JP2013197859A - Stray light countermeasure system and stray light countermeasure method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid a phenomenon in which the communication accuracy of single photon communication is not maintained and the cause of which is unknown in the case where a plurality of optical fibers were laid.SOLUTION: A stray light countermeasure system includes: a first optical fiber 11 through which a photon propagates; and a second optical fiber 12 adjacent to the first optical fiber. This system includes a stray light detection device 13 and a photon wavelength determination device 14. The stray light detection device 13 is a device to detect stray light having leaked from the second optical fiber and having penetrated into the first optical fiber. The photon wavelength determination device 14 is a device to obtain the wavelength of the photon used in the first optical fiber 11 by using information about the stray light detected by the stray light detection device 13. Because the stray light detection device 13 measures the stray light travelling from the second optical fiber 12 to the first optical fiber and the photon wavelength determination device 14 can determine a wavelength at which influence of the stray light is small, the stray light countermeasure system can perform information communication by use of the photon with the wavelength at which influence of such stray light is small.

Description

  The present invention relates to a stray light countermeasure system and a stray light countermeasure method. More specifically, the present invention relates to a stray light countermeasure system and a stray light countermeasure method that reduce the influence of stray light that leaks from an optical fiber adjacent to an optical fiber through which photons propagate and enters the optical fiber through which photons propagate.

  Optical fiber usually covers the core with light blocking material. For this reason, it was not assumed that light leaked from the optical fiber. Moreover, it was not assumed that the light once leaked from the optical fiber would enter another optical fiber. On the other hand, single photon communication is currently being performed in which information communication is performed using photons. In single photon communication, for example, a quantum key is created using a single photon, and encryption communication with extremely high confidentiality can be performed (Japanese Patent Laid-Open No. 2011-182283).

  In order to realize a quantum key distribution system, attempts have been made to install a plurality of optical fibers.

JP 2011-182283 A

  In fact, when multiple optical fibers were installed, a phenomenon of unknown reason occurred that could not maintain the communication accuracy of single photon communication. The present inventors have found that when a plurality of optical fibers are laid, light (stray light) that is emitted from an adjacent optical fiber and strays into a certain optical fiber becomes a major problem in performing single-photon communication. .

  Therefore, an object of the present invention is to provide a stray light countermeasure system and a stray light countermeasure method that can appropriately avoid the influence of the stray light.

  Even if a plurality of optical fibers are laid, the present invention basically detects stray light whose influence has not been studied so far, and uses photons of wavelengths that are not affected by the detected stray light. Based on the knowledge that the influence of stray light from adjacent optical fibers can be suppressed. Furthermore, the present invention is based on the knowledge that the stray light can be appropriately analyzed by using an analyzer that combines a wavelength tunable narrowband filter and a single photon detector.

  The first aspect of the present invention relates to an optical communication system using photons. This system is usually laid with multiple optical fibers. And the laid optical fiber has the 1st optical fiber 11 which a photon propagates, and the 2nd optical fiber 12 adjacent to a 1st optical fiber. This system includes a stray light detection device 13 and a photon wavelength determination device 14. The stray light detection device 13 is a device for detecting stray light that leaks from the second optical fiber and enters the first optical fiber. The photon wavelength determination device 14 is a device for obtaining the wavelength of a photon used in the first optical fiber 11 using information on stray light detected by the stray light detection device 13.

  In this system, since the stray light detection device 13 measures stray light and the photon wavelength determination device 14 can determine a wavelength that is less affected by stray light, light (photons) having a wavelength less affected by such stray light is used. The used information communication can be performed.

  In a preferred embodiment of this system, the stray light detection device 13 includes a variable wavelength narrow band filter 21 and a single photon detector 22. In a preferred embodiment of this system, the stray light detection device 13 relates to a device that measures the wavelength and intensity (count number) of stray light that has entered the first optical fiber.

  The single-photon detector 22 scans the photons output from the first optical fiber while the variable-wavelength narrowband filter 21 sweeps the wavelength region including the optical wavelength region used for communication in the adjacent optical fiber, for example. By detecting, the stray light spectrum which totaled the wavelength and count number of the stray light can be obtained. For this reason, the photon wavelength determination device 14 can select a wavelength region with less influence of stray light from a wavelength region in which a single photon can be output using information on the stray light spectrum.

  In the preferred embodiment of this system, the photon wavelength determining device 14 stores the wavelength region of photons that can be output to the first optical fiber 11, and the intensity of the stray light received from the stray light detection device 13 is stored in the stored wavelength region of the photons. The present invention relates to an apparatus for obtaining the wavelength of a photon used in a first optical fiber 11 by obtaining the smallest wavelength region or a wavelength region smaller than a predetermined threshold.

  The system of this aspect can automatically determine the wavelength of a photon with little influence of stray light and output such a photon by using, for example, a computer.

  A preferred embodiment of this system further includes a quantum key distribution device 31 that generates a quantum key using a photon having a wavelength obtained by the photon wavelength determination device 14 and outputs the quantum key to the first optical fiber 11.

  Since the system of this aspect can generate a quantum key having a photon wavelength with little influence of stray light, it functions as an extremely accurate quantum key communication system.

  The second aspect of the present invention determines the wavelength of a photon output to the first optical fiber 11 in an optical fiber system in which a plurality of optical fibers including the first optical fiber 11 through which photons propagate is laid. Regarding the method. Then, this method uses the stray light detection step for detecting stray light that has entered the first optical fiber, and the information on the stray light detected in the stray light detection step, to determine the wavelength of the photon used in the first optical fiber 11. And a desired photon wavelength determining step.

  The present invention provides a stray light countermeasure system and a stray light countermeasure method capable of appropriately avoiding the influence of stray light by appropriately grasping the wavelength and intensity of stray light and selecting light or photons having a wavelength that is less affected by stray light. Can do.

FIG. 1 is a conceptual diagram for explaining a single photon communication system of the present invention. FIG. 2 is a flowchart of the single photon communication of the present invention. FIG. 3 is a graph showing a stray light spectrum in the example.

  The present invention will be described below with reference to the drawings. The present invention is not limited to the following description, and includes modifications appropriately made within the scope obvious to those skilled in the art.

  FIG. 1 is a conceptual diagram for explaining a single photon communication system of the present invention. This system is usually laid with multiple optical fibers. As shown in FIG. 1, the installed optical fiber includes a first optical fiber 11 through which photons emitted from the light source 10 propagate and a second optical fiber 12 adjacent to the first optical fiber. Have. This system includes a stray light detection device 13 and a photon wavelength determination device 14. The single photon communication system is a system for performing information communication using a single photon. An example of a transmission path for propagating photons of this system is an optical fiber. An example of a single photon communication system is a quantum key distribution system. Quantum key distribution systems using single photons are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2011-182283, 2011-109302, 2011-66497, Japanese Patent No. 4625907, and Japanese Patent No. 4724035. As it is already known. This specification is incorporated by reference in its entirety. For example, the second optical fiber 12 adjacent to the first optical fiber may be in a state where the second optical fiber 12 is in contact with the first optical fiber, or the core of the first optical fiber. And the core of the second optical fiber 12 may be 1 cm or more and 1 m or less, or may be 1 cm or more and 10 cm or less. The optical fiber has a core part through which photons or light propagates and a part surrounding the core part. Although the number of cores in an optical fiber is usually one, one optical fiber may include a plurality of cores. In that case, each core functions as an optical fiber. For this reason, the present invention includes a case where two or more cores are included in one optical fiber, and a photon derived from one core strays into another core. That is, for so-called multi-core fibers, each core functions as the optical fiber of the present invention. Since the multi-core fiber has a short distance between the cores and does not separate the space, the system of the present invention can be used particularly effectively.

  In this system, since the stray light detection device 13 measures stray light and the photon wavelength determination device 14 can determine a wavelength that is less affected by stray light, light (photons) having a wavelength less affected by such stray light is used. The used information communication can be performed.

  The stray light detection device 13 is a device for detecting stray light that leaks from the second optical fiber and enters the first optical fiber. An example of the stray light detection device 13 includes a variable wavelength narrow band filter 21 and a single photon detector 22. The variable wavelength narrow band filter 21 is an optical filter that can change the wavelength region of the transmitted light. The light transmission region of the variable wavelength narrow band filter 21 may be any region that covers the wavelength of light propagating in the adjacent optical fiber. Since the wavelength tunable narrowband filter itself is already known, a known wavelength tunable narrowband filter can be appropriately used in the present invention. On the other hand, the tunable narrowband filter 21 of the present invention is preferably connected to a computer 30 to be described later so as to be able to control the transmitted wavelength region. The single photon detector 22 is a device that can measure the number of photons, for example. Then, the stray light detection device 13 selects a wavelength by the variable wavelength narrow band filter 21. A single photon detector 22 then counts the number of photons at the selected wavelength. In this way, the stray light detection device 13 can measure the wavelength and intensity (count number) of stray light that has entered the first optical fiber. That is, the single-photon detector 22 is output from the first optical fiber while the tunable narrowband filter 21 sweeps the wavelength region including the optical wavelength region used for communication in the adjacent optical fiber, for example. By detecting photons, it is possible to obtain a stray light spectrum in which the stray light wavelength and the number of counts are tabulated. For this reason, the photon wavelength determination device 14 can select a wavelength region with less influence of stray light from a wavelength region in which a single photon can be output using information on the stray light spectrum.

  The photon wavelength determination device 14 is a device for obtaining the wavelength of a photon used in the first optical fiber 11 using information on stray light detected by the stray light detection device 13. The photon wavelength determination device 14 stores, for example, a wavelength region of photons that can be output to the first optical fiber 11, and in the stored wavelength region of photons, a wavelength region in which the intensity of stray light received from the stray light detection device 13 is the smallest or The wavelength of a photon used in the first optical fiber 11 is obtained by obtaining a wavelength region smaller than a predetermined threshold. The photon wavelength determination device 14 is connected so that it can receive the output from the stray light detection device 13, for example. The photon wavelength determination device 14 is connected to, for example, a computer 30 that controls the operation of the stray light detection device 13. This computer has an input / output unit, a control unit, a calculation unit, and a storage unit, and appropriately reads out a control program from the storage unit based on the input order, and the control unit causes the calculation unit to perform a predetermined calculation. Can do. For this reason, the photon wavelength determination device 14 receives information on the stray light detected by the stray light detection device 13 and grasps a wavelength region where the stray light count is small. The computer 30 receives the output from the photon wavelength determining device 14 and adjusts the wavelength of the single photon output from the single photon generator that is the light source 10. In this way, this system can automatically determine the wavelength of a photon that is less affected by stray light and output such a photon by using a computer.

  A preferred embodiment of this system further includes a quantum key distribution device 31 that generates a quantum key using a photon having a wavelength obtained by the photon wavelength determination device 14 and outputs the quantum key to the first optical fiber 11. Since the system of this aspect can generate a quantum key having a photon wavelength with little influence of stray light, it functions as an extremely accurate quantum key communication system. Since a quantum key distribution device using a single photon is known, a known quantum key distribution device can be used.

  The quantum key distribution device 31 is a device for distributing quantum keys. The quantum key distribution device may be any device that can generate a quantum key and transmit it to a predetermined device. Examples of quantum key distribution devices are disclosed in Japanese Patent Application Laid-Open No. 2011-182283, Japanese Patent Application Laid-Open No. 2008-209933, Japanese Patent Application Laid-Open No. 2007-318445, and Japanese Patent Application Laid-Open No. 2012-4955. Is. Since it is already known, a known quantum key distribution device can be used as appropriate in the present invention. A method of encrypting information using a quantum key as a random number is also known.

  The quantum cryptography device disclosed in Japanese Patent Application Laid-Open No. 2008-205993 is a quantum cryptography device using an asymmetric Mach-Zehnder interferometer (AMZI). This quantum cryptography device can achieve the maximum drowning state of the polarization mode and the time-bin mode. This quantum cryptography device mainly carries information on the degree of freedom of a time bin pulse and propagates it through a transmission line such as a fiber. The operating principle of this quantum key distribution device is disclosed in Japanese Patent Laid-Open No. 2008-205993. Therefore, those skilled in the art can obtain the quantum key distribution device used in the present invention based on this publication.

  A quantum key distribution device disclosed in Japanese Patent Application Laid-Open No. 2007-318445 includes a light source that generates a entangled photon pair and a generated quantum entangled photon pair in two bands, a high frequency band and a low frequency band at the center frequency. There are those having a band separation filter that separates them. This device is a first communication device that receives separated high frequency band light, and divides the high frequency band light into a plurality of wavelength channels, and measures the high frequency band light in each wavelength channel. Have. Furthermore, this device is a second communication device that receives the separated light in the low frequency band, and divides the light in the low frequency band into a plurality of wavelength channels and measures the light in the low frequency band in each wavelength channel. Have a device to The operating principle of this quantum key distribution device is disclosed in Japanese Patent Application Laid-Open No. 2007-318445. Therefore, those skilled in the art can obtain the quantum key distribution device used in the present invention based on this publication.

  The quantum key distribution device disclosed in Japanese Patent Application Laid-Open No. 2012-4955 generates two types of photon pairs having different wavelengths, and enters each of them into an asymmetric Mach-Zehnder interferometer to make a time-bin drowning state. . The photon pair having two types of wavelengths has a wavelength suitable for optical fiber transmission and a wavelength suitable for free space transmission and photon detection. This quantum key distribution system can achieve quantum key distribution (QKD) over a long distance by performing swapping using a plurality of pairs of twisted photon pairs having two wavelengths. The operating principle of this quantum key distribution device is disclosed in Japanese Patent Application Laid-Open No. 2012-4955. Therefore, those skilled in the art can obtain the quantum key distribution device used in the present invention based on this publication.

  The quantum key in this specification may be a quantum key generated by a quantum key distribution device, or the quantum key distribution device includes a genuine random number generator or a pseudo-random number generator and is generated by these random number generators. It may be a key such as a common random number.

  In a preferred embodiment of the system of the present invention, the above stray light detection is performed at a predetermined frequency, and the wavelength of a single photon used for communication is automatically changed. On the other hand, in single photon communication, in addition to stray light, accurate communication may not be possible due to attacks from third parties. Therefore, a preferred embodiment of this system further includes a bit error rate measuring device 23 for measuring the bit error rate. That is, the system of this aspect measures the bit error rate at a predetermined frequency, and determines that the user is under attack when the bit error rate exceeds a predetermined threshold. Then, when the system is under attack, for example, the single photon communication is interrupted.

  The second aspect of the present invention determines the wavelength of a photon output to the first optical fiber 11 in an optical fiber system in which a plurality of optical fibers including the first optical fiber 11 through which photons propagate is laid. Regarding the method. This method uses a stray light detection step (S101) for detecting stray light that has entered the first optical fiber, and a photon used in the first optical fiber 11 by using information on stray light detected in the stray light detection step. And a photon wavelength determining step (S102) for determining the wavelength of the light. In this method, the photons having the wavelength determined in the photon wavelength determining step (S102) are output to the first optical fiber (the photon output step (S103) is performed using the photons having the wavelength that is less affected by stray light. However, in the present invention, the stray light detection step (S101) and the photon wavelength determination step (S102) are repeatedly performed to grasp the wavelength region where the influence of the stray light is small, and the influence of the stray light. It is preferable to output photons with few wavelengths to the first optical fiber, and to control the photons output to the first optical fiber by measuring the bit error rate described above in real time. Is preferred.

  An optical fiber was laid to make 100 round trips at approximately 45 km between the National Institute of Information and Communications Technology (Koganei City, Tokyo: NICT) and Otemachi. The 92nd, 97th, 98th and 99th loops returning to NICT were designated as optical fibers 1, 2, 3 and 4, respectively. A tunable narrowband filter and a single photon detector were connected to the optical fibers 1, 2, 3, and 4, respectively. In addition, the transmission band of the tunable narrowband filter was swept, and the photon count at each wavelength was detected using a single photon detector. The result is shown in FIG. In this example, it was found that the influence of stray light was small near 1549 nm. Therefore, the quantum channel was adjusted to a wavelength of 1549 nm as a single photon. By using a single photon with a wavelength that is less affected by stray light, a highly accurate quantum key distribution system could be realized.

  The present invention can be used in the field of information communication.

DESCRIPTION OF SYMBOLS 10 Light source; 11 1st optical fiber; 12 2nd optical fiber; 13 Stray light detection apparatus; 14 Photon wavelength determination apparatus;
21 Tunable narrowband filter; 22 Single photon detector; 23 Bit error rate measuring device;
30 computers; 31 quantum key distribution device

Claims (6)

A first optical fiber (11) through which photons propagate;
A second optical fiber (12) adjacent to the first optical fiber;
A stray light detection device (13) for detecting stray light leaking from the second optical fiber and entering the first optical fiber;
A photon wavelength determining device (14) for determining a wavelength of a photon used in the first optical fiber (11) using information on the stray light detected by the stray light detecting device (13);
Having a system. The system of claim 1, comprising:
The stray light detection device (13)
A tunable narrowband filter (21) and a single photon detector (22);
system. The system according to claim 1 or 2, wherein
The stray light detection device (13)
An apparatus for measuring the wavelength and intensity of stray light that has entered the first optical fiber.
system. A system according to claim 3, wherein
The photon wavelength determining device (14) includes:
A wavelength region of photons that can be output to the first optical fiber (11) is stored, and in the stored wavelength region of the photons, a wavelength region in which the intensity of stray light received from the stray light detection device (13) is the smallest or a predetermined value is stored. An apparatus for determining a wavelength of a photon used in the first optical fiber (11) by determining a wavelength region smaller than a threshold;
system. The system of claim 1, comprising:
A quantum key distribution device (31) for generating a quantum key using a photon having a wavelength determined by the photon wavelength determination device (14) and outputting the quantum key to the first optical fiber (11);
system. A method for determining a wavelength of a photon output to the first optical fiber (11) in an optical fiber system in which a plurality of optical fibers including a first optical fiber (11) through which a photon propagates is laid. ,
A stray light detection step of detecting stray light that has entered the first optical fiber;
A photon wavelength determining step for determining a wavelength of a photon used in the first optical fiber (11) using information on the stray light detected in the stray light detecting step;
Including methods.

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