JPH07119895B2 - Beam splitter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beam splitter, and more particularly to a beam splitter capable of changing the intensity ratio of transmitted light and reflected light with respect to incident light having a constant wavelength. It is about the splitter.

[0002]

2. Description of the Related Art A beam splitter is an optical element that divides incident light into two, and there are various types, and one of which uses a pair of right-angle prisms is known.
This is one in which the slanting surfaces of a pair of right-angle prisms are bonded together, and a film is formed between the slanting surfaces to reflect a part of light and transmit the rest. A beam splitter 30 of this type is shown in FIG. Incident light 35 is reflected by right-angle prism 31.
When the light is incident on, a part of the light is reflected by the reflection / transmission film 33, and the rest is transmitted to reach another rectangular prism 32. Then, the reflected light 36 is emitted from the prism 31 and the transmitted light 37 is emitted from the prism 32 in directions orthogonal to each other, and the incident light is separated.

In such a beam splitter, a multilayer film is usually used as a reflection / transmission film formed between slopes.
The material and thickness of this multilayer film are determined so as to obtain a desired intensity ratio of transmitted light and incident light according to the wavelength and incident angle of incident light used. Therefore, the specific
For a splitter, the ratio of transmittance to reflectance is uniquely determined with respect to the wavelength of incident light. Therefore, various beam splitters in which the reflection / transmission film is changed according to the wavelength of incident light and the ratio of the transmittance and the reflectance to be obtained are manufactured and used for various purposes.

[0004]

As described above, in the conventional beam splitter, the intensity ratio of transmitted light and reflected light is determined with respect to the wavelength of incident light. Therefore, when it is desired to change the intensity ratio of transmitted light and reflected light with respect to incident light having a constant wavelength, the beam splitter itself must be replaced. Further, when changing the wavelength of incident light to be used while keeping the intensity ratio constant, the beam splitter itself must be replaced. Therefore, it is necessary to prepare a number of beam splitters corresponding to the wavelength of the incident light and the desired intensity ratio of the reflected light and the transmitted light, which is expensive. Further, the beam splitter has to be replaced every time the wavelength of the incident light and the intensity ratio of the reflected light and the transmitted light to be obtained are changed, which is inconvenient.

On the other hand, even in the conventional beam splitter as shown in FIG. 6, if the incident angle of the incident light is changed, the intensity ratio of the reflected light and the transmitted light is changed for a constant wavelength of the incident light. be able to. However, in this case, since the outgoing directions of the transmitted light and the reflected light change, the position of the light receiving element has to be changed. Since such a position change is delicate, it is troublesome and a device for changing the position becomes complicated.

Further, as a method of changing the intensity ratio of the reflected light and the transmitted light without changing the incident angle, there is a combination of a polarization beam splitter and a 1/2 wavelength plate. This is because a half-wave plate is placed between the polarization beam splitter and the light source, and the half-wave plate is rotated to rotate the plane of polarization of the incident light so that the light is transmitted from the polarization beam splitter. The intensity ratio of light (p-polarized light) and reflected light (s-polarized light) is changed. However, when using such a polarization beam splitter, the incident light must be linearly polarized light,
In addition, the apparatus has to be used in combination with a half-wave plate, and the size and complexity of the apparatus have been increased, resulting in increased cost.

Therefore, an object of the present invention is to solve the above-mentioned problems, and it is possible to change the intensity ratio of transmitted light and reflected light with respect to incident light of a certain wavelength with a simple structure. It is to provide a beam splitter capable of doing so.

[0008]

In order to achieve the above object, a beam splitter of the present invention comprises a pair of optical blocks each having a surface facing each other, and an optical path portion of a light beam between the surfaces. an elastic body provided only in a region excluding, and means for applying a force to the elastic body to said surface perpendicular direction, the intensity ratio of the reflected light and transmitted light by changing the said surface distance It is characterized by changing the.

Here, the light between each surface of the optical block is
A layer having a refractive index lower than that of the optical block is provided in the optical path portion of the beam so that the incident light entering the optical block is emitted to the one surface of the optical block while satisfying the total reflection condition. Has become.

The low refractive index layer may be air as long as it has a refractive index lower than that of the optical block.

[0011]

When incident light enters one optical block and then exits while satisfying the total reflection condition for the one surface, total reflection is performed on that surface. But at the same time, the evanescent wave is on the other side of the plane, that is,
It occurs outside the optical block. The electric field of the evanescent wave has a property of exponentially decreasing as the distance from the surface increases. Then, the one surface of the other optical block is arranged to face the surface of the optical block. Therefore, an evanescent wave having an electric field of an amount corresponding to the inter-plane distance is captured by the other optical block, propagates in the optical block as transmitted light, and is emitted. Then, the evanescent wave that is not captured by this optical block becomes reflected light reflected by the one surface of the first optical block, propagates in the optical block, and is emitted.

When a force is applied to an elastic body provided between the respective surfaces of the pair of optical blocks in a direction perpendicular to these surfaces, the elastic body is elastically deformed, and the space between the surfaces of the optical blocks. The distance can be changed reversibly. Here, an evanescent wave having an electric field corresponding to this inter-plane distance propagates as transmitted light, and the intensity of reflected light also changes. Therefore, the intensity ratio of transmitted light and reflected light changes.

According to the present invention, since the interplanar distance is changed by elastically deforming the elastic body as described above, the interplanar distance can be continuously changed. Therefore, by slightly changing the interplanar distance, it is possible to catch the change in the electric field of the evanescent wave that exponentially sharply decreases with a slight change, and it is possible to change the intensity of the transmitted light.

Therefore, according to the beam splitter of the present invention, it is possible to change the intensity ratio of the reflected light and the transmitted light with respect to the incident light having a constant wavelength with a simple structure.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram for explaining a beam splitter 20 which is a first embodiment of the present invention. Beam splitter 2
Reference numeral 0 denotes a pair of right-angle prisms 1 and 2 arranged with their respective slopes 1a and 2a facing each other, an LB film (Langmuir-Blodgett film) 3 provided between these slopes, and a prism for holding these prisms. It consists of holder-4. The holder 4 includes a support frame 10 and holding portions 8b and 8a supported by the support frame 10 and holding the prisms 1 and 2, respectively. A screw 9 screwed into the support frame 10 is joined to the holding portion 8a.

Slopes 1a and 2a of the right angle prisms 1 and 2
One or both of them have the LB except for the central part which becomes the optical path.
The film 3 is formed. Holding part 8b and 8a of holder-4
, Sandwiches the right-angled portions 1b and 2b of the prisms 1 and 2, respectively. Of these, the holding portion 8b is a holder-4.
It is fixed to the support frame 10 of. On the other hand, the holding portion 8a is joined to a screw 9, and the screw 9 is screwed into a screw hole formed in the support frame 10. The screw 9 extends in a direction perpendicular to the slopes 1a and 2a of the prism, and its rotation moves in a direction perpendicular to the slopes 1a and 2a. A tensile force or a compressive force is applied to the LB film 3 via the prism 2 by the movement of the screw 9 and the holding portion 8a. As a result, the LB film 3 is elastically deformed, and the slopes 1a and 2a are
The distance can be changed. This change in the interplanar distance uses the elastic deformation of the LB film,
It is reversible and capable of subtle changes.

The beam splitter 20 is arranged so that the incident light 5 enters the prism 1 through its incident surface 1c and reaches the inclined surface 1a. Further, the material of the prism is selected so that the incident light 5 satisfies the condition of total reflection on the slope 1a. That is, for the wavelength of the incident light 5, the refractive index of the prism 1 is n ₁ , the refractive index of the layer formed between the slopes 1a and 2a is n ₂ , and the incident angle of the incident light on the slope 1a is When θ ₁ is set, the following formula 1 is obtained. In this embodiment, the beam splitter 2 has no light path portion between the slopes 1a and 2a.
When 0 is placed in the air, it is filled with air. Therefore, n ₂ is about 1. Further, since the beam splitter 20 is arranged so that the incident light 5 is vertically incident on the incident surface 1c of the prism 1, the incident angle θ ₁ is 45 degrees. From the above, n ₁ must satisfy the following formula 2. Therefore, in this embodiment, as the prisms 1 and 2, a BK7 glass (a borosilicate crown glass which is a kind of optical glass) having a refractive index n ₁ of greater than 1.5 in a wide wavelength range of incident light is used. One) is used. Therefore, it is possible to satisfy the total reflection condition with respect to incident light of various wavelengths.

[0019]

[Formula 1] n ₁ sin θ ₁ > n ₂

[0020]

[Equation 2] n ₁ > 2 ^1/2 n ₂ = 2 ^1/2

According to the first embodiment, the distance between the inclined surfaces 1a and 2a of the prism is changed based on the elastic deformation of the LB film 3 to change the intensity ratio of transmitted light and reflected light. Less than,
This principle will be described.

Incident light 5 satisfies the condition of total reflection and slope 1a
3A, an evanescent wave is generated on the other side of the slope 1a, that is, outside the prism 1, as shown in FIG. 3A. Therefore, when the prism 2 is not present, the incident light 5 causes the evanescent wave and then the reflected light 6
Then, the light propagates through the prism 1 again and exits from the exit surface 1d. The electric field E of the evanescent wave has a property of exponentially decreasing with distance from the slope 1a as shown in FIG. 3B, and is given by the following mathematical formula 3. Here, x is the distance from the inclined surface 1a, E ₀ is the electric field of the incident light 5, n ₂ is the refractive index outside the inclined surface 1a of the prism 1, and λ is the wavelength of the incident light 5 in vacuum.

[0023]

[Equation 3] E = E ₀ exp (−2πn ₂ x / λ)

Where an evanescent wave is generated,
As shown in FIG. 3A, when the prism 2 is moved closer, a part of the evanescent wave is captured by the prism 2 on the slope 2a. Then, the light propagates through the prism 2 as transmitted light and is emitted from the emission surface 2c.
The electric field of the transmitted light is equal to the electric field of the evanescent wave at the position of the slope 2a. Therefore, the electric field of the transmitted light is equal to the electric field E when x is the distance between the slopes 1a and 2a in FIG. 3B, and is given by the above mathematical expression 3.
Since the intensity I _{1 of} transmitted light is equal to the square of the electric field,
It is given by Equation 4 below.

[0025]

## EQU4 ## I ₁ = | E | ² = E ₀ ² exp (-4πn ₂ x /
λ)

On the other hand, the evanescent wave that is not captured by the prism 2 as transmitted light propagates in the prism 1 as reflected light 6 and is emitted from the emission surface 1d as shown in FIG. 3A. Here, the intensity of the incident light is I
_{Assuming that 0} (= | E ₀ | ² ), the intensity I _{2 of the} reflected light is given by Equation 5 below. Therefore, the intensity ratio I ₁ / I ₂ of the transmitted light and the reflected light is given by the following Equation 6.

[0027]

## EQU5 ## I ₂ = I ₀ -I ₁ = E ₀ ² {1-exp (-4πn ₂
x / λ)}

[0028]

## EQU6 ## I ₁ / I ₂ = E ₀ ² exp (-4πn ₂ x / λ) /
E ₀ ² {1-exp (-4πn ₂ x / λ)} = exp (-
4πn ₂ x / λ) / {1-exp (-4πn ₂ x / λ)}

As described above, the intensity ratio of the transmitted light and the reflected light changes according to the distance x between the slopes 1a and 2a. That is, the smaller the inter-slope distance x, the greater the intensity of transmitted light and the less intensity of reflected light. On the contrary, the larger the distance between the slopes, the smaller the intensity of the transmitted light and the greater the intensity of the reflected light.

In the present invention, the distance between the slopes is changed by applying a force to the LB film 3 as an elastic body formed between the slopes 1a and 2a by the rotation of the screw 9 to elastically deform it. When the screw 9 is rotated in the tightening direction with respect to the support frame 10, the intensity of transmitted light gradually increases and the intensity of reflected light decreases. On the contrary, when the screw 9 is rotated in the weakening direction with respect to the support frame 10, the intensity of the transmitted light gradually decreases and the intensity of the reflected light increases.

In this embodiment, since the distance between the slopes is changed by the elastic deformation of the LB film 3 as described above, the distance can be finely adjusted. Also, screw 9
Since the force is applied to the film by using, it is possible to continuously change the force and thus the interplanar distance. In addition, since elastic deformation is reversible, once the compressive force is applied to compress and reduce the distance between the slopes, if the force is reduced, the elastic membrane will surely extend and reduce the distance between the slopes. Can be increased. Furthermore, since the applied force and the amount of compression are determined one-to-one in elastic deformation, it is possible to adjust the distance between the slopes by adjusting the applied force, and thus the desired intensity of transmitted light and reflected light. The ratio is obtained.

Here, the LB film 3 of this embodiment is a Langmuir-Blodgett film (LB film) made of cadmium arachidate ((C ₁₉ H ₃₉ COO) ₂ Cd), and has a refractive index of 1. 52, the thickness is, for example, 2.75 [n
m]. LB film 3 of such cadmium arachidate
Has a proper elastic coefficient, and therefore, when used in the beam splitter 20 of the present invention, elastically deforms subtly in response to the rotation of the screw 9. Therefore, prisms 1 and 2
The distance x between the slopes 1a and 2a can be subtly changed. Therefore, the intensity ratio of the transmitted light 7 and the reflected light 6 can be continuously changed.

The inventors of the present invention conducted the following experiment on the change of the intensity ratio of the transmitted light and the reflected light by the beam splitter 20 of FIG. Here, right angle prisms 1, 2
For this, a right angle prism of BK7 in which each surface sandwiching a right angle is a square having a side of 31 [mm] is used. A He-Ne laser was used as a light source of incident light. The wavelength of incident light obtained by this light source was 632.8 [nm], and the incident power was 3.2 [mW]. An LB film of cadmium arachidate was formed to a thickness of 2.75 [nm] on each of the slopes 1a and 2a of the prisms 1 and 2 as shown in FIG. 4A. Therefore, the actual thickness of the LB film 3 is
It was 5.5 [nm].

In the beam splitter shown in FIG. 1,
The screw 9 was tightened first and then loosened. The results of this experiment are shown in Table 1 and FIG. 4B.

[0035]

[Table 1]

As is clear from Table 1 and FIG. 4B, when the screw is tightened to increase the compressive force applied to the LB film 3, the transmittance is increased and the reflectance is decreased. On the contrary, when the screw was loosened and the compressive force applied to the LB film 3 was reduced, the transmittance decreased and the reflectance increased. That is, it was proved that the intensity ratio of the transmitted light and the reflected light was changed by rotating the screw and changing the force applied to the LB film 3.

Next, a second embodiment of the present invention will be described with reference to FIG. The beam splitter 20 according to the second embodiment has substantially the same configuration as the beam splitter 20 according to the first embodiment. The same components as those of the first embodiment will be described with the same reference numerals.

The beam splitter 20 of the second embodiment has a pair of right-angled prisms 1 and 2 and a holder 4 for holding the prisms 1 and 2 as in the beam splitter 20 of the first embodiment. The LB film 3'which is an elastic body has the slopes 1a and 2 of the prism.
It is provided on both or one of a. Here, unlike the LB film 3 of the first embodiment, the LB film 3'is formed on the entire surface of the slopes 1a and / or 2a.

In such a case, the incident light 5 is totally reflected by the inclined surface 1a which is the interface between the prism 1 and the LB film 3 '.
The materials of the prism 1 and the LB film 3'are selected. For example, SFS1 glass (one of flint glass which is a kind of optical glass) is used as the prism 1, and a perfluoro type LB film is used as the LB film 3 ′. The SFS1 glass has a refractive index n ₁ larger than 1.9 with respect to incident light in a wide wavelength range. On the other hand, the perfluoro-type LB film 3'is the H-group of the cadmium arachidate as the F-group.
Substituted with a group ((C ₁₉ F ₃₉ COO) ₂ Cd)
It is an LB film produced using. Perfluoro LB
The film 3'has approximately the same thickness as the LB film 3 of the first embodiment . The LB film 3'is a material through which light is sufficiently transmitted. The refractive index n ₂ is about 1.3. Therefore, when the incident light 5 is incident on the slope 1a at an incident angle of 45 degrees, the total reflection condition (the following formula 7) is satisfied.

[0040]

[Equation 7] n ₁ > 2 ^1/2 n ₂

Therefore, when the incident light 5 is incident on the inclined surface 1a, an evanescent wave is generated in the LB film 3 ', and a part of the evanescent wave is caught by the prism 2 on the inclined surface 2a and propagates as transmitted light 7 in the prism 2 while remaining. Will propagate as reflected light 6 in the prism 1. The electric field of the transmitted light 7 is
It is equal to the electric field of the evanescent wave on the slope 2a. The electric field E of the evanescent wave is, as described above, the following Equation 8
Given in. Therefore, the intensity ratio of the transmitted light and the reflected light changes according to the distance x between the slopes 1a and 2a.

[0042]

[Equation 8] E = E ₀ exp (−2πn ₂ x / λ)

In the second embodiment, as in the first embodiment, the distance between the slopes is changed by elastically deforming the LB film 3 '. That is, since the perfluoro type LB film 3'of this embodiment also has an appropriate elastic coefficient, elastic deformation occurs by rotating the screw 9 of the holder-4 and applying a force thereto via the prism 2 The distance x can be changed subtly and continuously. The intensity ratio of the transmitted light and the reflected light changes according to the change in the distance.

The beam splitter of the present invention is not limited to the above embodiment, and various modifications can be made. For example, the elastic body provided between the slopes of the prism may be any that can be elastically deformed by applying a force. For example, one or a plurality of beads, such as beads, made of an elastic material may be arranged.

Further, the prisms 1 and 2 are not limited to the rectangular prism, and various optical blocks can be used as long as they have a flat surface 1a as a reflecting surface and a flat surface 2a as an incident surface. For example, a glass plate may be used.

Furthermore, in order to cause total reflection, the slope 1
What is interposed between a and 2a is not limited to the air or the LB film, and may be a layer having a refractive index lower than that of the prism 1.

Further, the means for applying a force to the elastic body provided between the prisms 1 and 2 is not limited to one that applies a force to the elastic body by rotating the screw 9 and pushing the prism. Anything that can continuously change the force may be used. If necessary, the elastic body may be discontinuously deformed by means of discontinuously changing force.

The beam splitter of the present invention can be further used in a control device for keeping the intensity ratio of transmitted light and reflected light constant when the wavelength of incident light changes. This control device will be described with reference to FIG.

The control device 45 comprises a light source 41, a space controller 42, and the beam splitter 20 of the present invention. The light source 41 causes light to enter the beam splitter 20. From the light source 41, information about the wavelength is input to the space controller 42. The space controller 42 stores in advance a look-up table regarding the relationship between the wavelength of incident light and the force to be applied to the beam splitter 20. The spacing controller 42 rotates the screw 9 on the holder 4 of the beam splitter 20 according to the lookup table to change the spacing between the slopes 1a and 2a.

According to the control device 45, even if the wavelength of the incident light changes, the distance between the slopes of the beam splitter can be adjusted accordingly. Therefore, it is possible to maintain a constant intensity ratio of transmitted light and reflected light even for incident light of different wavelengths.

The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

[0052]

As described above, according to the beam splitter of the present invention, the elastic body provided between the slopes of the pair of optical blocks facing each other is elastically deformed to reversibly reduce the distance between the slopes. By changing it, it is possible to change the intensity ratio of transmitted light and reflected light with respect to incident light having a constant wavelength. Further, the distance between the pair of prisms is changed by elastically deforming the elastic body, so that the distance can be changed subtly and continuously, and thus the intensity ratio can be changed subtly and continuously. Can be changed to. Further, since the distance can be slightly changed in this way, the intensity ratio of the transmitted light and the reflected light can be changed even for the incident light in the short wavelength region.

Further, by using the beam splitter of the present invention, it is possible to provide a control device capable of maintaining the intensity ratio of transmitted light and reflected light constant even if the wavelength of incident light varies.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a beam splitter according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a beam splitter according to a second embodiment of the present invention.

FIG. 3A is an explanatory diagram of the operation of the beam splitter of the present invention.

FIG. 3B is a diagram showing a relationship between a distance from a boundary surface that forms total reflection and an electric field of an evanescent wave.

FIG. 4A is an explanatory diagram showing a formation state of an LB film of a beam splitter used in an experiment.

FIG. 4B is a diagram of experimental results showing how the transmittance and reflectance change.

FIG. 5 is a diagram of a control device using the beam splitter of the present invention.

FIG. 6 is an explanatory diagram of a conventional beam splitter.

[Explanation of symbols]

 1 Right Angle Prism 2 Right Angle Prism 3 LB Film 3'Perfluoro LB Film 4 Holder 9 Screw 20 Beam Splitter 45 Controller 41 Light Source 42 Spacing Controller

Claims (4)

[Claims] 1. A pair of optical blocks, each surface of which is opposed to each other, an elastic body provided only in an area excluding an optical path portion of a light beam between the surfaces, and the elastic body. A beam splitter comprising: a means for applying a force in a direction perpendicular to a surface, wherein the intensity ratio between transmitted light and reflected light is changed by changing the distance between the surfaces. 2. A layer having a refractive index lower than that of the pair of optical blocks is provided in an optical path portion of a light beam between the surfaces of the pair of optical blocks. The beam splitter according to claim 1. 3. The beam splitter according to claim 1, wherein the elastic body is an LB film. 4. The beam splitter according to claim 1, wherein the optical block comprises a rectangular prism.