JPH0563657A - Double balanced polarized wave diversity receiver - Google Patents

Abstract

PURPOSE:To attain reception of a stable coherent light by combining advantages of the polarized wave diversity reception system and the double balanced mixing system with respect to the double balanced polarized wave diversity receiver in the coherent light communication. CONSTITUTION:The receiver is provided with a polarized light separate means 3 separating a reception light 1 and a local oscillation light 2 into polarized components orthogonal to each other, photocouplers 5, 6 receiving the same polarized component in the optical signals separated respectively by the polarized light separate means 3, a light receiving means 4 converting the output optical signal mixed by the photocouplers 5, 6 into an electric signal, and a synthesis means 7 synthesizing the electric signal by the light receiving means 4 with phase adjustment or the like.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dual balanced polarization diversity receiver in a coherent optical communication system. In the optical communication system, a direct detection system is generally used in which received light transmitted through an optical transmission line is directly received by a light receiving element and converted into an electric signal. Further, in the coherent optical communication system, a homodyne detection method or a heterodyne detection method in which a laser light source with high purity is used as a light source of local oscillation light and the received light and the local oscillation light are mixed is known, and the direct detection method is known. It can be expected that the receiving sensitivity will be improved in comparison. Therefore, it is possible to increase the repeater interval or reduce the number of repeaters in the optical transmission line. Further, when applied to a subscriber system or the like, an increase in the number of branches can be expected, so that the optical transmission line can be economically constructed. When using such a homodyne detection method or a heterodyne detection method, the reception efficiency due to the intensity noise of the local oscillation light and the reduction of the interference efficiency between the reception light and the local oscillation light due to the polarization fluctuation in the optical transmission line It is desired to suppress the decrease.

[0002]

2. Description of the Related Art FIG. 3 is an explanatory diagram of coherent optical communication, showing a homodyne detection system or a heterodyne detection system, in which an optical signal from a transmission unit 31 is received by an optical transmission line 32 formed of a single mode optical fiber. Transmitted to the department. In the receiving section, the received light and the local oscillation light from the local oscillation laser 34 are added to the optical mixer 33 and mixed, and the mixed output light is received by the light receiving element 3 such as a photodiode.
5 to convert the optical signal into an electric signal, and the amplifier 36
In addition to. The output signal of the amplifier 36 is, for example, an intermediate frequency signal of several GHz.

In such a coherent optical communication system, the optical signal sent from the transmitter 31 is a laser beam that is directly or indirectly modulated and becomes a linearly polarized light. Propagation through the long-distance optical transmission line 32 formed of a mode optical fiber may result in elliptical polarized light, or the polarization direction may rotate. If this fluctuation in the polarization direction is large, the efficiency of interference with the locally oscillated light in the optical mixer 33 decreases, and reception becomes impossible in the worst state.

The polarization diversity receiving system shown in FIG. 4 is intended to solve such a problem.
The received light received via the optical transmission line is converted into the polarization separation element 41.
The polarized light components are separated into orthogonal polarization components by, and the local oscillation light from the local oscillation laser 44 is mixed with the respective polarization components by the optical mixers 43 and 46. In this case, in order to form orthogonal polarization components from the same local oscillation light, means such as a ½ wavelength plate 45 is provided between the optical mixers 43 and 46, and the polarization separation element 41 and When the arrangement position with the light mixer 46 is as shown in the figure, the reflection plate 42 is provided and the optical signal separated by the polarization separation element 41 is supplied to the light mixer 46.
Is to be incident on.

The respective output lights of the light mixers 43 and 46 are incident on the light receiving elements 47 and 48, converted into electric signals, and amplified by the amplifiers 49 and 50. The synthesizing section 52 synthesizes the output signal of the amplifier 49 and the output signal of the amplifier 50 whose phase is controlled by the phase shifter 51, and shifts so that the synthesized output signal becomes maximum. The amount of phase shift of the phase shifter 51 is controlled. Therefore, even when the fluctuation of the polarization direction in the optical transmission line is large, the light receiving element 47,
Since the output signal can be obtained from any one of the 48, the reception is not impossible.

FIG. 5 shows C / due to the intensity noise of the local oscillation light.
An N characteristic curve diagram is shown, showing that the C / N or the minimum reception level is improved as the local oscillation light intensity of the local oscillation lasers 34, 44 is increased in the conventional example shown in FIG. 3 or 4. However, when the intensity noise of the locally oscillated light based on the stability of the locally oscillated lasers 34 and 44 is large, the shot noise limit that can be originally reached is not reached even if the intensity of the locally oscillated light is increased, so that the C / N is deteriorated. Sometimes. Therefore, it is necessary to allow the deterioration of C / N by using the local oscillation light having a small intensity noise or to suppress the intensity noise.

Therefore, a double balanced mixing system has been proposed.
This double equilibrium mixing method is, for example, as shown in FIG.
When the reception light 61 and the local oscillation light from the local oscillation laser 64 are added to and mixed with the optical mixer 62, two output lights are obtained. Therefore, the output lights are incident on the light receiving elements 63 and 65, respectively. It is converted into an electric signal, amplified by the amplifiers 66 and 67, and added to the subtractor 68.

The beat signal component of the received light and the local oscillation light which are incident on the light receiving elements 63 and 65 from the light mixer 62 is 1
Since the phase is shifted by 80 ° and the intensity noise components of the local oscillation light are in phase, the subtracter 68 causes the amplifiers 66 and 67 to operate.
By calculating the difference between the output signals of, the beat signal components are added and the intensity noise components are canceled, and the intensity noise of the local oscillation light can be greatly reduced.

FIG. 7 is an explanatory view of the double balanced light receiving section,
In (A), the received light and the local oscillation light are incident on the optical mixer 71, the mixed light is incident on the light receiving elements 72 and 73 connected in series, and a signal is amplified from the connection point of the light receiving elements 72 and 73 by the amplifier 74.
The light receiving elements 72 and 73 are connected in series, so that, like the subtracter 68 in FIG.
The beat signal components having a 180 ° phase difference are added, and on the contrary, the in-phase intensity noise components are canceled.

Further, FIG. 7B shows a case of a double balanced light receiving section using the 3 dB optical coupler 75 as an optical mixer, in which the received light and the local oscillation light are incident on the 3 dB optical coupler 75 and mixed. The output light thus generated is connected in series to the light receiving elements 76, 7 respectively.
The signal incident on the light receiving element 7 and amplified at the connection point between the light receiving elements 76 and 77 is amplified by the amplifier 78, and the intensity noise can be reduced as in the configuration shown in FIG.

[0011]

Problems to be Solved by the Invention FIG. 3 and FIG.
In the conventional example shown in (1), only one mixed output light from the optical mixers 33 and 46 is used, and the other mixed output light is lost. In addition, the C / N is deteriorated due to the intensity noise of the local oscillation light. Therefore, it is difficult to improve the reception sensitivity. Further, in the conventional example shown in FIG. 6, the intensity noise of the local oscillation light can be suppressed, but the fluctuation of the reception sensitivity due to the fluctuation of the polarization direction occurs, and in the worst case, the reception light and the local oscillation are generated. There is a drawback that the polarization direction of light becomes orthogonal to each other, and the reception becomes impossible. It is an object of the present invention to combine the advantages of the polarization diversity reception system and the double balanced mixing system to enable stable coherent optical reception.

[0012]

A dual balanced polarization diversity receiver according to the present invention will be described with reference to FIG.
Of the polarization components output from the polarization splitting means 3 and the polarization splitting means 3 formed of a birefringent crystal for splitting the received light 1 and the local oscillation light 2 into polarization components orthogonal to each other, the same polarization component Optical couplers 5 and 6 that mix and output a plurality of optical signals, light receiving means 4 that converts the output optical signals from these optical couplers 5 and 6 into an electrical signal having an intermediate frequency component, and the light receiving means from this light receiving means. And a synthesizing means 7 for synthesizing the electrical signals by adjusting their phases.

[0013]

The polarized light separating means 3 is composed of a birefringent crystal plate or block, and when the received light 1 and the locally oscillated light 2 are incident, they are separated into orthogonal polarized light components. The received light of the same polarization component and the locally oscillated light among them are converted into optical couplers 5, 6
Incident on and mixed. The mixed output optical signal is received by the light receiving means 4
When it is incident on, it is converted into an electric signal having an intermediate frequency component and input to the synthesizing means 7. Due to the connection configuration of the synthesizing means 7 or the light receiving means 4, the electric signals corresponding to the same polarization component are synthesized so that the intensity noise component of the local oscillation light is canceled and the beat signal component is added. Furthermore,
In the synthesizing means 7, the synthesized output signals of the electric signals corresponding to the same polarization component are phase-adjusted and synthesized and output.
This eliminates the effect of fluctuations in the polarization direction.

[0014]

EXAMPLE FIG. 2 is an explanatory view of an example of the present invention.
Is a transmission unit, 12 is an optical transmission line, 13 is a polarization beam splitter which constitutes polarization separation means, 14 is a local oscillation laser, 1
Reference numerals 5 and 16 are optical couplers such as 3 dB optical couplers, 17 to 20 are light receiving elements that constitute light receiving means, and 21 is a combining circuit that constitutes combining means.

The optical signal from the transmitter 11 is incident on the polarization beam splitter 13 of the receiver via the optical transmission line 12,
It is separated into orthogonal polarization components, that is, P wave and S wave.
Further, the locally oscillated light from the locally oscillated laser 14 is also incident on the polarization beam splitter 13, and orthogonal polarization components, that is,
It is separated into P wave and S wave. The polarization beam splitter 13 is made of a birefringent crystal and is illustrated as a single plate, but various shapes such as a block may be adopted according to the relational arrangement with the optical coupler 16. You can

The received light and the local oscillation light having the same polarization component of the orthogonal polarization components separated by the polarization beam splitter 13 are made incident on the optical couplers 15 and 16, respectively. For example, the S wave of the received light and the S wave of the locally oscillated light are incident on the optical coupler 15 and mixed, and the P wave of the received light and the P wave of the locally oscillated light are incident on the optical coupler 16 and mixed. These optical couplers 15 and 16 can use an optical waveguide type structure or the like.

The S wave mixed by the optical coupler 15 is incident on the light receiving elements 17 and 18, and the P wave mixed by the optical coupler 16 is incident on the light receiving elements 19 and 20 and converted into electric signals. The light receiving elements 17 and 18 are connected in series, the connection point is connected to the combining circuit 21, and the light receiving elements 19 and 29 are connected in series, and the connection point is connected to the combining circuit 21. In this case, the light receiving elements 17, 1
The intensity noise of the locally oscillated light entering 8 is in phase, and the intensity noise of the locally oscillated light incident on the light receiving elements 19 and 20 is also in phase. The beat signal components have a phase difference of 180 °. Therefore, the light receiving elements 1 connected in series
7, 18 and 19, 20 cancel the intensity noise of the local oscillation light and add the beat signal components.

The combining circuit 21 adjusts the phases of the output signals of the light receiving elements 17, 18 and 19, 20 and combines the output signals. In this case, the combined output signal becomes maximum as shown in FIG. Thus, it will control the phase shifter. As a result, the components of the P wave and the S wave are combined, so that stable reception is possible even if fluctuations in the polarization direction due to the optical transmission line 12 occur.

[0019]

As described above, according to the present invention, the polarized light separating means 3 made of a birefringent crystal separates the received light 1 and the locally oscillated light 2 into polarized light components which are orthogonal to each other. The same polarization components among them are mixed by the optical couplers 5 and 6, and the optical signals output from the optical couplers 5 and 6 are received by the light receiving means 4 including a photodiode or the like.
Is converted into an electric signal having an intermediate frequency component, and the synthesizing means 7 performs phase adjustment and the like to synthesize.
The intensity noise of the local oscillation light is input in phase with the light receiving means 4
This is canceled by obtaining the difference between the output signals of the light receiving elements. At that time, since the beat signal components are 180 ° in phase, they are added. Therefore, a semiconductor laser or the like having high noise intensity can be used as the local oscillation laser, and the cost can be reduced.

The influence of fluctuations in the polarization direction due to the transmission line or the like can be eliminated by combining the electric signals of the P wave and S wave components in the combining means 7 by adjusting the phase. Therefore, there is an advantage that stable coherent optical communication can be performed. Further, by forming the polarization separating means 3 in a plate shape as shown in the drawing and making the optical couplers 5 and 6 of the waveguide type, it becomes easy to directly optically couple them, and the optical couplers 5 and 6 can be easily coupled. It is also possible to directly optically couple 6 and each light receiving element of the light receiving means 4, and there is an advantage that the overall size can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of an example of the present invention.

FIG. 3 is an explanatory diagram of coherent optical communication.

FIG. 4 is an explanatory diagram of a polarization diversity receiving system.

FIG. 5 is a C / N characteristic curve diagram due to intensity noise of local oscillation light.

FIG. 6 is an explanatory diagram of a double equilibrium mixing system.

FIG. 7 is an explanatory diagram of a double balanced light receiving unit.

[Explanation of symbols]

 1 Received Light 2 Local Oscillation Light 3 Polarization Separation Means 4 Light Receiving Means 5, 6 Optical Coupler 7 Combining Means

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Claims (1)

[Claims] 1. A polarization separation means (3) made of a birefringent crystal for separating the received light (1) and the local oscillation light (2) into polarization components orthogonal to each other, and each polarization component output from the polarization separation means. Among them, an optical coupler (5, 6) that mixes the same polarization components and outputs a plurality of optical signals, and an output optical signal from the optical coupler (5, 6) into an electric signal having an intermediate frequency component. A dual balanced polarization diversity receiver comprising a light receiving means (4) for converting and a combining means (7) for phase-adjusting and combining the electric signals from the light receiving means (4).