JPH05158096A - Optical receiver for coherent light wave communication - Google Patents

Abstract

PURPOSE:To reduce the size of the optical receiver for coherent light wave communication which is suitable for a polarization diversity reception system and also suitable for the constitution of a double balanced type photodetector. CONSTITUTION:This receiver is provided with a polarization beam splitter (birefringent plate) 8 which polarizes and separates signal light, a branching means 10 which branches local luminescence, 1st and 2nd optical directional couplers 4 and 6, and 1st-4th photodetectors 18, 20, 22 and 24; and the 1st and 2nd optical directional couplers 4 and 6 are formed on the same waveguide substrate 2 and the respective photodetecting elements 18, 20, 22, and 24 are formed on a substrate 26 so that the photodetection parts of the respective photodetectors 18, 20, 22, and 24 are linearly positioned.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver for coherent lightwave communication which is suitable for implementing a polarization diversity receiving system and which is suitable for constructing a photodetector in a double balanced type.

The coherent lightwave communication system uses intensity modulation /
It is suitable for long-distance transmission because it can increase the receiving sensitivity compared to the direct detection method, and is also suitable for large-capacity transmission because it enables high-density frequency division multiplexing in the optical region.

In a system to which this method is applied,
The transmitted data is reproduced as follows. The signal light whose angle has been modulated by the transmitter is transmitted to the receiver via the optical transmission path, is added to the local oscillation light (local light) from the local light source at the receiver, and then enters the light receiver. When the signal light and the local light are incident on the light receiver, so-called light mixing occurs on the light receiving surface due to the square characteristic of the light receiver, and an intermediate frequency signal having a frequency corresponding to the difference between the frequency of the signal light and the frequency of the local light or Since a baseband signal (all are generally electric signals in the microwave range) is obtained, demodulation is performed based on this signal to reproduce the transmission data.

An optical receiver used in this type of system is required to have the following in principle of operation. (B) If the polarization planes of the signal light and the local light do not match on the light receiving surface of the light receiver, the amplitude of the intermediate frequency signal or the baseband signal becomes small, and in the worst case, reception becomes impossible. I need to deal with it. One of the technologies for that purpose is the polarization diversity reception system.

(B) Since the coherent lightwave communication optical receiver handles extremely high power local light as compared with the received signal light, it is desirable to deal with intensity noise of local light. It is a technique for this that the photoreceiver is constructed in a double-balanced type.

[0006]

2. Description of the Related Art FIG. 5 is a diagram showing two structural examples of a conventional optical receiver for coherent lightwave communication, which is suitable for reception of a polarization diversity reception system and is suitable for constructing a photodetector in a double balanced type. Is.

In the configuration shown in FIG. 5 (A), the signal light from the optical transmission line is polarized and separated by the polarization beam splitter 102, and two polarization components whose polarization planes are orthogonal to each other are polarization-maintaining type. Input to the optical couplers 104 and 106. On the other hand, the local light is distributed by the polarization maintaining type optical coupler 108 and input to the optical couplers 104 and 106, respectively.

The light output from the optical coupler 104 in the two optical paths is input to the double balanced type light receiving section 110, and the light output from the optical coupler 106 in the two optical paths is also input to the double balanced type light receiving section 112. input. The light receiving sections 110 and 112 are respectively configured to serially connect the light receiving sections 114 and 116 having the same characteristics, such as photodiodes, and amplify the potential change at the connection point by the amplifier 118 and output it.

In the configuration shown in FIG. 5B, the signal light and the local light are input to the polarization beam splitters 122 and 124 via the optical coupler 120, and the polarization components having the same polarization plane receive each other. The parts 110 and 112 perform photoelectric conversion.

According to any one of these conventional configurations, even if the fluctuation of the polarization state of the signal light is large, the light receiving section 11
Since a signal can be obtained from either 0 or 112, reception is not impossible (polarization diversity reception system).

If the optical path lengths from the optical couplers 104 and 106 or the optical coupler 120 to the light receiving sections 110 and 112 are set appropriately, the result of asymmetrical phase inversion in the optical couplers, for example, the light receiving section 110 receives light. Bowl 11
With respect to the set of 4 and 116, the signal components of the light incident on them are in opposite phase, and the intensity noise components of local light are in phase. Therefore, the signal components are added, the intensity noise components are canceled, and the influence of the intensity noise of the local light is suppressed (double balanced type).

[0012]

In the conventional structure,
Since the optical connection between the respective parts is made by the optical fiber or the spatial light beam system, there is a problem that the device becomes large.

The present invention was created in view of such circumstances, and an object thereof is to provide an optical receiver for coherent lightwave communication, which is suitable for miniaturization.

[0014]

According to the present invention, a polarization beam splitter for polarization-separating signal light into first and second polarization components whose polarization planes are orthogonal to each other, and first and second branching for local oscillation light. Branching means for branching into local light, a first optical directional coupler into which the first polarized light component and the first branched local light are input, the second polarized light component and the second branched local light A second optical directional coupler, a first and a second light receiving element for receiving light emitted from the first and second output ends of the first optical directional coupler, and the above An optical receiver for coherent lightwave communication, comprising: a third and a fourth light receiving element for receiving light emitted from the first and second output ends of the second optical directional coupler, respectively.
The first and second optical directional couplers are waveguide type optical couplers formed on the same waveguide substrate so that the respective optical directional couplers are parallel to each other. An optical receiver for coherent lightwave communication is provided in which the fourth light receiving element is formed on the same substrate so that the respective light receiving portions are located on a straight line.

[0015]

According to the present invention, since the waveguide type optical coupler formed on the same waveguide substrate so that the respective optical directional coupling portions are parallel to each other is used as the optical directional coupler. The optical axes of the light emitted from the four output ends of the optical directional coupler can be made substantially parallel. Therefore, it is possible to perform highly efficient optical coupling in a state where the first to fourth light receiving elements formed on the same substrate so that the respective light receiving portions are located on a straight line are brought close to the waveguide substrate. Miniaturization is achieved.

[0016]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a block diagram of an optical receiver for coherent lightwave communication showing a first embodiment of the present invention. Reference numeral 2 is a waveguide substrate made of an optical material such as quartz, and the surface layer of the waveguide substrate 2 has an optical coupler 4 as a first optical directional coupler and an optical coupler 4 as a second optical directional coupler. And a coupler 6 are formed. Optical directional coupling section 4A of optical coupler 4 and optical directional coupling section 6A of optical coupler 6
Are parallel to each other. 4B and 4C are input ports of the optical coupler 4, and 4D and 4E are output ports of the optical coupler 4. Further, 6B and 6C are input ports of the optical coupler 6, and 6D and 6E.
Is an output port of the optical coupler 6.

The birefringent plate 8 used as the polarization beam splitter is formed of a birefringent crystal such as rutile, and the birefringent plate 8 is fixed to the end face of the waveguide substrate 2 by, for example, an optical adhesive. .. Regarding the optical axis and thickness of the birefringent plate 8, the ordinary ray component of the signal light incident on the birefringent plate 8 is incident on the input port 4C of the optical coupler 4, and the extraordinary ray component is incident on the input port 6B of the optical coupler 6. Is set.

A polarization maintaining optical coupler 10 is used as a branching unit for branching the local light into the first and second branched local lights. The first branch station light emitted from the polarization maintaining optical coupler 10 enters the input port 4B of the optical coupler 4 via the polarization maintaining optical fiber 12 in a predetermined polarization state. Further, the second branch station light emitted from the polarization maintaining optical coupler 10 is incident on the input port 6C of the optical coupler 6 via the polarization maintaining fiber 14 in a predetermined polarization state.

The polarization-maintaining fiber 12 is optically coupled to the input port 4B so that the polarization plane of the first branch local oscillation light coincides with the polarization plane of the ordinary ray incident on the input port 4C. , And is optically coupled to the input port 6C so that the polarization plane of the second branched local light coincides with the polarization plane of the extraordinary ray incident on the input port 6B.

Reference numeral 18 is a first light receiving element for receiving the light emitted from the output port 4D, 20 is a second light receiving element for receiving the light emitted from the output port 4E, and 22 is the output port 6
A third light receiving element that receives the light emitted from D, and a fourth light receiving element 24 that receives the light emitted from the output port 6E. These first to fourth light receiving elements 18, 20, 2
2 and 24 are integrated so as to be arranged in a line on a common substrate 26, whereby the photodetector 16 is configured.

FIG. 2 is a diagram showing a cross-sectional structure of each of the light receiving elements shown in FIG. 1. In this embodiment, the surface of the substrate 26 made of InP on which the light is incident has a convex lens portion formed by etching or the like. 28 is formed. Reference numeral 30 denotes an n ⁺ -InP layer formed on the side of the substrate 26 opposite to the lens portion 28,
32 is GaInAs formed on the n ⁺ -InP layer 30.
Layer, 34 is an n ^-- I layer formed on the GaInAs layer 32.
nP layer, 36 is a Zn diffusion layer formed in the n ⁻ -InP layer 34, 38 is an n electrode in contact with the n ⁺ -InP layer 30, 4
Reference numeral 0 is a p-electrode in contact with the Zn diffusion layer 36. The diameter of each light receiving portion is, for example, about 20 μm, and the dimension between the light receiving portions is, for example, about 125 μm.

According to this structure of the light receiver, since the respective light receiving elements are integrally formed on the substrate 26 and the lens portion 28 is also integrally formed with the substrate 26, the light receiver can be formed in a small size. .. The surface of the substrate 26 on the light incident side may be directly fixed to the end surface of the waveguide substrate 2 without forming the lens portion 28.

According to this embodiment, since the photodetector 16 can be brought very close to the waveguide substrate 2 and the birefringent plate 8 as a polarization beam splitter is directly fixed to the waveguide substrate 2, the device Can be downsized.

Polarization control is performed so that the signal light is a linearly polarized light having a polarization plane such that the ordinary ray component incident on the input port 4C and the extraordinary ray component incident on the input port 6B have substantially equal intensities. This enables polarization diversity reception.

Further, by allowing the signal light to enter the birefringent plate 8 as circularly polarized light, image removal reception becomes possible. According to the present embodiment, since a large number of optical components are not arranged in the optical path to each light receiving element as in the conventional case, generation of reflected feedback light can be suppressed.

FIG. 3 is a block diagram of an optical receiver for coherent lightwave communication showing a second embodiment of the present invention. In this embodiment, another optical coupler 42 is formed on the waveguide substrate 2 upstream of the optical couplers 4 and 6, and this optical coupler 42 is formed.
Functions as a branching means. The optical directional coupling portion 42A of the optical coupler 42 is formed parallel to the optical directional coupling portions of the optical couplers 4 and 6.

In the optical coupler 42, 42B and 42C are input ports, and 42D and 42E are output ports. The output port 42D is connected to the input port 4C of the optical coupler 4,
The output port 42E is connected to the input port 6B of the optical coupler 6.

In this embodiment, the optical coupler 42 is provided between the input port 4B of the optical coupler 4 and the input port 6C of the optical coupler 6.
Since the input ports 42B and 42C are located, it is not practical to use the birefringent plate as the polarization beam splitter by directly fixing it to the waveguide substrate 2.

Therefore, the polarization separation prism 44 is used as the polarization beam splitter. Polarization separation prism 44
Is a pair of triangular prisms in which a polarization separation film made of a dielectric multilayer film or the like is interposed between the inclined surfaces.

The signal light incident on the polarization splitting prism 44 is split into P-polarized light and S-polarized light here, and the P-polarized light is passed through the polarization maintaining fiber 46 to the input port 4 of the optical coupler 4.
The B polarized light and the S polarized light respectively enter the input port 6C of the optical coupler 6 via the polarization maintaining fiber 48.

The polarization maintaining fibers 46 and 48 are arranged so that the respective polarization planes of the light incident on the input ports 4B and 6C coincide with the polarization planes of the local light (linearly polarized light) incident on the input port 42B. Optically coupled to the input ports 4B and 6C, respectively. Input port 42C of optical coupler 42
Is terminated by an optical fiber whose end face is provided with a non-reflection film or by directly applying the non-reflection film.

FIG. 4 is a block diagram of an optical receiver for coherent lightwave communication showing a third embodiment of the present invention. In this embodiment, as in the first embodiment, the birefringent plate 8 fixed to the end face of the waveguide substrate 2 is used as the polarization beam splitter, and the branching means is integrally formed on the waveguide substrate 2, As the branching means, the optical coupler 50 whose optical directional coupling section 50A faces the direction substantially perpendicular to the optical directional coupling sections 4A and 6A of the optical couplers 4 and 6 is used.

50B and 50C are input ports of the optical coupler 50, and 50D and 50E are output ports of the optical coupler 50. Local light is incident on the input port 50C. The output port 50D is connected to the input port 4B of the optical coupler 4, and the output port 50E is connected to the input port 6C of the optical coupler 6.

In this embodiment, the polarization planes of local light input from the optical coupler 50 to the optical couplers 4 and 6 are the same. On the other hand, the polarization planes of the signal lights input from the birefringent plate 8 to the optical couplers 4 and 6 via the input ports 4C and 6B are orthogonal to each other. Therefore, in order to effectively interfere the signal light and the local light on the light receiving surface, the input ports 4C and 6B are required.
It is desirable to interpose a half-wave plate between either one of the two and the birefringent plate 8 so that the polarization planes of the lights incident from the input ports 4C and 6B coincide with each other.

According to this embodiment, since both the branching means and the polarization beam splitter are integrated with the waveguide substrate, the size of the device can be further reduced as compared with the previous embodiments.

[0036]

As described above, according to the present invention,
It is possible to provide an optical receiver for coherent lightwave communication suitable for downsizing of the device.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical receiver for coherent lightwave communication showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a cross-sectional structure of a light receiving element in an example of the present invention.

FIG. 3 is a configuration diagram of an optical receiver for coherent lightwave communication showing a second embodiment of the present invention.

FIG. 4 is a configuration diagram of an optical receiver for coherent lightwave communication showing a third embodiment of the present invention.

FIG. 5 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 2 Waveguide substrate 4, 6, 42, 50 Optical coupler 8 Birefringent plate 16 Photodetector 18, 20, 22, 24 Photodetector

Claims (6)

[Claims] 1. A polarization beam splitter that polarizes and separates signal light into first and second polarization components whose polarization planes are orthogonal to each other.
(8,44) and branching means for branching the local light into the first and second branch local light
(10, 42, 50), the first polarization component and the first optical directional coupler (4) to which the first branch local light is input, the second polarization component and the second A second optical directional coupler (6) to which branch office light is input, and first and second output terminals of the first optical directional coupler (4).
First and second light receiving elements (18, 20) for respectively receiving the light emitted from (4D, 4E), and first and second output ends (of the second optical directional coupler (6) ( 6D, 6E) third and fourth light receiving elements (22, 24) for respectively receiving the light emitted from (6D, 6E)
In the optical receiver for coherent lightwave communication, the first and second optical directional couplers (4, 6) are the same waveguide substrate so that the respective optical directional couplers are parallel to each other. (2) A waveguide type optical coupler formed on the above, wherein the first to fourth light receiving elements (18, 20, 22, 24) are formed on the same substrate so that the respective light receiving portions are located on a straight line. An optical receiver for coherent lightwave communication, which is formed on the above. 2. The polarization beam splitter comprises the first
Said waveguide substrate (2) so as to face one input end (4C) of said optical directional coupler (4) and one input end (6B) of said second optical directional coupler (6) Is a birefringent plate (8) fixed to the second optical directional coupler (6) and the other input end (4B) of the first optical directional coupler (4). ) Is a polarization-maintaining optical coupler (10) connected to the other input terminal (6C) of (1) by a polarization-maintaining fiber (12, 14). Receiving machine. 3. The polarization beam splitter comprises the first
Of the optical directional coupler (4) and the one input end (6C) of the second optical directional coupler (6) by a polarization maintaining fiber (46, 48). Connected Polarization Separation Prism (44)
The optical receiver for coherent lightwave communication according to claim 1, wherein the branching means is a waveguide type optical coupler (42) formed on the waveguide substrate (2). 4. The polarization beam splitter is the first beam splitter.
Of the optical waveguide (2) so as to face one input end (4C) of the optical directional coupler (4) and one input end (6B) of the second optical directional coupler (6). ) Is fixed to the birefringent plate (8), and the branching means has an optical directional coupling section (50A) for the optical directional coupling of the first and second optical directional couplers (4, 6). Division (4
(A, 6A) so that it is oriented at a right angle to the waveguide substrate (2)
The optical receiver for coherent lightwave communication according to claim 1, which is a waveguide type optical coupler (50) formed above. 5. The signal light is input to the polarization beam splitter as linearly polarized light having a polarization plane such that the intensities of the first and second polarization components are substantially equal to each other. An optical receiver for coherent lightwave communication according to any one of 1. 6. The optical receiver for coherent lightwave communication according to claim 1, wherein the signal light is input to the polarization beam splitter as circularly polarized light.