JP2010035072A - Optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an optical transmission and reception method meeting the condition that safe communication can be performed in principle while using a difference between an authorized recipient and an unauthorized recipient when a bit error rate (BER) of the authorized recipient is smaller than that of the unauthorized recipient. <P>SOLUTION: While using carrier light accompanied with such fluctuation as to cause even an authorized recipient to make a bit error, a signal is transmitted and received while performing error-correction coding thereon. A base of n+1 or more is used for a 2<SP>n</SP>-valued signal ((n) is an integer of ≥1), and an authorized sender and an authorized recipient share a seed key beforehand and then perform communication while selecting a base using the shared key. In signal transmission, the signal and a random number are transmitted and the random number is used for selecting the next transmission base. An unauthorized recipient does not know the base but has to make 2<SP>n</SP>-valued determination for error-correction code decoding so that his/her BER becomes greater than that of the authorized recipient. A difference between the BERs of the authorized recipient and the unauthorized recipient ensures the quantity of information that is safe from a viewpoint of information theory. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical communication system, and more particularly to an optical communication system capable of improving safety in optical communication or general communication.

  The demand for confidentiality in communications has been an eternal theme from the past to the future, and in the recent network society, the demand has been secured by the development of cryptography. In general, ciphers are roughly divided into information-theoretic security and computational security according to the degree of security. The variable related to the transmission signal of the sender is X, the variable related to the reception signal of the regular receiver is Y, the variable related to the reception signal of the unauthorized receiver is Z, and the mutual information between the sender and the regular receiver is I (X; Y) , If the mutual information amount between the sender and the unauthorized recipient is I (X; Z), the information amount C in which information safety is guaranteed between the legitimate sender and receiver is C = I (X; Y) -Becomes I (X; Z). When the secret key of the information amount C is shared in advance between authorized senders / receivers and communication is performed with the information amount C or less between the authorized senders / receivers, secure communication in terms of information theory becomes possible. However, this requires an enormous amount of secret keys, so it is generally difficult to break down mathematically using a small amount of secret keys, and security is based on the fact that it takes unrealistic time to break down. Cryptography is often used (computational security). Computationally safe ciphers are difficult to decipher against the current cryptanalysis techniques and computer capabilities, but the security decreases with technological progress. In order to solve this problem, research on a method for generating a secret key with a sufficient amount of information C by using the properties of quantum mechanics is active. However, quantum mechanical properties appear in the microscopic world, and there are many restrictions when actually introduced.

  In general, the strength of encryption is evaluated based on how difficult the decryption is based on the assumption that a legitimate receiver and an eavesdropper obtain the same signal. However, there are noises in actual communication channels, and generally there are bit errors. That is, the assumption that the legitimate receiver and the eavesdropper get the same signal is not necessarily correct. It is known that a safe information amount C = I (X; Y) −I (X; Z) can be secured without using the quantum mechanical property by utilizing the point at which this bit error occurs. Since the mutual information amount decreases with an increase in the bit error rate (BER), communication that is theoretically safe becomes possible if a communication path in which the BER of the unauthorized recipient is larger than that of the regular recipient can be realized (Non-patent Document 1). ). It is also shown in information theory that there can be a method for obtaining a safe information amount C by post-processing using a public communication path even in a communication path where the BER of a regular receiver is larger than that of an unauthorized receiver (non-patent) Reference 2). However, no specific methodology has been established.

JP 2008-003339 A A. D. Wyner, “The wire-tap channel,” Bell Syst. Tech. J., 54, 1335 (1975). U. M. Maurer, “Secret key agreement by public discussion from common information,” IEEE Trans. Inf. Theory, 39, 733 (1993). C. H. Bennett, G. Brassard, and J.-M. Robert, SIAM J. Comput. 17, 210 (1988)) T. Tomaru, and M. Ban, “Secure optical communication using antisqueezing,” Phys. Rev. A 74, 032312 (2006) T. Tomaru, “LD light antisqueezing through fiber propagation in reflection-type interferometer,” Opt. Exp. 15, 11241 (2007))

  In general, an unauthorized receiver does not always eavesdrop on a condition worse than that of an authorized receiver, and whether the BER of the authorized receiver or the unauthorized receiver is larger depends on the transmission path and the unauthorized reception method. If it is possible to realize a communication path in which the BER of the illegal recipient is surely larger than that of the regular recipient, a safe information amount C can be ensured. Therefore, it is a problem to realize a communication path in which the BER of the unauthorized recipient is surely larger than that of the regular recipient. Another problem is how to estimate the mutual information amount I (X; Z) between the sender and the unauthorized receiver in the realized communication path in order to estimate the information amount that is safe in terms of information theory.

First, the means and principles for solving the problems are listed.
1. There is a bit error even for a legitimate receiver using light with fluctuation in the carrier light.

For this reason, it is transmitted after error correction coding.
2. If the signal during transmission is binary, the transmission basis number should be 2 or more.
3. It is assumed that the legitimate sender / receiver shares the seed key in advance, so that the legitimate sender / receiver knows which base is used to transmit / receive the signal. Therefore, a legitimate recipient can make a binary decision.
4). Since the unauthorized receiver does not know the transmission base, for example, if the base number is 2, it can only determine a 4-value.
5). Since there is a bit error, decoding of the error correction code is essential.
6). Since the error correction code is a code for a binary signal, a result of binary determination is required at the time of decoding.
7. Unauthorized recipients who can only make 4-value judgments must also make binary judgments for decryption.

Therefore, “bit error rate of unauthorized receiver> bit error rate of regular receiver” is realized.
8). This difference provides a safe amount of information between legitimate senders and receivers. By using a random number as the signal to be sent and received and reducing the number of bits from the number of bits of the random number signal sent and received by the privacy amplifier method to a safe amount of information, a new key that is theoretically secure between information senders and receivers could be shared Become.

That is, one-time-pad encryption communication is possible using this key.
9. Repeated use of a seed key that has been shared between authorized senders and receivers in advance allows decryption of illegally received signals, so signals (including random signals) and random numbers (separate from random signals) are alternated The transmission / reception base is renewed by using the transmitted / received random number as a new secret key.

The point of the present invention is that a bit error always exists even for a legitimate receiver using light with fluctuation (noise) in the carrier light. In order to correct bit errors, error correction codes are used during signal transmission. When superimposing a signal on carrier light, if n bits (n is an integer of 1 or more) are collectively set to 2 ⁿ values per modulation, the necessary number of transmission bases is n bases, but in the present invention, n + 1 It is assumed that the transmission base is used by sharing the seed key (random number sequence) in advance with a regular sender / receiver. Therefore, the normal receiver can perform normal reception, and the bit error is corrected when the error correction code is decoded. On the other hand, the unauthorized receiver does not know the seed key and must detect the signal light without having the basis information, so the 2 ⁿ value cannot be determined, and if the basis number is m (> n), the 2 ^m value Can only be determined. Because of the fluctuation, the unauthorized receiver must also decode the error correction code. However, since the error correcting code is not for the 2 ^m value but for the 2 ⁿ value, the unauthorized receiver must also determine the 2 ⁿ value. At that time, the unauthorized recipient is disadvantaged by not knowing the seed key, and the BER becomes larger than that of the authorized recipient.

  If the seed key has a finite length, if it is used repeatedly, there is a possibility that it will be decrypted in the same way as the stream cipher. Therefore, the transmitter is equipped with a random number generator, and signals and random numbers are transmitted alternately, and the seed key is renewed by using the transmitted and received random numbers as a new seed key for base selection at the next transmission. . If a signal and a random number are transmitted at a ratio of 1: 1, for example, the same base is used twice. If there is no bit error, the base may be deciphered if the same seed key is used twice, but if the carrier light fluctuates, even if the key is used twice, the effect of fluctuation still remains and illegal reception That the subscriber's BER is greater than the legitimate receiver's BER.

  The amount of information for which safety is guaranteed is only the difference between the mutual information amounts calculated from the BER of the legitimate receiver and the unauthorized receiver. Therefore, security is not guaranteed over all transmitted signals. Therefore, the signal itself is also composed of random numbers (this is called a random number signal), and the number of bits of the random number signal shared between the sender and receiver is reduced to a safe amount of information by a logical operation (by reducing the number of bits) Paying attention to the fact that the amount of secure information per bit increases, such a process to reduce the number of bits is called a privacy amplifier), and one-time-pad encryption communication as a secret key newly shared by the sender and receiver The method of doing is effective. That is, the above processing is a means for amplifying a small amount of secret key shared between the sender and the receiver in advance.

  According to the present invention, if a small amount of seed key is shared in advance between senders and receivers, it is possible to perform communication in which safety is guaranteed in terms of information theory using a normal optical communication path.

Embodiments of the present invention will be described below.
Example 1
The present invention can be applied to either the phase modulation method or the amplitude modulation method, but here, the case of the phase modulation method is shown. For simplicity, the signal is binary and the base number is 2. The phase modulation may be either phase-shift keying (PSK) that requires reference light or differential-type differential-phase-shift keying (DPSK). FIG. 1 is a block diagram showing an example of a basic part of the present invention. FIG. 2 shows the state of the signal in the phase space. Each crescent-shaped part schematically shows the fluctuation of each signal state. Since the signal is binary and the basis number is 2, the signal state is 4 values in total. When the base is selected for the q-axis, the phase 0 and π states with respect to the q-axis become “1” and “0” signals (FIG. 2 (a)), and the base is selected for the p-axis. In this case, the states of the phases π / 2 and 3π / 2 are “1” and “0” signals (FIG. 2B).

  Fluctuation is applied to the output light from the laser light source 110 in the transmitter 100 by the fluctuation generator 120 (a specific example of the fluctuation generator 120 is shown in the eighth embodiment). As a result, the crescent-shaped fluctuation shown in FIG. Assume that the sender and receiver share a seed key (random number sequence) in advance. Since the sender and receiver need to be authenticated before starting communication, it is considered that at least the authentication key is held, and this assumption is valid. The signal is encoded with an error correction code by an encoder 143. The base of the encoded signal is selected based on the seed key 141, and is superimposed on the carrier light accompanied by phase fluctuations by the modulator 130. The carrier light on which the signal is superimposed is sent to the receiver through the transmission path 201. In the receiver 300, a base is selected through the modulator 310 using a seed key 333 shared in advance between the sender and the receiver. Since the correct receiver can correctly select the basis, the receiver can always receive in the state shown in FIG. The detector 320 performs binary determination (a specific configuration of the detector 320 is shown in Example 10). The signal after the binary determination is error-corrected by the decoder 330. In the present invention, since the carrier light is accompanied by fluctuations, there is a bit error in the regular receiver, but the bit error is corrected because the error correction code is used. Basically, the signal can be transmitted and received.

  An unauthorized recipient does not know the seed key and cannot make a binary decision, but if the base number is 2, it can make a quaternary decision. Here, it is assumed that only phase measurement is performed instead of four-value determination. Since it is the phase measurement that the legitimate receiver is also making the binary decision, the amount of information is the same if both are interpreted as phase measurement. However, in the present invention, it is important to use carrier light accompanied by fluctuations. There is always a bit error due to fluctuation, and the error must be corrected. Since decoding is performed on a binary signal, it is not sufficient to perform phase measurement to obtain a final signal, and binary determination must be made. FIG. 2D schematically shows the fluctuation distribution of the signal received by the unauthorized receiver. If the fluctuation of each signal state is small, it becomes possible to make a quaternary determination using the dotted line as a boundary line. However, fluctuations of each signal state are overlapped and the BER of the unauthorized receiver is expected to be worse than that of the regular receiver. The decoding of the error correction code requires a binary determination result, so an unauthorized recipient must also make a binary determination rather than a 4-level determination prior to decoding, and the BER is determined at this point. Since illegitimate receivers do not know the basis, the BER is inevitably higher than that of authorized receivers, and the amount of mutual information between authorized receivers and senders and the amount of mutual information between unauthorized recipients and senders The information-theoretic safety is guaranteed.

FIG. 3 shows region classifications relating to binary determination of a regular recipient and an unauthorized recipient when the basis is the q axis and the signal is “1”. The judgment of the legitimate recipient in each area is indicated by B: “0” and B: “1”, and the judgment of the illegal recipient is indicated by E: “0” and E: “1”. Since the legitimate receiver knows the basis, q is judged positive or negative. The probability of q <0 for a signal of “1” is BER. Since the unauthorized recipient does not know the correct base, only four-value determination can be made, and in the case of FIG. 3, four areas separated by broken lines are determined (see FIG. 2 (d)). If “1” is assigned to the positive direction of the q-axis and p-axis, and “0” is assigned to the negative direction, the upper side of the broken line drawn from the upper left to the lower right of FIG. It will be determined as “0”. Since a signal of “1” is assumed, the probability of determining “0” is BER. It is when the measured value enters the shaded area in FIG. 3 that the regular receiver and the unauthorized receiver make a different determination. When the measured value is entered in the area 1, the authorized receiver is judged correctly, but the unauthorized recipient is judged wrong. If the probability that the measured value enters the area 1 is P ₁ , it is the disadvantage of the unauthorized recipient by that probability. On the other hand, when the measured value is entered in the area 2, the authorized receiver has made an erroneous determination, and the unauthorized receiver has made the correct determination. The probability that a measurement value enters the area 2 P ₂ Tosureba its probability amount corresponding unauthorized recipient becomes advantageous. Therefore, the difference (P ₁ −P ₂ ) in the probability that the measured value enters both areas is a disadvantage for the unauthorized recipient. If this probability difference is converted into a mutual information difference, an information amount whose safety is guaranteed in terms of information theory can be calculated.

In order to effectively use the BER difference generated between the regular receiver and the illegal receiver, it is necessary to appropriately set an error correction code. This is because if the error correction code used can correct the error even with respect to the BER of the unauthorized recipient, there will be no difference in the amount of information between the authorized recipient and the unauthorized recipient after decoding. Therefore, it is appropriate to set the threshold of whether error correction is possible or not to an intermediate value between the BER of the regular receiver and the BER of the unauthorized receiver. In addition, it is desirable that the error correction capability changes rapidly on both sides of the threshold. That is, it is desirable that error correction is almost certainly possible for BER lower than this threshold value, and error correction is almost impossible for BER higher than this threshold value. In general, the error correcting BER of an error correcting code and the BER threshold at which the correction capability changes abruptly not only depend on the individual code, but also change depending on the redundancy of the code, the degree of repetition, the degree of convolution, and the like. Therefore, the setting required in the present invention is performed by adjusting these parameters.
(Example 2)
In the first embodiment, the seed key is shared between the sender and the receiver, and the transmission base is selected using the seed key. Since the carrier light is fluctuated, it is effective to make the base multilevel. However, if the seed key is repeatedly used, the effect of fluctuation is reduced. This is because, generally, if the same signal with the same fluctuation is measured m times, the fluctuation is reduced to 1 / √m. Therefore, it is effective to reduce the number of times the same seed key is repeatedly used as much as possible. Therefore, it is considered that a signal and a random number are alternately sent and the transmitted random number is used as a secret key for base selection at the next transmission. If the signal and the random number are transmitted at a ratio of 1: 1, the same secret key is used twice. The effect of fluctuation is reduced to 1 / √2, but still large enough.

FIG. 4 is a block diagram specifically showing the present embodiment. The signal sequence _{1 s 1, s 2, s} 3, ..., r 1 an output random number sequence from the random number generator 140, r _2, r _3, synthesized in ... Tosureba combiner 142 column s _1, r _1, s ₂ , r ₂ , s ₃ , r ₃ ,... are formed and subjected to error correction decoding in the encoder 143.

Here, s _i and r _j (i, j = 1, 2, 3,...) Are not limited to 1 bit and may be any number of bits. The case where s _i and r _j are 1 bit corresponds to the case where random number bits and signal bits are alternately arranged, and the case where s _i and r _j are multiple bits corresponds to the case where they are arranged for each block. Furthermore, it is also effective to perform rearrangement so that the arrangement of random number bits and signal bits looks random. Since the outputs r ₁ , r ₂ , r ₃ ,... From the random number generator 140 are used as secret keys for base selection at the next transmission, they become seed keys 141. This is shown in FIG. On the receiver side, base selection is performed in the modulator 310 based on the seed key 333, binary determination is performed in the detector 320, and a combined sequence of random numbers and signals s ₁ , r ₁ , s ₂ , r ₂ , s in the decoder 330. _3, r _3, is decoded ... to. Next, the separator 331 separates the random number sequence 332 (r ₁ , r ₂ , r ₃ ,...) And the signal sequence 3 (s ₁ , s ₂ , s ₃ ,...). Since it is used as a secret key for base selection, it becomes a seed key 333.

When the synthesizer 142 rearranges the random number sequence and the signal sequence, the rearrangement rule is determined in advance, but the rearrangement rule is sequentially changed based on the random number output from the random number generator 140. Is also possible. Since the random number with the rearrangement rule changed is transmitted to the receiver side as described above, the receiver side can cope with the change of the rearrangement rule.
(Example 3)
In the first and second embodiments, it has been shown that a safe information amount C can be secured by using fluctuations in carrier light. However, the safe amount of information is only C = I (X; Y) -I (X; Z), and not all the transmitted bits are safe. Therefore, when it is necessary to guarantee the safety of all signals to be transmitted, another mechanism is necessary. An example is shown in FIG. The information used as the signal in the first and second embodiments is a random number output from the random number generator 1110 and is referred to as a random number signal 1 here. The random number signal 1 is transmitted from the transmitter 100 to the receiver 300 through the transmission path 201 by the procedure of the second embodiment, and is output as the random number signal 3. Since the random number signals 1 and 3 are random numbers, the information itself is not meaningful. Therefore, the amount of information is reduced to a safe amount of information C by repeating logical operations using the bit number reduction arithmetic units 1120 and 3120. For example, the random signal sequences 1 and 3 are represented as s ₁ , s ₂ , s ₃ , s ₄ , s ₅ , s ₆ , s ₇ , s ₈ ,..., T _i = s _2i-1 + s _2i (where i = 1, 2, 3, ..., + represents a logical sum) and the number of bits is halved. By repeating this process, the number of bits can be reduced to a desired amount of information. This process is exactly the same on the transmitter side and the receiver side, and is so-called Privacy amplification (Non-patent Document 3). Since the random number signals 1121 and 3121 with the reduced number of bits are the amount of information C whose safety is guaranteed in terms of information theory, encrypt the signal that you want to send as a one-time-pad secret key. Normal encryption communication. That is, the signal 11 that is actually desired to be sent is transmitted to the transmission line 202 by the optical transmitter 1130 using the encryption key 1121, and is uniformly decrypted by the encryption key 3121 in the optical receiver 3130, and the final output signal 31 is obtained. obtain.

In this embodiment, two transmission lines 201 and 202 are used, but normal transmission lines are sufficient for both. Both may be physically different transmission paths, or may be physically the same optical transmission path as in wavelength division multiplexing.
Example 4
In Example 3, a bit number reduction operation (Privacy amplification) was performed to secure a safe information amount in terms of information theory. However, information-theoretic safety is not always required in general communication, and it is conceivable to reduce the level of security in order to increase efficiency. If the cryptographic security (stream cipher, etc.) that is usually used frequently and carrier light with fluctuation are used in combination, even if information theoretical security cannot be achieved, the level is higher than normal computational security. Can be realized.

  FIG. 7 is a block diagram of an example for realizing it. A pseudo-random number generator 150 generates a pseudo-random number sequence using a part of the seed key 141 shared in advance between the sender and the receiver, and the encryptor 151 encrypts the signal 1. Thereafter, a synthesized sequence is formed in the synthesizer 142 from the output sequence from the random number generator 140 and the encrypted signal sequence by the same procedure as in the second embodiment, and error correction coding is performed by the encoder 143. And 120, the encoded signal is superimposed on the carrier light generated by 120. Base selection in the modulator 130 is performed using a part of the seed key 141. On the receiver side, using a part of the seed key 333, the base is selected in the modulator 310 in the same manner as in the second embodiment, the binary determination is made in the detector 320, and the error correction code is decoded in the decoder 330, The separator 331 separates the random number and the encrypted signal. The encrypted signal is decrypted by the decryptor 341 to obtain the output signal 3. In decryption, the pseudorandom number generator 340 is driven using a part of the seed key 333, and the output pseudorandom number sequence is used.

The output from the random number generator 140 is shared between the sender and receiver through the above transmission, and is used for base selection in the next signal transmission as in the second embodiment, and partly drives the pseudo random number generators 150 and 340. It is also used as a seed key update.
(Example 5)
In the fourth embodiment, a system using a combination of encryption using pseudo-random numbers and carrier light with fluctuation is described. The synthesis of the output sequence from the random number generator 140 and the signal sequence 1 was performed after the signal sequence 1 was encrypted. However, encryption can also be performed after the output random number sequence from the random number generator 140 and the signal sequence 1 are combined. FIG. 8 shows a block diagram in that case. The order of the encryptor 151 and the combiner 142 in the transmitter is switched compared to that in FIG. Similarly, the order of the separator 331 and the decoder 341 in the receiver is switched between FIG. 7 and FIG. The procedure of this example is the same as that of Example 4 except that this order is changed. In this embodiment, since encryption is performed including random numbers, decryption becomes more difficult.
(Example 6)
In the fourth and fifth embodiments, a system is described in which encryption using pseudo-random numbers and carrier light with fluctuation are used in combination. These systems can also be applied to the key generation system described in the third embodiment. That is, the transceiver of FIGS. 7 and 8 can be applied to the transceiver of FIG. In this case, the bit number reduction calculators 1120 and 3120 may determine the number of bits to be reduced according to the required level of security.
(Example 7)
Until Example 6, the output light from the laser light source 110 was converted into light accompanied by fluctuations by the fluctuation generator 120 and then the signal was superimposed in the modulator 130. However, fluctuations can be generated even after signal superposition. Therefore, it is possible to change the order of the fluctuation generator 120 and the modulator 130. For example, FIG. 9 shows what has been realized for the transmission / reception system of FIG. This replacement is possible for other system configurations.
(Example 8)
Although the fluctuation generator 120 can take various forms, a method using the Kerr effect of an optical fiber is convenient. An example is shown in FIG. Output light from the laser light source 110 is amplified by the optical amplifier 121, passes through the band filter 122, and propagates through the optical fiber 123. At this time, phase fluctuation is applied through the Kerr effect of the optical fiber. The laser output light can be described relatively well in the coherent state, and the shape of the fluctuation in the phase space is circular, but it becomes elliptical through the Kerr effect of the optical fiber, and further progresses into a crescent shape. Light in which the shape of the fluctuation is elliptical or crescent is called anti-squeezed light (Non-Patent Document 4 and Non-Patent Document 5). Since the Kerr effect increases in proportion to the light intensity, it is effective to increase the peak intensity using pulsed light. In this case, it is effective to suppress the pulse spread accompanying the fiber propagation, and it is preferable to select the pulse width, the light intensity, and the fiber dispersion value so as to satisfy the soliton condition (Patent Document 1). Further, if the light intensity is further increased than the above soliton condition, it becomes possible to satisfy the condition of higher-order soliton (Patent Document 1), and the pulse width reduction effect works to enhance the Kerr effect. Further, at that time, the spectrum width is expanded, and the spectrum expansion exhibits the same effect as the phase fluctuation in the phase detection, so that the effect of the fluctuation is further enhanced. Similar to the Kerr effect, the Raman effect is also effective in expanding phase fluctuations.

FIG. 11 shows an embodiment in which an optical circulator 124 and a Faraday mirror 125 are used to reciprocate the fiber propagation portion. The advantage is that the fiber length can be halved. Also, since the polarization rotates exactly 90 degrees when the fiber 123 is reciprocated once regardless of the polarization state during fiber propagation, it is effective when it is desired to stabilize the polarization at the output of the fluctuation generator. It is also effective to incorporate a fiber interferometer in the fluctuation generator 120 and increase the ratio of the phase fluctuation to the amplitude to increase the effect of the phase fluctuation (Non-Patent Document 5).
Example 9
In Example 8, it was shown that the fluctuation was generated by the Kerr effect of the optical fiber. Fluctuations can be generated in various ways, including other thermal fluctuations. On the other hand, it is also possible to add a phase corresponding to the fluctuation arithmetically using a random number generator. An example of realizing it for the system of FIG. 4 is shown in FIG. In order to make the output from the random number generator 115 fluctuate, the signal is converted into a multi-value (circuit 116), and the base selection signal based on the composite sequence and the seed key 141 which has been subjected to error correction coding is added in the circuit 144 to be added to the modulator 130. As an input signal. In this case, the fluctuation of the carrier light itself is basically a coherent state, but when many bits are averaged, a crescent shaped fluctuation as shown in FIG. 2 is obtained. The procedure for obtaining the signal sequence 3 from the signal sequence 1 is the same as in the second embodiment.

Further, as shown in FIG. 13, there is a method in which the laser is directly current-modulated using a multi-valued random number to make it appear as a fluctuation.
(Example 10)
In the case of the phase modulation method, the detector 320 can be a balanced homodyne detector. FIG. 14 shows a balanced homodyne detector corresponding to the DPSK system. Since the signal is superimposed on the phase difference between adjacent bits in the DPSK method, the two photodetectors 322 and 323 are used after interfering the signal light between adjacent bits using a 1-bit delay interferometer 321. Is converted into an electric signal. The differential circuit 324 takes a difference between two electrical signals, and this becomes an output signal. This output is obtained by projecting a signal in the phase space in a certain axial direction. The axial direction of projection is determined by the phase difference between the two optical paths of the 1-bit delay interferometer 321. In the case of FIG. 2 (c), the signal is determined based on the q-axis value, and the 1-bit delay interferometer 321 is set so that the projection axis is the q-axis. In the case of PSK, the 1-bit delay interferometer 321 is not necessary, but a reference light source is necessary, and the interference signal between the reference light and the signal light is converted into an electrical signal by two photodetectors.

In the case of a binary signal, one set of balanced homodyne detectors is sufficient as described above. However, if a quaternary signal is used, the output of two quadrature components is required, so two balanced homodyne detectors are used. Use. Even in the case of further multi-values, the coordinates can be determined in the two-dimensional phase space, so that signals can be detected by preparing two sets of balanced homodyne detectors as in the case of four-value signals.
(Example 11)
Up to the tenth embodiment, a binary and two-basis phase modulation method has been shown as an example. However, the present invention can also be applied to the intensity modulation system, and FIG. 15 shows the most basic block in the case of the intensity modulation system. FIG. 16 shows the intensity distribution function in the case of two values and two bases. In the intensity modulation method, the signal intensity of “0” and “1” varies depending on the base. In the case of two values and two bases, the signal state is a total of four values. The receiver changes the determination threshold depending on the selected base. As a result, the legitimate receiver can accurately receive other than a few bit errors caused by fluctuations, but since the illegal receiver does not know the basis, only a four-value determination can be made as shown in FIG. Bit errors increase because of the overlap in the probability distribution of quaternary signals.

  If it is thought that what the detector 320 is doing is intensity measurement rather than binary judgment or area judgment, what the authorized receiver and unauthorized recipient do is the same, and there is a difference in the amount of information held by both However, in the present invention, there is a bit error in order to use light with a large fluctuation, and the error correction code must be decoded. At that time, a binary decision must be made, and the BER increases by the amount that the unauthorized recipient does not know the basis. As a result, it is possible to secure a safe amount of information between authorized senders and receivers.

  As described above, even in the intensity modulation system, information-theoretically safe communication can be performed based on the same principle as in the case of the phase modulation system. FIG. 15 is obtained by changing the phase modulation method in FIG. 1 to the intensity modulation method. The main difference between the two systems is that the phase modulator 130 becomes an intensity modulator on the transmitter side. On the receiver side, instead of using the phase modulator 310 for base selection, the decision threshold is changed in the detector 320. Other procedures are the same for both the phase modulation method and the intensity modulation method. In both systems, the configuration of the individual components varies, but all the principles of the phase modulation method described up to the tenth embodiment can be applied even in the case of the intensity modulation method.

  The embodiment has been described by taking the phase modulation method as a main example. As mentioned in the eleventh embodiment, the present invention is established without distinction between the phase modulation method and the intensity modulation method. Further, although the embodiment has been described by taking the binary and the two bases as an example, the present invention can be applied to the case of the multivalue and the multi bases.

In the present invention, it has been shown that if the fluctuation is used, it is possible to perform secure communication in terms of information theory. The safety that is guaranteed in terms of information theory, not the computational safety, does not decrease the safety even if the computer performance is improved, and gives a very high level of safety. Therefore, the potential demand is considered high. In addition, the present invention can construct a sufficiently practical system even at the current technical level, and has high industrial applicability.
Note:
1. A random number generator, an encoder for error correction coding, a laser light source, means for generating phase fluctuation or intensity fluctuation in output light from the laser light source, and a transmitter comprising a modulator,
A receiver comprising means for measuring or determining the phase or intensity of the transmitted signal light and a decoder for an error correction code;
A transmission line provided between the transceiver and
A composite sequence obtained by adding the output random number sequence from the random number generator to the input signal sequence is encoded by the encoder, and the code is divided into n bits (n is an integer of 1 or more) to obtain 2 ⁿ values. Superimposed on the laser beam with the fluctuation in the modulator,
The number of transmission bases at the time of code superposition is n + 1 or more, and a seed key consisting of random numbers is shared in advance between the transmitter side and the receiver side, and the transmission base of the first code is the seed key The transmission base of the second and subsequent codes is determined by the seed key and / or the output random number sequence from the random number generator,
On the receiver side, the reception base of the first code is determined by the value of the seed key as in the transmission side, and the reception base of the second and subsequent codes is determined by the seed key and / or the first and subsequent keys. Determined by the output random number sequence from the random number generator constituting the transmitted code, the code superimposed light transmitted through the transmission path using the determined base is received, and the decoder Decrypted into a composite sequence, the decrypted composite sequence is separated into the random number sequence and the signal sequence, and the separated random number sequence is the second and subsequent seed keys that are newly shared between the sender and the receiver Used to determine the reception base of the code of
An optical communication system characterized in that the means for generating fluctuations comprises an optical fiber, and the output light from the laser light source is spectrum-expanded by a high-order soliton effect, and phase fluctuations are generated therethrough.
2. Random number generator, pseudo-random number generator, encoder for error correction encoding, laser light source, means for generating phase fluctuation or intensity fluctuation in output light from the laser light source, and transmitter comprising a modulator When,
Means for measuring or determining the phase or intensity of the transmitted signal light; a decoder for error correction code; and a receiver comprising a pseudo-random number generator;
A transmission line provided between the transceiver and
A seed key consisting of random numbers is shared in advance between the transmitter side and the receiver side, and an output pseudo-random number sequence is output from the pseudo-random number generator in the transmitter with a part of the seed key as an input, to the input signal sequence A composite sequence obtained by adding an output random number sequence from the random number generator is encrypted with the pseudo-random number sequence, the encrypted composite sequence is encoded by the encoder, and the code has n bits (n is 1 or more). for each integer) separated by being superimposed on the laser beam with the fluctuations in the modulator as a 2 ⁿ values, the number of transmission basis during code superimposed to the n + 1 or more, transmission of 1 th code The basis is determined by the remaining part of the seed key, the transmission base of the second and subsequent codes is determined by the seed key and / or the output random number sequence from the random number generator, and one of the output random number sequences. Is useful for renewing the input key to the pseudo-random number generator. It is,
On the receiver side, the reception base of the first code is determined by the value of the seed key, as in the transmission side, and the reception base of the second and subsequent codes is determined by the seed key and / or the first and subsequent keys. Determined by the output random number sequence from the random number generator constituting the transmitted code, the code superimposed light transmitted through the transmission path using the determined base is received, and the decoder The encrypted composite sequence is decrypted by the pseudo random number sequence output from the pseudo random number generator in the receiver using a part of the seed key in the same manner as the transmission side. The combined sequence that has been decrypted and decrypted is separated into the random number sequence and the signal sequence, and the separated random number sequence is the second and subsequent seed keys that are newly shared between the sender and the receiver Code base and pseudo-random number generator It is intended to be used to input key innovation,
An optical communication system characterized in that the means for generating fluctuation comprises an optical fiber, and the output light from the laser light source is expanded in spectrum by a high-order soliton effect, and phase fluctuation occurs through the spectrum.

The block diagram which shows an example of a structure with respect to the fundamental part of this invention. Figure (a, b) showing the fluctuation of each signal in the phase space in the case of binary signal and two bases in the phase modulation method, the reception signal state of the regular receiver (c) and the reception signal state of the unauthorized receiver ( The figure which shows d). The figure which shows "0" and "1" judgment with respect to each coordinate value in the phase space in the receiving side at the time of transmitting a signal of "1" on the q axis. “B” indicates an authorized recipient, and “E” indicates an unauthorized recipient. The block diagram which shows an example of a structure of this invention. The block diagram which shows the relationship between a base and a synthetic sequence. The block diagram which shows an example of a structure of this invention. The block diagram which shows an example of a structure of this invention. The block diagram which shows an example of a structure of this invention. The block diagram which shows an example of a structure of this invention. The block diagram which shows an example of a structure of a fluctuation generator. The block diagram which shows an example of a structure of a fluctuation generator. The block diagram which shows an example of a structure of this invention. The block diagram which shows an example of a structure of this invention. The block diagram which shows an example of a structure of a detector. The block diagram which shows an example of a structure with respect to the fundamental part of this invention in an intensity | strength modulation system. Binary signal in intensity modulation system, diagrams showing the intensity distribution of each signal value with fluctuations in the case of two bases (a, b), received signal status (c) of the regular receiver and received signal status of the unauthorized receiver ( The figure which shows d).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Signal (random number signal), 3 ... Signal (random number signal), 11 ... Signal, 31 ... Signal, 100 ... Transmitter, 110 ... Laser light source, 115 ... Random number generator, 116 ... Multi-valued circuit, 120 ... Fluctuation Generator 121, optical amplifier 122, band pass filter 123, optical fiber 124, circulator 125, Faraday mirror 130, modulator 140, random number generator 141, seed key 142, combiner 143 ... Encoder, 144 ... Adder circuit, 150 ... Pseudo random number generator, 151 ... Encryptor,
201: optical transmission path, 202: optical transmission path,
DESCRIPTION OF SYMBOLS 300 ... Receiver, 310 ... Modulator, 320 ... Detector, 321 ... 1 bit delay interferometer, 322 ... Photo detector, 323 ... Photo detector, 324 ... Differential output circuit, 330 ... Decoder, 331 ... Separation 332 ... random number, 333 ... seed key, 340 ... pseudo random number generator, 341 ... decoder,
1000 ... Transmitting device, 1110 ... Random number generator, 1120 ... Bit number reduction computing unit, 1121 ... Encryption key, 1130 ... Optical transmitter,
3000 ... receiving device, 3120 ... bit number reduction calculator, 3121 ... encryption key, 3130 ... optical receiver

Claims (20)

A random number generator, an encoder for error correction coding, a laser light source, means for generating phase fluctuation or intensity fluctuation in output light from the laser light source, and a transmitter comprising a modulator,
A receiver comprising means for measuring or determining the phase or intensity of the transmitted signal light and a decoder for error correction code;
A transmission path provided between the transceiver and
A composite sequence obtained by adding the output random number sequence from the random number generator to the input signal sequence is encoded by the encoder, and the code is divided into n bits (n is an integer of 1 or more) to obtain 2 ⁿ values. Superimposed on the laser beam with the fluctuation in the modulator,
The number of transmission bases at the time of code superposition is n + 1 or more, and a seed key consisting of random numbers is shared in advance between the transmitter side and the receiver side, and the transmission base of the first code is the seed key The transmission base of the second and subsequent codes is determined by the seed key and / or the output random number sequence from the random number generator,
On the receiver side, the receiving base of the first code is determined by the value of the seed key, and the receiving base of the second and subsequent codes is the seed key and / or the code transmitted after the first one. Determined by the output random number sequence from the random number generator constituting, and the code superimposed light transmitted through the transmission line using the determined basis is received and decoded into the combined sequence by the decoder Then, the decoded composite sequence is separated into the random number sequence and the signal sequence, and the separated random number sequence is used as a seed key shared between the sender and the receiver to determine the reception basis of the second and subsequent codes. An optical communication system, characterized in that it is used in an optical system.   2. The optical communication system according to claim 1, wherein the composite sequence formed from the random number sequence and the input signal sequence is subjected to the error correction coding after being rearranged.   2. The optical communication system according to claim 1, wherein an input signal sequence is encrypted before the composite sequence is formed.   2. The optical communication system according to claim 1, wherein the composite sequence is encrypted before the error correction coding.   The error correction code is set so that error correction is possible on the low BER side at a certain bit error rate (BER), but error correction is impossible on the high BER side, and the BER on the receiver side is set to the low BER side. The optical communication system according to claim 1, wherein an error correction code is set so as to satisfy. The input signal is a random number signal;
2. The optical communication system according to claim 1, wherein the number of bits of the random number signal is reduced by the privacy amplifier from the transmitted and received random number signal, and the random number signal of the reduced number of bits is used for encryption communication as an encryption key.   2. The optical communication system according to claim 1, wherein the laser light output through the means for generating fluctuation is anti-squeezed light.   2. The optical communication system according to claim 1, wherein said means for generating fluctuation comprises an optical fiber, and phase fluctuation is generated by Kerr effect.   2. The optical communication system according to claim 1, wherein said means for generating fluctuation comprises an optical fiber, and phase fluctuation is generated by a Raman effect.   The means for generating fluctuation comprises a second random number generator, and an output random number sequence from the second random number generator is multi-valued and input to the laser light source or the modulator. The optical communication system according to claim 1.   2. The optical communication system according to claim 1, wherein the means for measuring or determining the phase comprises one set or two sets of homodyne detectors.   2. The optical communication system according to claim 1, wherein said means for measuring or determining the phase comprises one set or two sets of balanced detectors composed of two photodetectors. Random number generator, pseudo-random number generator, encoder for error correction coding, laser light source, means for generating phase fluctuation or intensity fluctuation in output light from the laser light source, and transmitter having a modulator When,
Means for measuring or determining the phase or intensity of transmitted signal light; a decoder for error correction code; and a receiver comprising a pseudo-random number generator;
A transmission path provided between the transceiver and
A seed key consisting of random numbers is shared in advance between the transmitter side and the receiver side, and an output pseudo-random number sequence is output from the pseudo-random number generator in the transmitter with a part of the seed key as an input, to the input signal sequence A composite sequence obtained by adding an output random number sequence from the random number generator is encrypted with the pseudo-random number sequence, the encrypted composite sequence is encoded by the encoder, and the code has n bits (n is 1 or more). for each integer) separated by being superimposed on the laser beam with the fluctuations in the modulator as a 2 ⁿ values, the number of transmission basis during code superimposed to the n + 1 or more, transmission of 1 th code The basis is determined by the remaining part of the seed key, the transmission base of the second and subsequent codes is determined by the seed key and / or the output random number sequence from the random number generator, and one of the output random number sequences. Is useful for renewing the input key to the pseudo-random number generator. It is,
On the receiver side, the receiving base of the first code is determined by the value of the seed key, and the receiving base of the second and subsequent codes is the seed key and / or the code transmitted after the first one. Determined by the output random number sequence from the random number generator that constitutes the code superimposed light transmitted through the transmission path using the determined basis, and the combined sequence encrypted by the decoder The encrypted composite sequence is decrypted by the pseudo-random sequence output from the pseudo-random number generator in the receiver using a part of the seed key, and the composite is decrypted. The sequence is separated into the random number sequence and the signal sequence, and the separated random number sequence is used as a seed key newly shared between the sender and receiver, and the reception basis determination of the second and subsequent codes and the input of the pseudo random number generator Light used for key renovation Shin system.   The error correction code is set so that error correction is possible on the low BER side at a certain bit error rate (BER), but error correction is impossible on the high BER side, and the BER on the receiver side is set to the low BER side. 14. The optical communication system according to claim 13, wherein an error correction code is set so that   14. The optical communication system according to claim 13, wherein the laser light output through the means for generating fluctuation is anti-squeezed light.   14. The optical communication system according to claim 13, wherein the means for generating fluctuation comprises an optical fiber, and phase fluctuation is generated by the Kerr effect.   14. The optical communication system according to claim 13, wherein the means for generating fluctuation comprises an optical fiber, and phase fluctuation is generated by a Raman effect.   The means for generating fluctuation comprises a second random number generator, and an output random number sequence from the second random number generator is multi-valued and input to the laser light source or the modulator. The optical communication system according to claim 13.   14. The optical communication system according to claim 13, wherein the means for measuring or determining the phase comprises one set or two sets of homodyne detectors.   14. The optical communication system according to claim 13, wherein the means for measuring or determining the phase comprises one set or two sets of balanced detectors composed of two photodetectors.

Images