JP2571405B2 - Traveling waveform SAW light modulator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator that modulates the phase of light using ultrasonic waves, and more particularly, to a surface acoustic wave (SAW: By using Surface Acoustic Wave as a diffraction grating and arranging the SAW generation surface so as to be stacked at almost parallel intervals in the optical axis direction of the incident light, the phase modulation of the light wavefront can be performed with high efficiency. The present invention relates to a traveling wave SAW optical modulator.

[Conventional technology]

In a light modulator that spatially modulates the phase plane of light, the geometrical spatial positions of the light reflection points are set like an optical reflection grating, and the optical path difference is given to each reflected light from the deviation of those positions. It causes the desired phase delay (phase modulation), and changes the thickness of the material of the light transmitting part such as an optical lens and the refractive index to reduce the speed of the light. As a result,
Some cause a phase delay.

A phase modulation device that changes the refractive index of a medium that transmits light can easily and quickly generate a phase lag by applying an electric field or a magnetic field using an anisotropic crystal or the like as the light transmitting medium. There are many practical optical elements such as a modulation element that performs phase modulation by applying an electric field to an optical waveguide on a piezoelectric crystal substrate, and an optical switch that blinks light by interfering the modulated light of two elements. Has been developed.

Also, it is desired to generate, for example, a sinusoidal lattice phase change distribution over a material that does not cause a remarkable change in the refractive index due to changes in electrolysis, magnetic field, or the like, or that has a large light-transmitting area. In such a case, an acousto-optical method has been employed in which ultrasonic waves are emitted into a light transmitting medium and a change in the refractive index is caused by a change in the density of the medium due to the ultrasonic waves.

This acousto-optic phase modulation method can be roughly classified into those using bulk waves traveling inside the medium and those using surface acoustic waves (SAW) in which most of the energy is concentrated on the surface layer of the medium. And how to do it.

In the case of using a bulk wave, light can travel in an ultrasonic wave over a long distance, and as a result, the time (distance) of mutual interference between the ultrasonic wave and the light can be increased, and the phase change amount can be increased. Is large (high modulation efficiency), but it is difficult to increase the area of the light transmitting part, and there is a problem on the manufacturing surface such as the structure of the ultrasonic wave generation band is narrow, and the three-dimensional wave caused by the bulk wave. The incident angle of light with respect to the lattice-shaped refractive index change region is limited by the Bragg condition, and when the wavelength of the incident light or the wavelength of the ultrasonic wave changes, the incident angle needs to be adjusted accordingly. However, as a fixed-frequency light modulator, it is small, highly efficient, and one of the most practically used.

On the other hand, an optical modulator using SAW has a SAW generation mechanism suitable for high frequency and broadband, and the Bragg
Since a method of changing the launch direction of the SAW has been developed so as to be suitable for the (Bragg) condition, it has been used as a high-frequency, wideband optical modulator.

There are two ways to combine SAW and light: a method in which light is guided in a thin film onto the surface of the substrate where SAW is generated and propagated through the SAW for a long time (long distance) to increase the modulation efficiency. There is a method in which light is transmitted vertically to phase modulation in a short time (short distance). The method of guiding light to the SAW generation surface is
It is widely used mainly as a phase modulation element for thin-film optical ICs and has high utility value. On the other hand, the method to make light incident on the SAW generation surface
Phase modulation can be applied to the entire light beam having a wide width, and is often used like a phase diffraction grating in a general space propagation type optical system. In this case, a "progressive waveform surface acoustic wave light modulator" by the same applicant and partly the same inventor
As disclosed in Japanese Patent Application No. 62-77893, in order to increase the modulation efficiency of light by SAW, a method of laminating at an interval substantially parallel to the optical axis direction of incident light on the SAW generation surface is adopted. The method of making these lights incident has attracted attention as a different diffraction grating having a variable grating interval, which can change the lattice constant by changing the frequency of the SAW. Here, the modulation efficiency is obtained by forming an image of 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.

[Problems to be solved by the invention]

However, the method of laminating the SAW generation surface at a substantially parallel interval in the optical axis direction of the incident light is easy because heat generated by converting the energy of the SAW is easily accumulated because of the laminated structure,
This caused fluctuations in the operating band due to deterioration of the base material or changes in the SAW speed, and fluctuations in the diffraction angle due to wavelength changes.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to realize an optical modulator having a high modulation efficiency, stable operation, and a long life.

[Means for solving the problem]

As a means of solving the problem, a plurality of base materials that transmit light, and have an optical plane on which the SAW propagates on the front and back, are arranged so that each front and back face each other with an interval substantially parallel to each other, An acoustic wave generator for generating acoustic vibration is interposed in the gap between the base materials, and the acoustic vibration generated by the acoustic wave generator is applied to two optical planes sandwiching these generators at the same frequency and the same phase. A sound absorbing material that becomes SAW and propagates in the same direction, and absorbs SAW at both ends in the SAW propagation direction of a plurality of substrates and converts it into heat, and a radiator that emits heat converted by this sound absorbing material And a configuration including:

Further, a plurality of base materials each having the same acoustic specialty and having a sufficiently high transmittance of human light were used. Furthermore, it was emitted from each acoustic wave generator.
SAWs each have the same frequency and the same traveling direction, and when viewed through the optical axis direction of the incident light, each of the generators so that the phases of the plurality of SAWs are the same. The setting place of was decided.

[Action]

Since the SAW generation surface is stacked in the direction of the optical axis of the incident light, the light modulation efficiency increases, and the SAW propagating through each base material is absorbed by the sound absorbing material, exchanged for heat, and externally radiated by the radiator. Will be released.

〔Example〕

FIG. 1 shows a configuration of an embodiment of a traveling-wave SAW light modulator according to the present invention.

The present invention provides a substrate 1a, a substrate 1b, and a substrate 1c that propagate SAW.
And, between the base material 1a and the base material 1b, the acoustic wave generator 2a that generates SAW on both opposing surfaces of the base material 1a and the base material 1b, and is sandwiched between the base material 1b and the base material 1c. The acoustic wave generator 2b that generates SAW on both opposing surfaces of the base 1b and the base 1c, and the SAW propagates to the respective surfaces of the base 1a and the base 1b on which the acoustic wave generator 2a is provided. In order to create a space so that SAW propagates on the respective surfaces of the base 1b and the base 1c on the side where the acoustic wave generator 2b is provided, similarly to the spacer 3a provided to create a space so that Spacers 3b provided on the base material 1a, the base material 1b, and the SA that propagates through the base material 1c, respectively.
It is composed of a sound absorbing material 4a and a sound absorbing material 4b that absorb W and convert it to heat, and a radiator 5a and a radiator 5b that emit the heat converted by the sound absorbing material 4a and the sound absorbing material 4b to the outside. The spacers 3a and 3b have a thickness such that surfaces through which the SAW propagates are substantially parallel to each other.

In order to generate and propagate SAW on the surface of a substrate, a piezoelectric substrate is used as a substrate having optical transparency, and an interdigital electrode is formed as an acoustic wave generator on the surface of the substrate with a metal thin film of metal deposition There is. In this case, an interdigital electrode occurred
When SAW is absorbed by the sound absorbing material, it is provided as far away from the sound absorbing material as possible to prevent the influence of the heat generated.

In addition, in order to form the interdigital electrode, a metal thin film deposition technique and a semiconductor fine processing technique can be used.

Such a method using the interdigital electrodes of the piezoelectric substrate has the simplest structure as an element for phase-modulating light vertically incident on the SAW generation surface in a wide area.

There are two possible methods for forming the interdigital electrodes between the substrates. One is a method in which an acoustic wave generator 2a and an acoustic wave generator 2b for generating SAW are formed on both surfaces of a substrate 1b, and then both surfaces are sandwiched between the substrates 1a and 1c. Another method is a method in which an acoustic wave generator 2a and a sound generator 2b are formed on the surface of two of the substrates 1a, 1b, and 1c, and three substrates are stacked.

Spacer 3a and spacer 3b are the acoustic wave generator 2a,
Like 2b, it can be manufactured using a metal thin film deposition technique and a semiconductor fine processing technique.

The sound absorbing material 4a and the sound absorbing material 4b are provided at both ends of the three base materials, absorb the SAW propagating through each base material, prevent the reflection of the SAW, and exchange the SAW with heat. The spacer 3a and the spacer 3b are surrounded by the sound absorbing material 4a so that the SAW is not reflected.

The radiator 5a and the radiator 5b are a sound absorbing material 4a and a sound absorbing material, respectively.
It is provided near 4b, and is fixed to the base material 1a, the base material 1b, and the base material 1c with an adhesive. The sound absorbing material 4a and the sound absorbing material 4b are bonded to the fixed radiator 5a and radiator 5b in order to improve heat dissipation. In this case, the sound absorbing agent may also serve as the adhesive.

Next, the state of propagation of SAW generated on the surface of each base material, the incidence of light, and the phase modulation will be described with reference to FIG. FIG. 2 is a cross-sectional view of a plane including the optical axis of the incident light according to the present embodiment.

As shown in FIG. 1, in this embodiment, three substrates and two acoustic wave generators are used to cover four optical planes of the substrate.
SAW occurs.

As shown in FIG. 2, SAW1 and SAW
Name them 1 ', SAW2, SAW2'. If the oscillation frequency of each acoustic wave generator, the SAW emission direction, and the acoustic characteristics of each substrate are the same, all SAWs have the same wavelength and parallel phase plane (lattice plane when viewed from the optical axis direction). At the same speed. Therefore, in order for each SAW to give an equivalent phase modulation action to the incident light 6, it is only necessary that the phase plane of each SAW is in phase when seen through the optical axis direction. . Since SAW1 and SAW1 'are SAWs emitted from the same acoustic wave generator 2a, they completely maintain the same spatial phase. Similarly, it is apparent that SAW2 and SAW2 'have the same spatial phase.

Therefore, if the acoustic waves (SAW) emitted from the acoustic wave generators 2a and 2b are spatially in-phase, all the SAWs will be in phase when viewed from the optical axis direction.

There are several methods for adjusting the phase, but the simplest and most reliable method is to adjust the arrangement of the interdigital electrodes, which are the generating mechanisms of the acoustic wave generators, to match the phase of the SAW. That is, in the interdigital electrode as in the embodiment, it is sufficient that the position of each electrode in the SAW propagation direction can be adjusted with an accuracy of several to several tens of SAW wavelengths, and the actually required accuracy is several μm. Positioning within this degree of accuracy can be sufficiently realized with a current mask aligner for manufacturing semiconductor devices.

As another method, there is a method in which only the parallelism in the interdigital direction of the interdigital electrode is precisely matched, and the phase adjustment of the SAW is performed by individually delaying the phase of an electric signal applied to each electrode. Although this method slightly complicates the external circuit of the modulation element, it has the advantage that light can be actually incident after the light modulator is assembled and adjusted while observing the modulated output light. It is an effective means.

As described above, the two embodiments of the phase adjustment method of SAW, which is the basis of the present invention, have been described. In many cases, the refractive index of the substrate used is large, and when light is incident on the substrate in the air, the surface or internal reflectance is likely to be as high as several tens of percent. Therefore, it is necessary to form an optical antireflection film (optical coating) on an optical plane in the sense of preventing multiple reflection of light due to the laminated structure.

FIG. 3 shows an example in which the present invention is applied to a light deflecting device as a simple application example.

Substrate 1a by an electrical signal of a sine wave of frequency f _0, the substrate 1
b, SAW generated on the surface of the substrate 1c has a spatial period d corresponding to a lattice constant, and travels in the direction of the arrow at a speed v. When the incident light 6 passes through the base material from the left direction in the figure, the incident light undergoes phase modulation due to the unevenness of the surface of the base material due to the SAW and a change in the refractive index immediately below the base material surface. Since this phase modulation is periodic due to the repetition of the spatial period d, this light, like light transmitted through a normal sinusoidal phase grating, forms a diffracted image when focused on the focal plane 8 of the lens by the lens 7. Is generated. Here, if the incident light is monochromatic light having a wavelength λ, the diffraction image is a ± 1st-order diffraction luminescent spot whose position is determined by the lattice constant d. The position at which the diffraction luminescent spot is generated is located at a distance α from the optical axis on the focal plane 8, and the direction is equal to the SAW propagation direction. The value of α is represented by α = Fλ / d = f ₀ Fλ / v (1) where F is the focal length of the lens 7.

Here, the frequency of the sine wave electric signal is ± around the f ₀ Delta] f /
If two changes occur, the change amount Δα of the ± 1st-order diffraction luminescent spot on the focal plane 8 is Δα = ΔfFλ / v (2). As is apparent from the equation (2), the SAW propagation speed v is small, the focal length F of the lens is long, and the light wavelength λ
Is longer, the displacement amount Δα is larger, and changes in a linear relationship with the frequency of the electric signal.

According to the present invention, since the obstacle due to heat has been removed,
Spatial phase modulation with a variable lattice constant that can be applied efficiently to a spectrometer and a spatial phase modulator for opto-electrical signal processing devices such as acousto-optic correlators that can efficiently realize spatial phase modulation in a wide interview Becomes easier.

〔The invention's effect〕

As described above, according to the present invention, a substrate having optical transparency is arranged so as to sandwich an acoustic wave generator therebetween, and SA
The SAW generation surface is laminated in the light incident direction so that the spatial phases of W are aligned in the same phase.At both ends of the substrate, a sound absorbing material that absorbs the SAW and converts it into heat, and the heat converted by this sound absorbing material Is provided, the temperature rise of the base material can be suppressed to a low level while performing high-efficiency optical phase modulation.

As a result, the SAW propagation speed was stabilized, and distortion due to heat applied to the base material was reduced, so that an optical modulator having high modulation efficiency, stable operation, and long life was obtained.

[Brief description of the drawings]

FIG. 1 shows an embodiment of a traveling-wave SAW light modulator according to the present invention. FIG. 2 shows a positional relationship between the laminated substrate, a plurality of SAWs, and incident light in the embodiment of FIG. FIG. 3 shows a light deflection state when the present invention is applied to light deflection. In the figure, 1a and 1b and 1c are base materials, 2a and 2b are acoustic wave generators, 3a and 3b are spacers, 4a and 4b are sound absorbing materials, 5a and 5b are radiators, 6 is incident light, 7 is a lens, Reference numeral 8 denotes a focal plane.

Claims (1)

(57) [Claims] 1. A plurality of substrates (1a, 1b, 1c) having optical transparency, having the same acoustic characteristics, having optical planes on both sides, and being laminated with the optical planes facing each other. ,
An acoustic wave generator (2a, 2b) interposed between opposing optical planes of the plurality of base materials, and configured to generate surface acoustic waves having the same wavelength and the same phase in the same traveling direction on each of the opposing optical planes; A sound absorbing material (4a, 4b) for absorbing surface acoustic waves generated by the acoustic wave generating device and converting the same into heat; and a radiator (5a, 5) for releasing the heat converted by the sound absorbing material.
b) a stacked traveling-wave SAW light modulator that modulates light transmitted through the stacked optical planes.