASIC, first and second examples of modulation timing adjustment realized on the data processing unit will be described with reference to FIGS. 8 and 9.

[0036] <Example 1> As shown in FIG. 8, the data processing unit modulates the timing T M is initialized (operation S41). For example, an arbitrary start modulation timing is set to 0. Then, the currently set modulation timing T MThe variance V is calculated from the detected signal DS (operation S42), and the modulation timing T M and the variance V are stored in the storage unit 16 in association with each other on the time axis (Operation S43).

[0037] Next, the data processing unit calculates the modulation timing T M It is determined whether or not the shift amount has reached a predetermined amount from the initial value (operation S44). If it has not reached the predetermined amount (NO in operation S44), the modulation timing T M is shifted by a predetermined step Δt, and the process returns to step S42. M Steps S42 to S45 are repeated until the shift amount reaches a predetermined shift amount, which is two periods (2T) or more.

[0038] Modulation timing T M When the variance V at each modulation timing is calculated by shifting the modulation timing T by two periods (YES in operation S44), the data processing unit M From the corresponding data of V and variance V, the modulation timing T at which variance V reaches its maximum value in each period is M1 , T M2 , T M3 (Operation S46). Next, the data processing unit extracts adjacent modulation timing T Mi and T Mi+1 At the midpoint T MO is determined as the optimum modulation timing (operation S47).

[0039] <Example 2> In addition to the first example described above, it is also possible to determine the optimum modulation timing as a timing shifted by half a period (T / 2) from the modulation timing showing the maximum dispersion. Hereinafter, operations similar to those in the first example will be assigned the same reference numerals and explanations will be omitted, and operations different from the first example will mainly be explained.

[0040] As shown in Figure 9, the modulation timing T M and the variance V are stored in the storage unit 16 in association with each other on the time axis (operation S43), and then the data processing unit 14 stores the modulation timing TM It is determined whether or not the modulation timing T has reached one period T from the initial value (operation S51). If it has not reached T (NO in operation S51), the modulation timing T M is shifted by a predetermined step Δt, and the process returns to step S42. M The operations S42, S43, S51 and S45 are repeated until the shift amount reaches T for one period.

[0041] Modulation timing T M When the variance V at each modulation timing is calculated by shifting the modulation timing T for one period stored in the storage unit 16 (YES in operation S51), the data processing unit M From the corresponding data of V and variance V, the modulation timing T at which variance V reaches its maximum value is M1 (Operation S52).

[0042] 1 modulation timing T showing the maximum value M1 Once the modulation timing T M1 The timing shifted by half a period (T / 2) from the reference time is determined as the optimum modulation timing (operation S53).

[0043] 1.3) Effects As described above, according to this embodiment, the modulation timing T M The output optical pulse train P is modulated while shifting OUT The variance V of the detection signal DS is calculated, and the modulation timing at which the variance V is maximum is used as the criterion for determining the optimum modulation timing.

[0044] As an example, the two modulation timings that maximize the variance are used as the reference, and the midpoint between them is set as the optimal modulation timing T MO As another example, one modulation timing at which the dispersion is maximized is used as a reference, and the timing shifted by half a period (T / 2) from that is determined as the optimum modulation timing.

[0045] In this way, the optimal modulation timing is determined based on the variance of the detected signal, eliminating the need to calculate the error rate or the intensity of the combined light, as was previously required. Furthermore, the variance of the detected signal can be calculated separately from the stabilization control of the optical system, which has the advantage that only the modulation timing can be optimized even if the stabilization control of the optical system is insufficient.

[0046] 2. Working Example The modulation timing adjustment device 10 according to the above-described embodiment receives an input optical pulse train P IN is quadrature modulated, and the output optical pulse train P OUT The present invention is applicable to communication devices that detect a signal having a high frequency component. As an example, an example in which the above-described embodiment is applied to an optical communication system will be described below.

[0047] When the modulation timing adjustment system 1 illustrated in Fig. 1 is applied to an optical communication system, the modulator 11 and modulation control unit 12 can be installed in a communication device on the transmitting side, and the detection unit 13 and modulation timing adjustment device 10 can be installed in a communication device on the receiving side. In this case, the input optical pulse train P IN is output from the laser light source, and the output optical pulse train P OUT is transmitted to the receiving communication device through an optical transmission line.

[0048] The modulator 11 and the modulation control unit 12 may be installed in a communication device on the receiving side. In this case, the input optical pulse train P IN is the transmitted optical pulse train transmitted by the transmitting communication device through the optical transmission line, and the output optical pulse train P OUT is the received optical pulse train that is incident on the detector 13 via the optical transmission path in the receiving communication device.

[0049] The functions of the control unit 15, storage unit 16, and modulation timing control unit 17 of the modulation timing adjustment device 10 can be provided in either the transmitting communication device or the receiving communication device. Below, a continuous QKD system will be taken up as an example of an optical communication system.

[0050] 2.1) System Configuration 10, a continuous-quantum QKD system according to one embodiment of the present invention comprises a transmitter 100 and a receiver 200, which are connected by a quantum channel CH1 and a classical channel CH2. Here, quantum channel CH1 is a communication channel that transmits weak light from transmitter 100 to receiver 200, and the weak light has an optical power of 1 photon / bit or less. Therefore, the quantum channel is a channel with relatively large losses and noise, and is prone to errors, and is less reliable than a conventional channel.

[0051] The classical channel CH2 is a general communication channel, and transmits optical signals at, for example, normal optical power. Therefore, the classical channel CH2 is a substantially error-free communication channel and has sufficiently high communication reliability. For this reason, the classical channel CH2 is used to exchange information necessary for the transmitter 100 and the receiver 200 to perform basis matching, error correction, and privacy amplification in the QKD system. Note that the quantum channel CH1 and the classical channel CH2 may be physically separate transmission paths, or may be multiplexed onto a single optical transmission path.

[0052] The transmitter 100 according to this embodiment includes a quantum unit 101, a data processing unit 102, a communication unit 103, and a program memory 104. The data processing unit 102 executes a program stored in the program memory 104, thereby realizing the same function as the modulation control unit 12 in Fig. 1. The communication unit 103 communicates with the receiver 200 using ordinary light through a classical channel CH2.

[0053] The quantum unit 101 includes a laser light source 110, a non-polarizing beam splitter BSa, a phase modulator (PM) 111, a variable optical attenuator (VOA) 112, and a polarizing beam splitter PBSa. The laser light source 110 outputs an optical pulse train with a period T, and the beam splitter BSa splits each optical pulse from the laser light source 110 into a reference light path and a signal light path at a predetermined ratio. The split ratio is a value at which the reference light has a sufficiently higher intensity than the signal light, for example, reference light:signal light=99:1.

[0054] The reference optical pulse LO branched to the reference optical path is incident on the polarizing beam splitter PBSa as it is (or via a phase modulator, not shown). The reference optical pulse LO is reflected by the polarizing beam splitter PBSa to become a reference optical pulse of a predetermined linear polarization, and is transmitted to the receiver 200 via the quantum channel CH1. The signal optical pulse branched to the signal optical path is converted to a weak signal optical pulse L via the phase modulator 111 and the variable attenuator 112. Q The weak signal light pulse L is incident on the polarizing beam splitter PBSa. Q is transmitted through the polarizing beam splitter PBSa, becomes a linearly polarized signal light pulse orthogonal to the reference light pulse, and is transmitted to the receiver 200 through the quantum channel CH1.

[0055] In this embodiment, the phase modulator 111 is a QPSK modulator, and the data processing unit 102 applies phase modulation of 0, π / 2, π, or 3π / 2 to the signal light pulse in accordance with the key element K0 and a random number sequence of base A. The signal light pulse thus phase-modulated is converted by the variable attenuator 112 into a weak light pulse L of 1 photon / bit or less. Q and is transmitted to the receiver 200 via the polarizing beam splitter PBSa and the quantum channel CH1.

[0056] The receiver 200 according to this embodiment includes a quantum unit 201, a data processing unit 202, a communication unit 203, and a program memory 204. The data processing unit 202 realizes the above-mentioned modulation timing adjustment function by executing a program stored in the program memory 204. The communication unit 203 communicates with the communication unit 103 of the transmitter 100 using ordinary light through a classical channel CH2.

[0057] The quantum unit 201 includes a polarizing beam splitter PBSb, a phase modulator (PM) 210, a non-polarizing beam splitter BSb, photodetectors PD1 and PD2, a differential calculation unit SUB, and an analog-to-digital converter ADC. Each optical pulse of the optical pulse train arriving from the transmitter 100 through the quantum channel CH1 is split into a received signal optical pulse L by the polarizing beam splitter PBSb. QRCVand the received reference optical pulse LO RCV and separate into.

[0058] Received signal light pulse L QRCV The received reference optical pulse LO is incident on one input port of the beam splitter BSb directly (or via a phase modulator, not shown). RCV is phase-modulated by the phase modulator 210 and enters the other input port of the beam splitter BSb. 2 By this, the random number base B(x / In this embodiment, the basis B(x / p) x, p is the depth of phase modulation 0 , π / 2 The received signal light pulse L QRCV and the phase-modulated received reference optical pulse LO RCV and enters the beam splitter BSb.

[0059] The beam splitter BSb has equal optical transmittance and reflectance, and receives the signal optical pulse L QRCV and the received reference optical pulse LO RCV The two output beams are superimposed on each other and output to the photodetectors PD1 and PD2, respectively. Therefore, the beam splitter BSa of the transmitter 100 and the beam splitter BSb of the receiver 200 constitute one interferometer.

[0060] The signals detected by the photodetectors PD1 and PD2 are subjected to a difference calculation in the difference calculation unit SUB. The difference signal S RCV is quantized by the ADC to produce the detection signal S Q-RCV to the data processing unit 202.

[0061] The interferometer described above is configured to receive the signal light pulse L QRCV and the received reference optical pulse LO RCVThe receiving method that causes interference between the signal light and the local light is called self-homodyne detection, and has the advantage that it does not require compensation for the wavelength difference between the signal light and the local light. In addition, since it uses a reference light with high optical power, it can obtain an optical amplification effect for the signal light. Therefore, even if the power of the signal light is weak, less than 1 photon / bit, it can be detected using a general photodetector PD.

[0062] 2.2) Modulation timing adjustment The data processing unit 202 includes the functions of a measurement unit 220 and a modulation timing adjustment unit 221. The measurement unit 220 receives the detection signal S Q-RCV The variance V is calculated from the above and output to the modulation timing adjustment unit 221. Note that the variance calculation function of the measurement unit 220 can also use a function that is pre-installed in the receiving detector in the continuous quantity QKD system.

[0063] The modulation timing adjustment unit 221 adjusts the modulation timing T M While shifting, the variance V at each modulation timing is calculated, and the midpoint between the two modulation timings at which the variance V is maximum is defined as the optimal modulation timing T MO Therefore, the modulation timing adjustment unit 221 includes the functions of the control unit 15, the storage unit 16, and the modulation timing control unit 17 in FIG.

[0064] 11, the modulation timing adjustment unit 221 of the data processing unit 202 adjusts the modulation timing of the phase modulator 111 of the transmitter 100 through the communication units 203 and 103 (Operation S301). This modulation timing adjustment flow is as shown in Fig. 8 or 9. However, when adjusting the modulation timing of the phase modulator 111 on the transmitting side, the modulation of the phase modulator 210 on the receiving side is stopped.

[0065] When the modulation timing adjustment on the transmitting side is completed, the modulation timing adjustment unit 221 of the data processing unit 202 adjusts the modulation timing of the phase modulator 210 of the receiver 200 (Operation S302). This modulation timing adjustment flow is also as shown in Figure 8 or 9. In this way, the phase modulation timing of the transmitter 100 and the phase modulation timing of the receiver 201 can each be adjusted to the optimal modulation timing.

[0066] As described above, the modulation timing T M While shifting, the dispersion V at each modulation timing is calculated, and the modulation timing at which the dispersion V is maximum is used as the reference for setting the optimal modulation timing. Specifically, the first example (FIG. 8) or the second example (FIG. 9) described above can be applied. With this modulation timing adjustment method, even if the phase difference of the interferometer between quantum unit 101 and quantum unit 201 is unstable, the modulation timing can be optimized by monitoring the dispersion.

[0067] After the modulation timing is optimized and the phase difference of the interferometer between quantum unit 101 and quantum unit 201 is stabilized, the key generation process such as weak light transmission and basis matching in the QKD system is carried out as described below.

[0068] 2.3) Key Generation Process As illustrated in FIG. 12, the quantum unit 101 of the transmitter 100 receives weak light L that is phase-modulated according to a key element K0 and a basis A. Qis transmitted. The key element K0 is a random number sequence that is the base for generating the final key. Base A is also a random number sequence consisting of 0 / 1, but for ease of explanation, they are denoted as x / p. Such a 2-bit random number consisting of key element K0 and base A is placed at one of four signal points on the IQ plane. For example, if base A = "x", then key element K0 = "1" is placed at signal point (x,1), and K0 = "0" is placed at signal point (x,0). In other words, the I signal and Q signal, which have a 90° phase difference, correspond to the value of base A (x / p), and each signal value 0 / 1 corresponds to key element K0.

[0069] weak light L Q is given a phase modulation of 0°, 90°, 180°, or 270° in accordance with the key element K0 and the random number sequence of the base A, and is transmitted from quantum unit 101 to quantum unit 201 through quantum channel CH1.

[0070] If quantum fluctuations did not exist, there would be no variation in the measurements made by receiver 200, as seen in the transmit signal constellation shown in Figure 12. However, quantum fluctuations do cause variations in the amplitude measurements of the received signal. The received light in Figure 13 shows an example of the reception state caused by quantum fluctuations.

[0071] In FIG. 13, if the basis A (x or p) when the transmitter 100 generates weak light is known, the receiver 200 can obtain the detection signal S by performing basis matching using the basis information. Q-RCV In this embodiment, the basis A used in the transmitter 100 and the basis B used in the receiver 200 are matched through a classical channel CH2, and the detected signal S is generated using only the matched basis. Q-RCV Generate.

[0072] For example, if the bases A=B=x match, the detection signal S Q-RCVis obtained (correct basis). In this way, if the basis matching is correct, it becomes possible to make a soft decision as to which symbol it is. On the other hand, if the basis does not match (basis A = x, basis B = p), only a detection signal distributed near the origin of the IQ plane is obtained, and it is not possible to determine which symbol it is (incorrect basis).

[0073] After the basis matching described above, error correction and privacy amplification are performed, and the final encryption key can be shared between the transmitter 100 and the receiver 200.

[0074] 3. Notes Some or all of the above-described embodiments can be described as follows, but are not limited to these. (Appendix 1) An apparatus for adjusting modulation timing of a modulator that modulates an optical pulse train with a predetermined period, comprising: a variance calculation unit that calculates the variance of a detection signal of the output optical pulse of the modulator; a control unit that acquires a variance of a detection signal obtained by detecting the output optical pulse train at each modulation timing while shifting the modulation timing of the modulator, and adjusts the modulation timing based on the variance; wherein the control unit uses the modulation timing at which the dispersion is maximized as a criterion for determining the optimum modulation timing. (Appendix 2) The modulation timing adjustment device described in Appendix 1 is characterized in that the modulator performs modulation depending on which voltage value a modulation voltage having multiple voltage values ​​is, and the control unit changes the modulation voltage according to the predetermined period. (Appendix 3) 3. The modulation timing adjustment device according to claim 2, wherein the modulation voltage has a rising or falling transition portion. (Appendix 4) The modulation timing adjustment device described in Appendix 2, characterized in that when the control unit adjusts the modulation timing, it changes the modulation voltage between the highest voltage and the lowest voltage among the multiple voltage values. (Appendix 5) The modulation timing adjustment device according to any one of appendices 1 to 4, characterized in that the control unit sequentially shifts the modulation timing of the modulator by the predetermined period at predetermined intervals, and determines the optimal modulation timing to be a time point shifted by half the predetermined period from the modulation timing at which the dispersion is maximum within the predetermined period as a reference. (Appendix 6) The modulation timing adjustment device according to any one of appendices 1 to 4, characterized in that the control unit sequentially shifts the modulation timing of the modulator at predetermined intervals, and determines the midpoint between two adjacent modulation timings among the modulation timings at which the dispersion is maximum within the predetermined period as a reference, as the optimal modulation timing. (Appendix 7) A method for adjusting modulation timing of a modulator that modulates an optical pulse train with a predetermined period by a data processing unit, comprising: calculating a variance of a detection signal obtained by detecting an output optical pulse train of the modulator while shifting a modulation timing of the modulator; optimizing the modulation timing based on the modulation timing at which the variance is maximized; A modulation timing adjustment method comprising: (Appendix 8) The modulation timing adjustment method described in Appendix 7, characterized in that the modulator performs modulation depending on which voltage value of a modulation voltage having multiple voltage values ​​is, and changes the modulation voltage according to the predetermined period. (Appendix 9) 9. The modulation timing adjustment method according to claim 8, wherein the modulation voltage has a rising or falling transition portion. (Appendix 10) 9. The modulation timing adjustment method according to claim 8, wherein when adjusting the modulation timing, the modulation voltage is changed between a maximum voltage and a minimum voltage among the plurality of voltage values. (Appendix 11) the modulation timing of the modulator is sequentially shifted by the predetermined period at predetermined intervals, and the modulation timing at which the dispersion is maximized within the predetermined period is used as a reference, and a time point shifted by half the predetermined period from the reference is determined as the optimal modulation timing; 11. The modulation timing adjustment method according to any one of Supplementary Notes 7 to 10, (Appendix 12) The modulation timing of the modulator is sequentially shifted at predetermined intervals, and two adjacent modulation timings among the modulation timings at which the dispersion is maximized within the predetermined period are used as a reference, and the midpoint between the two references is determined as the optimal modulation timing. 11. The modulation timing adjustment method according to any one of Supplementary Notes 7 to 10, (Appendix 13) A program that causes a computer to function as a device that adjusts modulation timing of a modulator that modulates an optical pulse train with a predetermined period, a function of calculating the variance of a detection signal obtained by detecting an output optical pulse train of the modulator while shifting the modulation timing of the modulator; a function of optimizing modulation timing based on the modulation timing at which the dispersion is maximized; A program that causes the computer to realize the above. (Appendix 14) The program described in Appendix 13, characterized in that the modulator performs modulation depending on which voltage value a modulation voltage having multiple voltage values ​​has, and changes the modulation voltage according to the predetermined period. (Appendix 15) 15. The program of claim 14, wherein the modulation voltage has a rising or falling transition portion. (Appendix 16) 15. The program according to claim 14, wherein when adjusting the modulation timing, the modulation voltage is changed between a maximum voltage and a minimum voltage among the plurality of voltage values. (Appendix 17) the modulation timing of the modulator is sequentially shifted by the predetermined period at predetermined intervals, and the modulation timing at which the dispersion is maximized within the predetermined period is used as a reference, and a time point shifted by half the predetermined period from the reference is determined as the optimal modulation timing; 17. The program according to any one of appendices 13-16, (Appendix 18) The modulation timing of the modulator is sequentially shifted at predetermined intervals, and two adjacent modulation timings among the modulation timings at which the dispersion is maximized within the predetermined period are used as a reference, and the midpoint between the two references is determined as the optimal modulation timing. 17. The program according to any one of appendices 13-16, (Appendix 19) A receiver connected to a transmitter via an optical transmission line in an optical communication system, a receiving section that modulates an optical pulse train of a predetermined period using a modulator of the transmitter, receives the output optical pulse train of the modulator through the optical transmission line, detects the output optical pulse train, and outputs a detection signal; a variance calculation unit that calculates the variance of the detection signal; a control unit that acquires a variance of a detection signal obtained by detecting the output optical pulse train at each modulation timing while shifting the modulation timing of the modulator of the transmitter, and optimizes the modulation timing based on the modulation timing at which the variance is maximum; A receiver having: (Appendix 20) The receiver described in Appendix 19, characterized in that the modulator performs modulation depending on which voltage value of a modulation voltage having multiple voltage values ​​is, and the control unit changes the modulation voltage according to the predetermined period. (Appendix 21) 21. The receiver of claim 20, wherein the modulation voltage has a rising or falling transition portion. (Appendix 22) The receiver described in Supplementary Note 20, characterized in that when the control unit adjusts the modulation timing, the control unit changes the modulation voltage between the highest voltage and the lowest voltage among the multiple voltage values. (Appendix 23) The receiver according to any one of appendixes 19 to 22, characterized in that the control unit sequentially shifts the modulation timing of the modulator by the predetermined period at predetermined intervals, and determines the optimal modulation timing to be a time point shifted by half the predetermined period from the modulation timing at which the dispersion is maximized within the predetermined period as a reference. (Appendix 24) The receiver according to any one of Supplementary Notes 19 to 22, characterized in that the control unit sequentially shifts the modulation timing of the modulator at predetermined intervals, and determines the midpoint between two adjacent modulation timings among the modulation timings at which the dispersion is maximum within the predetermined period as a reference, as the optimal modulation timing. (Appendix 25) A receiver connected to a transmitter via an optical transmission line in an optical communication system, a receiving section that modulates an optical pulse train of a predetermined period using a first modulator of the transmitter and receives a first output optical pulse train of the first modulator through the optical transmission line; a second modulator that modulates the first output optical pulse train received from the transmitter through the optical transmission line; a variance calculation unit that calculates the variance of a detection signal obtained by detecting a second output optical pulse train of the second modulator; a control unit that acquires variance of the detection signal of the second output optical pulse train detected at each modulation timing while shifting the modulation timing of the first modulator in a state where the modulation operation of the second modulator is stopped, and optimizes the modulation timing of the first modulator based on the modulation timing at which the variance is maximized; A receiver having: (Appendix 26) 26. The receiver according to claim 25, wherein the control unit optimizes the modulation timing of the first modulator, and then shifts the modulation timing of the second modulator while acquiring variance of the detection signal of the second output optical pulse train detected at each modulation timing, and optimizes the modulation timing of the second modulator based on the modulation timing at which the variance is maximum. [Industrial Applicability]

[0075] The present invention is applicable to an optical communication system having a modulator that modulates optical pulses with a predetermined period. [Explanation of symbols]

[0076] 10 Modulation timing adjustment device 11 Modulator 12 Modulation control section 13 Detector 14 Variance calculation section 15 Control Unit 16 Memory section 17 Modulation timing control section

Claims

1. An apparatus for adjusting modulation timing of a modulator that modulates an optical pulse train with a predetermined period, comprising: a variance calculation unit that calculates the variance of a detection signal of the output optical pulse train of the modulator; a control unit that acquires a variance of a detection signal obtained by detecting the output optical pulse train at each modulation timing while shifting the modulation timing of the modulator, and adjusts the modulation timing based on the variance; wherein the control unit uses the modulation timing at which the dispersion is maximized as a criterion for determining the optimum modulation timing.

2. 2. The modulation timing adjustment device according to claim 1, wherein the modulator performs modulation depending on which voltage value a modulation voltage having a plurality of voltage values ​​has, and the control unit changes the modulation voltage according to the predetermined period.

3. 3. The modulation timing adjustment device according to claim 2, wherein the modulation voltage has a rising or falling transition portion.

4. 3. The modulation timing adjustment device according to claim 2, wherein the control unit, when adjusting the modulation timing, varies the modulation voltage between a maximum voltage and a minimum voltage among the plurality of voltage values.

5. The modulation timing adjustment device according to any one of claims 1 to 4, characterized in that the control unit sequentially shifts the modulation timing of the modulator by the predetermined period at predetermined intervals, and determines the optimal modulation timing to be a point shifted by half the predetermined period from the modulation timing at which the dispersion is maximum within the predetermined period as a reference.

6. The modulation timing adjustment device according to any one of claims 1 to 4, characterized in that the control unit sequentially shifts the modulation timing of the modulator at predetermined intervals, and determines the midpoint between two adjacent modulation timings among the modulation timings at which the dispersion is maximum within the predetermined period as a reference, as the optimal modulation timing.

7. A method for adjusting modulation timing of a modulator that modulates an optical pulse train with a predetermined period by a data processing unit, comprising: calculating a variance of a detection signal obtained by detecting an output optical pulse train of the modulator while shifting a modulation timing of the modulator; optimizing the modulation timing based on the modulation timing at which the variance is maximized; A modulation timing adjustment method comprising:

8. A program that causes a computer to function as a device that adjusts modulation timing of a modulator that modulates an optical pulse train with a predetermined period, a function of calculating the variance of a detection signal obtained by detecting an output optical pulse train of the modulator while shifting the modulation timing of the modulator; a function of optimizing modulation timing based on the modulation timing at which the dispersion is maximized; A program that causes the computer to realize the above.

9. A receiver connected to a transmitter via an optical transmission line in an optical communication system, a receiving section that modulates an optical pulse train of a predetermined period using a modulator of the transmitter, receives the output optical pulse train of the modulator through the optical transmission line, detects the output optical pulse train, and outputs a detection signal; a variance calculation unit that calculates the variance of the detection signal; a control unit that acquires a variance of a detection signal obtained by detecting the output optical pulse train at each modulation timing while shifting the modulation timing of the modulator of the transmitter, and optimizes the modulation timing based on the modulation timing at which the variance is maximum; A receiver having:

10. A receiver connected to a transmitter via an optical transmission line in an optical communication system, a receiving section that modulates an optical pulse train of a predetermined period using a first modulator of the transmitter and receives a first output optical pulse train of the first modulator through the optical transmission line; a second modulator that modulates the first output optical pulse train received from the transmitter through the optical transmission line; a variance calculation unit that calculates the variance of a detection signal obtained by detecting the second output optical pulse train of the second modulator; a control unit that adjusts modulation timings of the first modulator and the second modulator based on the dispersion; and the control unit acquiring a variance of the detection signal of the second output optical pulse train detected at each modulation timing while shifting the modulation timing of the first modulator in a state where the modulation operation of the second modulator is stopped, and optimizing the modulation timing of the first modulator based on the modulation timing at which the variance is maximized; After optimizing the modulation timing of the first modulator, the modulation timing of the second modulator is shifted while acquiring the variance of the detection signal of the second output optical pulse train detected at each modulation timing, and the modulation timing of the second modulator is optimized based on the modulation timing at which the variance is maximized. A receiver characterized by:

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