JPH01131530A - Optical modulator for progressive waveform saw - Google Patents

Abstract

PURPOSE:To prevent increase in the temp. of base materials having light transparency and to stabilizer the title modulator by disposing acoustic wave generators in the form of sandwiching the same between the base materials and laminating these base materials and generators in the state of matching the spacial phases with the same phase in the light incident direction, and further providing heat radiators which absorb surface acoustic waves, convert the waves to heat and release the converted heat to both ends of the base materials. CONSTITUTION:Plural pieces of the base materials 1a-1c having the optical planes which allow transmission of light and propagate the surface acoustic waves SAW on the front and rear are disposed in such a manner that the respective front and rear faces face each other while having the spacing nearly parallel with each other. The acoustic wave generators 2a, 2b are interposed in the spacings between the base materials 1a-1c so that the generated acoustic oscillation propagates in the same direction in the form of the SAW of the same frequency and the same phase on the two optical plane sandwiching these generators 2a, 2b. Furthermore, acoustical absorbents 4a, 4b which absorbs the SAW and convert the same to heat and the heat radiators 5a, 5b which release the heat converted by the acoustical absorbents 4a, 4b are provided at both ends in the SAW propagation direction of the plural base materials 1a-1c. The deterioration of the base materials by the heat or the instability of the operation by a change in the SAW speed is thereby prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical modulation device that modulates the phase of light using ultrasonic waves, and in particular, relates to a light modulation device that modulates the phase of light using ultrasonic waves, and in particular, the present invention relates to a light modulation device that modulates the phase of light using ultrasonic waves. W: 5 surface acoustic
wave) as a diffraction grating, and the SAW
The present invention relates to a traveling wave SAW optical modulator in which the phase modulation of a light wavefront can be performed with high efficiency by arranging the generation surfaces of the light waveforms so as to be stacked at substantially parallel intervals in the optical axis direction of the incident light.

[Conventional technology]

In a light modulation device that spatially modulates the phase plane of light, geometric spatial positions of light reflection points are set like an optical reflection grating, and the optical path difference of each reflected light is determined by the deviation of these positions. One is to generate a desired phase delay (phase modulation), and the other is to change the thickness and refractive index of the material of the part through which the light passes, such as in an optical lens, to slow down the speed of light. There are some things that cause phase lag.

In a phase modulation device that changes the refractive index of a medium that transmits light, it is possible to easily and quickly generate a phase delay by using an anisotropic crystal as the light transmitting medium and applying an electric or magnetic field. It is used in many practical optical elements, such as a modulation element that performs phase modulation by applying an electric field to a leading waveguide on a piezoelectric crystal substrate, and an optical switch that causes light to flicker by interfering with the modulated light of two elements. has been developed.

In addition, when there is a material whose refractive index does not change noticeably due to changes in electric field, magnetic field, etc., or where the area of the part through which light passes is large, and you want to generate, for example, a sinusoidal lattice-like phase change distribution in the entire part, An acousto-optic method has been used in which ultrasonic waves are emitted into a light-transmitting medium and the refractive index changes due to changes in the density of the medium caused by the ultrasonic waves.

This acousto-optic phase modulation method can be roughly divided into those that use bulk waves traveling inside the medium, and those that use surface acoustic waves (S
AW). Those that use bulk waves are able to propagate light over a long distance within the ultrasound, and as a result, the time (distance) for mutual interference between the ultrasound and light can be lengthened, and the amount of phase change can be increased. However, it is difficult to widen the area of the light-transmitting part, and the ultrasonic wave generation band is structurally narrow. The angle of incidence of light on a three-dimensional lattice-like refractive index changing region is limited by Bragg's condition,
When the wavelength of the incident light or the wavelength of the ultrasonic wave changes, it is necessary to adjust the incident angle accordingly.

However, as a fixed frequency optical modulation device, it is small and highly efficient, and is one of the most practically used devices.

On the other hand, in an optical modulation device that uses SAW, the SAW generation mechanism is suitable for high frequencies and wide bands, and the SAW is
As methods for changing the emission direction of light have also been developed, it has come to be used as a high-frequency, broadband optical modulation device.

There are two methods of combining SAW and light: one is to introduce light into a thin film onto the surface of the substrate where the SAW is generated and propagate through the SAW for a long time (long distance) to increase modulation efficiency; There is a method of transmitting light perpendicularly to the direction of the light and modulating the phase in a short time (short distance). The method of guiding light to the SAW generation surface is widely used and has high utility value mainly as a phase modulation element of thin film optical ICs. In addition, the method in which light is incident perpendicularly to the SAW generation surface can apply phase modulation to the entire wide beam, and can be used like a phase diffraction grating in general spatial propagation optical systems. There are many. In this case, SA
In order to increase the modulation efficiency of light by W, a method has been adopted in which the SAW generation surfaces are stacked at intervals substantially parallel to the optical axis direction of the incident light. These methods of vertically injecting light are SAW
It is possible to change the grating constant by changing the frequency of the phase diffraction grating, and it is attracting attention as a phase diffraction grating with variable grating spacing. The modulation efficiency referred to here is obtained by imaging the phase-modulated light, measuring the intensity of the diffracted light generated by the phase change, and determining what percentage of the incident light is diffracted.

[Problem that the invention seeks to solve]

However, in this method of laminating the SAW generation surfaces at almost parallel intervals in the optical axis direction of the incident light, the heat generated by converting the SAW energy is likely to accumulate due to the laminated structure, resulting in deterioration of the base material. Alternatively, variations in the operating band due to changes in the SAW speed and further variations in the diffraction angle due to changes in wavelength have been caused.

[Means for solving problems]

The present invention was made to solve such problems,
As a means of solving this problem, a plurality of base materials having optical planes on the front and back sides through which light is transmitted and SAW propagates are arranged so that each front and back surface faces each other with an interval substantially parallel to each other. Acoustic wave generators for generating acoustic vibrations are interposed in the gap between these base materials, and the acoustic vibrations generated by these acoustic wave generators are transmitted to two optical planes that sandwich these generators at the same frequency and the same phase. A sound absorbing material that absorbs the SAW and converts it into heat is installed at both ends of the SAW propagation direction of the plurality of base materials, and a heat dissipation material that releases the heat converted by this sound absorbing material. The structure is equipped with a container.

Furthermore, the plurality of base materials used were made of materials that each had the same acoustic characteristics and had sufficiently good transmittance to incident light. Furthermore, the S emitted from each acoustic wave generator
The AWs have the same frequency and the same direction of travel, and each of the generators is arranged so that the phases of the plurality of SAWs are the same when viewed from the optical axis direction of the incident light. The setting location was determined.

[Effect]

By the means described above, the SAW propagating through each base material is absorbed by the sound absorbing material and converted into heat, and moreover, it is immediately transmitted to a radiator near the sound absorbing material or fixed to the base material by the sound absorbing material. It becomes possible to propagate heat and reduce the temperature rise of the base material. As a result, the propagation speed of the SAW becomes stable, and the optical modulation device operates stably. Furthermore, since the distortion caused by the heat applied to the base material is also reduced, the device can have a long life.

〔Example〕

FIG. 1 shows a configuration diagram of an embodiment of a traveling waveform SAW optical modulator according to the present invention.

The present invention provides a substrate 1a that is sandwiched between a substrate La, a substrate lb, and a substrate IC that propagate SAW, and a substrate 1a and a substrate 1b.
and an acoustic wave generator 2a that generates SAW on both opposing surfaces of substrate 1b, and an acoustic wave generator that is sandwiched between substrate 1b and substrate 1c and generates SAW on both opposing surfaces of substrate 1b and substrate 1c. 2b, and a spacer 3a provided to create a space for the SAW to propagate on each surface of the base material 1a and the side where the acoustic wave generator 2a of the base material 1b is provided, and similarly, the base material 1b and Spacers 3 provided to create a space for the SAW to propagate on each surface of the base material 1c on the side where the acoustic wave generator 2b is provided.
b, a sound absorbing material 4a and a sound absorbing material 4b that absorb SAW propagating through the base material 1a, base material lb, and base material IC and converting it into heat, and transmitting the heat converted by the sound absorbing material 4a and the sound absorbing material 4b to the outside. It is composed of a heat radiator 5a and a heat radiator 5b.

In order to generate and propagate SAW on the surface of a base material, a piezoelectric substrate is used as a light-transmitting base material, and interdigital electrodes are formed on the surface of the substrate as an acoustic wave generator using a vapor-deposited metal film. There is. In this case, the interdigital electrodes are provided at a position as far away from the sound absorbing material as possible to prevent the influence of heat generated when the generated SAW is absorbed by the sound absorbing material.

Furthermore, metal thin film deposition technology and semiconductor microfabrication technology can be used to form the interdigital electrodes.

A method using such a piezoelectric substrate and interdigital electrodes is
This is the simplest structure for an element that phase-modulates the phase of light that is perpendicularly incident on the SAW generation surface over a wide area.

There are two possible ways to construct the base material forming the interdigital electrodes.

One is a method in which an acoustic wave generator W2a and an acoustic wave generator 2b that generate SAW are formed on both sides of a base material 1b, and then these both sides are sandwiched between a base material 1a and a base material 1c.

Another method is to use an acoustic wave generator 2a and an acoustic wave generator 2b on the surface of two of the base materials 1a, 1b, and lc.
This is a method of forming three base materials and stacking them. Spacer 3
a and the spacer 3b can be manufactured using metal thin film deposition technology and semiconductor microfabrication technology in the same way as the sound generating devices 2a and 2b.

The sound absorbing material 4a and the sound absorbing material 4b are provided at both ends of the three base materials,
It absorbs the SAW propagating through each substrate, prevents the reflection of the SAW, and converts the SAW into heat. Spacer 3a
and spacer 3b are respectively surrounded by sound absorbing material 4a and sound absorbing material 4b to prevent SAW from being reflected.

The heat radiator 5a and the heat radiator 5b are provided near the sound absorbing material 4a and the sound absorbing material 4b, respectively, and are glued to the base material 1a, base material 1b,
Fix it to the base IC. Fixed radiator 5a and radiator 5
In order to improve heat dissipation, the sound absorbing material 4a and the sound absorbing material 4b are bonded to b.

In this case, the sound absorbing agent may also serve as an adhesive.

Next, the propagation state of the SAW generated on the surface of each base material, the incidence of light, and the phase modulation will be explained using FIG. FIG. 2 shows a cross-sectional view on a plane including the SAW traveling direction axis and the incident optical axis of this embodiment.

As shown in Figure 1, in this example, three base materials and two
SA on the four optical planes of the base material by two acoustic generators
W occurs.

As shown in Figure 2, 5AWI,
Name them 5AW1', 5AW2.5AW2'. If the oscillation frequency and SAW emission direction of each acoustic generator and the acoustic characteristics of each substrate are the same, all SAWs have the same wavelength and parallel phase planes (lattice planes when viewed from the optical axis direction). Proceed at the same speed. Therefore, each SAW is
In order to give the same phase modulation effect to the incident light 6, it is sufficient that the phase planes of each SAW are aligned in the same phase when viewed from the optical axis direction. Since the 5AWI and 5AWI' are SAWs emitted from the same acoustic wave generator 2a, they maintain completely the same spatial phase. Similarly, it is clear that 5AW2.5AW2'' also has the same spatial phase.

Therefore, if the acoustic waves (SAW) emitted by the sound generators 2a and 2b are made to have the same spatial phase, all the SAWs will be aligned in the same phase when viewed from the optical axis direction.

Method of phase adjustment Yes (There are several possible methods, but the simplest and surest method is to adjust the arrangement of the electrodes that are the generation mechanism of each acoustic wave generator and match the phase of the SAW. In other words, the method of the embodiment In interdigital electrodes such as
It is sufficient to be able to adjust the position in the AW propagation direction with an accuracy of several to several tenths of the SAW wavelength, and the actual numerical accuracy required is several μm.
It is.

Positioning within this level of accuracy can be fully realized with current mask aligners for manufacturing semiconductor devices.

Another method is to match only the parallelism of the interdigital electrodes in the interdigital direction to the fine print, and adjust the SAW phase by individually delaying the phase of the electric signal applied to each electrode. Although this method has the difficulty of making the external circuit of the modulation element somewhat complicated, it also has the advantage that after assembling the light modulation device, it can be adjusted while actually inputting light and observing the modulated output light. , it is a practically effective means.

Above, three embodiments of the SAW generation method which is the basis of the present invention have been described. However, in the optical phase modulation device of the present invention in which the refractive index is changed by the density change of the base material due to the SAW, The refractive index of the base material used is often large, and when light is incident on it in the air, there is a high possibility that the surface or internal reflectance will be as high as several tens of percent.

Therefore, in order to prevent multiple reflections of light due to the laminated structure, it is necessary to form an optically flat optical antireflection film (optical coating).

FIG. 3 shows, as a simple application example of the present invention, an example in which the present invention is used in a light deflection device.

The SAW generated on the surfaces of the base materials 1a and 1b by a sinusoidal electric signal having a frequency fo has a spatial period d corresponding to a lattice constant, and travels at a speed V in the direction of the arrow. When incident light 6 passes through this base material from the left direction in the figure, this incident light undergoes phase modulation due to the unevenness of the base material surface due to the SAW and the change in refractive index just below the base material surface. Since this phase modulation is periodic due to the repetition of the spatial period d, when this light is focused on the focal plane 8 of the lens by the lens 7, it forms a diffraction image, just like light transmitted through a normal sine wave phase grating. will occur. Here, if the incident light is monochromatic light with a wavelength λ, the diffraction image becomes a ±1st-order diffraction bright spot image whose position is determined by the lattice constant d. The generation position of this diffraction bright spot is a distance α from the optical axis on the focal plane 8, and the direction is equal to the 5AWO propagation direction. If the focal length of lens 7 is different from F, the value of α is α=Fλ/d=foFλ/v −・−−=−(1)
It is expressed as

Here, the frequency of the sinusoidal electrical signal is ±Δ around fO
f/2 If it changes, ±1 on the focal plane 8
The displacement amount Δα of the next diffraction bright spot is Δα−ΔIFλ/V
・・・・・・・・・(2) becomes. As is clear from equation (2), the smaller the SAW propagation speed ■, the longer the focal length F of the lens, and the longer the wavelength λ of the light, the more the displacement Δ
It can be seen that α is large and changes in a linear relationship with the frequency of the electrical signal.

〔Effect of the invention〕

As described above, according to the present invention, since the acoustic wave generator is placed between the light-transmitting base materials, one acoustic wave generator can generate the same frequency and the same frequency on two surfaces. It can effectively generate phase SAW, and furthermore, these are stacked in the light incident direction with their spatial phases aligned in the same phase, and sound absorbing materials are provided at both ends of the base material to absorb SAW and convert it into heat. Since it is equipped with a radiator that releases the heat converted by this sound-absorbing material, it has become possible to achieve highly efficient optical phase modulation that was not possible with conventional devices. As a result, in the present invention, spatial phase modulation can be efficiently realized over a wide area, and it can be applied to a sine wave phase grating with a variable grating constant that can be applied to spectroscopic measurement equipment, and an optoelectrical signal processing device such as an acousto-optic correlator. The realization of a spatial phase modulator has become easier.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of a traveling waveform SAW optical modulator according to the present invention. FIG. 2 shows the positional relationship between the laminated base materials, a plurality of SAWs, and incident light in the embodiment of FIG. 1. Third
The figure shows the polarization state of light when the present invention is applied to light deflection. In the figure, 1a, 1b and 1c are base materials, 2a and 2b are sound wave generators, 3a and 3b are spacers, 4a and 4b are sound absorbing materials, 5a and 5b are radiators, 6 is incident light, and 7 is a lens. , 8 indicate focal planes, respectively.

Claims (1)

[Claims] A plurality of substrates (1a, 1b, 1c) having optical transparency and having the same acoustic properties, each having an optical plane on the front and back sides, and laminated with the optical planes facing each other; a plurality of acoustic wave generators (2a, 2b) that are interposed between opposing optical planes of the material and generate surface acoustic waves of the same wavelength and phase in the same traveling direction on each of the opposing optical planes; A plurality of sound absorbing materials (4a
, 4b); and a plurality of radiators (5a, 5b) for discharging heat converted by the plurality of sound absorbing materials, and a laminated traveling waveform modulating light transmitted through the laminated optical planes. SAW optical modulator.