JP3265487B2 - Polarization-independent optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used for optical communication and optical information processing, and more particularly to a polarization-independent optical isolator for specifying a traveling direction in one direction.

[0002]

2. Description of the Related Art In an optical fiber amplifier which is becoming increasingly important in an optical communication system, light reflected by an optical circuit element in a signal light path induces noise. In order to suppress this noise, a polarization independent optical isolator is used.

FIG. 4 is a diagram showing a configuration of an example of the above-mentioned polarization independent optical isolator. In the figure, reference numeral 31 denotes a birefringent crystal flat plate, which shifts extraordinary light vertically upward as indicated by a thick arrow. 32 is a 45 degree Faraday rotator. A birefringent crystal plate 33 also shifts extraordinary light clockwise from the horizontal direction by 45 °.
Reference numeral 34 denotes a birefringent crystal flat plate, which shifts extraordinary light in a 45 ° direction counterclockwise from the horizontal direction. The thickness of the three birefringent crystal flat plates 31, 33, 34 is √2: 1:
It is 1. As a result, the light that has entered the birefringent crystal flat plate 31 as ordinary light and the light that has entered as extraordinary light are once separated, but coincide after returning from the birefringent crystal flat plate 34. On the other hand, it can be seen that the light traveling in the reverse direction deviates from the optical path of the light incident in the forward direction in consideration of the non-reciprocity of the 45 ° Faraday rotator 32. The optical isolator thus configured has polarization independence, is easy to manufacture, and has high practicality.

However, when the above-mentioned optical isolator is installed in the signal light path, the birefringent crystal flat plate 31,
33 and 34 must be inclined by about 5 ° with respect to the optical path. Otherwise, the reflected light generated by the residual reflection on the light transmitting surface of the birefringent crystal flat plate is returned to the signal light path, and the optical isolator itself causes noise. Therefore, the optical isolator must be set at an angle to the signal light path.

[0005]

However, when the optical isolator of FIG. 4 is set at an angle, the polarization dependence of the forward loss increases and the isolation decreases. The reason is that the extraordinary light separation directions of the three birefringent crystal plates are different, and the extraordinary light separation distance must satisfy √2: 1: 1, so that the characteristics do not deteriorate in the state of being tilted. This is because the tilt direction and the tilt angle of the three birefringent crystal flat plates must be finely adjusted in each flat plate, and therefore cannot be performed in an actual manufacturing process.

Accordingly, the present invention is to provide an optical isolator which has a light transmission surface which is sufficiently inclined with respect to a signal optical path, is easy to manufacture, has high isolation, and has small polarization dependence of forward loss. is there.

[0007]

According to the present invention, when a uniaxial birefringent crystal having a shape of a parallel plate is arranged so that the surface normal of the parallel surface and the optical axis are appropriately inclined, light incident on the parallel surface is inclined. Is focused on that the extraordinary light component goes straight without refraction at the interface of the birefringent crystal plate, and the ordinary light component refracts according to Snell's law.

That is, according to the present invention, a first birefringent crystal flat plate, a 45-degree Faraday rotator, a 45-degree optical rotator, and a second birefringent crystal flat plate are arranged in the incident optical path in the stated order. Wherein the first and second birefringent crystal flat plates are provided at an angle such that an optical path of incident light passing through the birefringent crystal flat plate as extraordinary light does not refract at an interface. An independent optical isolator is obtained.

[0009]

With the above construction, the birefringent crystal flat plates at both ends allow extraordinary light to pass through without refraction, and the two non-reciprocal optical elements provided in the middle have a plane of polarization for light in the forward direction. 9
It rotates by 0 °, but acts to transmit light in the opposite direction without changing the polarization plane. Therefore, it has a sufficient function as an optical isolator.

[0010]

FIG. 1 shows the configuration of an embodiment of the present invention. First
Rutile single crystal flat plate 10, 45 degree Faraday rotator 1
1, 45-degree optical rotator 12 and second rutile single crystal flat plate 13
Are sequentially arranged obliquely in the optical path, and the incident light from the first optical fiber 14 passes through the above-mentioned four elements via the first lens 16 and the second optical fiber 15 via the second lens 17. To join. The light transmitting surface of the first rutile single crystal flat plate 10 is processed into a parallel plane, and its C axis (optical axis) 18 is inclined by 48 ° with respect to the normal to the interface 21. In this figure, the inclination direction is drawn as if it were in the plane of the paper. Further, the first rutile single crystal flat plate 10
It is inclined by 9.24 ° with respect to 0, and the inclination direction is in a plane including the C axis and the normal to the light transmitting surface. That is, in FIG. 1, the incident light path 20 is also a rutile single crystal flat plate C
Both the axis 18 and the normal to the interface 21 are in the plane of the paper. The direction of the magnetic field and the sign of the optical rotation for the 45-degree Faraday rotator 11 and the 45-degree optical rotator 12 are obtained by combining these two elements.
It is determined that the plane of polarization is rotated by 90 ° for light incident from the left side, and that the plane of polarization is not changed for light incident from the right side.

FIG. 2 is a diagram for explaining the operation when light is transmitted in the forward direction in the embodiment shown in FIG. The first chill single crystal flat plate 10 is installed so as to be inclined such that extraordinary light goes straight on the incident optical path. On the other hand, since the ordinary ray is refracted according to Snell's law, the outgoing light shifts in parallel with the incident light. By the way, the reflected light generated by the residual reflection on the light transmitting surface of the first rutile single crystal flat plate 10 is incident on the incident optical path 20.
, So that no noise is induced in the signal light. Next, the light incident on the 45-degree Faraday rotator 11 rotates the polarization plane by 45 degrees. Further, the polarization plane is rotated by 45 ° by the optical rotator 12. As a result, the light transmitted through the first rutile single crystal flat plate 10 as extraordinary light is ordinary light in the second rutile single crystal flat plate 13.

On the other hand, light transmitted through the first rutile single crystal flat plate 10 as ordinary light is extraordinary light in the second rutile single crystal flat plate 13. Two rutile single crystal flat plates 10
And 13 are the same and have the same inclination direction, so that the light separated into two optical paths by the rutile single-crystal flat plate 10 coincides again when exiting from the second rutile single-crystal flat plate 13. This indicates that the optical device of the present invention has polarization independence in the forward direction.

FIG. 3 is a diagram showing the operation when light is transmitted in the opposite direction in the embodiment shown in FIG. In FIG. 3, light incident as extraordinary light on the second rutile-edge single crystal flat plate 13 from the right side travels straight, and ordinary light is refracted according to Snell's law. By the way, since the second rutile single crystal flat plate 13 is sufficiently inclined with respect to the incident optical path, the reflected light from the light transmitting surface does not return to the incident optical path. Next, the light transmitted through the 45-degree optical rotator 12 has its polarization plane rotated by 45 °. Further, when the light passes through the 45-degree Faraday rotator 11, the polarization plane is rotated by 45 degrees in the opposite direction to the 45-degree optical rotator. As a result, the light transmitted through the second rutile single crystal flat plate 13 as extraordinary light is converted into the first rutile single crystal flat plate 10.
Is not coupled to the second optical fiber to transmit as extraordinary light. On the other hand, the light transmitted through the second rutile single crystal flat plate 13 as ordinary light is transmitted through the first rutile single crystal flat plate 10 as ordinary light, and is not coupled to the first optical fiber. These operations indicate that the optical device of the present invention acts as a polarization independent optical isolator.

In the above description, yttrium vanadate (YVO ₄ ) is used instead of the rutile single crystal of this embodiment.
The same applies to the case where a birefringent single crystal such as a single crystal is used.

[0015]

As described above, according to the present invention, a rutile single crystal flat plate can reliably operate as a polarization-independent optical isolator while having a light transmitting surface having an inclination such that light transmitted as extraordinary light is not refracted. Therefore, the reflected light from the rutile single crystal flat plate does not return to the signal light path, and the reflected light from the isolator itself does not induce noise in the signal light.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of one embodiment of an optical isolator according to the present invention.

FIG. 2 is a view for explaining the operation of the optical isolator of FIG. 1 when incident light travels in a forward direction.

FIG. 3 is a view for explaining the operation of the optical isolator of FIG. 1 when incident light travels in the opposite direction.

FIG. 4 is a diagram showing a configuration of an optical isolator according to a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 1st rutile single crystal flat plate 11 45 degree Faraday rotator 12 45 degree rotator 13 2nd rutile single crystal flat plate 14 1st optical fiber 15 2nd optical fiber 16 1st lens 17 2nd lens 18 C axis 19 C axis 20 Incident light path 21 Interface 22 Interface 31 Birefringent crystal flat plate 32 45 degree Faraday rotator 33 Birefringent crystal flat plate 34 Birefringent crystal flat plate

Claims (1)

(57) [Claims] 1. A first birefringent crystal flat plate, a 45-degree Faraday rotator, a 45-degree optical rotator, and a second birefringent crystal flat plate are arranged in an incident optical path in the stated order. A polarization-independent optical isolator, wherein the second birefringent crystal flat plate is provided at an angle such that an optical path of incident light transmitted through the birefringent crystal flat plate as extraordinary light does not refract at an interface. .