JP2553365B2 - Multi-layer polarization beam splitter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization separation device for separating a specific polarization component of light, and more particularly to a surface acoustic wave (SAW: Surface Acoustic Wave) propagating on a solid surface. 2) relates to a polarization diffractive element (multilayer polarization beam splitter) utilizing the function of forming an optical diffraction grating having polarization selectivity and capable of polarization-splitting incident light.

[Conventional technology]

As a conventional polarization separation optical component, there is a polarization beam splitter equipped with a dielectric multilayer film. This is the wavelength of light that can be used by changing the design value of the dielectric multilayer film, that is, the film thickness and refractive index. And a ratio for separating light of a specific polarization state (corresponding to the ratio of the amount of 0th-order light and the amount of ± 1st-order diffracted light; hereinafter referred to as polarization separation ratio) has the advantage of being freely selectable.

[Problems to be solved by the invention]

However, it has to be redesigned for light of another wavelength, its utility value is low for light sources with a wide wavelength band, and the polarization splitting ratio is determined by the design value and cannot be changed. I have

An object of the present invention is to realize a polarization diffraction element having no wavelength dependence and a variable polarization separation ratio.

[Means for solving problems]

As an optical-related device that has been proposed and put into practical use by utilizing SAW propagating on a solid surface, an optical deflection device that uses a sinusoidal unevenness of the surface formed by SAW and a periodic refractive index change as a diffraction grating (for example, , A light diffracting device utilizing surface acoustic waves (Japanese Patent Laid-Open No. 62-94831)) and a frequency shifter for light, all of which are based on Raman-Nurse diffraction.
It was applied by focusing on the first-order diffracted light. However, due to the diffraction state of SAW, the polarization state of the incident light causes
Nothing has been discussed about what kind of effect it will have, nor has any equipment been used.

This invention is based on that point, namely, the polarization state of incident light and
Focusing on the relationship with the diffraction effect of SAW, the polarization selectivity of the SAW element was confirmed by experiments described below, and it was decided to utilize it.

More specifically, the device of the present invention includes a light-transmissive first piezoelectric substrate and light having a polarization plane having a predetermined polarization among lights incident on the first piezoelectric substrate. A first cross finger shape provided on the surface of the first piezoelectric substrate so as to generate a surface acoustic wave on the first piezoelectric substrate that selectively diffracts only a part thereof. An electrode, a second light-transmitting piezoelectric substrate arranged so that light transmitted through the first piezoelectric substrate is incident, and a predetermined amount of light incident on the second piezoelectric substrate On the surface of the second piezoelectric substrate so as to generate a surface acoustic wave on the second piezoelectric substrate that selectively diffracts only a part of light having a polarization plane having And a second interdigitated electrode provided.

[Action]

Due to the surface acoustic waves generated on the surfaces of the first piezoelectric substrate and the second piezoelectric substrate, only a part of the incident light having a polarization plane with a predetermined polarization is selectively diffracted. To be done.

On the other hand, regarding the diffraction effect of the SAW element, the relationship between the wavelength λ of the incident light and the diffraction angle θ is 2d · sin θ = λ (1) when the grating spacing of the sinusoidal grating by SAW is d. Are known.

From this relationship, although the difference in wavelength affects the diffraction angle, there is no wavelength dependence in separating polarized light. If the diffraction angle θ is desired to be a desired angle, the grating spacing d may be set according to the wavelength λ of the incident light.
The lattice spacing d can be controlled by the frequency input to the interdigital electrodes provided to generate SAW. The polarization split ratio is
It can be controlled by the input power to the interdigital electrodes.

〔Example〕

FIG. 1 shows an embodiment of a multilayer polarization beam splitter according to the present invention.

In this embodiment, a piezoelectric substrate is used, which has a property that is confirmed in an experiment described later that only light having a polarization state perpendicular to the SAW propagation direction is partially diffracted.

Optically polished first piezoelectric substrate 3 having optical transparency
SAW is generated on the surface of the SAW element having the structure in which the first interdigital finger-shaped electrode 4 for generating SAW is provided on the surface of and the surface of the second piezoelectric substrate 5 which is also optically polished and has a light transmitting property. Second
There is a SAW element having a structure in which the interdigitated electrode 6 of
The piezoelectric substrate 3 and the second piezoelectric substrate 5 are arranged such that the SAW propagation directions are orthogonal to each other and the SAW propagation surfaces are parallel to each other. By arranging the first piezoelectric substrate 3 and the second piezoelectric substrate 5 in this way, the S-polarized component 8 of the incident light 7 is diffracted by the SAW on the first piezoelectric substrate 3 and the incident light The P-polarized component 9 of 7 will be diffracted by the SAW on the second piezoelectric substrate 5. For the absolute value and the ratio of the S-polarized component 8 and the P-polarized component 9 contained in the incident light 7, the diffraction efficiency with respect to the power input to the first interdigital electrode 4 and the second interdigital electrode 6 is measured in advance. It becomes possible to calculate it from the diffracted light amount by setting it in advance. Although a rectangular piezoelectric substrate is shown in FIG. 1, any shape may be used as long as it is a piezoelectric substrate having optical transparency.

In this embodiment, as shown in FIG. 1, the SA is provided on the same side surface of the first piezoelectric substrate 3 and the second piezoelectric substrate 5, respectively.
Since W is generated, the two piezoelectric substrates are separated from each other. However, if the SAW is generated on the opposite surface of the second piezoelectric substrate 5, it is not necessary to separate them.

Further, generally, the piezoelectric substrate has a high refractive index, so that the surface reflection of light is large and the intensity of transmitted light is often greatly reduced. As an example, the refractive index of the lithium niobate crystal used in the realization described later is that the ordinary ray refractive index is 2.286, the extraordinary ray refractive index is 2.200, the loss due to the reflection on the front and back surfaces, the transmitted light is about 70 of the incident light. Decay%.

In such a case, by attaching a light reflection prevention film on the front and back surfaces of the piezoelectric substrate with a material that has optical transparency such as silicon oxide and has little effect on the propagation of SAW, the light attenuation due to reflection can be reduced. It can be suppressed.

Here, an experiment for examining the polarization selectivity of the SAW element will be described.

The SAW element used in this experiment uses a lithium niobate crystal (LiNbO ₃ ) Y-cut substrate as the piezoelectric substrate, and the interdigital electrode (resonance frequency f ₀ = 107 MHz) on the piezoelectric substrate is SA.
It is arranged so that W propagates in the Z-axis direction. The wavelength of the light source is λ
= 0.633μm He-Ne laser (random polarization), once passed through a polarizer with an extinction ratio of 10 ^-4 or less, converted to linear polarization, and then made incident perpendicular to the SAW propagation plane. did. The measurement was performed by changing the angle of the plane of polarization of the incident linearly polarized light and measuring the amount of diffracted light at that time.

2 (a) and 2 (b) show the polarization state of incident light and the installation relationship of the SAW element.

1 is a piezoelectric substrate, 2 is an interdigital electrode, 7 is incident light, 8
Is an S-polarized component, 9 is a P-polarized component, and XYZ are crystal axes of the piezoelectric substrate. The lattice pattern shown in the center of the piezoelectric substrate 1 is drawn by modeling the periodical refractive index change due to SAW. As shown in FIG. 2A, when the incident light 7 is incident on the SAW optical diffraction grating of the piezoelectric substrate 1, only the S-polarized component of the incident light 7 is diffracted.

FIG. 3 (a) shows the measurement results of the relationship between the rotation angle of the polarization plane and the amount of diffracted light. However, the amount of diffracted light is normalized by the maximum value. In the figure, H is Horizontal type
, And V indicates the Vertical type. This measurement result shows that the amount of diffracted light is the largest when the light in the polarization state perpendicular to the SAW propagation direction is incident.

FIG. 3 (b) shows a comparison between the polarization state of the incident light and the polarization state of the diffracted light when the amount of diffracted light is set to the maximum, that is, the incident polarization plane is set perpendicular to the SAW propagation direction. The horizontal axis is the rotation angle of the rotating analyzer, and the vertical axis is the value obtained by normalizing the detected diffracted light amount by the maximum value. In the figure, a solid line indicates incident light and a point indicates diffracted light.

From the above measurement results, it was confirmed that the SAW element measured in this experiment has polarization selectivity in the diffracting action and the diffracted light retains the polarization state of the incident light.

〔The invention's effect〕

As described above, in the multilayer polarization beam splitter according to the present invention, it was confirmed by experiments that the SAW generated on the piezoelectric substrate forms an optical diffraction grating having polarization selectivity, and the property is utilized. By doing so, it is possible to realize a polarization separation element that does not depend on the wavelength of light and has a variable polarization separation ratio. By combining two such elements, the S polarization component included in the incident light A multilayer polarization beam splitter as a polarization separating optical component capable of separating the P polarization component has been realized.

[Brief description of drawings]

FIG. 1 shows an embodiment of the structure of a multi-layered polarization beam splitter according to the present invention. 2 (a) and 2 (b) show the polarization state of the incident light and the installation relationship of the SAW element. FIG. 3A shows the relationship between the rotation angle of the polarization plane and the amount of diffracted light. FIG. 3B shows a comparison between the polarization state of incident light and the polarization state of diffracted light. In the figure, 1 is a piezoelectric substrate, 2 is an interdigital electrode, 3 is a first piezoelectric substrate, 4 is a first interdigital electrode, and 5 is a second electrode.
Piezoelectric substrate, 6 is a second interdigital electrode, 7 is incident light,
8 indicates an S-polarized component, and 9 indicates a P-polarized component.

Claims (1)

(57) [Claims] 1. A first piezoelectric substrate (3) having optical transparency.
And a surface acoustic wave that selectively diffracts only a part of the light having a polarization plane having a predetermined polarization among the lights incident on the first piezoelectric substrate. As generated above, the first interdigitated electrode (4) provided on the surface of the first piezoelectric substrate and the light transmitted through the first piezoelectric substrate are arranged to enter. And a second piezoelectric substrate (5) having a light-transmitting property, and a part of the light incident on the second piezoelectric substrate, which has a polarization plane with a predetermined polarization, is selectively diffracted. A second interdigital electrode (6) provided on the surface of the second piezoelectric substrate so as to generate such surface acoustic waves on the second piezoelectric substrate. A multi-layered polarization beam splitter that diffracts and splits a portion of each of the rays of light into at least two different directions.