JPS63224427A - Method and device for polarization diversity optical reception - Google Patents

Abstract

PURPOSE:To realize high sensitivity, low cost, and miniaturization, by inputting reference light having a frequency different from the carrier frequency of signal light to a light receiver after mixing to the signal light, separating the output of the light receiver to two intermediate frequency signals, and adding them after converting to baseband signals respectively. CONSTITUTION:The reference light 1a and 1b are provided frequencies different from each other, and also, a polarized plane of almost 90 deg. is formed. An optical mixer 11 consisting of an optical coupler 12 mixes the reference light 1a and 1b and the signal light 2, and outputs mixed light 3. The light receiver 14 of a heterodyne detector 13 performs the heterodyne detection of the mixed light 3, and outputs a detecting signal 4. The branching filter 16 of a demodulator 15 branches the detecting signal 4. A band-pass filter 17a passes only the intermediate frequency signal 5a obtained from the signal light 2 and the reference light 1a, and a band-pass filter 17b passes only the intermediate frequency signal 5b obtained from the signal light 2 and the reference light 1b. The intermediate frequency signals 5a and 5b are converted to the baseband signals 6a and 6b by detectors 18a and 18b respectively, and the 6a and 6b are added at an adder 19, then, an output signal 7 can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarization diversity optical reception method and apparatus for heterodyne detection of signal light, and in particular, to
The present invention relates to a method and device suitable for realizing a low-cost and compact device.

[Conventional technology]

Coherent optical transmission, which uses the wave properties of light to transmit information, has the problem of reducing the signal-to-noise ratio (SNR) of the received signal due to fluctuations in the polarization state of signal light. To occur.

Countermeasures are important. As one of the above-mentioned measures, polarization diversity optical reception methods and devices have been proposed in the past. Polarization tie diversity optical reception suppresses a decrease in SNR by suppressing a decrease in the power of a received signal due to a change in the polarization state of signal light.

Regarding the conventional polarization diversity optical reception method and its device, see, for example, "I.O.C. J '83.
June 27-30, 1983, Proceedings No. 386-3
87 pages (100C'83. June 27-30.1
083. Technical Digest pp.
386. -387).

In the conventional method and device, first, a signal light is separated into two orthogonal polarization components by a polarization splitter. The two separated components are each mixed with a reference beam having the same frequency. The two mixed lights are heterodyne detected by different receivers. The two detected signals are added and output as one signal. As a result, if the polarization state of the signal light changes (1) For example, even if the signal light becomes linearly polarized and the signal light input to one receiver becomes zero, all the signal light will be received by the other receiver. Since the signal is input to the receiver, the received signal power does not become zero, that is, the decrease in SNH is suppressed.

[Problem that the invention seeks to solve]

The above-mentioned conventional technology has the following problem 61: Part of the signal light is reflected by the polarization splitter, and the intensity of the signal light input to the light receiver is reduced, resulting in a reduction in the sensitivity of the optical receiver.

2. Due to the loss of the polarization splitter, the signal light is attenuated in the process of passing through the polarization splitter, and the intensity of the signal light input to the optical receiver is reduced, resulting in a reduction in the sensitivity of the optical receiver.

3. In order to obtain two mixed lights, two mixers are required, which increases the cost of the optical receiver.

4. A photoreceiver and associated electrical equipment for heterodyne detection of the two mixed lights! ! (i
2 sources, amplifiers, etc.) and optical components (lenses, etc.).
As a result, a single optical receiver becomes larger and more expensive.

5. Since two sets of optical receivers and two electrical devices are each required, the power consumption of the devices increases and the operational cost of the optical receiver increases.

6. Since the power consumption of the optical receiving device is high, the amount of heat generated by the device is also large, which increases the cost required for cooling the device.

7. In order to make the characteristics of the two light receivers similar, it is necessary to select two light receivers with similar characteristics from a large number of light receivers, which increases the cost of the device.

The purpose of the present invention is to solve the above problems and provide high sensitivity.

The object of the present invention is to realize a low-cost and compact polarization diversity optical reception method and device.

[Means for solving problems]

The above purpose is to have two optical polarization planes with different frequencies at approximately 90°
A reference light and a signal light having different carrier frequencies are mixed, this mixed light is subjected to heterodyne detection, and this detected signal is transmitted to a first wave having a different carrier frequency. Separating into a second intermediate frequency signal, converting these two signals into first and second baseband signals, respectively,
This is achieved by adding these two baseband signals.

[Effect]

Heterodyne detection mixes a reference light with a frequency different from the carrier frequency of the signal light into the signal light, inputs this mixed light into a light receiver, and outputs the signal light, the transmitted wave, and the reference light as the output signals of the light receiver. This is to obtain an intermediate frequency signal having a carrier wave with a frequency difference between .

Here, the power of the intermediate frequency signal is proportional to the intensity of the component parallel to the polarization plane of the reference light in the signal light.

Therefore, when the intensity of the component parallel to the polarization plane of the reference light changes due to a change in the polarization state of the signal light, the power of the intermediate frequency signal also changes in proportion to it. The minimum value of the intermediate frequency signal power is zero, and the minimum value is obtained when the polarization plane of the signal light is orthogonal to the polarization plane of the reference light.

In the present invention, the sum of two intermediate frequency signals having different carrier frequencies is obtained as the photoreceiver output. This is because the reference light includes two lights with different frequencies. Furthermore, the powers of the two intermediate frequency signals are proportional to the intensities of the two orthogonal polarization components of the signal light. This is because the planes of polarization of the two lights forming the reference light form approximately 90 degrees. Therefore, one intermediate frequency signal is obtained from one of the lights forming the reference light and a component of the signal light parallel to the plane of polarization. Further, the other intermediate frequency signal is obtained from the remaining components of the signal light and the other light among the reference lights. Therefore, the sum of the powers of the two intermediate frequency signals does not become zero, no matter what polarization state variation of the signal light.

Furthermore, since the two intermediate frequency signals have different carrier frequencies, they can be separated by a filter. The separated intermediate frequency signals can be made into one signal by converting them into baseband signals (carrier frequency is zero) and then adding them.

Therefore, the power of this signal does not become zero regardless of the polarization state of the signal light. That is, the reduction in signal-to-noise ratio (SNR) is suppressed. Furthermore, the above effects can be achieved without using a polarization splitter and by using one light receiver.

〔Example〕

An embodiment of the present invention is shown in FIG. In the same figure, 1a
and 1b are reference lights, which have different frequencies, and
The plane of polarization forms approximately 90 degrees. 2 is a signal light. 3 is a mixed light of the reference lights 1a, lb and the signal light 2. 4 is
This is a detection signal obtained by heterodyne detection of the mixed light 3. 5a and 5b are intermediate frequency signals obtained by a signal light and two reference lights, respectively. 6a and 6b
are baseband signals obtained from intermediate frequency signals 5a and 5b, respectively. 7 is a baseband signal 6a
and 6b, and is the output signal of this device. 8 is an optical fiber through which the signal light 2 passes. 9 is a local oscillator that outputs reference beams 1a and 1b. 10
a and 10b are light sources constituting the local oscillator 9, which can be realized by a laser that oscillates in a single mode, etc. 6 For example, when a semiconductor laser is used, the pn junction planes of the respective active layers are approximately parallel to each other. If the semiconductor lasers are arranged at 90 degrees, the polarization planes of reference beams 1a and 1b will be approximately 90 degrees.
11 is the reference light la, lb and the signal light 2
This is an optical mixer that mixes the light and outputs mixed light 3. 12 is an optical coupler that constitutes the optical mixer 11. The optical coupler can be realized using a single mode optical fiber, a directional coupler, or the like. 13 is a heterodyne detector. , 14 is
It is a light receiver that heterodyne detects the mixer 3, and the PIN
- Can be realized with PD (pin-photo diode), APD (avalanche photo diode), etc. Va in the figure means the voltage applied to the light receiver 14. 15 is a demodulator that reconstructs the transmitted signal from the detected signal 4. 16 is a splitter that branches the detected signal 4. 1
7a and 17b are bandpass filters, respectively. The bandpass filter 17a passes only the intermediate frequency signal obtained from the signal light 2 and the reference light 1a, and the bandpass filter 17b passes only the intermediate frequency signal obtained from the signal light 2 and the reference light 1b. let 18a and 18b are envelope detectors, respectively. The intermediate frequency signal 5a is converted into baseband signals 6a and 6b by the detector 18b, and the intermediate frequency signal 5b is converted by the detector 18b, respectively. 19 is an adder that adds the baseband signals 6a and 6b.

The operation of this embodiment will be described below with reference to FIGS. 2 and 3. Figure 2 shows the polarization plane of light in each part and the third
The figure schematically represents the frequency spectrum of each signal. The polarization plane of the reference light 1a is shown in FIG. 2(a) (electric field amplitude: ELOV) + the polarization plane of the reference light 1b is shown in FIG. 2(b) (electric field amplitude: ELO), respectively. Since the polarization planes of 1b are approximately 90 degrees to each other,
The relationship between the two is as shown in FIG. 2(c).

On the other hand, if we consider as an example the case where the signal light 2 is a linearly polarized wave, the polarization plane of the signal light 2 will be as shown in FIG. 2(d). In the figure, Es is the electric field amplitude of the signal light 2. Esv
and ESH are E Lo among the components of Es, respectively.
t/ and the component parallel to E LOH. Esn and E
Let θ be the angle formed by s.

The polarization state of the signal light can be expressed by θ, and the following equation holds.

However, 0≦0≦90° The plane of polarization of the mixed light 3 is shown in FIG. 2(e). As shown in the same figure, EshiOv and Esv, and ELi')H and EsH are
They are parallel to each other. Therefore, the mixed light 3 is input to the photoreceiver, and the resulting detected signal I (t) is expressed by the following equation.

I (t)”Ia (t)+Ib (t)
...self-(2) itanshi, Ia (t)=D-Esv-E
Lov-cos [2g(fs-fv)+δa]1b(
t)=D−E−El, oH−cos[2πgea fn
)+δb] In equation (2), D is a constant determined by the receiver, and fv, fs and fs are the frequencies of the reference beams la and lb and the carrier wave frequency of the signal beam 2, respectively (Fig. 3(a), (see (b)). Also, δ8 and δ
b is the phase difference. Furthermore, Xa(t) is the reference beam 1a
represents an intermediate frequency signal obtained from signal light 2 and signal light 2, and its carrier frequency is (fs fv). Also,
h(t) represents an intermediate frequency signal obtained from the reference light 1b and the signal light 2, and the carrier wave frequency is (fs-fH). Therefore, equation (2) is expressed as the detected signal I (t)
are two intermediate frequency signals Ia(t) and Ib(t
). FIG. 3(C) shows the frequency spectrum of the detected signal I(t). In the figure, the spectrum within the dashed line on the right side represents Ia(t), and the spectrum within the dashed line on the left side represents Ib(t).

Therefore, the detected signal 4 is branched, respectively, as shown in FIG. 3(c).
When input to bandpass filters 17a and 17b having the pass characteristic shown by the broken line, the signal ra is output as the output of 17a.
(t), and the signal I+, (t) as the output of 17b.
can be obtained. Furthermore, the signals I a(t ) and Ib(t) are input to envelope detectors 18a and 1, respectively.
When input to 8b, the signal becomes a baseband signal, and its amplitude is expressed by the following formula.

Ia' (t) =D -Esv -ELov=D
Es -Esov-sinθ...(3) Ib' (t) =D-Eso・Et, oH=DEs-
Ia' (t) and ib' of Etano cos θ above formula
When (t) is added by the adder 19, a signal expressed by the following formula is obtained as the output signal 7.

I' (t) = Ia' (t) + Ib' (t) =
DEs @ (ELO!/ Fist sinθ+Et, on
-cos(J)-(4) The output signal of this embodiment expressed by equation (4) does not become zero for any value of 0. That is, the output signal I'(t) can be obtained for any polarization state of the signal light.

FIG. 4 shows another embodiment of the demodulator 15. In this example,
The light intensities of the reference beams 1a and 1b are made substantially equal to each other, and the baseband signals 6a and 6b are each substantially squared before being added. Therefore, square law detectors 20a and 20b are provided before the adder 19. Expressing the above conditions by a formula, we obtain the following formula.

(5) By rearranging the above equation (5) and substituting equation (3), the following equation is obtained as the output signal I' (t).

I' (t) cow (D・Es・
PLO)''...(6) The above equation (6) calculates the output signal I' (
t) indicates that it does not depend on θ, that is, the output signal has a constant value regardless of the polarization state of the signal light.

Therefore, the signal power is also constant. The condition of the first equation of equation (5) can be realized by adjusting the drive current of the light source that outputs ELOI/and ELOH. Further, the condition of the second equation of equation (5) can be realized by using a diode or a field effect transistor as a square law detector. According to this embodiment, the same effect as in the case of FIG. 1 is obtained, and at the same time, the power of the output signal can be made constant regardless of the polarization state of the signal light.

FIG. 5 shows another embodiment of one local oscillator 9.

In this embodiment, the local oscillator 9 is realized by one light source. In the figure, 10 is a light source such as a single mode semiconductor laser. 21 is a polarization separator such as a polarization beam splitter, and by inputting the output light of the light [10 to the polarization separator 21, the output light is separated into two lights 24a and 24b whose polarization planes are approximately 90 degrees. In particular, if the output light of the light source 10 is linearly polarized, and the angles of the polarization splitter 21 and the light source 1o are adjusted so that the plane of polarization and the polarization axis of the polarization splitter 21 form approximately 45 degrees, , it is possible to substantially match the intensities of the two lights. , 22 is light 2
This is a mirror that adjusts the traveling direction of 4a. 23 is light 2
623, which is an optical frequency converter that changes the frequency of 4a, is
This can be achieved by using an acousto-optic crystal or an electro-optic crystal. For example, optical frequency converters using acousto-optic crystals are commercially available. By using the light output from the optical frequency converter 23 as the reference light 1a and using the light 24b as the reference light 1b, the local oscillator 9 can be generated using one light source 10.
can be realized.

According to this embodiment, the same effect as in the case of FIG. 1 can be obtained, and at the same time, the number of light sources can be halved, so that the cost of the apparatus can be reduced.

〔Effect of the invention〕

According to the present invention, a polarization diversity optical reception method and apparatus having the following effects are obtained.

1. Since a polarization splitter, which conventionally causes signal light attenuation, is not used, the sensitivity of the optical receiver can be increased.

2. Since the number of mixers that conventionally required two can be reduced to one, the cost of the optical receiver can be reduced.

3. The number of optical receivers and associated electrical equipment and optical components that conventionally required two sets can be reduced to one set, making it possible to reduce the size and cost of the optical receiving device and at the same time reduce power consumption. Equipment operating costs can also be reduced, and
Since the amount of heat generated by the device can be reduced, the cost required for cooling the device can also be reduced.

4. The selection of light receivers that was conventionally required when using two light receivers is no longer necessary, and the cost of the optical receiver can be reduced.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention. Figure 2 is a diagram showing the polarization plane of light in each part in the embodiment shown in Figure 1, Figure 3 is a diagram showing the frequency spectrum of the signal in each part in the embodiment shown in Figure 1, and Figure 4 is a diagram showing different demodulators. FIG. 5 is a block diagram showing another embodiment of the local oscillator. 1...Reference light, 2...Signal light, 3...Mixed light, 4
Detection signal, 5 Intermediate frequency signal, 6 Baseband signal, 7 Output signal, 8 Optical fiber, 9 Local oscillator. 10... Light source, 11... Optical mixer, 12... Optical coupler, 13... Heterodyne detector, 14... Light receiver, 15... Demodulator, 16... Brancher, 17...
Bandpass filter, 18... Envelope detector, 19... Adder, 20... Square law detector, 21... Polarization separator, 2
2... Mirror, 23... Optical frequency converter.

Claims (1)

[Claims] 1. Mixing the first and second reference lights and the signal light in which the polarization planes of the two lights with different frequencies form approximately 90 degrees, performing heterodyne detection on this mixed light, and By separating the detected signal into first and second intermediate frequency signals with different carrier frequencies, converting these two intermediate frequency signals into first and second baseband signals, and adding these two baseband signals. A polarization diversity optical reception method characterized by obtaining an output signal. 2. Claim 1, characterized in that the light intensities of the first and second reference beams are made substantially equal, and the first and second baseband signals are each substantially squared before being added. A polarization diversity optical reception method. 3. In claim 1 or 2, the first and second reference beams split the output light of one single mode semiconductor laser into two, and at least one of the branched lights has a frequency. A polarization diversity optical reception method characterized by obtaining polarization diversity. 4. The polarization plane of the first and second reference beams with different frequencies is approximately 90
a means for outputting the output in such a way as to achieve a certain degree;
, means for mixing the second reference light and the signal light, means for heterodyne detection of the mixed light, means for separating the detected signal into first and second intermediate frequency signals having different carrier frequencies; A polarization diversity optical receiver comprising means for converting a second intermediate frequency signal into first and second baseband signals, respectively, and means for adding the first and second baseband signals.