WO2010092776A1 - Optical communication system, optical communication method, and optical communication device - Google Patents

Abstract

Disclosed is an optical communication system capable of applying a wavelength division multiplex technique to an optical interference system with any wavelength arrangement, independent of manufacturing error in the interferometer and without complicating the device configuration. The optical communication system, which includes optical interference systems that use the phase information of optical signals, comprises: an optical multiplexing means (101) that multiplexes optical signals of multiple wavelengths (λ1-λn) and outputs one multiplexed optical signal; an optical interference means (102) that temporally divides the multiplexed optical signal; a demultiplexing means (104) that demultiplexes the light output from the optical interference means and outputs demultiplexed optical signals of multiple wavelengths; multiple phase modulation means (PM1-PMn, 105) that phase modulate, wavelength-by-wavelength, optical signals of multiple wavelengths or the demultiplexed optical signals of multiple wavelengths output from the demultiplexing means; and a phase modulation control means (107) that individually controls the amount of modulation by the multiple phase modulation means according to the wavelength.

Description

Optical communication system, optical communication method, and optical communication apparatus

The present invention relates to an optical communication system that performs communication using phase information of an optical signal, and more particularly, to an optical communication system and method for transmitting signals of a plurality of wavelengths, an optical communication apparatus, and a phase modulation control method thereof.

In the current situation where the rapid growth of Internet traffic continues, improving the transmission bandwidth is one of the most important issues. This is no exception even in the backbone optical network, and various research institutions have increased the signal speed per carrier in order to expand the transmission capacity using the existing infrastructure without adding transmission lines and repeaters. In actual commercial transmission systems, the signal speed continues to increase.

In general optical communication technology, binary amplitude modulation (ASK: Amplitude Shift x Keying) is mainly used. Therefore, in order to improve the signal speed per carrier, an approach by shortening the time slot per bit is used. Was the mainstream. However, since the signal speed per carrier wave exceeds 10 Gb / s, the difficulty of speeding up with only ASK is becoming more prominent. One of the causes is waveform deterioration due to wavelength dispersion peculiar to the optical transmission line. The chromatic dispersion means a phenomenon in which the propagation delay time generated in the transmission line differs depending on the signal light wavelength. Since the signal light spectrum has a specific wavelength range, short wavelength components and long wavelength components in the same signal light accumulate different chromatic dispersion values during transmission, and after transmission, propagation delay differences are caused by this accumulated dispersion. That is, waveform distortion occurs. When compared with an ASK signal, the signal light spectrum is proportional to the modulation speed, so that the waveform distortion due to chromatic dispersion increases in proportion to the increase in the signal speed. On the other hand, since one time slot is shortened in proportion to an increase in signal speed, even when the same amount of waveform distortion (difference in propagation delay) is applied, this effect is more greatly affected as the signal is faster. As a result, the transmission characteristics deteriorate in proportion to the square of the signal speed.

As a result of the difficulty in speeding up only with ASK, as another approach for speeding up, a technique for improving the transmission band by multi-leveling the state represented by one time slot attracts attention. (See Non-Patent Documents 1, 2, and 3). Non-Patent Document 1 improves the transmission band by quaternary modulation of the phase of light, and Non-Patent Documents 2 and 3 describe one symbol by a method (APSK: Amplitude Phase Shift Keying) that modulates both the intensity and phase of light. It represents a larger number of digital bits (with one carrier and one time slot). However, phase information cannot be extracted simply by directly detecting the optical signal with a photoelectric conversion module such as a photodiode (PD) .Therefore, when modulating the phase of the optical signal, the receiving side interferes with the reference light and the signal light. It is necessary to adopt a detection method such as reading the light intensity after interference.

FIG. 11 is a schematic diagram showing an example of a differential phase shift (DPS: Differential Phase Shift) decoding method in which information is placed on a phase difference between adjacent bits. A 1-bit delay interferometer is provided on the receiving side to extract the phase information carried by interfering with the front and rear 1-bit optical signals. If the phase difference between the front and rear is 0, the optical signal after interference is output to port 0, and if the phase difference is π, the optical signal after interference is output to port 1. In this example, a Mach-Zehnder interferometer is used as a 1-bit delay interferometer, but a Michelson interferometer can also be used. The 1-bit delay interferometer is provided with a thermoelectric cooler (TEC: Thermo-Electric Cooler) as temperature adjusting means, as will be described later.

Quantum key distribution technology (QKD: Quantum Key Distribution) can also be cited as a communication system that uses phase modulation of optical signals. In QKD, a photon is generally used as a communication medium, and information is carried on the quantum state for transmission. An eavesdropper on the transmission path steals information by tapping the photons being transmitted, etc., but returns the photons that have been observed once to Heisenberg's uncertainty principle completely to the quantum state before observation. As a result, the statistical value of the received data detected by the legitimate receiver changes. By detecting this change, the receiver can detect an eavesdropper on the transmission path.

In the quantum cryptography key distribution method, an optical interferometer is organized by a sender and a receiver (hereinafter referred to as Alice and Bob), and phase modulation is randomly performed on each photon by Alice and Bob. Depending on the depth difference, an output of 0 or 1 can be obtained on the Bob side. Then, by collating a part of the conditions when the output data is measured between Alice and Bob, the same bit string can finally be shared between Alice and Bob. Hereinafter, this will be described in more detail with reference to FIG.

FIG. 12 is a system configuration diagram showing an example of an optical interferometer used for quantum key distribution. Here, it is assumed that Alice 21 which is a transmission side communication device is connected to Bob 23 which is a reception side communication device through an optical transmission path 22. This example is a one-way type optical interferometer, and 1- bit delay interferometers 213 and 232 are provided on both Alice 21 and Bob 23. The reason for this is that if information is placed between adjacent bits as in the above-described differential phase shift DPS, the eavesdropper (hereinafter referred to as “Eve”) prepares a local oscillator and there is a concern that the security of encryption key distribution may deteriorate. This is for the purpose. The 1- bit delay interferometers 213 and 232 are provided with temperature controllers 212 and 233, respectively.

In Alice 21, the optical pulse train (ΔT interval) generated by the pulse light source 211 is converted into a double pulse train having a delay amount Δt using a 1-bit delay interferometer 213, and a phase modulator 214 is used between each pulse pair. Modulate to give a phase difference of φA. In the most typical quantum key distribution algorithm called BB84 protocol described later, φA takes four values of 0, π, π / 2, and 3π / 2, and these four values are randomly assigned to each pulse pair. For this purpose, Alice 21 is provided with two systems of random number sources 215 and 216 and a digital-to-analog converter (DAC) 217 that adds these random numbers.

In Bob 23, the optical signal sent from Alice 21 is modulated by the phase modulator 231 so as to give a phase difference of φB between the pulse pairs again. Then, the pulse pair is caused to interfere using the 1-bit delay interferometer 232 having the delay amount Δt, and the interference result is read by the photon detectors 234 and 235. The phase modulation on the Bob side is performed with binary values of 0 and π / 2, and a random number source 236 for this purpose is held. At present, the repetition rate (1 / ΔT) of the system is increased to about 1 GHz due to the limitation of the operation speed of the photon detector, but the delay amount of the 1-bit delay interferometer corresponding to this operation speed is In order to obtain stable characteristics as long as ˜500 ps (˜10 cm), it is necessary to control the temperature of the interferometer in units of 0.01K.

FIG. 13 is an explanatory diagram conceptually showing the BB84 protocol disclosed in Non-Patent Document 4. Here, it is assumed that Alice 21 on the transmission side and Bob 23 on the reception side are connected by an optical transmission line 22 and perform quantum cryptography communication. In this method, four kinds of quantum states are used, Alice 21 has two random number sources, one random number 1 represents 0 or 1 encryption key data, and the other random number 2 encodes information of random number 1 To decide.

Specifically, in a quantum encryption key distribution method that performs four-state coding using a phase difference between two coherent pulses, a set of phase 0 is an encryption key “0” and phase π is an encryption key “1”. A coding set (hereinafter referred to as “X basis”) and a coding set (hereinafter referred to as “Y basis”) in which phase π / 2 represents the encryption key “0” and phase 3π / 2 represents the encryption key “1”. And 2) are selected with a random number 2. That is, four modulations of 0, π / 2, π, and 3π / 2 are randomly applied to one photon and transmitted to Bob23.

On the other hand, Bob 23 has a random number source (random number 3) corresponding to the base and decodes photons sent from Alice 21. When the value of the random number 3 is “0”, the phase 0 (X basis) is modulated on the photon, and when the value is “1”, the phase π / 2 (Y basis) is modulated. Here, a random number obtained as an output of the optical interferometer is set as a random number 4.

If the basis of modulation performed by both Alice 21 and Bob 23 is the same (random number 2 = random number 3), Bob 23 can correctly detect the value of random number 1 (random number 1 = random number 4), and if different ( For random number 2 ≠ random number 3), Bob 23 randomly obtains a value of 0/1 as random number 4 regardless of the value of random number 1.

Since both random numbers 1/2/3 are random numbers that change every bit, the probability that the bases match and the probability that they do not match are both 50%. However, since the bits whose bases do not match are deleted by the subsequent base collation (Basis Reconciliation), Alice 21 and Bob 23 can share the 0/1 bit string corresponding to the random number 1. In the quantum key distribution, the random number data before deleting the base mismatch bit is referred to as a “raw key”, and the random data after the base mismatch bit is deleted as a “selection key”.

FIG. 14 is a block diagram schematically showing the unidirectional QKD system disclosed in Non-Patent Document 5. The method shown in FIG. 14 uses a binary phase state and a binary time state.

First, using Alice 31, a 4 × 2 interferometer with 4 inputs and 2 outputs is used to input light pulses from the light sources LD1 to LD4 to each of the 4 input ports. When an optical pulse is input from the light source LD1, since the optical pulse passes only through the long path of the interferometer, only one pulse delayed in time is sent to the transmission path, and when an optical pulse is input from the light source LD4, Since the optical pulse passes only through the short path of the interferometer, only one pulse advanced in time is transmitted to the transmission path. When optical pulses are input from the light sources LD2 and LD3, an X basis or a Y basis can be generated by the phase difference between both paths of the 2 × 4 interferometer. In Bob 32, each base is decoded using a 2 × 4 interferometer having 2 inputs and 4 outputs, and detected by four photon detectors PD1 to PD4.

Hereinafter, as in the case of using the light sources LD1 and LD4, a coding set in which a photon exists on only one side of a double pulse is referred to as “Z basis”. Note that the sum of the light intensity of the double pulses when the X base or the Y base is selected needs to be equal to that when the Z base is selected, so the intensity of each pulse is halved.

That is, the Alice side selects which light source LD1 to LD4 generates the light pulse, and the Bob side simultaneously determines the bit and the base by detecting the photon with any of the photon detectors PD1 to PD4. can do. In this method, since no modulator is required on the Bob side, it is possible to increase the speed of encryption key generation.

FIG. 15 is a schematic block diagram of a quantum key distribution system using the DPS method disclosed in Non-Patent Document 6. Alice 41 of this system is provided with a pulse source 411, a phase modulator 412 and a random number source 413, but without an interferometer. In Bob 43, a phase modulator 431, a 1-bit delay interferometer 432, a temperature regulator 433, and photon detectors 434 and 435 are provided. The phase modulator 412 of Alice 41 randomly applies phase modulation (binary phase modulation of 0 and π) to the phase difference between adjacent bits.

Incidentally, in order to increase the speed of optical communication, in addition to the improvement of the repetition frequency as described above, a method of transmitting a plurality of signals in parallel can be considered. In particular, by using wavelength division multiplexing (WDM) technology, it is possible to improve the key generation speed in the QKD system with a single optical transmission fiber between Alice and Bob.

As a communication system that performs communication using a plurality of wavelengths, for example, Patent Document 1 discloses a wavelength multiplexed optical CDMA system. Here, a method is described in which a plurality of phase modulators are used to perform different phase modulation for each wavelength. However, it is assumed that a plurality of DPSK demodulators are used, and this DPSK demodulator has a 1-bit delay. It functions as an interferometer (see the paragraphs 0035 and 0048 of the specification of Patent Document 1 and FIG. 3).

Special table 2008-529422

However, when the WDM technique is applied to an optical interference system using phase modulation such as the quantum encryption key distribution technique, it is possible to provide a 1-bit delay interferometer for each wavelength as described in Patent Document 1, for example. As illustrated in FIG. 16, the apparatus configuration is complicated and disadvantageous in terms of cost.

FIG. 16 is a block diagram of a system in which WDM technology is simply applied to an optical interference system. The quantum cryptography system illustrated in FIG. 12 has a configuration in which a plurality of wavelengths are bundled using a WDM filter. In this configuration, the Alice 51 is connected to the Bob 53 through the optical transmission line fiber 52. In each of the Alice 51 and Bob 53, it is necessary to provide 1- bit delay interferometers 511 and 531 and their temperature regulators 512 and 532 for each wavelength. There is.

As described above, even if the repetition frequency (1 / ΔT) of the system is increased to about 1 GHz, the delay amount of the 1-bit delay interferometer corresponding to this operation speed is as high as ˜500 ps (˜10 cm). In order to obtain long and stable characteristics, it is necessary to control the temperature of the interferometer in units of 0.01K. Such a 1-bit delay interferometer and a temperature regulator for performing temperature control in units of 0.01K are very expensive. Therefore, as shown in FIG. 16, the configuration in which the 1-bit delay interferometer and the temperature controller are provided for each wavelength not only complicates the system configuration but also significantly increases the system cost.

FIG. 17A is a block diagram showing an experimental system for verifying the operating characteristics of the delay interferometer, FIG. 17B is a graph showing the relationship between temperature and light intensity, and FIG. It is a graph which shows the relationship between a wavelength and light intensity.

In FIG. 17A, the output of a continuous light source (CW light source) 601 is passed through a Mach-Zehnder interferometer 602, and the light intensity on one side of the output port is measured by an optical power meter 604. When the wavelength of the CW light source 601 is fixed at λ ₀ and the control temperature of the interferometer 602 is changed, the light intensity measured by the optical power meter 604 draws a sine wave as shown in FIG. . This is because when the temperature of the delay path of the interferometer 602 changes, the expansion and contraction of the delay path occurs, and the optical path difference between the optical signals passing through both paths of the interferometer changes, which is strengthened when combined. (The optical path difference is an integral multiple of the wavelength) or weakened (the optical path difference is "integer + 1/2" multiple of the wavelength). When the relationship shown in FIG. 17B is further viewed in the wavelength direction, the monitor light intensity changes with a certain slope as shown in FIG. 17C. This is because the delay amount at which the optical path difference of the interferometer 602 becomes an integral multiple of the wavelength differs depending on the wavelength.

Furthermore, the manufacturing error of the 1-bit delay interferometer must be taken into consideration. For example, consider two 1-bit delay interferometers with a delay amount (800 ps ± manufacturing error). A general specification value of the manufacturing error of the delay amount is about 800 ps ± 2%. As an example, consider a case where there is an error of 0.5% as shown in FIG. In order to simplify the calculation, the delay amount 800 ps is set to 16.00 cm, and the delay amount 804 ps when there is a manufacturing error of 0.5% is set to 16.08 cm.

In this case, as shown in FIG. 16, in an optical communication system having 1- bit delay interferometers 511 and 531 at both transmission and reception ends, the difference in delay amount between 1- bit delay interferometers 511 and 531 is 0 or π (+ wave integer). Times). In order to satisfy this condition at the specific wavelength λ ₀ , it is relatively easy to control the temperature of the delay amount of one interferometer.

However, if the control temperature is set such that the difference in delay amount between the 1- bit delay interferometers 511 and 531 is 0 (plus an integral multiple of the wavelength) at λ ₀ , the wavelength at which the difference in delay amount is “0 or π”. Is periodic as shown in FIG. It is possible to apply the WDM technique to the interference system if the wavelength arrangement is performed at this period, but this period changes due to a manufacturing error of the delay amount. Therefore, it is necessary to change the wavelength arrangement depending on the characteristics of the interferometer used, which is not suitable for mass production of the system. In addition, since WDM filters that perform wavelength multiplexing / demultiplexing are mostly manufactured according to the ITU grid (50 GHz interval or 100 GHz interval), it is also a problem that these commercially available products cannot be used.

Accordingly, the present invention provides an optical communication system, an optical communication system that can apply a wavelength division multiplexing technique to an optical interference system in an arbitrary wavelength arrangement without depending on manufacturing errors of the interferometer and without complicating the apparatus configuration. An object is to provide a communication method and an optical communication device.

An optical communication apparatus according to the present invention is an optical communication apparatus in an optical communication system including an optical interference system using phase information of an optical signal, and combines optical signals of a plurality of wavelengths and outputs one combined optical signal. An optical multiplexing unit; an optical interference unit that temporally separates the combined optical signal; a demultiplexing unit that demultiplexes output light from the optical interference unit and outputs the demultiplexed optical signals of the plurality of wavelengths; A plurality of phase modulation means for phase-modulating each of the wavelength-dependent optical signals on the input side of the wave means or the wavelength-demultiplexed optical signals on the output side of the light demultiplexing means for each wavelength; Phase modulation control means for individually controlling the amount of modulation by the phase modulation means according to the wavelength.

An optical communication method according to the present invention is an optical communication method in an optical communication system including an optical interference system using phase information of an optical signal, in which an optical multiplexing unit combines optical signals of a plurality of wavelengths to form one combined optical signal. The optical interferometer temporally separates the combined optical signal, and the optical demultiplexing means demultiplexes the output light from the optical interferometer to generate the demultiplexed optical signals of the plurality of wavelengths, A method of performing phase modulation on a wavelength-demultiplexed optical signal or an optical signal of a plurality of wavelengths, for each wavelength, with a modulation amount individually controlled according to the wavelength.

An optical communication system according to the present invention is an optical communication system including an optical interference system having a transmission-side communication device and a reception-side communication device and using phase information of an optical signal. A plurality of phase modulation means for phase-modulating each wavelength of the optical signal, and an optical multiplexing means for combining the phase-modulated optical signals of the plurality of wavelengths and transmitting one combined optical signal to the receiving-side communication device; The receiving-side communication device includes: a receiving-side optical interference unit that temporally separates a received optical signal; and the output light from the receiving-side optical interference unit is demultiplexed to demultiplex the plurality of wavelengths. Receiving-side optical demultiplexing means for outputting a wave signal, and the transmitting-side communication device further includes phase modulation control means for individually controlling the modulation amounts by the plurality of phase modulation means according to wavelengths. It is characterized by that.

A program according to the present invention is a program for causing a program control processor in an optical communication apparatus of an optical communication system including an optical interference system using phase information of an optical signal to function as a phase modulation control apparatus, wherein the optical multiplexing means has a plurality of wavelengths. The optical signal is multiplexed to generate one combined optical signal, the optical interferometer temporally separates the combined optical signal, and the optical demultiplexing means demultiplexes the output light from the optical interferometer to generate the combined optical signal. Generating a plurality of wavelength-demultiplexed optical signals, and individually controlling the modulation amounts of a plurality of phase modulation means for phase-modulating the plurality of wavelength-demultiplexed optical signals or the plurality of wavelength optical signals for each wavelength, according to the wavelength; The program control processor is made to function as described above.

As described above, according to the present invention, the wavelength division multiplexing technique can be applied to the optical interference system with any wavelength arrangement without depending on the manufacturing error of the optical interferometer and without complicating the apparatus configuration.

1 is a block diagram schematically showing the configuration of an optical communication system according to a first embodiment of the present invention. It is a block diagram which shows the schematic structure of the optical communication system by 2nd Embodiment of this invention. 1 is a block diagram schematically showing the configuration of an optical communication system according to a first embodiment of the present invention. It is a graph which shows the wavelength-phase difference for demonstrating the difference of the delay amount of two Mach-Zehnder interferometers used in 1st Example. (A) is a graph showing the waveform of the drive signal of the phase modulator when the phase control of the first embodiment is not performed, and (b) is the drive signal of the phase modulator when the phase control of the first embodiment is executed. It is a graph which shows a waveform. It is a block diagram which shows the structure of the optical communication system by 2nd Example of this invention roughly. (A) is a graph showing the waveform of the phase modulator drive signal based on random numbers in the second embodiment, and (b) is the waveform of the phase modulator drive signal when the offset phase control in the second embodiment is executed. There is a graph to show. It is a flowchart which shows the procedure which determines the amplitude of the drive signal of the phase modulator in 2nd Example. It is a block diagram which shows schematic structure of the transmitter of the optical communication system by 3rd Example of this invention. It is a block diagram which shows the structure of the optical communication system by 4th Example of this invention schematically. It is a schematic diagram which shows an example of the differential phase shift DPS decoding system which puts information on the phase difference between adjacent bits. It is a system block diagram for demonstrating an example of the optical interferometer used for quantum encryption key distribution, and its function. It is explanatory drawing which shows notionally BB84 protocol currently disclosed by the nonpatent literature 4. It is a block diagram which shows roughly the unidirectional QKD system disclosed by the nonpatent literature 5. FIG. FIG. 10 is a schematic block diagram of a quantum key distribution system using a DPS method disclosed in Non-Patent Document 6. 1 is a block diagram of a system in which WDM technology is simply applied to an optical interference system. (A) is a block diagram showing an experimental system for verifying the operating characteristics of a delay interferometer, (b) is a graph showing the relationship between temperature and light intensity, (c) is a graph of temperature, wavelength and light intensity. It is a graph which shows a relationship. (A) is a schematic configuration diagram of two 1-bit delay interferometers having a manufacturing error of the delay amount, and (b) is a graph showing a wavelength-phase difference for explaining a difference in delay amount of the 1-bit delay interferometer It is.

1. First Embodiment As shown in FIG. 1, an optical communication system according to a first embodiment of the present invention includes a wavelength division multiplexing (WDM) filter 101 for multiplexing / demultiplexing optical signals having a plurality of wavelengths λ ₁ to λ _n. , 104 and 106. A delay interferometer 102 is provided in a section in which optical signals between the WDM filter 101 and the WDM filter 104 are multiplexed, and a temperature regulator 103 that controls the temperature of the delay amount is provided. As the delay interferometer 102, for example, a 2-input 2-output Mach-Zehnder asymmetrical interferometer for time-separating an input optical signal is used. For example, a thermoelectric cooler (TEC) is used for the temperature regulator 103.

A phase modulator group 105 is provided in a section where the optical signals between the WDM filters 104 and 106 are demultiplexed for each wavelength, and the phase modulator group 105 corresponds to a plurality of optical signals having wavelengths λ ₁ to λ _n. Phase modulators PM ₁ to PM _n that respectively perform phase modulation. In these phase modulators PM ₁ to PM _n , a phase offset θ (λ) depending on the wavelength is set by the phase modulation control unit 107. For example, phase modulation is executed by adding a phase offset θ (λ) for compensating the wavelength-dependent delay amount of the delay interferometer 102 to the phase modulation amount φ according to given data, as will be described later. . A phase offset theta (lambda) is the offset value which is previously determined depending on the wavelength lambda, a phase offset value θ _{1 ~} θ _n is a phase modulator PM 1 _~ PM respectively corresponding to wavelengths λ _{1 ~} λ _n here _n , respectively. Note that the phase modulation φ and the phase offset θ can be executed by one phase modulator, or can be executed by separate phase modulators.

In this way, by compensating the wavelength dependence of the delay amount of the delay interferometer 102 by phase modulation, it becomes possible to set an arbitrary wavelength arrangement, and the delay interferometer 102 and its temperature adjuster 103 can be combined with light of a plurality of wavelengths. The signal can be shared.

As is clear from the configuration shown in FIG. 1, only one delay interferometer 102 (and its temperature regulator 103) needs to be provided for optical signals having a plurality of wavelengths λ ₁ to λ _n . As is clear from comparison with the configuration shown in FIG. 16, according to this embodiment, the device configuration can be greatly simplified even if wavelength division multiplexing is applied to the optical interference system. Furthermore, the phase offset values θ ₁ to θ _n are set in the phase modulators PM ₁ to PM _n , respectively, so that the manufacturing error of the delay amount of the delay interferometer 102 can be compensated.

According to the first embodiment, a transmission device or a reception device of an optical communication system can be configured. For example, a transmitter is configured by connecting a plurality of light sources that emit light of wavelengths λ ₁ to λ _n to local ports for each wavelength of the WDM filter 101 and connecting optical transmission fibers to multiple ports of the WDM filter 106. _If a photodetector that detects light of wavelengths λ ₁ to λ _n is connected to the local port of the WDM filter 101, a receiving device is configured.

Thus, according to this embodiment, when applying the WDM technique to an optical interference system using phase modulation such as a quantum cryptography key distribution technique, the delay amount of a 1-bit delay interferometer such as a Mach-Zehnder interferometer is reduced. By compensating the wavelength dependence by phase modulation, the interferometer and its temperature regulator can be shared by a plurality of wavelength signals, so that a system can be constructed with a simple configuration. As a result, the encryption key generation speed per transmission line can be improved in the quantum encryption key distribution system.

2. Second Embodiment As shown in FIG. 2, the optical communication system according to the second embodiment of the present invention also multiplexes / demultiplexes optical signals having a plurality of wavelengths λ ₁ to λ _n , as in the first embodiment. The WDM filters 101 and 104 are provided, a delay interferometer 102 is provided between the WDM filters 101 and 104, and a temperature regulator 103 for controlling the temperature of the delay amount is provided. For example, a two-input two-output Mach-Zehnder asymmetrical interferometer and a thermoelectric cooler (TEC) are used for the delay interferometer 102 and the temperature controller 103, respectively. However, in the optical communication system according to the first embodiment, one of the inputs and outputs is demultiplexed into optical signals of a plurality of wavelengths λ ₁ to λ _n and the other is a multiplexed signal, whereas the second embodiment The optical communication system is different in that it has an input / output demultiplexed into optical signals having a plurality of wavelengths λ ₁ to λ _n .

The port of each wavelength of the WDM filter 101 is connected phase modulator group 110, a phase modulator group 110 phase modulators PM ₁ ~ performing each phase modulation to light signals of a plurality of wavelengths λ _{1 ~} λ _n It consists of PM _n . These phase modulators PM ₁ to PM _n are subjected to phase modulation control by the phase modulation control unit 111 as in the first embodiment. Therefore, as in the first embodiment, the delay interferometer 102 and its temperature adjuster 103 can be shared by optical signals of a plurality of wavelengths by compensating the wavelength dependence of the delay amount of the delay interferometer 102 by phase modulation. Even if wavelength division multiplexing is applied to the optical interference system, the apparatus configuration can be greatly simplified. Furthermore, the phase offset values θ ₁ to θ _n are set in the phase modulators PM ₁ to PM _n , respectively, so that the manufacturing error of the delay amount of the delay interferometer 102 can be compensated.

According to the second embodiment, an optical communication system including a transmission device and a reception device can be configured. For example, the WDM filter 101, the phase modulator group 110, and the phase modulation control unit 111 are configured on the transmission side / reception side, and the WDM filter 104, the delay interferometer 102, and the temperature regulator 103 are configured on the reception side / transmission side. It is possible. In this case, the optical transmission line between the WDM filter 101 and the delay interferometer 102 is an optical fiber that connects the transmission device and the reception device.

Similarly to the first embodiment, according to this embodiment, when applying the WDM technique to an optical interference system using phase modulation such as a quantum cryptography key distribution technique, a 1-bit delay such as a Mach-Zehnder interferometer or the like is used. By compensating the wavelength dependence of the delay amount of the interferometer by phase modulation, the interferometer and its temperature controller can be shared by a plurality of wavelength signals, so that a system can be constructed with a simple configuration. As a result, the encryption key generation speed per transmission line can be improved in the quantum encryption key distribution system.

3. First Embodiment Next, an optical communication system according to a first embodiment of the present invention will be described in detail with reference to FIGS. In the present embodiment, the multi-level modulation level is adjusted, and an offset corresponding to the wavelength is superimposed on the phase difference applied between the double photon pulses. Hereinafter, although the number of wavelengths to be multiplexed is described as four as an example, it is needless to say that the number of wavelengths is not limited to this.

3.1) Configuration In FIG. 3, Alice 11 serving as a transmission device is connected to Bob 13 through an optical transmission line 12.

Alice 11 is a laser diode (LD) 1101-1104 that generates optical pulses of _four wavelengths λ ₁ to λ ₄ respectively, and a two-input two-output Mach-Zehnder asymmetric that generates a two-unit optical pulse by time-separating the optical pulses. An interferometer (hereinafter referred to as a Mach-Zehnder interferometer) 1105, a temperature controller 1106 that controls the temperature of the delay amount of the Mach-Zehnder interferometer 1105, and a two-time optical pulse that is time-separated into two predetermined positions. A phase modulator 1107-1110 for adding a phase difference, a clock source 1111 for driving the laser diode 1101-1104, a random number source 1112-1119, an offset storage unit 1120 for holding an offset amount for each wavelength, and a random number source 1112-1119 And a digital-analog converter DAC (Digital-t) that generates a modulation signal based on the offset amount of the offset storage unit 1120 o-Analog Converter) 1121-1124 and a WDM filter 1125-1127. Each of the DACs 1121-1124 receives two random numbers from two random number sources and an offset value from the offset storage unit 1120, and outputs a modulation signal for each wavelength to the phase modulator 1107-1110, respectively. Modulation φ + θ (λ) is performed.

A Mach-Zehnder interferometer 1105, which is a delay interferometer, is provided in a section in which optical signals between the WDM filters 1125 and 1126 are multiplexed, and a temperature regulator 1106 that controls the temperature of the delay amount is provided. . Phase modulators 1107-1110 that perform phase modulation on the optical signals of wavelengths λ ₁ to λ ₄ are connected in a section where the optical signals between the WDM filters 1126 and 1127 are demultiplexed for each wavelength. . Further, the random number source 1112-1119, the offset storage unit 1120, and the DAC 1121-1124 constitute the phase modulation control unit 107 in FIG. Alice 11 may be provided with an optical attenuator for attenuating each phase-modulated duplex optical pulse to a single photon level or less.

Bob 13 includes a phase modulator 1301-1304 that gives a phase difference again to the double photon pulse sent from Alice 11, a Mach-Zehnder interferometer 1305 that combines the double photon pulses, and a delay of the Mach-Zehnder interferometer 1305. It has a temperature regulator 1306 that controls the amount of temperature, two photon detectors 1307 and 1311, 1308 and 1312, 1309 and 1313, 1310 and 1314 for each wavelength, and a WDM filter 1315-1318.

A Mach-Zehnder interferometer 1305, which is a delay interferometer, is provided in a section where optical signals between the WDM filters 1316 and 1317/1318 are multiplexed, and a temperature regulator 1306 for controlling the temperature of the delay amount is provided. ing. A phase modulator 1301-1304 that performs phase modulation on the double photon pulses of wavelengths λ ₁ to λ ₄ is connected to a section where the optical signals between the WDM filters 1315 and 1316 are demultiplexed for each wavelength. Yes. Here, for the photon detectors 1307-1310 and 1311-1314, an avalanche photodiode (APD) is used in the gate mode here.

3.2) Operation In Alice 11, optical pulses of wavelengths λ ₁ to λ ₄ synchronized with a 625 MHz clock source 1111 are generated by the LD 1101-1104. Here, it is assumed that the wavelengths λ ₁ = 1550.92 nm, λ ₂ = 1551.72 nm, λ ₃ = 1552.52 nm, and λ ₄ = 1553.33 nm. Optical pulses having wavelengths λ ₁ to λ ₄ are multiplexed by a WDM filter 1125, converted into a double optical pulse time-separated by a Mach-Zehnder interferometer 1105, and then demultiplexed by a WDM filter 1126 again. Then, by modulating one optical phase of the duplex optical pulses of wavelengths λ ₁ to λ ₄ by the phase modulator 1107-1110, the relative phase difference φ _A of the duplex optical pulses of each wavelength is randomly generated. Then, the signal is multiplexed again by the WDM filter 1127 and sent to the transmission line 12. Performing described in detail later with respect to the phase modulation amount phi _A here.

The duplex optical pulse passes through the transmission line 12 to reach Bob 13 and is demultiplexed by the WDM filter 1315, and then one optical phase of the duplex optical pulses of wavelengths λ ₁ to λ ₄ is converted by the phase modulator 1301-1304. Each is modulated. The phase modulation here is performed randomly so that the relative phase difference (φ _B ) of the double light pulses is 0, π / 2. The duplex optical pulses are multiplexed by a WDM filter 1316 and then multiplexed by using a Mach-Zehnder interferometer 1305, so that a photon detector 1307-1310 or a photon detector is applied according to the modulation phase applied to Alice 11 and Bob 13, respectively. It is detected at 1311-1314.

Hereinafter, in order to explain specifically, the example shown in FIG. 18 shall be followed. That is, the Mach- Zehnder interferometers 1105 and 1305 are assumed to have delay amounts of 800.00 ps and 804.00 ps, respectively, as illustrated in FIG. FIG. 4 is a graph obtained by enlarging the graph of FIG. 18B in the wavelength band of λ ₁ to λ ₄ .

As shown in FIG. 4, since the difference in delay between the two Mach- Zehnder interferometers 1105 and 1305 is 4 ps, 1.65π for the wavelength λ ₁ , 1.11π for the wavelength λ ₂ , so that the phase difference of 0.05π occurs 0.58Pai, with respect to the wavelength lambda ₄ for wavelength lambda _3. This phase difference θ (λ) is measured in advance and stored as an offset in the offset amount storage unit 1120. A method for measuring the offset phase difference θ (λ) will be described later. Then, DAC corresponding to each wavelength, and inputs the two random numbers and the offset, and outputs a phase-modulated signal obtained by adding the offset theta _A phase modulation φ by two random numbers to the phase modulator.

With the configuration shown in FIG. 16, the phase difference φ obtained by referring to two random numbers for each of the four signals of wavelengths λ ₁ to λ ₄ is 0, π / 2, π, 3π / 2. While four values are given at random, in this embodiment, an offset is added for each wavelength. That is, for the wavelength _{λ 1, 0 + 1.65π, π} / 2 + 1.65π, π + 1.65π, the 4 phases of the 3π / 2 + 1.65π, to the wavelength _{λ 2, 0 + 1.11π, π} / 2 + 1. 11π, π + 1.11π, the 4 phases of the 3π / 2 + 1.11π, to the wavelength _{λ 3, 0 + 0.58π, π} / 2 + 0.58π, π + 0.58π, the 4 phases of the 3π / 2 + 0.58π, wavelength lambda _{For 4} , _four phases of 0 + 0.05π, π / 2 + 0.05π, π + 0.05π, and 3π / 2 + 0.05π are randomly given.

Next, the phase modulation control method in the present embodiment will be described in more detail with reference to FIG. 5, taking the wavelength λ ₁ as an example.

First, as shown in FIG. 16, in the configuration in which the double light pulses (pre-pulse and post-pulse) generated by the Mach-Zehnder interferometer provided for each wavelength are input to the phase modulator, FIG. As shown in the figure, random numbers RND1 = RND2 = 0 are both 0 and phase difference φ _A = 0, random numbers RND1 = 0, RND2 = 1, phase difference φ _A = π / 2, random numbers RND1 = 1, and RND2 = 0 are phase differences. When φ _A = π, random numbers RND 1 = 1, and RND 2 = 1, phase modulation with a phase difference φ _A = 3π / 2 is applied to the duplex light pulse. However, the above values are examples, and any number of equivalent modulation methods can be adopted, but all are quaternary modulation.

In contrast, according to this embodiment, as shown in FIG. 5 (b), the offset of 1.65π is applied to the wavelength lambda _1. Therefore, although added always phase modulation of 0 before pulse, the rear pulse, a random number RND1 = RND2 = 0 the phase difference φ _A = 1.65π, random number RND1 = 0, RND2 = 1 the phase difference phi _A = Phase modulation of π / 2 + 1.65π, random number RND1 = 1, RND2 = 0, phase difference φ _A = π + 1.65π, random number RND1 = 1, RND2 = 1, phase difference φ _A = 3π / 2 + 1.65π is a double beam Added to the pulse. As a result, the signal for driving the phase modulator 1107 is a quinary signal.

As described above, if the phase difference φ _A applied to Alice 11 with respect to the double light pulse, the phase difference φ _B applied to Bob 13, and the phase difference Δ between the interferometers 1105 and 1305, ΔΦ = φ _A + φ _B + Δ has four values of 0, π / 2, π, and 3π / 2 regardless of the wavelength. Therefore, in the quantum cryptography key distribution system shown in FIGS. 12 and 13, when ΔΦ is 0, a photon pulse of the corresponding wavelength of the photon detector 1307-1310 is detected, and the bit of the encryption key is It becomes “0”. When ΔΦ is π, the photon pulse of the corresponding wavelength of the photon detectors 1311-1314 is detected, and the corresponding bit of the encryption key is “1”. When ΔΦ is π / 2 or 3π / 2, photon pulses are detected by either photon detector with a probability of 1/2, but these detection bits are discarded in the base matching process as described above. The

It should be noted that such phase modulation control can be realized by executing a computer program on a program control processor such as a CPU.

3.3) Effect According to the present embodiment, when the WDM technique is applied to an optical interference system using phase modulation such as a quantum key distribution technique, a system can be constructed with an inexpensive configuration. This is because the interferometer and its temperature controller can be shared by a plurality of wavelength signals by compensating the wavelength dependence of the delay amount of a 1-bit delay interferometer such as a Mach-Zehnder interferometer by phase modulation. In this embodiment, the wavelength arrangement on the ITU grid is exemplified, but the wavelength arrangement is not limited to this, and the number of wavelengths is not limited to four wavelengths.

Further, in this embodiment, an example is shown in which the wavelength dependency of the delay amount of the interferometer 1105 is compensated by the phase modulator 1107-1110 on the transmission side, but this may be performed on the reception side. After adjusting the temperature regulators 1106 and 1306 so that the difference in delay between the two Mach- Zehnder interferometers 1105 and 1305 is also 0 or π at any of the wavelengths λ ₁ to λ ₄ , the remaining wavelengths An offset may be determined for.

4). Second Embodiment Next, an optical communication system according to a second embodiment of the present invention will be described in detail with reference to FIGS. Also in this embodiment, as in the first embodiment, the multi-level modulation level is adjusted to superimpose an offset corresponding to the wavelength on the phase difference applied between the double photon pulses. The difference is that a phase modulator for offset is connected in series. Hereinafter, although the number of wavelengths to be multiplexed is described as four as an example, it is needless to say that the number of wavelengths is not limited to this.

4.1) Configuration In FIG. 6, Alice 41 as a transmission device is connected to Bob 43 through an optical transmission line 42.

Alice 41 is a laser diode (LD) 4101-4104 that generates optical pulses of _four wavelengths λ ₁ to λ ₄ respectively, and a two-input two-output Mach-Zehnder asymmetric that generates a double optical pulse by time-separating the optical pulses. Interferometer (hereinafter referred to as “Mach-Zehnder interferometer”) 4105, temperature controller 4106 that controls the temperature of the delay amount of Mach-Zehnder interferometer 4105, and two-time optical pulses that are time-separated in accordance with random numbers. The phase modulator 4107-4110 for adding a predetermined phase difference and the phase modulator 4111-4114 for offset, the clock source 4115 for driving the laser diode 4101-4104, the random number source 4116-4123, and the offset amount for each wavelength Are stored in an offset storage unit 4124 and a random number source 4116-4123 to generate a modulation signal. It has a converter DAC (Digital-to-Analog Converter) 4125-4128 and a WDM filter 4129-4131. Each of the DACs 4125-4128 inputs the two random numbers from the two random number sources, and outputs the modulation signal for each wavelength to the phase modulator 4107-4110 to execute the above-described phase modulation φ, and stores it in the offset storage unit 4124. The stored offset amount is output to the phase modulator 4111-4114 for each wavelength, and the offset θ (λ) is executed. Therefore, phase modulation φ + θ (λ) is executed for each wavelength by the phase modulator 4107-4110 and the phase modulator 4111-4114.

A Mach-Zehnder interferometer 4105, which is a delay interferometer, is provided in a section in which optical signals between the WDM filters 4129 and 4130 are multiplexed, and a temperature regulator 4106 for controlling the temperature of the delay amount is provided. . In a section where the optical signals between the WDM filters 4130 and 4131 are demultiplexed for each wavelength, a phase modulator 4107-4110 and a phase modulator 4111 that respectively perform phase modulation on the optical signals of wavelengths λ ₁ to λ _4. -4114 is connected. The random number source 4116-4123, the offset storage unit 4124, and the DAC 4125-4128 constitute the phase modulation control unit 107 in FIG. The Alice 41 may be provided with an optical attenuator that attenuates each phase-modulated duplex optical pulse to a single photon level or less.

Bob 43 includes a phase modulator 4301-4304 that gives a phase difference again to the double photon pulse sent from Alice 41, a Mach-Zehnder interferometer 4305 that multiplexes the double photon pulses, and a delay of the Mach-Zehnder interferometer 4305. It has a temperature regulator 4306 for temperature control of the quantity, two photon detectors 4307-4310 and 4311-4314 for each wavelength, and a WDM filter 4315-4318.

A Mach-Zehnder interferometer 4305, which is a delay interferometer, is provided in a section in which optical signals between the WDM filters 4316 and 4317/4318 are multiplexed, and a temperature regulator 4306 for controlling the temperature of the delay amount is provided. ing. A phase modulator 4301-4304 that performs phase modulation on the double photon pulses of wavelengths λ ₁ to λ ₄ is connected to a section where the optical signal between the WDM filters 4315 and 4316 is demultiplexed for each wavelength. Yes. Note that, for the photon detectors 4307-4310 and 4311-4314, an avalanche photodiode (APD) is used in the gate mode here.

4.2) Operation Since the operations of the blocks other than the phase modulators 4107-4110 and 4111-4114 and the DAC 4125-4128 are the same as those in the first embodiment, the description thereof is omitted in this embodiment.

As in the first embodiment, the Mach- Zehnder interferometers 4105 and 4305 have delay amounts of 800.00 ps and 804.00 ps, respectively, as illustrated in FIG.

Since the difference in the delay amounts of the two Mach- Zehnder interferometer 4105 and 4305 are 4ps, 1.65π for wavelength λ _1, 1.11π for wavelength lambda _2, to the wavelength lambda ₃ 0.58Pai, so that the phase difference of 0.05π occurs with respect to the wavelength lambda _4. This phase difference θ (λ) is measured in advance and stored in the offset amount storage unit 4124 as an offset. A method for measuring the offset phase difference θ (λ) will be described later. The DAC corresponding to each wavelength inputs two random numbers and outputs a phase modulation signal that causes phase modulation φ to the phase modulator. For each of the four signals of wavelengths λ ₁ to λ ₄ , four values of 0, π / 2, π, and 3π / 2 are randomly given as the phase difference φ obtained by referring to two random numbers.

The duplex optical pulse generated by the Mach-Zehnder interferometer 4105 is phase-modulated by the phase modulator 4107-4110, and then subjected to offset phase modulation by the phase modulator 4111-4114. Specifically, 1.65 × V [pi for wavelength λ _1, 1.11 × Vπ for wavelength λ _2, 0.58 × Vπ for wavelength lambda _3, to the wavelength lambda ₄ Each of the phase modulators 4111 to 4114 is driven with an amplitude of 0.05 × Vπ. Here, Vπ represents a voltage applied to the phase modulator to apply phase modulation of π.

Next, the phase modulation control method in the present embodiment will be described in more detail with reference to FIG. 7, taking the wavelength λ ₁ as an example.

First, duplex optical pulses (pre-pulse and post-pulse) generated by the Mach-Zehnder interferometer 4105 are demultiplexed by the WDM filter 4130 and input to the phase modulators 4107 to 4110, respectively. As shown in FIG. 7A, when the random number RND1 = RND2 = 0, both are 0 and the phase difference φ _A = 0, the random number RND1 = 0, and when the random number RND2 = 1, the phase difference φ _A = π / 2, and the random number RND1 = 1. When RND2 = 0, phase modulation of phase difference φ _A = π, random number RND1 = 1, RND2 = 1, phase difference φ _A = 3π / 2 is applied to the duplex light pulse. However, the above values are examples, and any number of equivalent modulation methods can be adopted, but all are quaternary modulation.

Such phase-modulated duplicate optical pulses of a wavelength lambda _1, as shown in FIG. 7 (b), further offset 1.65π is added. That is, regardless of the values of the random numbers RND1 and RND2, the phase modulation with the offset θ ₁ = 1.65π is added to the double light pulse. Therefore, as a result, phase modulation φ + θ (λ) is performed on the double light pulse generated by the Mach-Zehnder interferometer 4105.

It should be noted that such phase modulation control can be realized by executing a computer program on a program control processor such as a CPU.

4.3) Effects According to the present embodiment, the following effects can be obtained in addition to the effects of the first embodiment. That is, the means for generating the phase modulator drive signal is simplified. The reason is that in the first embodiment, it was necessary to perform five-level phase modulation, but in this embodiment, the phase modulator is divided into a modulator that performs four-level modulation and a modulator that provides a phase offset. This is because the multi-value number can be suppressed. It is possible to further divide the phase modulator that performs the quaternary modulation into two to make all the drive signals binary signals. Of course, the system cost increases as the number of modulators increases.

In the present embodiment, an example is shown in which the wavelength dependency of the delay amount of the interferometer 4105 is compensated by the phase modulator 4111-4114 on the transmission side, but this may be performed on the reception side. After adjusting the temperature regulators 4106 and 4306 so that the difference in delay amount between the two Mach- Zehnder interferometers 4105 and 4305 is also 0 or π at any of the wavelengths λ ₁ to λ ₄ , the remaining wavelengths An offset may be determined for.

5). Procedure for Determining Phase Offset Amount Next, a method for determining the phase offset amounts θ ₁ to θ ₄ for each wavelength stored in the offset storage unit 1120 in the first embodiment and the offset storage unit 4124 in the second embodiment is illustrated. The configuration of the second embodiment shown in FIG. However, since the phase modulation depth of the phase modulator is determined by the voltage value of the drive signal, the amplitude of the drive signal of the phase modulator corresponding to each phase offset amount is actually determined.

In FIG. 8, first, when the system is introduced, the drive signal amplitudes of all the phase modulators 4107-4114, 4301-4304 are set to 0, and the initial values of the _four variables C ₁ -C ₄ are set to 0 (step S10). Subsequently, the drive signal amplitude of the phase modulator corresponding to each of the wavelengths λ ₁ to λ ₄ is determined. Since the basic procedure is the same for all wavelengths, the processing step S11 for the wavelength λ ₁ is used here. -S16 will be described.

First, with respect to the wavelength lambda ₁ of the signal, the drive signal amplitude was a is increased by a predetermined step [Delta] V (step S11) of the phase modulator 4111, measures the number of photon counts per unit time of the photon detectors 4307 and 4311 (Step S12). Subsequently, the count ratio C ₁ ′ is calculated using the respective count values C ₁₀ and C ₁₁ in the photon detectors 4307 and 4311 (step S13), and the calculated C ₁ ′ is compared with the current variable C _1. (Step S14).

If the calculated C ₁ ′ is equal to or greater than the current variable C ₁ (step S14: No), the calculated C ₁ ′ is newly set as the variable C ₁ (step S15), and the process returns to step S11. By repeating steps S11 to S15 in this way, the drive signal amplitude for obtaining C ₁ ′ greater than or equal to the previously calculated C ₁ ′ is updated in ΔV steps.

When the calculated C ₁ ′ becomes smaller than the current variable C ₁ (step S14: Yes), the peak has passed, so the drive signal amplitude of the phase modulator 4111 is reduced by ΔV and fixed, and the wavelength λ ₁ Is determined (step S16).

In this way, the drive signal amplitude of the phase modulator 4111 is scanned, and the drive signal amplitude is set so that the ratio of the photon detection count values of the corresponding two photon detectors 4307 and 4311 is maximized. At this time, it is possible to determine the phase offset amount with higher accuracy by reducing ΔV as much as possible. In the case of the other wavelengths λ ₂ to λ ₄ , the respective phase offset amounts can be determined by the same procedure in steps S21 to S26, steps S31 to S36, and steps S41 to S46.

6). Third Embodiment The present invention can also be applied to a quantum key distribution method (see FIG. 14) using a “Z base” as described in Non-Patent Document 5.

As shown in FIG. 9, in the transmission apparatus (Alice) in the optical communication system according to the third embodiment of the present invention, a four-input two-output 2 × 4 interferometer 5122 is used and multiple light source units are provided for each of four input ports. Is connected. The first multiple light source unit includes LDs 5101 to 5104 and WDM filters 5117 corresponding to the wavelengths λ ₁ to λ ₄ respectively, and the second multiple light source unit includes LDs 5105 to 5108 and WDM filters corresponding to the wavelengths λ ₁ to λ ₄ respectively. 5119, third multiplexing light source unit LD5109-5112 and the WDM filter 5120 correspond to the wavelengths λ ₁ ~ λ _4, LD5113-5116 and WDM filter fourth multiplexing light source unit correspond respectively to the wavelength lambda ₁ ~ lambda ₄ 5118.

Furthermore, phase modulators 5124-5127 are inserted in the demultiplexed sections between the WDM filters 5123 and 5128 corresponding to the respective wavelengths. The phase modulator 5124-5127 can compensate the wavelength dependence of the delay amount of the 2 × 4 interferometer 5122 by phase modulation as described above according to the offset amount from the offset storage unit 5129.

When an optical pulse is input to the 2 × 4 interferometer 5122 from the first multiple light source unit, the optical pulse passes through only the long path of the interferometer, and therefore, a phase modulator having a wavelength corresponding to only one pulse delayed in time. And sent to the transmission line. When an optical pulse is input from the fourth multiple light source unit, the optical pulse passes through only the short path of the interferometer, so that only one pulse advanced in time is sent to the transmission path. When an optical pulse is input from the second multiple light source unit or the third multiple light source unit, an X base or a Y base can be generated based on the phase difference between both paths of the 2 × 4 interferometer.

Thus, after the pulses of each wavelength are time-separated by the 2 × 4 interferometer 5122, the phase modulator 5124-5127 adds an offset corresponding to the wavelength according to the offset amount from the offset storage unit 5129. At this time, even if phase modulation in units of wavelengths is applied, there is no effect on the Z-base signal, so that the phase modulator 5124-5127 acts only on the X-base or Y-base signal.

7). Fourth Embodiment The present invention can also be applied to a quantum key distribution system (see FIG. 15) using the DPS method disclosed in Non-Patent Document 6.

As shown in FIG. 10, the transmission-side Alice of the optical communication system according to the fourth embodiment of the present invention includes a laser diode (LD) 6101-6104 that generates optical pulses of _four wavelengths λ ₁ to λ ₄ , and a wavelength A phase modulator 6105-6108 that modulates the phase of each of the optical pulses of λ ₁ to λ ₄ , a clock source 6109 that drives the laser diode 6101-6104, a random number source 6111-6114, and an offset amount for each wavelength An offset storage unit 6115, a random number source 6111-6114, a DAC 6116-6119 that generates a modulation signal based on the offset amount of the offset storage unit 6115, and a WDM filter 6120. Each of DACs 6116-6119 receives a random number from a corresponding random number source and an offset value from offset storage unit 6115, and outputs a modulated signal for each wavelength to phase modulator 6105-6108. The optical pulses having the wavelengths λ ₁ to λ ₄ thus phase-modulated are combined by the WDM filter 6120 and reach the receiving side Bob through the optical transmission path 6121.

The receiving-side Bob has a Mach-Zehnder interferometer 6122 that generates time-separated optical pulses by separating the optical pulses, a temperature adjuster 6123 that controls the temperature of the delay amount of the Mach-Zehnder interferometer 6122, and a wavelength for each wavelength. Each has two photon detectors 6125-6128 and 6129-6132 and WDM filters 6124 and 6133.

In this embodiment, the temperature adjustment is performed to control the temperature of the Mach-Zehnder interferometer 6122 and its delay amount in the section in which the optical signal between the WDM filter 6120 on the transmission side and the WDM filter 6124/6133 on the reception side is multiplexed. The instrument 6123 is equipped. Further, in the section where the optical signal between the laser diode (LD) 6101-6104 and the local port of the WDM filter 6120 is demultiplexed for each wavelength, phase modulation is performed on the optical signals of wavelengths λ ₁ to λ ₄ respectively. A phase modulator 6105-6108 to perform is connected.

In such a DPS quantum key distribution system, there is no interferometer on the transmission side, but a Mach-Zehnder interferometer provided on the reception side by applying different phase offsets between adjacent bits for each wavelength. The wavelength to be used can be determined without depending on the free spectral interval (FSR) of 6122.

The present invention can be used in an optical interference communication system using phase modulation represented by quantum key distribution technology. The protocol of the quantum cryptography key distribution method does not matter.

101 WDM filter
102 Interferometer
103 Temperature controller
104 WDM filter
105, 110 Phase modulator group
106 WDM filter
107, 111 Phase modulation controller
11, 41 Transmitter (Alice)
12, 42 Optical transmission line
13, 43 Receiver (Bob)
1101-1104, 4101-4104, 5101-5116, 6101-6104 Laser diode
1105, 1305, 4105, 4305, 6122 2-input 2-output Mach-Zehnder interferometer
1107-1110, 1301-1304, 4107-4114, 4301-4304, 5124-5127, 6105-6108 Phase modulator
1111, 4115, 6109 Clock source
1307-1314, 4307-4314, 6125-6132 Photon detector
5122 4-input 2-output asymmetric Mach-Zehnder interferometer
1106, 1306, 4106, 4306, 5121, 6123 Temperature controller
1120, 4124, 5129, 6115 Offset storage
1121-1124, 4125-4128, 6116-6119 Digital-to-analog converter (DAC)

Claims (20)

1. An optical communication apparatus in an optical communication system including an optical interference system using phase information of an optical signal,
   Optical multiplexing means for combining optical signals of a plurality of wavelengths and outputting one combined optical signal;
   Optical interference means for temporally separating the combined optical signal;
   Optical demultiplexing means for demultiplexing output light from the optical interference means and outputting the demultiplexed optical signals of the plurality of wavelengths;
   A plurality of phase modulation means for phase-modulating each of the optical signals of the plurality of wavelengths on the input side of the optical multiplexing means or the demultiplexed optical signals of the plurality of wavelengths on the output side of the optical demultiplexing means;
   Phase modulation control means for individually controlling the modulation amounts by the plurality of phase modulation means according to the wavelength;
   An optical communication device comprising:
2. 2. The optical communication apparatus according to claim 1, wherein the phase modulation control unit controls the modulation amount so as to compensate a wavelength dependency of a delay amount of the optical interference unit.
3. 3. The optical communication apparatus according to claim 1, wherein the phase modulation control unit adds the modulation amount to a modulation signal generated based on a random number and outputs the modulated signal to the plurality of phase modulation units.
4. Each of the plurality of phase modulation means includes a first phase modulator and a second phase modulator,
   3. The phase modulation control unit outputs a modulation signal generated based on a random number to the first phase modulator and outputs the modulation amount to the second phase modulator. An optical communication device according to 1.
5. 5. The optical communication apparatus according to claim 1, wherein the optical signal transmitted in the optical communication system has a light intensity of 1 or less photons per pulse.
6. An optical communication method in an optical communication system including an optical interference system using phase information of an optical signal,
   The optical multiplexing means combines optical signals of a plurality of wavelengths to generate one combined optical signal,
   An optical interferometer temporally separates the combined optical signal;
   An optical demultiplexing unit demultiplexes the output light from the optical interferometer to generate a demultiplexed optical signal of the plurality of wavelengths,
   Phase-modulating the demultiplexed optical signal of the plurality of wavelengths or the optical signal of the plurality of wavelengths for each wavelength with a modulation amount controlled individually according to the wavelength,
   An optical communication method characterized by the above.
7. The optical communication method according to claim 6, wherein the wavelength dependence of the delay amount of the optical interferometer is compensated by the modulation amount.
8. The optical communication method according to claim 6, wherein the modulation amount is added to the modulation signal generated based on the random number to perform phase modulation.
9. 8. The phase-modulated optical signal of the plurality of wavelengths or the optical signal of the plurality of wavelengths is modulated for each wavelength by a modulation signal generated based on a random number, and further phase-modulated by the modulation amount. An optical communication method according to claim 1.
10. The transmission-side communication device and the reception-side communication device of the optical communication system each have a transmission-side optical interferometer and a reception-side optical interferometer, and a difference in delay amount between the transmission-side optical interferometer and the reception-side optical interferometer The optical communication method according to claim 6, wherein: is compensated by the modulation amount.
11. The optical communication system includes a transmission-side communication device and a reception-side communication device, the optical interferometer is provided in the reception-side communication device, and the wavelength dependency of the delay amount of the optical interferometer is set in the transmission-side communication device. 8. The optical communication method according to claim 6, wherein compensation is performed by the modulation amount.
12. The optical communication method according to any one of claims 6 to 11, wherein the optical signal transmitted in the optical communication system has a light intensity of 1 or less photons per pulse.
13. In an optical communication system including an optical interference system using a phase information of an optical signal, having a transmission side communication device and a reception side communication device,
    The transmission-side communication device includes: a plurality of phase modulation units that phase-modulate optical signals of a plurality of wavelengths for each wavelength; and a plurality of phase-modulated optical signals that are combined to generate one combined optical signal. Optical multiplexing means for transmitting to the communication device,
    The receiving-side communication device includes: a receiving-side optical interference unit that temporally separates a received optical signal; and a receiving side that demultiplexes output light from the receiving-side optical interference unit and outputs the demultiplexed optical signals of the plurality of wavelengths. Optical demultiplexing means,
    The optical communication system, wherein the transmission-side communication device further includes phase modulation control means for individually controlling modulation amounts by the plurality of phase modulation means according to wavelengths.
14. 14. The optical communication system according to claim 13, wherein the wavelength dependence of the delay amount of the reception side optical interference means is compensated by the modulation amount.
15. The transmission-side communication apparatus includes: a transmission-side optical interference unit that temporally separates a transmission-multiplexed optical signal; and a transmission-side optical component that demultiplexes the output of the transmission-side optical interference unit and outputs the optical signals of the plurality of wavelengths. Wave means,
    15. The optical communication system according to claim 13, wherein the phase modulation control unit compensates for a difference in delay amount between the transmission side optical interference unit and the reception side optical interferometer with the modulation amount.
16. 16. The optical signal having a plurality of wavelengths transmitted from the transmitting communication device to the receiving communication device has a light intensity of 1 or less photons per pulse. The optical communication system according to any one of the above.
17. A program for causing a program control processor in an optical communication apparatus of an optical communication system including an optical interference system using phase information of an optical signal to function as a phase modulation control apparatus,
    The optical multiplexing means combines optical signals of a plurality of wavelengths to generate one combined optical signal,
    An optical interferometer temporally separates the combined optical signal;
    An optical demultiplexing unit demultiplexes the output light from the optical interferometer to generate a demultiplexed optical signal of the plurality of wavelengths,
    Individually controlling the amount of modulation of a plurality of phase modulation means for phase-modulating the wavelength-demultiplexed optical signal or the wavelength-dependent optical signal for each wavelength, according to the wavelength;
    A program for causing the program control processor to function as described above.
18. The program according to claim 17, wherein the wavelength dependence of the delay amount of the optical interferometer is compensated by the modulation amount.
19. The transmission-side communication device and the reception-side communication device of the optical communication system each have a transmission-side optical interferometer and a reception-side optical interferometer, and a difference in delay amount between the transmission-side optical interferometer and the reception-side optical interferometer The program according to claim 17 or 18, wherein the program is compensated by the modulation amount.
20. The optical communication system includes a transmission-side communication device and a reception-side communication device, the optical interferometer is provided in the reception-side communication device, and the wavelength dependency of the delay amount of the optical interferometer is set in the transmission-side communication device. The program according to claim 17 or 18, wherein compensation is performed by the modulation amount.

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