JPH0534633A - Optical circulator - Google Patents

Abstract

PURPOSE:To obtain the high isolation of optical circulator being independent upon polarization having a 4-terminal circuit used only double refraction crystals. CONSTITUTION:Double refraction crystals 10-1, 10-2 are provided at both sides of a non-reciprocal rotary polarization element 11 and the double refraction crystal 10-1 is provided with terminal modules 9-1, 9-3 and the double refraction crystal 10-2 is provided with terminal modules 9-2, 9-4 and by controlling the advancing direction of two optical beams being the same polarization direction from each terminal module, two 3-terminal circuits are combined to prepare a 4-terminal circuit.

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 with low loss and high isolation.

[0002]

2. Description of the Related Art In order to realize an optical circulator with low loss and high isolation, an error of each constituent element is made extremely small, or a structure in which the error and the influence of the error are extremely small is applied. There is a need to. Up to now, various types of optical circulators have been proposed and their effects have been confirmed for the purpose of structurally realizing low loss and high isolation.

However, in order to obtain higher isolation, it is necessary to reduce the error of each constituent element as well as to devise the structure. The main factors affecting isolation are incomplete polarization separation and polarization rotation errors. The degree of incomplete polarization separation depends on the type of polarization separation element, and in a polarization beam splitter composed of a dielectric multilayer film used in a normal optical circulator,
The extinction ratio is only about 20 to 30 dB. On the other hand, it is known that the use of a birefringent crystal enables polarization separation with an extinction ratio of about 60 dB.

However, while the polarization beam splitter made of the dielectric multilayer film described above operates as a 4-terminal circuit such as a directional coupler, the birefringent crystal operates as a 3-terminal circuit such as a Y-branch. Therefore, when this is applied to an optical circulator, a complete optical circulator cannot be conventionally constructed as shown in the example below. That is, the conventional birefringent crystal that performs polarization separation is limited to the quasi-optical circulator operation.

FIG. 7 shows the structure of a polarization-independent optical circulator with low loss and high isolation, which performs polarization separation with a conventional birefringent crystal (Japanese Patent Application No. 3-46849 "Optical Circuit Element"). In FIG. 7, 1-1 to 1-3 are birefringent crystals, 2 is a birefringent crystal, 3-1, 3-2 are reciprocal optical rotatory elements, 4 is a reciprocal optical rotatory element, 5 is a reciprocal optical rotatory element, and 6 is a non-reciprocal optical element. It is a reciprocal optical rotation element. The unpolarized light input to the birefringent crystal 1-1 is separated by the birefringent crystal 1-1 into two linearly polarized beams whose polarization directions are orthogonal to each other. The polarization azimuth of one of the linearly polarized beams is rotated by 90 degrees by the reciprocal optical rotation element 3-1 and both of the two light beams are incident on the birefringent crystal 2 as ordinary rays of the birefringent crystal 2. Therefore, the two light beams described above go straight through the birefringent crystal 2, and one light beam passes through the reciprocal optical rotation element 4 and the non-reciprocal optical rotation element 6, so that there is no rotation of the polarization azimuth and the other light beam reciprocates. Since the light passes through the optical rotatory element 5 and the non-reciprocal optical rotatory element 6, the polarization direction rotates by 90 degrees. As a result, the polarization directions of the two light beams are orthogonal to each other and are combined by the birefringent crystal 1-2.
On the other hand, the non-polarized light input to the birefringent crystal 1-2 is separated by the birefringent crystal 1-2 into two linearly polarized beams whose polarization directions are orthogonal to each other. Since one of the linearly polarized light beams passes through the reciprocal optical rotation element 4 and the non-reciprocal optical rotation element 6, the polarization direction is rotated by 90 degrees, and the other light beam passes through the reciprocal optical rotation element 5 and the non-reciprocal optical rotation element 6. There is no rotation of the polarization direction. As a result, the two light beams are
The extraordinary ray of the birefringent crystal 2 enters the birefringent crystal 2. The above-mentioned two light beams travel in the birefringent crystal 2 in the left oblique direction, and exit the birefringent crystal 2 from the point different from the point where the light beam from the birefringent crystal 1-1 enters. Since one of these light beams passes through the reciprocal optical rotation element 3-2 and the polarization direction is rotated by 90 degrees, the polarization directions of the two light beams are orthogonal and are combined by the birefringent crystal 1-3. .. In this way, the circuit shown in FIG. 7 operates in a non-reciprocal manner. Note that the terminals of the birefringent crystal 1-3 can be birefringent by using the same polarization rotation mechanism (composed of the reciprocal optical rotation elements 4 and 5 and the non-reciprocal optical rotation element 6) as the terminal of the birefringent crystal 1-2. It is obvious that the light input from the refraction crystal 1-3 is output from the other fourth terminal.

[0006]

However, in the optical circuit having the structure shown in FIG. 7, the light beam passes through the birefringent crystal 2.
A ray can go straight, and an extraordinary ray can go diagonally to the left.
The light travels in a zigzag manner, and in the case of the N-terminal optical circulator, there is a problem that there is no path of light from the N-th terminal to the first terminal and it can operate only as a quasi-optical circulator.

An object of the present invention is to realize a polarization-independent optical circulator with low loss and high isolation by using only a birefringent crystal capable of obtaining a high extinction ratio as a polarization separation element.

[0008]

The structure of the present invention for achieving the above object is a polarization-independent four-terminal circulator, which comprises a birefringent crystal and first and second non-reciprocal optical rotators. One surface of the first non-reciprocal optical rotator is arranged so as to face the other surface of the birefringent crystal where an ordinary ray is emitted from a predetermined position on one surface of the birefringent crystal. At the same time, one surface of the second non-reciprocal optical rotator is arranged so as to face the other surface of the birefringent crystal where the extraordinary ray exits, and the light incident on the birefringent crystal has a polarization direction. A terminal module that emits the same two light beams is formed, and four terminal modules are provided. The polarization directions of the light emitted by the first and fourth terminal modules are the same as those of the birefringent crystal that constitutes this terminal module. And the second and The polarization direction of the light emitted by the third terminal module is made to coincide with the extraordinary ray of the birefringent crystal that constitutes this terminal module, and further, the two birefringent crystals and the non-reciprocal optical rotator are connected to the first birefringent optical element. A refraction crystal, a non-reciprocal optical rotation element, and a second birefringent crystal are arranged in this order, the first and third terminal modules are arranged on the side of the first birefringent crystal, and the second and fourth terminals are arranged. Module as above
The birefringent crystal of each of the first, second, third and fourth terminal modules and the ordinary rays of the first and second birefringent crystals disposed on the birefringent crystal side of It is characterized by matching with a ray or an extraordinary ray.

[0009]

In a conventional polarization-independent optical circulator using only a birefringent crystal as a polarization splitting element, the traveling directions of two light beams with the same polarization direction from each terminal are caused by a single birefringent crystal. Had control. As long as light is incident at the same incident angle, the traveling direction of light in the birefringent crystal is
Since there are only two directions, that is, the direction in which the ordinary ray travels and the direction in which the extraordinary ray travels, as a result, in the optical circulator having such a configuration, the light coupling does not circulate completely between the terminals but circulates. It is limited to the operation of the quasi-optical circulator in which one of the couplings between the terminals is not performed. On the other hand, in the optical circulator of the present invention, the traveling directions of two light beams having the same polarization direction from each terminal are controlled by the nonreciprocal optical rotator and two birefringent crystals arranged on both sides thereof. As a result, a circuit for controlling the traveling direction of the light beam, which is a combination of two circuits with three terminals and acts as a circuit with four terminals, can be configured so that the link between the terminals circulates, and a complete optical circulator operation is achieved. Is realized.

[0010]

1 is a block diagram of a four-terminal optical circulator of an embodiment of the present invention, in which 7-1 to 7-4 are optical fibers and 8
-1 to 8-4 are lenses, 9-1 to 9-4 are terminal modules, 10-1 and 10-2 are birefringent crystals, and 11 is a non-reciprocal optical rotatory element. FIG. 2 shows the terminal module 9-1 shown in FIG.
9-9 is a diagram showing the configurations of optical fibers 7-1 to 7-4 and lenses 8-1 to 8-4, and 12-1 of the terminal modules 9-1 to 9-4. 12-4 are birefringent crystals, 13-1 to 13-4 are non-reciprocal optical rotators, 14-1
14-4 are non-reciprocal optical rotation elements.

For the following description, the birefringent crystals 10-1 and 10-2 shown in FIG. 1 have different lengths,
In both cases, the polarized light in the direction parallel to the paper surface of FIG. 1 is assumed to be an extraordinary ray, and the optical path of the extraordinary ray is moved to the right when viewed from the direction in which the light is directed. In addition, the non-reciprocal optical rotation element 11
Does not rotate the polarization direction of the light traveling from the birefringent crystal 10-1 to the birefringent crystal 10-2, but rotates the polarization direction of the light traveling from the birefringent crystal 10-2 to the birefringent crystal 10-1 by 90 degrees. I shall. Similarly, the terminal modules 9-1 to 9-4
The birefringent crystals 12-1 to 12-4 that compose are assumed to have polarized light in the direction parallel to the paper surface of FIG. 2 as an extraordinary ray and move the optical path of the extraordinary ray to the right when viewed from the direction in which the light is directed. To do. Further, the non-reciprocal optical rotation elements 13-1 to 13-4 do not rotate the polarization direction of the light from the birefringent crystals 12-1 to 12-4, and the light traveling toward the birefringent crystals 12-1 to 12-4 is not rotated. It is assumed that the polarization direction is rotated by 90 degrees. On the other hand, the non-reciprocal optical rotation elements 14-1 to 14-4 are the birefringent crystals 12-1 to 12-.
The polarization direction of the light from 4 is rotated by 90 degrees, and the birefringent crystal 12
It is assumed that the polarization directions of the light traveling from -1 to 12-4 do not rotate.

As is apparent from FIG. 2, the terminal modules 9-1 and 9-4 have the same structure, and the terminal modules 9-2 and 9-3 have the same structure. Terminal module 9
-2, 9-3 is different from the terminal modules 9-1 and 9-4 in the configuration in that the positions of the non-reciprocal optical rotation elements 13-1 to 13-4 and the non-reciprocal optical rotation elements 14-1 to 14-4 are different. It has been replaced. As a result, the terminal module 9-1
In (9-4), the light emitted from the optical fiber 7-1 (7-4) is polarized and separated into two light beams by the birefringent crystal 12-1 (12-4), and then the non-reciprocal Optical rotation element 13-1
(13-4), or the non-reciprocal optical rotation element 14-1 (1
4-4), both light beams are emitted with their polarization directions parallel to the plane of the paper of FIG. On the contrary, two light beams whose polarization directions are perpendicular to the paper surface of FIG. 2 enter the non-reciprocal optical rotation element 13-1 (13-4) or the non-reciprocal optical rotation element 14-1 (14-4). Birefringent crystal 12-1
When going to (12-4), the birefringent crystal 12-1
The optical fiber 7-1 (7-
It is clear that it binds to 4). On the other hand, in the terminal module 9-2 (9-3), the above-mentioned non-reciprocal optical rotation elements 13-2 and 13-3 and non-reciprocal optical rotation elements 14-2 and 1-3.
The optical fiber 7-2 (7-
The light emitted from 3) and polarized and separated into two light beams by the birefringent crystal 12-2 (12-3) is the non-reciprocal optical rotation element 13-2 (13-3) or the non-reciprocal optical rotation element. 14
-2 (14-3), both light beams,
The polarization direction is emitted perpendicular to the plane of the paper of FIG.
Further, two light beams whose polarization directions are parallel to the paper surface of FIG. 2 are generated by the non-reciprocal optical rotation element 13-2 (13-3), or
When the light enters the non-reciprocal optical rotation element 14-2 (14-3) and goes toward the birefringent crystal 12-2 (12-3), it is multiplexed by the birefringent crystal 12-2 (12-3), Optical fiber 7
-2 (7-3).

The forward operation of the optical circulator of this embodiment will be described with reference to FIG. Symbols 0 and e in FIG. 3 represent ordinary rays and extraordinary rays of the birefringent crystals 10-1 and 10-2, respectively. Birefringent crystal 10-1 of FIG.
10-2, the non-reciprocal optical rotation element 11 is a terminal module 9-
Since the two light beams emitted from 1 to 9-4 act in the same manner, only one of the light paths needs to be examined.
In FIG. 3, the optical path is shown from such a viewpoint. Now, assuming that the planes of FIG. 1 and FIG. 2 are parallel, the terminal module 9-1 (9-4) is
Light having a polarization direction parallel to the paper surface of FIG. 1 (the birefringent crystal 10-
1, 10-2 ordinary rays), and conversely, light whose polarization direction is perpendicular to the plane of FIG. 1 (birefringent crystals 10-1, 10-).
When the extraordinary ray 2) goes to the optical fiber 7-1 (7-4), the extraordinary ray is directed to the optical fiber 7-1 (7-4).
Join to.

Further, the terminal module 9-2 (9-3)
Emits an extraordinary ray of the birefringent crystals 10-1 and 10-2, and couples an ordinary ray toward the optical fiber 7-2 (7-3) to the optical fiber 7-2 (7-3).

The polarization direction of the light emitted from the optical fiber 7-1 and passing through the terminal module 9-1 is parallel to the paper surface of FIG. 3, and goes straight through the birefringent crystal 10-1. This light is
Since the polarization azimuth does not rotate even after passing through the non-reciprocal optical rotatory element 11, the birefringent crystal 10-2 goes straight and the polarization azimuth is changed as shown in FIG.
While continuing to be parallel to the paper surface of, proceed to the terminal module 9-2. Therefore, this light is coupled into the optical fiber 7-2. The light emitted from the optical fiber 7-2 and passing through the terminal module 9-2 is an extraordinary ray, which is moved rightward along the optical axis thereof by the birefringent crystal 10-2. When this light passes through the non-reciprocal optical rotation element 11, the polarization azimuth rotates by 90 degrees, so that it becomes an ordinary ray, goes straight through the birefringent crystal 10-1, and goes to the terminal module 9-3. Therefore, this light is transmitted by the optical fiber 7
Bind to -3. The light emitted from the optical fiber 7-3 and passing through the terminal module 9-3 is an extraordinary ray, which is moved rightward along the optical axis thereof by the birefringent crystal 10-1. Since this light does not rotate in the polarization direction even after passing through the non-reciprocal optical rotation element 11, the birefringent crystal 10-2 also has its optical axis moved to the right, and remains as an extraordinary ray in the terminal module 9 as well.
Go to -4. Therefore, this light is transmitted through the optical fiber 7-4.
Join to. The light emitted from the optical fiber 7-4 and having passed through the terminal module 9-4 is an ordinary ray, and goes straight through the birefringent crystal 10-2. This light passes through the non-reciprocal optical rotation element 11 and its polarization azimuth rotates by 90 degrees, so that it becomes an extraordinary ray, which is moved to the right on the optical axis by the birefringent crystal 10-1 and advances to the terminal module 9-1. .. Therefore, this light is coupled into the optical fiber 7-1.

As described above, the light input from the optical fiber 7-1 is output to the optical fiber 7-2, the light input from the optical fiber 7-2 is input to the optical fiber 7-3, and the optical fiber 7 is input. The light input from -3 enters the optical fiber 7-4,
The light input from the optical fiber 7-4 is output to the optical fiber 7-1. Therefore, the optical circuit configured as shown in FIG. 2 operates as a complete optical circulator.

Next, the optical circulator of this embodiment is
The polarization separation in the birefringent crystals 10-1 and 10-2 is complete and sufficient.
High isolation can be achieved if done in minutes
Will be described with reference to FIG. In other words, non-reciprocal
Optical rotation elements 13-1 to 13-4 or 14-1 to 14
-4, and the polarization rotation in the non-reciprocal optical rotation element is 0 °
If it is done without error, such as 90 degrees, it is not a problem at all.
High isolation can be achieved, but below 0 ° or 90 °
When α is outside, an error occurs, and for example, the optical path shown in FIG. 4 does not occur.
It As an example of the description, the optical fiber 7 of the optical fiber 7-4
Consider isolation from -1. Figure 4 is non-reciprocal
Optical rotation element 13-1 (non-reciprocal optical rotation element 14-1) and non-reciprocal
If the optical rotation element 11 has a polarization rotation error,
It shows the optical path of the bad light rays. Such a ray
There are three, non-reciprocal optical rotation element 13-1 (non-reciprocal optical rotation element 1
4-1) error of ray b, error of non-reciprocal optical rotation element 11
Rays a and c are generated in the terminal module 9-4
As is clear from FIG. 4, the non-reciprocal optical rotator
13-1 (non-reciprocal optical rotation element 14-1) and non-reciprocal optical rotation element
11 is the ray c resulting from both polarization rotation errors
Then, the optical power is emitted from the terminal module 9-1.
Based on the optical power²α)²To the extent of
It However, in this case, the non-reciprocal optical rotation element 13-1 (non-reciprocal
Polarization rotation of anti-optical rotatory element 14-1) and non-reciprocal optical rotatory element 11
It is assumed that the errors are equal and α. Moreover, this is Tsunemitsu
Since it is a line, the nonreciprocal optical rotation element 13-1 (nonreciprocal optical rotation element
14-1) has no error in polarization rotation, the optical fiber
Does not bind to Iva 7-4. Again, the non-reciprocal optical rotation element 1
3-4 (non-reciprocal optical rotation element 14-4)
If α, then (sin ²α)³Light power
Injury is coupled to the optical fiber 7-4.

Among conventional optical circulators using a polarization beam splitter made of a dielectric multilayer film, the isolation of a normal one is about (sin ² α) even if the deterioration due to the extinction ratio of the polarization beam splitter is ignored. Even with the high isolation type, it was about (sin ² α) ² . Therefore, the optical circulator of the present embodiment improves the extinction ratio of the polarization separation element and structurally significantly improves the influence of the polarization rotation error of its isolation as compared with the conventional one. Isolation of Optical Fiber 7-1 from Optical Fiber 7-2, Optical Fiber 7
It can be shown that the isolation from the optical fiber 7-3 of -2 and the isolation from the optical fiber 7-4 of the optical fiber 7-3 are similar.

In the embodiment shown in FIG. 1, the ordinary ray (extraordinary ray) of the birefringent crystal 10-1 and the birefringent crystal 10-2 coincide with each other, and the optical axis of the extraordinary ray moves in the same direction. did. However, this condition is not always necessary in the operation of the present invention, and the ordinary ray of the birefringent crystal 10-1 may coincide with the ordinary ray or the extraordinary ray of the birefringent crystal 10-2. FIG. 5 shows such a birefringent crystal 1.
This is an example showing that even when the condition of 0-1 and the ordinary ray (extraordinary ray) of the birefringent crystal 10-2 is changed, it functions normally. In this example, the birefringent crystal 10-1 and the birefringent crystal 10
The ordinary ray (extraordinary ray) of -2 is the same, but the direction in which the optical axis of the extraordinary ray moves is opposite because the birefringent crystal 10-1 is reversed from the case of FIG. Also in this case, as is apparent from FIG. 5, the coupling of light is performed from the optical fiber 7-1 to the optical fiber 7-2, from the optical fiber 7-2 to the optical fiber 7-3, and also to the optical fiber 7-. 3 to the optical fiber 7-4, and further from the optical fiber 7-4 to the optical fiber 7-1, and operates as an optical circulator. Further, its operation is basically the same as that of the embodiment of FIG. 1, so that high isolation can be obtained.

Further, when the conditions of the birefringent crystals 10-1 and 10-2 are changed and the ordinary ray of the birefringent crystal 10-1 corresponds to the extraordinary ray of the birefringent crystal 10-2 as shown in FIG. Is also shown to work properly. Symbol A in FIG.
If the positions of B, C, and D are made to correspond to the positions of the light emitted from the optical fibers 7-1, 7-2, 7-3, and 7-4 in FIG. It is obvious that a high isolation optical circulator can be realized.

As described above, according to the present invention, the traveling directions of two light beams having the same polarization direction from each terminal are determined by the nonreciprocal optical rotator and two birefringent crystals arranged on both sides thereof. Have control. As a result, the two 3-terminal circuits are coupled via the non-reciprocal optical rotation element that adjusts the polarization azimuth depending on the traveling direction of light, so that a 4-terminal circuit-like operation can be realized. As a result, a complete optical circulator can be realized by using a birefringent crystal having a high extinction ratio. Further, in the optical circulator of the present invention, light passes through three non-reciprocal optical rotation elements from one terminal to another terminal, but these three non-reciprocal optical elements are provided except between the terminals operating in the forward direction. Light is coupled only when there is a rotation error of the polarization direction in the optical rotatory element. That is, the optical circulator of the present invention can realize high isolation regardless of the rotation error of the polarization direction.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a polarization-independent optical circulator.

FIG. 2 is a diagram showing a configuration example of a terminal module.

FIG. 3 is a diagram showing that the optical path in the embodiment of FIG. 1 circulates.

FIG. 4 is a view for explaining that the embodiment of FIG. 1 can achieve high isolation.

FIG. 5 is a diagram showing that an optical path circulates when another birefringent crystal is arranged.

FIG. 6 is a diagram showing that the optical path is circulated in the case of disposing another birefringent crystal.

FIG. 7 is a diagram showing a configuration example of a conventional polarization-independent optical circulator.

[Explanation of symbols]

9-1 to 9-4 Terminal module 10-1, 10-2, 12-1 to 12-4 Birefringent crystal 11, 13-1 to 13-4, 14-1 to 14-4 Non-reciprocal optical rotation element

Claims (1)

Claim: What is claimed is: 1. A polarization-independent four-terminal circulator comprising a birefringent crystal and first and second non-reciprocal optical rotators, wherein one surface of the birefringent crystal is predetermined. One surface of the first non-reciprocal optical rotator is arranged to face the other surface of the birefringent crystal where the ordinary ray is emitted by the incident light from the position, and the extraordinary ray is emitted. One terminal of the second non-reciprocal optical rotator is arranged at the position of the other surface of the refraction crystal so as to face it, and the light incident on the birefringent crystal is emitted as two light beams having the same polarization direction. A module is formed, and four terminal modules are provided.
The polarization direction of the light emitted by the terminal module is matched with the ordinary ray of the birefringent crystal forming the terminal module, and the polarization directions of the light emitted by the second and third terminal modules are the same. While making the extraordinary ray of the above-mentioned birefringent crystal that composes the above, further dispose two birefringent crystals and a non-reciprocal optical rotation element in the order of a first birefringent crystal, a non-reciprocal optical rotation element and a second birefringent crystal Then, the first and third terminal modules are arranged on the first birefringent crystal side, the second and fourth terminal modules are arranged on the second birefringent crystal side, and the first and second terminal modules are arranged on the first birefringent crystal side. , Second, third and fourth
And the above-mentioned first and second birefringent crystals in each terminal module of
An optical circulator characterized in that the ordinary ray with the birefringent crystal of (1) is made to coincide with an ordinary ray or an extraordinary ray, respectively.