JPS6215523A - Optical heterodyne detecting device - Google Patents

Abstract

PURPOSE:To perform high-reliability optical heterodyne communication by providing two systems of device constitution for optical heterodyne detection equipped with a polarization control function in parallel and performing instantaneous switching when polarization control by one system becomes difficult while proceeding to the other system continuously. CONSTITUTION:A driving voltage V2l impressed to a polarization control element 4a is outputted from a polarization control element driving circuit 4b to a control part 12 from the moment the branch ratio of an optical branching device 8 reaches 0:1. The control part 12 receives it and controls a polarization control element driving circuit 3b so that the driving voltage V1l of a polarization control element 3a is V2l. Thus, the states of polarization control circuits 3 and 4 are inverted at the time of the transition of the flow of signal light L4d2 accompanying the approximation of the driving voltage V1 of the polarization control element 3a to a limit voltage. Then, the flow of the signal light L2 is changed alternately between the side of the 1st photocoupler 5 and the side of the 2nd photocoupler 6 according to the change of the polarization state of the signal light L2 to perform stable optical heterodyne detection which is not broken permanently.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applied to an optical heterogain communication system,
The present invention relates to an optical heterodyne detection device that operates stably over a long period of time.

[Prior art and its problems]

In an optical heterodyne communication system using a single mode fiber, when performing optical heterogain detection, it is necessary that the polarization state of the signal light and the polarization state of the local oscillation light match. However, when transmitting a single mode fiber,
The polarization state of the signal light after it has been transmitted changes due to disturbances such as temperature changes during that time, which causes the beat signal strength to fluctuate, leading to a decrease in system reliability and, in some cases, making it impossible to detect the signal light. Become. Therefore, in order to ensure high reliability in the above communication system, it is necessary to provide an optical heterodyne detection device having a polarization control function that matches the polarization states of the signal light and the local oscillation light. In the future, the polarization control function of the heterodyne detection device must be able to perform automatic control and operate stably over a long period of time without momentary interruptions.

Conventional devices for automatically controlling polarization include, for example, R.

Applied Fixtures Letters by Ulrich,
35, 1979, pp. 840-842. This device places an electromagnet on the side of a single-mode fiber and controls polarization using the action of an electromagnetic field.When the polarization state of light propagating in the single-mode fiber continues to change in a certain direction, the electromagnet The applied voltage continues to increase or decrease and eventually reaches the maximum or minimum limit voltage, making polarization control impossible, so when the applied voltage reaches a predetermined value, the It has a mechanism in which the control circuit is reset.

If such an automatic polarization control device is installed before the optical heterodyne detection circuit, stable polarization control will not be performed from the time the control circuit is reset until it reaches a steady state, and stable optical heterodyne detection cannot be performed during that time. This poses a problem in that the communication system may fall into a momentary interruption state.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical heterodyne detection device that can automatically and stably control the polarization of signal light over a long period of time without any momentary interruption, thereby increasing the operational reliability of a communication system. It is in.

[Structure of the invention]

The present invention provides an optical heterodyne detection device including a first optical splitter that splits a first incident light into two lights of approximately equal intensity, and a second optical splitter that branches a second incident light into two and whose branching ratio is adjusted. a second optical splitter that can be changed continuously; and first and second polarization control circuits that each include a polarization control element and its drive circuit and that manually control each part branched by the first optical branch; , a first optical coupler that combines the output light of the first polarization control circuit and one of the lights branched by the second optical splitter; and the output light of the second polarization control circuit. a second optical coupler that combines the other light branched by the second optical splitter; and first and second optical couplers that detect the outputs of the first and second optical couplers as electrical signals.
operations of a photodetector, a mixing circuit that adds signals output from the first and second photodetectors, a polarization control element drive circuit of the first and second polarization control circuits, and the second The present invention is characterized in that it is configured to include a control section that controls the branching ratio of the optical branching device.

[Effect]

In the optical heterodyne detection device according to the present invention, the polarization of either the signal light or the local oscillation light, which is the incident light, is compensated for by a polarization control circuit controlled by a control section,
Match the polarization states of both lights. In the following, a case will be described in which polarization compensation is performed on locally oscillated light. Two systems, a first system and a second system, are provided for the configuration consisting of two systems with matching polarization states.

Which system should be used for optical heterodyne detection?
It depends on which system the signal light is sent to by the second optical splitter. Note that the local oscillation light is always sent to both systems.

Assume that optical heterodyne detection is initially performed using the first system. In other words, the signal light is sent only to the first system,
An optical heterodyne detection signal is output from the photodetector in the first system. At this time, the drive voltage of the polarization control circuit in the second system is an appropriate value where the difference between the drive voltage value and the drive voltage value of the polarization control circuit in the first system is an integer multiple of 2π. It is controlled by the control unit as follows. Here, ■2π is the polarization angle or phase difference of light in the polarization control circuit in the first and second systems.
This is the voltage required to change by [rad]. Therefore, like the first system, the second system is always maintained in a state where polarization compensation of the locally oscillated light can be performed.

Now, as the voltage of the polarization control circuit of the first system approaches the limit voltage due to a change in the polarization state of the incident signal light, the branching ratio of the optical splitter is changed by the control unit, and the signal destination stream is continuously changed to the first one. Transfer from the first system to the second system. During this transition, electrical signals are output from each of the photodetectors in the first and second systems, and the sum of these signals becomes an optical heterodyne detection signal.

From the time the above transition is completed, optical heterodyne detection signals are output only from the photodetectors in the second system. Immediately after the completion of this transition, the drive voltage of the polarization control circuit in the first system is such that the difference between its drive voltage value and the value of the drive voltage of the polarization control circuit in the second system is an integer multiple of 2π. It is controlled by the control unit so that it becomes an appropriate value.

That is, the states of the first and second systems are reversed compared to the initial state. By continuing the operation involving the transition of optical heterogain detection between the first and second systems as described above, optical heterogain detection can be performed permanently without momentary interruption in principle, regardless of changes in the polarization state of the signal light. be able to.

Even in the case of polarization compensation of the signal light, the same effect as in the case of polarization compensation of the local oscillation light can be obtained by reversing the incidence of the signal light and the local oscillation light into the optical heterogain detection device according to the present invention. can.

By the way, in optical heterodyne detection, there is a level of locally oscillated light at which the detection sensitivity of signal light is maximum. Therefore, if the branching ratio of the signal light and the local oscillation light is changed at the same time when optical heterogain detection is transferred, the level of the local oscillation light in the photodetector will change during the transition, resulting in a decrease in the detection sensitivity of the signal light. There is a problem. According to the optical heterogain detection device according to the present invention, local oscillation light can always be supplied to the photodetector at a constant level, so detection can always be performed with maximum detection sensitivity.

〔Example〕

The present invention will be described in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical heterodyne detection device according to the present invention. The local oscillation light L1, which is the first human radiation excited and output by the local oscillation light source 1, is
The first light splitter 2 splits the light into two lights with approximately equal intensity, and then the first and second polarization control circuits 3.4 respectively
Then, the polarization state of the signal light is converted to match that of the signal light, and the signal light enters the first and second optical couplers 5 and 6. A leading waveguide 7 is used for transmitting the local oscillation light. On the other hand, the signal light L2 that is the second human incident light that has entered the second optical splitter 8 is split by the optical splitter 8 whose splitting ratio can be changed continuously, and then is optically transmitted via the leading waveguide 7. Combiner 5
．． 6 respectively. The optical coupler 5.6 combines the signal light L2 and the local oscillation light L1, and outputs the respective output lights to the first and second photodetectors 9.10. Each of these output lights is converted into an electrical signal by a photodetector 9.10, and then added in a mixing circuit 11 at an appropriate mixing ratio.

This added signal is output from the mixing circuit 11 as a detection output of the optical hetero gain detection device.

The polarization control circuits 3 and 4 each include a polarization control element 3.
a, 4 a and polarization control element drive circuit 3'b, 4
It consists of b. Reference numeral 12 denotes a control unit which inputs state signals from the polarization control element drive circuits 3b and 4b, as well as the branching ratio of the optical splitter 8, the mixing ratio of the mixing circuit 11, and the polarization control element drive circuit 3b.

Control signals are given to these to control the operations of 4b. The polarization control circuits 3 and 4 perform feedback control so that the detection output is maximized.

Next, a control operation for stably detecting the signal light L2 over a long period of time in the above configuration will be explained.

Generally, polarized light is expressed by a polarization angle θ and a phase difference φ between two mutually orthogonal modes, and the polarization state changes with a period of 2π rad for both the polarization angle θ and the phase difference φ. Here, the driving voltage of the polarization control elements 3a and 4a is
Each ■2. ! and V,,! <1=1.2.・
..., 1; i is a natural number), and polarization control circuits 3 and 4
It is assumed that the polarization state of the local oscillation light incident on is always constant. At this moment, the signal light L2 with a polarization angle θ5 and a phase difference φ5 enters this optical hetero gain detection device, and the polarization angle and phase difference of the local oscillation light L1 are set to θ5 and φ5, respectively, by the polarization control circuit 3. Suppose it has been converted. After that, the polarization angle and phase difference of the signal light are θ5+△θ and φ5, respectively.
If it changes to +△φ, the change in polarization angle and phase difference △
Change △V corresponding to θ and △φ, (j2=1.2°・
. . , 1;] is a natural number), the driving voltage of the polarization control element 3a changes. Therefore, when the magnitude of either or both of the polarization angle and phase difference of the signal light diverges to infinity, the magnitude of the drive voltage 11 of the polarization control element 3a also oscillates to infinity. The above also applies to the polarization control element 4a.

As can be seen from consideration of the periodicity of polarized light, the drive voltages V1. ! , V2A (, C=1.2°..., 1
: l is a natural number) exist infinitely, with intervals of the minimum voltage value required to theoretically change the polarization angle or phase difference of the locally oscillated light by 2π rad. However, in reality, 2. polarization control element drive circuit 3b.

The driving voltages v, , V2° of the polarization control elements 3a, 4a that can realize a constant change in the polarization state consisting of the output voltage of the polarization control elements 4b or the breakdown voltage of the polarization control elements 3a, 4a have finite values y, , respectively. min to V2. maM and y2. m”
5 to V27''M (, &-1, 2, . . ., 1:1 is a natural number).

In the polarization control circuits 3 and 4 in the optical heterodyne detection device according to the present invention, the polarization control element 3a is capable of realizing a constant change in polarization state as will be described later.

4a driving voltage ■1. ! and ■2. ! Since it is necessary to realize two different values of Vl,
and V2. At least two of each are required.

Now, let us consider a case where the optical splitter 8 is controlled by the control unit 12 so that the splitting ratio of the emitted light to the optical coupler 5 and the optical coupler 6 is 1:0. The signal light L2 that has entered the optical splitter 8 passes through the optical splitter 8 and is entirely incident on the optical coupler 5. On the other hand, the local oscillation light L1 that has been branched by the optical splitter 2 and entered the polarization control circuit 3 is converted into the same polarization state as the signal light L2, and then enters the optical coupler 5. The optical coupler 5 combines the signal light and the local oscillation light whose polarization states match each other. This combined light is detected by a photodetector 9, converted into an electrical signal, and then sent to a mixing circuit 1j without manual effort. Here, the mixing ratio of the input signals from the photodetector 9 and the photodetector 10 in the mixing circuit 11 is controlled by the control unit 12 so that it becomes 1:0. Therefore, the photodetector 9
The output signal is directly output from the mixing circuit 11. This output signal becomes the output of the optical heterodyne detection device.

In the above case, the drive voltage 1A applied to the polarization control element 3a is also output to the control section 12 from the polarization control element drive circuit 3b. In response to this, the control unit 12 determines that the driving voltage (2) of the polarization control element 4a is Vl,! ”(j
? =1.2. . . , I; 1 is a natural number). VIA here! ” is any one of a plurality of values different from the drive voltage Vl of the polarization control element 3a in the drive voltage Vl of the polarization control element 3a that can match the polarization state of the local oscillation light with the polarization state of the signal light. Therefore, the polarization state of the local oscillation light incident on the polarization control circuit 4 is always converted to match the polarization state of the signal light.In addition, in the above description of Vl/0, the polarization control element The voltage value explained by replacing the description regarding 3a1':l with the description regarding the polarization control element 4a is given by ■2.!*(β-1゜2,...
・, 1:1 is a natural number).

Due to the change in the polarization state of the signal light, the value of even one of the drive voltages V, , of the polarization control element 3a changes to Vl,'H-△■
(△■ is a positive voltage) or y, mrn+8■ or more, that is, when ■1° approaches the limit voltage,
The control unit 12 sets the branching ratio of the output light from the optical splitter 8 to the optical coupler 5 and the optical coupler 6 from 1;0 to (1-x):X (0<X
<1) and then continuously changes to 0;1. When the branching ratio of the optical splitter 8 is (1-X): Polarization control is performed by independently controlling the driving voltage.

The two locally oscillated lights, which are converted into a polarization state that matches the polarization state of the signal light by the polarization control circuits 3 and 4, are split into two signal lights whose intensity is (iX):X by the optical splitter 8, The respective signals are combined in an optical coupler 5.6. Each of the multiplexed lights is detected by photodetectors 9 and 10, converted into an electrical signal, and then input to a mixing circuit 11. The control unit 1 controls the mixing ratio of the input signals from the photodetector 9 and the photodetector 10 in the mixing circuit 11 to be (1-X):X.
2. The output signals of the photodetectors 9 and 10 are added together in a mixing circuit 11 and output as an output signal of the optical heterogain detection device. By controlling the mixing ratio in the mixing circuit 11 as described above, the level of the detected signal output from the mixing circuit 11 is kept constant regardless of the branching ratio of the optical splitter 8.

When the branching ratio of the optical splitter 8 becomes O:1, all of the signal light incident on the optical splitter 8 passes through the optical splitter 8 and enters the optical coupler 6. On the other hand, the local oscillation light branched by the optical splitter 2 and incident on the polarization control circuit 4' continues to have the same polarization state as the signal light since the branching ratio of the optical splitter 8 is (1-X):X. After being converted into , the light is input to the optical coupler 6 . The optical coupler 6 combines the signal light and the local oscillation light whose polarization states match each other. This combined light is detected by a photodetector 10, converted into an electrical signal, and then input to a mixing circuit 11. Photodetector 9 and photodetector 1 in mixing circuit 11
The control unit 12 controls the mixing ratio of the input signals from 0 to 1 to be O:1. Therefore, the output signal of the photodetector 10 is directly output from the mixing circuit 11. This output signal becomes the output of the optical hetero gain detection device.

From the time when the branching ratio of the optical splitter 8 becomes 0:1, the drive voltage (2) applied to the polarization control element 4a is also output from the polarization control element drive circuit 4b to the control unit 12. . In response to this, the control unit 12 sets the drive voltage Vl of the polarization control element 3a to 2. ! The polarization control element drive circuit 3b is controlled so that ".

In this way, the state of the polarization control circuit 3.4 is reversed before and after the shift in the flow of the signal light L2 as the drive voltage VB of the polarization control element 3a approaches the limit voltage. Thereafter, depending on the change in the polarization state of the signal light L2, the signal light L2
By continuing to alternately shift the flow between the first optical coupler 5 side and the second optical coupler 6 side, stable optical heterogain detection without permanent interruption can be performed.

Next, the operation of the polarization control system in the optical heterodyne detection device will be explained with reference to FIG. 2.

FIGS. 2(a) and 2(b) respectively show the drive voltages of the polarization control elements 3a and 4a that can match the polarization state of the locally oscillated light at each time with the polarization state of the signal light. .

V2* (indicated by a black circle (・)), Figure 2 (C)
2(d) shows the change over time in the branching ratios P1 and P2 of the output from the optical splitter 8 to the first optical coupler 5 side and the second optical coupler 6 side, and FIG. 2(d) shows the optical detection in the mixing circuit 11. Vessel 9
10 and 11 represent changes over time in the mixing ratios M1 and M2 of the input signals from the photodetector 10. Here, the sum of P and P2 and the sum of Ml and M2 are both determined to be 1.

Also Vlm, v2. Actual polarization control element 3
a, 4a drive voltage V, , V2. are shown by solid lines in FIGS. 2(a) and 2(b), respectively. Furthermore, V+
i"+V27" are respectively shown in FIG. 2(a).

(Indicated by a broken line in (b). Here, v, ”
Among ■11, the size was medium, or if there were two medium sizes, the one with the largest difference from ■11 was selected. Therefore, v,,'' may be the same size as ■1.

At time to, P+'= in the second optical splitter 8
1. P2=0, and the signal light incident on the optical splitter 8 is
The light enters the optical coupler 5. The polarization control circuit 3 converts the polarization state of the local oscillation light so that it matches the polarization state of the signal light, and inputs the polarization state to the optical coupler 5. The signal light and the local oscillation light are combined in the optical coupler 5. The combined light is detected by the photodetector 9 and converted into an electric signal. Mr = 1 and M2 = 0 in the mixing circuit 11, and the output signal of the photodetector 9 is directly output from the mixing circuit 11. At this time, the drive voltage (17) of the polarization control element 3a is output to the control unit 12.At this point, the polarization control element 3a' is driven to match the polarization state of the locally oscillated light with the polarization state of the signal light. There are six voltages ■1.!, among which the fourth one from the largest in magnitude is ■17*, and the drive voltage V2. of the polarization control element 4a is controlled so that it becomes v,,''. 12.

Q At time t1, the drive voltage of the polarization control element 3a ■1.
! is v4. ! 4"1-△■. From this point on, the ratio P1 of the output from the optical splitter 8 to the optical coupler 5 side begins to decrease, and conversely, the ratio P2 of the output to the optical coupler 6 side increases. Start. At time t2, P, = O.

P2-1, Ml-0, M2=1, and the execution of optical heterodyne detection is completely transferred to the optical coupler G side.

Between time t and t2, the driving voltages of the polarization control elements 3a and 4a are Vth, V3. are not controlled by the control unit 12, but are each controlled by feedback within the polarization control circuit 3.4. Further, between time t1 and time t12, the outputs from the photodetectors 9 and 10 are added in the mixing circuit 11, and the mixing ratio MI:M2 at this time is
is equal to Pl:P2.

After time t2, the driving voltage V2° of the polarization control element 4a is
It is output to the control section 12. Drive voltage of the polarization control element 4a that can match the polarization state of the locally oscillated light at time t2 with the polarization state of the signal light.2. ! There are six of them, and the fourth one from the largest one is V2. ! '', and the drive voltage (11) of the polarization control element 3a is controlled by the control unit 12 so that it becomes (2.*).

At time t3, the driving voltage of the polarization control element 4a is 2. I″tih+Δ■ is reached. From this point on, the ratio P2 of the output from the optical splitter 8 to the optical coupler 6 side starts to decrease, and conversely, the ratio P1 of the output to the optical coupler 5 side starts to increase. The time is also Pl=1.P, 2-0 and Ml-
1, M2=0, and the execution of optical hetero gain detection is completely transferred to the optical coupler 5 side. Between time t3 and t4, the driving voltages V+I and V2 of the polarization control elements 3a and 4a are
．． i! are not controlled by the control unit 12, but are each controlled by feedback within the polarization control circuit 3.4.

Further, between time t3 and t4, the outputs from the photodetectors 9 and 10 are added in the mixing circuit 11, and the mixing ratio M1:M2 at this time is equal to P,:P2.

Even after time t4, the first optical coupler 5 side and the second optical coupler side as described above are connected.
The transition of execution of optical hetero gain detection between the optical coupler 6 side and the optical coupler 6 side continues.

FIG. 3 is a block diagram showing in detail the configuration of the control section 12 in the optical heterodyne detection device 9 shown in FIG. The controller 12 is inside the broken line in the figure. first gate 1
3 inputs the drive voltages V, , V, for the polarization control elements 3a, 4a from the polarization control element drive circuits 3b, 4b, respectively, and the output side thereof outputs V1 or V2 or enters a high impedance state. Either you do it or you do it. The discrimination circuit 14 receives a vh signal from the gate 13 (n=1.2).
When vh, and ■h, ! are being output. ″″Jurisdiction-△V and ■. ,′″”h+△■(n=1.2), V,,,≧V,,,ma.

−△■ or ■l,,! Generates a trigger pulse when ≦■,,ノ″lh+△■.■9.!4 Generator 15
is ■ when ■- is output from the gate 13. ,!
V corresponding to. ,! 9 is generated and output to the second gate 1G. The gate roller operates in one of three operating states: when outputting the input voltage to the polarization control element drive circuit 3b, when outputting it to the polarization control element drive circuit 4b, and when both output sides are in a high impedance state. Take. A control circuit 17 that controls the optical branching device 8 and the mixing circuit 11 changes the branching ratio of the optical branching device 8 and the mixing ratio of the mixing circuit 11 when the trigger pulse is manually input from the discrimination circuit 14, and when this is completed, Generates a trigger pulse. Gate control circuit 1
8 puts the output of the first gate 13 into a high impedance state when a trigger pulse is manually applied from the discrimination circuit 14;
Both outputs of the second gate 16 are placed in a high impedance state. Next, when a trigger pulse is manually applied from the control circuit 17 to the gate control circuit 18, the output from the previous high impedance state at the gate 13 is changed to ■I,! ■2. ! When ■2 is ■7. ! control to output. Furthermore, at the gate 16, if human power has been output to the polarization control element drive circuit 3b before the previous high impedance state, the polarization control element drive circuit 4b
If the human power is output to the polarization control element drive circuit 4b, the polarization control element drive circuit 3b
Control output to .

Next, an example of the control operation of the control section 12 will be explained.

FIG. 4 shows the output state of the first gate 13 and the discrimination circuit 14 corresponding to the operation of the optical heterodyne detection device shown in FIG.
The trigger pulse output of the control circuit 17, the trigger pulse output of the control circuit 17, and the output state of the second gate 16 are shown.

Time t. to t, the drive voltage v1. of the polarization control element 3a input from the polarization control element drive circuit 3b. ! is output from the gate 13. Also, yh, *generator 15
VB2'' is generated and output from the gate 16 to the polarization control element drive circuit 4b. At time t1, V , -V , , +na'' - △■, and the trigger pulse is output from the discrimination circuit 14. In response to this, the outputs of the first and second gates 13.16 become in a high impedance state. At time t2, changes in the branching ratio of the optical splitter 8 and the mixing ratio of the mixing circuit 11 are completed, and a trigger pulse is output from the control circuit 17. In response, in the gate control circuit 1B, the gate 13 outputs the drive voltage ■2 for the polarization control element 4a inputted from the polarization control element drive circuit 4b, and the gate 16 outputs the drive voltage ■h, *generator 15
V2j caused by! " is outputted to the polarization control element drive circuit 3b. Thereafter, the same operation is continued.

In the above optical heterodyne detection device, the first and second
As a control method for the polarization control circuits 3 and 4, feedforward control is used in which the polarization state of the signal light L2 incident on the present device is monitored in advance, and the polarization state of the local oscillation light is controlled to match the polarization state. You may go. The local oscillation light L1 and the signal light L2 are reversely incident on the second and first optical splitters 8 and 2, respectively, and polarization control is performed so that the polarization state of the signal light matches the polarization state of the local oscillation light. It's okay.

The mixing ratio in the mixing circuit 11 is such that the higher level of the output signals from the first photodetector 9 and the second photodetector 10 is 1. The lower value may be set to 0. Alternatively, the mixing ratio may always be 0.5:0.5. In these two cases,
At worst, detection sensitivity is degraded by about 3 dB.

FIG. 5 is a plan view of an embodiment of the optical heterodyne detection device shown in FIG. 1.

By thermally diffusing Ti onto the lithium niobate substrate 19, a width of 1
A channel waveguide 20 of 0 μm is formed. This channel waveguide 20 functions as a single mode waveguide at a wavelength of 1.55 μm. After forming a 5102 film on the lithium niobate substrate 19 by the CVD method, a Cr-Aj? By forming the electrode, the first
and second optical splitter 21, 22, first and second mode converters 23, 24, first and second phase modulators 25, 2.
6, first and second optical couplers 27 and 28 are configured.

The waveguide end face on the input side of the optical splitter 21, 22 of the channel waveguide 20 and the waveguide end face on the output side of the optical coupler 27, 28 are each equipped with a laser beam having a built-in isolator, which is a local oscillation light source. Diode 291 Single mode fiber for human power 30. First and second photodetectors avalanche photodiodes 31,32 are coupled.

The output side of the avalanche photodiode 31 is connected to the input side of the phase compensation circuit 33. This phase compensation circuit 3
3 is for compensating the phase difference between the two detected signals caused by asymmetry such as the length of the optical waveguides in the optical coupler 27 and the optical coupler 28.

The output of the phase compensation circuit 33 and the output of the avalanche photodiode 32 are connected to the input side of the mixing circuit 34. The output of mixing circuit 34 is connected to the input of amplifier 35. A part of the output signal of the amplifier 35 is input to the polarization control element drive circuit 36, 37 and the phase compensation circuit 1433, forming a common feedback loop for the polarization control circuit (described later) and a feedback loop for the phase compensation circuit 33, respectively. There is. The phase compensation circuit 33 performs phase control so that the output of the amplifier 35 is maximized. The control unit 38 controls the polarization control element drive circuits 36 and 37, the branching ratio of the optical splitter 22, and the mixing ratio of the mixing circuit 34. The polarization control element 3 is formed by mode converters 23 and 24 that control the polarization angle of the locally oscillated light and phase modulators 25 and 26 that control the phase of the locally oscillated light.
9:4o is configured. Furthermore, polarization control element 39.40
The polarization control circuits 3 and 3 and the polarization control element drive circuits 36 and 37 and the common feedback loop described above constitute the polarization control circuits 3 and 4 described in FIG. 1, respectively. The polarization control method with the above configuration performs feedback control so that the output from the amplifier 35 is maximized.

In the above description, the optical splitter 21 has a Y-branch structure.

Further, the optical splitter 22 includes step I...delta beta...
It is based on distributed coupling using the reversal method, and the waveguide spacing is 3 μm and the total length is 5 mm. Mode converter 2
3.24 is one in which a comb-shaped electrode is provided on the channel waveguide 20, and the total length is 15 mm. The electrode spacing of the phase modulators 25 and 2.6 is 14 μm, and the total length is 1
It is 0mm. The optical couplers 27 and 28 are based on distributed coupling, and have a waveguide spacing of 3 μm and a total length of 5 mm.

Here, the optical splitter 22 and the optical couplers 27 and 28 have waveguide configurations that do not have polarization dependence.

The mode converters 23,24 can take values from 0 to 20V. Phase modulator 25.26 is from O■ to 30
It can take values up to V. In addition, mode converter 2
3.24, the driving voltage required to change the polarization angle of the incident light by 2π rad is 9■.

In the phase modulators 25 and 26, the phase difference of the incident light is set to 2π
The drive voltage required to change rad is 12V.

The mode converters 23, 24 have drive voltages of 18V or more or 2V or less, and the phase modulators 2,5. ．． 26 starts to perform optical heterodyne detection when its driving voltage becomes 28 V or more or 2 V or less. The time required to complete this transition is 1 μs.

The insertion loss for the signal light in the above-mentioned optical heterodyne detection device was 3 clB, and the shape of the substrate 19 made of tin niobate was 20×30 mm 2 , so that a small device with low loss was obtained.

In the optical hetero gain detection device described above, the signal light and the local oscillation light are reversely connected to the first and second optical couplers 21.2, respectively.
2 and perform polarization control so that the polarization state of the signal light matches the polarization state of the local oscillation light, the positions of the phase modulator 25 and the mode converter 23 are reversed. The same applies to the phase modulator 26 and mode converter 24.

Although the phase difference between the two detection signals is electrically compensated by the phase compensation circuit 33, it may be compensated for by compensating the phase of the output light from the optical couplers 27 and 28.

The substrate is not limited to lithium niobate, but may also be PLZT or Ga.
Any material having an electro-optic effect such as As may be used. In this case, the channel wave path is formed by an appropriate method. Furthermore, the light emitted from the laser diode 29 may not be directly incident on the end face of the channel waveguide, but may be made incident through a polarization maintaining fiber or the like. Furthermore, the optical splitter 21
may be based on distributed coupling.

Each waveguide type element may be realized by a micro optical element, for example, an optical splitter 21. The optical couplers 27 and 28 are fiber-type branching/coupling devices fused into a tapered shape, the optical branching device 22 is a liquid crystal switch, and the polarization control elements 39 and 4f) are devices that control polarization by applying pressure to the optical fibers, or electric devices. A material using an optical material may also be used.

〔Effect of the invention〕

As will be clear from the following explanation, according to the present invention, two systems of apparatus configuration for optical heterogain detection equipped with a polarization control function are provided in parallel, and one system performs detection while controlling polarization. When polarization compensation is possible in the other system, the incident light is maintained in a state where polarization compensation can be performed in the other system, and when polarization control becomes difficult in one system, the system continuously shifts to the other system and instantly switches to continue detection operation. Because it is configured as
Optical heterodyne detection can be performed stably and with low loss, and highly reliable optical heterodyne communication can be realized.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the optical heterodyne detection device according to the present invention, Fig. 2 is a time chart showing an example of the operation of the same party heterodyne detection device, and Fig. 3 is a block diagram showing the configuration of the control section. 4 is a time chart showing an example of the operation of the control section, and FIG. 5 is a plan view of a concrete implementation of the optical heterodyne detection device. 1 ・・・・・・・・・・・・・・・・・・・・・・・・
・Local oscillation light source 2, 8.2.1.22...
Optical splitter 3.4 ・・・・・・・・・・・・・・・・・・
-Polarization control circuit 3a, 4a, 39. ．． 40・
...Polarization control elements 3 b, 4 b, 36. ．． 37・
...Polarization control element drive circuit 5 +6. ,,27,28
・・・・・・Optical coupler 9.10 ・・・・・・・・・
...... Photodetector 11.34 ...
・・・・・・・・・・・・Mixed circuit 12.38
・・・・・・・・・・・・・・・・・・Control section 23.2
4 ・・・・・・・・・・・・・・・・・・Mode converter 25.26 ・・・・・・・・・・・・・・・・・・
・・Phase modulator 31.32 ・・・・・・・・・・・
...Avalanche photodiode

Claims (1)

[Claims] (1) In an optical heterodyne detection device, there is a first optical splitter that splits a first incident light into two lights of approximately equal intensity, and a second light splitter that branches a second incident light into two and makes the splitting ratio continuous. a second optical splitter that can change the polarization, and a first and second optical splitter that includes a polarization control element and its driving circuit, and that inputs each light branched by the first optical splitter, respectively.
a first optical coupler that combines the output light of the first polarization control circuit and one of the lights branched by the second optical splitter, and the second polarization control circuit. a second optical coupler that combines the output light of the circuit and the other light branched by the second optical splitter; and detects the outputs of the first and second optical couplers as electrical signals. first and second photodetectors, a mixing circuit that adds signals output from the first and second photodetectors, and a polarization control element drive circuit of the first and second polarization control circuits. An optical heterodyne detection device comprising: a control section that controls the operation and the branching ratio of the second optical branching device.