Optical receiver, its balance adjusting method and device

Abstract

An optical receiver 100 including an interferometer 101 for inputting interfering signal light SL and reference light LO comprises: a light intensity adjustment unit 102 for adjusting the intensity balance of first output light Lout 1 and second output light Lout2 of the interferometer; a first photodetector PD1 and a second photodetector PD2 for detecting the intensity-adjusted first output light and second output light, respectively; a storage unit 105 that stores a first output level value V1 of the first photodetector PD1 and a second output level value V2 of the second photodetector PD2 acquired at completion of the initial adjustment as a first reference value VR1 and a second reference value VR2; and a control unit 104 that controls the light intensity adjustment unit 102 so that the output level values V1 and V2 become equal to reference values VR1 and VR2, respectively, in a state in which the reference light at time of operation adjustment is inputted.

Description

Optical receiving device, and balance adjustment method and device therefor

[0001] The present invention relates to an optical receiving device in an optical transmission system, and more particularly to a balance adjustment technique for a balance detector that detects interference light between signal light and reference light.

[0002] Generally, a coherent optical receiving device causes interference between received signal light and local light, and detects the signal using a balance detector. In ordinary optical communications, digital signal processing is possible, so communications are possible even if the output signal of the balance detector contains some error. In contrast, in optical transmission systems that require high phase accuracy, the balance detector must be adjusted with high precision so that it operates without bias. An example of a method for adjusting such a balance detector is disclosed in Patent Document 1.

[0003] Patent Document 1 discloses a method for calibrating a balanced detector in quantum key distribution (hereinafter referred to as QKD). According to Patent Document 1, continuous wave laser light is input to a calibration circuit connected in front of the balanced detector, and phase modulation is performed alternately between 0° and 180°. This measures the extinction ratio at the output of the balanced detector, and adjusts the gain of the balanced detector so that the extinction ratio is maximized.

[0004] JP 2023-014764 A

[0005] Typically, due to instability in optical systems, balance adjustment of the balance detector is performed periodically. In the calibration method of Patent Document 1, a calibration circuit must be connected in front of the balance detector at each periodic adjustment to measure the extinction ratio and perform balance adjustment. This requires connecting a dedicated calibration circuit and measuring device at each periodic adjustment, making it difficult to perform periodic balance adjustment quickly and easily.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical receiver, a balance adjustment method and an apparatus that can quickly and easily adjust the balance of a photodetector.

[0007] According to a first aspect of the present invention, an optical receiving device includes an interferometer that inputs signal light and reference light to cause interference, and includes a light intensity adjuster that adjusts the intensity balance between first output light and second output light of the interferometer, a first photodetector and a second photodetector that detect the first output light and the second output light, respectively, whose intensities have been adjusted by the light intensity adjuster, a memory that stores a first output level value of the first photodetector and a second output level value of the second photodetector obtained when initial adjustment is completed, as a first reference value and a second reference value, respectively, and a control unit that controls the light intensity adjuster during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, whose intensity changes at a constant intensity or a predetermined frequency, is input to the interferometer. According to a second aspect of the present invention, there is provided a balance adjustment method for an optical receiving device including an interferometer that inputs signal light and reference light to cause them to interfere with each other, a first photodetector and a second photodetector that detect first output light and second output light, respectively, from the interferometer, wherein the processing unit adjusts the intensity balance between the first output light and the second output light, stores the first output level value of the first photodetector and the second output level value of the second photodetector obtained when initial adjustment is completed in a memory unit as a first reference value and a second reference value, respectively, and controls the optical intensity adjustment unit during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or an intensity that changes at a predetermined frequency, is input to the interferometer.According to a third aspect of the present invention, there is provided a balance adjustment device for an optical receiving device including an interferometer that inputs signal light and reference light to cause them to interfere with each other, and a first photodetector and a second photodetector that detect first output light and second output light of the interferometer and output first detection signals and second detection signals, respectively, the balance adjustment device including a light intensity adjuster that adjusts the intensity of at least one of the first output light and the second output light of the interferometer, a memory that stores a first output level value of the first photodetector and a second output level value of the second photodetector obtained at the time of completion of initial adjustment as first reference values ​​and second reference values, respectively, and a control unit that controls the light intensity adjuster during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or an intensity that changes at a predetermined frequency, is input to the interferometer. According to a fourth aspect of the present invention, there is provided a program that causes a computer to function as a balance adjustment device for an optical receiving device including an interferometer that inputs signal light and reference light to cause interference between them, and a first photodetector and a second photodetector that detect first output light and second output light, respectively, the program realizing in the computer the following functions: an intensity adjustment unit adjusts the intensity balance between the first output light and the second output light; a processing unit stores in a memory unit a first output level value of the first photodetector and a second output level value of the second photodetector, which are acquired when initial adjustment is completed, as first reference values ​​and second reference values, respectively; and a function that controls the optical intensity adjustment unit during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or an intensity that changes at a predetermined frequency, is input to the interferometer.

[0008] As described above, according to the present invention, balance adjustment of a balance detector in an optical receiving device can be performed quickly and easily.

[0009] FIG. 1 is a block diagram showing a schematic configuration of an optical receiving device according to the present disclosure. FIG. 2 is a block diagram showing an example configuration of a balance detector in an optical receiving device according to the present disclosure. FIG. 3 is a flowchart showing an example of a balance adjustment method for an optical receiving device according to the present disclosure. FIG. 4 is a block diagram showing a first example configuration of an optical receiving device according to the present disclosure. FIG. 5 is a block diagram showing a second example configuration of an optical receiving device according to the present disclosure. FIG. 6 is a block diagram showing a third example configuration of an optical receiving device according to the present disclosure. FIG. 7 is a schematic diagram for explaining a QKD key generation process. FIG. 8 is a schematic configuration diagram of a QKD system to which an example of an optical receiving device according to the present disclosure is applied. FIG. 9 is a schematic configuration diagram of a QKD system to which another example of an optical receiving device according to the present disclosure is applied.

[0010] <Overview of Embodiments> According to an embodiment of the present invention, during initial adjustment, initial reference light whose intensity varies at a predetermined frequency is incident on an interferometer. The intensity balance of two output light beams output from the interferometer is adjusted, and the output light beams are incident on a balance detector. By adjusting the intensity balance, it is possible to cancel a predetermined frequency component from the differential output of the balance detector, and the output levels of each of the pair of photodetectors in the balance detector at that time are stored as reference values. Subsequently, during operational adjustment, reference light whose intensity is constant or varies at a predetermined frequency (hereinafter referred to as reference light during operational adjustment) is incident on the interferometer, and the intensity balance of the interference light beams is adjusted so that the output levels of the pair of photodetectors in the balance detector each reach the corresponding reference value.

[0011] In this way, the output levels of the pair of photodetectors when the balance adjustment of the balance detector is completed during the initial adjustment are held as a reference for the balance adjustment, and thus, during periodic operational adjustment, accurate balance adjustment of the balance detector can be performed quickly and easily by simply adjusting the intensity balance of the interference light so that the output levels of the pair of photodetectors match the respective reference values.

[0012] Furthermore, the output level of each photodetector when balance adjustment of the balance detector is completed is used as the reference for balance adjustment, so that high-precision balance adjustment can be achieved even in a system in which signal light is transmitted at a signal transmission rate of a predetermined frequency.

[0013] Furthermore, since there is no need to install optical systems or equipment for measurement during operational adjustment, there is no need to secure installation space, which also enables cost reduction. Hereinafter, embodiments and examples of the present invention will be described in detail with reference to the drawings.

[0014] 1. One Embodiment 1.1) Configuration As illustrated in FIG. 1 , an optical receiving device 100 includes a detection system consisting of an interferometer 101, a light intensity adjustment unit 102, and a balanced detector (BD) 103, and a control system consisting of a control unit 104, a reference value storage unit 105, and a frequency analyzer 106.

[0015] The interferometer 101 causes interference between the signal light SL and the reference light LO, and outputs first and second output lights (hereinafter referred to as Lout1 and Lout2, respectively) to the balance detector 103 via the light intensity adjuster 102. The interferometer 101 can employ an optical element or optical system that has the function of equally dividing and combining the signal light SL and the reference light LO. The light intensity adjuster 102 can adjust the balance of the intensities incident on the balance detector 103 by changing the intensity of at least one of the output lights Lout1 and Lout2.

[0016] The balance detector 103 includes a pair of photodetectors PD1 and PD2 and a difference calculation unit SUB. The photodetectors PD1 and PD2 receive the output light Lout1 and Lout2, respectively, whose intensity balance has been adjusted by the light intensity adjustment unit 102, and output electrical signals according to the respective received light intensities to the difference calculation unit SUB. The difference calculation unit SUB outputs differential signals S, which are analog electrical signals corresponding to the output light Lout1 and Lout2, respectively. Ｒ to the control unit 104. The balance detector 103 may have a circuit configuration that has the same functions as the photodetectors PD1 and PD2 and the difference calculation unit SUB.

[0017] Furthermore, the balance detector 103 has terminals for outputting the output signals of the photodetectors PD1 and PD2 or the level values ​​V1 and V2 of each output signal to the outside. Here, the output level values ​​V1 and V2 are approximately constant voltage values ​​corresponding to the DC components of the signals output by the photodetectors PD1 and PD2, and can be obtained by averaging or smoothing the output signals. For example, the output level values ​​V1 and V2 can be obtained by passing the output signals of each photodetector through a low-pass filter (LPF). The balance detector 103 may incorporate an averaging circuit such as an LPF for the output signals of the photodetectors PD1 and PD2.

[0018] As shown in FIG. 2, if the balance detector 103 does not have an internal averaging circuit, the output level values ​​V1 and V2 can be obtained by connecting a similar averaging circuit (here, an LPF) externally.

[0019] The signal light SL is a laser light carrying a signal arriving from an optical transmitting device (not shown). The reference light LO may be a laser light arriving from the optical transmitting device, or may be a laser light output from a local oscillator light source (LO light source) provided in the optical receiving device 100. As will be described later, during initial adjustment, reference light LO (hereinafter referred to as initial reference light LO) intensity-modulated at a predetermined frequency is supplied to the interferometer 101. During operational adjustment, reference light LO during operational adjustment, which has a constant intensity or whose intensity changes at the same predetermined frequency as during initial adjustment, is supplied to the interferometer 101.

[0020] During operation, the signal light SL and the reference light LO from the optical transmitter are incident on the interferometer 101. In this case, the differential signal S output from the balance detector 103 is Ｒ corresponds to the signal component of the signal light SL. As will be described later, when the wavelengths of the signal light SL and the reference light LO are the same, it is called homodyne detection, and when they differ to some extent, it is called intradyne detection or heterodyne detection. Note that homodyne detection, intradyne detection, and heterodyne detection are collectively called coherent detection.

[0021] The control unit 104 includes the functions of analog-to-digital (AD) converters ADC1 and ADC2 and a balance adjustment unit 104.1. The AD converters ADC1 and ADC2 digitally convert the output level values ​​V1 and V2 of the balance detector 103, respectively, and output the converted values ​​to the balance adjustment unit 104.1. As described above, the output level values ​​V1 and V2 are smoothed DC voltages. Therefore, high-speed AD converters are not required for the AD converters ADC1 and ADC2; instead, general AD converters capable of periodically converting DC voltage values ​​into digital values ​​with a predetermined accuracy can be used. The AD converters ADC1 and ADC2 may be provided external to the control unit 104. In this case, the control unit 104 realizes the balance adjustment unit 104.1, and the digitally converted values ​​(V1 and V2) are supplied from the AD converters ADC1 and ADC2 to the balance adjustment unit 104.1.

[0022] In addition to the balance adjustment during the initial adjustment and operational adjustment, the control unit 104 can also have a function of processing received signals during operation. The functions of the control unit 104, including the balance adjustment unit 104.1, can be realized by executing a program stored in the program memory 107 on a processor such as a CPU (Central Processing Unit).

[0023] The reference value storage unit 105 stores the digitally converted values ​​of the output level values ​​V1 and V2 obtained after the initial adjustment as the reference values ​​VR1 and VR2, respectively. Ｒ and outputs the detected frequency component to the control unit 104. The control unit 104 controls the light intensity adjustment unit 102 to search for an intensity balance that cancels out the frequency component, and stores the output level values ​​V1 and V2 at that time in the reference value storage unit 105 as reference values ​​VR1 and VR2, respectively.

[0024] The control unit 104 compares the output level values ​​V1 and V2 detected during operational adjustment with the reference values ​​VR1 and VR2 stored in the reference value storage unit 105, and adjusts the intensity balance so that V1=VR1 and V2=VR2. The balance adjustment is completed when V1=VR1 and V2=VR2.

[0025] The frequency analyzer 106 may be attached externally during initial adjustment of the optical receiving device 100, or the optical receiving device 100 may have an equivalent frequency analysis function built in. The light intensity adjuster 102 is not limited to a single optical element. For example, if the interferometer 101 can adjust the intensities of the output lights Lout1 and Lout2, the interferometer 101 is considered to include the light intensity adjuster 102. The function of the balance adjuster 104.1 in the control unit 104 will be described in detail below with reference to FIG. 2.

[0026] 1.2) Balance Adjustment As illustrated in FIG. 3 , the balance adjustment method according to this embodiment is divided into initial adjustment and operational adjustment. Initial adjustment is a balance adjustment performed when the optical receiving device 100 is initially set up, but is not limited to this. It can also be performed after operational adjustment has been performed a predetermined number of times or when a predetermined time has elapsed since the start of operation. Operational adjustment is performed periodically at predetermined time intervals during the operation period of the optical receiving device 100, and the intensity balance is adjusted so that the detected digital conversion values ​​V1 and V2 are equal to the reference values ​​VR1 and VR2, respectively. This will be described in detail below.

[0027] 3, during initial adjustment, an initial reference beam LO intensity-modulated at a predetermined frequency F is supplied to the interferometer 101 (operation S120). Such initial reference beam LO can be obtained, for example, by intensity-modulating a laser beam of constant intensity at the frequency F by several percent or more. If deeper intensity modulation, for example, 100% intensity modulation (ON / OFF modulation), is performed, the predetermined frequency F component can be canceled more accurately. Furthermore, it is desirable that the frequency F be a frequency close to the signal transmission rate of the signal beam SL. The frequency F is, for example, 100 MHz.

[0028] Here, we assume an ideal detection system in which the interferometer 101 has an accurate equal division ratio with no delay, and the photodetectors PD1 and PD2 of the balanced detector 103 operate without bias. In this case, the initial reference light LO intensity-modulated at frequency F is divided into output lights Lout1 and Lout2 of equal intensity and received by the balanced detector 103. Therefore, the differential signal S output by the balanced detector 103 is ＲIdeally, it will be 0V.

[0029] However, if there is instability in the optical system or bias in the balance detector 103, the differential signal S Ｒ does not necessarily become 0 V. Also, even if the differential signal S Ｒ becomes 0 V, this does not necessarily mean that there is no bias in the balance detector 103. For example, if the photodetectors PD1 and PD2 of the balance detector 103 are biased in the same direction to the same degree, the differential signal S Ｒ This is because there are cases where the voltage becomes 0V.

[0030] Therefore, the control unit 104 calculates the difference signal S obtained by the frequency analyzer 106. Ｒ The component of a predetermined frequency F is monitored, and a differential signal S Ｒ The intensity balance of the light intensity adjuster 102 is controlled so that the predetermined frequency F component disappears from the output light Lout1 and the output light Lout2 (operation S121). That is, the intensity balance is adjusted by changing the intensity of at least one of the output light Lout1 and Lout2 incident on the balance detector 103.

[0031] Differential signal S Ｒ When the predetermined frequency F component is cancelled from V1, V2, the control unit 104 stores the output level values ​​V1 and V2 of the balance detector 103 at that time as reference values ​​VR1 and VR2 in the reference value storage unit 105 (Operation S122).

[0032] Thus, the differential signal S Ｒ The intensity balance when the predetermined frequency F component is cancelled from the optical intensity is maintained in the optical intensity adjuster 102, and the reference values ​​VR1 and VR2 at that time are stored in the reference value storage unit 105. Subsequently, as already described, the control unit 104 operates the optical receiving device 100 normally (operation S123). That is, the signal light SL from the optical transmitting device and the reference light LO during operation are incident on the interferometer 101, and the interference light Lout1 and Lout2 are incident on the photodetectors PD1 and PD2 of the balance detector 103, respectively, through the optical intensity adjuster 102. The balance detector 103 detects a differential signal S, which is a signal component. Ｒ to the control unit 104, and the control unit 104 acquires the received information through predetermined signal processing.

[0033] The control unit 104 measures the duration of the receiving operation using a timer, and when the timer expires, stops the signal light SL and starts the operation adjustment described below. Note that the control unit 104 can stop the signal light SL by notifying the optical transmitting device of the start of the operation adjustment.

[0034] When the operational adjustment is started, the control unit 104 supplies the reference light LO during the operational adjustment to the interferometer 101 (Operation S124). The reference light LO during the operational adjustment may be generated from a laser light arriving from the optical transmitting device, or may be generated from an output laser light of an LO light source provided in the optical receiving device 100.

[0035] The control unit 104 acquires the output level values ​​V1 and V2 of the balance detector 103 obtained when the reference light LO during operational adjustment is input, and the reference values ​​VR1 and VR2 stored in the reference value storage unit 105 (operation S125). Next, the control unit 104 adjusts the intensity balance of the light intensity adjuster 102 so that the level value V1 is equal to the reference value VR1 and the level value V2 is equal to the reference value VR2 (operations S125 to S127). If V1 = VR1 and V2 = VR2 (YES in operation S126), the control unit 104 fixes the intensity balance of the light intensity adjuster 102 at that time and returns to the normal reception operation S123. The control unit 104 executes the above operational adjustment operation at a predetermined interval during the normal reception operation.

[0036] 1.3) Effects As described above, according to this embodiment, the reference values ​​VR1 and VR2 for balance adjustment are obtained during initial adjustment, and the light intensity adjuster 102 is controlled so that the output level values ​​V1 and V2 of the balance detector 103 obtained during periodic operational adjustment correspond to the reference values ​​VR1 and VR2, respectively. This allows operational adjustment to be completed simply by referring to the reference values, making it possible to quickly and easily perform balance adjustment of the balance detector. Furthermore, by using the output level values ​​V1 and V2 of the photodetectors PD1 and PD2, respectively, of the balance detector 103, the state of the optical system and the balance photodetector 103 can be accurately reproduced, enabling more accurate and precise balance adjustment.

[0037] The reference values ​​VR1 and VR2 are the output voltage level values ​​of the photodetectors PD1 and PD2 when a component of a predetermined frequency is cancelled from the differential output of the balance detector 103. By using these as a reference for balance adjustment, high-precision balance adjustment becomes possible even in a system in which signal light is transmitted at a signal transmission rate of a predetermined frequency.

[0038] Furthermore, since there is no need to install optical systems or equipment for measurement during operational adjustment, there is no need to secure installation space, which also reduces costs.

[0039] 2. Examples Hereinafter, examples of configurations based on the above-described embodiments will be described. Components having the same functions will be assigned the same reference numerals, and the description will be simplified. Different components will be mainly described.

[0040] First Example As illustrated in FIG. 4, an optical receiving device 100A employs a beam splitter 101A as the interferometer 101 shown in FIG. 1, and variable attenuators VOA1 and VOA2 as the optical intensity adjuster 102.

[0041] The beam splitter 101A has a splitting ratio of 50:50 and has input ports for receiving the signal light SL and the reference light LO, and output ports for outputting the output lights Lout1 and Lout2. The equally split output lights Lout1 and Lout2 are incident on the photodetectors PD1 and PD2 of the balanced detector 103 via variable attenuators VOA1 and VOA2, respectively.

[0042] The attenuation amounts of the variable attenuators VOA1 and VOA2 are controlled by the control unit 104. Therefore, the control unit 104 can adjust the balance of the light intensities incident on the photodetectors PD1 and PD2 by adjusting the attenuation amount of at least one of the variable attenuators VOA1 and VOA2.

[0043] During initial adjustment, initial reference light LO intensity-modulated at a predetermined frequency is input from an external light source. The external light source may include a laser light source and an intensity modulator, and may be detachably attached to one of the input ports of the beam splitter 101A during initial adjustment.

[0044] During operation / operation adjustment, reference light LO, which has a constant intensity or whose intensity changes at a predetermined frequency, is incident on the input port of the beam splitter 101A from an optical transmitter or an internal LO light source.

[0045] 4 employs a beam splitter 101A with a splitting ratio of 50:50 and variable attenuators VOA1 and VOA2, which allows for easy adjustment of the intensity balance of output light Lout1 and Lout2 entering balance detector 103 with a simple configuration.

[0046] The frequency analyzer 106 may be external or built into the optical receiving device 100A. The circuit configuration of the balance detector 103 and the balance adjustment operation by the control unit 104 are as described in the above embodiment. Specifically, the control unit 104 acquires balance adjustment reference values ​​VR1 and VR2 during initial adjustment and stores them in the reference value storage unit 105. Subsequently, during periodic operational adjustment, the control unit 104 controls the attenuation amounts of the variable attenuators VOA1 and VOA2 so that the detected output level values ​​V1 and V2 match the reference values ​​VR1 and VR2, respectively. This allows operational adjustment to be completed simply by referring to the reference values, thereby enabling quick and easy balance adjustment of the balance detector. Furthermore, by using the output level values ​​V1 and V2 of the photodetectors PD1 and PD2, respectively, of the balance detector 103, the state of the optical system and the balance photodetector 103 can be accurately reproduced, enabling more accurate and precise balance adjustment.

[0047] 5, an optical receiving device 100B employs a beam splitter 101A, variable attenuators VOA1 and VOA2, and further incorporates an LO light source 110, an intensity modulator 111, and a frequency analyzer 106A. The ON / OFF of the modulation operation of the intensity modulator 111 is controlled by a control unit 104, thereby enabling the generation of reference light LO during initial adjustment / operational adjustment.

[0048] More specifically, during initial adjustment, the modulation operation is turned ON, causing the intensity modulator 111 to intensity-modulate the continuous laser light from the LO light source 110 at a predetermined frequency F. This allows the intensity modulator 111 to supply initial reference light LO to the beam splitter 101A. During operational adjustment, the modulation operation of the intensity modulator 111 may be turned OFF, allowing the continuous laser light from the LO light source 110 to enter the beam splitter 101A as reference light LO of a constant intensity. Alternatively, during operational adjustment, the modulation operation of the intensity modulator 111 may be turned ON, and reference light LO during operational adjustment that has been intensity-modulated at a predetermined frequency F may be supplied to the beam splitter 101A.

[0049] The frequency analyzer 106a performs frequency analysis during initial adjustment and outputs the component of a predetermined frequency F to the control unit 104. Therefore, both the initial adjustment and operational adjustment illustrated in FIG. 3 can be performed within the optical receiving device 100B.

[0050] As described above, according to the second example illustrated in FIG. 5 , similar to the first example, operational adjustment can be completed simply by controlling the attenuation of the variable attenuators VOA1 and VOA2 so that the output level values ​​V1 and V2 detected during operational adjustment match the stored reference values ​​VR1 and VR2, respectively. This allows accurate balance adjustment of the balance detector to be performed quickly and easily. Furthermore, according to the second example, the initial adjustment, normal receiving operation, and operational adjustment can be performed within the optical receiving device 100B, enabling stable receiving operation over a long period of time.

[0051] 6, an optical receiving device 100C according to a third example receives a signal light SL from an optical transmitting device. Each symbol of the signal light SL is mapped to a different signal point on the I (in phase)-Q (quadrature phase) plane. Hereinafter, I component signals and related blocks will be distinguished by adding (I) to the reference symbols, and Q component signals and related blocks will be distinguished by adding (Q) to the reference symbols.

[0052] As illustrated in Fig. 6, the optical receiving device 100C employs a 90° hybrid circuit 101B as the interferometer 101 shown in Fig. 1. The 90° hybrid 101B receives the signal light SL at a first input port and receives the reference light LO from the LO light source 110 at a second input port via the intensity modulator 111. As described in the second example above, the intensity modulator 111 intensity-modulates the continuous laser light from the LO light source 110 at a predetermined frequency F during initial adjustment and outputs the initial reference light LO to the 90° hybrid circuit 101B. During operational adjustment, the intensity modulator 111 outputs the continuous laser light from the LO light source 110 to the 90° hybrid circuit 101B either as a constant-intensity reference light LO or as a reference light LO intensity-modulated at the predetermined frequency.

[0053] The 90° hybrid circuit 101B outputs I-component output light Lout1(I) and Lout2(I) from the first and second output ports. The output light Lout1(I) and Lout2(I) are incident on photodetectors PD1 and PD2 of the balanced detector 103(I) through variable attenuators VOA1(I) and VOA2(I) (hereinafter collectively referred to as VOA(I)), respectively. The balanced detector 103(I) outputs an I-component differential signal S Ｒ The variable attenuator VOA(I) and the balance detector 103(I) output the output level values ​​V1(I) and V2(I) (hereinafter collectively referred to as V(I)) of the photodetectors PD1 and PD2 to the processing unit 112. The variable attenuator VOA(I) and the balance detector 103(I) have the same functions as the optical intensity adjusting unit 102 and the balance detector 103 described in the embodiment of FIG.

[0054] The processing unit 112 is equipped with a control unit 104(I) having the same function as the control unit 104 in Fig. 1. That is, the control unit 104(I) acquires reference values ​​VR1(I) and VR2(I) (hereinafter collectively referred to as VR(I)) for balance adjustment during initial adjustment and stores them in the reference value storage unit 105A. Subsequently, during periodic operational adjustment, the control unit 104(I) controls the attenuation amount of the variable attenuator VOA(I) so that the detected level value V(I) matches the reference value VR(I).

[0055] Similarly, the Q-component output light Lout1(Q) and Lout2(Q) are output from the third and fourth output ports of the 90° hybrid circuit 101B, and are incident on the balance detector 103(Q) through variable attenuators VOA1(Q) and VOA2(Q) (hereinafter collectively referred to as VOA(Q)), respectively. The balance detector 103(Q) has the same configuration as the balance detector 103(I) described above. The balance detector 103(Q) outputs a Q-component differential signal S Ｒ (Q) is output to the processing unit 112. The Q component difference signal S Ｒ (Q) and output level values ​​V1(Q) and V2(Q) (hereinafter collectively referred to as V(Q)) to the processing unit 112.

[0056] The processing unit 112 implements a control unit 104(Q) having the same functions as the control unit 104 in Fig. 1. The control unit 104(Q) acquires reference values ​​VR1(Q) and VR2(Q) (hereinafter collectively referred to as VR(Q)) for balance adjustment during initial adjustment and stores them in a reference value storage unit 105A. Subsequently, during periodic operational adjustment, the control unit 104(Q) controls the attenuation amount of the variable attenuator VOA(Q) so that the detected level value V(Q) matches the reference value VR(Q).

[0057] In this way, the detection systems and control systems for the I and Q components have the same configurations, and each performs balance adjustment similar to that in the above-described Examples 1 and 2. The processing unit 112 is a processor such as a CPU, and as described in the embodiments, can implement the functions of the control units 104(I) and 104(Q) by executing a program.

[0058] According to the third example having the above-described configuration, the processing unit 112 controls the intensity modulator 111 and controls the variable attenuators of the I and Q systems, respectively, thereby making it possible to perform initial adjustments and operational adjustments of both the I and Q paths.

[0059] Furthermore, similar to the first example, operational adjustment can be completed simply by controlling the attenuation of the variable attenuators VOA1 and VOA2 so that the level values ​​V(I) and V(Q) detected during operational adjustment match the stored reference values ​​VR(I) and VR(Q), respectively. This allows accurate balance adjustment of the balance detector to be performed quickly and easily.

[0060] Furthermore, with a simple configuration, it is possible to easily adjust the intensity balance of the output lights Lout1(I) and Lout2(I), Lout1(Q) and Lout2(Q) that are incident on the balance detectors 103(I) and 103(Q), respectively.

[0061] Furthermore, according to the third example, similarly to the second example, the initial adjustment, normal receiving operation, and operational adjustment can be performed within the optical receiving device 100C, enabling stable receiving operation for a long period of time.

[0062] 3. Application Examples A QKD transmission system to which the optical receiving device according to this embodiment is applied will now be described.

[0063] A typical QKD transmission system transmits random numbers, which are the basis of encryption keys, using weak light at the single photon level as a medium, thereby enabling secure key sharing between locations. Using such weak light quantum mechanically guarantees that encryption keys will not be leaked, achieving high levels of confidentiality. For this reason, it is expected to be used in encrypted communications that handle highly confidential information.

[0064] As shown in FIG. 7, QKD consists of four steps: (1) weak light transmission, (2) basis matching, (3) error correction, and (4) privacy amplification.

[0065] (1) In weak optical transmission, the transmitter (ALICE) modulates an optical signal with a transmission random number, which is the key element (random number), and transmits the weak light, attenuated to an intensity of one photon or less, through a quantum channel. Weak light is likely to be lost or cause errors during transmission, and the information (raw key) received by the receiver (BOB) through the quantum channel becomes part of the transmitted key element.

[0066] There are two types of weak optical transmission: discrete QKD, which transmits single photons through a quantum channel, and continuous QKD, which modulates the quadrature amplitude of light and transmits it. Discrete QKD uses a photon detector to detect the presence or absence of photons and generates a cryptographic key from the detected data. Therefore, special devices such as a single-photon source and a single-photon detector are required.

[0067] In contrast, continuous QKD uses coherent detection to measure the state of the optical field and generates an encryption key from the measurement data (see, for example, JP-A-2019-522394). Coherent detection is a technology commonly used in long-distance, high-capacity optical communications, and can be realized using standard optical components. Therefore, continuous QKD is expected to be less expensive than discrete QKD. Another advantage is that it allows wavelength multiplexing with optical signals for general communications.

[0068] (2) In basis matching, the sender (ALICE) and the receiver (BOB) transmit basis information through a classical channel, select the received information with a matching basis, and discard the other information. In this case, the sender (ALICE) and the receiver (BOB) must agree on which bit transmitted over the quantum channel the transmitted basis information corresponds to (establishment of bit position synchronization).

[0069] (3) Error correction is performed on the selection information (shift key) obtained by basis matching between the sender (ALICE) and the receiver (BOB). For example, the receiver (BOB) calculates error correction information (syndrome) from the received information and transmits it as communication information to the sender (ALICE) through a classical channel. According to the syndrome, the sender (ALICE) and the receiver (BOB) share the error-corrected shift key.

[0070] (4) Finally, privacy amplification is performed between the sender (ALICE) and the receiver (BOB). Privacy amplification is a process in which a random number (parity calculation bits) is generated within the sender (ALICE), and then the sender (ALICE) and the receiver (BOB) use this to generate a new random number (final key) by eliminating any information that may have been leaked during key distribution. An eavesdropper cannot know the final key after privacy amplification unless they know all of the parity calculation bits.

[0071] 8, the continuous-quantum QKD system is composed of a transmitting device 200 and a receiving device 300. The transmitting device 200 and the receiving device 300 function as the ALICE and BOB of the QKD, respectively. The receiving device 300 has the balance adjustment function of the optical receiving device 100 / 100A described above.

[0072] The transmitting device 200 and the receiving device 300 are connected by a quantum channel CH1 and a classical channel CH2. Here, the quantum channel CH1 is a communication channel that transmits weak light (signal light) and reference light from the transmitting device 200 to the receiving device 300, and the weak light has a weak optical power of 1 photon / bit or less. Therefore, the quantum channel is a channel with relatively large loss and noise, with many errors, and is less reliable than a conventional channel.

[0073] The classical channel CH2 is a general communication channel, and transmits an optical signal 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 transmitting device 200 and the receiving device 300 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 wavelength-multiplexed onto a single optical transmission path.

[0074] The transmitting device 200 includes a quantum unit 201, a processing unit 202, a communication unit 103, and a program memory 204. The processing unit 202 realizes a key generation function by executing a program stored in the program memory 204, and can also transmit initial reference light, which will be described later, and reference light during operation or operational adjustment, to the receiving device 300. The communication unit 203 communicates with the receiving device 300 using ordinary light through a classical channel CH2.

[0075] The quantum unit 201 includes a laser light source 210, a non-polarizing beam splitter BSa, a phase modulator (PM) 211, a variable optical attenuator (VOA) 212, and a polarizing beam splitter PBSa. The laser light source 210 outputs an optical pulse train with a predetermined repetition frequency F, and the beam splitter BSa splits each optical pulse from the laser light source 210 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. As an example, the optical pulse has a wavelength of 1550 nm, a repetition frequency F=10 MHz, and a pulse width of 5 ns.

[0076] The reference light pulse LO branched to the reference light path is incident on the polarizing beam splitter PBSa as it is (or via a phase modulator, not shown). The reference light pulse LO is reflected by the polarizing beam splitter PBSa to become a reference light pulse of a predetermined linear polarization, and is transmitted to the receiving device 300 via the quantum channel CH1. The signal light pulse SL branched to the signal light path is converted to a weak signal light pulse SL via the phase modulator 211 and the variable attenuator 212. Ｑ The weak signal light pulse SL is incident on the polarizing beam splitter PBSa. Ｑ is transmitted through the polarizing beam splitter PBSa, becomes a linearly polarized signal light pulse that is orthogonal to the reference light pulse LO, and is transmitted to the receiving device 300 through the quantum channel CH1.

[0077] If the phase modulator 211 is a QPSK modulator, the processing unit 202 applies a phase modulation of 0, π / 2, π, or 3π / 2 to the signal light pulse in accordance with the key element K0 and the random number sequence of the base A. The signal light pulse thus phase-modulated is converted by the variable attenuator 212 into a weak light pulse SL of 1 photon / bit or less. Ｑ and is transmitted to the receiving device 300 via the polarizing beam splitter PBSa and the quantum channel CH1.

[0078] The receiving device 300 includes a quantum unit 301, a processing unit 302, a communication unit 303, and a program memory 304. The processing unit 302 realizes the key generation function and the balance adjustment function described above by executing a program stored in the program memory 304. The communication unit 303 communicates with the communication unit 203 of the transmitting device 200 using ordinary light through classical channel CH2.

[0079] The quantum unit 301 includes a polarizing beam splitter PBSb, a phase modulator (PM) 310, a non-polarizing beam splitter BSb, variable attenuators VOA1 and VOA2, photodetectors PD1 and PD2, and a differential calculation unit SUB. According to the first example shown in Fig. 4, the beam splitter 101A corresponds to the non-polarizing beam splitter BSb, and the balance detector 103 corresponds to the photodetectors PD1 and PD2 and the differential calculation unit SUB.

[0080] When an optical pulse train arrives from the transmitter 200 through the quantum channel CH1, each optical pulse is split into a received signal optical pulse SL by the polarizing beam splitter PBSb. ＱＲ and the received reference light pulse LO Ｒ The received signal light pulse SL is separated into ＱＲ is incident on one input port of the beam splitter BSb as it is (or via a phase modulator, not shown). Ｒ is phase-modulated by the phase modulator 310 and enters the other input port of the beam splitter BSb.

[0081] As described above, in the case of QPSK modulation, the phase modulator 310 is driven by the processing unit 302 in accordance with a random number basis B(x, p). The basis B(x, p) corresponds to the depth of phase modulation (0, π / 2), respectively. ＱＲ and the phase-modulated received reference optical pulse LO Ｒ The light beam enters the beam splitter BSb.

[0082] The beam splitter BSb has equal light transmittance and reflectance, and receives the signal light pulse SL ＱＲ and the received reference light pulse LO Ｒ The beam splitter BSb outputs the output lights Lout1 and Lout2 to the photodetectors PD1 and PD2 via the variable attenuators VOA1 and VOA2, respectively. ＱＲ and the received reference light pulse LO Ｒ The beams propagate through the same optical path length between the beam splitter BSa of the transmitting device 200 and the beam splitter BSb of the receiving device 300, and interfere at the beam splitter BSb. Therefore, the beam splitters BSa and BSb constitute one interferometer.

[0083] The signals detected by the photodetectors PD1 and PD2 are subjected to a difference calculation in a difference calculation unit SUB. The difference signal S Ｒ are converted into digital signals and output to the processing unit 302. The output level values ​​V1 and V2 of the photodetectors PD1 and PD2, respectively, are supplied to the control unit 104 of the processing unit 302.

[0084] The processing unit 302 executes a program stored in the program memory 304, and thereby implements functions equivalent to those of the control unit 104 in the first example shown in Fig. 4 in addition to the key generation function. The reference light LO may be supplied from the transmitting device 200, or may be supplied from a reference light source for initial adjustment provided in the receiving device 300. When the reference light LO is supplied from the transmitting device 200, the processing unit 302 performs the following control.

[0085] First, for initial adjustment, an external or built-in frequency analyzer (106 or 106a) is connected to the processing unit 302. During the initial adjustment, the processing unit 302 notifies the processing unit 202 of the transmitting device 200 of the start of the initial adjustment via the communication units 303 and 203. In response, the processing unit 202 drives the laser light source 210, which then outputs an optical pulse at a predetermined frequency F. When the phase modulator 211 and the VOA 212 are stopped and the intensity of the signal optical pulse SLQ is set to zero, only the reference optical pulse LO at the predetermined frequency F is transmitted to the receiving device 300.

[0086] The reference optical pulse LO arriving from the transmitter 200 Ｒ is input to the beam splitter BSb. The output lights Lout1 and Lout2 of the beam splitter BSb pass through VOA1 and VOA2 and are incident on the photodetectors PD1 and PD2, respectively.

[0087] The processing unit 302 analyzes the difference signal S obtained by the frequency analyzer 106. Ｒ The component of a predetermined frequency F is monitored, and a differential signal S Ｒ The intensity balance between VOA1 and VOA2 is controlled so that a predetermined frequency F component disappears from the output.

[0088] Differential signal S Ｒ When the predetermined frequency F component is cancelled from the output level V1 and V2 of the balance detector 103 at that time, the processing unit 302 stores the output level values ​​V1 and V2 of the balance detector 103 in the reference value storage unit 105 as reference values ​​VR1 and VR2.

[0089] Thereafter, at the time of periodic operational adjustment, the processing unit 302 notifies the processing unit 202 of the transmitting device 200 of the start of operational adjustment via the communication units 303 and 203. Upon receiving this notification, the processing unit 202 outputs the signal light pulse SL Ｑ and controls the laser light source 210 to transmit reference light of an optical pulse of a predetermined frequency F as reference light LO during operational adjustment to the receiving device 300. This reference light LO during operational adjustment may be reference light of a constant intensity.

[0090] The quantum unit 301 of the receiving device 300 receives the reference light LO during operational adjustment from the transmitting device 200, and the output lights Lout1 and Lout2 of the beam splitter BSb at that time are incident on the photodetectors PD1 and PD2 through the VOA1 and VOA2, respectively. The processing unit 302 inputs the output level values ​​V1 and V2 of the photodetectors PD1 and PD2, and controls the intensity balance of the VOA1 and VOA2 so that these values ​​become equal to the reference values ​​VR1 and VR2 stored in the reference value storage unit 105, respectively.

[0091] In this way, the reference values ​​VR1 and VR2 are stored during initial adjustment, and the attenuation amounts of the variable attenuators VOA1 and VOA2 are controlled during periodic operational adjustments so that the detected output level values ​​V1 and V2 match the reference values ​​VR1 and VR2, respectively. This allows operational adjustments to be completed simply by referring to the reference values, making it possible to quickly and easily perform balance adjustment of the balance detector. Furthermore, by using the output level values ​​V1 and V2 of the photodetectors PD1 and PD2, respectively, of the balance detector 103, the state of the optical system and the balance photodetector 103 can be accurately reproduced, enabling more accurate and precise balance adjustments.

[0092] 3.2) Application Example 2 As illustrated in FIG. 9, the continuous quantity QKD system is composed of a transmitting device 400 and a receiving device 500. The transmitting device 400 and the receiving device 500 function as ALICE and BOB, respectively, of QKD. The receiving device 500 has the balance adjustment function of the optical receiving device 100 / 100C described above. Note that in FIG. 9, the frequency analyzer 106a and the reference value storage unit 105a in the optical receiving device 100C (third example) illustrated in FIG. 6 are omitted. The communication unit that performs normal communication between the transmitting device 400 and the receiving device 500 is also omitted.

[0093] In FIG. 9 , a transmitting device 400 includes a quantum unit 401 and a processing unit 401. The quantum unit 401 includes a laser light source 410, an IQ modulator 411, and a variable optical attenuator (VOA) 412. Note that while FIG. 9 illustrates an IQ modulator 411, a polarization multiplexed IQ modulator may also be used. In a polarization multiplexed IQ modulator, an IQ modulator is provided for each of the X polarization and the Y polarization, and the IQ-modulated optical signals are polarization-multiplexed and transmitted as weak light. One IQ modulator can be configured by connecting two Mach-Zehnder modulators in parallel.

[0094] The processing unit 402 controls the IQ modulator 411 in accordance with the key element K0 and the base A as described in Application Example 1. As a result, the IQ modulator 411 IQ-modulates the laser light emitted from the laser light source 410. The VOA 412 attenuates the IQ-modulated laser light to a weak state where the optical power is 1 photon / bit or less. The weak light L obtained in this way Ｑ is transmitted to the receiving device 500 through the quantum channel CH1.

[0095] The receiving device 500 has a quantum unit 501 and a processing unit 502, and the quantum unit 501 has the configuration exemplified in Fig. 6. However, the LO light source 510 in Fig. 9 includes the LO light source 110 and the intensity modulator 111 shown in Fig. 6. Furthermore, the VOA(I) of the quantum unit 501 is made up of VOA1(I) and VOA2(I) in Fig. 6, and the VOA(Q) is made up of VOA1(Q) and VOA2(Q) in Fig. 6. In addition, in the above-mentioned polarization multiplexing IQ modulation method, the weak light L arriving through the quantum channel CH1 ＱＲ is separated into X and Y polarized waves, and a detection module consisting of a 90° hybrid 101B, a VOA (I), a VOA (Q), and balanced detectors 103(I) and 103(Q) is provided for each of the X and Y polarized waves.

[0096] In the initial adjustment, the processing unit 502 controls the LO light source 510, which outputs to the 90° hybrid circuit 101B an initial reference light LO that has been intensity-modulated at a predetermined frequency F. In the operational adjustment, the processing unit 502 controls the LO light source 510, which outputs to the 90° hybrid circuit 101B a reference light LO during the operational adjustment.

[0097] The processing unit 502, like the processing unit 112 in FIG. 6 , executes a program to achieve the same functions as the control units 104(I) and 104(Q). That is, the processing unit 502 adjusts the intensity balance by changing the attenuation amounts of the variable attenuators VOA(I) and VOA(Q), respectively. As a result, as already described, the reference values ​​VR(I) and VR(Q) are stored in the reference value storage unit 105A during initial adjustment. During operational adjustment, the intensity balance of the VOA(I) and VOA(Q) is adjusted so that the output level values ​​V(I) and V(Q) of the balance detectors 103(I) and 103(Q) in the quantum unit 501 match the reference values ​​VR(I) and VR(Q), respectively.

[0098] If the receiving device 500 has a built-in frequency analyzer 106A, as described in the third example above, initial adjustment, normal receiving operation, and operational adjustment can be performed within the receiving device 500, enabling stable receiving operation for a long period of time.

[0099] Although the present disclosure has been described above with reference to embodiments and examples, the present disclosure is not limited to the above-described embodiments and examples. The configuration and details of the present disclosure may be modified in various ways that are understandable to those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment and each example may be combined with other embodiments and examples as appropriate.

[0100] 4. Supplementary Notes Some or all of the above-described embodiments may be described as in the following supplementary notes, but are not limited to these. (Supplementary Note 1) An optical receiving device including an interferometer that receives signal light and reference light and causes them to interfere with each other, comprising: a light intensity adjuster that adjusts the intensity balance between first output light and second output light of the interferometer, a first photodetector and a second photodetector that detect the first output light and the second output light, respectively, whose intensities have been adjusted by the light intensity adjuster, a storage unit that stores a first output level value of the first photodetector and a second output level value of the second photodetector obtained upon completion of initial adjustment as a first reference value and a second reference value, respectively, and a control unit that controls the light intensity adjuster during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, whose intensity changes at a constant intensity or a predetermined frequency, is input to the interferometer. (Supplementary Note 2) The optical receiving device according to Supplementary Note 1, further comprising a first filter that averages the output of the first photodetector to generate the first output level value and a second filter that averages the output of the second photodetector to generate the second output level value. (Supplementary Note 3) The optical receiving device according to Supplementary Note 1 or 2, wherein the first reference value and the second reference value are acquired as the first output level value of the first photodetector and the second output level value of the second photodetector when the intensity balance is adjusted during initial adjustment while initial reference light whose intensity varies at the predetermined frequency is input to the interferometer and the frequency component of the predetermined frequency in the difference between the outputs of the first photodetector and the second photodetector is canceled. (Supplementary Note 4) The optical receiving device according to any one of Supplementary Notes 1 to 3, wherein the interferometer equally divides the signal light and the reference light and combines them. (Supplementary Note 5) The optical receiving device according to Supplementary Note 4, wherein the interferometer is a beam splitter with a splitting ratio of 50:50. (Appendix 6) An optical receiving device described in any one of Appendices 1 to 5, wherein the light intensity adjusting unit is provided between the interferometer and the first photodetector and the second photodetector, and comprises a variable attenuator that attenuates the intensity of at least one of the first output light and the second output light.(Supplementary Note 7) The optical receiving device according to any one of Supplements 1 to 6, wherein the control unit monitors the component of the predetermined frequency in the difference between the outputs of the first photodetector and the second photodetector during the initial adjustment, and controls the intensity balance so that the component of the predetermined frequency is cancelled. (Supplementary Note 8) The optical receiving device according to any one of Supplements 1 to 7, wherein the reference light during the operational adjustment is supplied from an optical transmitting device together with the signal light. (Supplementary Note 9) The optical receiving device according to any one of Supplements 1 to 7, further comprising an intensity modulator that intensity-modulates continuous light of a predetermined intensity, wherein the intensity modulator outputs the initial reference light by intensity modulation during the initial adjustment in accordance with control of the control unit, and outputs reference light during operational adjustment whose intensity is constant or changes at the predetermined frequency during the operational adjustment. (Supplementary Note 10) The optical receiving device according to Supplementary Note 9, further comprising a local light source that outputs reference light during operational adjustment. (Supplementary Note 11) A continuous-quantum quantum key distribution system, in which the optical receiving device according to any one of Supplements 1 to 8 is connected to an optical transmitting device through an optical transmission line, the optical receiving device receives the initial reference light from the optical transmitting device during the initial adjustment, and receives the reference light for the operation adjustment from the optical transmitting device during the operation adjustment. (Supplementary Note 12) A continuous-quantum quantum key distribution system, in which the optical receiving device according to Supplementary Note 10 is connected to an optical transmitting device through an optical transmission line, and the optical transmitting device stops transmitting the signal light during the operation adjustment. (Supplementary Note 13) A balance adjustment method for an optical receiving device including an interferometer that inputs signal light and reference light to cause interference between them, a first photodetector and a second photodetector that detect first output light and second output light, respectively, of the interferometer, wherein the processing unit: adjusts the intensity balance between the first output light and the second output light, stores a first output level value of the first photodetector and a second output level value of the second photodetector obtained when an initial adjustment is completed in a memory unit as a first reference value and a second reference value, respectively, and controls the optical intensity adjustment unit during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or an intensity that changes at a predetermined frequency, is input to the interferometer.(Supplementary Note 14) The balance adjustment method according to Supplementary Note 13, wherein the first output level value is generated by averaging the output of the first photodetector using a first filter, and the second output level value is generated by averaging the output of the second photodetector using a second filter. (Supplementary Note 15) The balance adjustment method according to Supplementary Note 13 or 14, wherein, during initial adjustment, the intensity balance is adjusted with initial reference light whose intensity varies at the predetermined frequency being input to the interferometer, a frequency component of the predetermined frequency in a difference between the outputs of the first photodetector and the second photodetector is canceled, and the first output level value of the first photodetector and the second output level value of the second photodetector when the frequency component is canceled are stored as the first reference value and the second reference value. (Supplementary Note 16) The balance adjustment method according to any one of Supplementary Notes 13 to 15, wherein the processing unit monitors the component of the predetermined frequency in a difference between the outputs of the first photodetector and the second photodetector during the initial adjustment, and controls the intensity balance so that the component of the predetermined frequency is canceled. (Supplementary Note 17) The balance adjustment method according to any one of Supplementary Notes 13 to 16, wherein the processing unit controls a light intensity adjustment unit provided between the interferometer and the first photodetector and the second photodetector, and attenuates the intensity of at least one of the first output light and the second output light. (Supplementary Note 18) The balance adjustment method according to any one of Supplementary Notes 13 to 17, wherein the optical receiving device further has an intensity modulator that intensity-modulates continuous light of a predetermined intensity, and the processing unit controls the intensity modulator to generate the initial reference light during the initial adjustment and to generate the operational adjustment reference light during the operational adjustment.(Supplementary Note 19) A balance adjustment device for an optical receiving device including an interferometer that inputs signal light and reference light to cause interference between them, and a first photodetector and a second photodetector that detect first output light and second output light of the interferometer and output first detection signals and second detection signals, respectively, the balance adjustment device comprising: a light intensity adjuster that adjusts the intensity of at least one of the first output light and the second output light of the interferometer; a memory that stores a first output level value of the first photodetector and a second output level value of the second photodetector obtained at the time of completion of initial adjustment as a first reference value and a second reference value, respectively; and a control unit that controls the light intensity adjuster during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, the intensity of which changes at a constant intensity or a predetermined frequency, is input to the interferometer. (Supplementary Note 20) The balance adjustment device according to Supplementary Note 19, further comprising a first filter that averages outputs of the first photodetector to generate the first output level value and a second filter that averages outputs of the second photodetector to generate the second output level value. (Supplementary Note 21) The balance adjustment device according to Supplementary Note 19 or 20, wherein the intensity balance is adjusted during initial adjustment with initial reference light whose intensity varies at the predetermined frequency being input to the interferometer, and the first reference value and the second reference value are acquired as the first output level value of the first photodetector and the second output level value of the second photodetector when a frequency component of the predetermined frequency in a difference between the outputs of the first photodetector and the second photodetector is canceled. (Supplementary Note 22) The balance adjustment device according to any one of Supplementary Notes 19 to 21, further comprising an intensity modulator that intensity-modulates continuous light of a predetermined intensity, wherein the intensity modulator outputs the initial reference light by intensity modulation during the initial adjustment and outputs the operational adjustment reference light during the operational adjustment under control of the control unit.(Supplementary Note 23) A program that causes a computer to function as a balance adjustment device for an optical receiving device including an interferometer that inputs signal light and reference light to cause interference between them, and a first photodetector and a second photodetector that detect first output light and second output light of the interferometer, the program implementing on the computer a function of an intensity adjustment unit adjusting the intensity balance between the first output light and the second output light, a function of a processing unit storing in a memory unit a first output level value of the first photodetector and a second output level value of the second photodetector that are acquired when initial adjustment is completed, as a first reference value and a second reference value, respectively, and a function of controlling the optical intensity adjustment unit during operational adjustment so that the first output level value of the first photodetector and the second output level value of the second photodetector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or an intensity that changes at a predetermined frequency, is input to the interferometer. (Supplementary Note 24) The program according to Supplementary Note 23, wherein the processing unit, during initial adjustment, adjusts the intensity balance while initial reference light whose intensity varies at the predetermined frequency is input to the interferometer, cancels a frequency component of the predetermined frequency in a difference between the outputs of the first photodetector and the second photodetector, and stores a first output level value of the first photodetector and a second output level value of the second photodetector when the frequency component is canceled as the first reference value and the second reference value. (Supplementary Note 25) The program according to Supplementary Note 23 or 24, wherein the processing unit, during the initial adjustment, monitors the component of the predetermined frequency in a difference between the outputs of the first photodetector and the second photodetector, and controls the intensity balance so that the component of the predetermined frequency is canceled. (Supplementary Note 26) The program according to any one of Supplementary Notes 23 to 25, wherein the processing unit controls a light intensity adjuster provided between the interferometer and the first photodetector and the second photodetector, to attenuate the intensity of at least one of the first output light and the second output light.(Appendix 27) The program described in any one of Appendices 23 to 26, wherein the optical receiving device further has an intensity modulator that intensity-modulates continuous light of a predetermined intensity, and the processing unit controls the intensity modulator to generate the initial reference light during the initial adjustment and to generate the reference light during the operational adjustment.

[0101] The present invention is applicable to an optical receiving device including a balance detector that detects interference light between signal light and reference light.

[0102] 100, 100A, 100B, 100C Optical receiving device 101 Interferometer 102 Light intensity adjusting section 103 Balance detector PD1, PD2 Photodetector SUB Difference calculation section V1, V2 Output level value 104 Control section 104.1 Balance adjusting section 105, 105a Reference value storage section 106, 106a Frequency analyzer

Claims

1. An optical receiving device including an interferometer which inputs signal light and reference light to cause interference therebetween, comprising: a light intensity adjustment unit which adjusts the intensity balance between the first output light and the second output light of the interferometer; a first optical detector and a second optical detector which detect the first output light and the second output light respectively whose intensities have been adjusted by the light intensity adjustment unit; a memory unit which stores the first output level value of the first optical detector and the second output level value of the second optical detector obtained at the completion of initial adjustment as a first reference value and a second reference value respectively; and a control unit which controls the light intensity adjustment unit during operational adjustment so that the first output level value of the first optical detector and the second output level value of the second optical detector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or changes in intensity at a predetermined frequency, is input to the interferometer.

2. An optical receiving device as described in claim 1, further comprising a first filter that averages the output of the first optical detector to generate the first output level value, and a second filter that averages the output of the second optical detector to generate the second output level value.

3. An optical receiving device as described in claim 1 or 2, wherein the first reference value and the second reference value are obtained as the first output level value of the first optical detector and the second output level value of the second optical detector when the frequency component of the specified frequency in the difference between the outputs of the first optical detector and the second optical detector is cancelled by adjusting the intensity balance during initial adjustment while an initial reference light whose intensity changes at the specified frequency is input to the interferometer.

4. An optical receiving device according to any one of claims 1 to 3, wherein said interferometer equally divides said signal light and said reference light and combines them.

5. An optical receiver as claimed in claim 4, wherein said interferometer is a beam splitter with a split ratio of 50:

50.

6. An optical receiving device as claimed in any one of claims 1 to 5, wherein the optical intensity adjusting section is provided between the interferometer and the first and second optical detectors, and comprises a variable attenuator for attenuating the intensity of at least one of the first output light and the second output light.

7. An optical receiving device as claimed in any one of claims 1 to 6, wherein the control unit monitors the specified frequency component of the difference between the outputs of the first photodetector and the second photodetector during the initial adjustment, and controls the intensity balance so that the specified frequency component is cancelled.

8. An optical receiving device according to any one of claims 1 to 7, wherein the reference light during the operational adjustment is supplied from an optical transmitting device together with the signal light.

9. An optical receiving device as claimed in any one of claims 1 to 7, further comprising an intensity modulator which intensity-modulates continuous light of a predetermined intensity, said intensity modulator outputting said initial reference light by intensity modulation during said initial adjustment in accordance with the control of said control unit, and outputting said reference light during operational adjustment, the intensity of which changes at said constant intensity or at said predetermined frequency.

10. The optical receiving device according to claim 9, further comprising a local light source that outputs reference light during said operational adjustment.

11. A continuous quantum key distribution system in which an optical receiving device according to any one of claims 1 to 8 is connected to an optical transmitting device through an optical transmission path, and the optical receiving device receives the initial reference light from the optical transmitting device during the initial adjustment, and receives the reference light during the operational adjustment from the optical transmitting device during the operational adjustment.

12. A continuous quantum key distribution system in which the optical receiving device according to claim 10 is connected to an optical transmitting device through an optical transmission line, and the optical transmitting device stops transmitting the signal light during the operational adjustment.

13. A balance adjustment method for an optical receiving device including an interferometer that inputs signal light and reference light to cause interference therebetween, a first optical detector and a second optical detector that detect the first output light and the second output light, respectively, of the interferometer, wherein the processing unit adjusts the intensity balance between the first output light and the second output light, stores the first output level value of the first optical detector and the second output level value of the second optical detector obtained at the time of completion of initial adjustment in a memory unit as a first reference value and a second reference value, respectively, and controls the optical intensity adjustment unit during operational adjustment so that the first output level value of the first optical detector and the second output level value of the second optical detector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or changes in intensity at a predetermined frequency, is input to the interferometer.

14. A balance adjustment method as described in claim 13, further comprising averaging the output of the first photodetector with a first filter to generate the first output level value, and averaging the output of the second photodetector with a second filter to generate the second output level value.

15. A balance adjustment method as described in claim 13 or 14, comprising the steps of: adjusting the intensity balance during initial adjustment while an initial reference light whose intensity changes at the specified frequency is input to the interferometer; canceling a frequency component of the specified frequency in the difference between the outputs of the first photodetector and the second photodetector; and storing a first output level value of the first photodetector and a second output level value of the second photodetector when the frequency component has been canceled as the first reference value and the second reference value.

16. A balance adjustment device in an optical receiving device including an interferometer that inputs signal light and reference light to cause interference, and a first optical detector and a second optical detector that detect the first output light and the second output light of the interferometer and output a first detection signal and a second detection signal, respectively, the balance adjustment device including: a light intensity adjustment unit that adjusts the intensity of at least one of the first output light and the second output light of the interferometer; a memory unit that stores the first output level value of the first optical detector and the second output level value of the second optical detector obtained at the time of completion of initial adjustment as a first reference value and a second reference value, respectively; and a control unit that controls the light intensity adjustment unit during operational adjustment so that the first output level value of the first optical detector and the second output level value of the second optical detector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or changes in intensity at a predetermined frequency, is input to the interferometer.

17. A program that causes a computer to function as a balance adjustment device for an optical receiving device including an interferometer that inputs signal light and reference light to interfere with each other, and a first optical detector and a second optical detector that respectively detect the first output light and the second output light of the interferometer, wherein the program implements the following functions on the computer: an intensity adjustment unit adjusts the intensity balance of the first output light and the second output light; a processing unit stores in a memory unit the first output level value of the first optical detector and the second output level value of the second optical detector obtained at the time of completion of initial adjustment as first reference values ​​and second reference values, respectively; and a processing unit controls the optical intensity adjustment unit during operational adjustment so that the first output level value of the first optical detector and the second output level value of the second optical detector become the first reference value and the second reference value, respectively, when reference light during operational adjustment, which has a constant intensity or changes in intensity at a predetermined frequency, is input to the interferometer.

18. The program described in claim 17, wherein during initial adjustment, the processing unit adjusts the intensity balance while an initial reference light whose intensity changes at the predetermined frequency is input to the interferometer, cancels a frequency component of the predetermined frequency in the difference between the outputs of the first photodetector and the second photodetector, and stores the first output level value of the first photodetector and the second output level value of the second photodetector when the frequency component is canceled as the first reference value and the second reference value.

19. The program described in claim 18, wherein the optical receiving device further has an intensity modulator that intensity-modulates continuous light of a predetermined intensity, and the processing unit controls the intensity modulator to generate the initial reference light during the initial adjustment and to generate the reference light for the operational adjustment during the operational adjustment.

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