JPH01281415A - Optical isolator - Google Patents

Abstract

PURPOSE:To simplify the constitution and to reduce the propagation loss of light by connecting fibers maintaining the plane of polarization to both sides of a Farady rotator with lenses between them. CONSTITUTION:Fibers 2a and 2b maintaining the plane of polarization are connected to both sides of a Farady rotator FR with lenses 1a and 1b between them, and the plane of polarization in one direction of incident light can be propagated, and thereby, this device functions as a polarizer. When fibers 2a and 2b maintaining the plane of polarization are connected to lenses, the fiber 2a maintaining the plane of polarization on the incidence side is set to the plane of vertical polarization, and the angle of rotational emission of the Farady rotator FR is set to 45 deg. to the plane of vertical polarization, and that of the fiber 2b maintaining the plane of polarization on the exit side is set to 45 deg. to the plane of vertical polarization. Thus, the device is miniaturized and the propagation or connection loss of light is reduced.

Description

[Detailed Description of the Invention] "Industrial Application Field" (The invention relates to an optical isolator used in optical communication,
In particular, it facilitates connection to optical fibers.

"Prior art" For example, when trying to propagate light generated by a semiconductor laser as a light source through an optical fiber, it is difficult to propagate the light generated by a semiconductor laser as a light source. Reflected light may be generated and propagated back down the optical path to be coupled back into the light source semiconductor laser. In such a state, the oscillation of the semiconductor laser becomes unstable and the noise of the generated light increases, which is undesirable. Therefore, in such a case, an optical isolator is used to suppress the reflected light.

As shown in FIG. 4, the conventional isolator has polarizers P1 . This is a configuration in which P2 is arranged. In the same figure, the light incident from point a is converted into parallel light by lens B, and then polarizer P of the optical isolator
1. The polarizer Pi selectively passes only polarized light in a certain direction, for example, vertically polarized light, from the incident light. The light emitted from the polarizer P1 is YIG (Y20x, Fe203
The polarization direction is 45.
Normally, the Faraday circuit element FR is placed at the center of a cylindrical magnet M magnetized in the direction of the rotation axis as shown in the figure, and is magnetized in a direction substantially parallel to the optical path. The emitted light from the Faraday rotation element FR enters the analyzer P2, but the polarization direction of the analyzer P2 is 45° from the vertical direction.
It's leaning. Therefore, the light incident from the Faraday rotation element FR passes through the analyzer P2 and exits, passes through the lens 7, and produces an output focused on point b. Therefore, for example, if the end of the optical fiber is placed at point b, the light incident from point a can be coupled to the optical fiber.

On the other hand, as mentioned above, the reflected light generated in the optical fiber etc. enters the polarizer P2 (which acted as an analyzer last time) from point b through the lens 7, and the component light that matches the polarization direction of the polarizer P2 is polarized. The light passes through the child P2 and enters the Faraday rotation element FR. As is well known, in the Faraday rotation element FR, the rotation direction of the plane of polarization changes depending on the relationship between the incident direction of light and the magnetization direction of the Faraday rotation element material. In this case, the coordinate system of the arrangement is rotated by 45° in the same direction as the incident light, so the polarization direction of the output light of the Faraday rotation element FR is the propagation direction of the analyzer Pi (which acted as a polarizer in the previous case). perpendicular to. Therefore, the incident light from the Faraday rotary element FR is blocked by the analyzer PI and is not propagated to the side of a. Therefore, the reversely incident light coupled to the semiconductor laser placed at point a is blocked, and the S /N deterioration is prevented.

The optical isolator functions as described above, but when connecting it to an optical fiber, the light is taken out from the optical fiber and introduced into the optical isolator through a lens.

``Problems to be Solved by the Invention'' Conventionally, when connecting an optical isolator to an optical fiber, the connection is made after light has been extracted. Therefore, it is necessary to make fine adjustments so that the plane of polarization of the light incident on the polarizer of the optical isolator matches the plane of polarization of the polarizer, and it is necessary to provide a device for this purpose. Furthermore, since the light is made incident ξ on the Faraday rotation element via the polarizer of the optical isolator, it is not possible to reduce the propagation of light or the connection loss upward.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to simplify the configuration for connecting an optical isolator to an optical fiber, eliminate the need for fine adjustment of the plane of polarization, and further reduce the propagation loss of light.

"Means for Solving the Problems" The optical isolator of the present invention has polarization-maintaining fibers or fiber-type polarizers connected to both sides of a Faraday rotation element with a lens interposed on at least one of them. Note that the polarization-maintaining fiber or fiber-type polarizer is connected to the Faraday rotation element so as to have a predetermined polarization plane.

Polarization-maintaining fiber is an optical fiber that propagates while preserving the polarization plane of the fundamental mode of the polarization optical fiber by making the core elliptical or applying stress to the circular core, and can function as a polarizer. Note that a fiber-type polarizer is made by winding an optical fiber into a coil so that only one plane of polarization of incident light can propagate. In other words, a general optical fiber has two polarization plane modes, and even if one of them is selected and light is input, if the two waves travel at the same speed, the bending of the fiber, the pressure,
Depending on the refractive index, the two modes become larger together,
Mode conversion occurs and the plane of polarization changes. On the other hand, in a polarization-maintaining fiber or a fiber-type polarizer, the phase constants of the two modes are increased, so the two modes cancel each other out and combine, and the polarization plane does not change.

r Effect J In the optical isolator of the above means, the polarization-maintaining fiber or fiber-type polarizer on the input side allows only one polarization plane of the incident light to enter the Faraday rotation element. The plane of polarization is rotated by 45° by the Faraday rotator, and light with the same plane of polarization is output via a polarization-maintaining fiber or fiber-type polarizer on the output side.

On the other hand, the reflected light passing through the polarization-maintaining fiber or fiber-type polarized light T on the output side has its polarization plane rotated by 45° in the same direction as the previous time by the Faraday rotation element, and the polarization of the incident light through the polarization-maintaining fiber on the input side is Since the wavefront is rotated by 90 degrees, reflected light is not propagated to the light source side.

In addition, polarization maintaining fiber or fiber type polarized light f is
Since the fixed J-island is connected to the Faraday rotation element, all 3 ITJs of the polarization plane at the connection between the polarizer and the optical fiber are not required as in the case of conventional optical isolators.
Furthermore, propagation or connection loss is also reduced.

"Example" The optical isolator of the present invention will be explained with reference to FIG. 1.2.

Polarization maintaining fibers 2a and 2b are connected to both sides of the Faraday rotation element FR, which is composed of small crystals such as Y I G (an oxide whose main components are Y203 and Fe20), through lenses la and 1b, respectively. , furthermore, optical connectors 3a and 3b are connected to both ends of each,
It can be connected to optical fiber for optical communications.

The polarization-maintaining fibers 2a and 2b are capable of propagating only one polarization plane of the incident light, so that they can function as polarizers. In addition, the polarization maintaining fiber 2a,
When connecting + to the lens in 2, set its polarization plane to a predetermined angle.For example, set the polarization maintaining fiber 2d on the input side to the vertical polarization plane, and 7.
The rotational output angle of the polarization-preserving fiber 2b on the output side is set to be inclined at 45° with respect to the vertical polarization plane.

Note that instead of the polarization-maintaining fibers 2a and 2b, a fiber-type polarizer made by winding an optical fiber around a bobbin for about 2 m may be used.Also, although the lenses are placed on both sides of the Faraday rotation element, Since it is for converting into parallel light rays, it is not necessary to provide a lens on one side if the incident light is close to the f row.

Furthermore, in the above example, a single-stroke optical isolator was described in which a polarizer and an analyzer were provided on both sides of one rotating element.
It is also applicable to a two-stage optical isolator (which the present inventors proposed earlier and has already filed an application for), in which two identical configurations are connected. The system diagram in this case is as shown in Figure 3, where an analyzer 4 and a polarizer 5 are provided between two Faraday rotators FRI and FR2, and the polarization plane is on the outside of the two Faraday rotators FRI and FR2. They are connected to storage fibers or fiber-type polarizers 2a and 2b, respectively.

"Effects of the Invention" The uninvented optical isolator does not require a polarizer and an analyzer on both sides of the Faraday rotation element, but instead uses a polarization-maintaining fiber or a fiber-type polarizer to provide that function. It is possible to reduce the size of the device, and to reduce light propagation or connection loss. Further, the optical isolator can be connected to an optical fiber for optical communication via a connector, and there is no need for fine adjustment to match the plane of polarization of the polarizer to that of the optical fiber, as in the conventional art.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of an optical isolator of the present invention, Fig. 2 is a schematic diagram of an optical isolator, Fig. 3 is a system diagram of an optical isolator of another embodiment, and Fig. 4 is a system diagram of a conventional optical isolator. . la, lb; Lens FR; Faraday rotation elements 2a, 2b, polarization maintaining fiber Fig. 1

Claims (1)

[Claims] An optical isolator having a polarization function and an analysis function on both sides of a Faraday rotation element, characterized in that the polarization function and the analysis function are provided by a polarization preserving fiber or a fiber type polarizer.