JP3443810B2 - Polarization control circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical measurement technique and an optical communication technique, and more specifically to a polarization control circuit.

In optical technology such as coherent optical communication and optical interference measurement, coherent detection is used to read communication or measurement information carried on the phase or frequency of a detected light wave. Coherent detection is a method in which a detected light wave and a local oscillation wave prepared on the receiving side are superposed, and the information carried on the detected light wave is read from the resulting change in the phase or frequency of the electrical beat signal. At this time,
In order to effectively generate the electric beat signal, it is necessary that the polarization direction of the detected light wave and the polarization direction of the local oscillation wave match. However, the polarization direction of the detected light wave is not always kept constant, and in particular, when the detected light wave propagates through a single-mode optical fiber, the polarization direction is generally the time direction. Fluctuates against. In such a case, some means is required to match the polarization directions of the detected light wave and the local light wave.

Such means is generally called a polarization diversity technique. Specifically, after separating each of the detected light wave and the local oscillation wave into two orthogonal polarization components, coherent detection is performed for each component. Then, by adjusting the phase between the two electric beat signals caused by the respective polarization components and combining them, or by selecting only the component having a good SN ratio, a method for obtaining the final output electric signal is Generally applied.

[0004]

As described above, by using the polarization diversity technique, it is possible to effectively perform coherent detection and read information carried on the phase or frequency of the detected light wave. . However,
The polarization diversity technique has a problem that an electric circuit becomes complicated because phase control or component selection is performed in an electric signal processing stage after optical-electric conversion. Further, when the information is carried at a high bit rate, the response of the polarization diversity circuit must be made fast, and the circuit is further complicated due to this speedup.

An object of the present invention is to provide means for performing polarization diversity by a simple optical circuit without performing complicated electric circuit processing when performing coherent detection in optical communication or optical measurement. And

[0006]

In order to achieve the above-mentioned object, the polarization control circuit of the present invention converts an optical wave having an arbitrary polarization state input from the outside into two components having linear polarization components orthogonal to each other. A polarization splitting type beam splitter for separating, a light intensity detecting means for extracting a part of the light wave of each component and monitoring each light intensity, and comparing the light intensity of each component based on the monitored result. A processing circuit for performing the operation, a polarization plane rotation element for rotating the polarization direction of one component by 90 °, and a Mach-Zehnder type having each component as an optical input of two input ports and having a phase shifter inside Equipped with an optical interferometer, the phase shift amount in the phase shifter is changed according to the comparison result of the light intensity, and only one component is Mach-Zehnder type light. Characterized in that the output from the interferometer.

In the present invention as described above, a light wave having an arbitrary polarization state externally input to the polarization control circuit according to the present invention has linearly polarized light components orthogonal to each other.
Separated into components, part of each component is branched,
Thereby, the light intensity of each component is monitored, and the light intensity of both components is compared. The linear polarization direction of one component is rotated by 90 ° and becomes the same as that of the other component, and the respective components are input to different input ports of the Mach-Zehnder interferometer.

Generally, by inserting a phase shifter in one optical path of the Mach-Zehnder interferometer and controlling the amount of phase shift, the output to each output port of the Mach-Zehnder interferometer is controlled. Can be controlled.
Specifically, assuming that the two optical path lengths in the interferometer are equal, by selecting the phase shift amount to either 0 or π, either the output port corresponding to the input port or another output port is selected. Can be output to.

Therefore, by selecting either 0 or π as the phase shift amount in the phase shifter based on the result of the intensity comparison for the two separated components, for example,
Only a strong component can be continuously output to a specific output port. Here, since the linear polarization directions of both components are the same, the polarization state of the output light is always maintained in a constant direction regardless of the polarization state of the external input light. Furthermore, the decrease in light intensity can always be suppressed to a maximum of 3db or less (half or less). Further, by applying a Mach-Zehnder waveguide type optical intensity modulator using lithium niobate as a Mach-Zehnder type optical interferometer with a variable amount of phase shift,
It is possible to switch the output port at high speed (about 10 GHz).

Further, in the present invention, it is preferable to adjust the propagation lengths of the two component light waves from the polarization separation type beam splitter to the Mach-Zehnder interferometer so that they are equal. As a result, the difference in propagation delay time can be eliminated, and the timing of the signal carried on the external input light can be held accurately.

Further, in the present invention, a laser light source which outputs a light wave having the same linear polarization as that of one component is used as a light source for local light emission, and an external input light wave which is output through a polarization control circuit. After being combined by a multiplexer and converted into an electric signal, coherent detection can be effectively performed regardless of the polarization state of the external input light wave.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a first polarization control circuit according to the present invention.
It is explanatory drawing of an Example. Reference numeral 1 denotes a polarization splitting type beam splitter, which splits a light wave of an arbitrary polarization state input from the outside into an A component and a B component having linearly polarized light components orthogonal to each other. 2 is a light intensity detecting means, specifically, A
The optical splitters 3 and 4 for extracting a part of the component and B component light waves, and the light intensity detectors 5 and 6 for monitoring the respective light intensities.

A processing circuit 7 receives the light intensity detection result as an input, compares the light intensities of the A component and the B component, determines the component having the highest intensity, and outputs the result. If the two components have the same intensity, it is determined in advance which component should be selected. Reference numeral 8 denotes a polarization plane rotating element, and here, the polarization direction of the B component is rotated by 90 ° so that it becomes equal to the polarization direction of the A component. Reference numeral 9 denotes a Mach-Zehnder interferometer, which has first and second input ports 10 and 11 and first and second output ports 12 and 13, and is input from the input port 10 and transmitted through the beam splitter 14 at the entrance. It includes a phase shifter 15 for giving a phase shift to the component.

In the above configuration, in the optical path length of the Mach-Zehnder interferometer, the input from the first input port is output to the second output port when the phase shift is 0. Is pre-conditioned to output to the first output port. Further, the A and B components are input to the first and second ports 10 and 11, respectively, and are controlled as follows based on the output result from the processing circuit 7. That is, if the intensity of the A component is stronger than that of the B component, the phase shift amount is set to 0 and the A component is output from the second output port 13. If the intensity of the B component is stronger than that of the A component, the phase shift amount is π. Then, the B component is controlled to be output from the second output port 13.

The output light from the second output port 13 of the Mach-Zehnder interferometer is used as the external output light. The detected light input as the external input light has a power loss of 50% due to circuit transmission regardless of the polarization state at the time of input.
% Or more, it is always converted into light having a specific linear polarization state. The polarization separation type beam splitter 1
From Mach-Zehnder interferometer input port 10 or 11
By making the propagation lengths of both A and B components equal to each other, the difference in propagation delay time can be eliminated and the timing of the signal carried on the external input light can be held accurately.

Next, FIG. 2 shows a second embodiment of the polarization control circuit according to the present invention. Reference numeral 16 is a laser light source that outputs a light wave having the same linear polarization as that of the A component, and 17 is an optical multiplexer, which is a light wave output through the polarization control circuit of FIG. 1 and an output light wave from the laser light source 16. Combine. Reference numeral 18 denotes an optical receiver, which converts an optical signal output from the optical multiplexer 17 into an electric signal. The detected light input as the external input light is effectively coherently detected.

[0017]

As described above, according to the present invention, when performing coherent detection in optical communication, optical measurement, etc.,
Polarization diversity can be performed by a simple optical circuit without complicated electric circuit processing.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a first embodiment of a polarization control circuit according to the present invention.

FIG. 2 is a diagram illustrating a second embodiment of the polarization control circuit according to the present invention.

[Explanation of symbols]

1 Polarization separation type beam splitter 2 Light intensity detection means Three or four optical splitter 5, 6 Light intensity detector 7 Processing circuit 8 Polarization plane rotating element 9 Mach-Zehnder interferometer 10 First input port 11 Second input port 12 First output port 13 Second output port 14 Beam splitter 15 Phase shifter 16 laser light source 17 Optical multiplexer 18 Optical receiver

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Claims (3)

(57) [Claims] 1. A polarization splitting type beam splitter for separating a light wave having an arbitrary polarization state inputted from the outside into two components having linearly polarized light components orthogonal to each other, and extracting a part of the light wave of each component. Light intensity detecting means for monitoring the respective light intensities, a processing circuit for comparing the light intensities of the two components based on the monitored results, a polarization plane for rotating the polarization direction of one component by 90 ° A rotating element and a Mach-Zehnder type optical interferometer that uses each component as the optical input to each of the two input ports and has a phase shifter inside, and the phase shift amount in the phase shifter according to the comparison result of the light intensity The polarization control circuit is characterized in that only one of the components is changed to the output from the Mach-Zehnder interferometer. 2. The polarized light according to claim 1, wherein the light beams of the two components are adjusted to have equal propagation lengths from the polarization separation type beam splitter to the Mach-Zehnder interferometer. Control circuit. 3. A laser light source that outputs a light wave having the same linear polarization as one of the components, and an optical combiner for combining the light wave output through the polarization control circuit and the output light wave from the laser light source. The polarization control circuit according to claim 2, further comprising: an optical receiver and an optical receiver that converts an optical signal output from the optical multiplexer into an electrical signal.