JP2003114405A - Optical circulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical circulator with excellent optical characteristics without polarized wave dispersion. SOLUTION: In the optical circulator, two first double refraction crystals 1a, 1b of which two crystal axis directions 1aC, 1bC are in parallel with a overlapped surface and symmetrical to a traveling direction O of light, two second double refraction crystals 2a, 2b with the same shape as the first double refraction crystals 1a, 1b and arranged by being rotated with respect to the traveling direction O of the light by 90 degrees, non-reciprocal optical rotor 3, an optical element 4 for compensating polarized wave dispersion arranged on the optical path of one of the first double refraction crystals 1a, 1b and a third double refraction crystal 5 having the crystal axis 5C in the direction formed by synthesizing the direction of one of the double refraction crystal direction 1a of the first double refraction crystal with one of the double refraction crystal direction 2a of the second double refraction crystals are arranged in this order.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization independent optical circulator used as an optical passive component in an optical communication system.

[0002]

2. Description of the Related Art An optical circulator transmits an optical signal through different paths in a forward direction and a reverse direction, and is used for bidirectional communication, an amplifier, an optical signal branch, a dispersion compensation circuit, and the like. There are 3 terminal (3 port) type and 4 terminal (4 port) type depending on the application.

As a conventional optical circulator, Japanese Unexamined Patent Publication No.
In Japanese Patent Laid-Open No. 1-311754, two birefringent crystals having two birefringent crystals arranged in parallel with their crystal axes facing each other, a polarization rotating means, and a birefringent crystal having one single crystal axis are disclosed. An optical circulator in which a refraction crystal is arranged is disclosed. Japanese Patent No. 2977927 also describes an optical circulator having the same configuration. The light incident on the first port of these optical circulators is split into two polarizations, an ordinary ray and an extraordinary ray, by the first birefringent crystal that transmits first, and this ordinary ray is an extraordinary ray by the second birefringent crystal. ,
Further, the third birefringent crystal transmits as an extraordinary ray.
On the other hand, the extraordinary ray of the first birefringent crystal is transmitted as an ordinary ray in the second birefringent crystal, and is transmitted as an ordinary ray in the third birefringent crystal, which coincides with that transmitted as an extraordinary ray. That is, the first, second and third birefringent crystals are
One polarization is transmitted as ordinary ray → extraordinary ray → extraordinary ray, and the other polarization is transmitted as extraordinary ray → ordinary ray → ordinary ray. If the light beams that have passed through the first and second birefringent crystals are summed up, both polarizations will have the same optical path length (paths are different). It will be different.

[0004]

As described above, in the optical circulators described in the above-mentioned respective publications, since the optical paths of the two separated polarized lights are different from each other, the polarization which is a fatal defect in the optical separating / combining circuit. Since wave dispersion occurs, it cannot be used as a system component for long-distance transmission.

An object of the present invention is to provide an optical circulator having excellent optical characteristics without such polarization dispersion.

[0006]

An optical circulator of the present invention for achieving the above-mentioned object is, as shown in FIG. 1 corresponding to an embodiment, two birefringent crystal plates 1a and 1b which are polarized in a polarization separating direction. The first light separation / synthesis material 1 and the light beams separated by the first light separation / synthesis material 1 are different from each other.
A second light separating / combining material 2 incident on the birefringent crystal plates 2a and 2b and combined such that the polarization separating directions of the birefringent crystal plates 2a and 2b are opposite to each other, and a non-reciprocal optical rotator. 3 and a third light separation / synthesis material 5 composed of a birefringent crystal plate for synthesizing the light beams polarized and separated by the first and second light separation / synthesis materials 1 and 2 in the same optical path are arranged in this order. Further, an optical element 4 for compensating for polarization dispersion generated in the third light separating / combining material is arranged between the non-reciprocal optical rotator 3 and the third light separating / combining material 5.

In this optical circulator, the light incident on the birefringent crystal 1a of the first light separating / synthesizing material 1 is converted into the birefringent crystal 1a.
At a, it is separated into ordinary rays and extraordinary rays. This ordinary ray is refracted as an extraordinary ray by the birefringent crystal 2a of the second light separation / synthesis material 2, but the extraordinary ray refracted by the birefringent crystal 1a goes straight as an ordinary ray by the birefringent crystal 2b. The extraordinary ray refracted by the birefringent crystal 2a is refracted as an extraordinary ray by the birefringent crystal of the third optical separation / synthesis material 5 after the plane of polarization is rotated by the nonreciprocal optical rotator 3. The ordinary ray traveling straight on the birefringent crystal 2b has its plane of polarization rotated by the non-reciprocal optical rotator 3, and then the optical path length is lengthened by the polarization dispersion compensating optical element 4.
The birefringent crystal of the light separating / synthesizing material 5 goes straight as an ordinary ray.

As described above, the ordinary ray emitted from the third light separating / combining material 5 has its optical path length adjusted to the extraordinary ray by the polarization dispersion compensating optical element 4, so that polarization dispersion with the extraordinary ray does not occur. .

The optical circulator of the present invention, as shown in FIG. 3 corresponding to the embodiment, comprises a first optical separation / synthesis material 1
A second light separation / synthesis material 2, a non-reciprocal optical rotator 3,
An optical element 4 for arranging a third light separation / synthesis material 5 in this order, and for compensating for polarization dispersion between the second light separation / synthesis material 2 and the non-reciprocal optical rotator 3. May be arranged.

The birefringent crystals forming the first light separation / synthesis material 1, the second light separation / synthesis material 2, and the third light separation / synthesis material 5 are, for example, rutile and yttrium vanadate (YV).
O ₄ ), lithium niobate (LN), calcite, lithium borate (LBO).
The non-reciprocal optical rotator is a Faraday rotator composed of a magneto-optical crystal made of, for example, a bismuth-substituted rare earth iron garnet single crystal, and may or may not include a magnet. The polarization dispersion compensating optical element is, for example, rutile, yttrium vanadate (YVO ₄ ), lithium niobate (L
N), calcite, lithium borate (LBO) and the like can be used, and the birefringent crystal is not always necessary.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of an optical circulator to which the present invention is applied will be described in detail with reference to the drawings. FIG. 1 is a perspective view showing the configuration of a 3-port optical circulator of an embodiment to which the present invention is applied.

The optical circulator shown in this figure includes two pairs of first birefringent crystals 1a and 1b, two pairs of second birefringent crystals 2a and 2b, a non-reciprocal optical rotator 3, and a polarized wave. The dispersion compensating optical element 4 and the third birefringent crystal 5 are arranged in this order. The first birefringent crystals 1a and 1b, which are rutile crystals finished in the same shape, are stacked one above the other, and the directions 1aC and 1bC of the respective crystal axes are parallel to the interface between the two and symmetrical with respect to the light traveling direction O. It is pasted so that it becomes. The second birefringent crystals 2a and 2b are made of the same material and have the same shape as the first birefringent crystals 1a and 1b, but are rotated by 90 ° with respect to the traveling direction O of the light to form a boundary surface between the first birefringent crystals 1a and 1b. And the second birefringent crystals 2a and 2b are orthogonal to each other. The non-reciprocal light rotator 3 is a 45 ° Faraday rotator made of a bismuth-substituted rare earth iron garnet single crystal, and a magnet (not shown) is arranged around the outer periphery of the 45 ° Faraday rotator.

The polarization dispersion compensating optical element 4 is the upper half of the optical path.
Made of rutile crystal, which passes through the third birefringent crystal 5.
It has the thickness of the optical path difference between the ordinary ray and the extraordinary ray.
Specifically, it is calculated as follows. Third birefringent crystal
Let 5 be the thickness of L, and θ be the angle between the light traveling direction and the crystal axis.
Then, the difference in optical path length between the ordinary light traveling straight and the extraordinary light refracted is
L / Cos θ-L. The refractive index of the third birefringent crystal 5 is
Light n_O, Abnormal light is n_EThen, between ordinary light and extraordinary light
Transmission time difference is (L / c) (n_ECos θ-n_O). But
A polarization dispersion compensating optical element that compensates for this transmission time difference
4 is the same rutile crystal as the third birefringent crystal 5
, Its thickness is L / n_O(n_E/ Cos θ−n _O).

FIG. 2 shows a pair of first birefringent crystals 1a and 1b, a pair of second birefringent crystals 2a and 2b, a nonreciprocal optical rotator 3 and a polarization dispersion compensation in the optical circulator shown in FIG. It is the front view which looked at the optical element 4 and the 3rd birefringent crystal 5 from the port P1 and the port P3 side (left front side of FIG. 1).
A is the direction of the plane of polarization traveling from the port P1 to the port P2, and B is the direction of the plane of polarization traveling from the port P2 to the port P3. In both A and B, with respect to the first birefringent crystals 1a and 1b, the second birefringent crystal 2 has the polarization planes on the front surface (the light incident surface of the port P1) and the rear surface.
Regarding a.2b, the non-reciprocal optical rotator 3, the polarization dispersion compensating optical element 4, and the third birefringent crystal 5, the plane of polarization on the back surface is illustrated.

Optical fibers, lasers, light-receiving elements, etc. (not shown) are optically coupled to the input / output ports P1, P2, P3 by lenses.

The optical function of the optical circulator shown in FIG. 1 will be described below with reference to FIG.

As shown in FIG. 2A, from port P1 to the first
In the light incident on the birefringent crystal 1a, the ordinary ray (solid arrow) goes straight, and the extraordinary ray (broken arrow) is refracted by the crystal axis 1aC and is separated into two polarized lights. The ordinary ray is incident on the second birefringent crystal 2a and becomes an extraordinary ray with respect to the crystal axis 2aC thereof, so that it is refracted and transmitted. The extraordinary ray separated from the ordinary ray by the first birefringent crystal 1a and transmitted is the second birefringent crystal 2b.
To the crystal axis 2bC and becomes an ordinary ray, and thus goes straight. While the two polarized lights emitted from the second birefringent crystals 2a and 2b are transmitted through the non-reciprocal optical rotator 3, both polarized lights undergo Faraday rotation of 45 ° in the clockwise direction and are emitted. Here, the polarized light emitted from the upper half of the non-reciprocal optical rotator 3 passes through the polarization dispersion compensation optical element 4, but the polarized light emitted from the lower half does not pass through the polarization dispersion compensation optical element 4. Therefore, the polarized light transmitted through the polarization dispersion compensation optical element 4 is L / Cos θ.
-L increases the optical path length. Next, both polarized lights are incident on the third birefringent crystal 5, and the polarized light incident on the upper half is the crystal axis 5
Since it is an ordinary ray for C, it goes straight on and heads for port P2. The polarized light that has entered the lower half becomes an extraordinary ray with respect to the crystal axis 5C of the third birefringent crystal 5, so it is refracted and transmitted, and goes to the port P2. At this time, the optical path length is increased by L / Cos θ-L. At the port P2, the first birefringent crystal 1a to the third birefringent crystal 5 for both polarizations separated by the first birefringent crystal 1a
Since the optical path lengths up to are matched, both polarizations are combined and polarization dispersion does not occur.

As shown in FIG. 2B, the ports P2 through 3
In the light incident on the birefringent crystal 5, an ordinary ray (solid arrow) goes straight, and an extraordinary ray (broken arrow) is refracted by the crystal axis 5C, and is separated into two polarized lights and emitted. At this time, the extraordinary ray has a long optical path length of L / Cos θ-L. The linearly polarized light passes through the upper half polarization compensation optical element 4, but the refracted polarization does not pass through the polarization dispersion compensation optical element 4. Therefore, the polarized light transmitted through the polarization dispersion compensation optical element 4 is L / Cos.
The optical path length is increased by θ-L, and the optical path length difference generated when the light passes through the third birefringent crystal 5 is canceled. While passing through the non-reciprocal optical rotator 3, the two polarized lights both undergo Faraday rotation of 45 ° in the plane of polarization and are emitted, and enter the second birefringent crystals 2a and 2b. Second birefringent crystal 2
The polarized light incident on a becomes an ordinary ray with respect to its crystal axis 2aC, and thus goes straight. The polarized light that has entered the second birefringent crystal 2b becomes an extraordinary ray with respect to the crystal axis 2bC and is refracted and then transmitted. Then, the separated two polarized lights are incident on the first birefringent crystal 1b, the ordinary ray (solid arrow) goes straight, and the extraordinary ray (broken arrow) is refracted by the crystal axis 1bC and combined at the port P3. . The optical path lengths of both polarizations from the port P2 to the port P3 are the same, and the polarization dispersion of the light emitted from the port P3 does not occur.

FIG. 3 is a perspective view showing the structure of a 3-port optical circulator of another embodiment to which the present invention is applied. The constituent elements of this optical circulator are the same as those of the optical circulator shown in FIG. 1, but the order of arrangement of the polarization dispersion compensating optical element 4 and the nonreciprocal optical rotator 3 is exchanged. That is, the optical circulator shown in FIG. 3 has a pair of first birefringent crystals 1a and 1b and a pair of second birefringent crystals 2a and 2b.
b, polarization dispersion compensating optical element 4, non-reciprocal optical rotator 3, third
The birefringent crystals 5 are arranged in this order.

In FIG. 4, A indicates the direction of the plane of polarization traveling from the port P1 to the port P2, and B indicates the direction of the plane of polarization traveling from the port P2 to the port P3, respectively. Figure 4
The optical function of the optical circulator shown in FIG. 3 will be described with reference to FIG.

As shown in FIG. 4A, from port P1 to the first
In the light incident on the birefringent crystal 1a, the ordinary ray travels straight, and the extraordinary ray is refracted by the crystal axis 1aC and is separated into two polarized lights. The ordinary ray is incident on the second birefringent crystal 2a and becomes an extraordinary ray with respect to the crystal axis 2aC thereof, so that it is refracted and transmitted. The extraordinary ray separated from the ordinary ray and transmitted by the first birefringent crystal 1a is incident on the second birefringent crystal 2b, and its crystal axis 2b.
Since it is an ordinary ray for C, it goes straight. Of the two polarizations emitted from the second birefringent crystals 2a and 2b, the polarization emitted from the upper half is transmitted through the polarization dispersion compensation optical element 4, while the polarization emitted from the lower half is the polarization dispersion compensation optical element 4. Does not penetrate. Therefore, the optical path length of the polarized light transmitted through the polarization dispersion compensating optical element 4 is increased by L / Cos θ−L. Next, while the two polarized lights pass through the non-reciprocal optical rotator 3, both polarized lights undergo Faraday rotation of 45 ° clockwise and are emitted.
Both polarized lights whose polarization planes are inclined are incident on the third birefringent crystal 5, but the polarized light incident on the upper half is a normal ray with respect to the crystal axis 5C and goes straight to the port P2. The polarized light that enters the lower half becomes an extraordinary ray with respect to the crystal axis 5C of the third birefringent crystal 5, so it is refracted and transmitted, and goes to the port P2.
At this time, the optical path length is increased by L / Cos θ-L. Port P
In 2, the optical path lengths of the first polarized birefringent crystal 1a and the third polarized birefringent crystal 5 of the two polarized lights separated by the first birefringent crystal 1a are the same, so that both polarized lights are combined and the polarization dispersion is Does not happen.

As shown in FIG. 4B, the ports P2 through P3
In the light incident on the birefringent crystal 5, an ordinary ray travels straight, and an extraordinary ray is refracted by the crystal axis 5C, and is separated into two polarized lights and emitted. At this time, the extraordinary ray has a long optical path length of L / Cos θ-L. While both polarizations are transmitted through the non-reciprocal optical rotator 3, both polarization planes undergo Faraday rotation of 45 ° clockwise and are emitted. Of the two polarizations, the polarization emitted from the upper half is transmitted through the polarization dispersion compensation optical element 4, but the polarization emitted from the lower half is not transmitted through the polarization dispersion compensation optical element 4. Therefore, the optical path length of the polarized light transmitted through the polarization dispersion compensating optical element 4 is increased by L / Cos θ−L, and the optical path length difference generated when passing through the third birefringent crystal 5 is canceled.
Both polarized lights then enter the second birefringent crystals 2a and 2b. The polarized light incident on the second birefringent crystal 2a becomes an ordinary ray with respect to its crystal axis 2aC, and thus goes straight. The polarized light that has entered the second birefringent crystal 2b becomes an extraordinary ray with respect to the crystal axis 2bC and is refracted and then transmitted. Then, the separated two polarized lights enter the first birefringent crystal 1b, the ordinary ray travels straight, the extraordinary ray is refracted by the crystal axis 1bC, and the port P
Combined at 3. The optical path lengths of both polarizations from the port P2 to the port P3 are the same, and the polarization dispersion of the light emitted from the port P3 does not occur.

[0023]

As described above in detail, in the optical circulator of the present invention, since the optical paths of the two separated polarized lights are equal to each other, polarization dispersion does not occur, and therefore the optical transmission characteristics are extremely excellent. Therefore, this optical circulator is also optimal as a system component for long-distance transmission.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of an optical circulator to which the present invention is applied.

FIG. 2 is a diagram for explaining the operation of the optical circulator of the embodiment shown in FIG.

FIG. 3 is a perspective view of another embodiment of the optical circulator to which the present invention is applied.

FIG. 4 is a diagram for explaining the operation of the optical circulator of the embodiment shown in FIG.

[Explanation of symbols]

1a and 1b are first birefringent crystals, 1aC and 1bC are crystal axis directions of the first birefringent crystals, and 2a and 2b are second birefringent crystals,
2aC and 2bC are crystal axis directions of the second birefringent crystal, 3 is a non-reciprocal optical rotator, 4 is a polarization dispersion compensation optical element, 5 is a third birefringent crystal, and 5C is a crystal axis of the third birefringent crystal, P1 and P2
-P3 is an input / output port.

Claims (2)

[Claims] 1. A first light-separating / synthesizing material, which is formed by combining two birefringent crystal plates so that their polarization separation directions are opposite to each other, and is separated by the first light-separating / synthesizing material. A second light-splitting / combining material, in which light rays are incident on two different birefringent crystal plates, and the polarization separation directions of the birefringent crystal plates are opposite to each other, and a non-reciprocal optical rotator. A third light separation / combination material composed of a birefringent crystal plate for combining the light beams polarized and separated by the first and second light separation / combination materials in the same optical path, and the nonreciprocal light. An optical circulator, wherein an optical element for compensating for polarization dispersion generated in the third light separating / combining material is arranged between a rotor and the third light separating / combining material. 2. A first light separation / synthesis material according to claim 1, a second light separation / synthesis material, and a non-reciprocal optical rotator.
An optical element for arranging a third light separating / combining material in this order, and compensating for polarization dispersion according to claim 1 between the second light separating / combining material and the non-reciprocal optical rotator. An optical circulator characterized by arranging elements.