JP2507601Y2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is a polarization-independent optical isolator that is not influenced by the polarization direction for blocking reflected return light of an optical system in optical fiber communication using a semiconductor laser. Regarding

[Prior Art] With the development of optical communication centered on semiconductor lasers, optical measurement, etc., optical systems such as coupling lenses,
Various optical isolators have been proposed that block reflected return light by causing a problem that laser oscillation malfunctions due to reflected return light returning from an optical connector and other optical parts to destabilize high-speed, high-density signal transmission.

These optical isolators are composed of a polarizer, a Faraday rotator, an analyzer, and a permanent magnet for magnetizing the Faraday rotator. When it is incident, there is a drawback that transmitted light is significantly lost. As a configuration showing an isolation effect for all polarization planes independent of the polarization direction, a system in which a flat refraction crystal or a single optical rotation plate is combined has been proposed. For example, the system shown in FIG. 3 is a structure using a flat birefringent crystal (Japanese Patent Publication No. 60-
No. 51690), and the system shown in FIG. 4 has no polarization dependence (see Japanese Patent Publication No. 58-28561). In the latter case, the birefringent crystal of 1,1 'has the same thickness and 1'is a symmetrical structure in which 1'is rotated about the X axis by 180 °, and the Faraday rotator 5 and the optical rotator 7 are arranged between them. To rotate the plane of polarization. Quartz and tellurium dioxide (TeO ₂ ) are used as optical rotators. FIGS. 4 (a) and 4 (b) show the forward and backward light propagation states, respectively. In the forward direction, the light can be propagated again on the extension of the incident ray at the exit point. In the opposite direction, the return light has a displacement distance from the axis of the incident ray at the final incident point position, that is, is separated. However, in the system shown in FIG. 3, the position of the outgoing light is not parallel to the extension line of the incoming light, the incident deflecting surface is rotated by 45 ° on the outgoing side, and the incident light axis changes due to the temperature change of the Faraday rotator 5. There are inherent drawbacks such as the possibility that a light component returning to the upper side will occur and deterioration of the extinction characteristic is likely to occur. Moreover, in the method shown in Fig. 4, unlike the previous method, the outgoing light beam is coupled on the extension of the incident line. However, in addition to the birefringent material, an optically active crystal must be processed and assembled, which adds a complicated process. Quartz is a typical optical rotatory substance, but in order to rotate the plane of polarization at 45 °, the optical activity is about 4 ° / mm in the 1.3 μm band, and about 45 ° requires about 11.25 mm. It has a long optical path length, making it difficult to combine other optical systems such as spherical lenses and gradient index GRIN lenses, which causes large coupling loss and is not practical. On the other hand, TeO ₂ single crystal has an optical rotation of 1.3 mm.
The band is about 5 times larger, and it is practically about 2 mm because it rotates at 45 °, but it was expected to have difficulties in production such as complexity of processing and assembly and cost problems.

[Problems to be Solved by the Invention] Therefore, as disclosed in Japanese Patent Application Nos. 63-196340 and 1-469998, the present inventor has proposed three birefringent crystals and two Faraday rotations. The former has a structure in which the magnetization directions are the same in the two permanent magnets that magnetize the Faraday rotator, and the latter has a structure in which the magnetization directions are opposite, and the incoming and outgoing rays are on the same optical axis. We have proposed an optical isolator that does not depend on polarization. However, in these cases, when the required ray bundle is large, the spread of the Gaussian beam in the optical path becomes a problem, and it is necessary for improving the extinction characteristics that the spread portion of the ray in the opposite direction does not return to the ray axis. Was confirmed.

[Means for Solving the Problems] The present invention is characterized by being composed of the above-mentioned three birefringent crystals and two Faraday rotators, and the incident and outgoing rays being on the same optical axis. In an optical isolator without a filter, between the first birefringent crystal and the first Faraday rotator,
This is to arrange an elliptical slit through which an ordinary ray and an extraordinary ray separated by the first birefringent crystal can pass.

[Embodiment] FIG. 1 shows the above-mentioned three birefringent crystals 1, 2, 3 and two Faraday rotators 5, 6 and two permanent magnets whose directions are the same for magnetizing the Faraday rotator. FIG. 2 shows an optical isolator in which the elliptical slit 8 is arranged in the optical system configured in FIG. 2, and FIG. 2 shows an optical isolator in which the elliptical slit 8 is arranged in the optical system configured by two permanent magnets having opposite magnetization directions. Shows an isolator. In the figure, it can be seen that the light beam in the forward direction passes through the slit portion, and the light beam in the reverse direction is displaced from the optical axis and is blocked by the slit portion.

[Advantages of the Invention] According to the present invention, since the backward light beam is almost completely blocked by the slit portion, the optical isolator of this system does not depend on the polarization plane, and the positions of the incident ray and the outgoing ray are completely. It has the advantage of matching, and when it is inserted into an optical circuit, the original characteristics of extinction characteristics and polarization independence can be easily obtained without specially adjusting the optical axis.

[Brief description of drawings]

FIG. 1 shows a structure diagram of a polarization-independent optical isolator according to the present invention and polarization states of ordinary light and extraordinary light, where (a) shows a forward light propagation state and (b) shows a reverse light propagation state. FIG. 2 shows another structure diagram of the polarization-independent optical isolator according to the present invention and polarization states of ordinary light and extraordinary light.
Shows the light propagation state in the forward direction and (b) shows the light propagation state in the reverse direction. 3 and 4 show the structure of a conventional polarization-independent optical isolator and the polarization states of ordinary light and extraordinary light. 1; 2; 3: Birefringent crystal, 5; 6: Faraday rotator 7: Optical rotator, 8: Oval slit

Claims (1)

(57) [Scope of utility model registration request] 1. A first birefringent crystal whose crystal optical axis is tilted with respect to the surface, a first Faraday rotator for rotating a plane of polarization by 45 °, and a thickness √2 times that of the first birefringent crystal. And a second birefringent crystal which is arranged by rotating 180 ° about the X-axis and then rotating by 45 ° about the incident ray direction in the same or opposite direction as the first Faraday rotator. The magnetized second Faraday rotator, which has the same thickness as the first birefringent crystal, and which is rotated by 45 ° about the incident ray direction as the axis of rotation with respect to the second birefringent crystal. In the optical system composed of a third birefringent crystal that is rotated by 90 ° with respect to the optical axis of and the permanent magnet for magnetizing the Faraday rotator, the first birefringent crystal and the first Faraday rotation Ordinary rays and extraordinary rays separated by the first birefringent crystal can pass between the children An optical isolator characterized in that a oblong slit.