JPH0429042B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polarizing prism that performs polarization separation of light, and relates to a polarizing prism that is less susceptible to distortion by using a biaxial crystal instead of a conventional uniaxial crystal. In other words, in a polarizing prism made of a uniaxial crystal, the incident light does not experience birefringence, but as it travels through the crystal, it becomes elliptically polarized due to the influence of distortion, and the degree of separation deteriorates significantly, whereas in a biaxial crystal, the incident light does not experience birefringence. This is based on the fact that the incident light travels while experiencing birefringence, so even if there is distortion, the disturbance in polarization is minimal.

Conventionally, a prism or the like that polarizes and separates light has been used in a circulator or the like used in an optical circuit. As shown in Fig. 1, there is a method of separating these lights into light 2 in the straight direction and light 3 in the reflected direction by using a prism 1 having birefringence and utilizing reflection and transmission, and as shown in Fig. 2. There is a method of separating the light into light 5 and 6 by transmitting the light through a prism 4 having birefringence as shown in FIG. However, in the former case, since the light 2 traveling straight passes through the slopes 7 and 8 where the prisms are joined, there is a large loss of light, and the separation of the light is incomplete. Also, the latter has a small separation angle.

To this end, a method has been proposed in which a polarizing prism having birefringence as shown in FIG. 3 is used and the separation angle is increased. This is done by making one side of the right angle A, for example, side b, parallel to the optical axis of the crystal, and side b
The diagonal θ is taken as θ=tan ⁻¹ n _{e /} n _p , where the refractive index of the extraordinary ray is ne and the refractive index of the ordinary ray is n _p .

When light B enters perpendicularly to side a from the upper left of the figure,
Polarized light whose electric field is perpendicular to the plane of the paper is reflected at the base C at an incident angle θ equal to the incident angle θ and exits in the direction D. Further, polarized light whose electric field is within the plane of the paper is reflected at an angle of θ+α and emitted in the E direction. For example, a calcite prism has a wavelength of 1.3 μm.
When used in , the angle of θ is set to 42.06°, and the beam is totally reflected at the base C and emitted in the D direction. Polarized light whose electric field is within the plane of the paper is reflected at an angle of θ+α=42.06°+5.88° and exits in the E direction. The light exiting in the D direction is further refracted by 3.79 degrees by the prism and exits in the D′ direction, so
The separation angle of the light when it exits the prism is as large as 9.67°. However, this polarizing prism is a uniaxial crystal and has only one refractive index in the optical axis direction, which is a direction in which there is no birefringence. Therefore, polarized light whose electric field is perpendicular to the plane of the paper from side a and polarized light whose electric field is within the plane of the paper will travel to the base C with the same refractive index n _p . It is sufficient if the quality of the crystal of the polarizing prism is uniform, but in reality it is not possible to obtain a good crystal, and the crystal is slightly distorted.

Therefore, when light enters perpendicularly to side a and passes through a distorted part of the crystal on its way to base C, the light is affected by birefringence caused by the distortion, and becomes two polarized lights corresponding to this birefringence. The incident light is separated into
Each light is affected by a refractive index different from n _p and propagates with a different phase, resulting in a combination of polarized light different from that of the incident light, for example, elliptically polarized light.

Therefore, when using this prism as an optical switch element, if polarized light whose electric field is within the plane of the paper is incident on side a, if there is no crystal distortion, it will be reflected at side C and all of it will be emitted in the E direction. However, as mentioned above, when there is distortion, a component becomes elliptically polarized light that vibrates in the direction perpendicular to the plane of the paper, and this component
It separates and emits in the D′ direction. This has the disadvantage that crosstalk occurs in the optical switch, degrading the performance as a polarizer.

An object of the present invention is to provide a polarizing prism that uses an optical biaxial crystal prism and is not affected by crystal distortion.

The present invention is characterized by an optically biaxial crystal material, which forms a triangular prism with a right triangle cross section, and each of the sides that sandwich the right angle of the triangle cross section is parallel to the principal axis of refractive index of the crystal, and the diagonal of one side sandwiching the right angle is taken as the arctangent of the ratio of the refractive index of light whose polarization plane is parallel to that side and the refractive index of light whose polarization plane is parallel to the other side sandwiching the right angle, and When light is perpendicularly incident on the sides of the substance sandwiching a right angle, total reflection is performed on the oblique sides to separate the polarized light, thereby achieving the above-mentioned purpose.

FIG. 4 is a diagram for explaining one embodiment of the polarizing prism according to the present invention.

In the present invention, when total reflection occurs on one surface C' of a right-angled triangular prism (angle A' is a right angle) of an optical biaxial crystal, the refractive index for polarized light in the plane of the paper remains unchanged after reflection. The refractive index for light polarized with an electric field perpendicular to the plane of the paper changes before and after reflection.

An optically biaxial crystal has X, Y, and Z that are perpendicular to each other.
There are optical elastic axes (principal axes) with different refractive indices in the directions of the three straight lines.

One of the principal axes of the refractive index above is perpendicular to the plane of the paper, so light B is directed from the left side of the figure perpendicularly to side a'.
When , the refractive index of polarized light whose electric field is perpendicular to the plane of the paper does not change after reflection.

Next, consider a case where the electric fields of the incident polarized light and the electric fields of the reflected light are both in the direction of the principal axis of the refractive index within the plane of the paper.

If the incident angle is θ ₁ and the reflection angle is θ ₁ ′=θ ₁ +α ₁ , then from the above conditions, the electric fields of the incident and reflected light are both in the direction of the principal axis of the refractive index, so θ ₁ +θ ₁ ′=90°. be. Further, assuming that the refractive index of incident light is n ₁ and the refractive index of reflected light is n ₂ , n ₁ sinθ ₁ =n ₂ sinθ ₁ '. Therefore, n ₁ sin θ ₁ = n ₂ cos θ ₁ and tan θ ₁ = n ₂ /n ₁ . Therefore, θ ₁ = tan ^-1 n ₂ /n ₁ .

As a specific example, consider the case of a KN _b O ₃ single crystal. The refractive index of this crystal in the X, Y, and Z principal axis directions is 2.20, 2.31, and 2.37, respectively, and in this example, in order to increase the light separation angle, the maximum value of the refractive index is
2.37 and the minimum value 2.20 are used as n ₁ and n ₂ , respectively.

From θ ₁ =tan ⁻¹ n ₂ /n ₁ , θ ₁ =42.9°.

When light B is incident perpendicularly to side a' from the left side of the figure into the optical biaxial crystal prism formed in this way, the polarized light whose electric field is perpendicular to the plane of the paper (refractive index 2.31) is equal to θ ₁ It is reflected at the base C' at a reflection angle θ ₁ and exits in the G direction. Polarized light (refractive index 2.37) whose electric field is within the plane of the paper is reflected at an angle of θ ₁ +α ₁ and emitted in the F direction. The refractive index at this time is 2.20. The angle α is 2.20sin42.9° = 2.37sin (42.9° + α ₁ )... Therefore, α ₁ = 4.20°.

Therefore, the sum of the incident angle θ ₁ and the reflection angle θ ₁ + α ₁ is 42.9° × 2
+4.20° = 90°, and the light emitted in the F direction is perpendicular to side b'. Also, the equation becomes 1.39, which is larger than the value 1 that satisfies the condition for total reflection, so the light emitted in the G and F directions is totally reflected at the base C'.

Also, when entering perpendicularly to side b' from the upper right as shown in Figure 5, if the electric field is polarized within the plane of the paper, then
It takes the optical path opposite to that described above and is emitted in the H direction perpendicular to side a'. Further, when the electric field is polarized light perpendicular to the plane of the paper, it is reflected at a reflection angle θ ₁ ' equal to the incident angle θ ₁ ' and is emitted in the Q direction.
In this way, when light is incident in the opposite direction, the reflection angle of polarized light whose electric field is perpendicular to the plane of the paper is α ₁ larger than the reflection angle of polarized light whose electric field is within the plane of the paper, which is the opposite of the function when light is incident from the upper left. Become.

Also, since the light exiting in the G direction in Figure 4 and the Q direction in Figure 5 is refracted by the prism at the time of exit, the separation angle is both
9.7° and is emitted in the G′ and Q′ directions.

As described above, the light B incident perpendicularly to the side a' has the refractive index n ₃ = 2.31 of the G polarization perpendicular to the plane of the paper and the refractive index n ₁ = 2.37 of the P polarization within the plane of the paper. Therefore, they have a birefringence difference of n ₁ - n ₃ = 0.06, and they travel through the crystal with this difference.

When the magnitude of crystal distortion is δ and the magnitude of birefringence is Δ, the disturbance of polarization is expressed on the order of δ/Δ. Therefore, even if the crystal strain magnitude δ is the same, if the birefringence magnitude Δ is originally approximately 0, such as in a uniaxial crystal, the inherent birefringence magnitude n ₁ − n ₃ =
The value of the polarization disturbance δ/Δ is extremely large compared to the case of a biaxial crystal with a ratio of 0.06. Therefore, in the case of a biaxial crystal, the disturbance of polarization is extremely small compared to the case of a uniaxial crystal, and its value is about 10 ^-3 to ^{10 -4} .

As described above, this embodiment is a polarizing prism that can perform efficient light separation without loss because it totally reflects the incident light, and can also eliminate the effects of crystal distortion.

The polarizing prism of the present invention can be applied to isolators, circulators, switches, branching filters, etc. used in optical circuits.

The present invention has been described above with reference to the embodiments. According to the present invention, a prism is formed which has incident and output end faces perpendicular to the principal axis of refractive index of an optical biaxial crystal and separates polarized light by total reflection of light. As a result, there is no need to worry about elliptically polarized light caused by slight distortion of the refractive index due to crystal distortion, and the effect of obtaining a polarizer with good performance is significant.

[Brief explanation of drawings]

Figures 1 and 2 are explanatory diagrams of a conventional light separation method, and Figure 3 is an explanatory diagram of the operation of a conventional polarizing prism.
4 and 5 are operational diagrams of one embodiment of the polarizing prism according to the present invention.

Claims (1)

[Claims] 1 Form a triangular prism-shaped prism with a right triangle cross section using an optically biaxial crystal material, and make each of the sides that sandwich the right angle of the triangular cross section parallel to the principal axis of refractive index of the crystal, and the sides that sandwich the right angle Take the diagonal as the arctangent of the ratio of the refractive index of light with a polarization plane parallel to one side and the refractive index of light with a polarization plane parallel to the other side sandwiching the right angle, and sandwiching the right angle of the substance. A polarizing prism is characterized in that when light enters perpendicularly from the side, the polarized light is separated by total reflection at the oblique side.