JPS62200831A - Optical heterodyne/homodyne reception circuit - Google Patents

Abstract

PURPOSE:To prevent the deterioration in the reception sensitivity due to the fluctuation of polarized wave and due to the intensity noise of light source by providing a multiplexing means obtaining two multiplexed outputs, two polarized wave separation means and two differential multiplexing means and applying polarized wave diversity multiplexing to respective differential multiplexing outputs of the two differential multiplexing means. CONSTITUTION:Lights 3, 4 being the result of multiplexing of an input signal light 1 by a locally emitting light 2 by 1:1 at a multiplexer 5 are demultiplexed into two orthogonal polarized waves by polarized wave separators 6, 6'. The rays of light 7, 7' and 8, 8' after the separation are adjusted so as to be in the same polarized wave state at the incidence. That is, the output of photodetectors 9, 9' is subjected to proper phase adjustment by a phase adjustment device 15 to apply differential multiplexer by a differential multiplexer 16. The output of photodetectors 10, 10' is processed similarly. The signal differentially multiplexed is subjected to diversity multiplex by a diversity multiplex circuit 13. The incident light is divided into two orthogonal polarized waves and they are multiplexed differentially so as to neglect the intensity noise of a locally emitting light and the polarized wave diversity is applied in this way, then a reception level close to a shot noise limit is ensured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a receiving circuit when a polarization diversity receiving method is applied to an optical heterodyne/homodyne detection method.

(Prior art and its problems) An example of the configuration of a conventional polarization diversity reception system is shown in FIG. Signal light 1 having an arbitrary polarization state is multiplexed with local oscillation light 2 by 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 input section of the multiplexer 5.

The combined light 3 is divided into two orthogonal polarized waves 7 and 8 by a polarization separator 6, and received by a light receiver 9.10. The received signal is guided to diversity combining circuit 13. Diversity synthesis methods are described in References (1) [Okoshi, Kasa, Kikuchi "Polarization Diversity Optical Receiver for Heterogain/Coherent Optical Fiber Communication" IEICE Technical Report C383-221982], References (2) [Okoshi, Kikuchi] , Takita “Polarization diversity optical receiver for heterogain/coherent optical fiber communication” IEICE Technical Report 0QE8
4-251984), and only an overview will be given here. Combining methods include equal gain combining, maximum ratio combining, 9 selection combining, and switching combining. Further, there are IF synthesis and baseband synthesis at the stage of combining signals, and in IF synthesis, it is necessary to adjust the phase of signals 11 and 12 according to the polarization state. FIG. 2 shows the relationship between polarization fluctuation and S/N deterioration for each combination method. In maximum ratio combination, there is no deterioration due to the polarization state.

However, this is a case where the intensity noise of the light source can be ignored, and cannot be ignored when using an actual semiconductor laser or the like. An example of calculation of the relationship between the signal light and the local light to obtain a predetermined SN ratio is shown in Figure 3.The maximum receiving sensitivity is obtained when the relative noise intensity is -150 dB/Hz and the local light intensity is -7 dB+s. 0 or more with 3.5 dB deterioration from the shot noise limit As explained above, the intensity noise of the local light source cannot be ignored, and deterioration of the received light level is unavoidable 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.
W.

S, Chan+”No1se in how+o
dyne and heterodyne detect
ion”0PTIC5Letters, Vol.
8. lk3+March 1983). This means that signal light 1 and local light 2 are combined in a 1:1 ratio by multiplexer 5.
and obtains an output of 3.4. These lights are received, their phases are adjusted by a phase shifter 15, and then combined by a differential combiner 16.

Let the optical power of the signal light l and the local light 2 be P, respectively.

P, the reflectance of the multiplexer 5 is R, and the receiver is 9°9".
Let the quantum efficiencies of
The current 11+1g flowing in ° is each where: e: electron charge, h nblank constant.

C: Frequency, ωIF: Angular frequency of IF band.

φ(t): Signal.

11+l! The third term is the signal component, and there is a phase difference of π between 11 and 12. Therefore, the signal becomes the sum of currents by differential synthesis. On the other hand, if η1(1-R)=η2R is selected, 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.

(Objective of the Invention) The object of the present invention is to apply a differential multiplexing method that can ignore the deterioration of the intensity noise of the light source to the polarization diversity reception system, and to reduce the deterioration of the reception sensitivity due to the polarization variation and the reception sensitivity due to the intensity noise of the light source. An object of the present invention is to provide an optical heterodyne/homodyne receiving circuit that simultaneously solves the problem of deterioration of the optical heterodyne/homodyne receiving circuit.

(Feature 11k of the Invention) The main feature of the present invention is to simultaneously realize differential synthesis that suppresses intensity noise of a light source and polarization diversity that compensates for polarization fluctuations of an optical fiber.

In the past, there were proposals for individual methods, but there are examples of them being realized at the same time.

(Embodiment) FIG. 5 shows a first embodiment of the present invention, in which input signal light 1 is multiplexed with local light 2 by a multiplexer 5 at a ratio of 1:1. The combined lights 3 and 4 are split into two orthogonal polarized waves by polarization separators 6 and 6'. Here, the lights 7, 7° and 8 after separation
and 8° are adjusted so that the polarization state is the same at the time of incidence.

That is, the output of the light receiver 9°9° is appropriately phase-adjusted by the phase adjuster 15, and differentially combined by the differential combiner 16. The same procedure is applied to the output of the photoreceiver 10, 10'. The differentially combined signals are diversity combined by a diversity combining circuit 13.

Because of this configuration, the local light 2 is connected to the lights 7 and 7.
It is split 1:1 in degrees. Lights 8 and 8 are also split in the same way, but since the splitting ratio of lights 7 and 8 depends on the incident state of locally oscillated light 2, generally the ratio of lights 7 and 8 is 1:l. A variation in the incident polarization state of the input signal 1, which is selected to be split, appears as a phase difference between the beat signals in the output lights of 7.7' and 8.8'. However, the phase difference between the output lights 7 and 7' does not change due to changes in the polarization state, and there is no need to adjust the phase adjuster 15. The signal light synthesized in this way is controlled by the light source as shown in FIG. No deterioration due to intensity noise 0For example, if conventional diversity synthesis is performed on the output of the optical receiver 9.10, the gain changes depending on the level of the synthesis input in maximum ratio synthesis, so the local light intensity noise cannot be canceled out. In equal-gain synthesis, the phase of the combined input changes depending on the state of the incident polarization, so it is similarly impossible to cancel the local light intensity noise.

In the first embodiment, it is necessary to use a multiplexer 5 that has no polarization dependence, and the multiplexed signal 3.4 must be free from fluctuations in polarization state. This should be avoided. A second embodiment for this purpose is shown in FIG. It is divided into two polarized waves. One output from each of the polarization separators 6 and 6° is multiplexed by a tri multiplexer 5 to obtain two outputs. These outputs are converted into electrical signals by the optical receivers 9, 9', and the phase adjuster 15
After matching the phases, the differential combiner 16 performs differential combining.For the other output of the polarization separators 6 and 6°, the multiplexer 5°, the optical receiver 10, 10°, and the phase adjuster are used. 15'' and the differential combiner 16°, and the diversity combining circuit 13 is applied to the differentially combined signals.
to perform polarization diversity. It is clear that this configuration also provides the same effects as the first embodiment. In this configuration, since the polarization separators 6 and 6' can be constructed using polarization-maintaining fibers, etc., it has a characteristic of being strong against disturbances. Further, the polarization separator 6'' can be configured by a simple duplexer.

(Effects of the Invention) As explained above, the present invention divides incident light into two orthogonal polarization states, performs differential synthesis for each polarization state so that the intensity noise of the 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 explanation of drawings]

Figure 1 is a block diagram showing the configuration of a conventional polarization diversity receiver, Figure 2 is a diagram of polarization states and S/N ratio deterioration characteristics, and Figure 3 is a diagram of signal light and stations 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. FIG. 5 is a block diagram showing a first embodiment of the present invention. FIG. FIG. 3 is a block diagram showing a second embodiment. l...signal light, 2...local oscillation light, 3,4...
・Combined light, 5,5"...Multiplexer, 6.6"...Polarization separator, 7.7", 8.8'...Polarization separated light, 9
,9", 10.10'...Receiver, 11.12.
... Signal, 13... Polarization diversity synthesis circuit,
14...Baseband output, 15, 15'...
Phase adjuster, 16.16"...differential synthesizer

Claims (4)

[Claims] (1) In a receiving circuit that performs optical heterodyne or homodyne detection using polarization diversity, a combining means that combines input signal light with local oscillation light at a ratio of 1:1 to obtain two combined outputs; a first polarization splitter that separates one output of the multiplexing means into two orthogonal first polarizations by a first polarization separator;
a second polarization separation means for separating the other output of the multiplexing means into two second polarizations by a second polarization separator; a first phase that adjusts the phase of one of the waves and the other of the two second polarized waves and performs differential synthesis;
a second differential synthesizing means for differentially synthesizing the other of the two first polarized waves and one of the two second polarized waves by adjusting the phase thereof; 1. An optical heterodyne/homodyne receiving circuit comprising: diversity combining means for polarization diversity combining the differential combining outputs of the differential combining means and the second differential combining means. (2) In a receiving circuit that performs optical heterodyne or homodyne detection using polarization diversity, a first separating means separates an input signal light into two orthogonal polarized waves by a polarization splitter, and a first separating means separates the input signal light into two orthogonal polarized waves using a polarization splitter, and a second separating means that separates one of the two polarized wave outputs of the first separating means and one of the two outputs of the second separating means into two branch outputs;
a first combining means to obtain two outputs by combining the other of the two polarized wave outputs of the first separating means and the other of the two outputs of the second separating means in a ratio of 1:1; a second multiplexing means for obtaining two outputs; a first differential combining means for differentially combining the two received light outputs of the first multiplexing means by adjusting the phase thereof; a second differential combining means for differentially combining the received light outputs of the two outputs of the combining means; An optical heterodyne/homodyne receiving circuit characterized by comprising diversity combining means for performing polarization diversity combining of differential combining outputs. (3) The light according to claim 2, wherein the second separation means is constituted by a second polarization separator that takes out two orthogonal second polarization outputs. Heterodyne/homodyne receiver circuit. (4) The optical heterodyne/homodyne receiving circuit according to claim 2, wherein the second separation means is constituted by a 1:1 branch circuit.