JP2006166162A - Communication system provided with pulse waveform shaping function and communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a communication system provided with a pulse waveform shaping function for facilitating the timing design of the system, and its transmitter. <P>SOLUTION: A communication system transmits optical pulses from a transmitter 11 to a receiver 13 through a transmission line 12, wherein the transmitter 11 shapes the waveform of the optical pulses inputted from the receiver 13 by a dispersion compensator 113, then applies phase modulation to the shaped optical pulses by a phase modulator 112, and sends out the phase-modulated optical pulses through the dispersion compensator 113 to the transmission line. The receiver applies phase modulation to the optical pulses which arrive through the transmission line by a phase modulator 135. By a phase shift difference between a transmission side and a reception side, information is detected. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a communication system capable of compensating for the spread of transmission pulses in a transmission line and a transmission apparatus having a waveform shaping function. The present invention can be applied to, for example, a quantum cryptographic key distribution system whose security is guaranteed in quantum physics.

  The Internet, which continues to grow rapidly, is convenient, but has great concerns about its security, and the need for encryption technology is increasing to keep communication confidential. Currently used encryption methods are classified into secret key encryption such as DES (Data Encryption Standard) and Triple DES, and public key encryption such as RSA (Rivest Shamir Adleman) and elliptic curve encryption. However, these are cryptographic communication methods that guarantee the security based on the "complexity of computation". It is. Under such circumstances, the quantum encryption key distribution system (hereinafter referred to as QKD) is attracting attention as an encryption key distribution technique that is “never eavesdropped”.

  In QKD, a photon is generally used as a communication medium, and information is transmitted in quantum states such as polarization and phase. An eavesdropper on the transmission path steals information by tapping a photon being transmitted, but it is not possible to completely return a photon that has been observed once to the quantum state before the observation, due to the uncertainty principle of Heisenberg. Is possible. This observation (wiretapping) causes a change in the statistical value of the received data detected by the legitimate receiver, and this change enables the receiver to detect the presence or absence of an eavesdropper on the transmission path. That is, as long as there is no change in the statistical value of the received data, it is possible to prove that “absolutely” eavesdropping has not occurred.

  In the case of a quantum key distribution method using phase modulation, an interferometer is configured on the transmission side and the reception side (hereinafter, referred to as Alice and Bob, respectively, respectively), and each photon is randomly selected by Alice and Bob. Apply phase modulation. Depending on the difference in modulation phase depth, an output of 1 or 0 can be obtained. Then, by collating the conditions when the output data is modulated on each side of Alice and Bob for the detected output, the same bit string can finally be shared between Alice and Bob. As a procedure for sharing such secret information, for example, a BB84 (Bennett Brassard 84) protocol using four quantum states is known.

  JP 2000-517499 (Patent Document 1) and “Automated 'plug & play' quantum key distribution” (G. Ribordy, J. _D. Gautier, N. Gisin, O. Guinnard and H. Zbinden: Non-Patent Documents) 1) discloses a Plug & Play quantum encryption key distribution system that is frequently used as a configuration most suitable for practical use. Hereinafter, the Plug & Play method will be briefly described.

FIG. 12 is a block diagram showing a schematic configuration of a conventional Plug & Play quantum key distribution system. In the Plug & Play system, first, an optical pulse P is generated by a laser diode (LD) 735 of a quantum cipher key receiver 73 (Bob), which is divided into two by an optical coupler 733, and one optical pulse P1 has a short path P1. Through _S , the other optical pulse P2 is transmitted to the transmitter 71 (Alice) in succession through the long path P _L. Alice 71 sequentially receives the optical pulses P1 and P2 through the optical transmission line 72. The optical pulse P1 is reflected by the Faraday mirror 711 with the polarization state rotated by 90 °, and the optical power is attenuated slightly by the optical attenuator 713 and returned to the Bob 73. On the other hand, the optical pulse P2 is similarly reflected by the Faraday mirror and phase-modulated (φ _A ) by the phase modulator 712, and the phase-modulated optical pulse P2 ^{* A} is similarly weakly attenuated by the optical attenuator 713. Returned to Bob73.

In Bob73, obtain a phase modulation (phi _B), and phase-modulated optical pulse P1 ^{* B} by the phase modulator 732 with passing the long path P _L that is different from the time of transmission of the light pulses P1 received from Alice71. On the other hand, passed through a short path P _S which is different from the time of the optical pulse P2 * ^A phase-modulated transmission at Alice71, to interfere with the optical pulse P1 * ^B that is phase-modulated by Bob73, the results in photon detector To detect. As a whole, since the optical pulses P1 and P2 divided into two in the Bob 73 pass through the same optical path with the Alice 71 and interfere with each other, the interference result observed by the photon detector is the delay variation of the optical transmission line 72. Etc. are canceled out, and depends on the difference between the phase modulation φ _A of Alice 71 and the phase modulation φ _{B of} Bob 73. In such a folded configuration, accurate interference can be realized if the interval between the photon pulses P1 and P2 generated by the interferometer including the short path P _S and the long path P _L is kept constant for a time longer than the round trip time.

  Japanese Unexamined Patent Application Publication No. 2004-93561 (Patent Document 2) describes a virtual image using a 3D mirror that compensates for chromatic dispersion of light caused by light propagating through an optical transmission line by adding “reverse dispersion”. A phased array (VIPA) device is disclosed.

  Japanese Patent Laid-Open No. 2000-183815 (Patent Document 3) discloses crosstalk and optical transmission from an adjacent channel by giving a wavelength dispersion different from that of the adjacent channel after optical modulation of each channel on the transmission side of the wavelength division multiplexing transmission system. A system for compensating the chromatic dispersion of a path is disclosed.

  In addition, as a prior document related to the components used in the embodiments of the present invention, “Balanced, gated-mode photon detector for quantum-bit discrimination at 1550 nm” (Non-Patent Document 2) by Tomita et al. A mode photon detector is described. In “Temperature independent QKD system using alternative-shifted phase modulation” (Non-patent Document 3) by Tanaka et al., A light pulse is circulated by a PBS loop mirror, and according to the direction of rotation. An alternative-shifted phase modulation method has been disclosed in which a modulation phase difference of half a wavelength (π) is given to obtain the same effect as a rotating action by a Faraday mirror.

Special Table 2000-517499 JP 2004-93561 A JP 2000-183815 A "Automated 'plug & play' quantum key distribution" G. Ribordy, J. _D. Gautier, N. Gisin, O. Guinnard and H. Zbinden "Balanced, gated-mode photon detector for quantum-bit discrimination at 1550nm" Optics letters, vol. 27 (2002) pp1827-1829 A. Tomita, K. Nakamura "Temperature independent QKD system using alternative-shifted phase modulation" A. Tanaka, A. Tomita, A. Tajima, T. Takeuchi, S. Takahashi, Y. Nambu (European Conference On Optical Communication 2004, Proceedings Vol.2 pp. 260 )

  However, in the above prior art, no consideration is given to the spread of pulses caused by the influence of wavelength dispersion or the like of the transmission path. In particular, when an optical pulse is generated by directly modulating a laser diode (LD) to obtain a high extinction ratio, the pulse spread is likely to occur due to chromatic dispersion due to high chirping associated with a change in the refractive index of the semiconductor laser medium. .

  In the optical pulse folding configuration such as the Plug & Play method described with reference to FIG. 12, as described above, the interval between the transmitted photon pulses P1 and P2 must be maintained at a predetermined interval or more during at least reciprocation. Since the phase modulator is used in both directions, the higher the optical pulse transmission repetition rate, the more difficult the system timing design becomes. In the state where the optical pulse spreads, it may become impossible to design the timing for performing appropriate phase modulation.

  It is also possible to provide a known dispersion compensating fiber or dispersion compensating module for compensating for the spread of the pulse in the transmission line. However, for example, if it is simply placed in the transmission path of the quantum cryptographic key distribution system, a new problem arises that the cryptographic key generation speed deteriorates due to the loss.

  An object of the present invention is to provide a communication system having a pulse waveform shaping function and a transmission apparatus thereof that facilitate system timing design.

  Another object of the present invention is to provide a communication system having a pulse waveform shaping function that facilitates system timing design and achieves efficient information transmission, and a transmission apparatus thereof.

  According to the present invention, the first communication device in a system for transmitting an optical pulse from a first communication device to a second communication device through a transmission line includes: a pulse compression unit that compresses an optical pulse to be transmitted; and the pulse compression unit. Transmission processing means for performing a first process on a predetermined parameter of the optical pulse compressed by the above and transmitting it to the second communication device, wherein the second communication device transmits the optical pulse that has reached through the transmission path. It has a reception processing means for detecting information by performing a second process on the predetermined parameter. A typical parameter of the light pulse is typically a phase state or a polarization state. The pulse compression means is preferably a dispersion compensator that performs accumulated dispersion compensation in the transmission line.

  Furthermore, as a preferred embodiment, the light intensity of the light pulse transmitted to the second communication device is weakened to a predetermined level using the light intensity attenuation characteristic of the pulse compression means. The first communication device may further include a light attenuating unit that weakens the light intensity of the compressed light pulse to a predetermined level. In the method in which the optical pulse reciprocates between the first communication device and the second communication device, the optical pulse output from the transmission processing unit is transmitted again to the second communication device through the pulse compression unit, so that the optical pulse is transmitted. It is possible to weaken the intensity to a predetermined level.

  As yet another embodiment, the first communication device includes a measuring unit that measures the length of the transmission line, and a control that changes a dispersion compensation setting value of the dispersion compensator according to the measured transmission line length. Means.

  As still another embodiment, the first communication device inputs the optical pulse to be transmitted from the second communication device through the transmission path in a predetermined time zone, and the pulse compression means is provided in the transmission path. It is a dispersion compensating fiber that performs accumulated dispersion compensation, and it is desirable that the length of the dispersion compensating fiber is long enough for a series of optical pulses generated in the predetermined time zone to be accommodated in the dispersion compensating fiber.

  According to another aspect of the present invention, a method of transmitting an optical pulse from a first communication device to a second communication device through a transmission line is obtained by compressing an optical pulse to be transmitted by the first communication device. A first process is performed on a predetermined parameter of the optical pulse and transmitted to the second communication device, and a second process is performed on the predetermined parameter of the optical pulse reached by the second communication device through the transmission path. In this way, information is detected.

  According to an embodiment, the first communication device inputs an optical pulse from the second communication device through the transmission line, compresses the input optical pulse by a pulse compressor, and selects a predetermined optical pulse. A transmission light pulse is generated by performing a first process on the parameter, and the transmission light pulse is compressed by the pulse compressor and transmitted to the transmission line. Furthermore, the second communication device outputs an optical pulse through the transmission path to the first communication device, and when the optical pulse is returned from the first communication device through the transmission path, the predetermined parameter of the optical pulse. The information is detected by performing the second process on.

  As described above, according to the present invention, the first communication device on the transmission side performs processing such as modulation on parameters such as the phase of the optical pulse after the optical pulse is compressed. Therefore, the response operation can be executed in a state where the spread of the optical pulse is suppressed, the drive voltage when driving the modulator can be easily stabilized, and the operation timing can be easily designed. Similarly, since the pulse is compressed and transmitted, the spread of the pulse waveform when the optical pulse propagates through the transmission path is suppressed. Therefore, the timing design of the processing for the received optical pulse is facilitated also in the second communication device on the receiving side. This makes it possible to design a detection circuit on the receiving side with high accuracy and facilitates improving detection accuracy.

  When the present invention is applied to a quantum key distribution system, the system timing can be easily designed without deteriorating the quantum key generation rate. This is because an optical pulse with a compressed pulse width is input to the phase modulator and the receiver by arranging a pulse compression medium in front of the phase modulator of the transmitter.

  Further, since the dispersion compensator as the pulse compression means has a loss, this loss can be used to weaken the transmission light pulse intensity. Thereby, the optical attenuator in the transmitter can be made unnecessary. Even when an optical attenuator is used, an inexpensive device can be provided because it is not necessary to select one having a large attenuation. This is because the attenuation amount of the optical attenuator used in the transmitter can be reduced.

  Hereinafter, an embodiment in which a communication system having a pulse waveform shaping function according to the present invention is applied to a plug & play quantum encryption key distribution system will be described in detail. However, the basic configuration and operation of the Plug & Play system are appropriately described using symbols used in the system of FIG.

1. 1. First Embodiment 1.1) Configuration FIG. 1 is a block diagram showing a Plug & Play quantum key distribution system according to a first embodiment of the present invention. In the present embodiment, a transmitter (Alice) 11 and a receiver (Bob) 13 are connected through a transmission path 12 such as an optical fiber.

  Alice 11 includes a Faraday mirror (FM) 111, a phase modulator 112, and a dispersion compensator 113, and is configured to perform dispersion compensation by the dispersion compensator 113 before performing phase modulation. As will be described later, the dispersion compensator 113 is set to be inverse dispersion of the accumulated dispersion in the transmission line 12, and performs pulse compression on the optical pulses (P1 and P2) received from the Bob 13 to perform phase modulation. Output to the instrument 112.

The phase modulator 112 performs phase modulation (φ _A ) according to the random number data on the Alice side at one timing of the optical pulses P1 and P2 at a predetermined interval, and the phase-modulated optical pulse (here, P2 ^{* A} ) And the optical pulse (here P1) which was not phase-modulated is reflected by the Faraday mirror 111. The optical pulses whose polarizations are rotated by 90 ° due to reflection are pulse-compressed again by the dispersion compensator 113. At the same time, a large loss generally possessed by the dispersion compensator 113 is used to form a weak pulse in a single photon state to Bob13. Sent out.

Bob 13 includes a polarization beam splitter (PBS) 131, a phase modulator 132, an optical coupler 133, an optical circulator 134, a laser diode (LD) 135, and an avalanche photodiode APD ₁ constituting a balanced gate mode photon detector 136. Has APD ₂ .

An optical pulse emitted by applying a voltage to the laser diode 135 is sent to the optical coupler 133 by the optical circulator 134. The optical pulse is branched into two by the optical coupler 133, and one pulse P 1 is directly transmitted to the polarization beam splitter 131, transmitted as it is, and transmitted to the transmission path 12. The other pulse P2 is sent to the polarization beam splitter 131 via the phase modulator 132, reflected by the polarization beam splitter 131, and sent to the transmission path 12. As described above, by connecting the optical coupler 113 and the polarization beam splitter 131 with the short path P _S and the long path P _L , it is possible to generate two optical pulses P1 and P2 that are temporally divided. .

  The polarization beam splitter 131 sends out these time-divided optical pulses P1 and P2 to the transmission line 12, respectively. The optical pulses P1 and P2 pass through the transmission line 12 and are input to Alice 11. When the light pulses P1 and P2 arrive at Alice 11, the waveforms spread due to accumulation of chromatic dispersion in the transmission line 12.

As described above, the optical power of Bob 13 is transmitted through the transmission line 12 through the transmission line 12 with the optical pulse P2 ^{* A} and the phase-modulated optical pulse P1 whose polarization is rotated by 90 ° due to reflection by the Faraday mirror 111 of Alice11. Receive in a weak state. Since polarization is rotated 90 ° by reflection by the Faraday mirror 111 of Alice11, the optical pulse P2 ^{* A} reaches the optical coupler 133 through a short path P _S transmitted through the polarization beam splitter 131. On the other hand, the optical pulse P1 is reflected by the polarization beam splitter 131, passes through a long path P _L, is phase-modulated (φ _B ) by the phase modulator 132 according to the random data on the Bob side, and the phase-modulated optical pulse P1 ^{* B} is It reaches the optical coupler 133.

The optical pulses P1 and P2 bifurcated in this way by the optical coupler 133 return to the optical coupler 133 through the same path as a whole, and the optical pulses P2 ^{* A} phase-modulated by Alice 11 and the optical pulse P1 phase-modulated by Bob ^{* B} interferes and the result is observed by the photon detector 136. As described above, the interference result observed by the balanced gate mode photon detector 136 is dependent on the difference between the phase modulation φ _A of Alice 11 and the phase modulation φ _{B of} Bob 13 because the delay variation of the transmission line 12 is canceled. To do.

  As described above, in the forward path from Bob to Alice, the pulse spread due to the accumulated dispersion in the transmission line 12 is compensated by the dispersion compensator 113 of Alice 11, and at the same time, the dispersion compensator 113 makes the dispersion overcompensated state by the dispersion compensator 113. The light is made into a single photon state by the attenuation action of the compensator 113 and sent out to the transmission line 12. Therefore, when the light pulse passes through the transmission line 12, the optical pulse is in a state of being shaped in a so-called manner.

  Here, the dispersion compensator 113 generally has a large loss. For example, when the VIPA described in Patent Document 2 is used, the insertion loss is about 8 dB. In a reciprocating system such as the Plug & Play system, a loss of 16 dB can be created. That is, the insertion loss that should be avoided in normal optical communication can be used as a means for creating a single photon state in quantum cryptography communication. Accordingly, it is not necessary to have an optical attenuator in Alice 11, and a single photon state can be generated only by the loss of dispersion compensator 113 of Alice 11 and the output adjustment of laser diode (LD) 135 of Bob 13.

1.2) Operation FIG. 2 is a flowchart schematically showing main operations of the Plug & Play quantum key distribution system according to the first embodiment of the present invention.

  First, Bob as a receiver generates an optical pulse train (step S1) and transmits it to Alice as a sender (step S2). Alice compresses the optical pulse width of the transmitted optical pulse using a pulse compression medium (dispersion compensator 113 in FIG. 1), and attenuates the light intensity due to the loss of the pulse compression medium ( Step S3).

Subsequently, Alice performs the first phase modulation (φ _A ) on the optical pulse (step S4), reflects the optical pulse using the Faraday mirror (step S5), and repulses the optical pulse. The compression medium compresses the pulse width and attenuates the light intensity to obtain a single photon state (step S6), and sends it back to Bob as a light pulse with weak light power (step S7).

Bob performs the second phase modulation (φ _B ) on the returned optical pulse (step S8), and the difference between the first phase modulation (φ _A ) and the second phase modulation (φ _B ). Δφ is detected (step S9).

  In the conventional method, since the pulse width compression step of step S3 and step S6 is not included in the above flow, the phase modulation of steps S4 and S8 is performed on the pulse in a state where the pulse width is widened. The timing design was difficult. In this embodiment, the difficulty of timing design is avoided by inserting the pulse width compression step S3 between steps S2 and S4 and the pulse width compression step S6 between steps S5 and S7, respectively. . In addition, creating a single photon state by attenuating the light intensity in the pulse width compression steps S3 and S6 has the advantage that the conventionally required variable optical attenuator becomes unnecessary. Hereinafter, the effect of the present invention that the timing design is facilitated will be further described.

1.3) Improvement of timing margin FIG. 3 is a waveform diagram showing a change in timing margin of an optical pulse reciprocating between Bob and Alice. FIG. 3A is a waveform in a conventional system without an optical pulse compression medium. FIG. 4B is a waveform diagram in the system according to the present embodiment provided with an optical pulse compression medium.

  The pulse width of the optical pulse at the time of Bob output is widened by transmitting through the transmission line, and the pulse waveform is widened at the time of Alice input. Here, in the conventional case where Alice does not include the optical pulse compression medium, as shown in FIG. 3 (A), both the input of the phase modulator in Alice and the output of Alice have no change in the optical pulse shape. The expanded waveform is returned to Bob. As a matter of course, the optical pulse having a broadened waveform is further expanded in pulse width by being transmitted through the transmission line, and the timing margin of the preceding and succeeding optical pulses is extremely small when reaching the Bob.

  On the other hand, in the case of this embodiment in which the optical pulse compression medium is provided in the preceding stage of the Alice phase modulator, the optical pulse is compressed by the input of the Alice phase modulator as shown in FIG. When the Alice is output, the pulse compression is again performed and the pulse width is further narrowed. Even if the optical pulse in this state is transmitted through the transmission line and the pulse waveform spreads, the timing margin of the preceding and following optical pulses is still maintained sufficiently wide when reaching the Bob.

  This means that the circuit timing design in Bob becomes very easy. Furthermore, it can be seen that even in Alice, since the input waveform of the phase modulator is pulse-compressed, the drive timing design of the phase modulator is much easier than in the prior art.

  Specific numerical examples are shown below. A directly modulated laser diode (LD) having a wavelength of 1550 nm is used as the pulse light source, and a single mode fiber (SMF) usually laid on the transmission line is used. It is known that an optical signal emitted by LD direct modulation generally has a large “α parameter” representing a time change of a carrier frequency within a pulse and takes a positive value. The local dispersion value of a single-mode fiber having a wavelength of 1550 nm is about +17 ps / nm / km, and since it is an anomalous dispersion region, pulse broadening occurs in combination with a positive α parameter.

  FIG. 4 is a network configuration diagram for experimentally showing pulse broadening. A laser diode (LD) 61 was driven to generate an optical pulse having a pulse width of 500 ps and transmitted through a 40 km single mode fiber 62, and then the waveform was measured with an oscilloscope 63.

  Comparing the optical pulse waveform 64 before transmission with the waveform 65 after transmission, it can be seen that the pulse has a pulse width of 500 ps as set before transmission, whereas the pulse spreads up to 1200 ps after transmission. That is, it can be seen that in such a state, phase modulation cannot be accurately performed at a repetition rate of 830 MHz (= 1/1200 ps) or more. Conversely, if the repetition rate is fixed at 830 MHz (one time slot 1200 ps), the timing margin can be secured at 700 ps when the pulse width is 500 ps, whereas the timing margin becomes almost zero when the pulse width increases to 1200 ps. Become.

  Furthermore, the time during which one light pulse passes through the phase modulator must continue to apply a constant voltage to the phase modulator. If the voltage value that drives the phase modulator fluctuates, that is, it indicates a fluctuation in the amount of phase shift applied by the phase modulator, resulting in a deterioration in the extinction characteristics of the interferometer. In general, when performing binary modulation of ON / OFF, the saturation characteristic of the output voltage of an IC or the like is used, so that it is relatively easy to create a stable voltage. However, a typical quantum key distribution system employing the BB84 method needs to perform multilevel modulation. When performing multi-level modulation, it becomes much more difficult to stabilize the signal voltage of the multi-level signal waveform for a certain period of time compared to binary modulation.

  In the present embodiment, by providing the dispersion compensator 113 in the previous stage of the phase modulator 112 of the transmitter (Alice) 11, the input optical pulse width of the phase modulator 112 is narrowed, the timing margin is improved, and the phase modulator The time for stabilizing the driving voltage 112 can be shortened. This facilitates analog design, and as a result, it is easy to obtain high interference characteristics.

  In order to receive a single photon, it is necessary to drive the avalanche photodiode (APD) constituting the photon detector of the receiver (Bob) 13 in the gate mode. In such a gate mode drive, in general, avalanche multiplication is most likely to occur when a photon is incident immediately after the gate voltage is applied, and the avalanche multiplication is decreased when the timing of photon incidence deviates from that, and thus the output current is reduced. Decreases and the signal-to-noise ratio deteriorates. Since the variation in the incident timing of individual photons is equivalent to the pulse width of the optical pulse, the timing design is similarly facilitated in the receiver (Bob) 13 if the optical pulse width is compressed.

  Further, in the present embodiment, by using the loss generated in the dispersion compensator 113, it is possible to create a single photon state by attenuating the light intensity, and there is an advantage that a variable optical attenuator becomes unnecessary.

  Various dispersion compensators 113 in the present embodiment can be used. The VIPA described in Patent Document 2 can be used, or an optical device using a fiber Bragg grating, a dispersion compensator using a planar waveguide, or a dispersion compensating fiber can be used. I do not care. The configuration of the transmitter (Alice) 11 may be any configuration as long as pulse width compression means such as a dispersion compensator is provided in the preceding stage of the modulator so that the pulse width of the input pulse of the modulator is compressed. It is not necessary if a means for realizing an equivalent function can be used.

2. Second Embodiment 2.1) Configuration FIG. 5 is a block diagram showing a plug & play quantum encryption key distribution system according to a second embodiment of the present invention. However, the same components as those in the first embodiment in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the present embodiment, the configuration of the transmitter (Alice) 21 is different from that of the first embodiment. That is, Alice 21 includes a phase modulator 211, a polarization beam splitter 212, a dispersion compensation fiber 213, and an optical attenuator 214. The polarization beam splitter 212 is optically connected to the transmission line 12 through the dispersion compensation fiber 213 and the optical attenuator 214, and the polarization beam splitter 212 and the phase modulator 211 constitute a PBS-loop mirror.

  As described above, in the Plug & Play system, since the optical pulse is turned back from Bob → Alice → Bob, backscattered light generated in the transmission path 12 appears as a noise signal. If the noise signal is large, quantum key distribution cannot be performed accurately and safely. Therefore, in the system according to the second embodiment shown in FIG. 5, a storage fiber 213 is provided in the transmitter (Alice) 21 in order to avoid the influence of backscattered light. Regarding the storage fiber, Patent Document 2 describes a method of dividing a time zone in which an optical pulse is continuously sent from Bob and a time zone in which a regular signal reflected by Alice returns to Bob.

  FIG. 6 is a network diagram showing signal transmission time zones in order to explain the operation of the storage fiber. FIG. 6A shows a time period in which a series of pulse signal light 1014 is transmitted from Bob 13. It is assumed that a reflection point 1016 exists in the transmission line 12.

  When the signal light 1014 is output from the Bob 13, as shown in FIG. 6B, a part of the reflected light 1027 of the signal light 1014 returns to the Bob 13 by the reflection point 1016, and the signal light 1014 is stored in the storage fiber 213 inside the Alice 21. Accumulated in. Then, the signal light 1034 returns to Bob 13 at the timing when the reflected light 1027 disappears.

  In this manner, the timing of the signal light and the reflected light can be separated using the storage fiber 213, and it becomes possible to prevent transmission characteristic deterioration due to the reflected light. In the present embodiment, a dispersion compensating fiber having a wavelength dispersion characteristic opposite to the accumulated dispersion in the transmission line 12 is used as the storage fiber 213.

In this embodiment, instead of performing polarization rotation and reflection using a Faraday mirror, a selective phase shift modulation method described in Non-Patent Document 3 is employed. Specifically, the optical pulses P1 and P2 received from Bob 13 are each divided by the polarization beam splitter 212 according to the polarization component (P1 → P1 _V + P1 _H , P2 → P2 _V + P2 _H ). Divided P1 _V and P1 _H are orbiting the PBS- loop mirror opposite direction, P1 without adding a phase modulation for _V, P1 _H is the phase modulation of the half-wavelength ([pi) fraction was added to the re-polarization beam splitter At 212, the signal is multiplexed and returned to the dispersion compensating fiber 213. On the other hand, P2 _V and P2 _H similarly rotate in the reverse direction around the PBS-loop mirror, but phase modulation of φ _A is applied to P2 _V and (φ _A + π) is applied to P2 _H , respectively. Therefore, the optical pulse returning to the dispersion compensating fiber 213 is given a modulation phase difference corresponding to a half wavelength (π) according to the circulation direction. This is equivalent to the rotating action by the Faraday mirror.

2.2) Timing Design Next, timing design in the present embodiment will be described.

  FIG. 7 is a schematic waveform diagram showing the modulation timing conditions required for the Alice phase modulator. First, when the two optical pulses time-divided in Bob 13 arrive at Alice 21, they are divided in the opposite directions in the polarization beam splitter 212, for a total of four optical pulses.

In the phase modulation method in the PBS-loop mirror, with respect to these four optical pulses, another optical pulse does not reach the phase modulator 221 in a time zone in which one optical pulse passes through the phase modulator 211. It is necessary to give different phase shifts. Assuming that the optical pulse width is T _P seconds, the optical path length of the phase modulator 211 is T _M seconds, and the pulse interval time-divided by Bob 13 is T _B seconds, a certain optical pulse reaches the phase modulator 211 and finishes passing. It can be seen that (T _M + T _P ) seconds are required. Therefore, the restrictions on the pulse timing are as follows.

The optical pulse 615 must not reach the phase modulator 211 for (T _M + T _P ) seconds from when the optical pulse 612 starts to pass through the phase modulator 211 until it finishes passing;
Similarly, the optical pulse 613 must not reach the phase modulator 211 for (T _M + T _P ) seconds from when the optical pulse 615 starts to pass through the phase modulator 211 to when it finishes passing;
- distance of the light pulses 612 and 613 are T _B seconds.

From the above three conditions, conditions required for the pulse interval T _B which are time division Bob13 is
T _B > 2 × (T _M + T _P )
It becomes.

If the optical pulse width T _P is 500 ps and the electrode length of the phase modulator 211 is 40 mm (T _M = 200 ps), T _B needs to be 1400 ps or more, and the repetition rate of the system is 1 / (2T _B ) or less. That is, it is limited to 350 MHz or less. Here, if the optical pulse width T _P has spread to 1200 ps, the upper limit of the repetition rate and the same calculation is reduced to less than 178 MHz. Actually, since it is necessary to consider the time zone during which the drive voltage passes through the phase modulator 211 and the modulation timing margin, the upper limit of the repetition rate is further reduced.

2.3) Light attenuation In the single photon state used in the quantum key distribution system, it is necessary to set the light intensity strictly. If the light intensity is higher than the set value, the risk of eavesdropping increases and the safety is impaired. On the other hand, if the value is lower than the set value, the number of photons that reach Bob 13 from Alice 21 decreases, and the encryption key generation speed deteriorates. Therefore, it is desirable that the variable optical attenuator 214 used for generating the single photon state is a device that can set the light intensity with high accuracy.

  On the other hand, in order to attenuate the output light pulse of the laser diode (LD) 135 to a single photon state, the maximum attenuation must be high, and the device used to satisfy the above two requirements becomes expensive. Therefore, in this embodiment, the dispersion compensation medium 213 having a relatively large loss is used as a part of the optical attenuation means. As a result, the requirement of “high maximum attenuation” for the variable optical attenuator 214 is relaxed, the range of usable optical devices is widened, and inexpensive devices can be used.

  In this embodiment, the connection order of the dispersion compensating fiber 213 and the variable optical attenuator 214 may be reversed. Further, the Alice 21 can be configured to use a Faraday mirror as in the first embodiment.

3. Third Embodiment FIG. 8 is a block diagram showing a Plug & Play quantum key distribution system according to a third embodiment of the present invention. However, the same components as those in the first embodiment in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the present embodiment, the configuration of the transmitter (Alice) 31 is different from that of the first embodiment. That is, Alice 31 has a dispersion having optical fiber distance measuring means such as a variable dispersion compensator 313, a variable optical attenuator 314, and an OTDR (Optical Time Domain Reflectometer) in addition to the Faraday mirror (FM) 311 and the phase modulator 312. It has a compensation controller 315 and a wavelength division multiplexing (WDM) coupler 316.

  The probe light used in the OTDR of the dispersion compensation controller 315 has a wavelength different from the wavelength of the pulse light source 135 of Bob 13 and is wavelength-multiplexed on the transmission line 12 by the WDM coupler 316 to measure the transmission distance between Alice and Bob. . The dispersion compensation controller 315 calculates the accumulated dispersion in the transmission line 12 according to the measured transmission distance, and changes the set dispersion value of the variable dispersion compensator 313 according to the calculation result. The set dispersion value does not need to be a value that exactly compensates the accumulated dispersion in the transmission line 12, and a dispersion value that causes pulse compression may be set.

  Here, the connection order of the tunable dispersion compensator 313 and the tunable optical attenuator 314 may be reversed. Also, the configuration of Alice 31 can use means for realizing the equivalent function instead of the Faraday mirror.

  In addition, by dividing the system operation into a “phase for measuring the distance of the transmission path” and a “phase for performing encryption key generation communication”, the probe light for distance measurement having the same wavelength as the pulse light source 135 is used. It is also possible.

4). Fourth Embodiment FIG. 9 is a block diagram showing a one-way quantum key distribution system according to a fourth embodiment of the present invention. In this embodiment, a transmitter (Alice) 41 and a receiver (Bob) 43 are connected through a transmission line 12 such as an optical fiber, and an optical pulse is only transmitted in one direction from Alice to Bob. It does not reciprocate like the first to third embodiments.

  In Alice 41, a direct-modulation laser diode (LD) 411 is connected to an optical coupler 413 through a dispersion compensator 412, and a phase modulator 414 and a delay line 415 are connected in parallel between the optical couplers 413 and 416. . The output end of the optical coupler 416 is connected to the transmission line 12 via the variable optical attenuator 417. On the other hand, in Bob 43, the input end of the optical coupler 431 is connected to the transmission line 12, and the phase modulator 433 and the delay line 432 are connected in parallel between the optical couplers 431 and 434.

The optical pulse emitted from the direct modulation laser diode (LD) 411 is pulse-compressed by the dispersion compensator 412 and branched into two by the optical coupler 413. One of the branched pulses is phase-shifted (φ _A ) by the phase modulator 414 and output to the optical coupler 416, and the other pulse is delayed by a predetermined delay amount by the delay line 415 and then output to the optical coupler 416. Is done. The phase-shifted pulse and the delayed pulse divided in this way are attenuated to a single photon state by the variable optical attenuator 417 through the optical coupler 416 and sequentially sent out to the transmission line 12. As already described, the dispersion compensation value of the dispersion compensator 412 is set so that the pulse spread of the optical pulse does not occur through the transmission line 12.

The phase-shifted pulse that reaches Bob 43 through the transmission line 12 and the subsequent delayed pulse are branched into two by the optical coupler 431. One of them is delayed by a delay line 432, and the other is phase-shifted (φ _B ) by a phase modulator 433 and then multiplexed by an optical coupler 434. Therefore, if the length of the delay line 415 of Alice 41 is set equal to the length of the delay line 432 of Bob 43, interference occurs when two optical pulses arrived at the optical coupler 434 are combined, and Alice 41 and Bob 43 The output port after the interference is determined by the difference between the phase shift amounts applied in step (b).

  FIG. 10 is a flowchart schematically showing main operations of the one-way quantum key distribution system according to the fourth embodiment of the present invention.

First, the sender Alice 41 generates an optical pulse train (step S11), compresses the optical pulse width using the dispersion compensator 412, and attenuates the light intensity due to the loss of the pulse compression medium of the dispersion compensator 412. (Step S12). Subsequently, the first phase modulation (φ _A ) is performed on one of the branched optical pulses (step S 13), and the phase-modulated optical pulse is transmitted by the variable optical attenuator 417 together with the delayed optical pulse. The light intensity is attenuated to the one-photon state (step S14) and transmitted to Bob 43, which is the receiver (step S15).

The Bob 43 performs the second phase modulation (φ _B ) on the optical pulse sent from the Alice 41 (step S16), and the phase difference Δφ between the first phase modulation and the second phase modulation (here, 0 or π) is detected (step S17).

  In the conventional method, since the pulse width compression step of step S12 is not included in the above flow, the phase modulation of steps S13 and S16 is performed on the pulse having a wide pulse width, and the system timing design is performed. Has become difficult. In this embodiment, the difficulty of timing design is avoided by inserting the pulse width compression step S12 before step S13.

  In addition, since the light intensity is also attenuated in the pulse width compression step S12, there is an advantage that it is not necessary to use an expensive variable optical attenuator having a large attenuation amount in the variable optical attenuation step S14.

  As described above, according to this embodiment, by performing pulse compression before performing modulation processing on an optical pulse, the design of an analog circuit that stabilizes the driving voltage of the phase modulator 414 and the repetition rate of the system are increased. Timing design to keep it easier. The variable optical attenuator 417 may be disposed between the laser diode (LD) 411 and the optical coupler 413 because the light intensity at the time of output from the Alice 41 only needs to be in a single photon state. it can.

5. Fifth Embodiment FIG. 11 is a block diagram showing a one-way quantum key distribution system according to a fifth embodiment of the present invention. In the present embodiment, unlike the above-described embodiment in which information is put by phase modulation, a method of putting information in the polarization state of a photon is adopted. That is, the optical pulse emitted from the pulse light source 511 of Alice 51 as a transmitter is subjected to pulse compression in the dispersion compensator 512, and the pulse-compressed optical pulse is set to a polarization state according to random data by the polarization rotator 513. The Then, the intensity of the optical pulse is attenuated to a single photon state by the variable optical attenuator 514 and transmitted to Bob 53 through the transmission line 12. Bob 53 sets the polarization direction of the polarizer 531 according to a random number, and detects the presence or absence of a photon of a light pulse that has passed through the polarizer 531 by the photodiode 532.

  Similar to the above-described embodiment, the present embodiment also avoids the difficulty of timing design by providing a dispersion compensator 512 as a pulse width compression unit before the polarization rotator 513. In addition, since the light intensity is attenuated by the dispersion compensator 512, there is an advantage that it is not necessary to use an expensive variable optical attenuator 514 having a large attenuation.

  The variable optical attenuator 514 may be installed between the pulse light source 511 and the polarization rotator 513 as long as the light intensity at the time of output from the Alice 51 is in a single photon state.

  The present invention can be applied to the quantum key distribution system as described above, but is not limited thereto, and can be applied to general systems that perform communication through a transmission path in which a pulse waveform spreads by propagation.

1 is a block diagram illustrating a Plug & Play quantum key distribution system according to a first embodiment of the present invention. 3 is a flowchart schematically showing main operations of the Plug & Play quantum key distribution system according to the first embodiment of the present invention. It is a wave form diagram which shows the change of the timing margin of the optical pulse reciprocating between Bob and Alice, (A) is a wave form diagram in the conventional system without an optical pulse compression medium, (B) is an optical pulse compression medium. It is a wave form diagram in the system by this embodiment provided. It is a network block diagram for showing a pulse spread experimentally. FIG. 6 is a block diagram illustrating a Plug & Play quantum key distribution system according to a second embodiment of the present invention. It is the network figure which showed the time slot | zone of signal transmission in order to demonstrate the effect | action of a storage fiber. It is a typical wave form diagram which shows the modulation timing conditions requested | required of the phase modulator of Alice. It is a block diagram showing a plug & play quantum encryption key distribution system according to a third embodiment of the present invention. It is a block diagram which shows the one-way type | mold quantum encryption key distribution system by 4th Embodiment of this invention. 10 is a flowchart schematically showing main operations of a one-way quantum key distribution system according to a fourth embodiment of the present invention. FIG. 10 is a block diagram illustrating a one-way quantum key distribution system according to a fifth embodiment of the present invention. It is a block diagram which shows schematic structure of the conventional Plug & Play system quantum encryption key distribution system.

Explanation of symbols

11 Transmitter (Alice)
111 Faraday mirror 112 Phase modulator 113 Dispersion compensator 12 Transmission path 13 Receiver (Bob)
131 Polarizing beam splitter 132 Phase modulator 133 Optical coupler 134 Optical circulator 135 Laser diode (pulse light source)
136 Balanced Gate Mode Photon Detector 21 Transmitter (Alice)
211 Phase modulator 212 Polarizing beam splitter 213 Dispersion compensating fiber 214 Optical attenuator 31 Transmitter (Alice)
311 Faraday mirror 312 Phase modulator 313 Variable dispersion compensator 314 Optical attenuator 315 Dispersion compensation controller 316 WDM coupler 41 Transmitter (Alice)
411 Laser diode (pulse light source)
412 Dispersion Compensator 413 Optical Coupler 414 Phase Modulator 415 Delay Line 416 Optical Coupler 417 Optical Attenuator 43 Receiver (Bob)
431 Optical coupler 432 Delay line 433 Phase modulator 434 Optical coupler 51 Transmitter (Alice)
511 Laser diode (pulse light source)
512 Dispersion Compensator 513 Polarization Rotator 514 Optical Attenuator 53 Receiver (Bob)
531 Polarizer 532 Photon Detector 71 Transmitter (Alice)
711 Faraday mirror 712 Phase modulator 713 Optical attenuator 72 Transmission path 73 Receiver (Bob)
731 Polarizing beam splitter 732 Phase modulator 733 Optical coupler 734 Optical circulator 735 Laser diode (pulse light source)

Claims (16)

In a communication system for transmitting an optical pulse through a transmission path from a first communication device to a second communication device,
The first communication device is
Pulse compression means for compressing optical pulses to be transmitted;
A transmission processing means for performing a first process on a predetermined parameter of the optical pulse compressed by the pulse compression means and transmitting to the second communication device;
Have
The second communication device includes a reception processing unit that detects information by performing a second process on the predetermined parameter of the optical pulse that has reached through the transmission path.
A communication system characterized by the above.   The communication system according to claim 1, wherein the pulse compression unit is a dispersion compensator that performs accumulated dispersion compensation in the transmission path.   2. The communication system according to claim 1, wherein the light intensity of the light pulse transmitted to the second communication device is weakened to a predetermined level using the light intensity attenuation characteristic of the pulse compression means.   4. The communication system according to claim 3, wherein the first communication device further includes an optical attenuation unit that weakens the light intensity of the compressed optical pulse to a predetermined level. 5.   4. The communication according to claim 3, wherein an optical pulse output from the transmission processing unit is transmitted again to the second communication device through the pulse compression unit to weaken the light intensity to a predetermined level. 5. system. The first communication device is
Measuring means for measuring the length of the transmission line;
Control means for changing a dispersion compensation setting value of the dispersion compensator according to the measured transmission line length;
The communication system according to claim 2, further comprising: The first communication device inputs the optical pulse to be transmitted from the second communication device through the transmission path in a predetermined time zone,
The pulse compression means is a dispersion compensating fiber that performs accumulated dispersion compensation in the transmission line, and the length of the dispersion compensating fiber is such that a series of optical pulses generated in the predetermined time zone is contained in the dispersion compensating fiber. Is long enough,
The communication system according to claim 1.   The communication system according to claim 1, wherein the predetermined parameter is at least one of a phase state and a polarization state of an optical pulse. In a communication device that transmits an optical pulse to another communication device through a transmission line,
Pulse compression means for compressing optical pulses to be transmitted;
Transmission processing means for performing predetermined processing on a predetermined parameter of the optical pulse compressed by the pulse compression means and transmitting to the other communication device;
A communication apparatus comprising:   The communication apparatus according to claim 9, wherein the pulse compression unit is a dispersion compensator that performs accumulated dispersion compensation in the transmission path.   10. The communication device according to claim 9, wherein the light intensity of the light pulse transmitted to the other communication device is weakened to a predetermined level using the light intensity attenuation characteristic of the pulse compression means.   The communication apparatus according to claim 9, wherein the transmission processing unit includes a phase modulator that shifts a phase of an optical pulse.   The communication apparatus according to claim 9, wherein the transmission processing unit includes a polarizer that sets a polarization direction of an optical pulse. In a method for transmitting an optical pulse through a transmission path from a first communication device to a second communication device,
The first communication device compresses an optical pulse to be transmitted, performs a first process on a predetermined parameter of the compressed optical pulse, and transmits to the second communication device;
The second communication device detects information by performing a second process on the predetermined parameter of the optical pulse that has reached through the transmission path;
A communication method characterized by the above. The first communication device is
An optical pulse is input from the second communication device through the transmission path,
The input light pulse is compressed by a pulse compressor,
A transmission light pulse is generated by performing a first process on a predetermined parameter of the compressed light pulse,
The transmission light pulse is compressed by the pulse compressor and sent to the transmission line.
The communication method according to claim 14. The second communication device is
Outputting an optical pulse through the transmission path to the first communication device;
When an optical pulse is returned from the first communication device through the transmission path, information is detected by performing a second process on the predetermined parameter of the optical pulse.
The communication method according to claim 15.

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