JPH0246419A - Optical isolator - Google Patents

Abstract

PURPOSE:To align the optical axes in incident and exit positions without having dependency on polarized light by constituting an optical system of 3 pieces of double refractive crystals and 2 pieces of Faraday rotors and specifying the thickness of the crystals and the inclination of the crystal axes. CONSTITUTION:The optical isolator is constituted of the three double refractive crystals 1-3 and the two Faraday rotors 5, 6. The thickness of the respective crystals 2, 3 is specified to 1:2<1/2>:1 when the thickness in the optical axis direction of the 1st crystal 1 is 1. The 2nd crystal 2 is disposed by rotating the 1st crystal 1 180 deg. around the (x) axis to put the optical axis of the 2nd crystal into specular symmetry, then rotating the same 45 deg. in the incident ray direction with respect to the optical axis of the 1st crystal 1. The disposition of the 3rd crystal 3 is attained by rotating the optical axis thereof 45 deg. around the (x) axis of the 2nd crystal 2 and rotating this optical axis 90 deg. with the optical axis of the 1st crystal 1.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides polarization-independent light that is not affected by the polarization direction and is used to prevent reflected light from an optical system in optical fiber communication using a semiconductor laser. Regarding isolators.

[Prior Art] With the development of optical communication, optical measurement, etc. centered on semiconductor lasers, laser oscillations may malfunction due to reflected return light from optical systems such as coupling lenses, optical connectors, and other optical components, causing high-speed, high-speed The problem of destabilizing density signal transmission has arisen, and various optical isolators have been proposed to block the reflected return light.

These optical isolators include polarizers, Faraday rotators,
It consists of a permanent magnet to magnetize the analyzer and Faraday rotator, and it is generally effective only for a certain plane of polarization, so if light that does not match the polarization direction of the optical isolator is incident, the transmitted light will be significantly lost. was there. A system in which a tabular birefringent crystal or a single optically active crystal is combined has been proposed as a structure that exhibits an isolation effect for all polarization planes without depending on the polarization direction.

For example, the system shown in Fig. 2 has a structure using a tabular birefringent crystal (see Japanese Patent Publication No. 60-51690), and the system shown in Fig. 3 has no polarization dependence (special (See Publication No. 58-28561). In the latter case, the birefringent crystals of 1.1° have the same thickness and have a symmetrical structure rotated by 180° around the X axis, with 1° being 1°, and a Faraday rotator 5. An optical rotator 7 is arranged to rotate the plane of polarization. Crystal or tellurium dioxide (
■e02) etc. are used. Figure 3 (a), (b
) indicate the propagation state of light in the forward and reverse directions, respectively; in the forward direction, the light can propagate again on an extension of the incident light ray at the exit point. In the opposite direction, the return light from the incident beam axis at the final incident point position has a certain displacement distance, ie is separated.

[Problem to be Solved by the Invention 1] However, in the method shown in Fig. 2, the position of the emitted light is not on the extension line of the incident light ray but moves in parallel, the incident polarization plane is rotated by 45 degrees on the output side, and Faraday rotation Unlike the previous method, the method shown in FIG. There is an advantage that the emitted light beam is combined on the extension of the input light beam, but in addition to the birefringent material, optically active crystals must also be processed and assembled, which adds a complicated process. Of these, crystal is the most representative, but 45
°To rotate the plane of polarization, the optical rotation power is approximately 4° with 1.3 sails.
/, and at 45°, approximately 11.25M is required, resulting in a long optical path length, and a ball lens, gradient index type G
It is difficult to connect other optical systems with RIN lenses, etc.
The coupling loss increases, making it impractical. On the other hand, the e02 single crystal has an optical rotation power of 13 degrees, which is 5 times more powerful, and it rotates at 45 degrees, so it is about 2 cocoons, which is practical, but it causes problems such as the complexity of processing and assembly and the cost. Difficulties in production were expected.

[Means for Solving the Problems] The present invention provides a high-performance optical isolator that does not have polarization dependence and emits light on an extension of the incident light beam using only a birefringent crystal and a Faraday rotator, which could not be obtained with the conventional method described above. It is something to do.

Considering the supply power of IC materials, price, and performance, it is preferable to use calcite as the tabular birefringent crystal because its open plane can be used as a single plate. Of course, the present invention also encompasses the use of other birefringent crystal materials 9 such as rutile.

FIG. 1 shows a basic configuration diagram of the present invention. In the drawings, numerals 1 to 3 indicate tabular birefringent crystals, and numerals 5 and 6 are Faraday rotators magnetized in the same direction by permanent magnets. If the thickness of the first tabular birefringent crystal 1 in the direction of the light beam is 1, then the thickness of each birefringent crystal becomes a ratio of 1.fi.1. In addition, the optical axis of each crystal is tilted (approximately 45 degrees with respect to the surface) of the first tabular birefringent crystal, whereas the optical axis of the second tabular birefringent crystal 2 is tilted to that of the first tabular birefringent crystal. The layered crystal 1 is rotated 180 degrees around the X axis to arrange it in a crystal-structural mirror symmetry, and then rotated 45 degrees around the direction of the incident light beam (Z axis) to form a third flat plate. The optical axis of the birefringent crystal 3 is rotated by 45 degrees with respect to the second plate-shaped birefringent crystal 2 about the Z axis, and rotated by 90 degrees with respect to the optical axis of the first plate-shaped birefringent crystal 1. The present invention is achieved by the arrangement.

[Example] FIG. 1(a) shows a trace of the propagation state of an incident light beam in the forward direction. The separated polarized lights are recombined at the exit surface position, and the polarized light plane is in the same polarized state as when it was incident. This property holds true for all polarized light. On the other hand, in the opposite direction, as shown in FIG. 1(b), the two polarized light components are separated from each other and are displaced by a certain distance from the center line, so that they are not combined into the ray path on the incident side.

In other words, the light entering from the incident side is not coupled onto the center line even on the output side. From this, the separation distance d is expressed by the following formula. is the thickness of the first tabular birefringent crystal, no, n
e is the refractive index of ordinary light and extraordinary light in the used wavelength range, respectively, and θ is the angle between the optical axis of the crystal and the plane of the flat plate.

The value of d depends on the optical isolator coupling method of this configuration. For example, when coupling a semiconductor laser with a ball lens, the displacement from the center axis of the ball lens and semiconductor laser is ±
It has been reported that when the axis misalignment exceeds 80, the coupling loss reaches as much as -40 dB (IEICE paper C-84, 54-350), and it is sufficient to obtain a d value of ±100 or more. An isolation effect can be obtained. In this case, the Faraday rotator is liquid phase epitaxial (LPE method).
If we use the material deposited on a garnet substrate by using the material deposited on the garnet substrate, and if we use a single plate using the open plane of calcite as the birefringent crystal, d = 0.1 cylinder, θ-44, 6° no = 1.6
58. J-0,65 as ne = 1.486
Therefore, the thickness of the second and third decks is 0.92 mm,
It becomes 0.65#III+, and even if the LPE film is 1# (substrate thickness -0.5111111, film thickness -400), it can be achieved if the total thickness is about 5M.

[Effects of the Invention] As described above, the optical isolator based on the present invention has the advantage that it does not depend on the plane of polarization and the positions of the incident light beam and the output light beam completely match, and when inserted into an optical circuit, it has the advantage that The original performance can be easily obtained without performing precise optical axis adjustment.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle structure of a polarization-independent optical isolator according to the present invention, in which (a) shows the state of light propagation in the forward direction and (b) shows the state of light propagation in the reverse direction. FIGS. 2 and 3 show the principle structure of a conventional polarization-independent optical isolator. 1:2:3: Tabular birefringent crystal 5:6: Faraday rotator 7: Optical rotator Patent applicant Namiki Precision Jewel Co., Ltd.

Claims (1)

[Claims] A first tabular birefringent crystal whose optical axis is tilted with respect to the surface;
a first Faraday rotator for rotating the plane of polarization by 45°, having a thickness √2 times that of the first tabular birefringent crystal;
Further, a second plate-shaped birefringent crystal is arranged after being rotated by 180 degrees around the X axis and then rotated by 45 degrees around the direction of the incident light beam; The Faraday rotator has the same thickness as the first tabular birefringent crystal, and is rotated by 45 degrees with respect to the second tabular birefringent crystal about the direction of the incident light beam as the rotation axis. An optical isolator comprising a third flat birefringent crystal arranged 90 degrees rotated with respect to the optical axis of the crystal, and a permanent magnet for magnetizing a Faraday rotator.