JPH0534634A - Optical circulator - Google Patents

Abstract

PURPOSE:To obtain high isolation and also to further more facilitate the optical connection between each port and an optical waveguide. CONSTITUTION:This circulator is provided with first and second double refractive crystal plates 5, 6 being arranged by keeping a specified space along the advancing direction of light beams, a reciprocal rotor 8 and a non-reciprocal rotor 7 being inserted between the first and second double refractive crystal plates 5, 6 and the reciprocal rotor 8 and the non-reciprocal rotor 7 being rotated an electric field vibration plane of the light beams by the equal angle is provided and also the directions of each crystal axis of the first and second double refractive crystal plates 5, 6 are characterized by being coincided mutually.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circulator used for optical communication, and more particularly to a polarization maintaining type optical circulator.

[0002]

2. Description of the Related Art FIG. 12 is a perspective view showing the structure of a conventional polarization-maintaining optical circulator. This type of optical circulator includes a Faraday rotator 1 that rotates an electric field oscillation surface of incident light by 45 degrees and emits the incident light, as shown in FIG.
And the incident surface 2a and the reflecting surface 2 of the Faraday rotator 1.
It is composed of two rectangular parallelepiped polarizing prisms 3 and 4 arranged so as to face b.

The deflecting prisms 3 and 4 have an optical axis (Z in the figure).
About the axis) and are inclined to each other by 45 degrees. In order to facilitate the following description, here, three ridge lines orthogonal to each other at predetermined apexes of the deflecting prism 3 are respectively denoted by X.
Consider an XYZ coordinate system that is parallel to the axes, the Y axis, and the Z axis. One surface (a plane parallel to the XZ plane) of the deflecting prism 3 serves as a first port P1, and the other surface (a plane parallel to the XY plane) serves as a third port P3. , One surface of the polarization prism 4 (orthogonal to the XY plane, and X-Z
The plane intersecting the plane at 45 degrees is used as the second port P2, and the other plane (the plane parallel to the XY plane) is used as the fourth port P4.

In the above conventional structure, the polarization prism 3
Linearly polarized light L vibrating in the X-axis direction from the first port P1 of
When i1 is incident, the second port P of the polarization prism 4
The linearly polarized light Lo1 oscillated from 2 around the Z axis in the direction in which the Y axis is rotated by 45 degrees is emitted. In addition, the deflection prism 4
When linearly polarized light Li2 oscillating in the direction in which the Y axis is rotated 45 degrees around the Z axis is incident from the second port P2 of
Linearly polarized light Lo2 oscillating in the Y-axis direction is emitted from the third port P3 of the polarization prism 3.

Further, the third port P of the polarization prism 3
If linearly polarized light Li3 vibrating in the Y-axis direction from 3 is made incident, linearly polarized light Lo vibrating in the direction in which the Y-axis is rotated 45 degrees around the Z-axis from the fourth port P4 of the polarizing prism 4 will be described.
3 is emitted. Furthermore, if linearly polarized light Li4 vibrating in the direction in which the Y-axis is rotated by 45 degrees around the Z-axis is made incident from the fourth port P4 of the polarizing prism 4, it vibrates in the X-axis direction from the first port P1. The linearly polarized light Lo4 is emitted.

[0006]

By the way, in the conventional optical circulator, the polarization prism (polarization beam
Since splitters 3 and 4 are the basic components,
It was technically difficult to suppress the amount of leakage of the P wave component to be transmitted into the S wave component to be reflected by the polarization prisms 3 and 4 (hereinafter referred to as isolation) to −30 dB or less. Therefore, there is a drawback that the isolation cannot be increased.

Further, the above-mentioned input / output ports P1, P2, P
There is also a drawback that sophisticated technology is required for optical coupling between 3, P4 and the optical waveguide. Generally, in the coupling of an optical waveguide, particularly a single-mode waveguide with light, an angular error between the propagation direction of light and the propagation direction in the optical waveguide greatly affects the coupling efficiency. Therefore, if the traveling direction of light is changed by reflection as in the conventional configuration, the angle error is less likely to occur.
Therefore, there is a problem that the coupling efficiency is likely to be deteriorated, and that a higher level of technology is required to suppress the deterioration.

The present invention has been made in view of the above circumstances, and provides an optical circulator capable of obtaining high isolation and easily and optically coupling with an optical waveguide. It is an object.

[0009]

In order to solve the above-mentioned problems, the invention of claim 1 provides a first and a second birefringent crystal arranged at a predetermined interval along the traveling direction of a light beam. A plate and a reciprocal rotator and a non-reciprocal rotator that are inserted between the first and second birefringent crystal plates and rotate the electric field oscillation surface of the light beam by the same angle.
And the directions of the respective crystal axes of the second birefringent crystal plate are aligned with each other.

Further, in the invention of claim 2, the first and second birefringent crystal plates are arranged at a predetermined interval along the traveling direction of the light beam, and the first and second birefringent crystals. A non-reciprocal rotator that is inserted between the crystalline plates to rotate the electric field oscillation plane of the light beam by a predetermined angle, and the direction of each crystal axis of the first and second birefringent crystal plates is the predetermined direction. The feature is that they are different from each other by the angle.

[0011]

According to the structure of the present invention, high isolation can be obtained, and the optical coupling between each port and the optical waveguide can be further facilitated.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a view showing the arrangement of an optical circulator according to the first embodiment of the present invention. The optical circulator of this example includes two birefringent crystal plates 5 and 6 arranged at a predetermined interval along the traveling direction of a light beam, a non-reciprocal 45-degree rotator 7 and a reciprocal 45-degree rotator. It is composed of 8. The birefringent crystal plate 5 is provided with light incident / exiting ports Port1 and Port3, and the birefringent crystal plate 6 is provided with two light incident / exiting ports Port2 and Port4.

Here, the left and right directions in the figure are the Z axis (the direction from left to right is the positive direction of the Z axis), and the up and down directions are the Y axis (the direction from bottom to top is the positive direction of the Y axis). It is described using the Cartesian coordinate system. The birefringent crystal plate 5
Means that the extraordinary ray λe moves in the positive direction of the Y axis while the ordinary ray λo propagates in the crystal plate 5 parallel to the Z axis.
The orientation of the crystal axis is selected so as to propagate in the positive direction of the axis.

Further, the birefringent crystal plate 6 is the crystal plate 6 concerned.
The orientation of the crystal axis is selected so that the ordinary ray λo propagates in the Z axis direction and the extraordinary ray λe propagates in the Z axis positive direction while moving in the Y axis negative direction. ing. Non-reciprocal 45 degree rotor 7 and reciprocal 45 degree rotor 8
Is arranged so that when light propagates in the positive direction of the Z-axis, the direction of rotation of the plane of polarization by these rotors 7 and 8 is clockwise, looking at the direction of propagation. With this arrangement, when light propagates in the negative direction of the Z-axis, when viewed from the same direction as above, clockwise in the non-reciprocal 45-degree rotor 7 and in the reciprocal 45-degree rotor 8. The vibrating surface rotates counterclockwise.

As the birefringent crystal plates 5 and 6, calcite and rutile crystals are suitable. Further, the non-reciprocal 45-degree rotor 7 has a Y.V. I. G crystals and bismuth substitution type Y. I. G
A Faraday rotator using a crystal and a reciprocal 45 ° rotator 8 are preferably made of a crystal rotator, a half-wave plate, or a liquid crystal.

Next, the operation of the optical circulator having the above configuration will be described with reference to FIGS. In these figures, symbols Z1 to Z5 represent spatial positions, and correspond to the spatial positions Z1 to Z5 shown in FIG. 1, respectively. First, FIG. 2 is a diagram in which the polarization state of light at each spatial position (Z1 to Z5) on the optical path from the light incident / exit port Port1 (Z1) to the light incident / exit port Port2 (Z5) is observed from the incident side. .

As shown in the figure, the light beam entrance / exit port Port
From 1 (Z1), when a light ray whose electric field vector oscillates parallel to the X axis is vertically incident, this light ray travels straight in the birefringent crystal plate 5 as an ordinary ray λo without changing the vibration direction (Z2). ). The light beam that has passed through the birefringent crystal plate 5 is rotated by 45 degrees by the non-reciprocal 45-degree rotator 7 (Z
3) Then, it is rotated by 45 degrees by the reciprocal 45 degree rotor 8. After all, since the electric field vibration direction is rotated 90 degrees from the initial direction by the non-reciprocal 45-degree rotator 7 and the reciprocal 45-degree rotator 8 (Z4), the light beam incident on the birefringent folded crystal plate 6 is The extraordinary ray λe propagates in the positive direction of the Z axis while moving in the negative direction of the Y axis in the birefringent folded crystal plate 6, and is emitted from the ray entrance / exit port Port2 (Z5).

Next, referring to FIG. 3, the light beam entrance / exit port
The operation of the optical path (Z-axis negative direction propagating light) traveling from the port 2 (Z5) to the light incident / exit port Port3 (Z1) will be described. FIG. 3 is a diagram in which the polarization state of the light beam at each spatial position is observed from the emission side. Beam entrance / exit port Port2 (Z
5) When the light is made incident from the above, if the electric field vibration direction of the incident light is set to be parallel to the Y-axis, the inside of the birefringent folded crystal plate 6 will be an extraordinary ray λe in the positive direction of the Y-axis. It propagates in the negative Z-axis direction while moving (Z4). The light beam that has passed through the birefringent crystal plate 6 is rotated counterclockwise by 45 degrees by the reciprocal 45 degree rotator 8 (Z3), but is rotated by 45 degrees clockwise by the next non-reciprocal 45 degree rotator 7. Since it is rotated, the electric field vibration direction does not change after passing through the non-reciprocal 45-degree rotor 7 (Z2). Since the electric field oscillation direction has not changed, the light also propagates in the birefringent folded crystal plate 5 as an extraordinary ray λe while moving in the negative direction of the Y-axis, and is emitted from the ray entrance / exit port Port3.

Next, referring to FIG. 4, a light beam entrance / exit port
The operation of the optical path (Z-axis positive direction propagating light) from the port 3 (Z1) to the light incident / exit port Port4 (Z5) will be described. FIG. 4 is a diagram in which the polarization state of the light beam at each spatial position is observed from the incident side. Beam entrance / exit port Port3 (Z
Even when the light is incident from 1), if the electric field oscillation direction of the incident light is set to be parallel to the Y-axis, the Z-axis is moved in the positive direction of the Y-axis as an extraordinary ray λe in the birefringent folded crystal plate 5. Propagate in the positive axial direction (Z2). The light rays that have passed through the birefringent crystal plate 5 have the non-reciprocity of 4 due to the same action as in FIG.
Since the electric field oscillation direction is rotated by 90 degrees from the initial direction by the 5 degree rotor 7 and the reciprocal 45 degree rotor 8, (Z
4), the ray incident on the birefringent folded crystal plate 6 is the ordinary ray λo
Is propagated in the birefringent folded crystal plate 6 along the Z axis and is emitted from the light incident / exiting port Port4 (Z5).

Next, referring to FIG. 5, a light beam entering / exiting port
The operation of the optical path (light propagating in the Z-axis negative direction) from the port 4 (Z5) to the light incident / exit port Port1 (Z1) will be described. FIG. 5 is a diagram in which the polarization state of the light beam at each spatial position is observed from the emission side. Beam entrance / exit port Port4 (Z
When light is incident from 5), if the electric field vibration direction of the incident light is set to be parallel to the X axis, it propagates in the birefringent folded crystal plate 6 as the ordinary ray λo in the negative direction of the Z axis ( Z
4). The light ray passing through the birefringent crystal plate 6 does not change the electric field vibration direction when passing through the non-reciprocal 45-degree rotor 7 (Z2) by the same action as in the case of FIG. Therefore, the light propagates in the birefringent folded crystal plate 5 as the ordinary ray λo along the Z axis and is emitted from the ray entrance / exit port Port1 (Z1).

In the above-described operation, the isolation between the light incident / exiting port Port1 and the light incident / exiting port Port3 and the isolation between the light incident / exiting port Port2 and the light incident / exiting port Port4 are the birefringent folded crystal plates 5, 6 (for example, , Calcite) and the separation width of ordinary and extraordinary rays and the extinction ratio of both. The separation width can be widened, for example, by using long calcite. Extinction ratio is 50d for calcite
B or more can be secured. 60 dB for crystals with less distortion
Some show the above. Therefore, high isolation of 50 dB or more can be realized.

In addition, each of the light incident / exiting ports Port1 to Port4
The propagation directions of the light emitted from are all parallel, and in this example, parallel to the Z axis. In the case of light propagating as an extraordinary ray in calcite, the direction of the pointing vector indicating the direction of energy propagation is not parallel to the Z axis, but the direction of the wavefront normal vector is parallel to the Z axis and the refractive index is elliptical. The pointing vector is also parallel to the Z axis when it is emitted into the air due to its body. In this way, since the propagation directions of the light emitted from the respective ports are all parallel, it is possible to couple with the optical waveguide based on the reference line parallel to the Z axis. Therefore, it is possible to easily couple light to the optical waveguide with low loss.

By reversing the rotation direction of either the non-reciprocal 45-degree rotator 7 or the reciprocal 45-degree rotator 8, the light ray entrance / exit port Port2 and the light ray entrance / exit port Port4.
You can switch the positions of.

(Modification) FIG. 6 is a view showing the arrangement of an optical circulator which is a modification of the above embodiment. The optical circulator according to this modification is largely different from the optical circulator according to the above-mentioned embodiment in that the light incident / exit port Po
Lenses L1 and rt1, Port2, Port3 and Port4 respectively
L2, L3, L4 and polarization maintaining optical fibers PF1, PF
2, PF3 and PF4 are provided coaxially.

(Second Embodiment) FIG. 7 is a view showing the arrangement of an optical circulator according to the second embodiment of the present invention. The optical circulator of this example differs from that of the first embodiment (FIG. 1) in that in this example, the reciprocal 45-degree rotator 8 is removed and the crystal of the birefringent crystal plate 6 is used. The point is that the axis is rotated by 45 degrees with respect to the optical axis of the optical system.

The operation of the optical circulator of this example will be described below with reference to FIGS. In these figures, symbols Z1 to Z4 represent spatial positions, and correspond to the spatial positions Z1 to Z4 shown in FIG. 7, respectively. First, referring to FIG. 8, an optical path from the light incident / exiting port port1 to the light incident / exiting port Port2 (Z-axis positive direction propagating light).
The operation will be described. FIG. 8 is a diagram in which the polarization state of the light beam at each spatial position is observed from the incident side. As shown in the figure, when a light beam whose electric field vector vibrates parallel to the X axis is vertically incident from the light incident / exit port Port1 (Z1), this light beam does not change its vibrating direction and the birefringent crystal plate It travels straight as an ordinary ray λo in 5 (Z2). The light ray that has passed through the birefringent crystal plate 5 is rotated 45 degrees clockwise by the non-reciprocal 45-degree rotator 7 (Z3). This vibration direction is a direction that becomes the ordinary ray λo in the birefringent crystal plate 6, so that the light ray that has passed through the non-reciprocal 45-degree rotator 7 passes through the birefringent crystal plate 6 in the positive direction of the Z axis. And is emitted from the light incident / exiting port Port2 (Z4).

Next, referring to FIG. 9, a light beam entrance / exit port.
The operation of the optical path (Z-axis negative direction propagating light) traveling from Port 2 to the light incident / exiting port Port 3 will be described. FIG. 9 is a diagram in which the polarization state of light rays at each spatial position is observed from the emission side. When entering light from the light incident / exiting port Port2 (Z4), if the electric field oscillation direction of the incident light is set to be inclined by 45 degrees counterclockwise from the Y axis, the inside of the birefringent crystal plate 6 will be kept constant. The light ray λo propagates in the negative direction of the Z axis (Z3). The light beam that has passed through the birefringent crystal plate 6 is rotated 45 degrees clockwise by the non-reciprocal 45-degree rotator 7, so that the light vibrating in the Y-axis direction is emitted from the non-reciprocal 45-degree rotator 7. It The emitted light vibrating in the Y-axis direction propagates in the birefringent crystal plate 5 as an extraordinary ray λe while moving in the negative direction of the Y-axis, and the ray entrance / exit port Port3 (Z
It is emitted from 1).

Next, referring to FIG. 10, a light path (Z) from the light incident / exiting port Port3 to the light incident / exiting port Port4.
The operation will be described with respect to the axial positive direction propagation light). Figure 10
It is the figure which observed the polarization state of the light beam in each space position from the emission side. When light is incident from the light incident / exit port Port3 (Z1), the electric field oscillation direction of the incident light is set to be parallel to the Y axis. The light whose vibrating surface is set in this way propagates in the birefringent crystal plate 5 as an extraordinary ray λe. And, by the action of the non-reciprocal 45-degree rotor 7,
The vibration direction of light is rotated 45 degrees clockwise from the Y axis (Z3). This emitted light propagates in the Z axis positive direction while moving in the Y axis negative direction as an extraordinary ray λe in the birefringent crystal plate 6, and is emitted from the ray input / output port Port4 (Z
4).

Next, referring to FIG. 11, an optical path (Z) from the light incident / exiting port Port4 to the light incident / exiting port Port1.
The operation will be described with respect to the negative axial propagation light). FIG. 11 shows
It is the figure which observed the polarization state of the light beam in each spatial position from the emission side. When light is incident from the light incident / exiting port Port4, if the electric field oscillation direction of the incident light is set to be inclined by 45 degrees clockwise from the Y-axis, the inside of the birefringent crystal plate 6 will be regarded as an extraordinary ray λe. Propagate in the negative direction of the axis (Z3). The light beam that has passed through the birefringent crystal plate 6 is rotated 45 degrees clockwise by the non-reciprocal 45-degree rotator 7, so that the light oscillating in the X-axis direction is emitted from the non-reciprocal 45-degree rotator 7. (Z2). This emitted light oscillating in the X-axis direction is Z as an ordinary ray λo in the birefringent crystal plate 5.
Propagate in the negative direction along the axis, and the ray input / output port Port1
It is emitted from (Z1).

In this example, the light beam entering / exiting port
In order to make the separation width between the ports 2-4 in the Y-axis direction equal to the separation width between the light incident / exiting ports Port 1-3, the length of the birefringent crystal plate 6 should be 2 ^1/2 of that of the birefringent crystal plate 5. It should be doubled.

According to the above structure, the same effect as that of the first embodiment can be obtained. Further, since the reciprocal rotator is unnecessary, the optical circulator can be constructed with a smaller number of parts as compared with the first embodiment.

As a modified example, it is possible to provide a polarization maintaining optical waveguide at each port. In this case, since all the light emitted from each port is parallel to the Z axis, a light coupling system with the optical waveguide can be easily mounted.

[0033]

As described above, according to the present invention, it is possible to obtain high isolation and
Optical coupling between each port and the optical waveguide can be further facilitated.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical circulator that is a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the embodiment.

FIG. 3 is a diagram for explaining the operation of the embodiment.

FIG. 4 is a diagram for explaining the operation of the embodiment.

FIG. 5 is a diagram for explaining the operation of the embodiment.

FIG. 6 is a diagram showing a configuration of an optical circulator that is a modified example of the same embodiment.

FIG. 7 is a diagram showing a configuration of an optical circulator that is a second embodiment of the present invention.

FIG. 8 is a diagram for explaining the operation of the embodiment.

FIG. 9 is a diagram for explaining the operation of the embodiment.

FIG. 10 is a diagram for explaining the operation of the embodiment.

FIG. 11 is a diagram for explaining the operation of the embodiment.

FIG. 12 is a perspective view showing a configuration of a conventional polarization-maintaining optical circulator.

[Explanation of symbols]

5 Birefringent crystal plate (first birefringent crystal plate) 6 Birefringent crystal plate (second birefringent crystal plate) 7 Non-reciprocal 45 degree rotor (non-reciprocal rotor) 8 Reciprocal 45 degree rotor (reciprocal rotor) Port1 Light input / output port Port2 Light input / output port Port3 Light input / output port Port4 Light input / output port λo ordinary ray (ray) λe extraordinary ray (ray)

Claims (2)

[Claims] 1. A first birefringent crystal plate and a second birefringent crystal plate, which are arranged at a predetermined interval along a traveling direction of a light beam, and are inserted between the first and second birefringent crystal plates, A reciprocal rotator and a non-reciprocal rotator for rotating the electric field oscillation plane of the light beam by the same angle, and the directions of the respective crystal axes of the first and second birefringent crystal plates coincide with each other. An optical circulator characterized in that 2. A first and a second birefringent crystal plate, which are arranged at a predetermined interval along a traveling direction of a light beam, and are inserted between the first and the second birefringent crystal plates, A non-reciprocal rotator that rotates the electric field vibration surface of the light beam by a predetermined angle, and the directions of the crystal axes of the first and second birefringent crystal plates are different from each other by the predetermined angle. An optical circulator characterized in that