JPS63115144A - Acoustooptic switch - Google Patents

Abstract

PURPOSE:To reduce polarization dependency by using a polarization plane maintaining fiber as the 1st optical fiber and providing a 1/4-wavelength plate. CONSTITUTION:The 1/4-wavelength plate 90 is provided on the optical axis between an acoustooptic element 50 using As2Se3 and a convergent rod lens 42 so that a linear polarized light beam emitted from the polarization plane maintaining fiber becomes a circular polarized light beam. A single mode fiber 20, therefore, emits circular polarized light. When this acoustooptic switch is used for a pulse tester, back scattered light from a fiber to be measured also becomes nearly circular polarized light, which passes through the 1/4-wavelength plate 90 again and is incident on the element 50 as a linear polarized light beam, so waveform fluctuations due to polarization characteristics are reducible. Thus, the circular polarized light passes through the acoustooptic element, so linear polarized beams having all angles of polarization can be present on an average, so the polarization dependency can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an acousto-optic switch suitable for an optical arm tester, etc., and particularly to an acousto-optic switch for single mode fiber.

[Conventional technology]

The progress in optical fiber communication technology has been remarkable.

Many fiber optic communication systems are in use.

Various measuring instruments are used to maintain the transmission quality of these optical fiber communications.

In particular, an optical pulse tester is an important measuring instrument for locating failure points such as break points and discontinuity points in optical cables, and for measuring optical loss per unit length, connection point status, etc.

This type of optical pulse tester makes optical pulses emitted from a semiconductor laser driven by an arm pulse generator enter the optical fiber to be measured via an optical directional coupler, and measures the generated light within the optical fiber. The backscattered light or the Fresnel reflected light generated at the end face of the optical fiber is incident on the detector via the optical directional coupler again.

A method was used to convert it into an electric no. but.

In this method, especially when the fiber under test is a single mode fiber, the level of backscattered light is small and the difference with the Fresnel reflection level becomes large, the dynamic range decreases, the unobservable area expands, and the resolution deteriorates. It had the following drawback. Another problem is that an optical directional coupler with low loss and low crosstalk is required in order to separate the light incident on the optical fiber from the light source and the backscattered light.

In order to solve the above problems, a method using an acousto-optic switch instead of an optical directional coupler has been developed. In this case, since the Schiffresnel reflection can be masked by the optical switch, the unobserved area can be improved.

Conventionally, as shown in FIG. 2, this type of acousto-optic switch includes a single mode fiber 10 that is first and second optical fibers facing each other with an acousto-optic element 50 such as PbMoO4 or Teo2 in between. .20, respectively nth
The incident light from the second single mode fiber 20 is diffracted by being coupled by a parallel beam system by the second lens 41 , 42 and operating the acousto-optic element 50 , and is then transmitted through the third lens 43 . The optical path was switched to a third optical fiber, a focusing multimode fiber 30 (Kanayama et al., National Techn.
ical, Report.

Vol, 29. A6. PIOo, December 1983).

[Problem that the invention seeks to solve]

However, in the pulse tester using the acousto-optic switch described above, the acousto-optic element used in the acousto-optic switch has different photoelastic constants in the direction in which the ultrasound propagates and in the direction perpendicular to it. The diffraction efficiency changes depending on the polarization state of the light beam incident on the beam. On the other hand, the polarization of light propagating in a single mode fiber is disturbed by a slight external force or structural imperfections within the core or at the boundary between the core and cladding, so backscattered light also changes over the propagation distance of the optical fiber. The polarization state will change accordingly.

Therefore, when the backscattered light enters the acousto-optic switch,
Since the diffraction efficiency of the acousto-optic element is polarization dependent, it has the disadvantage that the loss changes over time. In other words, in a pulse tester using an acousto-optic switch,
Waveform fluctuations occur due to the polarization characteristics of backscattered light,
The problem was that it was difficult to measure minute changes in loss distribution and continuous loss.

In view of the above drawbacks, one object of the present invention is to provide an acousto-optic switch in which the polarization dependence of twice-diffracted light is small.

[Means for solving problems]

According to the present invention, a polarization-maintaining fiber as a first optical fiber and a single-mode fiber as a second optical fiber are arranged opposite to each other, and the polarization-maintaining fiber and the single-mode fiber are optically connected to each other. the first and second
an acousto-optic element provided on the optical axis between the first and second lenses, and a third lens that receives a light beam incident from the single mode fiber and deflected within the acousto-optic element. a lens, a third optical fiber, and a 1/2-inch optical fiber provided on one of the optical axes between the first lens and the acousto-optic element and between the acousto-optic element and the second lens. An acousto-optic switch characterized by having a four-wavelength plate is obtained. That is, by making the first optical fiber a polarization-maintaining fiber and providing a quarter-wave plate, the light beam that enters the acousto-optic element from the second optical fiber, which is a single mode fiber, is The light beam becomes approximately circularly polarized, and the diffraction efficiency of the acousto-optic element no longer has dependence on incident polarization and becomes approximately constant.

Therefore, when the acousto-optic switch according to the present invention is used in a pulse tester, the backscattered light from the fiber under test also becomes circularly polarized light, so waveform distortion due to the polarization characteristics of the fiber under test will hardly be observed. .

〔Example〕

The present invention will be explained in detail below using two drawings.

FIG. 1 is a sectional view showing an embodiment of an acousto-optic switch according to the present invention. The polarization maintaining fiber 11 which is the first optical fiber and the single mode fiber 20 which is the second optical fiber are connected to the converging rod lens 41 which is the first lens.
and the converging rod lens 42, which is the second lens, are optically efficiently coupled. An acousto-optic element 50 using As2Se3 is provided on the optical axis between the focusing rod lenses 41 and 42. Furthermore, the light incident from the single mode fiber 20 and diffracted by the acousto-optic element 50 is
f The optical path is changed by the IJ beam 70, and the beam enters the focusing multimode fiber 30 having a core diameter of 50 μm, which is the third optical fiber, through the focusing rod lens 43, which is the third lens. Further, on the optical axis between the acousto-optic element 50 and the focusing rod lens 42, a quarter-wave plate is arranged so that the linearly polarized light beam emitted from the polarization maintaining fiber 11 becomes a circularly polarized light beam. 90 are provided. Therefore.

Circularly polarized light is emitted from the single mode fiber 20. When this acousto-optic switch is used in a Norms tester, the backscattered light from the fiber under test also becomes substantially circularly polarized light, passes through the quarter-wave plate 90 again, and is transmitted to the acousto-optic element 50 as a linearly polarized light beam. Since the light is incident, waveform fluctuations caused by polarization characteristics can be reduced. When we actually observed the waveform fluctuation due to the polarization characteristics of the fiber under test, we were able to reduce it to about 0.02 dB or less, which is several tenths of the conventional level.

The 174 wavelength plate 90 may be provided between the focusing rod lens 41 and the acousto-optic element 50, and the light beam passing through the acousto-optic element 50 becomes circularly polarized light instead of linearly polarized light. Furthermore, the quarter wavelength plate 90 may be fixed to one end surface of the focusing rod lens 41 or 42.

〔Effect of the invention〕

Above, the acousto-optic switch according to the present invention has been described in detail based on the embodiments. Circularly polarized light passes through the acousto-optic element, and linearly polarized light having all polarization angles can be present almost on average, which has the effect of reducing polarization dependence. Therefore, in the Norms tester using the acousto-optic switch according to the present invention, the polarization of the backscattered light from the fiber under test is averaged, and waveform fluctuations caused by the polarization dependence of the backscattered light are sufficiently suppressed. There is an effect that can be done.

[Brief explanation of the drawing]

FIG. 1 is a plan view schematically showing an embodiment of an acousto-optic switch of the present invention, and FIG. 2 is a plan view schematically showing a conventional acousto-optic switch. 10.20...Single mode fiber, 11...Polarization maintaining fiber, 30...Focusing multimode fiber. 41.42.43...Focusing rod lens, 50...
・Acousto-optic element, 60...Matching circuit, 70...
・Prism, 80...electrical input terminal, 90...1/
4 wavelength plate. Figure 1 n

Claims (1)

[Claims] 1. First and second optical fibers arranged to face each other, first and second lenses that optically couple the first and second optical fibers, and the first and second lenses. an acousto-optic element provided on the optical axis between the two, and a third lens and a third optical fiber that receive a light beam incident from the second optical fiber and deflected within the acousto-optic element, By driving the acousto-optic element, the second
The optical path from the optical fiber to the first optical fiber is connected to the second optical fiber.
In the acousto-optic switch for switching an optical path from an optical fiber to the third optical fiber, the first optical fiber is a polarization maintaining fiber, the second optical fiber is a single mode fiber, and the first optical fiber is a polarization maintaining fiber, and the second optical fiber is a single mode fiber. 1. An acousto-optic switch characterized in that a quarter-wave plate is provided between one of the first lenses and the acousto-optic element and between the acousto-optic element and the second lens.