JPH01179013A - Polarization plane converting device - Google Patents

Abstract

PURPOSE:To control a plane of polarization nearly without exerting any influence upon the quantity of input information light by providing a processor which generates the driving signal of a phase device according to separated TE-mode component light and TM-mode component light. CONSTITUTION:An information beam which is converted into a linear polarized light beam by the polarization plane converting device 3 is entered into a splitting device 4 which splits the information light into the TE-mode component light and TM-mode component light and the TE component light is entered into a receiver 5 to obtain an information signal. The TM component light, on the other hand, is converted by a photodetector 6 into an electric signal, and a processor 7 generates a control signal according to the input signal and supplies the signal to a motor driving circuit 8, thereby supplying the driving signal to rotary driving motors 10a and 11a for lambda/4 plates 10b and 11b of phase devices 10 and 11 mounted in the polarization plate converting device 3. Consequently, a slight quantity of light in the information light is only extracted for a control signal generating process and such trouble that the quantity of the information light entering the receiver decreases can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a polarization conversion device, particularly a polarization conversion device suitable for controlling the polarization state of information light from a transmission optical fiber. be.

(Prior Art) Optical communication systems and optical measurement systems using semiconductor lasers that oscillate in a single-axis mode and single-mode optical fibers are being put into practical use. In this optical communication system, when the optical fiber that transmits the information light has birefringence, or when pressure is applied to the optical fiber or temperature change is applied, the light emitted from the optical fiber becomes an arbitrary elliptically polarized wave. If this information light with a changed polarization state is directly coupled to an optical communication device, noise will increase and a major problem will occur in the system.

Conventionally, to control the polarization state of light emitted from a transmission optical fiber, there have been methods that use a polarization-maintaining fiber in the optical fiber that transmits information light, and systems that generate stress birefringence in the core of the optical fiber. There is a method that maintains one polarization excitation.

Furthermore, methods using electro-optic effects and magneto-optic effects have also been proposed.

Furthermore, a method has been proposed in which a phase compensation plate is connected to a motor and the polarization state is controlled by rotating the phase compensation plate according to the phase difference.

(Problems to be Solved by the Invention) In the above-mentioned transmission method using a polarization-maintaining fiber or method that utilizes stress birefringence that occurs in the fiber, the polarization state of information light remains constant. It is difficult to transmit information light, and there are problems with stability. In addition, in the method of controlling the phase compensation plate by rotating it, the phase compensation plate is connected to the motor via various coupling devices such as pulleys, which not only has problems with response speed but also has problems in positioning the phase compensation plate. The problem is that the accuracy is poor and it is difficult to perform high-precision phase control. Furthermore, the TE mode of the polarization plane and 7M
Since both modes are handled simultaneously, it is necessary to take out a portion of the output light from the fiber and analyze the polarization state of the transmitted light for both the TE mode component and TM mode component, and input it to the receiving device. This has the disadvantage that not only the amount of light of the information signal to be transmitted is reduced, but also that the configuration of the control system becomes complicated.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a polarization plane conversion device which eliminates the above-mentioned drawbacks, allows stable and high-speed access, and which can control the polarization plane without substantially affecting the amount of information light input to a receiving device. It is something to do.

(Means for Solving the Problems) A polarization plane conversion device according to the present invention includes a first phase shifter that converts polarized light to be polarized into linearly polarized light, and a first phase shifter that converts the converted linearly polarized light into circularly polarized light. a λ/4 plate that converts the circularly polarized light into linearly polarized light;
a separating device that separates the polarized light that has passed through the plate into TE mode component light and TM mode component light; and driving the first and second phase shifters based on the separated TE mode component light or TM mode component light. The invention is characterized in that it includes a processing device that generates a drive signal for the purpose of the invention.

(Function) In an optical communication system, when information light of, for example, TE mode component is transmitted from the transmitting side through a single mode fiber,
Owing to the pressure acting on the single mode fiber, the receiving side receives an elliptically polarized wave consisting of a TE mode component and a TM mode component. Therefore, in the present invention, the transmitted information light is separated into TE mode component light and TM mode component light using a separation device such as a Rochon prism, and the separated T
Control signals for two phase shifters are created from the M-mode component light. Based on the control signal, the transmitted information light is converted into linearly polarized light by a first phase shifter, then converted into circularly polarized light by a second phase shifter, and further converted into circularly polarized light using a λ/4 plate. Converts into information light consisting only of the original TE mode component. It is structured like this.

If this is done, since the TE mode component light generated by the single mode fiber is minute, the plane of polarization can be converted at high speed without reducing the amount of information light.

(Example) The optical signal sent from the transmitting side is linearly polarized light (TE mode component) that vibrates along the X-axis direction in a plane orthogonal to the propagation direction, as shown in Figure 3a. Linearly polarized light (TM mode component) that vibrates along the y-axis orthogonal to the y-axis due to the effects of pressure, temperature, etc. acting on the optical fiber.
is generated and transmitted to the receiving side as arbitrary elliptically polarized light.

FIG. 1 is a diagram showing the overall configuration of a polarization plane conversion device according to the present invention. Information light transmitted from a single mode fiber is converted into a parallel beam via a collimator device 2, and this parallel beam is made incident on a polarization plane conversion device 3. Since the incident information beam becomes an arbitrary elliptical polarized light depending on the pressure and temperature acting on the fiber, the polarization plane conversion device 3
The information beam is converted into a linearly polarized beam identical to the original information beam. The converted information beam enters a separation device 4 that separates the information light into TE mode component light and TM mode component light, and the TE component light enters a receiver 5 where it is photoelectrically converted into an information signal. On the other hand, the TM component light is converted into an electrical signal by a photodetector 6 and input to a post-processing device 7 . This processing device 7 is constituted by, for example, a personal computer, and generates a control signal for rotationally driving a λ/4 plate installed in the polarization plane conversion device 3 based on an input signal. The control signal generated by the processing device 7 is sent to the motor drive circuit 8.
This motor drive circuit 8 generates a motor drive signal for rotationally driving the λ/4 plate, and this drive signal is used for rotationally driving the λ/4 plate installed in the polarization plane conversion device 3. Supplies the motor.

FIG. 2 is a diagram showing the configuration of an example of a polarization plane conversion device. The polarization plane conversion device includes a first phase shifter 10, a second phase shifter 11 disposed at the subsequent stage, and a λ/
Consists of 4 boards, 1 board and 2 boards. First and second phase shifter 10
and 11 are step motors 10a and lla, respectively.
and λ/4 plates 10b and llb mounted within the rotating shaft thereof. As shown in FIG. 3a, the information light on the transmitting side is linearly polarized light (TE mode component) vibrating in the in-plane X-axis direction perpendicular to the propagation direction. As described above, elliptically polarized light as shown in the figure is incident on the polarization plane conversion device. The step motor 10a of the first phase shifter 10 rotates its rotation axis in response to the supplied drive signal until the crystal axis of the λ14 plate 10b built in the rotation axis coincides with the principal axis of the elliptically polarized light. This rotational operation of the first phase shifter causes the first
The light beam that has passed through the phase shifter 10 is converted into linearly polarized light that vibrates along the principal axis of the ellipse, as shown in FIG. 3C, and then enters the second phase shifter 11.

The step motor lla of the second phase shifter 11 is driven by the λ/4 plate 1 built in its rotating shaft by the supplied drive signal.
The crystal axis of 1b is π/2 with respect to the principal axis of the incident linearly polarized light.
Rotate to a position that forms an angle of . As a result, the incident linearly polarized light is converted into circularly polarized light as shown in FIG. 3d. This circularly polarized light is converted into linearly polarized light that vibrates along the X-axis direction on a λ/4 plate 2 whose crystal axis is arranged at an angle of π/4 with respect to the X-axis. This linearly polarized information light is transmitted to a separation device 4 composed of, for example, a Rossigon prism.
After passing through, the signal is input to the receiver 5, and after being photoelectrically converted, an electrical information signal is generated.

Next, a process for creating control signals for the first and second phase shifters will be described. When the polarization state of the information light changes during transmission, a TE acid component light wave is generated in the information light incident on the receiving side. This TE component light is separated by a separation device 4, photoelectrically converted by a photodetector 6, and then converted into a digital signal by a ^/D converter 13 before being input to a processing device 7. First, a process for creating a control signal for the first phase shifter 10 will be described. Since the magnitude of the light wave before being separated by the separation device 4 is the sum of the respective magnitudes of the TE component light and the TM component light after separation, the following normalization formula holds true.

TE+TM=1 Here, TE is the absolute value TM of the magnitude of the TE component light.
The absolute value of the magnitude of the component light Therefore, the TE component light is determined by the following equation using the detected TM acid component.

TE=1-TM The action of the first phase shifter is to convert arbitrary elliptically polarized light into linearly polarized light, so due to the properties of the λ/4 plate, the crystal axis of the λ/4 plate is aligned with the principal axis of the elliptically polarized light. Just let it happen. Since it is sufficient for the step motor to rotate ±45°, the maximum rotation angle is set to 45°. There is the following relationship between the rotation angle and the components of the light wave.

1 45° φ= (1-l TE-TM l ) x45° Here, the azimuth angle φ is the angle that the principal axis of the ellipse makes with the X axis. T.E.
can be detected at the receiver side, and TM can be detected by the photodetector 7. Therefore, the processing device 7 can determine the azimuth angle φ based on the detected TE value and TM value.

If the azimuth of the λ/4 plate before inputting the control signal is φ0, the step motor is φ−φ. All you have to do is rotate it by the angle . Note that the rotation direction of the step motor can be arbitrarily determined; for example, it can be set counterclockwise when TE-TM<O, and clockwise when TE-TM>O. Next, the operation of the second phase shifter will be explained. Since the second phase shifter functions to convert linearly polarized light into circularly polarized light, it is only necessary to tilt the first phase shifter λ by an angle of 45° with respect to the azimuth angle φ of the plate. Therefore, it is sufficient to interlock the detected azimuth of the λ/4 plate of the first phase shifter with an angular shift of 45 degrees.

FIG. 4 is a flowchart showing the data processing steps in the processing device. In the initial settings (A), set the rotation angle per step of the motor, whether or not to save data, and limit values, import the TM acid component signal (B), and then
Compare TE-TM l and the limit value (C), and
If I < limit value, return to CB), I TE-TM I >
If it is the limit value, proceed to (D).

In (D), the rotation direction is determined by the method described above, and if the rotation is forward, the process goes to (E), and if the rotation is reverse, the process goes to (F).

In (E) and (F), the rotation angle of the motor 10a is determined and a pulse signal output (G) is provided. Furthermore, in (I()), the rotation angle of the motor lla is determined by the method described above, the necessary pulses are outputted (I), and the process returns to (B).

Note that the screen of the personal computer can display the rotation angle, ellipticity, phase difference between TE and TM, etc. of each motor, and can also display the state of the polarization plane.

The present invention is not limited to the embodiments described above, and various modifications are possible. For example, in the above-described embodiment, TE polarized light is transmitted as information light from the transmitting side, and the TM polarized light component in the information light is extracted on the receiving side to create a control signal, but 7M polarized light is transmitted as information light. On the receiving side, it is also possible to extract the TE polarization component and create a control signal.

(Effects of the Invention) As explained above, according to the present invention, the TM acid component of the received information light is extracted, and the λ/4 plate incorporated in the two phase shifters is rotated based on this TM acid component. Since it is configured to form a control signal for control, it is only necessary to extract a small amount of the information light for the control signal creation process, which eliminates the problem of a decrease in the amount of information light input to the receiving device. It can be resolved.

In addition, since the plate is rotated to λ built in the rotating shaft of the step motor based on the control signal, the response speed can be further increased.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the overall configuration of the polarization plane conversion device according to the present invention, Fig. 2 is a diagram showing the detailed configuration of the polarization conversion device, and Fig. 3-
U;! J a - e are diagrams showing each polarization state in the conversion process, and FIG. 4 is a flowchart showing the steps in the processing device. 1... Single mode fiber 2... Collimator device 3... Polarization plane conversion device 4
Separation device 5 Receiver 6 Photodetector 7 Processing device 8 Motor drive circuit 10 First phase shifter 11 Second phase shifter
12...λ14 plate Figure 2

Claims (1)

[Claims] 1. A first phase shifter that converts the polarized light to be polarized into linearly polarized light, a second phase shifter that converts the converted linearly polarized light into circularly polarized light, and a second phase shifter that converts the converted linearly polarized light into circularly polarized light. a λ/4 plate for conversion, a separation device for separating the polarized wave passing through the λ/4 plate into TE mode component light and TM mode component light;
A polarization plane conversion device comprising: a processing device that generates a drive signal for driving the first and second phase shifters based on the separated TE mode component light or TM mode component light.