JP2011146787A - Optical communication device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical communication device capable of reducing crosstalk due to a nonlinear optical effect. <P>SOLUTION: This optical communication device includes: a first transmitter for transmitting first light in predetermined polarization; a second transmitter for transmitting second light stronger than the first light in predetermined polarization; a multiplexer for multiplexing the first light and the second light as polarized waves orthogonal to each other; an optical fiber transmission path for transmitting output light from the multiplexer; a polarization controller for controlling the polarization state of the output light from the optical fiber transmission path into a predetermined state; a demultiplexer for demultiplexing the output light from the polarization controller into first light and second light; a first receiver for receiving the first light from the demultiplexer; and a second receiver for receiving the second light from the demultiplexer. Here, the polarization controller is used to collectively control the first light and the second light into a predetermined polarization state by monitoring the polarization state of the second light to control the second light into the predetermined polarization state. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical communication device that multiplex-transmits extremely weak light and strong light, such as a quantum cryptography communication device.

  For example, in quantum cryptography communication that transmits highly weak signal light and performs highly confidential communication that is difficult to eavesdrop, in order to transmit a clock and other control information between the transmission device and the reception device, It has been considered that signal light of normal intensity is transmitted as a separate channel. Hereinafter, weak intensity light that performs quantum communication utilizing quantum mechanical phenomena is referred to as quantum light, and normal intensity light that performs classical dynamic communication is referred to as classical light.

FIG. 6 is a block diagram showing a general quantum cryptography communication device.
In FIG. 6, this quantum cryptography communication device includes a quantum optical transmitter 51, a classical optical transmitter 52, a wavelength multiplexer 53, an optical fiber transmission line 54, a wavelength demultiplexer 55, a quantum optical receiver 56, and a classical optical receiver. Instrument 57.

  The quantum light transmitter 51 and the classical light transmitter 52 output quantum light and classical light having different wavelengths to the wavelength multiplexer 53, respectively. The wavelength multiplexer 53 multiplexes the quantum light and the classical light, transmits the optical fiber transmission path 54, and outputs it to the wavelength demultiplexer 55. The wavelength demultiplexer 55 demultiplexes the combined light into quantum light and classical light, and outputs them to the quantum light receiver 56 and classical light receiver 57, respectively. At this time, as shown in FIG. 7, the quantum light and the classical light can be transmitted through the same optical fiber transmission line 54 by arranging them at different wavelengths.

The transmission intensity of quantum light is 10 log {(hc / λ) × f × when, for example, light having a wavelength λ = 1.55 μm and 1-bit information is transmitted with a repetition frequency f = 100 MHz by a single photon. It becomes a very weak value of 10 ³ } = − 78.9 dBm. However, c (speed of light) = 3.0 × 10 ⁸ m / s and h (Planck's constant) = 6.6 × 10 ⁻³⁴ J · s.

  On the other hand, the transmission intensity of classical light is, for example, −10 dBm to 0 dBm. In this example, the intensity is 50 dB or more stronger than quantum light. Here, when classical light is transmitted through the same optical fiber transmission line as quantum light, even if slight crosstalk from classical light to quantum light occurs, the transmission error rate of weak quantum light is greatly degraded. There was a problem.

  Therefore, in order to solve such problems, there is known a crosstalk elimination method that reduces crosstalk from classical light to quantum light by optimizing the transmission characteristics of the wavelength multiplexer and wavelength demultiplexer. (For example, refer to Patent Document 1).

JP 2006-101491 A

However, the prior art has the following problems.
When light is transmitted through an optical fiber transmission line, crosstalk due to a nonlinear optical effect occurs in light between different wavelengths. As a specific example, a phenomenon called stimulated Raman scattering is known, and crosstalk occurs when the energy of classical light transitions to the wavelength of quantum light.

  Photons generated by such crosstalk have the same wavelength as quantum light. Therefore, it cannot be distinguished and removed by the difference in wavelength. That is, the crosstalk elimination method based on the optimization of the transmission characteristics of the wavelength multiplexer and the wavelength demultiplexer disclosed in Patent Document 1 has a problem that the influence of the crosstalk light due to the nonlinear optical effect cannot be avoided. is there.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain an optical communication apparatus capable of reducing crosstalk due to a nonlinear optical effect.

  The optical communication apparatus according to the present invention includes a first transmitter that transmits first light with a predetermined polarization, and a second transmitter that transmits second light having a stronger intensity than the first light with a predetermined polarization. A transmitter, a multiplexer that combines the first light and the second light as polarizations orthogonal to each other, an optical fiber transmission path that transmits output light from the multiplexer, and an optical fiber transmission path A polarization controller that controls the polarization state of the output light from the polarization controller to a predetermined state, a demultiplexer that demultiplexes the output light from the polarization controller into the first light and the second light, The polarization controller includes a first receiver that receives the first light from the demultiplexer and a second receiver that receives the second light from the demultiplexer. By monitoring the wave state and controlling the second light to a predetermined polarization state, the first light and the second light are collectively controlled to a predetermined polarization state.

According to the optical communication device of the present invention, the multiplexer synthesizes the first light from the first transmitter and the second light from the second transmitter as orthogonal polarizations, Output to the optical fiber transmission line. In addition, the polarization controller monitors the polarization state of the second light out of the output light from the optical fiber transmission line, and controls the second light to a predetermined polarization state, thereby controlling the first light. The light and the second light are collectively controlled to a predetermined polarization state.
Therefore, an optical communication device that can reduce crosstalk due to the nonlinear optical effect can be obtained. In addition, stable optical communication that does not depend on fluctuations in the polarization state can be realized.

It is a block block diagram which shows the quantum cryptography communication apparatus which concerns on Embodiment 1 of this invention. It is a block block diagram which shows the polarization controller of the quantum cryptography communication apparatus concerning Embodiment 1 of this invention. It is explanatory drawing for demonstrating the function of the polarization controller of the quantum cryptography communication apparatus which concerns on Embodiment 1 of this invention. It is a block block diagram which shows the quantum cryptography communication apparatus which concerns on Embodiment 2 of this invention. It is a block block diagram which shows the quantum cryptography communication apparatus which concerns on Embodiment 3 of this invention. It is a block block diagram which shows a general quantum cryptography communication apparatus. It is explanatory drawing which shows the relationship between the quantum light and classical light which are each output from the quantum light transmitter and classical light transmitter which were shown in FIG.

Hereinafter, preferred embodiments of the optical communication apparatus of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts will be described with the same reference numerals.
In the following embodiment, a case will be described in which this optical communication device is a quantum cryptography communication device.

Embodiment 1 FIG.
FIG. 1 is a block configuration diagram showing a quantum cryptography communication apparatus according to Embodiment 1 of the present invention.
In FIG. 1, this quantum cryptography communication device includes a quantum light transmitter 1 (first transmitter), a classical light transmitter 2 (second transmitter), a polarization beam combiner 3 (PBC: Polarization Beam Combiner), optical Fiber transmission path 4, polarization controller 5, polarization splitter 6 (PBS: Polarization Beam Splitter), quantum light receiver 7 (first receiver), and classical light receiver 8 (second receiver) ing.

Hereinafter, the function of each part of the quantum cryptography communication device will be described.
The quantum light transmitter 1 outputs quantum light (first light) having a weak intensity with a predetermined linearly polarized wave (see an arrow A in FIG. 1). The classical light transmitter 2 outputs classical light (second light) having a stronger intensity than the quantum light in a predetermined linearly polarized wave (see arrow B in FIG. 1).

  The polarization multiplexer 3 combines the quantum light from the quantum light transmitter 1 and the classical light from the classical light transmitter 2 as linearly polarized waves (see arrow C in FIG. 1) orthogonal to each other, Output to the optical fiber transmission line 4. As the polarization multiplexer 3, a bulk crystal type, a fiber fusion drawing type coupler, or the like can be applied. An arrow C in FIG. 1 schematically shows that the quantum light and the classical light are in a polarization state orthogonal to each other. Note that the path from the quantum optical transmitter 1 and the classical optical transmitter 2 to the polarization multiplexer 3 is preferably stabilized using a polarization maintaining fiber in order to keep the light in a predetermined polarization state. .

  The optical fiber transmission line 4 transmits the quantum light combined with the polarization multiplexer 3 and the classical light and outputs them to the polarization controller 5. The polarization state of quantum light and classical light changes along the longitudinal direction of the optical fiber transmission line 4 during propagation (see arrow D in FIG. 1). In addition, since the amount of change in the polarization state in the optical fiber transmission line 4 fluctuates due to changes in conditions such as temperature, the polarization state of the quantum light and classical light after propagation through the optical fiber transmission line 4 is It becomes an unstable state that is not fixed with respect to time.

  The polarization controller 5 controls the polarization state to stabilize and control the quantum light and classical light to linearly polarized waves orthogonal to each other (see arrow E in FIG. 1). The detailed configuration and function of the polarization controller 5 will be described later. The polarization demultiplexer 6 has a function of demultiplexing two linearly polarized waves orthogonal to each other, and demultiplexes the combined light into quantum light and classical light (arrows F, G), and output to the quantum optical receiver 7 and the classical optical receiver 8, respectively. The polarization demultiplexer 6 can use the same components as the polarization multiplexer 3, and a bulk crystal type, a fiber fusion stretched coupler, or the like can be applied.

  The quantum light receiver 7 receives the quantum light from the polarization demultiplexer 6, and the classical light receiver 8 receives the classical light from the polarization demultiplexer 6. Information modulated into quantum light and classical light by the quantum light transmitter 1 and the classical light transmitter 2 is demodulated by the quantum light receiver 7 and the classical light receiver 8, respectively. In addition to realizing encrypted communication, it is possible to transmit clocks and other control information necessary for classical light using classical light.

Next, the detailed configuration and function of the polarization controller 5 will be described.
FIG. 2 is a block configuration diagram showing the polarization controller 5 of the quantum cryptography communication apparatus according to Embodiment 1 of the present invention.
In FIG. 2, the polarization controller 5 controls the polarization state of the combined quantum light and classical light transmitted through the optical fiber transmission line 4 and input to the input terminal 21, and the quantum light and classical light are controlled. The light is stabilized and controlled to linearly polarized waves orthogonal to each other, and output from the output terminal 22.

The polarization controller 5 includes a polarization controller 23, a branching device 24, a wavelength filter 25, a polarizer 26, a photodiode 27, and a control circuit 28.
The polarization controller 23 changes the polarization state of the input quantum light and classical light. The branching device 24 branches the output light from the polarization controller 23 into output light from the polarization controller 5 and monitor light. The wavelength filter 25 passes only light that satisfies a specific wavelength condition with respect to the monitor light from the branching device 24.

  The polarizer 26 allows only light having a specific linear polarization to pass through the output light from the wavelength filter 25. The photodiode 27 receives the output light from the polarizer 26 and outputs an electrical signal corresponding to the received light intensity. The control circuit 28 controls the polarization controller 23 so that the electric signal from the photodiode 27 is maximized, and controls the output polarization from the polarization controller 5 to a predetermined linear polarization.

  Here, the wavelength filter 25 is a filter having a wavelength characteristic that allows classical light to pass through but blocks quantum light, and thus the polarization state of the classical light can be accurately monitored. Note that the wavelength filter 25 can be omitted because the intensity of the quantum light is extremely weak and has almost no influence on the classical light. In the polarization controller 5 of FIG. 2, the wavelength filter 25 is provided between the branching device 24 and the polarizer 26, but is not limited thereto, and the order of the polarizer 26 is changed, A wavelength filter 25 may be provided between the polarizer 26 and the photodiode 27.

Next, the polarization states of quantum light and classical light will be described.
First, in order to describe the polarization state, a plane orthogonal to the propagation direction is considered, and two orthogonal linear axes x and y are assumed as shown in FIG. The linear axes x and y are axes along the delay amount given by the polarization controller 23.

  For example, when the quantum light is a linearly polarized wave or an elliptically polarized wave along the x axis of the polarization controller 23, the polarization is as shown in FIGS. In addition, when the classical light is linearly polarized light or elliptically polarized light orthogonal to the quantum light, the polarized light is as shown by c and d in FIG. 3, respectively. The polarization state of quantum light and classical light is determined by the phase difference between the two polarization components.

The polarization controller 23 changes the polarization state by giving a delay amount between the x and y components and changing the phase difference of the polarization components. For example, in the case of light in the wavelength λ = 1.55 μm band, the polarization controller 23 gives a delay amount of (c / λ) ⁻¹ = 0.005 ps, resulting in a phase difference of 360 ° and the polarization state. Go round. That is, the polarization controller 23 can obtain an arbitrary polarization state by changing the delay amount and the axis.

Here, when the wavelengths of the quantum light and the classical light are different from each other, the amount of change in the polarization state is also slightly different. For example, when the wavelength difference between quantum light and classical light is Δλ = 1 nm, the frequency difference is Δν = (c / λ ² ) × Δλ = 0.12 THz. At this time, if the delay amount that the polarization state makes a round is expressed as 360 °, the change amount of the polarization state of the quantum light and the classical light is 360 ° × (0.12 THz) × 0 by the delay amount of 0.005 ps. A small value of 0.005 ps = 0.2 °.

  Therefore, in the case where an arbitrary polarization state is generated by the polarization controller 23, the amount of change in the polarization state of the quantum light and the classical light is almost the same, and only a difference of about 0.2 ° occurs. It can be seen that the orthogonal relationship between light and classical light is maintained almost perfectly.

  That is, the control circuit 28 controls the polarization controller 23 so that the output of the classical light polarizer 26 (electric signal from the photodiode 27) included in the monitor light is maximized. Classical light having a predetermined linear polarization is output from the output end 22. As described above, the wavelength filter 25 may be omitted, but even in that case, the presence of quantum light with extremely weak intensity does not affect the control.

Next, the principle by which the crosstalk due to the nonlinear optical effect is reduced by the quantum cryptography communication device according to the first embodiment will be described.
Crosstalk due to the nonlinear optical effect that occurs in light between different wavelengths in the optical fiber transmission line 4, such as the stimulated Raman scattering described above, is a phenomenon that occurs strongly between light having the same polarization state. It is known that no generation occurs in the polarization state in which the quantum light and the classical light are orthogonal to each other. Therefore, by inputting the quantum light and the classical light into the optical fiber transmission line 4 in a polarization state in which they are orthogonal to each other, crosstalk due to the nonlinear optical effect can be greatly reduced.

  In addition, during the propagation of the optical fiber transmission line 4, a polarization mode dispersion (PMD: Polarization Mode Dispersion) of the optical fiber itself causes a delay between the polarization components of the quantum light and the classical light, and the polarization state Fluctuates and rotates. However, when the difference in wavelength between quantum light and classical light is small, there is no significant difference in the amount of change in the polarization state of quantum light and classical light, and the orthogonal relationship between quantum light and classical light is It is maintained at a considerable transmission distance. This is the same as that in the polarization controller 5 described above, the amount of change in the polarization state of the quantum light and the classical light is substantially the same, and the orthogonal relationship between the quantum light and the classical light is maintained.

  Further, even when the amount of change in the polarization state of the quantum light and classical light generated in the optical fiber transmission line 4 is fluctuated due to fluctuations in conditions such as temperature, as described above, in the polarization controller 5, Since the polarization orthogonal relationship between the quantum light and the classical light is maintained, the quantum light is also output from the output end 22 with the prescribed linear polarization by controlling the output polarization of the classical light to the prescribed linear polarization. Is done. Therefore, at the output end 22 of the polarization controller 5, the quantum light and the classical light are obtained as predetermined linearly polarized waves that are orthogonal to each other. Therefore, the polarization splitter 6 can separate and output the quantum light and the classical light with stable intensity.

As described above, according to the first embodiment, the polarization multiplexer combines the first light from the first transmitter and the second light from the second transmitter that are orthogonal to each other. And output to the optical fiber transmission line. In addition, the polarization controller monitors the polarization state of the second light out of the output light from the optical fiber transmission line, and controls the second light to a predetermined polarization state, thereby controlling the first light. The light and the second light are collectively controlled to a predetermined polarization state.
Further, the polarization controller collectively gives the polarization states of the first light and the second light by giving delay amounts to the different polarization components of the input first light and second light. And a control circuit that controls the polarization controller to be changed so that the monitor light branched by the branching unit satisfies a predetermined condition.
Therefore, an optical communication device that can reduce crosstalk due to the nonlinear optical effect can be obtained. In addition, stable optical communication that does not depend on fluctuations in the polarization state can be realized.

Embodiment 2. FIG.
FIG. 4 is a block diagram showing a quantum cryptography communication apparatus according to Embodiment 2 of the present invention.
In FIG. 4, this quantum cryptography communication device includes a wavelength multiplexer 9 and a wavelength demultiplexer 10, respectively, instead of the polarization multiplexer 3 and the polarization demultiplexer 6 shown in FIG.

  The wavelength multiplexer 9 multiplexes light having different wavelengths while maintaining the polarization axis. The wavelength multiplexer 9 is, for example, a combination of a constant polarization optical fiber and a wavelength multiplexing filter fixed by adjusting the polarization axis, or a fusion-extension type optical coupler of the constant polarization optical fiber. be able to. Here, in the wavelength multiplexer 9, quantum light and classical light having different wavelengths are multiplexed in a polarization state orthogonal to each other.

The wavelength demultiplexer 10 demultiplexes light having different wavelengths while maintaining the polarization axis. The wavelength demultiplexer 10 can use the same components as the wavelength multiplexer 9.
The other configuration is the same as in FIG. 1, and the crosstalk due to the nonlinear optical effect is greatly reduced by inputting the quantum light and the classical light into the optical fiber transmission line 4 in a polarization state in which the light is orthogonal to each other. can do.
As described above, according to the second embodiment, the same effect as in the first embodiment can be obtained.

Embodiment 3 FIG.
FIG. 5 is a block configuration diagram showing a quantum cryptography communication apparatus according to Embodiment 3 of the present invention.
In FIG. 5, this quantum cryptography communication device includes a line switch 11 in addition to the quantum cryptography communication device shown in FIG. The optical fiber transmission line 4, the polarization controller 5, the polarization demultiplexer 6, the quantum light receiver 7 and the classical light receiver 8 after the line switch 11 are divided into two paths, A and B are added after the distinction. Other configurations are the same as those in FIG.

  The line switch 11 selects and switches the propagation path of quantum light and classical light. As the line switch 11, for example, a wavelength selective switch or the like can be applied in addition to a mechanical or magneto-optical switch. By selectively switching the path of the optical fiber transmission line 4A or 4B with the line switch 11, the quantum light receiver 7A and the classical light receiver 8A, the quantum light receiver 7B, and the classical light receiver 8B are switched to provide quantum light. And can transmit classical light.

  At the time of path switching, the polarization controllers 5A and 5B control the quantum light and the classical light to a predetermined polarization state, respectively, so that the quantum light receiver 7A and the classical light receiver 8A, or the quantum light receiver The 7B and the classical light receiver 8B can receive light of stable intensity without being affected by the change in the polarization state. Further, by inputting quantum light and classical light into the optical fiber transmission line 4A or 4B in a polarization state in which they are orthogonal to each other, crosstalk due to the nonlinear optical effect can be significantly reduced.

As described above, according to the third embodiment, the line switch that switches the propagation paths of the first light and the second light is provided between the multiplexer and the optical fiber transmission line.
Therefore, the same effect as in the first embodiment can be obtained, and the first light and the second light can be transmitted by switching the output destination.

  In the first to third embodiments, two linearly polarized waves are given as examples of orthogonal polarized waves, and the polarization states of quantum light and classical light are described. However, the present invention is not limited to this, and as long as the polarizations are orthogonal to each other, for example, the same effects as those in the first to third embodiments can be obtained for the left circular polarization and the right circular polarization.

  In the first to third embodiments, a case where a total of two channels of classical light of one channel and quantum light of one channel are multiplexed and transmitted is described as an example. However, the present invention is not limited to this, and even when multiple channels of classical light with different wavelengths or multiple channels of quantum light with different wavelengths are multiplexed and transmitted, the polarization of the quantum light and classical light can be orthogonalized to The same effect as in the first to third embodiments can be obtained.

  In the first to third embodiments, an example of quantum cryptography communication has been described. However, any optical communication device that multiplex-transmits a plurality of light beams having greatly different intensities is not limited to quantum cryptography communication. The same effect as in the first to third embodiments can be obtained.

  DESCRIPTION OF SYMBOLS 1 Quantum optical transmitter (1st transmitter), 2 Classical optical transmitter (2nd transmitter), 3 Polarization multiplexer, 4, 4A, 4B Optical fiber transmission line 5, 5A, 5B Polarization controller , 6, 6A, 6B Polarization demultiplexer, 7, 7A, 7B Quantum optical receiver (first receiver), 8, 8A, 8B Classical optical receiver (second receiver), 9 wavelength multiplexer, DESCRIPTION OF SYMBOLS 10 Wavelength demultiplexer, 11 Line switch, 23 Polarization controller, 24 Branching device, 25 Wavelength filter, 26 Polarizer, 27 Photodiode, 28 Control circuit.

Claims (8)

A first transmitter for transmitting the first light with a predetermined polarization;
A second transmitter for transmitting a second light having a stronger intensity than the first light with a predetermined polarization;
A multiplexer that combines the first light and the second light as polarizations orthogonal to each other;
An optical fiber transmission line for transmitting output light from the multiplexer;
A polarization controller for controlling the polarization state of the output light from the optical fiber transmission line to a predetermined state;
A demultiplexer for demultiplexing the output light from the polarization controller into the first light and the second light;
A first receiver for receiving the first light from the duplexer;
A second receiver for receiving the second light from the duplexer,
The polarization controller monitors the polarization state of the second light and controls the second light to a predetermined polarization state, thereby changing the first light and the second light. An optical communication device characterized in that it is controlled collectively to a predetermined polarization state.   The optical communication apparatus according to claim 1, wherein the multiplexer is a polarization multiplexer that combines linearly polarized lights orthogonal to each other. The first light and the second light are light having different wavelengths.
The multiplexer is a wavelength multiplexer that multiplexes light having different wavelengths while maintaining a polarization state in which the first light and the second light are orthogonal to each other. The optical communication apparatus according to claim 1.   4. The polarization demultiplexer according to claim 1, wherein the demultiplexer is a polarization demultiplexer that demultiplexes output light from the polarization controller into two orthogonal polarization components. The optical communication device according to any one of the above.   The optical demultiplexer according to any one of claims 1 to 3, wherein the demultiplexer is a wavelength demultiplexer that demultiplexes light having different wavelengths. The polarization controller is
By applying a delay amount to different polarization components of the input first light and second light, the polarization states of the first light and second light are collectively changed. A polarization controller;
A branching device for branching the output light from the polarization controller into output light from the polarization controller and monitor light;
For the monitor light from the splitter, a polarizer that passes only light of a specific polarization, and
A photodiode that receives output light from the polarizer and outputs an electrical signal corresponding to the received light intensity;
A control circuit that controls the polarization controller so that an electrical signal from the photodiode satisfies a predetermined condition;
The optical communication apparatus according to claim 1, further comprising:   The polarization controller blocks the first light and allows only the second light to pass between the splitter and the polarizer or between the polarizer and the photodiode. The optical communication apparatus according to claim 6, further comprising a wavelength filter having wavelength characteristics.   8. A line switch for switching a propagation path of the first light and the second light is provided between the multiplexer and the optical fiber transmission line. The optical communication device according to any one of the above.

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