JP2572627B2 - Optical isolator and optical circulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to an optical isolator and an optical circulator having no polarization dependency used in an optical measurement system or an optical communication system.

(Prior Art) In general, in a parallel plate-shaped birefringent crystal having a thickness d and an optical axis (c-axis) formed to be about θ = 45 ° with respect to the light transmission direction, light is transmitted to the crystal. On the other hand, when incident perpendicularly, as shown in FIG. 9, the polarization component in the direction perpendicular to the paper (ordinary light)
Transmits through this birefringent crystal 40 without any refraction,
When entering the birefringent crystal 40, the polarization component (extraordinary light) in the direction of the paper is refracted as shown in FIG. 3, and exits at a position separated by the separation distance a in parallel with the ordinary light.

In a birefringent crystal such as rutile or calcite, the separation distance a can be expressed by the following equation (1).

| a | = k × | d | (1) For example, when the wavelength is 1.3 μm and θ = 45 °, the value of k is about 0.1 for both rutile and calcite.

By the way, in an optical isolator, which is a type of non-reciprocal optical device, conventionally, only a specific polarization component is transmitted in the forward direction of light, and the remaining light component is discarded. There was a problem that was not.

For this reason, conventionally, an optical isolator that transmits all polarized light components has been proposed.

A conventional example of such an optical isolator (Japanese Patent Publication No. 58-2856)
No. 1) will be described with reference to FIGS. 10 to 12.

The optical isolator main body 50 shown in FIG. 10 is arranged between a first optical fiber 31 as a first optical waveguide and a second optical fiber 32 as a second optical waveguide. A condensing lens 33 is disposed between the optical isolator body 50 and the first optical fiber 31.

The optical isolator main body 50 is composed of three parallel plate-like birefringent crystals 51, 52, 53 indicated by reference numerals, and a 45 ° Faraday rotator 5 joined and disposed between the birefringent crystals 51, 52.
4 is provided.

Here, three orthogonal axes of X, Y, and Z with respect to the optical isolator body 50 are set as shown in FIG. 10, and the forward direction of the light traveling direction is defined as the −Z direction as shown in FIG. Further, when the three-dimensional angle of the optical axis (C axis) of each of the birefringent crystals 51, 52, 53 is defined as shown in FIG.
Table 1 shows the transmission state of the light isolator body 50 for the light of = 0 ° and the light of the polarization angle of ψ = 90 °.

Here, the angle ψ is a projection angle of the C axis on the X and Y planes.
The thickness of the birefringent crystals 51, 52, 53 is in order d, d are set.

In the optical isolator body 50 thus configured,
FIG. 11A shows the relationship between the center positions of the polarized beams on the X and Y planes in the forward direction.

As is clear from FIG. 11 (a) and Table 11, the light having the polarization angle ψ = 0 ° in the first optical fiber 31 has the birefringent crystal 51 as the extraordinary light, and After the 45 ° polarization plane is rotated by the 45 ° Faraday rotator 54, the birefringent crystal 52 passes as an extraordinary light, and passes with a separating distance of a. The light passes through and enters the second optical fiber 32.

Further, the light having a polarization angle ψ = 90 ° in the first optical fiber 31 passes through the birefringent crystal 51 as ordinary light, then the 45 ° polarization plane is rotated by the 45 ° Faraday rotator 54, and the birefringent crystal 52 is further rotated. It passes as ordinary light, passes through the birefringent crystal 53 as extraordinary light, and has a separation distance a from ordinary light, and enters the second optical fiber 32.

As a result, the light having a polarization angle of ψ = 0 ゜ and the light having a polarization angle of ψ90 ゜ are once separated in the optical isolator main body 50, but finally enter the same position of the second optical fiber 53.

On the other hand, focusing on the state of transmission of light in the opposite direction to the optical isolator body 50, as shown in FIG.
The light with a polarization angle of {45} emitted from the optical fiber 32 passes through the birefringent crystal 53 as ordinary light, then passes through the birefringent crystal 52 as extraordinary light, and with a separation distance a from ordinary light, 45
偏 The plane of polarization is rotated by 45 ° by the Faraday rotator 54, passes through the birefringent crystal 51 as ordinary light, and travels in the X direction from the first optical fiber 31. Light is emitted to a remote position.

In addition, the polarization angle ψ = emitted from the second optical fiber 32 =
The 135 ° light passes through the birefringent crystal 53 as extraordinary light and with a separation distance a, passes through the birefringent crystal 52 as ordinary light, and then rotates the 45 ° polarization plane by the 45 ° Faraday rotator 54 to change the polarization angle 180 °.゜ = 0, and the birefringent crystal 51
As extraordinary light and separation distance From the first optical fiber 31 in the X direction Light is emitted to a remote position.

As a result, no light is incident on the first optical fiber 31 in the reverse direction.

As described above, the light from the first optical fiber 31 enters the second optical fiber 32 regardless of the presence or absence of the polarized light.

However, when the light with the polarization angle ψ = 0 ° emitted from the first optical fiber 31 passes through the birefringent crystal groups 51, 52, 53 of the optical isolator body 50, Is transmitted as extraordinary light, and the distance d is transmitted as ordinary light.

On the other hand, light with a polarization angle of ψ = 90 ° has an extraordinary light at a distance of d. Is transmitted as ordinary light.

In this case, the birefringent crystals 51, 52,
Since the refractive indices 53 are different from each other, a substantial difference in the optical path length occurs when the above-mentioned two polarized lights pass through the optical isolator main body 50, and as shown in FIG. There is a problem that the polarization is different, and coupling loss occurs when such two polarized lights are received by the second optical fiber 32.

Next, another example of the conventional optical isolator (Japanese Patent Publication No. 61-5880)
No. 9) will be described with reference to FIG.

The optical isolator body 60 shown in the figure has tapered birefringent crystals 61 and 62 arranged on both sides of a Faraday rotator 63,
Polarization separation is performed by utilizing the difference in the refractive index of the birefringent crystals 61 and 62 with respect to ordinary light and extraordinary light. In FIG. 13, reference numeral 34 denotes a condenser lens. However, in such an optical isolator body 60, there is a problem in that the tapered birefringent crystals 61 and 62 are difficult to process as compared with the parallel plate type and lack mass productivity, so that the entire manufacturing cost is high.

(Problems to be Solved by the Invention) As described above, the conventional optical isolator has a problem that a coupling loss occurs when light is received by the second optical fiber, and that the manufacturing cost rises.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical isolator and an optical circulator that have low loss, do not have polarization dependence, and do not cause a rise in manufacturing cost.

[Constitution of the Invention] (Means for solving the problem) The optical isolator according to claim 1, wherein at least one 45 ° Faraday rotator and at least one lens are provided between two opposing optical waveguides. , At least four parallel-plate birefringent crystals, and when light in the forward direction from one optical waveguide to the other optical waveguide passes through each of the parallel-plate birefringent crystals, a certain linear polarization (1) is transmitted as ordinary light or extraordinary light, and linearly polarized light (2) orthogonal to the linearly polarized light (1) is transmitted as extraordinary light and extraordinary light when the linearly polarized light (1) is transmitted as ordinary light. In this case, the linearly polarized light is transmitted as ordinary light, and the sum of the optical passage lengths when the linearly polarized light (1) transmits through all of the birefringent crystals of the parallel plate is orthogonal to the linearly polarized light (1). (2) is the parallel plate Are set to be equal to the sum of the optical transmission lengths when transmitting through all of the birefringent crystals, and the emission positions of the two polarized lights coincide, and the optical path lengths of the two polarized lights entering the Faraday rotator are different. Further, only the Faraday rotator is used as an element for rotating the plane of polarization.

The optical isolator according to claim 2, wherein the parallel-plate birefringent crystals are all made of a single crystal of the same material, and the angle between each optical axis and the normal vector of the light input / output surface is equal, and Each optical axis is set at an angle rotated by an integral multiple of 45 ° with the normal vector as the central axis, and the total thickness of the birefringent crystal that transmits the linearly polarized light (1) as ordinary light, and the linearly polarized light (1). The total thickness of the birefringent crystal that transmits the linearly polarized light (2) orthogonal to 1) as ordinary light is equal, and the total thickness of the birefringent crystal that transmits the aforementioned linearly polarized light (1) as extraordinary light is the same as the linearly polarized light. The total thickness of the birefringent crystal that transmits linearly polarized light (2) orthogonal to (1) as extraordinary light is equal.

The optical circulator according to claim 3, wherein the first and third two optical waveguides arranged side by side are provided on one side, and the second optical waveguide is provided on the other side in a state where they face each other. At least one and a 45 ° Faraday rotator between the optical waveguides of
A linearly polarized light (1) having at least one lens and at least four parallel-plate birefringent crystals and being included in light traveling from the second optical waveguide to the third optical waveguide;
Means that each of the parallel-plate birefringent crystals is transmitted as ordinary light or extraordinary light, and the linearly polarized light (2) orthogonal to the linearly polarized light (1) is abnormal if the linearly polarized light (1) is transmitted as ordinary light. When transmitted as light, extraordinary light is transmitted as ordinary light, and the sum of the optical transmission lengths when the linearly polarized light (1) passes through all of the birefringent crystals of the parallel plate is the linearly polarized light. The linearly polarized light (2) orthogonal to (1) is equal to the sum of the optical passage lengths when transmitting all of the birefringent crystals of the parallel plate, and the emission positions of the two polarized lights coincide with each other. The optical path lengths of the two polarized lights incident on the Faraday rotator are set differently, and only the Faraday rotator is used as an element for rotating the plane of polarization.

The optical circulator according to claim 4, wherein the parallel plate birefringent crystals are all made of a single crystal of the same material, and the angle between each optical axis and the normal vector of the light input / output surface is equal, and Each optical axis is set at an angle rotated by an integral multiple of 45 ° with respect to the normal vector as a central axis, and the total thickness of the birefringent crystal that transmits the linearly polarized light (1) as ordinary light and the linearly polarized light (1) ), The total thickness of the birefringent crystals transmitting the linearly polarized light (2) orthogonal to ordinary light is equal, and the total thickness of the birefringent crystals transmitting the linearly polarized light (1) as extraordinary light is equal to the total thickness of the linearly polarized light (2). The total thickness of the birefringent crystal that transmits linearly polarized light (2) orthogonal to 1) as extraordinary light is equal.

(Operation) The operation of the optical isolator and optical circulator having the above configuration will be described below.

According to the optical isolator of the first aspect, each birefringent crystal transmits different polarized components of the light in the forward direction as ordinary light or extraordinary light through the birefringent crystal, and has the same transmission length of both polarized components. Since the thickness and the direction of the optical axis are set, different polarization components of the light in the forward direction enter the same position of the other optical waveguide, and there is no polarization dependency of the light in the forward direction. Losses are also reduced.

In addition, since this optical isolator uses a parallel-plate birefringent crystal, the manufacturing cost is low.

According to the optical isolator according to the second aspect, all the optical axes are made of a single crystal of the same material, and all the optical axes are equal at a predetermined angle, and the total when both linearly polarized lights are transmitted as ordinary light or extraordinary light. Since the birefringent crystals having the same thickness are used, there is no dependency on the polarization of light, the coupling loss can be reduced, and the manufacturing cost can be reduced.

According to the optical circulator of the third aspect, different polarization components of light traveling from the second optical waveguide to the third optical waveguide are transmitted as ordinary light or extraordinary light through the birefringent crystal, and both polarization components are transmitted. Since the thickness of each birefringent crystal and the direction of the optical axis are set so that the lengths are equal, different polarization components in the forward light enter the same position in the other optical waveguide, and the forward light Is not dependent on polarization, and coupling loss is reduced.

Further, since this optical circulator uses a parallel-plate birefringent crystal, the manufacturing cost is low.

According to the optical isolator according to the fourth aspect, the optical isolator is made of a single crystal of the same material, all optical axes are equal at a predetermined angle, and the total when both linearly polarized lights are transmitted as ordinary light or extraordinary light. Since each refraction crystal having the same thickness is used, there is no dependence on the polarization of light, the coupling loss can be reduced, and the manufacturing cost can be reduced.

(Example) Hereinafter, an example of the present invention will be described in detail.

First, a first embodiment will be described with reference to FIGS. 1 and 2 (a) and 2 (b).

FIG. 1 shows an optical isolator of the present invention.
A nonreciprocal optical device main body 10 and a condensing lens 33 are arranged between a first optical fiber 31 as a first optical waveguide and a second optical fiber 32 as a second optical waveguide. . The non-reciprocal optical device main body 10 includes four birefringent crystals 1, 2, 3, and 4 indicated by reference numerals,,, and a 45 ° Faraday rotator 5 bonded and arranged between the birefringent crystals 1 and 2. Is provided.

X, Y, Z orthogonal 3 to this non-reciprocal optical device main body 10
The following description will be made assuming that the axis, the C-axis, and the polarization angle ψ are the same as those shown in FIG. 10, and the forward and reverse directions of the light are the same as those shown in FIG.

Table 2 shows the transmission state of the light of ψ = 0 ゜ and the light of ψ = 90 ゜ in the nonreciprocal optical device main body 10 in this case.

As shown in Table 2, the thickness of each birefringent crystal 1, 2, 3, 4 is
In order Is set to

In the non-reciprocal optical device main body 10, in the forward direction,
FIG. 2A shows the relationship between the center positions of the polarized beams on the X and Y planes.

As is clear from FIG. 2 (a) and Table 2, the light having a polarization angle of ψ = 0 ° from the first optical fiber 31 has the birefringent crystal 1 as extraordinary light, and After the 45 ° Faraday rotator 5 rotates the 45 ° polarization plane, the light passes through the birefringent crystal 2 as extraordinary light and at a separation distance of a, and further passes through the birefringent crystals 3 and 4. The light passes as ordinary light and enters the second optical fiber 32.

The light having a polarization angle of ψ = 90 ° from the first optical fiber 31 passes through the birefringent crystal 1 as ordinary light, and after the 45 ° polarization plane is rotated by the 45 ° Faraday rotator 5, the birefringent crystal 2 is rotated. Pass as ordinary light. Further, this ordinary light reaches the birefringent crystal 3, where it is regarded as extraordinary light, and And the extraordinary light at the birefringent crystal 4, and And enters the second optical fiber 32.

As a result, in the non-reciprocal optical device main body 10, the light of と し て = 0 ゜ and the light of ψ = 90 ゜ both have a transmission length as extraordinary light and a transmission length as ordinary light. Thus, the condensing positions of both lights in the second optical fiber 32 become the same, and the coupling loss is reduced.

On the other hand, the transmission state of light in the opposite direction to the nonreciprocal optical device main body 10 is as shown in FIG. 2 (b).

That is, the light of ψ = 45 ° from the second optical fiber 32 passes through the birefringent crystals 3 and 4 as ordinary light, then passes through the birefringent crystal 2 as extraordinary light, and with a separation distance of a,
The 45 ° polarization plane is rotated by the 45 ° Faraday rotator 5, passes through the birefringent crystal 1 as ordinary light, and travels in the X direction from the first optical fiber 31. Light is emitted to a remote position.

Further, the light of ψ = 135 第 from the second optical fiber 4 is
Birefringent crystal 4 as extraordinary light, and , The birefringent crystal 3 as extraordinary light, and the separation distance After passing through the birefringent crystal 2 as ordinary light, and rotating the 45 ° polarization plane by the 45 ° Faraday rotator 5, the birefringent crystal 1 is regarded as extraordinary light, and From the first optical fiber 31 and X
In the direction Light is emitted to a remote position.

As a result, neither of the above-mentioned light beams from the second optical fiber 32 enters the first optical fiber 31, and the non-reciprocal optical device main body 10 has its original light as an optical isolator with respect to light in the opposite direction. Function will be demonstrated.

Next, an optical isolator according to a second embodiment will be described with reference to FIGS. 3 and 4 (a) and (b).

Table 3 shows the thickness and the light transmission state of the nonreciprocal optical device main body 10A shown in FIG. 3, and FIGS. 4 (a) and 4 (b) show the relationship between the polarized beams on the X and Y planes.

The nonreciprocal optical device main body 10A has four birefringent crystals 11, 12, 13, and 14 (indicated by reference numerals,,).
And two 45 ° Faraday rotators 5A and 5B. The thickness of each birefringent crystal 11, 12, 13, 14 is Is set to

In the case of the nonreciprocal optical device main body 10A as well, as is clear from Table 3 and FIG.
The light of = 0 ° and the forward light of 方向 = 90 ° both have an optical path length 2a that passes as extraordinary light, and the second optical fiber
It is incident on 32 identical positions.

Also, the light in the opposite direction from the second optical fiber 32 is
The result is as shown in FIG. That is, one polarized light passes through the four birefringent crystals 14, 13, 12, and 11 as extraordinary light and does not reach the first optical fiber 31, and the other polarized light does not cause a separation distance. It passes through birefringent crystals 14, 13, 12, 11 as ordinary light.

According to the non-reciprocal optical device main body 10A, the same function as that of the non-reciprocal optical device main body 10 can be exhibited, and the two 45 ° Faraday rotators 5A and 5B are used. Characteristics and wavelength characteristics can be further improved.

Next, a third embodiment of the present invention will be described with reference to FIG.

The non-reciprocal optical device main body 20 shown in the figure has seven parallel plate-shaped birefringent crystals 21 to 27 denoted by reference numerals and
It is configured by joining and joining three 45 ° Faraday rotators 5A, 5B, 5C.

Table 4 shows the thickness and the light transmission state of the nonreciprocal optical device main body 20, and FIG. 6 shows the relationship between the polarized beams on the X and Y planes.

Also in the case of the non-reciprocal optical device main body 20, as shown in Table 4 and FIG. 6, the forward light of ψ = 0 ° and ゜ = 90 ° from the optical fiber 31 is: In any case, the optical path length passing as extraordinary light is Then, the light enters the same position of the second optical fiber 32.

In addition, any polarized light of the opposite direction from the second optical fiber 32 does not reach the first optical fiber 31.

Next, an application example of the present invention will be described with reference to FIGS.

FIG. 7 shows an optical system in which the third optical fiber 32A is arranged near the second optical fiber 32 using the nonreciprocal optical device main body 10.

In this optical system, if a certain polarization component emitted from the third optical fiber 32A is arranged so as to enter the first optical fiber 31 through the nonreciprocal optical device main body 10, the third
Optical fiber 32A → non-reciprocal optical device main body 10 → first optical fiber 31, first optical fiber 31 → non-reciprocal optical device main body 10
→ An optical circulator that forms an optical path of the second optical fiber 32 can be configured.

In this case, the optical path from the third optical fiber 32A to the first optical fiber 31 has polarization dependence, but the optical path from the first optical fiber 31 to the second optical fiber 32 has polarization dependence. Accordingly, an optical circulator suitable for bidirectional communication and optical measurement can be provided.

Further, even if a similar optical system is configured using the nonreciprocal optical device main body 10A as shown in FIG. 8, an optical circulator having the same function as that shown in FIG. 7 can be provided.

The present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the invention.

[Effects of the Invention] According to the present invention described in detail above, there is no polarization dependence of light traveling from one optical waveguide to the other optical waveguide, the coupling loss is small, and the manufacturing cost is reduced. Thus, an optical isolator that can be provided can be provided.

Further, according to the present invention, there is provided an optical circulator which has no polarization dependence of light traveling from the second optical waveguide to the third optical waveguide, has small coupling loss, and can be manufactured at low cost. Can be provided.

Furthermore, in the present invention, by using a configuration in which the two polarized lights incident on at least one Faraday rotator have a difference in the optical path length, the number of birefringent crystals used is minimized. The effect of contributing to cost reduction is extremely large, and is an invention having high industrial utility.

Further, since only the Faraday rotator is used as the element for rotating the plane of polarization, it is not necessary to use an expensive and large light-transmitting crystal plate, so that the optical isolator and the optical device can be reduced in cost and size. A circulator can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic structural view showing a first embodiment of the present invention, and FIG.
FIGS. 3A and 3B are explanatory diagrams showing forward and reverse separation distances of the respective polarized beams on the X and Y planes of the optical isolator shown in FIG. 1, and FIG. 3 is a second embodiment of the present invention. FIGS. 4 (a) and 4 (b) are schematic diagrams showing an embodiment, and FIGS. 4 (a) and 4 (b) are explanatory views showing forward and reverse separation distances of each polarized light beam on the X and Y planes of the optical isolator shown in FIG. 3, respectively. FIG. 5 is a schematic configuration diagram showing a third embodiment of the present invention, FIG. 6 is an explanatory diagram showing a forward separation distance of a polarized beam in the X and Y planes of the optical isolator shown in FIG. FIG. 8 and FIG. 8 are schematic diagrams showing an optical circulator as an application example of the present invention, FIG. 9 is an explanatory diagram showing the principle of polarization separation of a birefringent crystal, and FIG. 10 is a diagram showing the configuration of a conventional optical isolator. FIGS. 11 (a) and 11 (b) show the optical isolator shown in FIG. 10 in the X and Y planes, respectively. FIG. 12 is an explanatory diagram showing forward and reverse separation distances of a polarized beam, FIG. 12 is an explanatory diagram showing a difference in light-collecting position in the optical system shown in FIG. 10, and FIG. 13 is a schematic diagram showing another example of a conventional example. It is a block diagram. 10, 10A, 20 ... non-reciprocal optical device main body, 1 to 4, 11 to 14 ... birefringent crystal, 5, 5A, 5B, 5C ... 45 ° Faraday rotator.

Claims (4)

(57) [Claims] At least one 45 ° Faraday rotator, at least one lens, and at least four parallel plate birefringent crystals are provided between two opposing optical waveguides. When light in the forward direction from the optical waveguide to the other optical waveguide passes through each of the birefringent crystals of the parallel plate, certain linearly polarized light (1) is transmitted as ordinary light or extraordinary light, and the linearly polarized light (1) is transmitted. Linear polarized light (2) orthogonal to the above is transmitted as extraordinary light when the linearly polarized light (1) is transmitted as ordinary light, and is transmitted as ordinary light when transmitted as extraordinary light,
The sum of the optical transmission lengths when the linearly polarized light (1) passes through each of the birefringent crystals of the parallel plate is linear linearly polarized light (2) orthogonal to the linearly polarized light (1). It is set so as to be equal to the sum of the optical passing lengths when transmitting all the birefringent crystals, and to match the emission positions of the two polarized lights, and to have different optical path lengths of the two polarized lights incident on the Faraday rotator. An optical isolator using only the Faraday rotator as an element for rotating the plane of polarization. 2. The birefringent crystal of the parallel plate is made of a single crystal of the same material, and the angle between each optical axis and the normal vector of the light input / output surface is equal, and the normal vector 45 as the central axis
Each optical axis is set at an angle rotated by an integral multiple of ゜, the total thickness of the birefringent crystal that transmits the linearly polarized light (1) as ordinary light, and the linearly polarized light (2) orthogonal to the linearly polarized light (1). ) Is equal to the total thickness of the birefringent crystal that transmits as ordinary light, and the total thickness of the birefringent crystal that transmits the linearly polarized light (1) as extraordinary light is the same as the linearly polarized light that is orthogonal to the linearly polarized light (1). 2. The optical isolator according to claim 1, wherein the total thickness of the birefringent crystal transmitting as extraordinary light is equal to 2). 3. The optical waveguide according to claim 1, wherein the first and third two optical waveguides are arranged on one side in a state where the second optical waveguide is opposed to the other side. And at least one 45 ° Faraday rotator, at least one lens, and at least four parallel-plate birefringent crystals, between the second optical waveguide and the third optical waveguide. The linearly polarized light (1) included in the traveling light transmits each of the birefringent crystals of the parallel plate as ordinary light or extraordinary light, and the linearly polarized light (2) orthogonal to the linearly polarized light (1) is the linearly polarized light. When (1) is transmitted as ordinary light, it is transmitted as extraordinary light, and when it is transmitted as extraordinary light, it is transmitted as ordinary light. The optics when the linearly polarized light (1) passes through each of the parallel plate birefringent crystals. Of the total pass lengths are orthogonal to the linearly polarized light (1) Linearly polarized light (2) is equal to the sum of the optical path length as it passes through each whole birefringent crystals of the parallel plate, and with the output position of the second polarization match,
An optical circulator in which the optical path lengths of the two polarized lights incident on the Faraday rotator are set to be different, and only the Faraday rotator is used as an element for rotating the plane of polarization. 4. The parallel plate birefringent crystals are all made of a single crystal of the same material, the angle between each optical axis and the normal vector of the light entrance / exit surface is equal, and the normal vector is As the central 7 axes
Each optical axis is set at an angle rotated by an integral multiple of 45 °, the total thickness of the birefringent crystal that transmits the linearly polarized light (1) as ordinary light, and the linearly polarized light (orthogonal to the linearly polarized light (1)). 2) The total thickness of the birefringent crystal that transmits as ordinary light is equal, and the total thickness of the birefringent crystal that transmits the linearly polarized light (1) as extraordinary light, and the linearly polarized light that is orthogonal to the linearly polarized light (1) 4. The optical circulator according to claim 3, wherein (2) has the same total thickness of the birefringent crystal transmitting as extraordinary light.