JPH05257084A - Optical isolator - Google Patents

Abstract

PURPOSE:To prevent reflected light from being generated without depending upon the wavelength of input light. CONSTITUTION:The sum of the polarization rotation angles of a 1st Faraday rotator 34 and a 2nd Faraday rotator 35 is 45 deg. and the sum of the differential values of the angle of polarization rotation by the 1st Faraday rotator 34 and the angle of polarization rotation by the 2nd Faraday rotator 35 corresponding to reference wavelength is zero; and the sum of the polarization rotation angles of a 3rd Faraday rotator 36 and a 4th Faraday rotator 37 is 45 deg. and the sum of the differential values of the angle of polarization rotation by the 4th Faraday rotator 37 and the angle of polarization rotation by the 4th Faraday rotator 37 corresponding to the reference wavelength is zero.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator.

[0002]

2. Description of the Related Art Conventionally, in optical communication using a semiconductor laser as a light source, a writable video disk or the like, an optical isolator has been used as a device for preventing reflected light from the end faces of optical fibers, lens systems and connectors.

FIG. 2 shows an example of a conventional optical isolator. In the figure, 1 and 2 are birefringent crystal elements, 3 is a half-wave plate,
Reference numeral 4 is a Faraday rotator (magneto-optical crystal), 5 is a permanent magnet, 6 is a case, 7 is a light input section, and 8 is a light output section.

The state of light propagation in this optical isolator will be described with reference to FIG. FIG. 3A shows the state of polarization when light is incident in the forward direction. In this case, the input light 10 input from the light input unit 7 passes through the birefringent crystal element 1 and is then an ordinary light component 11 that is a polarization component that is not refracted.
a and the extraordinary light component 11b which is the refracted polarization component 2
It is split into two orthogonal polarizations. These two light components 11
a and 11b have passed through the half-wave plate 3, whereby their polarization directions have been rotated by 45 degrees (12a and 12b, respectively).
b) Further, it passes through the Faraday rotator 4 to which the magnetic field A in the direction of the arrow is applied by the permanent magnet 5 (not shown here), thereby rotating its polarization direction by 45 degrees (1
3a, 13b). Therefore, the polarization directions of the lights 13 a and 13 b output from the Faraday rotator 4 are deviated from the input light 10 by 90 degrees. The birefringent crystal element 2 is arranged such that its crystal axis is shifted by 90 degrees with respect to the birefringent crystal element 1, and the lights 13 a and 13 b are respectively an extraordinary light component and an ordinary light component with respect to the birefringent crystal element 2. Therefore, they are combined and output as the output light 14. This output light 1
4 has a polarization direction rotated by 90 degrees with respect to the input light 10.

FIG. 3B shows the case where light is incident in the opposite direction. In this case, the input light 20 passes through the birefringent crystal element 2 and then enters the ordinary light components 21a and 21b.
It is split into two orthogonal linearly polarized lights. The two light components 21a and 21b pass through the Faraday rotator 4, which rotates their polarization directions by 45 degrees (22
a, 22b) and further through the half-wave plate 3, whereby the polarization direction is rotated by 45 degrees (23a, 23b).
b). Here, the rotation direction of the polarization direction by the Faraday rotator 4 is determined by only the direction of the magnetic field A regardless of the incident direction of light because of the non-reciprocity of the Faraday rotator 4.
In this case, the direction of rotation by the Faraday rotator 4 and the direction of rotation by the half-wave plate 3 are opposite. Therefore, the half-wave plate 3
The polarization directions of the lights 23a and 23b output from the optical components are not rotated with respect to the lights 21a and 21b output from the birefringent crystal element 2. The output lights 23a and 23b of the half-wave plate 3 become an ordinary light component and an extraordinary light component with respect to the birefringent crystal element 1, so that the light 23a goes straight (24a), and the light 23b is incident in the forward direction. The position is changed in the opposite direction (24b). The output lights 24a and 24b are respectively output to positions different from the position 25 when the light is incident in the forward direction.

Therefore, if the light input section 7 in FIG. 2 is formed into a pinhole, it is possible to block the input light in the opposite direction, that is, the reflected light. In this way, the optical isolator only allows light to pass in the forward direction and does not depend on the polarization of the incident light.

FIG. 4 shows the path of light in the optical isolator. FIG. 4A shows the case where the light is incident in the forward direction, and FIG. 4B shows the case when the light is incident in the reverse direction. Each one is shown. In the figure, ● (black circle) indicates an extraordinary light component, and | (vertical line) indicates an ordinary light component.

As shown in the figure, the light input from the light input unit 7 is output to the light output unit 8 regardless of the polarization direction. However, the light input from the light output unit 8 is in any polarization direction. Is also output to a position deviated from the light input unit 7. It can be said that this optical isolator utilizes not the complete blocking of light but the change of the spot position of light.

In another conventional example, the half-wave plate 3 is not used and two birefringent crystal plates are combined instead of the birefringent crystal element 2 so that the polarization directions differ by 90 degrees.
One that synthesizes two separated lights (Japanese Patent Publication Sho 5)
8-28561).

[0010]

By the way, the Faraday rotation angle of the Faraday rotator 4 has wavelength dependence as shown in FIG. Therefore, when the wavelength of the input light changes in the optical isolator, the polarization rotation angle by the Faraday rotator 4 deviates from 45 degrees, and the light returns to the light input unit 7, that is, the isolation sharply decreases. There was a problem that it did.

The object of the present invention is to solve the above problems.
An object of the present invention is to provide an optical isolator capable of preventing reflected light without depending on the wavelength of input light.

[0012]

In order to achieve the above object, in a first aspect of the present invention, a first polarization separation element for separating and combining two orthogonal polarizations and a second polarization separating element for separating and combining two orthogonal polarizations. And a third polarization separation element that separates and combines two orthogonal polarizations, and a non-reciprocal second polarization separation element is provided between the first polarization separation element and the second polarization separation element. One polarization rotator and a non-reciprocal second polarization rotator made of a material different from that of the first polarization rotator are arranged, and the second polarization separation element and the third polarization rotator are arranged.
A non-reciprocal third polarization rotator and a non-reciprocal fourth polarization rotator made of a material different from that of the third polarization rotator. , The optical axis of the second polarization splitting element is rotated by 45 degrees with respect to the first polarization splitting element, and the third polarization splitting element is rotated with respect to the optical axis of the first polarization splitting element. 90 degrees, the thickness of the first polarization separation element is the same as that of the third polarization separation element, and the thickness of the second polarization separation element is √ of the first polarization separation element. It is double.

According to a second aspect, the sum of the polarization rotation angles of the first polarization rotator and the second polarization rotator is 45 degrees,
And the sum of the differentials of the polarization rotation angle by the first polarization rotator and the polarization rotation angle by the second polarization rotator at the reference wavelength is zero, and the third polarization rotator and the fourth polarization rotator The sum of the polarization rotation angles is 45 degrees, and the sum of the polarization rotation angle by the third polarization rotator and the differentiation of the polarization rotation angle by the fourth polarization rotator at the reference wavelength is zero.

According to a third aspect of the present invention, an incident optical fiber, a first lens for collimating the light emitted from the incident optical fiber into a parallel beam, and a first polarization separation element made of a tapered birefringent material. , A non-reciprocal first polarization rotator that rotates the polarized light at a reference wavelength by α degrees, a non-reciprocal second polarization rotator that rotates the polarized light at a reference wavelength by β degrees, and a taper A second polarization separation element made of a birefringent material, a second lens for focusing parallel light emitted from the second polarization separation element on a light reception optical fiber, and a light reception optical fiber. The top and bottom of the tapered first polarization separation element face the bottom and the top of the tapered second polarization separation element, respectively, and the corresponding surfaces are installed parallel to each other. Is the optical axis of the first polarization separation element It is rotated by 45 degrees with respect to, and the forward light beam from the incident optical fiber is focused on the light receiving optical fiber when focused by the second lens, and the backward light beam is focused by the first lens. When tied, the deflection angles of the light rays were set so that the ordinary ray and the extraordinary ray were separated from each other by at least the diameter of the incident optical fiber.

According to a fourth aspect, in the third aspect, the polarization rotation angle by the first polarization rotator and the second polarization rotator is 45 degrees (α + β = 45), and the first polarization rotator and The sum of the differentials of the polarization rotation angle characteristics of the second polarization rotator at the reference wavelength was set to zero.

According to a fifth aspect of the present invention, a first polarization separation element made of a tapered birefringent material, a non-reciprocal first polarization rotator for rotating polarized light by α degrees at a reference wavelength, and a reference. A first unit provided with a second polarization rotator having a non-reciprocal property, which rotates polarized light with respect to a certain wavelength, and a second polarization separation element made of a tapered birefringent substance. And a third polarization separation element made of a tapered birefringent material, a third polarization rotator that rotates the polarized light at a reference wavelength by α degrees, and a polarized light that rotates by β degrees with respect to the reference wavelength. A second polarization rotator and a fourth polarization separation element made of a tapered birefringent material
The units are arranged in series, and the taper inclination directions of the first and second polarization separation elements and the taper inclination directions of the third and fourth polarization separation elements are shifted by 90 degrees.

[0017]

According to the first aspect, the first polarization rotator and the second polarization rotator are arranged in series, the sum of the polarization rotation angles of the two polarization rotators is set to 45 degrees, and at the reference wavelength. Since the sum of the differentials of the polarization rotation angles of the two polarization rotators is set to zero, the polarization rotation angle after passing through the two polarization rotators is close to the reference wavelength even if the wavelength of the incident light changes. Four
It is possible to configure an isolator that does not change from 5 degrees and has no wavelength dependence.

According to the second aspect, a two-stage optical isolator can be easily constructed by using the second polarization separation / combination element, and high isolation can be achieved.

According to the third aspect, the light incident in the forward direction is separated into ordinary light and extraordinary light by the first tapered polarization separation element, and is refracted in different directions. Further, rotated by 45 degrees from the first and second polarization rotators,
It is output in parallel by the second tapered polarization separation element.
The output light is focused by a lens and coupled into an optical fiber.
The light incident in the opposite direction is separated into ordinary light and extraordinary light by the second tapered polarization separation element and refracted in different directions. Further, the second and first polarization rotators rotate 45 degrees in the same direction as when the light enters in the forward direction, and the ordinary light and the extraordinary light are refracted in different directions by the first tapered polarization separation element, and are output. In this case, it cannot be focused by the lens and coupled to the optical fiber.

According to the fourth aspect, when light having a wavelength different from the reference wavelength is input, the polarization rotation angle wavelength dependences of the first polarization rotator and the second polarization rotator are mutually compensated. Since they are set to match each other, the same operation as when the reference wavelength is incident is performed.

Further, according to claim 5, by arranging the isolator according to claim 3 in two stages in series, a higher isolation can be realized.

[0022]

1 is a block diagram showing a first embodiment of an optical isolator according to the present invention. In FIG. 1, 31 is orthogonal 2
First as a polarization splitting / combining element for separating and combining two polarized lights
, A second birefringent crystal plate 32,
3 is also a third birefringent crystal plate, 34 is a first Faraday rotator as a non-reciprocal polarization rotator, 35 is a second Faraday rotator, 36 is a third Faraday rotator, 3
7 is a fourth Faraday rotator, 38 and 39 are first and second lenses, 40 is an optical fiber for incidence, and 41 is an optical fiber for emission.

This optical isolator comprises an incident optical fiber 40, a first lens 38, a first birefringent crystal plate 31, a first Faraday rotator 34, and a second Faraday rotator 3.
5, the second birefringent crystal plate 32, the third Faraday rotator 36, the fourth Faraday rotator 37, the third birefringent crystal plate 33, the second lens 39, and the output optical fiber 41 are shown. The optical axes of the second birefringent crystal plate 32 are arranged in order, and the optical axis of the second birefringent crystal plate 32 is rotated by 45 degrees with respect to the first birefringent crystal plate 31. The birefringent crystal plate 31 of
The birefringent crystal plate 31 and the third birefringent crystal plate 33 are made equal in thickness, and the thickness of the second birefringent crystal plate 32 is √2 times the thickness of the first birefringent crystal plate 31. Composed.
Further, the first and third Faraday rotators 34, 36 are made of members having similar wavelength characteristics, and
The Faraday rotators 35 and 37 are made of materials different from those of the Faraday rotators 34 and 36 and have members having similar wavelength characteristics. Further, the polarization rotation angle by the first Faraday rotator 34 and the second Faraday rotator 35 is 45 degrees, and the differentiation of the polarization rotation angle characteristic of the first Faraday rotator 34 and the second Faraday rotator at the reference wavelength. The sum of the derivatives of the polarization rotation angle characteristics of 35 is set to zero, and the polarization rotation angle by the third Faraday rotator 36 and the fourth Faraday rotator 37 is 45 degrees, and The sum of the derivative of the polarization rotation angle characteristic of the Faraday rotator 36 of No. 3 and the derivative of the polarization rotation angle characteristic of the fourth Faraday rotator 37 is set to zero.

First, the case of light having a reference wavelength will be described. FIG. 5 shows a polarization state and a beam position state of the optical isolator of the present invention. The forward direction means the propagation of light from the left to the right direction in the figure, and the backward direction means the direction opposite to the forward direction. Show. FIG. 5A illustrates the case where light is incident in the forward direction. In the figure, + and-indicate the direction in which the extraordinary light component moves, + indicates the movement direction when light is incident from the forward direction, and-indicates the movement direction when light is incident from the opposite direction.

The output light from the optical fiber 40 is supplied to the lens 3
The light is converted into parallel light by 8 (50) and input to the first birefringent crystal plate 31. This input light is split into two lights, an ordinary light 51a and an extraordinary light 51b, in the first birefringent crystal plate 31. Next, the polarization of these two lights is rotated by α degrees by the first Faraday rotator 34 (52a, 5a).
2b). The polarized light is rotated by β degrees by the second Faraday rotator 35 (53a, 53b). Where 2
The polarization rotation angle by the four Faraday rotators 34 and 35 is 4
Since it is set to 5 degrees, α + β = 45 degrees. The crystal axis of the second birefringent crystal plate 32 is rotated 45 degrees with respect to the first birefringent crystal plate 31, and it is assumed that the second birefringent crystal plate 32 rotates in the opposite direction to the 45 degree rotation by the Faraday rotator.
When passing through the second birefringent crystal plate 32, the light 53a becomes an extraordinary light component and the light 53b becomes an ordinary light component. Therefore, after passing through the second birefringent crystal plate 32, the position of only the extraordinary light component changes, and the lights 54a and 54b are output to different positions.
Two lights 54a, 5 which have passed through the second birefringent crystal plate 32
The polarized light 4b is rotated by α degrees by the third Faraday rotator 36 (55a, 55b). Further, the polarization is rotated by β degrees by the fourth Faraday rotator 37 (5
6a, 56b). Therefore, the fourth Faraday rotator 37
After passing through, the polarized light is rotated by 90 degrees with respect to after passing through the first birefringent crystal plate 31. On the other hand, since the crystal axis of the third birefringent crystal plate 33 is rotated by 90 degrees with respect to the first birefringent crystal plate 31, the light 56a becomes the ordinary light component and the light 56b becomes the extraordinary light component. Therefore, after passing through the third birefringent crystal plate 33, the position of only the ordinary component changes, and
The two lights are combined and output (57).

Next, the operation when light is incident in the opposite direction will be described with reference to FIG. The output light from the output optical fiber 41 is collimated by the second lens 39 and then input to the third birefringent crystal plate 33. When passing through the third birefringent crystal plate 33, it is divided into ordinary light 61a and extraordinary light 61b. The light that has passed through the third birefringent crystal plate 33 passes through the fourth Faraday rotator 37. At that time, the polarized light is rotated by β degrees in the same direction as when passing in the forward direction (62a, 62b). Further, when passing through the third Faraday rotator 36, the polarized light rotates by α degrees in the same direction as when passing in the forward direction (63a, 63b). Therefore, when passing through the second birefringent crystal plate 32, the light 63a becomes the ordinary light component and the light 63b becomes the extraordinary light component, and the positions of only the extraordinary light component are changed (64a, 64b). The two lights that have passed through the second birefringent crystal plate 32 are then input to the second Faraday rotator 35. In the second Faraday rotator 35, the polarized light is rotated by β degrees (65a, 65b). Further, when passing through the first Faraday rotator 34, the polarization of the two lights is rotated by α degrees (66a, 66b).
The light that has passed through the first Faraday rotator 34 passes through the first birefringent crystal plate 31. At that time, the light 66a becomes an ordinary light component and the light 66b becomes an extraordinary light component. The position of only the extraordinary light component is changed by the first birefringent crystal plate 31. Light output from the first birefringent crystal plate 31 (67a, 67b)
Cannot be coupled to the fiber 40 by the first lens 38 because it is in a different position than when it was incident. From the above, it can be seen that this configuration operates as an optical isolator.

Next, a case where light having a wavelength different from the reference wavelength is incident will be described. FIG. 6 shows a polarization state and a beam position state in the optical isolator when light having a wavelength different from the reference wavelength is incident on the optical isolator of the present invention.
FIG. 6A illustrates the case where light is incident in the forward direction. In the figure, + and-indicate the direction in which the extraordinary light component moves, + indicates the movement direction when light is incident from the forward direction, and-indicates the movement direction when light is incident from the opposite direction.

The output light from the optical fiber 40 is converted into parallel light by the first lens 38 (70) and input to the first birefringent crystal plate 31. In the first birefringent crystal plate 31, this input light is divided into the ordinary light 71a and the extraordinary light 71b.
It is separated into two lights. Next, the polarization of these two lights is rotated by α + Δθ ₁ degree by the first Faraday rotator 34 (72a, 72b). Here, Δθ ₁ indicates the change of the polarization rotation angle due to the deviation from the reference wavelength. Further, the polarized light is rotated by β + Δθ ₂ degrees by the second Faraday rotator 35 (73a, 73b). Δθ ₂ indicates the change in the polarization rotation angle due to the deviation from the reference wavelength. Near the reference wavelength,
Since the sum of the derivatives of the polarization rotation characteristics of the first and second Faraday rotators 34 and 35 is set to zero, Δθ ₁ +
Δθ ₂ = 0 and the two Faraday rotators 34 and 35
The polarization due to is rotated by α + β = 45 degrees. The second birefringent crystal plate 32 has its crystal axis rotated 45 degrees with respect to the first birefringent crystal plate 31, and is assumed to rotate in a direction opposite to the 45 degree rotation by the Faraday rotator. When passing through the birefringent crystal plate 32, the light 73a becomes an extraordinary light component and the light 73b becomes an ordinary light component. Therefore, after passing through the second birefringent crystal plate 32, the position of only the extraordinary light component changes, and the lights 74a and 74b are output to different positions. Second
Two lights 74a, 74b that have passed through the birefringent crystal plate 32 of
Is polarized by the third Faraday rotator 36.
It is rotated by + Δθ ₁ degree (75a, 75b). Further, the polarization is β + Δθ by the fourth Faraday rotator 37.
Only _twice is rotated (76a, 76b). Therefore, the fourth
After passing through the Faraday rotator 37, the polarized light is rotated by 90 degrees with respect to after passing through the first birefringent crystal plate 31. On the other hand, since the crystal axis of the third birefringent crystal plate 33 is rotated 90 degrees with respect to the first birefringent crystal plate 31,
76a is an ordinary light component and 76b is an extraordinary light component. Therefore, after passing through the third birefringent crystal plate 33, the position of only the extraordinary light component changes, and the two lights are combined and output (77).

Next, the operation when light is incident in the opposite direction will be described with reference to FIG. The output light from the output optical fiber 41 is collimated by the second lens 39 and then input to the third birefringent crystal plate 33. When passing through the third birefringent crystal plate 33, it is divided into ordinary light 81a and extraordinary light 81b. The light that has passed through the third birefringent crystal plate 33 passes through the fourth Faraday rotator 37. At that time, the polarized light is rotated by β + Δθ ₂ degrees in the same direction as when passing in the forward direction (82a, 82b). Further, when passing through the third Faraday rotator 36, the polarized light is rotated by α + Δθ ₁ degree in the same direction as when passing in the forward direction (83a, 83b).
Therefore, when passing through the second birefringent crystal plate 32, the light 83a becomes the ordinary light component and the light 83b becomes the extraordinary light component, and the positions of only the extraordinary light component are changed (84a, 84b). The two lights that have passed through the second birefringent crystal plate 32 are
Is input to the Faraday rotator 35. In the second Faraday rotator 35, the polarized light is rotated by β degrees (85
a, 85b). Further, when passing through the first Faraday rotator 34, the polarizations of the two lights are rotated by α + Δθ ₁ degrees (86a, 86b). First Faraday rotator 34
The light that has passed through passes through the first birefringent crystal plate 31. At that time, the light 86a becomes an ordinary light component and the light 86b becomes an extraordinary light component. The position of only the extraordinary light component is changed by the first birefringent crystal plate 31. The lights 87a and 87b output from the first birefringent crystal plate 31 have different positions from those at the time of incidence, and therefore cannot be coupled to the optical fiber by the first lens 38.

From the above, it can be seen that even when an optical isolator having a wavelength different from the reference wavelength is incident, the optical isolator operates similarly.

FIG. 7 shows an example of wavelength characteristics of materials used as the first and second Faraday rotators 34, 35 or 36, 37. This figure shows the one-wavelength characteristic of the Faraday rotation coefficient of the Faraday rotator. Here, YIG used in the 1 μm band isolator,
The characteristics of Ga-substituted YIG, Bi, and Ga-substituted YIG are shown.
Thus, the direction of Faraday rotation can be changed by changing the substance to be replaced, etc., and a pair of Faraday rotators capable of compensating for wavelength dependence can be configured.

FIG. 8 shows a second embodiment of the optical isolator of the present invention. Nonreciprocal first and second Faraday rotators 133 and 134 are arranged between the first and second tapered birefringent crystal plates 131 and 132. 135 and 136 are the first and second lenses, and 13
Reference numerals 7 and 138 denote incident and outgoing optical fibers. When light passes through the tapered first and second birefringent crystal plates 131 and 132, polarization angles can be separated because ordinary light and extraordinary light have different refraction angles.

First, the operation when light of the reference wavelength is incident on the optical isolator will be described with reference to FIG.

FIG. 9 shows the state of polarization and the state of the position of the beam. In FIG.
Further, the forward direction refers to the propagation direction of light from left to right in the figure.

A case where light propagates in the forward direction will be described with reference to FIG. The output light 140 from the incident optical fiber 137 is collimated by the first lens 135 (141). When passing through the first birefringent crystal plate 131 having a tapered shape, the ordinary light and the extraordinary light have different refractive indices, so that they are refracted in different directions and output (142a, 1).
42b). In the first Faraday rotator 133, the two lights respectively rotate α degrees clockwise in the z direction (143a, 143b). Further, in the second Faraday rotator 134, the polarization directions of the two lights are rotated clockwise by β degrees in the z direction (144a, 144a,
144b). Here, the polarization rotation angle (α + β) by the first and second Faraday rotators is set to 45 degrees. The output light from the second Faraday rotator 134 is input to the second tapered birefringent crystal plate 132. The optical axis of the second tapered birefringent crystal plate 132 is rotated 45 degrees clockwise in the z direction with respect to the optical axis of the first tapered birefringent crystal plate 131. Therefore, light 14
4a corresponds to ordinary light, and light 144b corresponds to extraordinary light. The ordinary ray and the extraordinary ray that have passed through the tapered second birefringent crystal plate 132 are output in parallel with each other (145a, 145).
b). The two parallel lights are condensed by the second lens 136 (146) and input to the emission optical fiber 138.

The case where light propagates in the opposite direction will be described with reference to FIG. The output light 150 from the output optical fiber 138 is collimated by the second lens 136 (1
51), and input to the tapered second birefringent crystal plate 132. In the second tapered birefringent crystal plate 132, since the ordinary light and the extraordinary light have different refractive indices, they are divided into ordinary light and extraordinary light, refracted in different directions, and output (152a, 1).
52b). In the second Faraday rotator 134, since the polarization directions of the two lights are non-reciprocal, the lights are rotated by β degrees clockwise with respect to the z direction and output (153a, 153).
b). In the first Faraday rotator 133, the polarization directions of the two lights are each rotated by α degrees in the clockwise direction with respect to the z direction (154a, 154b). For the first birefringent crystal plate 131 having a tapered shape, the light 154b is extraordinary light and the light 1
54a becomes ordinary light. The two lights are refracted in different directions and enter the lens 135. At this time, two lights 15
Since the traveling directions of the lights 5a and 155b are different from the traveling directions of the incident light, they are not condensed by the first lens 135 and are not coupled to the optical fiber 137 (156a and 156b).

Next, a case where light different from the reference wavelength enters the optical isolator will be described with reference to FIG. Figure 1
(A) of 0 shows the case where light is incident in the forward direction. The output light 160 from the optical fiber 137 is collimated by the lens 135 (161). When passing through the tapered birefringent crystal plate 131, the ordinary light and the extraordinary light have different refractive indices, and thus are refracted in different directions and output (162a, 162b). First Faraday rotator 13
In FIG. 3, the polarization directions of the two lights are rotated in the z direction by α + Δθ1 degrees clockwise (163a, 16a).
3b). Furthermore, in the second Faraday rotator 134, the polarization directions of the two lights are β + Δθ2 in the clockwise direction, respectively.
It is rotated once (164a, 164b). Where Δθ1
Since the setting is such that = −Δθ2, the state of polarization after passing through the first and second Faraday rotators is the same as when the reference wavelength is incident. The output light from the second Faraday rotator 134 is input to the second tapered birefringent crystal plate 132. Tapered second birefringent crystal plate 1
The optical axis of 32 is rotated 45 degrees clockwise in the z direction with respect to the optical axis of the tapered first birefringent crystal plate 131. Therefore, the light 164a corresponds to the ordinary light and the light 164b corresponds to the extraordinary light. Tapered second birefringent crystal plate 132
After being transmitted, the two lights 165a and 165b become lights parallel to the traveling direction at the time of incidence, are condensed by the second lens 136, and are coupled to the emission optical fiber 138 (16
6).

Next, the case where light is incident in the opposite direction is shown in FIG.
(B) of will be explained. The light 170 output from the emitting optical fiber 138 is collimated by the second lens 136 (171), and the tapered second birefringent crystal plate 1 is formed.
Enter in 32. Tapered second birefringent crystal plate 132
Since the ordinary light and the extraordinary light have different refractive indices, the ordinary light and the extraordinary light are divided and refracted in different directions and output (172
a, 172b). In the second Faraday rotator 134,
The polarization directions of the two lights are β + Δθ with respect to the z direction.
It is output by rotating it twice clockwise (173a, 173).
b). In the first Faraday rotator 133, the polarization directions of the two lights are α + Δθ in the clockwise direction with respect to the z direction, respectively.
It is rotated once (174a, 174b). Therefore, Δθ
Since the thicknesses of the two Faraday rotators are adjusted so that 1 = −Δθ2, the tapered birefringent crystal plate 13
1, the light 174b becomes extraordinary light and the light 174a becomes ordinary light. Therefore, the lights 174 a and 174 b are refracted by the tapered birefringent crystal plate 131 and are incident on the first lens 135. At this time, since the traveling directions of the two lights 175a and 175b are different from the traveling direction of the incident light, the optical fiber 1
It does not bind to 37 (176a, 176b). Therefore, even when light with a wavelength different from the reference wavelength is input, the two Faraday rotators compensate each other for the wavelength dependence of the polarization rotation angle, and the isolation does not depend on the wavelength.

As the tapered birefringent crystal plate, rutile crystal, calcite crystal or the like is used.

Also in the above-mentioned embodiment, it is said that the materials used as the first and second Faraday rotators 133 and 134 have wavelength characteristics that compensate each other as shown in FIG. There is no end.

FIG. 11 shows a third embodiment of the present invention. Here, 181, 182, 183 and 184 are tapered birefringent crystal plates, and the first and second Faraday rotators 185 are provided between the first and second tapered birefringent crystal plates 181 and 182. 186 is disposed, and third and fourth Faraday rotators 187 and 188 are disposed between the third and fourth tapered polarization rotators 183 and 184. 189 and 190 are the first and second lenses, 191 and
Reference numeral 192 denotes an optical fiber for incidence and an optical fiber for emission. In this optical isolator, the birefringent crystal plates of the optical isolator of the second embodiment shown in FIG. 8 are connected in two stages by shifting the taper inclination direction by 90 degrees. The inclination direction of the taper of the first and second tapered birefringent crystal plates 181 and 182 and the inclination direction of the third and fourth tapered birefringent crystal plates 183 and 184 are shifted from each other by 90 degrees. FIG. 12 illustrates the operation of the optical isolator when light of the reference wavelength is incident. In the figure, circle 11 indicates the corresponding positional relationship. First, the case of incident light in the forward direction will be described with reference to FIG. The output light 201 from the incident fiber 191 is collimated by the first lens 189 (202).
When passing through the tapered first birefringent crystal plate 181,
Since ordinary light and extraordinary light have different refractive indexes, they are refracted in different directions and output (203a, 203b). In the first Faraday rotator 185, the two lights rotate α degrees clockwise in the z direction (204).
a, 204b). In addition, the second Faraday rotator 18
In Fig. 6, the polarization directions of the two lights are β clockwise.
It is rotated once (205a, 205b). Where α + β
= 45 degrees. The output light from the second Faraday rotator 186 is input to the second tapered birefringent crystal plate 182. The optical axis of the tapered second birefringent crystal plate 182 is the tapered first birefringent crystal plate 1.
4 clockwise in the z direction with respect to the 81 optical axis
It rotates 5 degrees. Therefore, the light 205a is the ordinary light, and the 205
b corresponds to abnormal light. Second tapered birefringent crystal plate 1
The ordinary ray and the extraordinary ray that have passed through 82 are parallel to each other and are output in parallel to the incident light. Further, the output light 206
a and 206b are transmitted through the tapered third birefringent crystal plate 183. In that case, the output lights 206a and 206b are both
Since there are ordinary light and extraordinary light components and the refraction directions of ordinary light and extraordinary light are different, light divided into four is output (207).
a-207d). Four lights are the third Faraday rotator 1
It is rotated α degrees clockwise with respect to the z-axis direction by 87 (208a to 208d). Further, the polarization is rotated by β degrees by the fourth Faraday rotator 188 (209a
~ 209d). The optical axis of the fourth tapered birefringent crystal plate 184 is rotated 45 degrees clockwise in the z direction with respect to the optical axis of the tapered birefringent crystal plate 181. Therefore, the lights 209a and 209c are ordinary light, 209b,
209d corresponds to extraordinary light. The ordinary light and the extraordinary light transmitted through the tapered birefringent crystal plate 184 are output in parallel with each other (210a to 210d). The four parallel lights are condensed by the second lens 190 and coupled to the emitting optical fiber 192 (211).

Next, the case of propagating in the opposite direction will be described. The output light 221 from the output optical fiber 192 is collimated (222) by the second lens 190, and is input to the fourth tapered birefringent crystal plate 184. In the fourth tapered birefringent crystal plate 184, Since the ordinary light and the extraordinary light have different refractive indices, they are divided into ordinary light and extraordinary light, refracted in different directions, and output (223a, 223b). In the fourth Faraday rotator 188, the polarization directions of the two lights are z
It is output by rotating β degrees clockwise with respect to each direction (224a, 224b).

In the third Faraday rotator 187, the polarization directions of the two lights are rotated by α degrees in the clockwise direction with respect to the z direction (225a, 225b). For the tapered third birefringent crystal plate 183, the light 225a becomes extraordinary light and the light 225b becomes ordinary light. The refracting directions of the two lights are different, and the two lights are refracted in directions away from each other. In the second tapered birefringent crystal plate 182, two light beams 226
Since a and 226b both have an ordinary light component and an extraordinary light component,
It is separated into four lights (227a to 227d). In the second Faraday rotator 186, the polarized light is rotated by β degrees (228a to 228d). Further, the first Faraday rotator 185 is rotated by α degrees clockwise (229
a-229d). Further, in the tapered first birefringent crystal plate 181, the lights 230a and 230b are extraordinary lights and the light 23
0c and 230d correspond to extraordinary light. Therefore, after passing through the tapered first birefringent crystal plate 181, the four light beams 2
30a to 230d are refracted in directions away from each other. Therefore, it cannot be coupled to the incident optical fiber 191 by the first lens 189.

A description will be omitted of the case where light different from the reference wavelength is incident. However, since the Faraday rotators also compensate each other for the wavelength dependence, the isolation does not depend on the wavelength change. Furthermore, since the two isolators are connected in series, high isolation can be obtained.

[0045]

As described above, according to the first and second aspects, by using two polarization rotators, the polarization is rotated by 45 degrees during passage, and the polarization rotation angles due to wavelengths are mutually compensated. By using this configuration, it is possible to realize an optical isolator having no wavelength dependence with respect to the input light. Further, this configuration can be made into two stages to achieve high isolation. According to the third, fourth, and fifth aspects, in addition to the above effect, the length of the entire device can be shortened by making the polarization separation element tapered.

[Brief description of drawings]

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

FIG. 2 is a configuration diagram showing an example of a conventional optical isolator.

FIG. 3 is an explanatory diagram showing how light propagates in the optical isolator shown in FIG.

FIG. 4 is an explanatory diagram showing a path of light in the optical isolator of FIG.

FIG. 5 is an explanatory diagram showing a state of propagation of light having a reference wavelength in the configuration of FIG.

FIG. 6 is an explanatory diagram showing how a light having a wavelength different from a reference wavelength propagates in the configuration of FIG. 1;

FIG. 7 is a graph showing an example of wavelength characteristics of a Faraday rotator.

FIG. 8 is a configuration diagram showing a second embodiment of the optical isolator of the present invention.

FIG. 9 is an explanatory diagram showing a state of propagation of light having a reference wavelength.

FIG. 10 is an explanatory diagram showing a state of propagation of light having a wavelength other than the reference.

FIG. 11 is a configuration diagram showing a third embodiment of the optical isolator of the present invention.

FIG. 12 is an operation explanatory diagram when light of a reference wavelength is incident on the optical isolator of FIG. 11.

[Explanation of symbols]

31 ... 1st birefringent crystal plate, 32 ... 2nd birefringent crystal plate, 33 ... 3rd birefringent crystal plate, 34 ... 1st Faraday rotator, 35 ... 2nd Faraday rotator, 36 ... Third
Faraday rotator, 37 ... Fourth Faraday rotator,
38 ... 1st lens, 39 ... 2nd lens, 40 ... Incident optical fiber, 41 ... Emitting optical fiber.

Claims (5)

[Claims] 1. A first method for separating and combining two orthogonal polarizations.
And a third polarization separation element that separates and combines two orthogonal polarizations, and a third polarization separation element that separates and combines two orthogonal polarizations, the first polarization separation element And the second polarization separation element, a non-reciprocal first polarization rotator and a non-reciprocal second polarization rotator made of a material different from that of the first polarization rotator. And a non-reciprocal third polarization rotator and a material different from that of the third polarization rotator between the second polarization separation element and the third polarization separation element. A non-reciprocal fourth polarization rotator, the optical axis of the second polarization splitting element being rotated by 45 degrees with respect to the first polarization splitting element, The polarization separation element is rotated by 90 degrees with respect to the optical axis of the first polarization separation element, The thickness of the first polarization separating element is the same as in the third polarization separating element, the thickness of the second polarization separating element is the first
The optical isolator is √2 times larger than the polarization separation element of 2. The sum of the polarization rotation angles of the first polarization rotator and the second polarization rotator is 45 degrees, and the polarization rotation angle by the first polarization rotator and the second polarization rotator at a reference wavelength. The sum of the derivatives of the polarization rotation angles by the polarization rotator is zero, the sum of the polarization rotation angles of the third polarization rotator and the fourth polarization rotator is 45 degrees, and the third polarization at the reference wavelength 2. The optical isolator according to claim 1, wherein the sum of the polarization rotation angle by the rotator and the derivative of the polarization rotation angle by the fourth polarization rotator is zero. 3. An incident optical fiber and a first lens for collimating the light emitted from the incident optical fiber,
A first polarization separation element made of a tapered birefringent material;
A non-reciprocal first polarization rotator that rotates the polarized light by α degrees at the reference wavelength, a non-reciprocal second polarization rotator that rotates the polarized light by β degrees with respect to the reference wavelength, and a tapered shape A second polarization splitting element made of a birefringent substance, a second lens for focusing parallel light emitted from the second polarization splitting element on a light receiving optical fiber, and a light receiving optical fiber, The top and the bottom of the tapered first polarization separation element face the bottom and the top of the tapered second polarization separation element, respectively, and the corresponding surfaces are installed parallel to each other. The optical axis is rotated by 45 degrees with respect to the optical axis of the first polarization separation element, and the forward light beam from the incident optical fiber is focused on the light receiving optical fiber when focused by the second lens. , When the backward ray is focused by the first lens , The deflection angle of the beam optical isolator characterized in that the away larger than the diameter of the mutually incident optical fiber between the ordinary light and extraordinary light. 4. The polarization rotation angle by the first polarization rotator and the second polarization rotator is 45 degrees (α + β = 4).
5. The optical isolator according to claim 3, wherein the sum of the differentials of the polarization rotation angle characteristics of the first polarization rotator and the second polarization rotator at the reference wavelength is zero. 5. A first polarization splitting element made of a tapered birefringent material, a non-reciprocal first polarization rotator that rotates polarized light by α degrees at a reference wavelength, and a reference wavelength. A first unit having a second polarization rotator that rotates the polarized light by β degrees and has non-reciprocity, and a second polarization separation element made of a tapered birefringent material; A third polarization separation element made of a birefringent substance of
A third polarization rotator that rotates the polarized light by α degrees at the reference wavelength, a fourth polarization rotator that rotates the polarized light by β degrees with respect to the reference wavelength, and a fourth polarization rotator composed of a tapered birefringent substance. And a second unit in which the polarized light separating element of FIG. 3 is arranged in series, and the taper inclination direction of the first and second polarization separating elements and the taper inclination direction of the third and fourth polarization separating elements. Is an optical isolator characterized by being shifted by 90 degrees.