JPH05127125A - Optical receiver for coherent light wave communication - Google Patents

Abstract

PURPOSE:To hardly occur crosstalk and to facilitate the optimization of branching ratio for an optical coupler by slanting a polarized light separation direction at a specific angle to a yz plane. CONSTITUTION:The optical receiver for coherent light wave communication provided with an optical coupler 2, a polarization beam splitter 18, and a photodetector 28 having 1st-4th photodetection parts 20, 22, 24, and 26 is constituted so that a plane containing the main axes of 1st and 2nd mutually parallel polarized light beams 106 and 108 and a plane containing the main axes of 3rd and 4th mutually parallel polarized light beams 110 and 112 slant at almost 45 to a plane (yz plane) containing the main axes of 1st and 2nd branch light beams 102 and 104. The four photodetection parts 20, 22, 24, and 26 are therefor positioned at the respective vertices of a parallelogram having no 90 vertical angle. Consequently, when two double balanced type photodetectors are constituted, their signal lead-out points do not face each other, crosstalk is hardly generated, and the branching ratio is optimized.

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 linear 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 transmission information is reproduced as follows. The signal light whose angle is modulated in the transmitter is transmitted to the receiver through the optical transmission line,
The light is added to the locally oscillated 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 or a baseband signal having a frequency that is the difference between the frequency of the signal light and the frequency of the local light. Since a signal (all are generally electric signals in the microwave region) is obtained, the transmission information is reproduced by performing demodulation based on this signal.

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 necessary to deal with the intensity noise of the 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 a configuration example of a conventional optical receiver for coherent lightwave communication, which is suitable for implementing a polarization diversity receiving system and is suitable for constructing a photodetector in a double balanced type. .. Reference numeral 2 is an optical coupler based on evanescent wave coupling in the optical waveguide.
It has input ports 4 and 6 and first and second output ports 8 and 10. An optical fiber 12 for transmitting signal light is connected to the first input port 4, and a polarization-maintaining optical fiber 14 'for transmitting local light is connected to the second input port 6. The optical coupler 2 adds the input signal light and local light and branches them at a predetermined branching ratio (usually 1: 1) to output the first and second branched lights 102 and 104 to the first and second outputs, respectively. Output from ports 8 and 10.

Reference numeral 16 is a lens such as a flat plate microlens for converting the first and second branched lights into a parallel light beam. 1
Reference numeral 8'denotes a birefringent crystal plate used as a polarization beam splitter, and this birefringent crystal plate 18 'is used for the first branched light 1
02 is the first and second polarized beams 1 whose polarization planes are orthogonal to each other
06 'and 108' are polarized and separated, and the second branched light 1
04 is the third and fourth polarized beams 1 whose polarization planes are orthogonal to each other
The polarized light is separated into 10 'and 112'.

Reference numerals 20 ', 22', 24 'and 26' denote first to fourth polarized beams 106 ', 108' and 11 respectively.
First to fourth light receiving portions for receiving 0 ', 112',
These light receiving portions are integrally formed on the light receiving element 28 '.

Hereinafter, in order to explain the positional relationship of each member, etc., the first or second light emitted from the optical coupler 2 in parallel with each other.
Principal axes of the light beams of the branched lights 102 and 104 (in the case of a cylindrical light beam, correspond to the direction of the light beam located at the center)
Is the z-axis, the direction parallel to the plane including the principal axes of the first and second branched lights 102 and 104 and perpendicular to these principal axes is the y-axis, and the direction perpendicular to the z-axis and the y-axis is the x-axis. Set a three-dimensional coordinate system. The x-axis is directed from the back side to the front side of the paper, the y-axis is directed from the lower side to the upper side in the figure, and the z-axis is directed from the left side to the right side in the figure.

FIG. 6 is an explanatory view of the optical axis of the birefringent crystal plate 18 'of FIG. (A) is a view of the birefringent crystal plate 18 'viewed from the positive side of the z-axis, that is, a view of the birefringent crystal plate 18' viewed from the left side to the right side in FIG.
(B) is a view of the birefringent crystal plate 18 'as viewed toward the positive side of the y-axis. The optical axis c'of the birefringent crystal plate 18 'is perpendicular to the y axis, and the optical axis c'is parallel to the plane represented by x = z.

In this case, the planes containing the principal axes of the first and second polarized beams 106 'and 108' which are parallel to each other are parallel to the xz plane, and the third and fourth polarized beams 110 'and 112 which are also parallel to each other. The plane including the principal axis of 'is also parallel to the xz plane. Therefore, the four light receiving parts 20 ', 22', 2
4'and 26 'are located at each vertex of the rectangle.

FIG. 7 is a diagram for explaining the planar positional relationship and connection state of the respective light receiving portions of FIG. First light receiving section 2
0'and the third light receiving portion 24 'are connected in series, and the potential change at the connection point is amplified by the amplifier 32 to be extracted as an intermediate frequency signal or a baseband signal. Further, the second light receiving portion 22 'and the fourth light receiving portion 26' are also connected in series, and the potential change at the connection point is amplified by the amplifier 34 and taken out.

According to the conventional configuration described above, the first light receiving portion 20 'is provided even if the fluctuation of the polarization state of the signal light is large.
And a signal can be obtained from either the set of the third light receiving unit 24 'or the set of the second light receiving unit 22' and the fourth light receiving unit 26 ', so that reception is not possible (polarization diversity reception system). ).

On the other hand, if the optical path length from the optical coupler 2 to each light receiving portion is set appropriately, as a result of asymmetrical phase inversion in the optical coupler 2, for example, the first light receiving portion 20 'and the third light receiving portion.
With respect to the set of the light receiving sections 24 ', the signal components of the light incident on them are in the opposite phase, and the intensity noise components of the local light are in the same phase. Therefore, the signal components are added, the intensity noise component is canceled, and the influence of the intensity noise of the local light is suppressed (double balanced type photoreceiver).

[0015]

In the conventional optical receiver for coherent lightwave communication, as shown in FIG.
Since the two light receiving portions are arranged so as to be located at the vertices of the rectangle, the two signal extraction points face each other, and there is a problem that crosstalk is likely to occur.

In the double-balanced photodetector, in order to effectively suppress the local oscillation intensity noise, it is desirable that the branching ratio of the local oscillation light in the optical coupler is exactly 1: 1. Specifically, in the conventional configuration, the polarization plane is set so that the polarization plane of the local light incident on the optical coupler 2 is parallel to the plane represented by x = y or the plane represented by x = −y. Since the storage optical fiber 14 'is connected to the optical coupler 2, both the local polarization component having the polarization plane parallel to the yz plane and the polarization component of the local light polarization having the polarization plane parallel to the xz plane are transmitted. It is required that the branching ratio in the coupler 2 be exactly 1: 1.

However, in a waveguide type optical coupler, the branching ratio generally changes depending on the polarization state, so that it is not easy to satisfy the above condition. The present invention was created in view of such technical problems,
It is an object of the present invention to provide an optical receiver for coherent lightwave communication in which crosstalk is unlikely to occur.

It is also an object of the present invention to provide an optical receiver for coherent lightwave communication in which the branching ratio in the optical coupler can be easily optimized.

[0019]

In the optical receiver for coherent lightwave communication of the present invention, the signal light and the local light which are respectively input to the first and second input ports are added together and branched at a predetermined branching ratio. An optical coupler that outputs the first and second branched lights from the first and second output ports, respectively, and the first branched light is polarized and separated into first and second polarized beams whose polarization planes are orthogonal to each other. A polarization beam splitter for splitting and splitting the branched light into third and fourth polarized beams whose polarization planes are orthogonal to each other, and a light receiving element having first to fourth light receiving portions for respectively receiving the first to fourth polarized beams are provided. In an optical receiver for coherent lightwave communication, the plane including the principal axes of the first and second polarized beams parallel to each other and the plane including the principal axes of the third and fourth polarized beams parallel to each other. There configured so as to outline 45 ° inclined relative to a plane including the principal axis of the first and second branch light.

[0020]

According to the present invention, the plane containing the principal axes of the first and second polarized beams and the plane containing the principal axes of the third and fourth polarized beams with respect to the plane containing the principal axes of the first and second branched beams. Since the light receiving portions are inclined at about 45 °, the four light receiving portions are located at the vertices of the parallelogram whose apex angle is not 90 °. As a result, when two sets of double-balanced optical receivers are configured, the signal extraction points do not face each other, and crosstalk is less likely to occur. Further, the fact that it becomes easy to optimize the branching ratio in the optical coupler will be described in detail in the embodiments.

[0021]

The preferred embodiments of the present invention will be described below. FIG. 1 is an overall perspective view of an optical receiver for coherent lightwave communication showing a preferred embodiment of the present invention. The parts substantially the same as those in the conventional example are designated by the same reference numerals, and the description thereof is partially omitted. Further, the above-mentioned x, y, z coordinate axes are used in describing the arrangement relationship of each member and the like.

The structure of this embodiment is different from the conventional structure in the following points. A polarization-maintaining optical fiber 14 for local light is connected to the optical coupler 2 such that the polarization plane of the local light incident on the second input port 6 of the optical coupler 2 is parallel or perpendicular to the yz plane.

A plane including the principal axes of the first and second polarized beams 106 and 108 and the third and fourth polarized beams 110 and 112.
The plane including the principal axis of the first and second branched lights 102, 104
The birefringent crystal plate 18 that is inclined by about 45 ° with respect to the plane including the principal axis of (1) (yz plane) is used. A configuration example of such a birefringent crystal plate will be described later.

First to fourth polarized beams 106, 10
The first to fourth light receiving parts 20, 22, 24, 26 for receiving the light of 8, 110, 112 respectively are formed such that their respective central parts are located at the respective vertices of the parallelogram whose apex angle is not 90 °. There is.

FIG. 2 is a diagram showing a sectional structure of each light receiving portion. Each light receiving portion is composed of a PIN photodiode, and these light receiving portions are integrally formed on a substrate 36 made of InP. Reference numeral 38 denotes a convex lens portion formed on the surface of the substrate 36 on which light is incident, 40 is an n ⁺ -InP layer formed on the opposite side of the lens portion 38 of the substrate 36, and 42 is n ⁺ -InP. GaInAs layer formed on the layer 40, 4
4 is an n ⁻ -InP formed on the GaInAs layer 42.
Layer, 46 is a Zn diffusion layer formed in the n ⁻ -InP layer 44, 48 is an n electrode in contact with the n ⁺ -InP layer 40, 50
Is a p electrode in contact with the Zn diffusion layer 46. The diameter of each light receiving part is about 20 μm, and the dimension between the light receiving parts is about 125 μm.
And

According to this embodiment, since each light receiving portion is formed integrally on the substrate 36 and the lens portion 38 is formed integrally with the substrate 36, the light receiving element can be formed in a small size.

FIG. 3 is an explanatory view of the optical axis of the birefringent crystal plate 18 of FIG. 3 (A) and 3 (B) are shown in FIG.
It corresponds to (A) and (B). In this embodiment, the projection of the optical axis c of the birefringent crystal plate 18 onto the main surface (the surface of the birefringent crystal plate on which the first and second branched lights 102 and 104 are incident) 18A is performed by the first and second branched lights. It is inclined at approximately 45 ° with respect to a plane (yz plane) including the principal axes of the lights 102 and 104.
The projection of the optical axis c on the bottom surface 18B of the birefringent crystal plate is
Similarly, it is inclined at approximately 45 ° with respect to a plane (yz plane) including the principal axes of the first and second branched lights 102 and 104. Specifically, the optical axis c of the birefringent crystal plate 18 is x = −y = z
It is parallel to the straight line represented by.

Such a birefringent crystal plate 18 can be obtained by rotating the birefringent crystal plate 18 'shown in FIG. 5 by 45 ° with respect to the z-axis. When assembling each member into a module, For this purpose, as shown in FIG. 1, it is desirable to dispose the rectangular parallelepiped birefringent crystal plate 18 in parallel with other rectangular parallelepiped members.

When a birefringent crystal plate having such an optical axis is used, a plane including the principal axes of the first and second polarized beams 106 and 108 and a plane including the principal axes of the third and fourth polarized beams 110 and 112. Is approximately 45 ° with respect to the xz plane
It becomes inclined.

FIG. 4 is a diagram for explaining the arrangement of the respective light receiving portions when viewed from the back surface side of the light receiving element 28 (the arrangement when viewed from the positive side to the negative side of the z-axis) and the connection relationship. The connection relationship is the same as that in FIG. 7, and the description thereof will be omitted.

In this embodiment, since the birefringent crystal plate 18 having the above-mentioned specific structure is used, the first to fourth light receiving beams on which the first to fourth polarized beams 106, 108, 110 and 112 respectively enter. Since the portions 20, 22, 24, and 26 are located at the vertices of the parallelogram whose vertex angle on the substrate surface of the light receiving element is not 90 °, the first and third light receiving portions 20,
The signal extraction point of 24 and the signal extraction points of the second and fourth light receiving portions 22 and 26 do not face each other, and the occurrence of crosstalk is suppressed.

In this embodiment, since the polarization plane of the local light incident on the optical coupler 2 is parallel or perpendicular to the yz plane, the branching ratio of the optical coupler 2 is the polarization plane of the incident light. The dependent change does not cause any problem in setting the branching ratio for local light to a desired value. Therefore, it is possible to effectively prevent the influence of the intensity noise of the local light by setting the branching ratio of the local light to 1: 1.

According to the present embodiment, the polarization-maintaining optical fiber 1 is compared with the case where the polarization plane of the local light incident on the optical coupler 2 is inclined by 45 ° with respect to the yz plane.
It is easy to connect 4 to the optical coupler 2. As described above, according to the present embodiment, in the optical receiver for coherent lightwave communication, which is suitable for implementing the polarization diversity receiving method and suitable for configuring the receiver in the double balanced type, the optimization of the branching ratio of the optical coupler is performed. It is possible to facilitate the above, and it is possible to prevent crosstalk from occurring easily.

In this embodiment, when local light is made incident on the optical coupler 2 as circularly polarized light, an image-removal type optical receiver for coherent lightwave communication can be realized. By forming the amplifiers 32 and 34 for front-end amplification on the substrate 30 to which the light receiving element 28 is fixed, it is easy to downsize the device. In this case, the electrical connection between the light receiving section and the amplifier can be made in the shortest distance, and the broadband optical signal can be received.

Optical deterioration of the light receiving portion can be suppressed by hermetically assembling the light receiving element 28 and the substrate 30 with a cap having, for example, sapphire as a transmission window. The birefringent crystal plate 18, the lens 16 or the optical coupler 2 can also be used as the transmission window of the package. This makes it possible to realize a compact optical receiver.

[0036]

As described above, according to the present invention,
It is possible to provide an optical receiver for coherent lightwave communication with less crosstalk. Further, according to the preferred embodiment of the present invention, in addition to the above effects, there is an effect that the branching ratio in the optical coupler can be easily optimized.

[Brief description of drawings]

FIG. 1 is an overall perspective view of an optical receiver for coherent lightwave communication showing a preferred embodiment of the present invention.

FIG. 2 is a diagram showing a cross-sectional configuration of a light receiving unit of FIG.

FIG. 3 is an explanatory diagram of an optical axis of the birefringent crystal plate of FIG.

FIG. 4 is a diagram for explaining the arrangement and connection of the light receiving units of FIG.

FIG. 5 is an overall perspective view of a conventional optical receiver for coherent lightwave communication.

6 is an explanatory diagram of an optical axis of the birefringent crystal plate of FIG.

FIG. 7 is a diagram for explaining the arrangement and connection of the light receiving units of FIG.

[Explanation of symbols]

 2 Optical Coupler 18 Polarizing Beam Splitter (Birefringent Crystal Plate) 28 Photodetector

Claims (5)

[Claims] 1. The signal light and the local light input to the first and second input ports (4, 6) are added together and branched at a predetermined branching ratio, and these first and second branched lights are respectively divided into first and second beams. And optical coupler that outputs from the second output port (8,10)
(2) and the first and second split light beams whose polarization planes are orthogonal to each other
A polarization beam splitter (18) that splits the polarized beam into polarized beams and splits the second branched light into third and fourth polarized beams whose polarization planes are orthogonal to each other, and receives the first to fourth polarized beams, respectively. An optical receiver for coherent lightwave communication comprising a light receiving element (28) having first to fourth light receiving parts (20, 22, 24, 26), wherein the principal axes of the first and second polarized beams parallel to each other are A coherent lightwave characterized in that a plane including the planes including the main axes of the third and fourth polarized beams parallel to each other and a plane including the main axes of the first and second branched lights are substantially 45 ° with respect to each other. Optical receiver for communication. 2. The first to fourth light receiving parts (20, 22, 24, 26)
Are formed on the same substrate (36), and the apex angle on the substrate surface is 90.
The optical receiver for coherent lightwave communication according to claim 1, wherein the center of each of the light receiving portions is located at each vertex of a parallelogram that is not °. 3. The polarization beam splitter is a birefringent crystal plate (18), and a principal plane (1) of an optical axis in the birefringent crystal plate (1).
8. The optical receiver for coherent lightwave communication according to claim 2, wherein the projection onto 8A) is inclined by about 45 ° with respect to the plane including the principal axes of the first and second branched lights. 4. The optical receiver for coherent lightwave communication according to claim 1, wherein the local light is circularly polarized light. 5. The local light is linearly polarized light, and its polarization plane is perpendicular or parallel to the plane containing the principal axes of the first and second branched lights. An optical receiver for coherent lightwave communication according to Crab.