JPH05323234A - Three port type optical circulator - Google Patents

Abstract

PURPOSE:To unnecessitate high precision working of constituent optical elements, to reduce a number of parts, to facilitate assembling and adjustment, to attain low price and to put to practical use. CONSTITUTION:A second port P2, a non-reversing part 10 and a third port P3 are arranged in this order and a first port P1 is arranged in the proximity of the third prot P3. The non-reversing part is constituted by arranging the first and the second wedge-like birefringent prisms 18, 19 so that respective thick and thin parts are confronted to each other. Both birefringent prisms are made of a rutile single crystal having the vertex angle of 11 to 13 degrees. The optical axis of the second birefringent prism 19 rotates by 45 degrees to that of the first birefringent prism 18. A signal light incident from the second port P2 couples to the third port P3 while a signal light with linear polarization from the first port P1 couples to the second port P2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a wedge-shaped birefringent prism made of rutile single crystal and having an apex angle of 11 to 31 degrees, located on both sides of a 45 degree Faraday rotator and having a thick portion of both birefringent prisms. The present invention relates to a three-port type optical circulator having a non-reciprocal portion that is combined with a thin portion so as to face each other.

[0002]

2. Description of the Related Art An optical circulator is a multi-port passive non-reciprocal element used for optical communication systems, optical measurement, etc. and having a function of emitting incident light from a certain port only to another port in a specific direction.

As a typical conventional optical circulator, there is a 4-port type disclosed in Japanese Patent Laid-Open No. 55-93120. This 4-port type optical circulator has a Faraday rotator and a 1/2
This is a configuration in which a wave plate is arranged. Here, the polarization beam splitter is formed by bonding a polarization separation film and a prism whose total reflection surface is processed separately with an adhesive or the like. If the processing accuracy of the polarization beam splitter is poor, the polarization separation film and total reflection surface
A predetermined reflection angle cannot be obtained for the P-polarized light and the S-polarized light, and the coupling between the P-polarized light and the S-polarized light emitted to each port becomes insufficient. Even if the optical axis is adjusted, there is a limit, and the desired characteristics may not be obtained depending on the level of processing accuracy. Therefore, since the polarization beam splitter requires high-precision processing, it is a very expensive device.

Most of the practical optical circulators use only three ports. 4 above
The port type optical circulator does not require one port, which is a great waste considering that the device is expensive. As a 3-port type optical circulator,
There is also an example using a triangular prism-shaped reflector (JP-A-55-117124), but it is difficult to say that it is practical because the structure is complicated and all ports have polarization dependence.

Therefore, recently, a 3-port type optical circulator having a simplified structure using a birefringent plate has been proposed (1990).
Proceedings of the Autumn National Conference of the Institute of Electrical, Information and Communication Engineers C-183). This utilizes the lateral deviation of the optical axis when extraordinary light passes through the birefringent plate whose optical axis forms an angle of about 45 degrees with the plane, and the non-reciprocity of the Faraday rotator. Three
Two birefringent plates (thickness ratio is approximately 1: 2 ^1/2 : 1) are used, and their optical axes are arranged so that they are rotated by 45 degrees around each other, and between them are 45 degrees. Faraday rotator is installed. And 2 on one side of the combination
1 optical fibers are arranged side by side, and a lens is placed on the other side 1
This is a configuration in which a book optical fiber is provided. Each of the three optical fibers serves as a port.

[0006]

However, the above-mentioned three-port type optical circulator uses a parallel-plane birefringent plate as a polarizing element, and therefore has a problem that the port interval is very narrow. This is because the birefringent plate shifts the light rays, that is, the separation distance between the ordinary ray and the extraordinary ray is extremely small. Therefore, fixing the port is not easy. To increase the amount of light beam shift, it is conceivable to increase the thickness of the birefringent plate, but this would not only increase the size of the device but also increase the cost.

Further, in the above structure, since it is difficult to provide a space between the ports, two adjacent optical fibers have a structure in which two optical fiber core wires (outer diameter size 125 μm) are bundled, and one lens is provided for each optical fiber. Cannot be placed individually. Generally, to increase the coupling efficiency, 1
It is desirable to combine one lens with one optical fiber. However, the above-mentioned optical circulator has only one lens incorporated into three optical fibers, so that there is a limit in improving the light coupling efficiency.

The object of the present invention is to eliminate the need for high-precision machining of constituent optical parts, to reduce the number of parts, and to widen the space between adjacent ports, so that assembly adjustment and fixing can be easily performed, and a lens can be attached to each port. It is an object of the present invention to provide an inexpensive and practical 3-port type optical circulator having a high light coupling efficiency because it can be incorporated.

[0009]

SUMMARY OF THE INVENTION According to the present invention, wedge-shaped birefringent prisms made of rutile single crystal and having an apex angle of 11 to 31 degrees are located on both sides of a 45 degree Faraday rotator and have a thick portion and a thin portion. And a non-reciprocal part which are combined so as to face each other, and second and third fiber collimators which are respectively arranged at second and third ports on both sides of the non-reciprocal part, and which are close to the third port. First placed on the first port
It is a 3-port type optical circulator equipped with a fiber collimator. Here, the second and third fiber collimators each have a lens and an optical fiber, and the first fiber collimator has a lens and a polarization-maintaining optical fiber. The signal light incident from the second port passes through the non-reciprocal portion without polarization dependence and is coupled to the third port, and the linearly polarized signal light from the first port passes through the non-reciprocal portion to the second port. Join.

[0010]

The optical signal incident from the second port is collimated by the fiber collimator and is separated into ordinary light and extraordinary light by the first birefringent prism. The polarization plane of the separated light is rotated by 45 degrees by the Faraday rotator, and becomes parallel light by the second birefringent prism.
It is emitted to the port of and is condensed on the optical fiber by the lens. Therefore, when the light is emitted from the second port to the third port, it is polarization independent.

When the linearly polarized signal light enters from the first port, it is refracted by the second birefringent prism and reaches the Faraday rotator. Due to the non-reciprocity of the Faraday rotator, the plane of polarization is rotated by 45 degrees, and the plane of polarization of the first birefringent prism is rotated by 45 degrees with respect to the optical axis of the second birefringent prism. The surface will be orthogonal to the optical axis of the first birefringent prism. That is, ordinary light is converted into extraordinary light, or abnormal light is converted into ordinary light. The optical signal thus converted is refracted by the first birefringent prism and emitted to the second port side. Conversely speaking, the position of the first port is determined so that the converted light is exactly emitted to the second port. Therefore, when the light is emitted from the first port to the second port, it depends on the polarization.

Due to the large apex angle of the birefringent prism, the third
The distance between the port and the first port also increases, and the degree of freedom in fixing the fiber collimator increases. However, if the apex angle is too large, the separation distance between the ordinary ray and the extraordinary ray incident on the third port also increases, making it difficult to combine the two. Therefore, it is effective for the optical circulator to set the apex angle to 11 to 31 degrees.

[0013]

1 shows the structure of a 3-port type optical circulator of the present invention. The second port P ₂ , the non-reciprocal portion 10,
In this structure, the third port P ₃ and the first port P ₁ are arranged in this order. The non-reciprocal portion 10 accommodates a magneto-optical element 14 in a cylindrical permanent magnet 12 to form a Faraday rotator 16, and a first wedge-shaped birefringent prism 18 and a second wedge-shaped birefringent prism 18 are provided on both sides of the Faraday rotator 16. The refraction prism 19 is located, and the first and second birefringence prisms 18 and 19 are arranged so that their thick portions and thin portions face each other. The optical axis of the second birefringent prism 19 is rotated by 45 degrees with respect to the optical axis of the first birefringent prism 18. Both birefringent prisms 18 and 19 are made of rutile single crystal, and the apex angle thereof is set within the range of 11 to 31 degrees.

The magneto-optical element 14 is, for example, a substrate on which an LPE (liquid phase epitaxial growth) film of a magneto-optical crystal is formed. The birefringent prisms 18 and 19 are made of, for example, rutile single crystal, and are cut out with a slope so that the optical axis lies in a plane perpendicular to the incident light beam. The lenses 22, 32, 42 are attached to the respective ports P ₂ , P ₃ , P _1.
The fiber collimators 20, 30, and 40 in which the optical fibers with ferrules 24, 34, and 44 are housed in the metal sleeves 26, 36, and 46 are arranged. Ports P ₂ and P
The optical fibers of the fiber collimators 20 and 30 arranged at ₃ are both single-mode fibers (SMF), and the optical fiber of the fiber collimator 40 arranged at port P ₁ is a polarization maintaining fiber (PMF). ..

The optical signal incident from the second port P ₂ (shown by a solid line) is output to the third port P ₃ without polarization. That is, the incident light from the _second port P ₂ becomes parallel light by the lens 22 of the second fiber collimator 20, and is separated by the first birefringent prism 18 into ordinary light and extraordinary light whose polarization planes are orthogonal to each other. .. Ordinary light is linearly polarized light whose polarization direction is perpendicular to the plane containing the wavefront normal and the optical axis (paper surface in the figure), and extraordinary light is linearly polarized light whose polarization direction is parallel to the plane containing the wavefront normal and the optical axis. is there. This separated light is
The Faraday rotator 16 rotates the plane of polarization by 45 degrees, and the second
The parallel refracting prisms 19 collimate the parallel light, and the collimated light is emitted to the third fiber collimator 30.
Is focused by the optical fiber 34. That is, the light is emitted to the third port. On the other hand, the reflected return light from the opposite direction, which is incident from the third port P ₃ , is separated into ordinary light and extraordinary light by the second birefringent prism 19, and the Faraday rotator 16
The plane of polarization is rotated by 45 degrees due to the non-reciprocity. The polarization plane of the first birefringent prism 18 is the second birefringent prism 19
Since it is rotated by 45 degrees with respect to the optical axis of, the separated light is orthogonal to the optical axis of the first birefringent prism 18. Therefore, in the first birefringent prism 18, the ordinary light refracts as extraordinary light, and the extraordinary light refracts as ordinary light. Therefore, after passing through the first birefringent prism 18, the ordinary light does not become parallel light, and Of the fiber collimator 20 is not reached.

Applying the principle of this reflected return light, the linearly polarized signal light (shown by the dotted line) from the first port P ₁ is made incident on the nonreciprocal portion and is emitted to the second port P ₂ . 1 port P ₁ is arranged. It is assumed that the light incident on the nonreciprocal portion 10 from the first port P ₁ has the same polarization plane as the extraordinary light. The light is refracted by the second birefringent prism 19, and the plane of polarization is rotated by 45 degrees due to the non-reciprocity of the Faraday rotator 16. Since the polarization plane of the first birefringent prism 18 is rotated by 45 degrees with respect to the optical axis of the second birefringent prism 19, the separated light is orthogonal to the optical axis of the first birefringent prism 18. ing. Therefore, the extraordinary light is refracted as the ordinary light in the first birefringent prism 18, is emitted to the fiber collimator 20, and is condensed on the optical fiber 24 by the lens 22.

In this embodiment, the Faraday rotator 16 is used.
Is slightly tilted with respect to the optical axis, which reduces reflected return light.

Dimensions required when assembling the optical circulator of the present invention, that is, as shown in FIG. 1, the apex angle α of the wedge-shaped birefringent prism, the incident angle θ of the incident light from the _first port P _{1 with} respect to the optical axis, distance L1 of the non-reciprocal unit and the first port P ₁ of the optical axis direction, and the third port P ₃ the first port P
An interval L2 of ₁ and a separation distance δ of ordinary light and extraordinary light incident on the third port P ₃ were calculated. Here, the third port P ₃ the first distance L2 port P _1, since the outer diameter is using those 2~2.5mm as fiber collimator is required than 2.5 mm. The distance L1 between the wedge-shaped birefringent prism in the optical axis direction and the first port P ₁ is set to 25.
If the mm, that the third port P ₃ of the first port spacing L2 of P ₁ can be secured more 2.5mm has been confirmed. Therefore, L1 = 25 mm is fixed here. The results are shown in Table 1.

When the apex angle α of the prism is increased, the distance L2 between the third port P ₃ and the first port P ₁ is also increased, and the fixing degree of freedom of the fiber collimator is increased. However, at the same time, the separation distance δ between the ordinary ray and the extraordinary ray incident on the port P ₃ is also increased, so that the ordinary ray and the extraordinary ray are difficult to be coupled, and the coupling efficiency is deteriorated. Since the outer diameter of the current fiber collimator is 2 to 2.5 mm,
The apex angle of the prism must be 21 degrees or more. It is expected that the outer diameter of the fiber collimator will be reduced in the future and the outer diameter dimension will be about 1.0 mm or less, and the apex angle will be within the allowable range up to about 11 degrees.

The apex angle of the wedge-shaped birefringent prism can be easily and accurately processed by using a jig. Compared with the conventional assembly process of the polarization beam splitter of the optical circulator, the manufacturing process is extremely low because the process of processing the polarization separation film and the total reflection surface and the joining of the prisms with an adhesive are unnecessary.

[0021]

[Table 1]

Table 2 shows the results of measuring the isolation characteristics of the optical circulator by arranging each component with the values shown in Table 1 when the apex angle of the wedge-shaped birefringent prism is 21 degrees. The unit is dB.

[Table 2]

The isolation characteristics from the _second port P ₂ to the third port P ₃ and from the third port P ₃ to the first port P ₁ both show high values, while
The insertion loss in the direction from the second port P ₂ to the third port P ₃ and from the first port P ₁ to the second port P ₂ is very small. Therefore, the present invention is applicable to, for example, the second port P.
₂ transmission line, a third port the light receiving element to P _3, the first port P ₁ connects the light emitting element, the first port P ₁ →
It is suitable for bidirectional communication from the second port P _{2 to} the third port P ₃ . Further, it can be applied to optical measurement such as an optical fiber analyzer (OTDR).

[0024]

In the three-port type optical circulator of the present invention, the wedge-shaped birefringent prisms made of rutile single crystal and having an apex angle of 11 to 31 degrees are located on both sides of the 45 degree Faraday rotator, and the thickness of both birefringent prisms is large. Since the structure has a non-reciprocal portion that is combined so that the thin portion and the thin portion face each other,
Since the ports arranged on the same side of the non-reciprocal portion can be widened, the assembly adjustment and fixing can be easily performed, and the light coupling efficiency can be improved because the lens can be incorporated in each port. Further, the vertical angle of the birefringent prism can be easily processed by using a jig, high precision processing is not required, and the structure is simple and the number of parts is small, so that the cost can be reduced.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an embodiment of a 3-port type optical circulator according to the present invention.

[Explanation of symbols]

10 non-reciprocal part 12 permanent magnet 14 magneto-optical element 16 Faraday rotator 18 first birefringent prism 19 second birefringent prism 20 second fiber collimator 30 third fiber collimator 40 first fiber collimator P ₁ first 1st port P ₂ 2nd port P ₃ 3rd port

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[Procedure amendment]

[Submission date] May 28, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

As a typical conventional optical circulator, there is a 4-port type disclosed in Japanese Patent Laid-Open No. 55-93120. This 4-port type optical circulator has a Faraday rotator and a 1/2
This is a configuration in which a wave plate is arranged. Here, the polarization beam splitter is formed by bonding a polarization separation film and a prism whose total reflection surface is processed separately with an adhesive or the like. If the processing accuracy of the polarization beam splitter is poor, the polarization separation film and total reflection surface
A predetermined reflection angle cannot be obtained for the P-polarized light and the S-polarized light, and the coupling between the P-polarized light and the S-polarized light emitted to each port becomes insufficient. Even if the optical axis is adjusted, there is a limit, and desired characteristics may not be obtained depending on the level of processing accuracy. Therefore,
Since the polarization beam splitter requires high-precision processing, it is an extremely expensive device.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0005

[Correction method] Change

[Correction content]

Therefore, recently, a 3-port type optical circulator having a simplified structure using a birefringent plate has been proposed (1990).
Proceedings of the Autumn National Conference of the Institute of Electrical, Information and Communication Engineers C-183). This utilizes the fact that the optical axis shifts laterally when extraordinary light passes through the birefringent plate whose optical axis forms an angle of about 45 degrees with the plane, and the non-reciprocity of the Faraday rotator. Using three birefringent plates (thickness ratio is approximately 1: 2 ^1/2 : 1),
Place by around the rotation 45 degrees their optical axes are optical axes with each other, interposing the respective 45-degree Faraday rotator between them. Then, two optical fibers are juxtaposed on one side of the combined body, and one optical fiber is provided on the other side via a lens. Each of the three optical fibers serves as a port.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0015

[Correction method] Change

[Correction content]

The optical signal incident from the second port P ₂ (shown by a solid line) is output to the third port P ₃ without polarization. That is, the incident light from the _second port P ₂ becomes parallel light by the lens 22 of the second fiber collimator 20, and is separated by the first birefringent prism 18 into ordinary light and extraordinary light whose polarization planes are orthogonal to each other. .. Ordinary light is linearly polarized light whose polarization direction is perpendicular to the plane containing the wavefront normal and the optical axis, and extraordinary light is linearly polarized light whose polarization direction is parallel to the plane containing the wavefront normal and the optical axis. Since the polarization plane of the separated light is rotated 45 degrees by the Faraday rotator 16 and becomes parallel light by the second birefringent prism 19, the separated light is emitted to the third fiber collimator 30 and condensed on the optical fiber 34 by the lens 32. To be done. That is, the light is emitted to the third port.
On the other hand, the reflected return light from the opposite direction incident from the third port P ₃ is separated into ordinary light and extraordinary light by the second birefringent prism 19, and the plane of polarization is 45 degrees due to the non-reciprocity of the Faraday rotator 16. Rotate. First birefringent prism 1
Since the polarization plane of No. 8 is rotated 45 degrees with respect to the optical axis of the second birefringent prism 19, the separated light is orthogonal to the optical axis of the first birefringent prism 18. Therefore, in the first birefringent prism 18, the ordinary light refracts as extraordinary light, and the extraordinary light refracts as ordinary light. Therefore, after passing through the first birefringent prism 18, the ordinary light does not become parallel light, and Of the fiber collimator 20 is not reached.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

In this embodiment, the Faraday rotator 16 is used.
Is slightly tilted with respect to the optical axis , which reduces reflected return light.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0018

[Correction method] Change

[Correction content]

Dimensions required when assembling the optical circulator of the present invention, that is, as shown in FIG. 1, the apex angle α of the wedge-shaped birefringent prism and the optical axis of the incident light from the _first port P _1. Angle of incidence θ, non-reciprocal part in the optical axis direction and the first port P
The distance L1 of _1, the distance L2 of the third port P ₃ and the first port P ₁ , and the separation distance δ of the ordinary light and the extraordinary light incident on the third port P ₃ were calculated. Where the third port P
₃ a first distance L2 port P _1, since the outer diameter is using those 2~2.5mm as fiber collimator is required than 2.5 mm. When the distance L1 between the wedge-shaped birefringent prism in the optical axis direction and the first port P ₁ is set to 25 mm, the distance L2 between the third port P ₃ and the first port P ₁
It has been confirmed that the gap can be secured at 2.5 mm or more. Therefore, L1 = 25 mm is fixed here. The results are shown in Table 1.

[Procedure Amendment 6]

[Document name to be amended] Statement

[Name of item to be corrected] 0022

[Correction method] Change

[Correction content]

Table 2 shows the results of measuring the isolation characteristics of the optical circulator by arranging each component with the values shown in Table 1 when the apex angle of the wedge-shaped birefringent prism is 21 degrees. The unit is dB.

[Table 2]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Ikuo Maeda 5 36-11 Shinbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. In the company

Claims (1)

[Claims] 1. A rutile single crystal having an apex angle of 11 to 31.
The wedge-shaped birefringent prisms are positioned on both sides of the 45-degree Faraday rotator and are combined so that their thick and thin portions face each other. Second and third fiber collimators arranged at the second and third ports on both sides of the reciprocal portion, and a first port having a lens and a polarization-maintaining optical fiber and being close to the third port. And a first fiber collimator arranged, the signal light incident from the second port is coupled to the third port through the non-reciprocal portion without polarization dependence, and the linearly polarized signal from the first port is provided. A three-port type optical circulator characterized in that light is coupled to the second port through the non-reciprocal portion.