JPH0293524A - Optical receiver, photodetector, quantum state controller, and optical communication equipment - Google Patents

Abstract

PURPOSE:To improve the SN ratio or bit error characteristics of a system by providing a received quantum state control function for a coherent optical communication. CONSTITUTION:Laser light emitted by the light source 5 of an optical transmitter 1 is inputted to a quantum state controller 8 through an optical modulator 6 and a transmission line 2 for communication. Here, the signal light is converted from a coherent state to a squeezed state by a squeezer 9 and injection synchronization is carried out by a self-oscillating slave laser 10 with the signal light outputted by the squeezer 9. Then an optical homodyne detecting device 3 multiplexes signal light outputted by the controller with local oscillation light outputted by a local oscillation light source 12 by an optical multiplexer 11. The multiplexed signal light is restored by the photodetector 13 to a received signal 14 nearly equal to a sent signal, then outputted. Consequently, the SN ratio of the signal light is improved and the code error rate of the received signal is decreased.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides an optical receiving device and an optical receiving device that significantly improve the signal-to-noise ratio or bit error characteristics of a system by providing a reception quantum state control function to coherent optical communication. The present invention relates to a detection device, a quantum state control device, and an optical communication device.

[Prior art] The signal-to-noise ratio of a coherent optical communication system is determined by the quantum noise of the signal light, the local oscillation 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 referred to as Document 1).In recent years, the quantum state of light has been controlled to further improve the signal-to-noise ratio. Applications of physical phenomena have been proposed to reduce the quantum noise of signal light. For this application, a technology called squeezed state (or two-photon coherent state) generation technology is used (Literature, Fizcal Review, Ni, Volume 13. Number 6 (1976), pp. 2226 to 2243). (Ph
physical Review A13 Na6 Jun
e 1976, PP2226-2243)). Here, the squeezed state is a state in which two orthogonal amplitude components of light have different quantum noises. On the contrary,
Light conventionally used in coherent optical communications is called a coherent state, and 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. The quantum noise for two orthogonal amplitude components of the signal light are both 1 (normalized power) in the coherent state, but in the squeezed state, the quantum noise of one component is i e-z and the other is upper eZ. However, Z is a squeezed parameter. As a result, when the quantum state of the transmitted light is a coherent state (general coherent optical communication), the theoretical value of the signal-to-noise ratio of the system is 4〈
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), where <n> is 0 or more, which is the average number of photons of the pulsed signal light.By putting the signal light in a squeezed state, the signal-to-noise ratio can be greatly improved. For details, see Reference 1. It is described in Chapter 2.

[Problems to be Solved by the Invention] In the application technology for quantum state control described above, if energy loss occurs in the communication transmission line, the squeezed state of the signal light is destroyed, the signal-to-noise ratio deteriorates, and the quantum state control becomes difficult. This completely eliminates the advantages of this method, resulting in serious difficulties in the application of actual communication systems. Details of this ILJ can be found in Reference 1. It is described in Chapter 2.

The purpose 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 receiving device and a photodetecting device that obtain much higher signal-to-noise characteristics than conventional coherent optical communication systems. be.

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 for Solving the Problem] The above object is to provide a device (squeezer) that converts the quantum state of signal light into a squeezed state and a signal output from the squeezer in a conventional coherent communication optical receiver. This is realized by providing a slave laser that is injection-locked by light and an optical homodyne detection device that performs optical homodyne detection of the signal light output from the slave laser.

The other objects described above are achieved by providing a squeezer and a slave laser injection-locked by the signal light output from the squeezer.

The other objects described above are achieved by providing a conventional optical transmitter, a conventional communication transmission line for transmitting signal light, and the above-mentioned apparatus for receiving signal light output from the transmission line.

[Operation] The present invention is based on a new discovery of the operation of a receiving quantum state controller.

For example, a light source for an optical transmitter has a coherent state, and the signal is PSK (phase shift keying) using a cosine component of a light wave. However, even in the case of ASK (amplitude shift keying) using the cosine component of light waves, the PS
The same effect as in the case of K can be obtained. 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> roar, where 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 converted into squeezed light by a squeezer. The signal of the cosine component of the converted light and the quantum noise are G
It is amplified twice, and 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<nR>. 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 line axis represents the amplitude of the sine component.

ER is the electric field amplitude of the cosine component of the squeezer human-powered light, which is equal to Tσ.Next, an optical injection locking system using the squeezer output light as the injection signal light is constructed. Hereinafter, E, ω, and φ are respectively Amplitude of electric field.

Let it represent frequency and phase. The slave laser outputs E when there is no injected light. Self-sustained oscillation occurs at cos (ω.t+φ.). This laser has EtCO8(ωmut+
When the light of φ1) is injected, the slave laser is synchronized with the injected light and the Elcos (
ω, t+φ,). The principle of injection locking is described in detail in Chapter 4 of Reference 1.

Here we present the theory of quantum noise characteristics of optical injection-locked systems. To simplify the theoretical formula, EI<Eo, ω=ω.

Assume that At this time, E x = E. It can be considered as Below, the analysis will be performed based on this assumption.

To demonstrate the quantum fluctuation characteristics of optical injection-locked laser output,
- Apply the theory of analyzing the photon coherence characteristics of laser light to m. The fluctuation of the number of photons in a laser is generally expressed as Δn"=E"(1+hE"), where h is a parameter representing excess noise with respect to the ideal laser. When h=o, the ideal laser light and h=1 When , it corresponds to noise-like light. In the case of injection locking, the excess noise is caused by fluctuations in the photon number and phase of the injected light.

Further, the phase fluctuation is the sum of the phase fluctuation of the injection light and the phase fluctuation of the slave laser. As a result, the photon number fluctuation and phase fluctuation of the injection-locked laser output light are Δna2=Eo” (1+ (a+β)E02](
1) Δφ2=Δφin2+Δφ. ′(2). Here, α is a parameter of excessive fluctuation caused by fluctuation in the number of photons of the injected light, β is a parameter of excessive fluctuation caused by phase fluctuation of the injected light, ΔφIn 9
Δφ. ' is the phase fluctuation during self-sustained oscillation of the injected light and the slave laser. If (α+β)=1, the output light of the synchronized laser becomes noise-like light (same as light from a light bulb). If α+β)=0, this corresponds to light in a coherent state. As a characteristic of injection locking, if the slave laser operates with high excitation, the formula (1
) can be regarded as O. On the other hand, β is not suppressed because it is directly affected by the phase fluctuation of the injected light, and β
n''.Equation (1).

If (2) is found, the quantum noise of the cosine component of the injection-locked laser output light shows the noise characteristics of the injection-locked system when the squeezer output light is used as the injection light using the above theory.

The signal amplitude +1qiEn of the communication transmission line output becomes f+11 in the squeezer output, which becomes Et. On the other hand, the quantum noise has a cosine component multiplied by G and a sine component multiplied by one.

This corresponds to the fact that the photon number fluctuation of the cosine component becomes extremely large and the phase fluctuation becomes small. When such light is injected into the slave laser, the quantum noise of the cosine component of the output light after synchronization can be calculated from equations (1) to (3) by detecting only the cosine component of this output light using optical homodyne detection. The signal-to-noise ratio (SNRtc) at that time can be found as follows.

However, <no>=Eo"+<nR): ER2. The S N RR of the optical fiber output light is 4<nR>, and the SNR improvement it (Δ5NR) according to the present invention is given by the following equation.

Equation (7) is a condition under which ΔSNR in Equation (6) becomes larger than 0 (dB), that is, a condition under which the SNR is improved by the present invention. Since 1>(nR)/<00> usually holds, the right side of Equation 1 (7) is approximately equal to Δ(dB). FIG. 9 shows the squeezer gain G(d
The relationship between B) and the amount of improvement Δ5NR (dB) in the signal-to-noise ratio is shown. The parameters are <nR> / <no> (d
B). When <nR>/<n-> is -ω (dB), ΔSNR is proportional to the square of G. On the other hand, <nR>
/<n,> is larger than −ψ(dB), small G
ΔSNR is proportional to the square of G, but as G increases, ΔSNR asymptotically approaches <n,>/<nR>0. For example, when <nR> / <n,> = -60 (dB) In this case, the ΔSNR asymptotically approaches 60 (dB) as G increases.

[Embodiment] Hereinafter, a first embodiment of an optical receiving device, a photodetecting device, a quantum state control device, and an optical communication device of the present invention will be described with reference to FIG. 1. Reference numeral 61 denotes an optical transmitting device. Reference numeral 5 denotes a light source, which can be realized by a laser that oscillates at a single frequency, such as a distributed feedback (DFB) semiconductor laser, a distributed Bragg reflection (DBR) semiconductor laser, a semiconductor laser with an external cavity, and a gas laser. 6 is an optical modulator, which is oscillated by a transmission signal 7; When obtaining PSK-modulated signal light, 6 is a phase modulator, which can be realized by, for example, a modulator using commercially available lithium niobate (L x N b O3). At this time, 6 applies PSK modulation to the coherent light output from 5 using phase 0 radian and redundant radian. When obtaining ASK-modulated signal light, an optical switch or an optical intensity modulator is used as 6. At this time, 6 turns on (passing) and off (cutting) the light output from 5, or switches the light intensity.

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 the quantum state control in! of the present invention! It is. 9 is a device (squeezer) that converts the quantum state of the signal light output from 2 from a coherent state to a squeezed state (power amplification factor G). The squeezer 9 is realized by using, for example, degenerate parametric amplification or degenerate four-wave mixing, and for details of the embodiment, see Physical Review Letters, Volume 57, Number 20. 25th
20-2523, 1986 (Rhys, Re
viewLett,, vol, 57. Na2O,p
p, 2520-pp, 2523.1986),
Regarding the latter, see Science, Vol. 18, No. 7, pp. 48-58.

Discussed in 1988 (Nikkei Science Publishing). Reference numeral 10 denotes a slave laser that self-oscillates with high excitation, and can be realized by a single frequency laser similar to the light source 5. 10 is injection-locked by the signal light output from 9.

The signal light output from 10 becomes the output of quantum state control device 8.

3 is an optical homodyne detection device. The signal light output from the local light source 12 is multiplexed with the local light output from the local light source 12 by an optical multiplexer 11 such as an optical coupler.

12 can be realized by a single frequency laser similar to light 'g5. Here, the No. 48 light and the local light are combined in a state in which their respective polarization states and phases substantially match. Reference numeral 13 denotes a photodetector, which restores and outputs a received signal 14 that is substantially equal to the transmitted signal.

13 can be realized using at least a photo diode (PD). 13 may include circuits included in a normal optical receiving device such as an amplifier and a filter.

Reference numeral 15 denotes an optical receiving device or a detecting device of the present invention, which is composed of at least 8 and 3. The optical communication device of the present invention is composed of at least 1.2 and 15. According to the quantum state control device of this embodiment, it is possible to improve the signal-to-noise ratio of signal light. Moreover, the optical receiving device of this embodiment.

According to the photodetection device and the optical communication device, since optical homodyne detection of the signal light is performed immediately after controlling the quantum state of the signal light, the influence of energy loss in the communication transmission path can be avoided. For signal light having the same bit rate and intensity, it is possible to obtain a received signal with a higher signal-to-noise ratio than in a conventional coherent optical communication system. As a result, the following effects are obtained.

(2) If the intensity and bit rate of the signal light are made the same as in conventional coherent optical communication systems, the bit error rate of the received signal can be lowered than in the conventional system.

(2) If the bit rate and code error rate of the received signal are kept the same as before, the loss allowed for one communication transmission path can be increased. That is, the transmission distance can be increased.

■If the code error rate of the received signal and the intensity of the signal light are kept the same as before, the bit rate can be higher than before.

■By keeping the bit rate and code error rate of the received signal the same as before, the signal light can be split into multiple parts.

A plurality of optical receivers or optical detectors can simultaneously receive signal light.

FIG. 5 shows a second embodiment of the optical communication device of the present invention.
1-1.1-2 and 1-m are optical transmitters, and signal lights of different carrier frequencies output from each are multiplexed by an optical star coupler 22. After the multiplexed signal light is transmitted through the communication transmission line 2, it is branched at the optical star coupler 23 and sent to each optical receiver 115-
1. Enter into 15-2 and 15-n. Each optical receiver can select and receive a desired signal light from the multiplexed signal light by adjusting the frequency of the local light. Here, one of m and r' may be 1, and both may or may not be equal to 1.

According to this embodiment, the same effects as in the first embodiment are obtained, and at the same time, the cost of the optical communication device per signal is reduced because a plurality of optical transmitting devices and optical receiving devices share the same communication transmission path. Get the effect of being able to do it.

FIG. 6 shows a second embodiment of the quantum state control device of the present invention, 8 represents the device, 9 a squeezer,
Further, 10 represents a slave laser. , 16A
, 16B and 16C are polarization plane control devices;
0 and the polarization state of the signal light input to the optical homodyne detection device is approximately optimized. Further, when a change occurs in the polarization state of the signal light due to disturbance, the influence of the change is suppressed.

According to this embodiment, the same effects as in the first embodiment can be obtained, and at the same time, even if the characteristics of the squeezer, slave laser, and optical homodyne detector depend on the polarization state of the signal light, Even if the polarization state of the signal light changes due to disturbances or the like, the effect is always stable. The same effect can be obtained even if chairs 16A, 16B, and 16C are omitted if necessary. Further, the optical receiving device, photodetecting device, and optical communication device using the quantum state control device of this embodiment have the same effects as those of the first and second embodiments, and also have the above-mentioned effects.

FIG. 7 shows a third embodiment of the quantum state control device of the present invention. 8, 9 and 10 are the same as in FIG. 1
Optical isolators 7A, 17B, and 17c suppress the signal light from returning in the opposite direction due to reflection or the like.

According to this embodiment, the same effects as in the first embodiment can be obtained, and at the same time, the characteristic deterioration of the squeezer and slave laser due to reflected light can be prevented, and the frequency generated in the resonator formed by two or more reflection points can be reduced. Generation of amplitude conversion noise can be suppressed. A similar effect is found in 17A, 17B and 17
It can be obtained even if any of C is omitted as necessary. Also,
Similar effects can be obtained by using the polarization plane control device of the second embodiment in combination and placing optical isolation before and after or on one side thereof. Further, the optical receiving device, photodetecting device, and optical communication device using the quantum state control device of this embodiment have the same effects as those of the first and second embodiments, and also have the above-mentioned effects.

FIG. 8 shows an embodiment of a balanced optical homodyne detection device. 12 is a local light source which outputs local light. 1
Reference numeral 8 denotes a phase control device, which operates so that the phase of the signal light input to the optical coupler 11 and the phase of the local light beam are combined in a state where they substantially match. 16 is a polarization plane control device;
Similarly, it operates so that the signal light and the local light are combined in a state where their polarization states substantially match. 16 and 18 may branch and detect a portion of the received signal to control it, or the order of 16 and 18 may be reversed or omitted as necessary.

19A and 19B are PDs that respectively detect the two combined lights. 1 By connecting both as shown in Fig. 7, a signal 20 can be obtained by subtracting the detection signal of one from the detection signal of the other. Can be done. At the same time, the intensity noise of the local light can be suppressed during the subtraction process. Moreover, the power of single station light emission can be used effectively. 20 is an amplifier 21
After being amplified at , it is output as a received signal 14. 2
1 may be omitted.

When the optical homodyne detection device of this embodiment is applied to the above-mentioned optical receiving device, photodetecting device, and optical communication device, the power of the local light can be effectively used, and the intensity noise of the local light can be suppressed, resulting in higher The effect is that a received signal having a high signal-to-noise ratio can be obtained.

[Effects of the Invention] According to the quantum state control device of the present invention, it is possible to improve the signal-to-noise ratio of signal light.

According to the optical receiving device, the optical detecting device, and the optical communication device of the present invention, the built-in quantum state control device can improve the signal-to-noise ratio of the signal light, so that the following effects can be obtained.

(2) If the intensity and bit rate of the signal light are made the same as in conventional coherent optical communication systems, 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 loss allowed in the communication transmission path can be increased more than before. That is, the transmission distance can be increased.

■If the code error rate of the received signal and the intensity of the signal light are kept the same as before, the bit rate can be 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 drawings]

Fig. 1 shows a first embodiment of a quantum state control device, an optical receiver, a photodetector, and an optical communication device of the present invention, Fig. 2 is a block diagram of a conventional coherent optical communication system, and Fig. 3 shows a conventional quantum FIG. 4 is a diagram showing the relationship between squeezer human power, output signal light, and quantum noise; FIG. 5 is a second embodiment of the optical communication device of the present invention; FIG. A second embodiment of the quantum state control device of the present invention, FIG. 7 shows a third embodiment of the quantum state control device of the present invention, FIG. 8 shows an embodiment of an optical homodyne detection device, and FIG. 9 shows optical fiber output light. FIG. Explanation of symbols 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...A device that converts the quantum state of signal light from a coherent state to a squeezed state! (squeezer), 10...
- Slave laser, 11... optical multiplexer, 12... local light source. 13... Photodetector, 14... Received signal, 15...
Optical receiver or photodetector, 16... Polarization control device, 17... Optical isolator, 18... Phase control device, 19... PD (photo diode), 20.
...Signal obtained by subtracting the detection signal by 19B from the detection signal by 19A, 21...Amplifier, 22... Optical star coupler, 23... Optical star coupler. Fig. 2 Fig. 3

Claims (1)

[Claims] 1. A squeezer that converts the quantum state of signal light into a squeezed state, a slave laser that is injection-locked by the signal light output from the squeezer, and a slave laser that converts the signal light output from the slave laser into an optical An optical receiver comprising at least an optical homodyne detection device that performs homodyne detection. 2. Consists of at least a squeezer that converts the quantum state of signal light into a squeezed state, and a slave laser that is injection-locked by the signal light output from the squeezer, and outputs the signal light output from the slave laser. An optical receiver comprising: a quantum state control device; and an optical homodyne detection device that performs optical homodyne detection of an output from the quantum state control device. 3. An optical receiving device, characterized in that the quantum state control device according to claim 2 is placed upstream of an optical homodyne detection device. 4. A squeezer that converts the quantum state of signal light into a squeezed state, a slave laser that is injection-locked by the signal light output from the squeezer, and an optical homodyne detection of the signal light output from the slave laser to achieve the above A photodetection device comprising at least an optical homodyne detection device that detects signal light. 5. A quantum state control device comprising at least a squeezer that converts the quantum state of signal light into a squeezed state, and a slave laser injection-locked by the signal light output from the squeezer. 6. In the optical receiving device according to any one of claims 1 to 3, the gain G of the squeezer is set such that G≧√[(1/(1−
<n_R>/<n_l>)] (where <n_R> is the number of photons per pulse of the signal light, and <n_l> is the number of photons per pulse of the signal light output from the slave laser). optical receiver. 7. The optical receiver according to any one of claims 1 to 3, wherein the polarization state of at least one signal light among the signal light, the signal light output from the squeezer, and the signal light output from the slave laser. 1. An optical receiver comprising at least one polarization state controller for controlling the polarization state controller. 8. The optical receiver according to any one of claims 1 to 3, wherein at least one of the signal light, the signal light converted to the squeezed state, and the signal light output from the slave laser is at least one signal light. An optical receiving device characterized by passing through an optical isolator. 9. The quantum state control device according to claim 5, wherein at least one signal light among the signal light, the signal light converted into the squeezed state, and the signal light output from the slave laser passes through at least one optical isolator. A quantum state control device characterized by passing through. 10. The optical receiving device according to any one of claims 1 to 3, wherein the optical homodyne detection device includes an optical coupler that combines the signal light output from the slave laser and the local light, and an optical coupler that combines the signal light output from the slave laser and the local light output. first and second optical detectors that convert the first and second multiplexed lights into first and second detection signals, respectively; and a subtracter that subtracts the second detection signal from the first detection signal. An optical receiving device comprising: 11. An optical transmitter comprising at least a light source that outputs light of a single frequency, and an optical phase modulator that receives the output light from the light source and is driven by a transmission signal, and this optical transmitter a communication transmission line that transmits signal light output from the communication transmission line; a squeezer that converts the quantum state of the signal light output from the communication transmission line into a squeezed state; and a squeezer that is injection-locked by the signal light output from the squeezer. An optical communication device comprising at least a slave laser and an optical homodyne detection device that performs optical homodyne detection of a signal light output from the slave laser. 12. An optical transmitter comprising at least a light source that outputs light of a single frequency, and an optical switch or optical intensity modulator that receives the output light from the light source and is driven by a transmission signal; A communication transmission line that transmits the signal light output from the transmitting device, a squeezer that converts the quantum state of the signal light output from the communication transmission line into a squeezed state, and an injected signal light output from the squeezer. An optical communication device comprising: an optical receiving device comprising at least a synchronized slave laser; and an optical homodyne detection device that performs optical homodyne detection of signal light output from the slave laser. 13. One or more optical transmitting devices comprising at least a light source that outputs light of a single frequency, and an optical phase modulator that receives the output light from the light source and is driven by a transmission signal. , an optical multiplexer that multiplexes the signal light output from the optical multiplexer, one or more communication transmission lines that transmit the signal light output from the optical multiplexer, and an output from the communication transmission line. a squeezer that selects and receives a desired signal light from the signal light output from the optical splitter and converts the quantum state of the signal light into a squeezed state; one or more optical receivers comprising at least a slave laser that is injection-locked by the signal light output from the squeezer, and an optical homodyne detection device that performs optical homodyne detection of the signal light output from the slave laser; An optical communication device having 14. One or more optical devices consisting of at least a light source that outputs light of a single frequency, and an optical switch or optical intensity modulator that receives the output light from the light source and is driven by a transmission signal. a transmitter, an optical multiplexer that multiplexes the signal light output from the optical multiplexer, one or more communication transmission lines that transmit the signal light output from the optical multiplexer, and the communication transmission. an optical splitter that branches the signal light output from the optical splitter; and a squeezer that selects and receives a desired signal light from the signal light output from the optical splitter and converts the quantum state of the signal light into a squeezed state. and one or more optical receivers comprising at least a slave laser that is injection-locked by the signal light output from the squeezer, and an optical homodyne detection device that performs optical homodyne detection of the signal light output from the slave laser. An optical communication device comprising: a device;