JPH02287444A - Quantum state control device, optical receiver and optical communication device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a receiving quantum state control device, an optical receiving device, and an optical communication device that aim to significantly improve the receiving sensitivity and bit error rate of coherent optical communications.

[Conventional technology]

The signal-to-noise ratio of a coherent optical communication system is determined by the quantum noise of the signal light, the local light noise of the optical receiver, and the dark current. Its limits are determined by circuit thermal noise. Currently, the signal-to-noise ratio of such systems has been reached to the limit based on the quantum noise of the signal light. FIG. 2 shows an example of the configuration of a conventional coherent optical communication system. In the figure, reference numeral 1 denotes an optical communication device, which outputs signal light in a coherent state. 2 is a communication transmission line for transmitting signal light, and 3 is an optical homodyne detection device. Details are explained in the document "Optical Communication Theory and Its Applications", Chapter 6, Morikita Publishing, 1988 (hereinafter abbreviated as Document 1). In recent years, in order to further improve the signal-to-noise ratio, applications of physical phenomena that control the quantum state of light and reduce the quantum noise of signal light have been proposed. For this application, a technology called squeezed state (or two-photon coherent state) generation technology is used (Literature, Fiscal Review, Ni, Volume 13. Number 6 (1976), pp. 2226 to 2243). (Ph
ysical Review A 13 No. 6
June 1976, pp, 2226-2243)). Here, the squeezed state is a state in which two orthogonal amplitude components of light have different quantum noises. In contrast, light conventionally used in coherent optical communications is called a coherent state, and the two amplitude components have the same quantum noise. Currently, as an applied method of quantum state control, a method has been proposed in which the quantum state of signal light is controlled in an optical transmitter as shown in FIG. In the figure, reference numeral 4 denotes an optical transmitter that outputs signal light in a squeezed state. 2 and 3 are the second
Same as the figure. When the quantum noise for two orthogonal amplitude components of the signal light is in a coherent state, they are both -(normalized power), where Z is a squeezed parameter. As a result,
If the quantum state of the transmitted light is a coherent state (-coherent optical communication), the theoretical value of the signal-to-noise ratio of the system is 4.
It becomes <n>. On the other hand, when the information signal is the one with smaller quantum noise in the squeezed state, the signal-to-noise ratio is 4(n) (
(n)+1). However, <n> is greater than or equal to 0, which is the average number of photons of the pulse signal light, and by putting the signal light in a squeezed state, the signal-to-noise ratio can be significantly improved. For details, see Reference 1. It is described in Chapter 2. In the application technology of quantum state control mentioned above, if energy loss occurs in the communication transmission line, the squeezed state of the signal light will be destroyed, the signal-to-noise ratio will deteriorate, and the advantage of quantum state control will be completely lost, so it is difficult to use in actual communication. Significant difficulties arise in the application of the system. Details of this phenomenon can be found in Reference 1. It is described in Chapter 2. [Problems to be Solved by the Invention] Conventional quantum state control technology aimed at improving the characteristics of coherent optical communication loses its advantages due to energy loss in the communication transmission line. An object of the present invention is to solve the problem of the deterioration of the advantages of quantum state control due to energy loss, and to realize an optical receiver that obtains much higher signal-to-noise characteristics than conventional coherent optical communication systems. Another object of the present invention is to realize a quantum state control device used in the above device. Another object of the present invention is to realize an optical communication device using the above device.

[Means to solve the problem]

The optical receiver for the above purpose includes an optical branching circuit that branches a signal light into first and second branched lights, a pumping laser that is injection-locked by the second branched light, and an output light from the pumping laser. a double harmonic generator that doubles the frequency of the second harmonic, and a device that is bombed by the output light from the double harmonic generator and that converts the quantum state of the first branched light into a squeezed state and outputs the same. This is achieved by providing a (squeezer) and an optical homodyne detection device for homodyne detecting the output light from the squeezer. The quantum state control device for the other purpose described above includes an optical branching circuit that branches a signal light into first and second branched lights, a pumping laser that is injection-locked by the second branched light, and a pumping laser that separates the signal light into first and second branched lights. Double the frequency of the output light of 2
A double harmonic generator and a device (squeezer) that is pumped by the output light from the double harmonic generator and converts the quantum state of the first branched light into a squeezed state and outputs the same.
This is achieved by providing The optical communication device for the other purpose described above is achieved by providing an optical transmitter, a communication transmission line, and the optical receiver.

OPERATION The present invention is based on new discoveries in the operation of receiver quantum state controllers. The output light from the light source for the optical transmitter has a coherent state, and the signal is binary PSK (phase shift keying) for the phase of the light wave. However, for information 1 and O, P
The phase of the SK signal is assumed to be 90° with respect to Oo. In other words, the optical signal has the amplitude NXc of the cosine component of the light and the amplitude 1t of the sine component of the light.
Compatible with liiXs. When this coherent state light propagates through a communication transmission line with energy loss, its output quantum state also becomes a coherent state. If the signal is detected immediately, the signal-to-noise ratio will be 4 (nR). Here, nR> is the number of photons per optical pulse in the communication transmission line output. This signal-to-noise ratio is commonly referred to as the shot noise limit. In the present invention, coherent light output from a communication transmission line is split into two beams by an optical branching circuit, one beam is sent to the signal light input end of the squeezer, and the other beam is sent to the pumping laser which is the pumping light source of the squeezer.・Inject to the edge. A squeezer is a device that squeezes the cosine component or sine component of incident light (amplifying one and attenuating the other). If the phase of the pumping light is Oo, the cosine component signal and quantum noise of the light are amplified by a factor of G by the squeezer,
The sine component is attenuated to -. The signal-to-noise ratio of the cosine component, which is the information signal, is equal to that of the manual squeezer, which is 4.
<nn>. However, the relationship between signal and noise is as shown in FIG. In the figure, the horizontal axis of the orthogonal coordinates represents the amplitude of the cosine component of the electric field of the signal light, and the vertical axis represents the amplitude of the sine component. ER is the electric field amplitude of the cosine component of the squeezer-human power light, and is equal to J<nR>. On the other hand, when the phase of the pumping light is 180'' (7c), the view, knee, volume 38. Number 9 (1988) pages 4696 to 4711 (Physical Rev.
iew A 38, No. 9, Nov. 1988, pp. 4696-4711)). Therefore, the quantum state control device of the present invention outputs the signal light shown in FIG. 5 in response to the input light shown in FIG. The operating principle of our device, which uses the above characteristics to significantly improve the bit error characteristics of coherent optical communications, is described below. The transmitted light is Er cos(ωst+φ5)=Xc cos ωst
The +Xs sin ω5t modulation signal is a binary PSK with phases φS=O radians and -radians. At this time, the amplitudes of the corresponding transmitted lights are Xc and Xs, respectively, and their absolute values are both equal to the amplitude ET. ωS is the angular frequency of the transmitted signal. The signal light output from the communication transmission line is J KET cos(ωs t+φ5) EERCo5(
(11st+φs) (2) Input to the input end of the sensor. The other signal is injected as an injection locking signal into a laser pumping a squeezer having an oscillation frequency ωS. This injection-locked system requires a frequency double of 2ωS. Therefore, the output of the bombing laser injection-locked to the signal light is generated by a double harmonic generator with a frequency of 2ωS and a phase of 2φS.
is converted to If the output power of the pumping laser is made sufficiently large, the harmonics will also have sufficient power to pump the squeezer. Therefore, the relationship between signal and noise is as shown in FIG. However, JK is the transparency of the transmission path, and JKET=ER holds true. One side is VηERCO8(ω
st+φs), and the other is v/1−ηE*cos(ωst+
φS). However, Vη represents the rate of branching, and 0×η holds true. When the v'qEn component is used as a signal light, it becomes π. Therefore, when the signal component input to the input end of the squeezer is φs=0, the signal and quantum noise are as follows. GS is the degree of squeezing of the squeezer. Also, XC, , , and ΔX 2Cl a ut
+ is the amplitude of the cosine component of the signal light emitted from the squeezer and the dispersion of quantum noise. Similarly, It is expressed as
” c +., )). (Reference 1,
Chapter 6). The H-number determination method for optical homodyne-detected signal output is based on the transmission signal being 1 (corresponding to φS=O) and O (φS=π/
2), the characteristics of the noise distribution become as shown in Figure 6, so the two thresholds +XTH and - as shown in this figure are used.
XT) l. That is, if the detected output is within the range of +XT)l, the signal is determined to be 0, otherwise it is determined to be 1. The error probability Pe at this time is ■ However, erfc represents the number of error interpolations, where XT)
The selection of I is 2xvo when the number of received photons ηER is fixed.
-exp [-2r+E+/] = erfc [74
Eu”].However, the right side of the above equation is the conventional optical PS
This is the error probability characteristic of the K system. At this time, formula (7)%
Equation % From the above, by making Gs sufficiently large, a system with much higher reliability than the conventional optical PSK system can be obtained. FIG. 7 shows the theoretical value of the relative error rate H of the present invention with respect to conventional optical P/SK homodyne detection. The parameters are ER2: (ny+) = 9 and GS,. ,=
5270. The same figure. By increasing K = Gs/Gs+t+++ to greater than 1, the error rate can be improved compared to the conventional method (H<1)
It is shown that. To summarize the above, the effects of the present invention are as follows. (2) If the intensity and bit rate of the signal light are made the same as in the conventional coherent optical communication system, the bit error rate of the received signal can be lowered than in the conventional system. ■ If the bit rate and code error rate of the received signal are kept the same as before, the receiving sensitivity will be improved, so the loss allowed in the communication transmission path can be increased. That is, the transmission distance can be increased. ■ Bit error rate of received signal and intensity of signal light Pe (
l l O) g. By keeping Gs-+ω the same as before, the bit rate can be made higher than before. (2) If the bit rate and code error rate of the received signal are kept the same as before, the signal light can be branched into multiple parts, and multiple optical receivers or photodetectors can receive the signal light at the same time.

Embodiment 1 Hereinafter, one embodiment of an optical receiving device, a quantum state control device, and an optical communication device of the present invention will be described with reference to FIG. 1 is an optical transmitter. 5 is a light source, for example, a distributed feedback type (D
FB) It can be realized by a laser that oscillates at a single frequency, such as a semiconductor laser, a distributed Bragg reflection type (DBR) semiconductor laser, a semiconductor laser with an external cavity, and a gas laser. Reference numeral 6 denotes an optical modulator, which is operated by a transmission signal 7. When obtaining PSK-modulated signal light, 6 is a phase modulator, which can be realized, for example, by a modulator using commercially available lithium niobate (LiNb○). At this time, 6 applies PSK modulation to the coherent state light outputted from 5 using a phase of 0 radian and a zia-one. Reference numeral 2 denotes a communication transmission line for transmitting signal light, which can be realized by, for example, an optical fiber. 2 may be a space. 8 is a quantum state control device of the present invention. Reference numeral 9 denotes an optical branching circuit that branches the communication transmission line output light into two waves. 9 can be realized by, for example, a commercially available optical coupler or half mirror. 10 is a device (
(squeezer) (power amplification factor G). squeezer-10
For example, this is realized using degenerate parametric amplification, degenerate four-wave mixing, etc. For details of the examples, see
Regarding the former, Physical Review Letter No. 57
Volume, number 20. Pages 2520-2523, 198
6 years (Phys, Review. Lett,, Vol, 57, No, 20. pp
, 2520-pp, 2523.1986), and the latter in Science, Volume 18, Number 7.
Discussed in ゜pages 48-58, 1988 (joined Nikkei Science). Reference numeral 11 denotes a highly excited, self-oscillating laser for pumping the squeezer. 11 is injection-locked by the signal light output from 9. 12 is an optical double harmonic generator, which doubles the frequency of the output light of 11; 12 can be realized using a material having optically nonlinear characteristics. The output of 12 is used as a pumping light source for the squeezer 10. At this time, the phase φS of the output light of No. 11 is also doubled. When the transmitted signal is 1′1” (φS=O radian),
The phase of the output light from 12 is 0 radian, while when the transmission signal is "O" (φS=π/2 radian), the phase of the output light from 12 is π radian. Therefore, when the transmitted signal is '1', the squeezer amplifies the cosine component of the signal light, while in the case of II Ojfi,
Amplify the sine component. As a result, the device 8
Signal light in a coherent state (FIG. 8) can be converted into signal light in a squeezed state (FIG. 5) and output. The signal light output from the squeezer 10 becomes the output of the quantum state control device 8. 3 is an optical homodyne detection device. The signal light output from the local light source 14 is multiplexed with the local light output from the local light source 14 by an optical multiplexer 13 such as an optical coupler. 14 can be realized by a single frequency laser similar to light source 5. Here, the signal light and the local light are combined in a state in which their respective polarization states and phases substantially match. 15 is a photodetector, which receives a received signal 1 that is approximately equal to the transmitted signal.
6 is restored and output. 15 can be realized using at least a photo diode (PD). 15 may include circuits included in a normal optical receiving device such as an amplifier and a filter. Reference numeral 17 denotes an optical receiving device of the present invention, and the optical communication device of the present invention is composed of at least 8 and 3. The optical communication device of the present invention is composed of at least 1.2 and 17. According to this embodiment, it is possible to realize a quantum state control device, an optical receiving device, and an optical communication device having the effects described in the [Operation] section with a simple configuration. [Effect of the Invention 1] According to the quantum state control device of the present invention, it is possible to improve the bit error rate characteristics of received light. According to the optical receiving device and the optical communication device of the present invention, since the built-in quantum state control device can improve the bit error rate characteristics of received light, the following effects can be obtained. (2) If the intensity and bit rate of the signal light are made the same as in the conventional coherent optical communication system, the bit error rate of the received signal can be lowered than in the conventional system. ■ If the bit rate and code error rate of the received signal are kept the same as before, the reception sensitivity will be improved, so the loss allowed in the communication transmission path can be greater than before. That is, the transmission distance can be increased. ■ By keeping the bit error rate of the received signal and the intensity of the signal light the same as before, the bit rate can be made higher than before. (2) If the bit rate and code error rate of the received signal are kept the same as before, the signal light can be branched into multiple parts, and multiple pre-receiving devices or photodetectors can receive the signal light at the same time.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a block diagram of a conventional coherent optical communication system, Fig. 3 is a diagram showing a conventional application method of quantum state control, and Fig. 4 is a diagram of a squeezer. FIG. 5 is a diagram showing the relationship between input and output optical signals and noise; FIG. 5 is a diagram showing the relationship between optical signals and noise output from the quantum state control device of the present invention; FIG. Figure 7 shows conventional optical PSK
FIG. 8 is a diagram showing the relative error rate of the present invention with respect to homodyne detection, and FIG. 8 is a diagram showing the relationship between the optical signal and noise that are present in the quantum state control device of the present invention. 1... Optical transmitter, 2... Communication transmission line, 3...
Optical homodyne detection device, 4... Optical transmission device that outputs signal light in a squeezed state, 5... Light source, 6... Optical modulator, 7... Transmission signal, 8... Quantum state control device ,
9... Optical branch circuit, 10... Squeezer-11...
Pumping laser, 12... Optical double harmonic generator, 13... Optical multiplexer, 14... Local light source, 15...
Optical detector, 16... Received signal, 17... Optical receiver of the present invention.

Claims (1)

[Claims] 1. An optical branching circuit that branches a signal light into first and second branched lights, a pumping laser that is injection-locked by the second branched light, and an output light from the pumping laser. a double harmonic generator that doubles the frequency; and a squeezer that is pumped by the output light from the double harmonic generator and that converts the quantum state of the first branched light into a squeezed state and outputs the squeezed state. A quantum state control device comprising: 2. An optical receiver comprising the quantum state control device according to claim 1 and an optical homodyne detection device for homodyne detecting the squeezed state light output from the quantum state control device. 3. An optical communication device comprising an optical transmitter, a communication transmission line, and the optical receiver according to claim 2.