JPH05224267A - Optical receiver - Google Patents

Abstract

PURPOSE:To enable polarized wave diversity reception by homodyne reception by separating the polarized wave of signal light into two orthogonal polarized waves and then receiving the respective polarized waves with local emitted light having an independent phase control system. CONSTITUTION:The receiver consists of a polarized wave separating element 7 which separates the sent signal light 1 into the two orthogonal polarized waves, optical hybrids 8a and 8b, photodetectors 9a and 9b, PLL controllers 10a and 10b, and local emitted light generation parts 11a and 11b which are provided for the separated optical systems, and a multiplexing part 12. The photodetectors 9a and 9b performs the homodyne detection of the signal light mixed by the optical hybrids 9a and 9b and outputs the detection signal to the multiplexing part 12 and PLL controllers 10a and 10b. Thus, the two light beams separated by the polarized light separating element 7 are independently processed by homodyne detection and brought under phase control, so the polarized wave diversity reception by the homodyne system is performed to obtain sufficient signal intensity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver used for coherent optical communication, and more particularly to a technique for performing polarization diversity by homodyne.

[0002]

2. Description of the Related Art Generally, in coherent optical communication, on the receiving side, transmitted signal light is mixed with local oscillator light generated in a receiver, and the interference wave is received and used for transmission of information. At this time, the polarizations of the signal light and the local light must match, and if they do not match, sufficient interference does not occur and the reception sensitivity deteriorates.

Therefore, as a method of matching the polarization of the signal light and the local oscillation light, conventionally, polarization active control and mixing for controlling one polarization of the signal light or the local oscillation light to match the other polarization are mixed. There have been proposed polarization diversity in which light is split into two polarizations orthogonal to each other and received, and polarization scrambling in which signal light is transmitted in a format in which two polarizations orthogonal to each other in time division are switched.

Of these, polarization diversity is relatively popular because it operates stably and operates at high speed. FIG. 6 is a configuration diagram showing a conventional example of a coherent optical receiver using such polarization diversity, and the principle thereof will be described below based on the same figure.

In FIG. 1, a signal light 1 and a local light generator 2 are provided.
The local light emitted from is supplied to the coupler 3 and mixed. Then, the outgoing wave of the coupler 3 is separated into two polarizations orthogonal to each other by the polarization separation element 4, and each polarization is received by the individual optical receivers 5a and 5b. At this time,
The polarization of the local light is adjusted in advance so that the same power is incident on the optical receivers 5a and 5b.

Since the light incident on each of the optical receivers 5a and 5b has the same polarization determined by the axis of the polarization separation element for both the signal light and the local light, the total power of the incident light interferes. It will be. After that, the two optical receivers 5a,
By adding the outputs of 5b, it becomes possible to receive all of the transmitted signal light, and theoretically, there is no deterioration of the reception sensitivity due to polarization fluctuation.

According to such a principle, the polarization diversity can sufficiently cope with the high-speed polarization fluctuation, the fluctuation of the light receiving sensitivity is small, and the signal will not be lost no matter how the polarization changes. As a result, reliable signal reception is guaranteed.

Coherent optical communication is roughly classified into two types, a heterodyne system and a homodyne system. Among them, in the heterodyne, the optical frequencies of the signal light and the local light are different, and the detector detects the intermediate frequency, and the demodulator demodulates the baseband signal. In homodyne, the signal detected by the detector at the same optical frequency as the signal light and the local light is a baseband signal. Therefore, in homodyne, if the local light and the signal light are out of phase,
Sufficient signal strength cannot be obtained even with interference.

Although homodyne has drawbacks such as a stricter requirement for laser line width than heterodyne, it has excellent light-reception sensitivity and detects a baseband signal at the time of light reception. It has the advantage of being less demanding than heterodyne, which uses intermediate frequencies.

[0010]

However, when the polarization diversity is attempted by homodyne, the polarization fluctuation corresponds to the fluctuations of the phases of the two orthogonal polarizations. The phase difference between the signal light and the local light changes due to the polarization fluctuation of the signal light before entering the coupler. Therefore, phase synchronization cannot be applied and reception is difficult.

Therefore, the conventional method has a drawback that the polarization diversity cannot be performed by homodyne.

The present invention has been made to solve such a conventional problem, and an object of the present invention is to provide a polarization diversity optical receiver capable of homodyne reception.

[0013]

In order to achieve the above object, the present invention is an optical receiver for receiving an optical signal by a homodyne system, which comprises a first signal light and a first signal light whose polarizations are orthogonal to each other. A polarized light splitting means for splitting into two signal lights;
Means for emitting a first local oscillation light whose polarization coincides with that of the signal light, and a first optical detection means for receiving a signal in which the first signal light and the first local oscillation light are mixed, A means for controlling the phase of the first local oscillation light by using a part of the output of the first optical detection means to maximize the data output of the first optical detection means, and the second signal light. Means for emitting a second local oscillation light whose polarization is the same, second optical detection means for receiving a signal obtained by mixing the second signal light and the second local oscillation light, and the second optical detection means. A means for controlling the phase of the second local oscillation light by using a part of the output of the light detection means to maximize the data of the second light detection means, the first light detection means and the second light detection means. And a synthesizing means for synthesizing the output from the photodetecting means.

[0014]

With the above-described structure, the phase synchronization of the local oscillation light can be individually performed with respect to the two signal lights separated into the orthogonal polarizations. Therefore, it becomes possible to perform polarization diversity by the homodyne method.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a first embodiment of an optical receiver to which the present invention is applied.

As shown in the figure, this receiver comprises a polarization splitting element (PBS) 7 for splitting the transmitted signal light 1 into two polarizations orthogonal to each other, and an optical hybrid provided for each split optical system. 8a, 8b, photodetectors 9a, 9b, PLL
Controllers 10a and 10b, local light generation units 11a and 1
1b, and a combining unit 12 that combines the signals obtained for each optical system.

The optical hybrids 8a and 8b are composed of the optical signal separated by the polarization separation element 7 and the local light generation units 11a and 11b.
The local light output from the above is mixed and output to the photodetectors 9a and 9b.

The photodetectors 9a and 9b are the optical hybrid 8
The signal lights mixed in a and 8b are subjected to homodyne detection, and the detected signals are combined into the combining unit 12 and the PLL controller 10.
a and 10b.

The PLL controllers 10a and 10b use the detection signals from the photodetectors 9a and 9b to control the phase of the local oscillator light output from the local oscillator generators 11a and 11b to synchronize with the phase of the signal light. To do.

In the receiver configured as described above,
The two lights separated by the polarization separation element 7 are detected by homodyne detection independently of each other, and the phase can be controlled. Therefore, in the homodyne system, polarization diversity can be performed and sufficient signal strength can be obtained.

The optical hybrid 8a shown in FIG.
8b, photodetectors 9a and 9b, and PLL controller 1
The configurations of 0a and 10b differ depending on the form of the phase control system. For example, one of them uses a phase control system Costas loop as shown in FIG. Since the Costas loop shown in FIG. 2 is well known, its description is omitted.

Besides, there are various phase control systems such as decision-directed and pilot carriers, and the present invention can be applied to any phase control system. Furthermore, the phase control of local light may use an external phase controller, or may directly perform modulation. At this time, the direct modulation has an advantage that it has an infinite phase tracking property as compared with the case of using an external modulator.

Further, in the homodyne system, conventionally, active polarization control having a feedback loop in order to match two polarizations was often performed, but in the present embodiment,
The availability of polarization diversity eliminates the need for a feedback loop for polarization control.

FIG. 3 is a block diagram showing the configuration of the second embodiment of the present invention, which is an example in which only one local oscillator generating section 2 is used. In this example, in the local light source, one local light is split into two systems by a coupler, a beam splitter, etc., one of which is a phase modulator 14a and the other is a phase modulator 14b.
Supply to. Then, after individually performing phase control in each phase modulator 14a, 14b, each optical hybrid 8a,
8b. As a result, the number of local laser generators 2 is reduced to one, and the configuration can be simplified.

In the polarization diversity, the signal voltage detected by the light receiving portion of each branch is proportional to the amplitude of the signal light incident on each branch. Therefore, even if they are added as they are, the sum is not constant and is affected by the polarization fluctuation of the signal light. Its power to stay constant,
Alternatively, it must be converted into an amount proportional to the power and added. In heterodyne detection where polarization diversity has been often used so far, square wave detection is often used when demodulating a baseband signal from an intermediate frequency, and the square wave detector output is proportional to power and must be added as is. I was able to. However, in homodyne, the output of the photodiode is the baseband signal as it is, and there is no need for further demodulation, but in order to keep the combined output constant, maximum ratio combining or the like must be used when combining the two branches. .. Maximum ratio combining is a method in which signals in each branch are weighted in proportion to their amplitude and then added.

When synthesizing two branches of polarization diversity, if the output of one branch is small enough to be buried in noise, a method of separating the branch may be considered. In the present invention,
If the signal in one branch is very small or no signal is coming in that branch, the phase control system may not operate correctly or may not operate at all. At this time, depending on the operation of the phase control system, the local oscillation light may generate a large amount of noise.Therefore, when the power of the signal light entering one of the branches is very small, the output of that branch is cut off. It is effective to leave it.

FIG. 4 is a block diagram showing a modification of the second embodiment shown in FIG. 3. In this example, either one system is directly subjected to phase control and frequency control, and the other system is a station. An external phase modulator 14b is added to the branch output from the light emission generator 2.
Is used for phase control. With this structure, the structure can be further simplified.

FIG. 5 is a block diagram showing the configuration of the third embodiment of the present invention, which is an example in which a balanced receiver 16 is adopted in the photodiode portion of the photodetector. When the local light and the signal light are mixed by the optical hybrid 8a shown in FIG.
Due to the nature of the light, two lights in which the phase of the interference light is shifted by 180 degrees appear as outgoing lights (in the case of homodyne, two outgoing lights in which 1 and 0 of data are inverted appear). Then, normally, only one of them is received, but in this example, since the balanced receiver 16 can receive both signals, it is possible to effectively receive the optical power,
It becomes possible to suppress intensity noise of local light.

Further, when the signal of one system is very small or no signal enters its branch, the phase control system may not operate properly or may not operate at all. At this time, the phase control system Depending on the operation of, the local light may generate a large amount of noise, so when the power of the signal light is very small, it is effective to disconnect the output of that branch.

[0030]

As described above, according to the present invention,
When performing homodyne reception, polarization diversity reception is possible by separating the polarization of the signal light into two orthogonal polarizations, and then receiving each polarization by local oscillation with an independent phase control system. Becomes As a result, it is possible to cope with high-speed polarization fluctuation, and there is little fluctuation in reception sensitivity due to polarization fluctuation
The effect that stable reception is possible is obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of an optical receiver to which the present invention is applied.

FIG. 2 is a configuration diagram showing an example when a Costas loop is used as the phase control system of the first embodiment.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing a modification of the second embodiment.

FIG. 5 is a configuration diagram showing a third embodiment of the present invention.

FIG. 6 is a configuration diagram showing a conventional example.

[Explanation of symbols]

 1 Signal Light 7 Polarization Separation Element 8a, 8b Optical Hybrid 9a, 9b Photodetector 10a, 10b PLL Controller 11a, 11b Optical Laser Generator 12 Combiner 13a, 13b PLL Controller 14a, 14b Phase Modulator 15 Frequency Controller 16 Balance Receiver 17 amplifier 21a, 21b optical hybrid 22a, 22b photodetector 23a, 23b local light emission generator 24a, 24b loop filter 25 combiner

Claims (5)

[Claims] 1. An optical receiver for receiving an optical signal by a homodyne system, comprising: polarization splitting means for splitting the signal light into first signal light and second signal light whose polarizations are orthogonal to each other; Means for emitting a first local oscillation light whose polarization matches that of the first signal light, and a first means for receiving a signal in which the first signal light and the first local oscillation light are mixed
And a means for maximizing the data output of the first optical detection means by controlling the phase of the first local oscillation light by using a part of the output of the first optical detection means. Means for emitting a second local oscillation light whose polarization matches that of the second signal light;
Second optical detecting means for receiving a signal in which the second signal light and the second local light are mixed, and the second station using a part of the output of the second optical detecting means Means for controlling the phase of the emitted light of the emitted light to maximize the data of the second photodetection means, and synthesizing means for synthesizing the outputs of the first photodetection means and the second photodetection means. An optical receiver having. 2. The means for emitting the first and second local oscillators is composed of one laser device, and the emitted light of the laser device is divided into a first local oscillator and a second local oscillator. The optical receiver according to claim 1, wherein the optical receivers are controlled by different phase control loops. 3. The optical receiver according to claim 1, wherein the first and second optical detection means have a balanced receiving portion. 4. The optical receiver according to claim 1, wherein said combining means performs maximum ratio combining on the outputs of said first and second optical detection means. 5. The synthesizing means detects the output levels of the first and second photodetecting means, and when one of them is a noise level, the output of the noise level is separated from the synthesizing means, and the other The optical receiver according to claim 1, wherein only the output of the optical receiver is input.