JPH02251937A - Acoustooptic switch - Google Patents

Abstract

PURPOSE:To suppress sufficiently waveform fluctuations due to the polarization dependency of the diffraction efficiency of an acoustooptic element by providing a Faraday rotator whose polarizing direction can be rotated by + or -45 deg.. CONSTITUTION:The Faraday rotator 60 which has such a thickness that the polarizing direction is rotated by 45 deg. in a saturated magnetic field is fixed on one end surface of a converging rod lens 41. Further, the Faraday rotator 60 is applied with the saturated magnetic field by feeding electricity to coil 70 earlier than the light pulse emission signal (sent light pulse) of a light pulse tester. The waveform distortion of back scattered light, guided to the convergence type fiber 30, due to the polarization dependency of the diffraction efficiency of the acoustooptic element 50 is inverted when an observation is made with a 1st light pulse and a 2nd light pulse. For the purpose, the light signal which is guided to a convergence type fiber 30 is averaged by performing measurement plural times to eliminate the waveform distortion due to the polarization dependency of the acoustooptic element 50.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an acousto-optic switch used in an optical pulse tester or the like.

[Conventional technology]

In general, various measuring instruments are used to maintain the transmission quality of optical fiber communications, and these measuring instruments particularly measure the break point of the optical cable.

Optical pulse testers are important for detecting fault points, locating discontinuities, and measuring optical propagation loss, connection point conditions, etc.

Generally, in an optical pulse tester, an optical pulse from a semiconductor laser driven by a pulse generator is input into an optical fiber to be measured via an optical directional coupler, and backscattered light or backscattered light generated within the optical fiber is The Fresnel reflected light generated at the end face of the optical fiber enters the detector again via the optical directional coupler and is converted into an electric pulse. However.

In the case of such an optical pulse tester, especially when the fiber under test is single mode, the level of backscattered light is small (,
As a result, the difference from the Fresnel reflected light level is large.
Dynamic range decreases. As a result, the unobservable region expands and the resolution deteriorates.

Furthermore, in order to separate the light incident on the optical fiber from the light source and the backscattered light, an optical directional coupler with low loss and low crosstalk is required.

Therefore, instead of the optical directional coupler, an optical pulse tester using a so-called acousto-optic switch has been proposed, and by using the acousto-optic switch, it is possible to completely mask the 7-Renel reflection.

As a result, the unobservable area can be improved.

By the way, the one shown in FIG. 5 is conventionally known as this type of acousto-optic switch. As shown in Figure 6, the conventional acousto-optic switch is PbMoO4 or TeO2.
The first and second 71' ribs 10 and 20 facing each other with the acousto-optic element 50 sandwiched between the first and second
The incident light from the second fiber 20 is diffracted by the parallel beam system using the lenses 41 and 42, and the acousto-optic element 50 is fully operated. (For example, Kanayama et al., Nationa
l Technical Report.

VOl, 29. NO, 6,100 pages, 198
(December 3rd year).

[Problem that the invention seeks to solve]

However, in the case of conventional acousto-optic switches.

Since the acousto-optic element has different photoelastic constants in the direction in which the ultrasound propagates and in the direction perpendicular thereto, the diffraction efficiency changes depending on the polarization state of the light beam incident on the acousto-optic element.

On the other hand, the polarization of light propagating in a single mode fiber is disturbed by slight external forces and structural imperfections within the core and at the boundary between the core and cladding, so backscattered light also varies depending on the propagation distance of the optical fiber. The polarization state will change.

Therefore, when backscattered light enters an acousto-optic switch, there is a problem in that the loss changes over time due to the polarization dependence of the diffraction efficiency of the acousto-optic element. Tsumashi, acousto-optic switch.

A pulse tester using a chip has the problem that waveform fluctuations occur due to the polarization characteristics of backscattered light, making it difficult to measure minute changes in loss distribution and splice loss.

An object of the present invention is to provide an acousto-optic switch whose loss does not change over time.

[Means for solving problems]

The acousto-optic switch of the present invention includes first and second single mode fibers arranged opposite to each other, and first and second lenses for optically coupling the first and second fibers. and an acousto-optic element provided on the optical axis between the first and second lenses, and between the first lens and the acousto-optic element on the optical axis between the first and second lenses. 7 provided
A Faraday rotator, a coil for applying a magnetic field to the Faraday rotator, a fifth lens and a third fiber that receive a light beam incident from a second fiber and deflected within the acousto-optic element. Characterized by having money.

〔Example〕

Two examples of the present invention will be explained below.

Referring to FIG. 1, the first single mode fiber 10 and the second single mode fiber 20 have a convergent rod lens 41 as a first lens and a convergent rod lens 42 as a second lens. are efficiently coupled optically via the As is on the optical axis between the focusing rod lenses 41142.
An acousto-optic element 50 using 2Ae5 is provided.

By applying a high frequency current to the electrode 50a from the electric circuit 100 through the matching circuit 90, the acousto-optic element 5
Ultrasonic waves are applied to 0. On one end surface (right end surface) of the focusing rod lens 41.

A Faraday rotator 60 made of Bi-substituted iron garnet having a thickness such that the direction of polarization rotates by 45 degrees in a saturated magnetic field is fixed, and a coil 70 is provided as a core of the focusing rod lens 41t. There is.

Emitted from the second single mode fiber 20.

The optical path of the light diffracted by the acousto-optic element 50 is changed by a prism 80, and the converging rod lens 4, which is a sixth lens,
3 and enters the focusing fiber 30, which is the sixth fiber.

Referring also to FIG. 2, the conditions for energizing the coil 70 are set as follows. First, the coil 70 is energized to apply a saturation magnetic field to the Faraday rotator 60.

The first optical pulse (first optical pulse) of the optical pulse tester is rotated by 45° in the polarization direction and is transferred to the single mode fiber 20.
The signal is sent to a fiber under test (not shown) connected to a fiber under test (not shown). The energization of the coil is reversed after the first light pulse is sent out and before the second light pulse (second light pulse) is emitted. Therefore, the second light pulse will now be sent out with its polarization direction rotated by 145°. Here, the polarization direction of the backscattered light returning from the fiber under test changes depending on the polarization direction of the transmitted optical pulse, and since the polarization directions of the first optical pulse and the second optical pulse differ by 90 degrees,
The polarization direction of the backscattered light caused by the second light pulse and the polarization direction of the backscattered light caused by the first light pulse also differ by 90°. Therefore, the waveform distortion of the backscattered light guided to the Kento type fiber 60 due to the polarization dependence of the diffraction efficiency of the acousto-optic element 50 is different when observed with the first optical pulse and when observed with the second optical pulse. So we're going to do two inversions.

Therefore.

By performing measurements multiple times at the above timing and averaging the optical signals guided to the focusing fiber 30, the waveform distortion caused by the polarization dependence of the acousto-optic element 50 is eliminated, and the time of the pure backscattered light is Only changes will be observed. Furthermore, in general, a Faraday rotator that has not reached a magnetic saturation state becomes a source of light scattering, but when using a Faraday rotator under the above conditions, the Faraday rotator becomes magnetically Since light does not pass through the saturator, fluctuations in lens system coupling efficiency due to light scattering do not affect the observation of backscattered light.

When the acousto-optic switch of this example was used in an optical pulse tester to observe waveform fluctuations due to the polarization characteristics of the fiber under test, it was approximately 0.02 dB after 24 averagings.
It was possible to reduce it to below, which is several tens of minutes lower than the conventional value.

〔Effect of the invention〕

The acousto-optic switch according to the present invention has been described above in detail based on the embodiments.2 The acousto-optic switch according to the present invention is equipped with a Faraday rotator that can rotate the polarization direction by ±45 degrees. In the optical pulse tester using the acousto-optic switch according to the present invention, the polarization direction of the transmitted light pulse is changed by 90 degrees, and by averaging the time changes of the backscattered light multiple times, the acousto-optic element is This has the effect of sufficiently suppressing waveform fluctuations caused by the polarization dependence of diffraction efficiency.

[Brief explanation of drawings]

FIG. 1 is a plan view schematically showing an embodiment of the acousto-optic switch of the present invention, FIG. 2 is a timing chart for explaining all the energizing conditions of the coil used in the acousto-optic switch of the present invention, and FIG. FIG. 2 is a plan view schematically showing a conventional acousto-optic switch. 10.20... Single mode fiber, 30... Focusing multimode fiber, 41, 42.45... Focusing rod lens, 50... Acousto-optic element, 60... Faraday rotator, 70... Coil, 80... Prism. 90... Matching circuit, 100... Electric circuit. Figure 3

Claims (1)

[Claims] 1. first and second single mode fibers arranged opposite to each other, first and second lenses that optically couple the first and second single mode fibers; and an acousto-optic element and a Faraday rotator provided on the optical axis between the second lenses; an applying means for applying a magnetic field to the Faraday rotator; The third part receives the optical beam deflected within the element.
and a third fiber, and by driving the acousto-optic element, the optical path from the second single mode fiber to the first single mode fiber is changed from the second single mode fiber to the third fiber. An acousto-optic switch characterized in that the optical path is switched to the optical fiber of No. 3.