JPH06100740B2 - Progressive waveform surface acoustic wave optical 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 in particular to a surface acoustic wave (SAW: Surface acoustic wave) is used as a diffraction grating, and by arranging the generation surfaces of the SAW so as to be stacked at approximately parallel intervals in the optical axis direction of the incident light, it is possible to perform phase modulation of the light wavefront with high efficiency. The present invention relates to a traveling wave surface acoustic wave light modulator.

[Conventional technology]

In a light modulator that spatially modulates the phase plane of light, a geometric spatial position of a light reflection point is set like an optical reflection grating, and an optical path is set to each reflected light from the deviation of those positions. What causes the desired phase delay (phase modulation) by making a difference, and the thickness of the material through which light passes, such as an optical lens, and the refractive index are changed to slow down the speed of light. ，
Some cause a phase delay. In a phase modulator that changes the refractive index of a medium that transmits light, an anisotropic crystal or the like is used as the light transmitting medium, and it is possible to easily and rapidly generate a phase delay by applying an electric field or magnetic field. It is possible, and many practical applications such as a modulator that applies an electric field to an optical waveguide on a piezoelectric crystal substrate to perform phase modulation, and an optical switch that causes modulated light of two elements to interfere and blink the light Optical elements have been developed.

In addition, when the area of a material that does not significantly change the refractive index due to changes in the electric field or magnetic field, or the area through which light is transmitted is large and it is desired to generate, for example, a sinusoidal phase change distribution in the entire area. Has used an acousto-optical method in which ultrasonic waves are radiated into a light-transmitting medium and the refractive index changes due to the density change of the medium caused by the ultrasonic waves.

This acousto-optical phase modulation method is roughly classified into a method using a bulk wave traveling inside the medium and a surface acoustic wave (SA) in which most of the energy is concentrated on the surface layer of the medium.
W) can be divided into two methods.

In the case of using the bulk wave, the light can be propagated in the ultrasonic wave over a long distance. As a result, the time (distance) of mutual interference between the ultrasonic wave and the light can be lengthened, and the phase change amount can be increased. Is large (modulation efficiency is high), but it is difficult to widen the area of the light transmitting part, and the generation band of ultrasonic waves is structurally narrow. The incident angle of light to the three-dimensional lattice-shaped refractive index changing region was limited by the Bragg condition, and when the wavelength of incident light or the wavelength of ultrasonic waves changed, the incident angle had to be adjusted accordingly.
However, it is one of the most practically used fixed-frequency optical modulators because of its small size and high efficiency. On the other hand, in the optical modulator using SAW, the SAW generation mechanism is suitable for high frequency and wide band, and
Since a method of changing the SAW emission direction to suit the g (Bragg) condition has also been developed, it has been used as a high-frequency, wide-band optical modulator. The method of combining SAW and light is to guide the light in a thin film onto the surface of the substrate where SAW is generated and propagate it through SAW for a long time (long distance) to increase the modulation efficiency. There is a method in which light is transmitted perpendicularly to phase modulation in a short time (short distance). The method of guiding light to the SAW generation surface is mainly used as a phase modulation element for thin-film optical ICs and has high utility value. In addition, the method of injecting light perpendicularly to the SAW generation surface can apply phase modulation to the entire wide light flux, and it should be used like a phase diffraction grating in a general space propagation type optical system. There are many. In this case, SAW
It has been attracting attention as a phase diffraction grating with a variable grating interval, which can change the grating constant by changing the frequency.

[Problems to be solved by the invention]

However, it is difficult to increase the efficiency of phase modulation in such an optical modulator that makes the light incident vertically on the SAW generation surface, because the interaction time between the SAW and the light is shorter than in other systems. It was

In the modulator using the bulk wave, if the Bragg condition is satisfied, a modulation efficiency of about 80% can be expected, but in the modulator in which light is vertically incident on the SAW, it is about several%. It was extremely low and had very limited practical aspects. (Note: The modulation efficiency referred to here is the percentage of incident light diffracted by measuring the intensity of the diffracted light generated by the phase change after imaging the light after phase modulation. [Means for Solving Problems] The present invention has been made to solve these problems.
As a means of solving the problem, a plurality of base materials having front and back optical planes that transmit light and propagate SAW are arranged so that the front and back surfaces of the base materials 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 these acoustic wave generators has the same frequency and the same phase on two optical planes sandwiching these generators. The SAW becomes a SAW and propagates in the same direction. For this reason, a plurality of base materials made of materials having the same acoustic characteristics and sufficiently good incident light transmittance were used. Furthermore, the SAWs emitted from the respective acoustic wave generators have the same frequency and the same traveling direction, respectively, and when viewed from the optical axis direction of the incident light, the plurality of SAWs are generated.
The respective setting locations of the generator are set so that the phases of the above are the same.

[Action]

The SAs stacked in the optical axis direction of the incident light by the above means
The light passes through the W-propagating substrate in a short time that can be ignored with respect to the SAW propagation velocity, and the phase generated at each substrate surface has the same frequency and is spatially aligned in the same plane ( After that, the same phase modulation is repeatedly received by multiple SAWs having the same spatial phase), and the modulation efficiency can be increased according to the number of SAW propagation base materials.

〔Example〕

FIG. 1 shows a first embodiment of the traveling waveform surface acoustic wave optical modulator of the present invention.
The block diagram in the Example of this is shown.

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

To easily realize a structure in which an acoustic wave generator is arranged between a plurality of substrates, an acousto-optical phase modulation element using SAW which has been conventionally used, for example, by the same applicant and the same inventor Invention "Diffraction device of light using surface acoustic wave"
(Japanese Patent Application No. 60-234812) and the like can be considered as a method of combining with other base materials having the same acoustic characteristics according to the gist of the present invention. In this "optical diffraction device using SAW", as a structure for performing the basic operation of optical phase modulation,
A piezoelectric substrate is used as a light-transmitting base material, and interdigital electrodes are formed on the surface of the substrate as a sound generating device by vapor deposition of metal.

As shown in FIG. 1, the base material 1a is a piezoelectric substrate, and an interdigital finger electrode formed by a metal thin film deposition technique and a semiconductor fine processing technique is arranged as a sound generating device 2a on one surface of the substrate. It is the configured configuration.

This optical phase modulation element using the piezoelectric substrate and the interdigital electrodes is the simplest structure and widely known as an element for phase modulating the light vertically incident on the SAW generation surface in a wide area.

Now, referring again to the embodiment shown in FIG. 1 based on such a phase modulation element, this embodiment shows two phase modulation elements by SAW having the same characteristics, that is, the substrate 1a and the sound generator 2a. , And a structure in which a base material 1b having acoustic characteristics equivalent to those of the base materials 1a and 1b is sandwiched between two phase modulation elements composed of a combination of the base material 1c and the sound generator 2b. I can say. In this case, if the surfaces of the acoustic wave generators 2a and 2b are superposed so as to be in close contact with the front and back optical planes of the substrate 1b, the acoustic wave generators 2a and 2b can also be used for the front and back surfaces of the substrate 1b.
It is possible to generate AW.

Experimentally, the vibration amplitude of SAW is several Å to several tens of Å, and
Since the film thickness of the interdigital finger electrode that is usually used is about several μm, even if the interdigital electrode is sandwiched between the base materials and laminated and pressure bonded, the The voids between the substrates are sufficiently spaced to allow SAW propagation in their opposing optical planes.

Also, as shown in the figure, a simple method of keeping the width of the void constant and stacking the base materials substantially parallel to each other is, as shown in FIG. It is preferable to form small pieces of a vapor deposition film as the spacers 3a and 3b on the opposite side. In this embodiment, the spacers 3a and 3b are formed so as to be oblique with respect to the traveling direction of the SAW so that unnecessary reflection of the SAW does not return to the acoustic wave generators 2a and 2b.

Next, the state of propagation of SAW generated on the surface of each substrate, the incidence of light, and the incidence of light are shown in FIG. 2 which is a sectional view in a plane including the SAW traveling direction axis and the incident optical axis of the present embodiment. The phase modulation will be described.

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

As shown in Fig. 2, SAW1, SAW1 ', S
Name them AW2 and SAW2 ′. If the oscillation frequency of each acoustic generator and the SAW emission direction and the acoustic characteristics of each substrate are the same, all SAWs have parallel phase planes (lattice planes when viewed from the optical axis direction) with the same wavelength. Proceed at the same speed.

Therefore, in order for each SAW to give the same phase modulation action to the incident light 4, it suffices if the phase planes of each SAW are aligned in phase when viewed from the optical axis direction. . Since SAW1 and SAW1 'are SAWs emitted from the same acoustic wave generator, they completely maintain the same spatial phase. Similarly, it is clear that SAW2 and SAW2 'have the same spatial phase.

Therefore, if the acoustic waves (SAW) emitted from the acoustic generators 2a and 2b are spatially in phase,
All SAWs are aligned in phase when viewed from the optical axis direction.

There are several possible methods for adjusting the phase, but the simplest and surest method is to adjust the arrangement of the generating mechanism of each acoustic wave generator and adjust the SAW phase.

In the interdigitated 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 tenths of the SAW wavelength, and the numerical accuracy actually required is several μm. Positioning within this degree of accuracy can be sufficiently realized by the current mask aligner for manufacturing semiconductor devices.

As another method, there is a method in which only the parallelism of the interdigital electrodes in the interdigital direction is precisely matched, and the phase of the SAW is adjusted by individually delaying the phase of the electric signal applied to each electrode. Although this method makes it difficult to make the external circuit of the modulator a little complicated, it also has the feature that it can be adjusted while actually injecting light after observing the modulated output light after assembling the optical modulator. , Is a practically effective means.

Next, some construction methods in the embodiments of the present invention will be described.

FIG. 3 shows a block diagram of the second embodiment. This example
In the simplest form of the present invention, a phase modulation element 5 having an acoustic wave generating device 2 on one surface and a base material 1b of the same quality as this element 5 are closely contacted so that the acoustic wave generating device is sandwiched from both sides. It was done. Here, the shape of the base material 1b is made smaller than that of the base material 1a of the phase modulation element 5 because of a device for exposing a part of the electrodes of the acoustic wave generator 2 to facilitate signal supply. , It is not directly related to the basic operation of optical phase modulation.

FIG. 4 shows a block diagram of the third embodiment.

In this embodiment, two acoustic wave generators 2a and 2b are provided, in which the same type of base materials 1a and 1b are adhered from both sides of a double-sided phase modulation element 6 having acoustic wave generators 2a and 2b on both sides. The operation of generating SAW on the four surfaces of the three light-transmitting base materials 1a, 1b, 1c is the same as that of the first embodiment shown in FIG. However, the technique used in the manufacturing process is different, and in the first embodiment, the assembly for matching the positions of the acoustic wave generators 2a and 2b at the stage of combining the two phase modulation elements and adjusting the SAW phase to the same phase. On the other hand, the third embodiment requires double-sided mask alignment for matching the spatial phase of the acoustic wave generators 2a and 2b when manufacturing the double-sided phase modulation element 6. Technology is important. At present, the construction method of the first embodiment is technically superior, but the construction method shown in the third embodiment is considered to be effective for simplifying the manufacturing process and making the quality of finished products uniform. To be

The three embodiments of the SAW generation method, which is the basis of the present invention, have been described above. However, when the present invention is put into practical use, it is also shown in the above-mentioned cited invention "a light diffracting device using surface acoustic waves". As described above, the arrangement of ultrasonic absorbing members that prevent unnecessary reflection of SAW and measures against heat generation when SAW disappears as heat are also important items. Furthermore, as in the present invention, in the optical phase modulator in which the refractive index change is caused by the density change of the base material by SAW, the refractive index of the base material used is often large, and in the air When light is incident, 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. 5 shows an example applied to a light deflector as a simple application example of the present invention.

The SAW generated on the surfaces of the base materials 1a and 1b by the sinusoidal electric signal of the frequency f ₀ has a spatial period d corresponding to the lattice constant, and advances at the velocity v in the direction of the arrow. When the incident light 4 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 SAW and the change in the refractive index immediately below the surface of the base material. Since this phase modulation is periodic due to the repetition of the spatial period d, this light is the same as the light transmitted through the ordinary sinusoidal phase grating, and when it is imaged on the focal plane 8 of the lens by the lens 7, the diffraction image is obtained. Cause Here, if the incident light is monochromatic light of wavelength λ, the diffraction image is a ± 1st-order diffracted bright spot image whose position is determined by the lattice constant d. The position where the diffracted bright spot occurs is a position separated from the optical axis on the focal plane 8 by a distance α, and the direction is the same as the SAW propagation direction. α
The value of is expressed by α = Fλ / d = f ₀ Fλ / v (1) where F is the focal length of the lens 7. Here, assuming that the frequency of the sine wave electric signal changes ± Δf / 2 around f ₀ , the displacement amount Δα of the ± 1st-order diffracted bright spot on the focal plane 8 is Δα = ΔfFλ / v ... … (2) As is clear from the equation (2), the SAW propagation velocity v
It can be seen that the displacement amount Δα increases as the focal length F of the lens increases, the focal length F of the lens increases, and the wavelength λ of the light increases, and the displacement Δα changes in a linear relationship with the frequency of the electric signal.

〔The invention's effect〕

As described above, according to the present invention, the acoustic wave generator is arranged between the light-transmissive base material so that the acoustic wave generator is sandwiched between the two substrates. Phase SAW can be effectively generated, and these are stacked in the light incident direction, and the spatial phase is aligned to the same phase state, so a highly efficient optical phase that could not be obtained with conventional devices. It has become possible to realize modulation.

As a result, according to the present invention, spatial phase modulation in a wide area can be efficiently realized, and a sine wave phase grating having a variable grating constant applicable to a spectroscopic measurement device or an optoelectronic signal processing device such as an acousto-optic correlator can be used. The realization of spatial phase modulators has become easier.

[Brief description of drawings]

FIG. 1 shows a configuration of a traveling waveform surface acoustic wave optical modulator according to a first embodiment of the present invention. FIG. 2 shows the positional relationship between the laminated base materials in the first embodiment of FIG. 1, a plurality of SAWs, and incident light. FIG. 3 shows a configuration of a traveling waveform surface acoustic wave optical modulator according to a second embodiment of the present invention. FIG. 4 shows a structure of a traveling waveform surface acoustic wave optical modulator according to a third embodiment of the present invention. FIG. 5 shows the deflection 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 acoustic wave generators, 3a and 3b are spacers, 4 is incident light, 5 is a phase modulation element combining 1a and 2, 6 is 1b and 2a and A double-sided phase modulation element combining 2b, 7 is a lens, and 8 is a focal plane.

Claims (1)

[Claims] 1. A plurality of base materials which are light transmissive and have the same acoustic characteristics, have front and back optical planes, respectively, and are laminated with the optical planes facing each other, and a plurality of the base materials. An acoustic wave generator, which is interposed between opposing optical planes and generates surface acoustic waves of the same wavelength and the same phase in the same traveling direction on each of the opposing optical planes, transmits through the laminated optical planes. A laminated traveling-wave surface acoustic wave light modulator that modulates light.