JPS62245741A - Polarization diversity optial reception - Google Patents

Abstract

PURPOSE:To apply the ratio scaler multiplex without using the electric square characteristic by adjusting the polarization of a local oscillated light so that the branch ratio to the 1st and 2nd optical beams by a polarization separation element of the local oscillation light is equal to the branch ratio of the signal light. CONSTITUTION:Part of the 1st and 2nd electric signals 12, 13 is branched, a division circuit 22 detects its ratio to obtain the 1st output signal 24. On the other hand, a division circuit 23 detects the ratio of the lst and 2nd DC components 20,21, the signal 25 obtained is given to a nonlinear circuit 26 having the square characteristic to obtain the 2nd output signal 27 proportional to the square of the branch ratio of the local oscillation light 2 to the 1st and 2nd light beams 8,9. The difference obtained by inputting the 1st and 2nd output signals 24,27 to a differential amplifier 28 is utilized as a control signal 29 to the polarization control element 3. The control signal 29 turns the polariza tion control element 3 to change the polarized direction of the local oscillation light 2, the branch ratio of the local oscillation light 2 by a polarization separa tion element 3 is changed to make the branch ratio of the signal light 4 to the local oscillation light 2 euqal.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a polarization diversity optical receiver used in optical communication systems, optical measurement systems, etc. that use optical heterodyne detection or optical homodyne detection.

(Prior technology) In general, the optical heterogain detection method has the great feature of increasing reception sensitivity by 10 to 100 times compared to the conventional optical direct detection method, so it is used in long-distance optical communication trunk systems and various high-speed This is an effective optical detection method for sensitive optical sensors.

In order to achieve high reception sensitivity in this optical detection method, efficient multiplexing of the signal light and local oscillation light is required. The beam diameter etc. must be matched. Here, in order to match the propagation direction and beam diameter of both the signal light and the local oscillation light with low loss, it is effective to use an optical fiber coupler, and the use of an optical fiber coupler as an optical multiplexer is generally used. It has become. Regarding the polarization state, in the case of optical communications, the polarization state of signal light propagated over a long distance in an optical fiber is affected by various disturbances applied to the optical fiber, and the polarization state changes over time. In the case of measurement systems, the polarization state of the signal light changes depending on the state of the object to be measured.
A device is required to always match the polarization states of the signal light and the local oscillation light.

Conventionally, the following three types of devices have been considered.

One of them is a fiber with good polarization preservation. This makes it possible to prevent the polarization state of the signal light from changing. The other monitors the polarization state of the signal light during reception.
This is a device that controls the polarization state of signal light or local oscillation light, and attempts to achieve polarization matching between signal light and local oscillation light by controlling the polarization state. The remaining one is a polarization diversity destination receiving device related to the present invention. In this method, a signal light and a local oscillation light are combined by an optical multiplexer, and the combined light is incident on a polarization splitting element and split into two linearly polarized first and second light beams with planes of polarization perpendicular to each other. , are incident on separate light-receiving elements and converted into electrical signals, and the electrical signals are processed and synthesized by a processing section. In this case, the first
, each signal light component and local oscillation light component of the second light beam are:
Since the light beams are linearly polarized and have the same polarization direction, no signal loss occurs in the first and second light beams due to mismatch in polarization state.

In a normal polarization diversity optical receiver, the polarization state of the locally oscillated light is set so that the split ratio of the locally oscillated light into the first and second light beams by the polarization separation element is 1=1. There is. In the simplest polarization diversity optical receiver, first and second light beams are received by first and second light receiving sections, respectively, and the electrical signals obtained there are combined in a processing section at an intensity ratio of 1:1.

FIG. 2 shows the power penalty that occurs in this synthesis method. Here, if the polarization state of the signal light is such that it is separated by the polarization separation element at an intensity ratio of 1:1, no deterioration in reception sensitivity occurs due to polarization diversity reception. On the other hand, if the signal light is split into only one beam by the polarization splitter, the signal cannot be extracted from the receiving system where the signal light is not incident, and only noise is added by electrical system synthesis. I will do it. In this case, compared to the case where the signal is demodulated using a single receiving system, only the noise is doubled, resulting in a 3 dB power penalty. In order to prevent such deterioration, the electrical synthesis in the processing section is performed by the following equipment, that is, the first and second
When the magnitude of the signals from the receiving section of the
A maximum ratio synthesis) device is being considered. (Okoshi, Kasa, and Tsuiike, ``Polarization diversity optical receiver for heterogain coherent optical fiber communication'' IEICE Communication Systems Study Group Material C383-22, 1983). If this ratio scaler synthesis is ideally performed, the signal can be demodulated without deterioration regardless of the polarization state of the signal light.

(Problems to be solved by the invention) However, in reality, for example, the literature by Joint Venture et al. As shown in National Convention 279), the square characteristic of a diode, etc. is used to realize the square of the amplitude ratio.

When passing through a circuit with such a square-law characteristic, there is a problem in that the noise component is emphasized more than the signal component due to the square-law characteristic, resulting in a deterioration of the S/N ratio (Fan, Dare, Tweel, (A, V, D., Ziel) Taki, Narutori, Tamiya Translated Noise Chapter 13, published by Radio Operator Education Association). SUMMARY OF THE INVENTION An object of the present invention is to solve this problem and provide a polarization diversity optical receiver that can perform ratio scaler combining without using electrical square characteristics.

(Means for Solving the Problems) A polarization diversity optical receiver of the present invention includes a light source for locally oscillated light, a signal light, and a locally oscillated light having a stable polarization state emitted from the light source. An optical multiplexing unit that combines the waves, a polarization separation element that separates the combined light obtained by the optical multiplexing unit into first and second light beams whose polarization planes are orthogonal to each other, and receives the first and second light beams. and a processing section that detects signal output from each electrical signal from the first and second light receiving sections. The diversity optical receiver is further equipped with a polarization control element that changes the polarization state of the locally oscillated light and a means for detecting the splitting ratio of the signal light into the first and second light beams in the polarization separation element, thereby achieving polarization control. The element is provided with a function of adjusting the polarization of the locally oscillated light so that the splitting ratio of the locally oscillated light into the first and second light beams by the polarization splitting element is equal to the splitting ratio of the signal light. This is achieved by

(Function) In the present invention, the polarization control element changes the polarization state of the locally oscillated light based on the information obtained by the means for detecting the branching ratio of the signal light into the first and second light beams. By changing the polarization state, the first local oscillation light produced by the polarization separation element is
Although the branching ratio to the second light beam changes, the polarization control element operates so that the branching ratio of the locally oscillated light becomes equal to the branching ratio of the signal light. As a result, the ratio of the outputs from the first and second light receiving sections becomes proportional to the square of the signal splitting ratio by the polarization splitting element. Therefore, the electrical signals from the first and second light receiving sections are linearly added together in the processing section to obtain a signal for signal demodulation. If the local oscillation light is strong enough, the largest noise component that determines the S/N of the system is the shot noise due to the local oscillation light, but the weak local oscillation light Since only a small amount of light is added, the noise generated at the light receiving section is small. Therefore, in the polarization diversity optical receiver of the present invention, even if the electrical signals from the light receiving section with the weaker signal are added together in the processing section, the system's S/N
hardly deteriorates. Moreover, the electric signals to be added are proportional to the square of the ratio of signal magnitudes.

Therefore, according to the present invention, it is possible to perform ratio scaler synthesis by optical control using a polarization control element without using an electrical nonlinear element conventionally used in a processing section, and polarization with less deterioration due to polarization fluctuations. A diversity receiving device can be realized.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the invention. Local oscillation light 2 with linear polarization emitted from local oscillation light source 1 is 1
After passing through a polarization control element 3 consisting of a /2 wavelength plate, the signal light 4 is sent to an optical multiplexer 5 consisting of a beam splitter.
It is combined with The combined output light 6 obtained by this is
The polarization separation element 7 separates the light beam into first and second light beams 8 and 9 having mutually orthogonal linear polarizations.

These first and second light beams 8 and 9 are heterodyne detected by first and second light receiving sections 10.11, respectively.
It is converted into a second electrical signal 12.13. The first of these,
The second electrical signals 12 and 13 are both intermediate frequency electrical signals having a frequency corresponding to the frequency difference between the local oscillation light 2 and the signal light 4 as a carrier frequency. These first and second electrical signals 12.13 are transmitted through first and second delay lines 14, 15,
The signal is demodulated by a processing section consisting of a synthesizer 16 and a detection circuit 18. That is, the first and second electrical signals 12.13 are matched in phase by the first and second delay lines 14.15, and then combined by the combiner 16. The intermediate frequency signal 17 output from the synthesizer 16 is detected by a detection circuit 18 and output as a baseband demodulated signal 19.

In this embodiment, the first,
Detection of the splitting ratio into the second light beams 8 and 9 and determination of the magnitude of the control signal 29 to the polarization control element 3 were simultaneously realized by the following means. When performing normal optical heterodyne detection, the size of the local oscillation light 2 is sufficiently large compared to the size of the signal light 4. Therefore, the first and second light receiving sections 10
．． The photocurrent flowing through 11 is caused by locally oscillated light 2 . Therefore, the first
, by monitoring the second DC component 20.21,
The branching ratio of the locally oscillated light 2 into the first and second light beams 8 and 9 by the polarization separation element 7 can be known. In the present invention, it is necessary to control the branching ratio of the locally oscillated light 2 into the first and second light beams 8 and 9 to be equal to the branching ratio of the signal light 4. Therefore, in this embodiment, control was performed using the following means.

Signal light 4 and local oscillation light control? If the split ratios are equal, the first
, the ratio of the signal amplitudes of the second electrical signal 12.13 is the first,
It is equal to the square of the ratio of the first and second DC components 20 and 21 in the second light receiving section 10.11, that is, the branching ratio of the locally oscillated light. Therefore, both error signals are used as the control signal 29. That is, in this embodiment, first, a part of the first and second electric signals 12 and 13 is branched, and the first dividing circuit 2
2, the ratio was detected and a first output signal 24 was obtained. On the other hand, in the second division circuit 23, the first and second light receiving sections 10.1
The ratio of the first and second DC components at 20.21 at 1 was detected. By passing the signal 25 obtained here through a nonlinear circuit 26 having a square characteristic, the first and second local oscillation lights 2 are
A second output signal 27 was obtained which was proportional to the square of the splitting ratio into the light beams 8 and 9. These first and second output signals 24°2
The difference in magnitude between the first and second output signals 24 and 27 obtained by inputting the signal 7 into the differential amplifier 28 was used as the control signal 29 to the polarization control element 3. This control signal 29 causes V
By rotating the characteristic axis of the polarization control element 3 consisting of two plates, the polarization direction of the locally oscillated light 2 changes, and the splitting ratio of the locally oscillated light 2 by the polarization separation element 7 changes, so that the signal light 4 and the locally oscillated light are separated. The branching ratio of the light 2 is controlled to be equal.

Note that at this time, the branching ratio of the locally oscillated light 2 can be monitored by the monitor output 30 of the second division circuit 23. It is possible to know the variation in the branching ratio of the signal light 4 from the variation in the monitor output.

In this embodiment, a semiconductor laser was used as the local oscillation light source 1, and a multilayer film deposited on a prism was used as the polarization separation element 7. First and second light receiving sections 1
0.11 is composed of a high-speed photodiode, a preamplifier, a main amplifier, etc., and as the detection circuit 18, since the signal light 4 is amplitude modulated light, an envelope detection circuit is used.

Note that the first and second delay lines 14, 15, synthesizer 16, detection circuit 18, etc. used are those used in ordinary microwave communication equipment. FIG. 3 shows the results of measuring the power penalty in the polarization diversity optical receiver of the present invention using the signal light branching ratio as a parameter. The penalty when the signal is biased toward one output terminal is caused by the fact that the local oscillation light output on the side where the signal is output is thick (and therefore, the influence of the intensity noise of the local oscillation light is eliminated). However, the penalty is less than 1 dB. Considering that the conventional system had a power penalty of 3 dB at worst as shown in FIG. 2, the present invention improves the worst value by more than 2 dB.

At this time, a 100 Mb/s binary amplitude modulated signal light propagated through a long distance optical fiber was used as the signal light 4. This signal light 4 is accompanied by polarization fluctuations due to propagation through the optical fiber, but when the code error rate of the demodulated signal 19 was detected, no large fluctuation of more than one digit in the error rate was observed over a long period of time.

In addition to the above-described embodiments, various modifications can be made to the present invention. For example, various lasers other than a semiconductor laser can be used as the local oscillation light source 1, and the signal light 4 may be propagated through space or another leading wave path instead of the one propagated through the single mode optical fiber 3. good. Furthermore, it is also possible to use an optical fiber coupler as the optical multiplexer. In this case, the polarization control element 3 may be one in which an optical fiber is wound into a ring shape, or an element that applies pressure to the side surface of the optical fiber. The polarization separation element 7 may be a Rochon prism or the like using an optical crystal. An appropriate detection circuit 18 is used depending on the modulation format of the signal, for example, an envelope detection circuit or a synchronous detection circuit for optical amplitude modulation, a frequency discrimination circuit, a filter and an envelope detection circuit for optical frequency modulation, etc. For combinational or optical phase modulation, a delay detection circuit or a synchronous detection circuit is used.

It is also possible to control the polarization state of the locally oscillated light 2 in steps. That is, the local oscillation light is kept constant, and the polarization direction of the local oscillation light 2 is changed when the output of the differential amplifier 28 exceeds a certain level. For example, the local oscillation light 2 can be polarized in three directions (vertical, horizontal, 45°).
), the penalty is 1 dB.
It can be held below.

(Effects of the Invention) As described above, by using the polarization diversity optical receiver of the present invention, it is possible to change the combination ratio of received signal components by optically changing one parameter of the polarization of local oscillation light. . Furthermore, since electrical nonlinear elements are not passed through, the deterioration caused by changing the synthesis ratio is small. Therefore, according to the present invention, it is possible to obtain a polarization diversity optical receiver for optical heterodyne detection with little deterioration due to polarization fluctuations.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a diagram showing the power penalty in conventional simple polarization diversity reception, and Fig. 3 is a diagram showing the power penalty in the polarization diversity optical receiver of the present invention. FIG. In the figure, 1...Local oscillation light source, 3...Polarization control element, 4
...Signal light, 5.. Optical multiplexer, 7.. Polarization separation element, 10.11.. Light receiving section, 14.15.. Delay line, 16.. Combiner, 18..・Detection circuit,
22.23...Division circuit, 26...Nonlinear circuit,
28...Differential amplifier, K 1 Figure

Claims (1)

[Claims] a light source for local oscillation light; an optical multiplexing section that multiplexes the signal light and the local oscillation light with a stable polarization state emitted from the light source; a polarization separation element that separates the light beams into first and second light beams orthogonal to each other; and first and second light beams that receive the first and second light beams and convert them into first and second electrical signals, respectively. a light receiving section, a processing section that detects a signal output from each electric signal from the first and second light receiving sections, a polarization control element that changes the polarization state of the locally oscillated light, and a polarization control element that changes the polarization state of the locally oscillated light; and at least means for detecting a branching ratio of the signal light into the first and second light beams, the polarization control element:
operating to adjust the polarization of the locally oscillated light so that a splitting ratio of the locally oscillated light into the first and second light beams by the polarization separation element is equal to a splitting ratio of the signal light; A polarization diversity optical receiver featuring: