JP2553358B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of use] The present invention relates to a configuration of an optical isolator utilizing the Faraday effect used for optical communication, optical measurement and the like.

[Prior Art and its Problems] Optical isolators are generally used to stabilize the oscillation of laser light by using the Faraday effect to prevent the reflected light from the optical fiber end and the light receiving element from returning to the laser. .

However, the conventional optical isolator is not satisfied in the following points. (1) In order to prevent the reflected light from the optical isolator from returning to the laser, each element or the optical isolator is tilted and used. Since the incident light and the emitted light are not on the same straight line, it is difficult to align the ray axes.

(2) Since an optical isolator constructed by using a polarizing prism such as a Rochon prism, a Glan-Thompson prism, or a polarizing beam splitter has a polarization characteristic, it can pass only a polarized beam of one polarization plane in the forward direction. Therefore, when it is used for a semiconductor laser, it is necessary to align the optical isolator with the polarization direction of the semiconductor laser. Further, when an optical isolator is used for a circularly polarized beam or an elliptically polarized beam, the polarized beam having a component orthogonal to the transmitted polarization direction must be discarded and used.

(3) Generally, the Faraday rotation angle of the Faraday element changes depending on the temperature, and has a temperature characteristic of, for example, about 0.06 deg / ° C., and the Faraday rotation angle deviates from 45 ° by an angle Δθ.
Isolation deteriorates. The characteristic is shown by the following equation.

Isolation = −10 log (sin ² Δθ) An object of the present invention is to provide a configuration of an optical system of an optical isolator that solves the above problems.

[Means for Solving Problems] The present invention, as shown in FIG. 1, includes a first birefringent prism 1, a first Faraday element 5, and a second birefringent prism 2.
And third birefringent prism 3, second Faraday element
6, In the configuration of the optical system including the fourth birefringent prism 4, the birefringent prisms 1, 2, 3, 4 have the same prism angle α,
The respective optical axes are perpendicular to the axis AB of the optical system, and the optical axes of the first birefringent prism 1 and the second birefringent prism 2 are shifted by 45 °, Birefringent prism 2
Since the optical axes of the and the third birefringent prism 3 are parallel to each other, the directions of the magnetic fields H applied to the Faraday elements 5 and 6 are opposite to each other, so that 6 has a polarization plane rotated by 45 ° in the opposite direction to 5. Then
Therefore, the optical axis of the fourth birefringent prism 4 is offset from the third birefringent prism 3 by 45 ° in the opposite direction to the relationship between 1 and 2.

The Faraday rotation angles of the Faraday elements 5 and 6 are 45 °, respectively, and the directions of the magnetic fields H applied to the Faraday elements 5 and 6 are opposite to each other, and the entrance surface and the exit surface of the first birefringent prism 1 are the second birefringent surfaces. The exit surface and the entrance surface of the refraction body prism 2 are parallel, and the entrance surface of the second birefringent body prism 2 and the exit surface of the third birefringence body prism 3 are parallel, and the second birefringence body The exit angle of the prism 2 and the incident angle of the third birefringent prism 3 are the same and have a positive and negative relationship, and the incident surface and the exit surface of the third birefringent prism 3 are respectively the fourth birefringent surface. The configuration of the optical isolator parallel to the exit surface and the entrance surface of the refraction prism 4 prevents the reflected return light from each element from returning to the incident light beam, cancels the temperature characteristics and the polarization characteristics, and further forward incidence. Light and outgoing light are arranged on the same straight line. It is transmitted without changing the ordinary and extraordinary rays, reverse light by repeating the transformation of the ordinary and extraordinary rays, and does not return to the incident light.

EXAMPLE FIG. 1 is an example of the present invention. Calcite is used as the birefringent material of the prisms 1, 2, 3, and 4, the prism angle α is 30 °, and the optical axis direction is the axis A- of the optical system. It is in a plane perpendicular to B, and 1 is + 22.5 °, 2 with respect to the bottom surface S of the birefringent prism 1.
Is -22.5 °, 3 is -22.5 °, and 4 is + 22.5 °, and the Faraday elements 5 and 6 are processed to have a thickness such that the polarization plane rotates by 45 ° at a wavelength of 1.3 μm. An antireflection film was formed on each element. An Sm-Co permanent magnet was used as the applied magnetic field source so that the Faraday element was sufficiently saturated, and the directions of the magnetic fields H were reversed for the 5 and 6 respectively. In the forward light, the incident light and the outgoing light were on the same straight line, and the polarization dependence was 0.1 dB.
This is because the incident surface is inclined and therefore the transmission characteristics of P wave and S wave are different. However, the polarization dependence when using a polarizing prism is 40 to 50 dB, which is a significant improvement. When the prism angle α is increased, the transmission characteristics of the P wave and the S wave become closer, so that the polarization dependence is further improved. In addition, the forward insertion loss had almost no temperature dependence. On the other hand, the temperature dependence of the reverse insertion loss in the reverse direction was 0.5 dB in the temperature range of 0 to 60 °. The temperature dependence when using a polarizing prism is 5 dB, which is a significant improvement.

[Operation] In the configuration of the optical system of the present invention, (a) shows a forward optical path and (b) shows a reverse optical path. For the forward light, the light ray incident from the A direction is incident on 1, and is separated into an ordinary ray and an extraordinary ray in the prism. The separated rays are incident on the Faraday element 5, and the polarization planes of the respective rays are both rotated by 45 ° and are incident on 2. Since the optical axes of the birefringent prisms 1 and 2 are rotated by 45 ° in the rotation direction of the plane of polarization of 5, the relationship between the ordinary ray and the extraordinary ray is maintained. The incident light on 1 and the outgoing light from 2 are parallel. Birefringent prism
The relationship between the optical axes of 2 and 3 is 0 °, and similarly the relationship between the ordinary ray and the extraordinary ray is maintained. The plane of polarization of the light beam incident on the Faraday element 6 rotates 45 ° in the direction opposite to the direction of rotation of the plane of polarization of 5. Since the optical axes of the birefringent prisms 3 and 4 are rotated by 45 ° in the direction of rotation of the plane of polarization of 6, the ordinary ray and the extraordinary ray are not converted, and eventually the birefringent prism 1 The incident light and the light emitted from the birefringent prism 4 are on the same straight line. Moreover, the incident light is transmitted regardless of the plane of polarization. (However, since the incident surface is inclined, it has a slight polarization characteristic due to the difference in reflection loss of P-wave and S-wave.) As is clear from the drawing, all incident angles at the end face of each element are other than 0 °. Therefore, the reflected light from each end face does not return to the incident light beam, and therefore the reflected light from the optical isolator of the present invention does not return to the light source.

With respect to the backward light, the light ray incident from the B direction is incident on 4, is separated into an ordinary ray and an extraordinary ray within the prism, and is incident on the Faraday element 6. Within 6, the plane of polarization rotates 45 ° in the same direction that it rotated during forward transmission. Since the optical axes of the birefringent prisms 3 and 4 are rotated by 45 ° in the Faraday rotation direction in the forward direction, they are viewed from the rotation angle of the polarization plane viewed from the traveling direction of the backward light and the optical axis of 4. The rotation angle of the optical axis of 3 has a relation of 45 ° in the opposite direction. That is, the planes of polarization of the ordinary ray and the extraordinary ray are 0 ° and 90 ° with respect to the optical axis of 3, respectively. Since the plane of polarization of the ordinary ray is perpendicular to the plane containing the optical axis and the plane of polarization of the extraordinary ray lies in the plane containing the optical axis, the ordinary ray changes into an extraordinary ray and the extraordinary ray changes into an ordinary ray within 3. Therefore, since the refraction angle changes, the incident light to the birefringent prism 4 and the outgoing light from the birefringent prism 1 in the backward light are not parallel, and the ordinary ray and the extraordinary ray are emitted with a certain separation angle. become. 2,5,
The same is true for 1, but since the directions of the magnetic fields H applied to the two Faraday elements are opposite to each other, even if the Faraday rotation angle deviates from 45 ° due to the temperature change of the Faraday element, If the temperature and the temperature characteristic of the element are the same, the angles deviated by the temperature change cancel each other. Therefore, the forward insertion loss and isolation are improved.

[Advantages of the Invention] According to the present invention, the incident light and the emitted light are on the same straight line, so that the ray axes can be easily aligned, the incident light can be transmitted through any polarization plane, and has no polarization characteristic. When used in a semiconductor laser, an optical isolator can be coupled regardless of the polarization direction of the semiconductor laser. In addition, when an optical isolator is used for a circularly polarized beam or an elliptically polarized beam, all can be transmitted. Furthermore, since the directions of the magnetic fields applied to the two Faraday elements are opposite to each other, even if the Faraday rotation angle deviates from 45 ° due to the temperature change of the Faraday element, the angles offset by the temperature change cancel each other out. As a result, the forward insertion loss and isolation are improved.

Further, in terms of cost, the conventional polarizing prism type requires 8 (= 2 × 2 × 2) prisms, but in the present invention, 4 (= 2 × 2) prisms suffice, which is an expensive calcite. If the number of parts of the prism formed in 1 is reduced to half and the prism angle α is larger than 45 °, the optical isolator can be thinned.

[Brief description of drawings]

 FIG. 1 is a schematic diagram of an optical system showing an example of the present invention. (A): Forward optical path diagram, (b): Reverse optical path diagram 1,2,3,4: Birefringent prism 5,6: Faraday element α: Prism angle, H: Applied magnetic field AB : Optical axis of optical system S: Bottom of birefringent prism 1

Claims (1)

(57) [Claims] 1. A first birefringent prism, a first Faraday element, a second birefringent prism and a third birefringent prism, a second Faraday element, and a fourth birefringent prism. In the configuration of the optical system, the prism angles of all the birefringent prisms are the same, their respective optical axes are perpendicular to the axis of the optical system, and the first birefringent prism and the second birefringent prism are The optical axis of the second birefringent prism and the third birefringent prism are parallel to each other, and the optical axis of the second birefringent prism is parallel to that of the third birefringent prism. The optical axes of the two are shifted by 45 °, the Faraday rotation angles of the two Faraday elements are each 45 °, the directions of the magnetic fields applied to them are opposite, and the incident surface of the first birefringent prism is The exit surface is the exit surface and the entrance surface of the second birefringent prism, respectively. In parallel, the entrance surface of the second birefringent prism and the exit surface of the third birefringent prism are parallel, and the exit angle of the second birefringent prism and the entrance of the third birefringent prism are parallel. Is the same as a horn,
An optical isolator having positive and negative reciprocal relations, wherein the entrance surface and the exit surface of the third birefringent prism are parallel to the exit surface and the entrance surface of the fourth birefringent prism, respectively.