JPH06252848A - Optical receiver - Google Patents

Abstract

PURPOSE:To receive a signal without mistake even when a polarized state of the signal light is fluctuated at a high speed by adopting the configuration such that a device to control a polarized state is not used for the receiver when a signal light subject to polarized modulation with a binary digital signal is received. CONSTITUTION:A power of an incident signal light to the receiver is divided equally into three by a 3-branch circuit 1. A 1st output 2 is given to a 1st polarized light separate element 6. A 2nd output 3 is given to a 2nd polarized light separate element 7, and the major axis of the 2nd polarized light separate element 7 is tilted by 45 deg. to the major axis of the 1st polarized light separate element 7. A 3rd output 4 is given to a 1/4 wavelength plate 5, and its output is given to the 3rd polarized light separate element 8. Two outputs from the three polarized light separate elements are received by two photodetectors 15 and electric outputs from them are decoded by a delay decoder 19, an amplitude adjustment device 26 makes the signal amplitude constant and an identification device 27 is used for identification. By this constitution, the polarized light modulation optical receiver unnecessitating a polarized light controller is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiver circuit used in optical fiber communication using a polarization modulation method.

[0002]

2. Description of the Related Art In conventional optical fiber communication, an intensity modulation-direct detection system has been used in which information is assigned to the intensity of signal light and the intensity is directly detected by a photodetector. However, it is known that when the signal light is transmitted by this method, the non-linear optical effect generated in the optical fiber, particularly the self-phase modulation effect or the cross-phase modulation effect causes deterioration of the transmission quality. The self-phase modulation effect and the cross-phase modulation effect are due to the intensity change of the signal light.

A polarization modulation method can be cited as a communication method for avoiding these two effects. The polarization modulation method is a method of transmitting information by allocating information to the polarization state of transmitted light. In such a polarization modulation optical fiber communication system, since the intensity distribution of the transmitted light is constant, the self-phase modulation effect and the mutual phase modulation effect are less likely to occur, and the transmission deterioration is small. As a configuration of a binary digital communication device using a polarization modulation system, there is one that has been applied for a patent under the name of "optical communication system" (Fukaya, JP-A-1-208920). In this "optical communication" transmitter, the transmitted "0" and "1" bits are assigned to two polarizations of transmitted light that are in an orthogonal relationship. this is,
Since the orthogonal relationship between the two polarizations is always maintained in the fiber,
This is because the mutual interference between codes after transmission is minimized.
In the receiver, first, the polarization of the signal light is directly converted into the polarized light by the polarization controller, and the two polarized lights corresponding to 0 and 1 are separated by the polarization separating element. The two lights after the polarization separation element are complementary intensity modulation signals (when one is in the on state, the other is in the off state). After these two signal lights are condensed by a lens, they are photoelectrically converted by two light receivers (photodetectors), and a difference component between the obtained two electric signals is detected.

[0004]

The polarization modulation optical receiver having the above structure requires some kind of polarization control device. Currently, as a polarization control device that enables automatic control,
Controlling the polarization state by applying stress to an optical fiber with a squeezer (Shimizu et al., Journal of L
lightwave Technology LT-9
No. 10 p. 1217), which controls the polarization state of guided light by an electro-optic effect by applying a control voltage on an optical waveguide formed on a lithium niobate substrate (F. Heissmann et al., Photonics Technology).
y Letter PTL-4 No. 5 p. Fifty
3) is proposed. However, in any of these devices, the speed of polarization control is limited by the feedback circuit for control, and the polarization state of light that fluctuates at or above this speed cannot be completely controlled. Therefore, when the above-mentioned optical receiver receives the polarization that fluctuates over the control speed, it becomes impossible to completely separate the two polarizations corresponding to "0" and "1". There is an error.

[0005]

In order to solve the above-mentioned problems, an optical receiving apparatus provided by the present invention is an optical branch circuit for dividing received light into three equal parts, and an output of this optical branch circuit. Quarter-wave plate that converts the state of one polarization of the optical splitter, a first polarization splitting element that splits the output light from one of the outputs of the optical splitting circuit into two polarizations, and the output of the optical splitting circuit From the second polarization splitting element that splits the output light from the other one of the two polarizations into two polarizations with the axis inclined by 45 degrees with respect to the polarization splitting axis of the first polarization splitting element, and the quarter wavelength plate. A third polarization separation element that separates the output light of the second polarization direction into two polarization directions;
And first and second polarization detectors provided corresponding to each of the first, second and third polarization separation elements for receiving two signal lights respectively output from the second polarization separation element and the third polarization separation element. And a photodetector.

[0006]

First, the light Z having the polarization state shown in FIG. 2 (a) enters the polarization separation element 6 shown in FIG. 2 (b), and the two outputs X and Y of the polarization separation element 6 are two lights. Consider the time when the light enters the photodetector circuit including the detector 15. Here, FIG.
The X and Y arms in (b) are outputs obtained by cutting out the polarized light in the X and Y directions in FIG. 2 (b), respectively. In this photodetector circuit, the detected photocurrent Iz is given by the difference between the received light power on the X arm side and the received light power on the Y arm side.
Here, assuming that the polarized light Z is represented by the points shown in FIG. 2C on the Poincare sphere, Iz is given by the following equation.

Iz = kz × Io Here, Io is a photocurrent value detected when linearly polarized light in the X direction of FIG. 2A is incident, and kz is shown in FIG.
It is the coordinates of the orthographic projection point from the Z point to the XY axis when the X point in (c) is 1 and the Y point is -1. in this way,
The output signal when the polarization-modulated light is received by this photodetector circuit is as an orthogonal projection point from the point that gives the incident polarization state on the Poincare sphere to the axis (in this case, the XY axis) given by the polarization separation element. expressed.

Next, let us consider a case where the optical receiver of the present invention receives polarization-modulated light which becomes "1" when the polarization state is switched and "0" when the polarization state is not changed.
Although the polarization state of this signal light can be arbitrarily changed in the transmission path, the two polarization components in the orthogonal relationship maintain the orthogonal relationship even after transmission. Therefore, the two polarization components contained in the incident light to the light receiving device are represented by two opposing points on the Poincare sphere, such as the ZZ 'point in FIG. This optical receiving device uses three photodetector circuits shown in FIG. 2B. With the above configuration,
The photodetector circuit including the second and third polarization separation elements detects the polarization state in three axial directions that are orthogonal to each other on the Poincare sphere. As a result, regardless of the state of the polarized light incident on the light receiving device, the switching of the polarized light is always detected in the detection signals from one or more photodetection circuits. As described above, in the present invention, since it is not necessary to use the polarization controller in the optical receiving device, the response speed to the polarization fluctuation is not limited by the polarization controller, and the response speed to the polarization fluctuation is significantly improved. Theoretically, half the speed at which the polarization changes to the orthogonal state within one time slot.
It is possible to correctly recognize the code if:

[0009]

1 is a diagram showing the configuration of an embodiment of the present invention. The signal light that has entered the optical receiver of this embodiment through the entrance fiber 30 is divided into three equal powers by the three-branching circuit 1. The first output 2 from the branch circuit 1 is incident on the first polarization separation element 6 at an angle of 0 degree. Here, the angle refers to the angle formed by the direction of the linearly polarized light observed in the branched output when a certain linearly polarized light is incident on the input fiber 30 and the principal axis direction of the polarization separation element. The second output 3 from the branch circuit 1 enters the second polarization separation element 7 at an angle of 45 degrees. The third output 4 from the branch circuit 1 enters the quarter-wave plate 5 and then enters the third polarization separation element 8. At this time, the two elements are arranged so that the angle formed by the principal axis of the quarter-wave plate 5 (the direction in which the linearly polarized light output when circularly polarized light is incident) and the principal axis of the polarization separation element 8 is 0 degree. Connect. Two signal lights output from each of the polarization separation elements 6, 7, and 8 are respectively received by the two photodetectors 15, and the differential outputs of the photodetectors 15 are output as electrical signals 16, 17, and 18. . Three electrical signals 1
6, 17 and 18 are exclusive ORed with the output from the 1-bit delay unit 20 in the delay decoder 19 and the exclusive OR circuit 21. After the sum of the three converted electric signals 22, 23, 24 is taken by the adder 25, the signal amplitude is made constant by the amplitude adjuster 26, and the signal is discriminated by the discriminator 27.

On the optical transmitter side, the modulated signal is encoded in advance using the circuit shown in FIG. 4, so that the polarization state does not change for "0" and the polarization changes for "1". Is modulated. In the optical receiver, the receiver incident polarized light that outputs the strongest light to the arm 9 in FIG.
Let it be the X point inside. Then, Y, P, Q, S, shown in FIG.
When the polarized light indicated by the point R is incident, the strongest light is output to the arms indicated by reference numerals 10, 11, 12, 13, and 14 in FIG. 1, respectively. In this optical receiver, the ports XY, P of FIG.
Detection signals in the Q and SR axis directions are output.

When the polarization-modulated light switching between ZZ 'as shown in FIG. 3 is incident on the optical receiver, as shown in FIG. 5 (a), all three ports of 16, 17, 18 are provided. The sign of the output signal is inverted. Here, when the signal from each port is decoded by the delay decoder 19, the output becomes “1” at each port of 22, 23 and 24 when the polarization state is switched, and when not switched, all output amplitudes are It becomes "0". After that, a signal can be obtained by adding all the outputs, adjusting the amplitude, and then identifying.

As a special case, let us consider a case where ZZ 'is the S and R points in FIG. 3 as shown in FIG. 5 (b).
In this case, when the polarization state switches, the ports 16, 1
No signal switching detected on port 7, but port 18
Switching can be detected with. Therefore, a correct signal can be obtained by taking the logical sum after decoding. FIG. 6 shows a case where polarization-modulated light actually sent through an optical fiber is received using this optical receiving device.
The waveforms at each point of the receiver and the output signal of the receiver are measured. The polarization state of the signal light transmitted during this experiment was changed as shown in FIG. However, as shown in FIG. 6B, the switching of the polarization state is always detected by any one or more of the three photodetection circuits regardless of the polarization state of the received light, and the transmission code must be received correctly. I was able to. As described above, by using the optical receiver of the present embodiment, it was possible to receive a correct signal regardless of the polarized light incident on the optical receiver.

[0013]

As described in detail above, by using the configuration of the present invention, the polarization modulated signal light can be received without using the polarization controller, so that the response speed to the polarization fluctuation can be greatly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram for explaining an operation principle of an optical receiving circuit.

FIG. 3 is a diagram showing a Poincare sphere for representing the polarization state of received polarization-modulated light.

FIG. 4 is a configuration diagram of an optical transmitter that performs encoding.

5 is a diagram showing the output signal amplitude at each point in the embodiment of FIG.

FIG. 6 is a diagram showing an experimental result of the output of the optical receiver in the case of receiving the signal light whose polarization state is changed.

[Explanation of symbols]

 1 Three-branch optical branch circuit 2 First output of optical branch circuit 3 Second output of optical branch circuit 4 Third output of optical branch circuit 5 Quarter wave plate 6 First polarization separation element 7 Second Polarization separation element 8 Third polarization separation element 9 X-arm 10 Y-arm 11 P-arm 12 Q-arm 13 S-arm 14 R-arm 15 Photodetector 16 First output signal 17 Second output signal 18 Third output Signal 19 Delay decoder 20 1-bit delay device 21 Exclusive OR circuit 22 First decoding output 23 Second decoding output 24 Third decoding output 25 Adder 26 Amplitude adjuster 27 Discriminator 28 Poincare sphere

Claims (2)

[Claims] 1. An optical branching circuit that divides received light into three equal parts, a first polarization splitting element that splits light from a first output of the optical branching circuit into two polarizations, and a first polarization splitting circuit of the optical splitting circuit. The light from the output of 2 is 4 with respect to the polarization splitting axis of the first polarization splitting element.
A second polarization splitting element that splits the light into two polarized lights with an axis inclined by 5 degrees, a quarter-wave plate that converts the polarization state of light from the third output of the optical branching circuit, and the quarter wavelength plate. A third polarization separation element that separates the output light from the one-wave plate into two polarization directions, and two signal lights that are output from each of the first, second, and third polarization separation elements are received. Therefore, the optical receiving device is provided with a first photodetector and a second photodetector provided corresponding to each of the first, second and third polarization separation elements. 2. A delay decoder for decoding the difference between the outputs of the first and second photodetectors, and three delay decoding elements corresponding to the first, second and third polarization separation elements, respectively. The optical receiving apparatus according to claim 1, wherein the received light is decoded by adding the outputs of the digitizers with an adder and identifying the outputs of the adders with an identifier.