JPS59122140A - Optical heterodyne detector - Google Patents

Abstract

PURPOSE:To attain optical heterodyne detection stable in the detecting characteristic by separating a synthesized wave of an input signal and a local oscillating light into two optical beams where polarized planes are orthogonal. CONSTITUTION:An output light 2 of a local oscillator 1 is synthesized with a signal light 6 at an optical wave synthesizer 7 as a circular polarized light at a circular polarized element 3 of a polarized light converting element 3. A synthesized output light 8 is separated into the 1st and the 2nd optical beams 10, 11 having a linearly polarized wave orthogonal with each other by a polarized light separating element 9. Each optical beam is converted into electric signals 14, 15 at the 1st and the 2nd photodetecting sections 12, 13, and inputted to detecting circuits 16, 17. Each detected output is converted into base band signals 18, 19, synthesized at a synthesizer 22 and reduced to a demodulated output 23. Thus, the stable detecting characteristic is attained independently of the polarized state of the signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical heterodyne detection device used in optical communication systems, optical information processing systems, and the like.

In general, the optical heterodyne detection method has the great advantage of increasing reception sensitivity by 10 to 100 times compared to the conventional optical direct detection method, so it is effective for long-distance optical communication trunk systems and various high-sensitivity optical sensors. It uses a light detection method.

In order to achieve high reception g with this optical detection method, efficient multiplexing of signal light and local oscillation light is required.
For this purpose, the propagation directions, polarization states, beam diameters, etc. of these two lights must be matched. However, in optical communications, the polarization state of signal light propagated over a long distance through an optical fiber is affected by various disturbances applied to the optical fiber, and the polarization state changes over time. To,
! 'The polarization state fluctuates. Therefore, stable and efficient multiplexing cannot be performed as is.

Traditionally, the following two methods have been studied to solve this problem.
It is considered a ball. One method is to use a fiber with good polarization preservation. This can be achieved by increasing the birefringence of the fiber by, for example, making the cross-sectional shapes of the fiber core and cladding oval, or by giving the fiber a stress distribution. The aim is to improve wave sustaining characteristics. However, the conservation and polarization stability during long-distance propagation have not been confirmed at present. Further, when connecting fibers, the characteristic axes must be aligned, which causes a problem in that connection work when installing optical fibers is difficult. Furthermore, span fibers that preserve unicircularly polarized waves have been considered, but this has the drawback of being weak against external forces.

Another method is to monitor the polarization state of the signal light during reception, and then control the polarization state of the signal light or local oscillation light to achieve polarization matching between the signal light and the local oscillation light. be. However, this method requires a complicated configuration of the device that controls the polarization state, has a large insertion loss of 5 dB or more, and is difficult to detect due to the low level of the signal light. There are a number of problems, including the need for highly sensitive detectors.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical heterodyne detection device which eliminates these drawbacks, provides stable detection characteristics regardless of the polarization state of a signal, and has a simple configuration.

The optical heterodyne detection device of the present invention includes an optical multiplexing section that multiplexes input signal light and local oscillation light with a stable polarization state, and a multiplexed light obtained by this optical multiplexing section whose polarization planes are orthogonal to each other. 1st. a polarization splitting element that separates the light beam into a second light beam; receiving the second light beam and receiving the first and second light beams respectively; The first electrical signal is converted into a second electrical signal. a second light receiving section;
These first. and a processing section that detects a signal output from each electric signal of the second light receiving section.

In the present invention, signal light with an undefined polarization direction propagated through an optical fiber is multiplexed with local oscillation light with a stable polarization state in an optical multiplexer, and this combined light enters a polarization separation element. , the first . of linearly polarized light having mutually orthogonal polarization planes.
The second light beam is split into two, each incident on a separate light receiving element, and the first and second light beams are split into two. converted into a second electrical signal.

In this case, 1. Each signal light component and local oscillation light component of the second light beam are linearly polarized lights and have the same polarization direction. Furthermore, since the polarization state of the locally oscillated light is stable, the light intensity of the locally oscillated light component is also stable. Therefore, the first. The second light beam is the first. The respective conversion efficiencies when converted into second electrical signals are constant. However, since the polarization direction of the signal light is indeterminate, the first. The light intensity of the signal light component of the second light beam is unstable and fluctuates greatly, so the intensity of the first and second electric signals also fluctuates in response to the fluctuations in the signal light component.

By the way, number one. The sum of the light intensities of the signal light components of each of the second light beams is equal to the light intensity of the signal light before entering the polarization separation element, and it is considered that the sum of the light intensities is almost stable. This is fine because the conversion efficiency of the light receiving section to electrical signals is constant.

1st. If the sum of the respective powers of the second electrical signal is also taken, it can be stabilized to a nearly constant value.

In addition, 1. If one of the second electrical signals is selected with a higher signal strength, the signal power is 1/1/2 of the sum of each electrical signal.
You can also get 2 or more. Therefore, the ideal polarization state (
In contrast to the case of heterodyne detection of a signal light (linearly polarized wave in a constant polarization direction), there is a strong Lou deterioration or S/N of the electrical signal.
Deterioration can be suppressed to 3 dB or less.

As described above, the present invention provides an optical heterodyne detection device that is simple, has low loss, and has stable detection characteristics independent of the polarization state of signal light without using a polarization-maintaining fiber or a polarization control device. can get.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a block diagram for explaining a first embodiment of the present invention. First, the output light 2 of the local oscillator 1 enters the polarization conversion element 3 and is converted into circularly polarized locally oscillated light 4.

This circularly polarized locally oscillated light 4 is combined with a signal light 6 emitted from an optical fiber 5 by an optical multiplexer 7. This combined output light 8 is passed through a polarization splitting element 9 into a first wave having linearly polarized waves orthogonal to each other. It is split into a second light beam 10.11.

These first. The second light beams 10.11 are respectively
．． The light enters the second light receiving sections 12 and 13 and is subjected to heterodyne detection. second electrical signal], 4, 15
is converted to These first. Second electrical signal 14.15
Both are electric signals having a carrier frequency corresponding to the frequency difference between the local oscillation light 4 and the signal light 6.

These first. The second electric signal 14.15 is detected by the first and second detection circuits 16 and 17, and the second electric signal 14.15 is detected by the first and second detection circuits 16 and 17. It is converted into a second baseband signal 18.19. These first. Second
The baseband signal 18.19 of the first. second delay line 2
After the phases are matched by 0.21, the signals are combined in a combiner 22 and output as a stable demodulated signal 23.

In this example, a semiconductor laser with stabilized output was used as the local oscillator 1, and a Pavinet-Soleil phase compensator was used as the polarization conversion element 3, so that stable circularly polarized locally oscillated light 4 was obtained. Further, as the optical multiplexer 7, a mirror 24 having a transmittance of about 70% and a reflectance of about 30% is used. An angle of 45° was created with respect to the signal light 6, and both parties were combined. Further, as the polarization separation element 9, a prism with a multilayer film deposited thereon is used. The second light receiving section 12.1-3 includes a high-speed photodiode, a preamplifier,
It consists of a main amplifier etc., and the first. Second detection circuit 16.17
Since the signal light 6 was amplitude modulated light, an envelope detection circuit was used. Note that the first and second detection circuits 16 and 17
, 1st. Second delay line 20.21. The combiner 22 and other components used in ordinary microwave communication equipment were used.

Further, as the synthesizer 22, a ratio scaler synthesizer was used which synthesizes the amplitudes of each baseband signal using a ratio obtained by squaring the amplitude ratio.

In such a configuration, when signal light is supplied to a single mode fiber (optical fiber) with a length of 10 km, the output signal light 6 from the optical fiber 5 has a polarization state of the optical fiber 5.
However, the intensity of the first and second electric signals 14 and 15 also changed in response to changes in the polarization state of the signal light 6.
As the output demodulated signal of the ratio scaler synthesizer 22, a signal with almost no S/N deterioration and no output fluctuation could be obtained.

FIG. 2 is a block diagram of a second embodiment of the present invention3.
In this embodiment, the output light 2 of the local oscillator 1 is converted into a linearly polarized local oscillation light 4 having an inclination of 45° with respect to the optical axis direction of the polarization separation element 9 by the polarization conversion element 3.
However, the other optical systems are the same as in the first embodiment. The configuration in which this embodiment differs from the first embodiment is as follows. The problem lies in the processing of signals after the second light receiving section 12.13. That is, 1st. The first and second electric signals 14.15 from the second light receiving section 12.13 have their intensities detected by a comparison circuit 25, and this detection signal causes the switching section 2
Move 6 to 1st. Of the second electrical signals 14.degree. 15, only the one with greater intensity is sent to the detection circuit 27.

The polarization state of the signal light 6 from the optical fiber 5 varies greatly depending on bending or twisting applied to the optical fiber 5, ambient temperature, etc. At this time, the first and second light beams 10.
One of the 11 signal light components always has an optical intensity that is 4 or more than the optical intensity of the signal light 6, and therefore the demodulated signal 23 in this case has a maximum level fluctuation of 3 dB. However, this level fluctuation of the demodulated signal 23 can be sufficiently stabilized by using an automatic gain control circuit or the like. Since the fluctuations of the second electrical signals 14.15 are relatively slow in terms of time, the comparison circuit 25. The switching unit 26 can also sufficiently follow the fluctuations. Therefore, in this embodiment, as in the first embodiment, a stable demodulated signal can be obtained with a relatively simple configuration.

In addition to the above-described embodiments, various modifications can be made to the present invention. For example, the local oscillation light 4 may be any elliptically polarized light as long as it has an inclination of 45 degrees with respect to the polarization axis direction of the polarization separation element 9;
Even if the polarization has a slope other than °, the first. As long as the ratio of the local oscillation light components of the second light beam 10.11 can be detected and the corresponding correction can be applied by the electric circuit, it will be stable in any polarization state as in the embodiment. A demodulated signal can be obtained. The signal light 6 may not be propagated through an optical fiber, but may be propagated through space or another optical waveguide. As the optical multiplexer 7, in addition to one using the mirror 24, various types such as one using a proximity waveguide, one using a diffraction grating, etc. can be used. The polarization separation element 9 may be a Rochon prism or the like using an optical crystal. Further, the detection circuits 16, 17, and 27 are appropriately selected depending on the modulation format of the signal, such as an envelope detection circuit or a synchronous detection circuit for optical amplitude modulation, and a frequency discrimination circuit for optical frequency modulation.

For optical phase modulation, a delay detection circuit or a synchronous detection circuit is used. In the first embodiment, the first. second electrical signal 1
4.15 may be combined and then detected, and in the second embodiment, the first. Comparison of signal levels after detecting the second electrical signal 14.15.

Switching may also be performed. In addition, in the first embodiment, the first
．． A simply configured synthesizer in which the synthesis ratio of the second electrical signals 14 and 15 is kept constant may be used.

Although the amplitude fluctuation in this case is 3 dB at maximum, a multistable demodulated signal can be obtained by using an automatic gain control circuit or the like. Furthermore, the first. It is also possible to select the one with the highest S/N among the second electric signal 14.15 and a composite signal obtained by combining these electric signals 14.15 with their phases matched. According to this configuration, the S It is possible to suppress the variation in /N to within 0.7 dB.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the invention, and FIG. 2 is a block diagram of a second embodiment of the invention. In the figure, 1...Local oscillator, 2...Local oscillator output light, 3...Polarization conversion element, 4...
Local excavation light, 5... Optical fiber, 6...
・Signal light, 7... Optical multiplexer, 8... Combined light, 9... Polarization separation element, 10.11...
...Light beam, 12.13... Light receiving section, 14
．． 15... Electric signal, 16, 17.27-°-
. Detection circuit, 18.19...Baseband signal,
20゜21...delay line, 22...combiner, 23...demodulated signal, 24...mirror, 25...comparison circuit , 26... is a switching section.

Claims (3)

[Claims] (1) An optical multiplexer that multiplexes input signal light and local oscillation light with a stable polarization state, and a first optical multiplexer whose polarization planes are orthogonal to each other. a polarization splitting element that separates the light beam into a second light beam; receiving the second light beam and receiving the first and second light beams respectively; a first and second light receiving section that converts the first and second electrical signals into a second electric signal; An optical heterodyne detection device including a processing section that detects a signal output from each electrical signal of the second light receiving section. (2) The processing section is the first. 2. The optical heterodyne detection device according to claim 1, further comprising a signal combining section that adds the second electrical signal. (3) The processing unit is the first one. 2. The optical heterogain detection device according to claim 1, further comprising a signal selection section that selects one of the second electrical signals having a larger signal strength.