JPS6294832A - Optical heterodyne detecting device - Google Patents

Abstract

PURPOSE:To eliminate a decrease in reception sensitivity and to carry out stable optical heterodyne detection by multiplexing local oscillation light under the polarization control of a waveguide type polarization control element and signal light by a fiber type coupler which has small input loss and no dependency in branching ratio. CONSTITUTION:The local oscillation light from a laser diode 104 is applied to an optical branching device 200 and branched to the channel waveguide which has the 1st polarization control element 112a and the 3rd phase modulator 203 and the channel waveguide which has the 2nd polarization control element 112b under the control of a control part 202. Further, the modulator 203 compensates the phase difference between both waveguides and the light beams are coupled by a photocoupler 201 and applied to one single mode fiber of the fiber type optical coupler 105. The signal light is made incident on the signal light incidence side single mode fiber 107 of this coupler 105. Then, the coupler 105 multiplexes the signal light and continuously varied local oscillation light and an avalanche photodiode 109 detects the multiplexed light to reduce a decrease in reception sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical heterodyne detection device used in an optical heterodyne communication system.

[Conventional technology]

In an optical heterodyne communication system using a single mode fiber, when performing optical heterodyne detection, it is necessary that the polarization state of the signal light and the polarization state of the local oscillation light match. However, because the polarization state of the signal light changes after propagating through a single mode fiber due to disturbances such as temperature changes, the beat signal strength fluctuates, leading to a decrease in system reliability and, in some cases, making it impossible to detect the signal light. Sometimes.

Therefore, in order to ensure reliability, an optical heterodyne detection device is required that has a polarization control function that matches the polarization states of the signal light and the local oscillation light. Waveguide type polarization control elements for polarization control are in particular demand from the viewpoint of miniaturization, low voltage, and the like.

Until now, there has been no report on an optical heterodyne detection device that includes a waveguide-type polarization control element, but an example of the structure of the device is W, A.
, Stallard et al., 3rd European Conference on Ingrated Optics, 1
985, pp. 164-168. This is a device in which a waveguide-type polarization control element, an optical coupler, etc. are integrated on the same substrate, and a photodetector is coupled to the end face of the leading waveguide. After the polarization state of the signal light incident on the optical waveguide is converted by the polarization control element to match the polarization state of the local oscillation light by coupling by butting the end faces of the single mode fiber and the optical waveguide, both lights become optical. The signals are combined by a coupler. This combined light is converted into an electrical signal by a photodetector.

[Problem that the invention seeks to solve]

In an optical heterodyne detection device having a configuration in which signal light is coupled to a waveguide type element as described above, the signal light suffers from coupling loss between the single mode fiber and the guide waveguide, propagation loss in the guide waveguide, and bending loss in the optical waveguide. etc. will be received.

The total loss suffered by these signal lights is thought to reach 2 to 3 dB, and there is a problem in that the reception sensitivity is indescribably lowered.

An object of the present invention is to provide an optical heterodyne detection device that solves these problems.

[Means for solving problems]

An optical heterodyne detection device according to the present invention includes: a local oscillation light source; a waveguide type polarization control element that converts the local oscillation light emitted from the local oscillation light source into a desired polarization state; The optical fiber optic coupler includes a fiber type optical coupler that combines the emitted light and the signal light, and a photodetector that detects the emitted light from the fiber type optical coupler.

[Effect]

In the optical heterodyne detection device according to the present invention, after the polarization state of the locally oscillated light is converted to match the polarization state of the signal light by a waveguide-type polarization control element, both lights are combined by a fiber-type optical coupler.

The combined light is converted into an electrical signal by a photodetector, and an optical heterodyne detection signal is obtained.

As described above, the signal light propagates through the single mode fiber for transmission and then enters the photodetector through only the fiber type optical coupler. Since the insertion loss of a fiber type optical coupler is very small, the signal light suffers almost no loss. In addition, in optical heterodyne detection, there is a coupling ratio between the signal light and the local oscillation light that maximizes the detection sensitivity of the signal light, but in fiber-type optical couplers, the coupling ratio has almost no polarization dependence. , the signal light and the local oscillation light are always combined at a substantially constant rate.

Therefore, in the optical heterodyne detection device according to the present invention, stable optical heterodyne detection can be performed with almost no reduction in reception sensitivity due to loss of signal light.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a plan view of an optical heterodyne detection device showing a first embodiment of the present invention.

A channel waveguide 101 having a width of 8 μm is formed on a lithium niobate substrate 100 by thermally diffusing a Ti film patterned by sputtering and photolithography.

The channel waveguide 101 has T at a wavelength of 1.55 μm.
The basic mode is maintained for E mode and TM mode.

A buffer layer of SiO2 is deposited on the lithium niobate substrate 100 on which the channel waveguide 101 is formed using the CVD method.
Form a film, and then apply vacuum evaporation method and photo) IJ on top of it.
A mode converter 102 and a phase modulator 103, both of which use electro-optic effects, are constructed by selectively forming a Cr-Aj2 film serving as an electrode using a lithography method.

The mode converter 102 has finger-shaped electrodes provided on both sides of the channel waveguide 101, and the electrode spacing is 12 μm and the total length is 15 mm. Phase modulator 1
03 is one in which rectangular electrodes are provided on the channel waveguide 101, the electrode spacing is 5 μm, and the total length is F3 mm. The mode converter 102 and the phase modulator 103 are elements that convert the polarization angle and phase difference of guided light, respectively, and a pair of these constitutes a polarization control pad 112.

Mode converter 102 in channel waveguide 101
A laser diode 104 with a built-in isolator, which is a local oscillation light source, and a fiber-type optical coupler 1 are installed at the input end and the output side of the phase modulator 103, respectively.
Local oscillation light human emission side single mode fiber 10 in 05
6 are combined. Here, the fiber type optical coupler 10
No. 5 is constructed by fusing the sides of two fibers, the insertion loss is 0.2 dB, and the fluctuation of the output light intensity with respect to the polarization state of the incident light is 1% or less.

The laser diode 104 is connected to the channel waveguide 1 in TE or TM mode so that the light emitted therefrom undergoes most efficient polarization angle conversion by the mode converter 102.
01. An output side single mode fiber 108 in the fiber type optical coupler 105 is coupled to an avalanche photodiode 109 which is a photodetector.

A part of the output of the amplifier 110 connected to the output side of the avalanche photodiode 109 is input to the polarization control element control circuit 111 .

Local oscillation light that has entered the channel waveguide 101 from the laser diode 104 is converted by the mode converter 102 and the phase modulator 103 so that its polarization state matches that of the signal light, and then the local oscillation light enters the input side of the local oscillation light. Single mode fiber 106 to fiber type optical coupler 10
5. On the other hand, the signal light enters the fiber type optical coupler 105 from the single mode fiber 107 on the signal light input side. The local oscillation light and the signal light are multiplexed in a fiber type optical coupler, and then passed through an avalanche photodiode 10.
9, it is converted into an electrical signal. The output from the avalanche photodiode 109 is an optical heterodyne detection signal, and the intermediate frequency output is (selected).The heterodyne detection signal is then amplified by an amplifier 110, and a part of the output is sent to a polarization control element control circuit 111 The polarization control element control circuit 111 controls the drive voltages of the mode converter 102 and the phase modulator 103 so that the optical heterodyne detection signal strength is maximized.

The insertion loss of this optical heterodyne detection device for signal light is 0.2 dB, and it operates stably even when the polarization state of signal light changes.

However, the optical heterodyne detection device of this embodiment has the following problems. That is, when the polarization state of the signal light continues to change in a certain direction, the driving voltage of the mode converter 102 or the phase modulator 103 also increases in order to change the polarization state of the local oscillation light accordingly. Continue to do so.

Therefore, the drive voltage eventually reaches its limit and it becomes impossible to control the polarization of the locally oscillated light.

FIG. 2 is a plan view of an optical heterodyne detection device showing a second embodiment of the present invention that solves such problems. This embodiment replaces the polarization control element 112 in the first embodiment with a first polarization control element 112a consisting of a first mode converter 102a and a first phase modulation local section 103a, and a second polarization control element 112a consisting of a second mode converter 102b and a first phase modulation local section 103a. The first and second mode converters 102a are replaced with the second polarization control element 112b consisting of two phase modulators 103b.
, 102b and the output sides of the first and second phase modulators 103a, 103b are coupled by an optical splitter 200 and an optical coupler 201, respectively, which can continuously change the branching ratio. It becomes. Optical splitter 2
00 is based on distributed coupling using the step delta beta reversal method, and the waveguide spacing is 5 μm and the length of the coupling part is 5 mun. The optical coupler 201 has a Y-branch structure. Moreover, the third phase modulator 20
3 is for compensating the phase difference between the guided light caused by the asymmetry of the channel waveguides on the first polarization control element 112a side and the second polarization element 112b side.

First, all of the local oscillation light is incident on the first polarization control element 112a by the optical splitter 200, and after being polarized, it passes through the optical coupler 201 and enters the fiber type optical coupler 105. Thereafter, optical heterodyne detection is performed by the same operation as in the first embodiment. At this time, the driving voltages of the second mode converter 102b and the second phase modulator 103b are determined by the respective values and the driving voltages of the first mode converter 102b.
02a and the driving voltage of the first phase modulator 103a is controlled by the control unit 202 so that it becomes an appropriate value that is an integral multiple of vA and VA. Here V
A and VA are the first and second mode converters 102a, 102t!, respectively. and first and second phase modulators 103a.

103b, the polarization angle and phase difference of the guided light are set to 2π
This is the voltage required to change by rad.

Therefore, similar to the first polarization control element 112a, the second polarization control element 112a
The polarization control element 112b is always in a state where polarization control of locally oscillated light can be performed.

Now, when the driving voltage of the first mode converter 102a or the first phase modulator 103a approaches the limit voltage due to a change in the polarization state of the signal light, the branching ratio of the optical splitter 200 is changed by the control unit 202, and the local The flow of oscillated light is continuously transferred from the first polarization control element 112a side to the second polarization control element 112b side. At this time of transition, the divided local oscillation light is subjected to polarization control in the first and second polarization control elements 112a and 112b, respectively, and then
The fiber-type optical coupler 105 is coupled by the optical coupler 201.
incident on . Thereafter, optical heterodyne detection is performed by the same operation as in the first embodiment. When this transition of the flow of the locally oscillated light is completed, all the locally oscillated light enters the second polarization control element 112 b, and after being polarized, the optical coupler 2
The light passes through the old fiber optic coupler 105 and enters the fiber type optical coupler 105.

Thereafter, optical heterodyne detection is performed by the same operation as in the first embodiment. Also, immediately after the completion of the flow transition, the first
mode converter 102a and first phase modulator 1
The drive voltage of 03a is determined by the values of those values, the second mode converter 102b and the second phase modulator 10, respectively.
The control unit 202 controls the difference between the drive voltage of the drive voltage 3b and the drive voltage of the drive voltage 3b to an appropriate value that is an integral multiple of the voltage V&. That is, the first and second polarization control elements 1
The state between 12a and 112b is reversed compared to the initial state.

The first and second polarization control elements 112a as described above,
By continuing the operation involving the transition of the flow of local oscillation light between 112b and 112b, permanently stable optical heterodyne detection can be performed in principle without momentary interruption, regardless of the polarization state of the signal light.

In the first and second embodiments described above, the substrate is not limited to lithium niobate, but may be any material having an electro-optic effect such as PLZT or GaAs. In this case, the channel waveguide is formed by a suitable method. Furthermore, the light emitted from the laser diode 104 may be made to enter through a polarization maintaining fiber instead of being made to enter the end face of the channel waveguide directly. Furthermore, fiber-type optical couplers are two fibers whose sides are polished and joined together.
It is possible to use a method in which two fibers are etched and then twisted and placed close to each other.

〔Effect of the invention〕

In the optical heterodyne detection device according to the present invention, local oscillation light whose polarization is controlled by a waveguide-type polarization control element and signal light are combined by a fiber-type optical coupler with low insertion loss and almost no polarization dependence of the splitting ratio. are doing. therefore,
Since stable optical heterodyne detection can be performed with almost no reduction in receiving sensitivity due to loss of signal light, highly reliable optical heterodyne communication can be easily realized.

[Brief explanation of drawings]

FIGS. 1 and 2 are plan views of optical heterodyne detection apparatuses which are first and second embodiments of the present invention, respectively. DESCRIPTION OF SYMBOLS 100... Lithium niobate substrate 101... Channel waveguide 102, 102a, 102b-%-domain converter 103, 103a, 103b, 203... Phase modulator 104... Laser diode 105... Fiber type Optical coupler 106... Local oscillation light input side single mode fiber 10
7... Signal light human emission side single mode fiber 108...
Output side m! - Mode fiber 109... Avalanche photodiode 110... Amplifier 111... Polarization control element control circuit 112, 112a, 112b - Polarization control element 200...
・・Optical splitter 201 ・・Optical coupler 202 ・・Control unit

Claims (1)

[Claims] (1) A local oscillation light source, a waveguide type polarization control element that converts the local oscillation light emitted from the local oscillation light source into a desired polarization state, and a signal light and a waveguide type polarization control element that converts the local oscillation light emitted from the local oscillation light source into a desired polarization state. What is claimed is: 1. An optical heterodyne detection device comprising: a fiber-type optical coupler for multiplexing; and a photodetector for detecting light emitted from the fiber-type optical coupler.