JPS63244018A - Standing wave type surface acoustic wave light modulator - Google Patents

Abstract

PURPOSE:To provide modulated light which flickers with high efficiency by laminating surfaces for generating surface acoustic waves (SAW) apart at nearly a parallel spacing in the optical axis direction of incident light and disposing plural acoustic wave generators in such a manner that the progressing directions of the SAW are reversed. CONSTITUTION:Plural pieces of base materials 1a, 1b, 1c having the optical planes in which the SAWs propagate on the front and rear are disposed to face each other apart at nearly the parallel spacing and the acoustic wave generators 2a, 2b are interposed in the spacing between these base materials so that acoustic oscillations propagate in the form of the SAWs having the same frequency and the same phase in the same direction in the two optical planes sandwiching the generators 2a, 2b. The positions of plural pieces of the acoustic wave generators are so set that the SAWs between the two groups are reversed in the direction only in the progressing direction and intersect with each other spacially in the light transmission regions on the SAW propagation base materials. The intermittent phase modulation function at a high frequency is thereby permitted.

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: 5surface Acoust
icWave) as a diffraction grating, and the SA
The generation surfaces of W are arranged so as to be stacked at substantially parallel intervals in the optical axis direction of the incident light, and the plurality of acoustic wave generators that generate SAW are arranged so that the traveling directions of the SAWs are opposite to each other. When viewed from the optical axis direction, the SAW standing wave produces an effect that looks as if it is repeating the generation and disappearance of a SAW standing wave, and as a result, it is possible to obtain modulated light that flickers with high efficiency. The present invention relates to a surface acoustic wave optical modulator.

[Conventional technology]

A light modulation device that spatially modulates the phase plane of light has the geometrical spatial positions of the light reflection points set like an optical reflection grating, and the difference in each reflected light from the deviation of those positions. There are those that create a desired phase delay (phase modulation) by creating an optical path difference, and those that slow the speed of light by changing the thickness and refractive index of the material of the part through which the light passes, such as optical lenses. As a result, there are cases where a one-phase delay occurs. In a phase modulation device that changes the refractive index of a medium that transmits light, an anisotropic crystal or the like is used as the light transmission medium, and by applying an electric or magnetic field, it is possible to easily and quickly generate a phase delay. This is possible, and there are many practical optical applications such as modulators that perform phase modulation by applying an electric field to optical waveguides on piezoelectric crystal substrates, and optical switches that cause light to flicker by interfering with the modulated light of two elements. devices have been developed.

Also, when using a material that does not cause a noticeable change in refractive index due to changes in electric or magnetic fields, or when the area through which light passes is large, and you want to generate, for example, a sine wave lattice-like phase change distribution over the entire area, teeth.

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 can propagate light over long distances within ultrasonic waves.

As a result, the time (distance) for mutual interference between ultrasonic waves and light can be lengthened, and the amount of 1 phase change is large (high modulation efficiency). However, it is difficult to increase the area of the light transmitting part. In addition, there are problems in production, such as the ultrasonic generation band being structurally narrow, and the angle of incidence of light on the three-dimensional lattice-like refractive index changing region generated by bulk waves is limited by Bragg conditions. When the wavelength of the incident light or the wavelength of the ultrasonic wave changes, it is necessary to adjust the angle of incidence 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, optical modulation devices that use SAW have a SAW generation mechanism that is suitable for high frequencies and wide bands, and we have also developed a method to change the direction of SAW emission to suit the Bragg conditions. As a result, it has been used as a high-frequency, broadband optical modulator. A method of combining SAW and light involves guiding light in a thin film onto the surface of the substrate where SAW is generated.
There is a method in which light is propagated in W for a long time (long distance) to increase modulation efficiency, and a method in which light is transmitted perpendicularly to the generation surface of the SAW to perform phase modulation in a short time (short distance). How to guide light to the SAW generation surface?

It is mainly used as a phase modulation element in thin-film optical ICs and has high utility value. 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, it is possible to change the grating constant by changing the SAW frequency, and is attracting attention as a phase diffraction grating having a variable grating interval.

[Problem that the invention seeks to solve]

However, in such an optical modulation device in which light is perpendicularly incident on the SAW generation surface, the interaction time between the SAW and the light is shorter than in other methods, making it difficult to increase the efficiency of phase modulation. Ta.

In the modulation device using the bulk wave, the Bragg
If the (Bragg) condition is satisfied, a modulation efficiency of about 80% can be expected, but in a modulation device that injects light perpendicularly to the SAW, it is extremely low at only a few percent, and its practical use is extremely limited. It had become a thing.

(Note: The modulation efficiency referred to here is determined by imaging the light after phase modulation and measuring the intensity of the diffracted light generated by 9 phase changes to determine what percentage of the incident light is diffracted. ) [Means for solving the problems] The present invention has been made to solve the 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 the two optical surfaces that sandwich these generators at the same frequency and in the same manner. The configuration is such that the SAW becomes a phase SAW and propagates in the same direction. For this reason, a plurality of base materials were used that each had the same acoustic characteristics and had sufficiently good transmittance for incident light. Furthermore, the plurality of acoustic wave generators are divided into two groups of approximately half each, and in each group, the SAWs advance in the same direction at the same frequency and at the same speed, and each SAW
When viewed from the optical axis direction, they overlap in phase.

The SAW generation functions (characteristics) and placement locations of the plurality of generators were set. Furthermore, the SAW between these two groups has one frequency, the traveling speed is the same, and the two phase planes are spatially aligned parallel to each other, and only the traveling direction is set to be opposite to each other in the light transmission area on the SAW propagation base material. The positions of the sound generators in each group were set so that they spatially intersected.

[Effect]

By the above means, S
Light passes through the AW propagation base material in a short time that is negligible compared to the 5AWO propagation speed, and the same frequency and spatially aligned phase (hereinafter referred to as , spatially the same phase) is repeatedly subjected to similar phase modulation by multiple SAWs, and the SAW
It has become possible to increase modulation efficiency according to the number of propagation substrates. In addition, since multiple SAWs are divided almost equally and moved backwards, when looking through the overlapping SAWs from the optical axis direction, the moment the SAWs moving backwards overlap at the same phase position, the apparent Up.

All the SAWs strengthen each other, resulting in a state where maximum intensity phase modulation is applied, and conversely, when they overlap with opposite phases, they weaken each other, for example, each SAW of two groups
When the sum of the amplitude intensities is equal, a state occurs in which no five-phase modulation is applied, that is, a state in which the apparent distribution of refractive index changes becomes uniform. Such changes in the phase modulation state result in a spatially standing wave state when the two intersecting SAWs have the same frequency and speed, and temporally, the SAWs move at twice the relative speed. Since they intersect, the entire spatial standing wave repeatedly generates and disappears at a time frequency twice that of the SAW. Therefore, if the plane wave light that has passed through, for example, nine devices is converged by a lens and a Fraunhofer diffraction image is observed, the first bright spot will be a bright spot blinking at twice the temporal frequency of the SAW.

Therefore, 1. the present invention increases the efficiency of phase modulation by laminating base materials, and 2.
It combines a phase modulator section that intermittents at a high frequency.

〔Example〕

FIG. 1 shows the first part of the standing waveform surface acoustic wave optical modulator of the present invention.
1 shows a configuration diagram in an embodiment of the present invention.

This figure shows the gist of the present invention, and shows a plurality of base materials 1a+lb, l
This figure illustrates a combination of the acoustic wave generator 2a and the acoustic wave generators 2a and 2b.

In order to easily realize a structure in which an acoustic wave generator is placed between multiple base materials, a conventional acousto-optic phase modulation element using SAW, for example, by the same applicant and inventor A conceivable method is to combine the invention "Light diffraction device using surface acoustic waves" (Japanese Patent Application No. 60-234812) with another base material having the same acoustic characteristics according to the principles of the present invention. In this optical diffraction device using rSAW, a piezoelectric substrate is used as the optically transparent base material to perform the basic operation of optical phase modulation, and the surface of the substrate is used as an acoustic wave generator. The interdigital electrodes are formed from a vapor-deposited metal film.

As shown in FIG. 1, the base material 1a is a piezoelectric substrate, and interdigital electrodes formed on one surface thereof by metal thin film deposition technology, semiconductor microfabrication technology, etc. are used as an acoustic wave generator 2a. This is the arranged configuration.

This optical phase modulation element using a piezoelectric substrate and interdigital electrodes has the simplest structure and is a widely known system as an element that phase modulates the light incident perpendicularly to the SAW generation surface over a wide area.

Now, to explain the embodiment shown in FIG. 1 again based on such a phase modulation element, this embodiment has two SAW phase modulation elements having the same characteristics, namely, the base material 1a and the sound generating device 2a+. and a structure in which a base material 1b having acoustic characteristics equivalent to the base material 1a and the IC is sandwiched between two phase modulation elements constituted by a combination of a base material 1c and an acoustic wave generator 2b. I can say it. In this case, if the surfaces of the acoustic wave generators 2a and 'lb are stacked so that they are in close contact with the optical planes on the front and back sides of the base material 1b, the acoustic wave generators 2a and 2
b, it is possible to generate SAW on both the front and back surfaces of the base material 1b.

Experimentally, the vibration amplitude of SAW is from several people to several tens of people, and the film thickness of the commonly used interdigital electrodes is about several μm, so only the interdigital electrodes are sandwiched between the base materials. Even when laminated and crimped by itself, the gap between each base material caused by the thickness of the interdigital electrodes is sufficient to allow 5AWO propagation on the opposing optical planes.

In addition, as shown in the same figure, a simple method of keeping the width of this gap constant and stacking each base material almost in parallel is when forming an interdigital electrode. It is preferable to form small pieces of vapor deposited film as spacers 3a and 3b on the opposite side. In this embodiment, these spacers 3a and 3b are
The SA is formed obliquely to the direction of movement of the AW.
This is to prevent unnecessary reflection of W from returning to the acoustic wave generators 2a and 2b.

Furthermore, the acoustic wave generators 2a and 2b are placed at positions facing each other across the light transmission area in the center of the base material, and the spacing thereof is determined in consideration of the size of the luminous flux of the incident light to be transmitted. do. The practical maximum value of this distance is, assuming that the base material is a piezoelectric substrate such as lithium niobate and the SAW frequency is approximately 200 MHz, the transmission loss is 0.3 dB.
It can be estimated that the diameter is about 10 cm, considering the maximum diameter of currently usable substrates and the capacity of manufacturing equipment such as mask aligners. This value is much larger than the shape of optical diffraction gratings currently used in the form of scored lines on glass materials, and is sufficient for practical use.

Next, the state of 5AWO propagation, light incidence, and phase modulation will be explained using FIG. 2, which shows a cross-sectional view on a plane including the SAW traveling direction axis and the incident optical axis of the fifth embodiment.

As shown in FIG. 1, in this embodiment, three base materials and two acoustic wave generators generate SAW on four optical planes of the base materials.

As shown in Fig. 2, 5AWL,
Name them 5AWI° and 5AW2.5AW2°. The 5AWI and 5AWI' are the same acoustic wave generator 2
Since it is a fired SAW, it maintains completely the same spatial phase. Similarly, it is clear that 5AW2.5AW2' also has the same spatial phase. Therefore, the acoustic waves (SAW) emitted by the acoustic wave generators 2a and 2b travel in spatially opposite directions, and are visible from the optical axis direction in the light transmission area located approximately in the center of the base material. In this case, it is necessary to adjust the arrangement so that the directions of the phase planes (lattice directions) are aligned in the same direction and intersect.

There are several methods for phase adjustment, but the simplest and most reliable method is to adjust the arrangement of the generation mechanism of each acoustic wave generator to match the phase planes of the SAWs.

In the interdigital electrodes as in the embodiment, it is only necessary to adjust the position of each electrode in the SAW propagation direction with an accuracy of several to several tenths of the SAW wavelength, and the actual numerical accuracy required is several μm.

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

FIG. 3 shows a configuration diagram of the second embodiment.

In this embodiment, a double-sided phase modulation element 6 equipped with acoustic wave generators 2a and 2b on both sides is provided with homogeneous base materials la and lc.
Two acoustic wave generators 2a and 2b are arranged in close contact with each other.
and three base materials la, lb, and lc having light transmittance, 4
The operation of generating SAW on the surface is similar to that of the first embodiment shown in FIG. 1 above. However, the technology used in the manufacturing process is different, and in the first embodiment, the positions of the acoustic wave generators 2a and 'lb are matched at the stage of combining the two phase modulation elements, and the SAW traveling direction and phase plane (grating direction)
On the other hand, in this embodiment, when manufacturing the double-sided phase modulation element 6,
A double-sided mask alignment technique for spatially matching the phases of the acoustic wave generators 2a and 2b is important. At present, the construction method of the first embodiment is technically superior, but the construction method shown in this embodiment is considered to be effective in simplifying the manufacturing process and making the quality of the finished product uniform.

Examples of the SAW generation method which is the basis of the present invention have been described above, but in practical application of the present invention, the above-cited invention "Light diffraction device using surface acoustic waves"
As shown in the above, important items include the arrangement of ultrasonic absorbing members to prevent unnecessary reflection of the SAW, and measures against heat generation when the SAW is dissipated as heat. moreover,
In optical phase modulators that change the refractive index by changing the density of the base material due to SAW, as in the present invention, the refractive index of the base material used is often large. When performing
There is a high possibility that the rate will be as high as 0%.

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. 4 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 base materials 1a and lb are
The two sets of SAWs generated on the opposing surfaces of the substrates 1b and lc have a spatial period d corresponding to the lattice constant, and advance at a speed V in the direction of the arrow so as to intersect with each other. When incident light 4 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 the 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 S
It is equal to the propagation direction of AW. The value of α is α=Fλ/d=foFλ/v...='' (1
). Here, the frequency of the sine wave electrical signal is f
If it changes by ±Δf/2 around o, the displacement Δα of the ±1st-order diffraction bright spot on the focal plane 8 is Δα;
ΔfFλ/V . . . [2).

As is clear from equation (2), the smaller the propagation speed V of the SAW, the longer the focal ratio 1iffF of the lens, and the longer the wavelength λ of light, the larger the displacement Δα, and it has a linear relationship with the frequency of the electrical signal. You can see that it changes.

Also, as mentioned in the [effect] section above.

The two sets of SAWs that intersect and travel can be thought of as standing waves with wavelength d from the optical axis direction, and this standing wave overall occurs and disappears at twice the frequency of the SAWs. It is something that is repeated.

Therefore, the diffraction bright spot also repeats blinking at the same period as the generation period of this standing wave.

〔Effect of the invention〕

As described above, according to the present invention, two or more acoustic wave generators are sandwiched between light-transmitting base materials, and one acoustic wave generator can generate two or more base material surfaces. generate SAWs with the same frequency and the same phase, and half of the acoustic wave generators are placed at opposing positions across the light transmission area, so that the SAWs generated by them are spatially the same in the light transmission area. By making the phase planes (lattice directions) intersect and pass each other, it is possible to achieve highly efficient phase modulation that could not be obtained with conventional devices.In addition, the intersecting SAWs can generate and dissipate a standing wave-like phase that periodically repeats generation and disappearance. We were able to form a lattice. If the plane wave light spatially and temporally modulated by this phase grating is converted into a diffraction image using a lens, for example, the frequency is twice that of the SAW (several hundreds of MHz).
z to several GHz), a diffraction bright spot that can repeatedly blink at high speed is obtained.

Since this blinking cycle is synchronized with the SAW cycle, it can be used as a high-speed optical chopper in optical information processing systems. Furthermore, the two devices are characterized by being able to handle light (luminous flux) having a wide area, and can be used in conjunction with the frequency tunability of SAW to also be used as a phase grating with a variable lattice constant.

Furthermore, it is also possible to use it as a light deflection device or to utilize diffracted light that can repeatedly blink at high speed as a strobe light source.

[Brief explanation of the drawing]

FIG. 1 shows the configuration of a first embodiment of a standing waveform surface acoustic wave 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 first embodiment shown in FIG. FIG. 3 shows the configuration of a second embodiment of the invention. FIG. 4 shows the polarization state of light when the present invention is applied to light deflection. In the figure, 1a, 1b and IC are base materials, 2a and 2b are acoustic wave generators, 3a and 3b are spacers, 4 is incident light,
Reference numeral 6 indicates a double-sided phase modulation element including 1b, 2a, and 2b, 7 indicates a lens, and 8 indicates a focal plane.

Claims (1)

[Scope of Claims] A plurality of base materials that have optical transparency, have the same acoustic properties, have optical planes on the front and back sides, and are laminated with the optical planes facing each other, and the plurality of base materials. a first acoustic wave generating device that is interposed between at least one of the opposing optical planes and generates a first surface acoustic wave having a predetermined wavelength and a predetermined phase in a predetermined traveling direction on the optical plane; one or more two or more that are interposed between opposing optical planes of a plurality of base materials, have the same wavelength as the predetermined wavelength in a direction opposite to the predetermined traveling direction, and have one or the same phase with each other; a second acoustic wave generating device that generates a second surface acoustic wave on an optical plane; Wave light modulator.