JPH0716175B2 - Optical heterodyne / homodyne detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical communication device, and more particularly to an optical heterodyne / homodyne detection communication device.

[Conventional technology]

The optical heterodyne / homodyne detection method, which combines the signal light and the local oscillation light, detects the intermediate frequency component, and demodulates the signal light, is an effect of the thermal noise of the optical detection device that causes deterioration of the signal light detection sensitivity. Therefore, the signal light detection sensitivity can be significantly improved as compared with the method of directly detecting the signal light. Therefore, its application to the measurement of a small amount of light and the field of optical communication is being studied.

In such an optical heterodyne / homodyne detection method, as the amount of local oscillation light is increased, the influence of the thermal noise is reduced, but if it is increased too much, the influence of light intensity noise of the local oscillation light increases. The signal light detection sensitivity deteriorates. Therefore, conventionally, the local oscillation light is set so that the signal light detection sensitivity is maximized, that is, the ratio of the sum of the noise consisting of thermal noise, light intensity noise, and shot noise to the detection signal is maximized. Was there.

On the other hand, while further improvement in signal light detection sensitivity is desired,
A balanced receiver type optical heterodyne / homodyne detector (hereinafter abbreviated as balanced receiver) having an effect of suppressing optical intensity noise has begun to attract attention. The configuration of this balanced receiver is based on the optics
Letters, Vol. 18, 1983, pp. 177-179, Proceedings of the 60th General Meeting of the Institute of Electronics and Communication Engineers of 1985 by Machida 4-
For details, see the literature on pages 306-307.

FIG. 4 is a block diagram showing the structure of the balanced receiver. In this balanced receiver, when the local oscillation light 92 emitted from the local oscillation light source 99 and the signal light 91 emitted from the signal light source 98 are combined via the optical coupler 95 having a branching ratio of 1: 1, the two combined lights By utilizing the fact that the relationship between the phase of the intermediate frequency component of 93, 94 and the light intensity noise component differs by 180 ° between the combined lights 93, 94, only the light intensity noise is canceled.

That is, the intermediate frequency component is 180 at the port I consisting of the photodetector (detector) 96 and the port II consisting of the photodetector (detector) 97.
It has a phase difference and is demodulated by the differential synthesizer 100. on the other hand,
Since the light intensity noise is in phase with each port, a differential combiner
Suppressed by 100. Therefore, the balanced receiver reduces the influence of the optical intensity noise even if the local oscillation light 92 is increased, and the influence of the thermal noise is almost eliminated, so that the signal light detection sensitivity is limited only by the shot noise. . That is, it approaches the ideal limit.

[Problems to be solved by the invention]

In the balanced receiver of FIG. 4, in order to maximize the signal light detection sensitivity (hereinafter referred to as the maximum signal light detection sensitivity) corresponding to the amount of local oscillation light, the local light input to the 2-series photodetectors 96 and 97 Oscillation light intensity ratio k (hereafter called light intensity ratio between two series)
To 1: 1 and the gains of the two series photodetectors must be the same.

In addition, when the branching ratio of the optical coupler 95 is not 1: 1, in order to obtain the best signal light detection sensitivity corresponding to the locally oscillated light quantity, according to the light quantity ratio k between the two series, between the two series photodetectors. An optimum gain difference should be given. In this case, a signal light detection sensitivity almost equal to the maximum signal light detection sensitivity can be obtained.

However, since the branching ratio of the optical coupler 95 generally has wavelength dependency, the wavelength shift of the signal light source 98 and the local oscillation light source 99 due to the temperature change may change the light amount ratio k between the two series. In addition, k may fluctuate due to changes over time in the photodetectors 96 and 97 and positional displacement due to impact.

Such a variation in the light intensity ratio k between the two series deteriorates the signal light detection sensitivity, but conventionally there has been no device capable of keeping the reception sensitivity constant against such a variation.

It is an object of the present invention to provide a balanced receiver type optical heterodyne / homodyne detection device that can always obtain the best signal light detection sensitivity in response to a change in the light intensity ratio k between two series.

[Means for solving problems]

The present invention detects two combined lights obtained by combining the local oscillation light and the signal light by a directional optical coupler, detects each with two series of photodetectors, converts them into an electric signal, and then outputs the electric signal. 18 ingredients
In a balanced receiver type optical heterodyne / homodyne detection device for differentially combining with a 0 ° phase difference, a means for monitoring the ratio of the light amounts of the combined light input to the two series of photodetectors, and And a means for varying the electric gain difference between the two series of photodetectors in accordance with the ratio of the light quantities.

〔Example〕

 Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram for explaining the configuration and operating principle of a first embodiment of an optical heterodyne / homodyne detector according to the present invention.

This optical heterodyne / homodyne detector is a signal light source.
18, a local oscillation light source 19, an optical fiber coupler 15 made of an optical fiber, and two series of photodetectors (detectors) 16, 17
And a differential amplifier 24, and adopts a balanced receiver type configuration. The detector of the present embodiment further includes a divider 23 for monitoring the ratio of the light amounts of the combined light input to each of the two series of photodetectors, and two series corresponding to the monitored light amount ratios. A variable gain amplifier 22 for varying the electric gain difference between the photodetectors 16 and 17 is provided.

In the optical heterodyne / homodyne detector having the above configuration, the signal light 11 and the local oscillation light 12 emitted from the signal light source 18 and the local oscillation light source 19 are input to the optical fibers 20 and 21, respectively, and then combined by the optical fiber coupler 15. To be done. The obtained combined lights 13 and 14 are demultiplexed at a rate determined by the branching ratio of the optical fiber coupler 15 and input to the photodetectors 16 and 17 of each detector series.

At this time, if the signal light intensity I _S, a local oscillation light amount I _L, 2 series between the light amount ratio is k, the intermediate frequency component intensity of each photodetector 16, 17 outputs are the same, k (1- k) I _S I _L.

On the other hand, in the optical heterodyne / homodyne detection method, I _L >> I _S is generally satisfied, and the optical intensity noise is almost equal to that of the local oscillation light source.
caused by. Therefore, the optical intensity noise (<i _L ² >) is divided according to the branching ratio of the optical fiber coupler 15, k <i _L > for the I-series photodetector 16 and (1-k for the II-series photodetector 17). ) <
Input with amplitude ratio of i _L >.

Therefore, in order to obtain the best signal light detection sensitivity, in order to cancel the respective intensity noises input to the I and II series photodetectors 16 and 17, that is, in order to make the intensities the two series photodetectors, It is necessary to give an optimal gain difference between them.

FIG. 2 is a characteristic diagram for explaining the principle used by the optical heterodyne / homodyne detector according to the present invention.

FIG. 2 shows the change in the power penalty with respect to the gain difference between the two-series photodetectors when the total amount of local oscillation light input to the two-series photodetector is set to 20 mW using the two-series light intensity ratio k as a parameter. It is a thing. Here, the power penalty is defined as the deterioration amount from the signal light detection sensitivity determined only by the shot noise.

It can be seen from FIG. 2 that when the light intensity ratio k between the two series varies, the gain difference between the two series photodetectors giving the best signal light detection sensitivity shifts. That is, if the gain difference between the two series photodetectors is kept constant, the light quantity ratio k between the two series is k.
Fluctuations in the signal light cause deterioration of the signal light detection sensitivity.

The optical heterodyne / homodyne detection device according to the present invention shown in FIG. 1 is configured to always provide the best signal light detection sensitivity with respect to fluctuations in the light amount ratio k between two series. That is, the change in the light amount ratio k between the two series is determined by monitoring the light quantity input to each detector series by the photodetectors 16 and 17 and then calculating the divider 23.
It is read by inputting into and dividing by the divider 23. After this, the read light quantity ratio k between the two series k
In response to the above, the gain of the variable gain amplifier 22 is controlled to give a gain difference between the two series photodetectors so that the best signal light detection sensitivity can always be obtained. Therefore, the signal light detection sensitivity obtained by the output from the differential amplifier 24 is the light quantity ratio k between the two series.
The best performance is always obtained.

FIG. 3 shows a second embodiment of the optical heterodyne / homodyne detector according to the present invention.

This optical heterodyne / homodyne detector is further provided with optical fiber couplers 25, 26 and monitor photodetectors 27, 28 in the configuration of FIG. 1, and the divider 23 is provided with these photodetectors 27, 28. The amount of light detected from is input. Other configurations are similar to those of the embodiment shown in FIG. 1, and therefore, the same components are designated by the same reference numerals as in FIG.

In the present embodiment, the light quantity ratio k between the two series is such that a part of the combined light demultiplexed by the optical fiber coupler 15 is used as the optical fiber coupler 2.
After being taken out as monitor light 29,30 using 5,26, it is detected by the photodetectors 27,28 and read by the divider 23. It is desirable to use the same branching ratios for the optical fiber couplers 25 and 26. However, if the branching ratios are not the same, if they are known, a correction value may be given to the divider in advance.
In addition, the branching ratio of the optical fiber couplers 25 and 26 is
Set so that the output of 14 is sufficiently higher than the output of monitor light 29, 30.

Therefore, even in the configuration of the embodiment shown in FIG.
The signal light detection sensitivity obtained by the output from 24 always obtains the best performance corresponding to the light intensity ratio k between the two series.

〔The invention's effect〕

As described above, according to the present invention, in the balanced receiver type optical heterodyne / homodyne detector,
The light intensity ratio k between the two series is monitored so that the best signal light detection sensitivity is obtained corresponding to the light intensity ratio k between the two sequences, and the gain difference between the two light detectors of the balanced receiver is monitored according to this value. Therefore, there is an effect that the best signal light detection sensitivity can always be obtained even if the light amount ratio between the two series fluctuates.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of an optical heterodyne / homodyne detector according to the present invention, FIG. 2 is a characteristic diagram for explaining the principle of the present invention, and FIG. 3 is a second embodiment according to the present invention. An example block diagram, FIG. 4 is a block diagram of a conventional balanced receiver type optical heterodyne / homodyne detector. 11 …… Signal light 12 …… Local oscillation light 13,14 …… Composite light of signal light and local oscillation light 15,25,26 …… Optical fiber coupler 16,17,27,28 …… Photodetector 18 …… Signal light source 19 …… Local oscillation light source 20,21 …… Optical fiber 22 …… Variable gain amplifier 23 …… Divider 24 …… Differential amplifier 29,30 …… Monitor light

Claims (1)

[Claims] 1. A two-series photodetector detects two combined lights obtained by combining a local oscillation light and a signal light by a directional optical coupler, converts them into an electric signal, and then outputs the electric signal. Signal components
In a balanced receiver type optical heterodyne / homodyne detection device that differentially combines with a 180 ° phase difference, a means for monitoring the ratio of the light amounts of the combined light input to the two series of photodetectors, and And a means for varying the electric gain difference between the two series of photodetectors corresponding to the ratio of the light quantities.