JPH0685507B2 - Optical heterodyne / homodyne receiver circuit - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a receiving circuit when a polarization diversity receiving system is applied to an optical heterodyne / homodyne detection system.

(Conventional Technology and Problems Thereof) FIG. 1 shows a configuration example of a conventional polarization diversity reception system. The signal light 1 having an arbitrary polarization state is combined with the local oscillation light 2 by the multiplexer 5. Although the branching ratio of the multiplexer 5 is arbitrary, it is generally selected so that the maximum light receiving sensitivity can be obtained at the incident portion of the multiplexer 5. The multiplexed light 3 is split into two orthogonal polarizations 7 and 8 by the polarization splitter 6 and received by the photodetectors 9 and 10. The received signal is guided to the diversity combining circuit 13. As a diversity combining method, there are literatures (1) [Okoshi, Kasa, Kikuchi "Polarization diversity optical receiver for heterodyne / coherent optical fiber communication" IEICE Tech. , Kikuchi, Takita "Polarization diversity optical receiver for heterodyne / coherent optical fiber communication" IEICE Technical Report OQE 84
-25] [1984], and only an outline is given here. The combining methods include equal gain combining, maximum ratio combining, selective combining and switching combining. In addition, there are IF synthesis and baseband synthesis at the stage of signals to be synthesized.
It is necessary to adjust the phase with and 12 according to the polarization state. Figure 2 shows the relationship between polarization fluctuation and S / N degradation for each combination method. There is no deterioration due to the polarization state in maximum ratio combining. However, this is a case where the intensity noise of the light source can be ignored, and cannot be ignored when an actual semiconductor laser or the like is used. FIG. 3 shows a calculation example of the relationship between the signal light and the local light for obtaining the predetermined SN ratio. The maximum receiving sensitivity is obtained when the relative noise intensity is −150 dB / Hz and the local light intensity is −7 dBm, but there is 3.5 dB deterioration from the shot noise limit. As described above, generally, the intensity noise of the local light source cannot be detected, and the deterioration of the light receiving level cannot be avoided unless a low-noise preamplifier is used.

On the other hand, in order to suppress the intensity noise of the light source, a configuration as shown in Fig. 4 has been proposed [3] [HP Yuen and VWSChan, "Noise
in homodyne and heterodyne detection "OPTICS Letter
s, Vol.8, No.3, March 1983]. In this, the signal light 1 and the local light 2 are multiplexed by the multiplexer 5 at a ratio of 1: 1 to obtain outputs 3 and 4. Receive these, adjust the phase with the phase shifter 15,
Combined by the differential combiner 16.

Let the optical powers of the signal light 1 and the local light 2 be P _s and P _e , respectively,
Assuming that the reflectivity of the multiplexer 5 is R and the quantum efficiencies of the receivers 9 and 9'are η ₁ and η ₂ , respectively, the current i flowing through the photodetectors 9 and 9 '
₁ and i ₂ are Here, e: charge of electron, h: Planck's constant, ν: frequency, ω _IF : angular frequency of IF band, φ (t): signal.

The third term of i _1, i ₂ is the signal component, the i ₁ and i ₂ the phase π
There is only a difference. Therefore, the signal becomes the sum of currents by the differential combination. On the other hand, if the selection is such that η ₁ (1-R) = η ₂ R, the intensity noise of the local light can be ignored.

In order to realize the above state, it is necessary to keep the polarization state of the incident light 1 constant.

(Object of the Invention) An object of the present invention is to apply a differential multiplexing method that can ignore deterioration of intensity noise of a light source to a polarization diversity reception system, and to deteriorate reception sensitivity due to polarization fluctuation and reception sensitivity due to intensity noise of the light source. Another object of the present invention is to provide an optical heterodyne / homodyne receiver circuit that simultaneously solves the above deterioration.

(Characteristics of the Invention) The present invention is mainly characterized in that differential synthesis for suppressing intensity noise of a light source and polarization diversity for compensating for polarization fluctuation of an optical fiber are simultaneously realized. Conventionally, there have been proposals for individual methods, but there is no example of realizing them at the same time.

(Embodiment) FIG. 5 shows a first embodiment of the present invention, in which an input signal light 1 is combined with a local light 2 by a multiplexer 5 at a ratio of 1: 1. The multiplexed lights 3 and 4 are split into two polarizations orthogonal to each other by polarization splitters 6 and 6 '. Here, the separated lights 7 and 7'and 8 and 8'are adjusted so as to have the same polarization state upon incidence. That is, the outputs of the photo detectors 9 and 9'are properly phase-adjusted by the phase adjuster 15 and differentially combined by the differential combiner 16. The same applies to the outputs of the photo detectors 10 and 10 '. The diversity-combined signals are diversity-combined by the diversity composing circuit 13.

Due to such a structure, the local light 2 is split into the lights 7 and 7'in a ratio of 1: 1. Similarly, the light 8 and the light 8'are also branched, but since the branching ratio of the light 7 and the light 8 depends on the incident state of the local oscillation light 2, the light 7 and the light 8 are generally 1: 1. Choose to branch. The fluctuation of the incident polarization state in the input signal 1 appears in the output light of 7,7 ', 8,8' as a phase difference of the beat signal. However, the phase difference between the output lights 7 and 7'has no change due to the polarization state variation, and the phase adjuster 15 does not need to be adjusted. The signal light thus combined does not deteriorate due to the intensity noise of the light source, as shown in FIG. For example, the conventional diversity combination can
In the case of the output of, the maximum ratio combining cannot cancel the local oscillation intensity noise because the gain changes depending on the level of the combining input. Further, in the equal gain combination, the phase of the combined input changes depending on the state of the incident polarized wave, and similarly, the local light intensity noise cannot be canceled.

In the first embodiment, it is necessary to use the multiplexer 5 that does not have polarization dependency, and the multiplexed signals 3 and 4 must be such that the polarization state does not change. A second embodiment for avoiding this is shown in FIG. In this case, the input signal light 1 is split into two polarizations by the polarization splitter 6 and the local oscillation light 2 is split into two polarizations by the polarization splitter 6 '. 1: 1 multiplexer 5 for each of the outputs of polarization splitters 6 and 6 '
And combine to obtain two outputs. These outputs are received by the receiver 9,
The signal is converted into an electric signal by 9 ', the phase is adjusted by the phase adjuster 15, and then the differential combiner 16 performs differential combination. For the other output of the polarization splitters 6 and 6 ', the multiplexer 5'and the photoreceiver 1
The same operation is performed by 0, 10 ', the phase adjuster 15', and the differential combiner 16 ', and the diversity combining circuit 13 performs polarization diversity on the differentially combined signals.
Even with this configuration, it is clear that the same effect as that of the first embodiment can be obtained. In this configuration, the polarization splitter 6,
Since 6'and later can be constructed using polarization-maintaining fiber, etc., it has the property of being strong against disturbance. Further, the polarization separator 6'can be configured by a simple demultiplexer.

(Effects of the Invention) As described above, the present invention divides incident light into two orthogonal polarization states, and differentially combines the polarization states so that intensity noise of local light can be ignored. Since polarization diversity is performed, polarization diversity is possible while ensuring a reception level close to the shot noise limit.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a conventional polarization diversity receiver, FIG. 2 is a polarization state and S / N ratio deterioration characteristic diagram, and FIG.
Is a characteristic diagram showing the relationship between signal light and local light for satisfying a predetermined S / N ratio, FIG. 4 is a block diagram showing an example of a receiver configuration for suppressing intensity noise of a light source, and FIG. FIG. 6 is a block diagram showing a first embodiment, and FIG. 6 is a block diagram showing a second embodiment of the present invention. 1 ... Signal light, 2 ... Local oscillation light, 3, 4 ... Combined light, 5,
5 '... Multiplexer, 6,6' ... Polarization splitter, 7,7 ', 8,8' ...
… Polarization split light, 9,9 ′, 10,10 ′ …… Receiver, 11,12 …… Signal, 13 …… Polarization diversity combining circuit, 14 …… Baseband output, 15,15 ′ …… Phase Regulator, 16, 16 '... Differential combiner

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 59-13434 (JP, A) JP 59-122140 (JP, A) JP 58-149025 (JP, A) JP 60- 47524 (JP, A)

Claims (4)

[Claims] 1. A receiving circuit that performs optical heterodyne or homodyne detection using polarization diversity, and combines the input signal light with the local oscillation light at a ratio of 1: 1 to obtain two combined outputs. A first polarization separating means for separating one output of the combining means into two first polarized waves orthogonal to each other by a first polarization separator, and the other output of the combining means 2nd polarization splitting into 2nd polarization by 2nd polarization splitter
Polarization separating means, first differential combining means for phase-adjusting one of the two first polarized waves and the other of the two second polarized waves, and differentially combining the two first polarized waves; Second differential combining means for phase-adjusting the other of the first polarized waves and one of the two second polarized waves to differentially combine, the first differential combining means, and the second An optical heterodyne / homodyne receiver circuit comprising diversity combining means for performing polarization diversity combining on the respective differential combined outputs of the differential combining means. 2. In a receiving circuit for performing optical heterodyne or homodyne detection using polarization diversity, first demultiplexing means for demultiplexing input signal light into two orthogonal polarizations by a polarization demultiplexer and local oscillation light. Of the two polarized outputs of the first separating means and one of the two outputs of the second separating means are combined at a ratio of 1: 1. First combining means for obtaining two outputs, the other of the two polarized outputs of the first separating means and the other of the two outputs of the second separating means are combined at a ratio of 1: 1 to obtain two outputs. A second combining means for obtaining the above, a first differential combining means for phase-adjusting respective light reception outputs of the two outputs of the first combining means, and a second combining means; Second differential synthesizing means for differentially synthesizing the respective received light outputs of the two outputs of the first differential synthesizing means and the first differential synthesizing means. Optical heterodyne homodyne receiver circuit, characterized in that the respective differential combining outputs with a diversity synthesizing means for performing polarization diversity combining means and said second differential combining means. 3. A pair of second second separation means which are orthogonal to each other.
3. An optical heterodyne / homodyne receiver circuit according to claim 2, wherein the optical heterodyne / homodyne receiver circuit is constituted by a second polarization separator for extracting the polarization output of the above. 4. An optical heterodyne / homodyne receiver circuit according to claim 2, wherein said second separating means is composed of a 1: 1 branch circuit.