JPH1062827A - Acousto-optic switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the waveform fluctuation of light receiving pulses by the polarization characteristic of fibers to be measured. SOLUTION: This acousto-optic switch has first and second optical fibers 1, 2 which are arranged to face each other on the same optical axis, a third optical fiber 3 which is arranged by varying its optical axis from the optical axis of these first and second optical fibers 1, 2 and an acousto-optic element 6 which is interposed between the respective optical fibers 1 to 3 and is capable of switching the optical path from the second optical fiber 2 to the first optical fiber 1 to the optical path of the third optical fiber 3 by acoustic-optic effect. The second optical fiber 2 is formed by connecting two pieces of constant polarization fibers 9, 10 in the state of rotating the respective main axes 45 deg.. A quarter-wave plate 7 is interposed in the optical path between the second optical fiber 2 and the acousto-optic element 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acousto-optic switch applied to an optical pulse tester or the like for measuring a breaking point, a failure point, and a propagation loss of an optical fiber.

[0002]

2. Description of the Related Art A switch disclosed in, for example, Japanese Patent Application Laid-Open No. 64-44922 is known as this type of acousto-optic switch. In the acousto-optic switch disclosed in this publication, as shown in FIG. 2, an input optical fiber 1 and a transmitted light output optical fiber 2 which are single mode fibers are optically connected via convergent rod lenses 5 and 8. The acousto-optic element 6 is arranged on the optical axis of the converging rod lenses 5 and 8. The light receiving optical fiber 3 is arranged so that the optical axis is changed by the prism 12, and the light receiving optical fiber 3 is connected to the acousto-optical element 6 and the transmitted light output optical fiber 2 via the prism 12 and the converging rod lens 13. Optically connected. The transmitted light output optical fiber 2 is composed of a constant polarization fiber, and is provided with a connecting portion 14 in the middle thereof for coupling the main shaft by rotating the main shaft by 45 degrees.

The acousto-optic device 6 has a phenomenon that the diffraction efficiency changes depending on the polarization state of the light beam incident on the acousto-optic device, because the photoelastic constant in the direction in which the ultrasonic wave propagates and the direction perpendicular thereto are different. . On the other hand, when a single mode fiber is used as the second optical fiber, the light propagating in the single mode fiber is disturbed in polarization even by a slight external force, and is connected to the transmitted light output optical fiber 2. The polarization state of the backscattered light from the measured fiber 11 changes according to the propagation distance of the optical fiber. Therefore,
When the backscattered light is incident on the acousto-optic switch, the diffraction efficiency of the acousto-optic element 6 has a polarization dependence as described above, so that there is a problem that the loss changes with time.

Therefore, in the acousto-optic switch disclosed in the above publication, the transmitted light output optical fiber 2 is composed of a constant polarization fiber, and a connecting portion 14 for rotating the main axis by 45 degrees is provided in the constant polarization fiber. Since the linearly polarized light is provided, the polarization of the linearly polarized light is disturbed by the group delay time difference between the two fundamental modes of the constant polarization fiber, and the light is almost non-polarized light and enters the acousto-optic element 6. Accordingly, the diffraction efficiency of the acousto-optic device 6 becomes constant, and the fluctuation of the waveform of the received light pulse due to the polarization characteristics of the measured fiber can be suppressed as much as possible.

The effect of converting linearly polarized light into unpolarized light depends on the number of connecting portions 14 of the constant polarization fiber and the number of connecting portions 1.
Depends on the spacing between the four.

[0006]

However, in the above-described conventional acousto-optical switch, since the connecting portion 14 for connecting the constant polarization fiber by rotating the main axis by 45 degrees has a distortion caused by the connection, the connecting portion 14 is distorted. There is a problem that the light does not completely enter the non-polarized state due to the 14 passages.
Therefore, even with the conventional acousto-optic switch, the fluctuation of the waveform of the received light pulse due to the polarization characteristics of the measured fiber cannot be sufficiently suppressed. According to the actual measurement by the present applicant, the waveform fluctuation due to the conventional acousto-optic switch was 0.1 dB at the minimum value.

SUMMARY OF THE INVENTION An object of the present invention is to provide an acousto-optic switch capable of eliminating waveform fluctuation of a received light pulse due to polarization characteristics of a measured fiber.

[0008]

According to the present invention, there are provided first and second optical fibers opposed to each other on the same optical axis, and different optical axes for the first and second optical fibers. A third optical fiber disposed and interposed between the respective optical fibers, and an optical path from the second optical fiber to the first optical fiber can be switched to an optical path of the third optical fiber by an acousto-optic effect. The present invention is applied to an acousto-optic switch including an acousto-optic element. Then, the second optical fiber is assumed to be connected in a state where the two constant polarization fibers have their main axes rotated by 45 degrees, and the second optical fiber is connected to the optical path of the second optical fiber and the acousto-optic element by 1 /. It is characterized in that a four-wave plate is interposed. In the present invention, it is preferable that two constant polarization fibers are fused to each other.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the same components as those in the above-described conventional example are denoted by the same reference numerals.

FIG. 1 is a block diagram showing the overall configuration of an acousto-optic switch according to an embodiment of the present invention. In FIG. 1, an incident optical fiber 1, a transmitted light optical fiber 2,
The light receiving optical fiber 3 serves as an optical path of an optical signal, and is fixed to the case 4 by a screw or an adhesive. An acousto-optic element 6 is arranged at the center of the case 4 that forms the outer shape of the acousto-optic switch of the present embodiment. The acousto-optic element 6 is driven by a drive circuit (not shown).

The lens 5 converts the light beam emitted from the incident optical fiber 1 into a parallel beam and guides the light beam to the acousto-optic device 6.
After adjusting the optical axis for each axis, it is fixed to the case 4. Similarly, the lens 8 converges the light beam from the acousto-optic element 6 and guides it to the transmitted light output optical fiber 2, and adjusts the transmitted light output optical fiber 2 to X, Y, and Z axes. After that, it is fixed to the case 4.
A 波長 wavelength plate 7 is interposed in the optical path between the acousto-optic element 6 and the lens 8.

The transmitted light output optical fiber 2 of the present embodiment comprises:
It is composed of two constant polarization fibers 9 and 10, and the constant polarization fibers 9 and 10 are rotated by 45 degrees with respect to each other's main axes at a connecting portion (fused portion) 14 and fused. .

The lens 13 guides the diffracted light from the acousto-optic element 6 to the light receiving fiber 3 via the prism 12, and the optical axis of the light receiving fiber 3 is adjusted to the X, Y, and Z axes. After that, it is fixed to the case 4.

In the above-described configuration, the fiber under test 11 is connected to the transmitted light output optical fiber 2, an optical pulse is incident from the incident optical fiber 1 side, and the measured fiber 11 is transmitted through the transmitted light output optical fiber 2. When an optical pulse is input to 11, Fresnel reflection from the fiber under test 11 (break point, connection point, etc.) propagates through the fiber under test 11 and is guided to the transmitted light output optical fiber 2.

Here, since the constant polarization fibers 9 and 10 are fused by being rotated by 45 degrees with respect to their main axes, a difference in group delay time between the two fundamental modes of the constant polarization fibers 9 and 10 is obtained. The polarization state of the light pulse returned to the transmitted light output optical fiber 2 is disturbed, and the light pulse is again incident on the acousto-optic element 6 in a state close to non-polarized light. Here, the state close to non-polarized light means that the state is not completely non-polarized because there is distortion caused by fusion of the two constant polarization fibers 9 and 10. However, the fusion splicing has a smaller fiber stress associated with the splicing than the other splicing methods, resulting in a state closer to non-polarization.

The light pulse re-entering the acousto-optic element 6 has a polarization plane of 45
The light is rotated by degrees and becomes completely unpolarized light, and is incident on the acousto-optic element 6. The acousto-optic element 6 diffracts the incident light and guides it to the light receiving fiber 3 via the prism 12 and the lens 13.

Therefore, since the light guided to the acousto-optic element 6 is in a completely non-polarized state, there is no fluctuation in the diffraction efficiency, and the waveform fluctuation due to the polarization characteristics of the fiber under test 11 can be eliminated.

[0018]

As is apparent from the above description, according to the present invention, the second optical fiber is formed by connecting two constant polarization fibers in a state where their main axes are rotated by 45 degrees. In addition, since the 1/4 wavelength plate is interposed in the optical path between the second optical fiber and the acousto-optic element, the light incident on the acousto-optic element can be completely unpolarized, and Waveform fluctuation of the received light pulse due to the polarization characteristic can be completely eliminated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an overall configuration of an acousto-optic switch according to an embodiment of the present invention.

FIG. 2 is a block diagram showing an example of a conventional acousto-optical switch.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Incident optical fiber 2 Transmitted light output optical fiber 3 Receiving fiber 6 Acousto-optic element 7 Quarter wave plate 9, 10 Constant polarization fiber 14 Connection part

Claims (2)

[Claims] 1. A first optical fiber and a second optical fiber which are arranged on the same optical axis to face each other, and a third optical fiber which is arranged so that the optical axes of the first and second optical fibers are different from each other. When,
An acousto-optic element interposed between the respective optical fibers, and capable of switching an optical path from the second optical fiber to the first optical fiber to an optical path of the third optical fiber by an acousto-optic effect; The second optical fiber is connected with two constant-polarization fibers in a state where their main axes are rotated by 45 degrees, and has a 1/4 wavelength in the optical path between the second optical fiber and the acousto-optic element. An acousto-optic switch comprising a plate interposed therebetween. 2. The acousto-optic switch according to claim 1, wherein said two constant polarization fibers are fused to each other.