JPH0529882B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a Glan-Thompson prism that is applied to optical components such as optical isolators.

(Prior Art) In an optical communication system, various types of information are transmitted to a desired communication destination via an optical fiber using light from a laser light source. In this case, if the transmitted laser light is reflected from the outside for some reason and returns to the laser light source,
The oscillation of the laser light source becomes unsettling, and communication becomes affected. For this reason, an optical isolator is used to prevent reflected light from entering the laser light source and to ensure stable laser oscillation.

FIG. 6 shows an example of such an optical isolator, where 1 and 2 are first and second polarizers made of prisms, 3 is a Faraday rotator disposed between them, and 4 and 5 are first and second polarizers. A polarizer holder 6 that supports the second polarizers 1 and 2 is a permanent magnet.

According to this structure, when light is incident on the first polarizer 1 in the forward direction from the laser light source, only the light component having a fixed polarization plane with respect to the optical axis L shown by the broken line passes through the incident light. and components with a plane of polarization perpendicular to it are not passed. Similarly, when the reflected light is incident on the second polarizer 2 in the opposite direction, only light components having a certain plane of polarization are passed through. The direction of the polarization plane of the light that has passed through the first and second polarizers 1 and 2 in the forward direction and in the reverse direction is rotated by 45 degrees by the Faraday rotator 3. Since it is irreversible with respect to the direction, the polarization plane of the light exiting the Faraday rotator 3 in the forward and reverse directions is
The difference will be 90°. Therefore, in the first and second polarizers 1 and 2, light from opposite directions does not return to the incident direction, but is reflected by the polarizers on the slopes of the prisms. As a result, only the polarized light component in the forward direction is focused on the optical fiber, and in the reverse direction, both polarized light components are prevented from returning to the incident direction, thereby making it possible to prevent the influence of reflected light.

In such an optical isolator, the first or second polarizer 1 or 2, which acts to pass only the light having a specific polarization plane among the incident light, is a gran as shown in FIG. Thomson prisms are often used. This Glan-Thompson prism 10 is made of a first rutile single crystal, for example.
The prism piece 10a and the second prism piece 10b are integrally formed by an adhesive layer 11. Rutile single crystal has a high birefringence of 0.29, is not water-soluble, is resistant to acids and alkalis, and is chemically stable, making it an excellent prism material. First and second prism pieces 10a, 1
For example, 0b is the refractive index of ordinary light for a wavelength of 1.3 μm.
n _O =2.44 and an extraordinary refractive index n _E =2.69, while the adhesive layer 11 has a refractive index of 1.4 to 1.6.
Note that the apex angle θ _t of the first and second prism pieces 10a and 10b is set to 30° to 40°.

Now, the operating principle of the Glan-Thompson prism will be explained.

In the following, we will consider a case where n _O < n _E , such as in a rutile single crystal, but the principle is the same even in a case where n _O > n _E , such as in calcite, except that the roles of ordinary and extraordinary light are reversed.

Generally, when light is incident from a medium with a high refractive index to a medium with a low refractive index, total reflection occurs when the angle of incidence exceeds a certain value.

In the Glan-Thompson prism shown in Figure 4, the first
When light enters the adhesive layer ₁₁ from _the prism piece 10a _of n _O ...(2).

However, θin: the angle between the normal l of the adhesive layer and the optical axis L.

n _a : refractive index of adhesive layer.

_nO : refractive index for ordinary light.

n _E : refractive index for extraordinary light.

When n _O < n _E , extraordinary light is totally reflected, but there is an incident angle at which ordinary light is not totally reflected, and when light is incident at such an angle, it operates as a polarizer. Expressed as a formula, it is in the range of sin ^-1 (n _a /n _E )<θin<sin ^-1 (n _a /n _O ) (3).

Next, the relationship between the incident angle θin _O of light at the boundary between air and the prism piece 10a and θin is as follows, if θin _O is defined as shown in Fig. 1, θin = θ _t + sin ^-1 (sin θin _O /n _P ) It is indicated by.

However, θ _t : apex angle of the prism, n _P : refractive index of the prism, so the operating angle range for θin _O is sin ^-1 {n _E sin (sin ^-1 (n _a /n _E )−θ _t ) }＜θin _O ＜sin
^-1 {n _O sin (sin ^-1 (n _a /n _O )−θ _t )}...(4).

For example, when a wavelength of 1.3 μm is incident on a rutile single crystal, n _O = 2.44, n _E = 2.69, and if an adhesive layer with a refractive index n _a = 1.54 is used, the operating incident angle range with respect to θin is 34.9° < If θin < 39.1 and the apex angle is set to θ _t = 37°, then -5.6° < θin _O < 5.2°, and if the apex angle is set to θ _t = 34°, it will operate at 2.5° < θin _O < 12.6°. become.

Of course, normally θ _t is determined so that when light enters the Glan-Thompson prism perpendicularly, that is, θin _O = 0°, it operates as a polarizer, but in the case of optical isolators, etc., the reflection from the prism The prism may be placed diagonally to prevent the light from returning to its original position _.
It is necessary to decide.

Furthermore, since n _O and n _E change depending on the wavelength, it is necessary to design according to the wavelength used.

As a result, by using the Glan-Thompson prism 10 as a polarizer, only light having a specific polarization plane among the incident light can be passed.

Note that when such a Glan-Thompson prism is used as a polarizer, an extinction ratio of 40 dB or more is usually required.

(Problem to be solved by the invention) By the way, in the conventional Glan-Thompson prism,
Since the conditions for total reflection change depending on the thickness of the adhesive layer, there is a problem that the extinction ratio characteristics become unstable. In other words, the thickness of the adhesive layer was conventionally considered to be irrelevant as a condition for total reflection of extraordinary light, but the inventors' experiments revealed that the conditions for total reflection differ depending on the thickness of the adhesive layer. It was confirmed. For example, when the adhesive layer 11 is formed with a smaller thickness than that shown in FIG. 4 as shown in FIG. It was confirmed that there was no total reflection. For this reason, the extraordinary light is not completely extinguished and the extinction ratio deteriorates.

The present invention was made in response to the above-mentioned problems, and it is an object of the present invention to provide a Glan-Thompson prism that provides stable extinction ratio characteristics.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention comprises two prism pieces each having a right triangular cross section, whose hypotenuses are bonded together with an adhesive layer. In the Glan-Thompson prism, the thickness d of the adhesive layer is expressed by the following formula: However, λ: wavelength of the light used, n ₁ : the larger value of the refractive index for ordinary light and the refractive index for extraordinary light of the crystal of the prism piece used, n _a : refractive index of the adhesive layer, θin: adhesive surface The angle between the normal line and the optical axis is set to satisfy the following.

(Function) By setting the film thickness of the adhesive layer so as to satisfy the above formula, the extraordinary light can be almost completely reflected, thereby making it possible to stabilize the extinction ratio characteristic. Therefore, by using such a Glan-Thompson prism as a polarizing prism and applying it to an optical isolator or the like, a highly reliable optical component can be obtained.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing an embodiment of the Glan-Thompson prism of the present invention, in which 10a and 10b are a first prism piece and a second prism piece each made of rutile single crystal and having a right triangular cross section. It has an apex angle θ _t of °. Reference numeral 11 denotes an adhesive layer, which forms the Glan-Thompson prism 10 by bonding and integrating the oblique sides of each prism piece 10a, 10b. The thickness d of this adhesive layer 11 is set as described below. That is, it is set to a value that stabilizes the extinction ratio, and this value can be determined by solving Maxwell's equation.

Briefly, it is determined by utilizing the fact that the incident electric field Ein and the outgoing electric field Eout generated by light propagating through the adhesive layer 11 are attenuated as shown by the following equation.

Here, yd is the depth at which the light penetrates, and is expressed by the following equation.

However, λ: wavelength of the light used, n ₁ : refractive index of the prism used for ordinary light n _O
and the refractive index n _E for extraordinary light, whichever is larger, n _a : refractive index of the adhesive layer, θin : angle between the normal l of the adhesive surface and the optical axis L.

Further, the extinction ratio can be estimated by squaring the above equation (5) as follows.

Since an extinction ratio of 40 dB or more is normally required for use as a polarizing prism, taking this condition into account, equation (7) can be expressed as the following equation.

As a result, d>4.6×yd is obtained, and d is set as follows: Figure 2 is a characteristic diagram showing an example of the calculation results. Rutile prism, wavelength 1.31 μm, n _O = 2.44, n _E = 2.69,
This is the case when n _a =1.54 and θ _t =34°. The vertical axis shows the extinction ratio [dB], the horizontal axis shows the incident angle θin _O [°], and the film thickness d
is shown as a parameter. In order to maintain an extinction ratio of 40 dB or more over a wide range of incident angles, it is advantageous to form the film with a large thickness d.

For example, in order to obtain an extinction ratio of 40 dB or more when θin _O =4°, the thickness d of the adhesive layer 11 must be 3 μm or more. (9)
Calculating with the formula, if θin _O = 4° and θ _t = 34°, θin =
35.5°, and by substituting λ=1.31 μm, n ₁ =2.69, and n _a =1.54, d>3.7 μm. Note that Figures 2 and 3 were obtained by precisely solving Maxwell's equation, and although the values are slightly different from the simple equation (9), it has been confirmed that there is no major difference.

According to this embodiment, when the first and second prism pieces 10a and 10b are bonded together by the adhesive layer 11, the adhesive layer 1
By setting the film thickness d of 1, it is possible to stably secure an extinction ratio of 40 dB or more, which is acceptable for practical use as a polarizing prism. Therefore, by using the Glan-Thompson prism as in this embodiment as a polarizing prism for an optical isolator or the like, a highly reliable optical component can be obtained.

[Effects of the Invention] As described above, according to the present invention, since the thickness of the adhesive layer that adheres each prism piece together is set to satisfy a predetermined relationship, stable extinction ratio characteristics can be obtained. I can do it.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing an embodiment of the Glan-Thompson prism of the present invention, Figs. 2 and 3 are characteristic diagrams obtained by this embodiment, and Figs. 4 and 5 show a conventional prism. 6 is a cross-sectional view of an optical isolator to which a Glan-Thompson prism is applied. 10...Glan Thompson prism, 10a...
First prism piece, 10b... Second prism piece, 11... Adhesive layer, d... Film thickness of adhesive layer, n _E ...
...Refractive index for extraordinary light, n _O ...Refractive index for ordinary light, n _a ...Refractive index for adhesive layer.

Claims (1)

[Claims] 1. In a Glan-Thompson prism formed by bonding the hypotenuses of two prism pieces each having a right triangular cross section with an adhesive layer, the film thickness d of the adhesive layer is expressed as follows: However, λ: wavelength of the light used, n ₁ : the larger value of the refractive index for ordinary light and the refractive index for extraordinary light of the crystal of the prism piece used, n _a : refractive index of the adhesive layer, θin: adhesive surface A Glan-Thompson prism characterized by being set to satisfy the angle between the normal line and the optical axis. 2. The Glan-Thompson prism according to claim 1, wherein the two prism pieces are made of rutile single crystal.