JPH06100741B2 - Standing 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 waves) are used as diffraction gratings, and the SAW generation surfaces are arranged so as to be stacked at approximately parallel intervals in the optical axis direction of the incident light, and a plurality of acoustic wave generators that generate SAWs are also provided. When the SAWs are arranged so that their traveling directions are opposite to each other and seen through from the direction of the optical axis, the SAW standing wave is repeatedly generated and extinguished. The present invention relates to a standing wave surface acoustic wave light modulator that can obtain modulated light that blinks efficiently.

[Prior Art] In a light modulator that spatially modulates a phase plane of light, a geometrical spatial position of a light reflection point is set like an optical reflection grating, and each of them is adjusted from the shift of the position. That produces a desired phase delay (phase modulation) by causing an optical path difference in the reflected light, and the speed of light by changing the thickness and the refractive index of the material of the part through which light passes, such as an optical lens. , Which results in a phase delay. In the phase modulation device of the type that changes the refractive index of the medium that transmits light,
Anisotropic crystal is used as the light transmission medium, and it is possible to generate a phase delay easily and at high speed by applying an electric field or magnetic field. Many practical optical elements have been developed, such as a modulation element that performs modulation, and an optical switch that causes the modulated light of two elements to interfere to blink the light.

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 high rate 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 also 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 agg condition has also been developed, it has been used as a high-frequency, broadband 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). SAW
The method of guiding light to the generating surface is mainly used as a phase modulator 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, attention is being paid to a phase diffraction grating that can change the SAW frequency to change the grating constant and has a variable grating interval.

[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 plurality of acoustic wave generators are divided into two groups, each of which is about half, and in each group, SAWs travel in the same direction at the same frequency and at the same speed, and each SAW is directed in the optical axis direction. The SAW generation functions (characteristics) and location of the generators were set so that they would overlap in phase when viewed more.
Furthermore, in SAW between these two groups, the frequency and the traveling speed are equal, the phase planes are spatially aligned in parallel, and only the traveling direction is the opposite direction in the light transmission region on the SAW propagation substrate. The positions of the sound generators of each group were set so as to intersect spatially.

[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 was repeatedly received by multiple SAWs with the same spatial phase), and it became possible to increase the modulation efficiency according to the number of SAW propagation substrates. In addition, since multiple SAWs were divided into almost equal parts and moved backward, when the SAWs overlapping in the optical axis direction were seen through, it was apparent that the backward SAWs overlap at the in-phase position. On the other hand, the SAW of the whole is mutually strengthened, and the phase modulation of the maximum intensity is applied, and on the contrary, when they overlap in opposite phases, they weaken each other, for example, the sum of the SAW amplitude intensity of each of the two groups is equal. In some cases, a state where no phase modulation is applied, that is, a state in which the distribution of apparent refractive index changes becomes uniform appears. When the frequency and velocity of two intersecting SAWs are equal, such a change in the phase modulation state results in a spatially standing wave state, and in terms of time, the SAW is relatively doubled in velocity. Since they intersect, the entire spatial standing wave is repeatedly generated and extinguished at a time frequency twice that of SAW. Therefore, for example, if the plane wave light that has passed through this device is converged by a lens and the Fraunhofer diffraction image is observed, the ± first-order luminescent spot becomes a luminescent spot blinking at twice the time frequency of SAW.

Therefore, according to the present invention, the efficiency of phase modulation is increased by stacking the base materials, the SAW propagation direction is reversed, and the phase modulation function of interrupting at a high frequency is combined.

〔Example〕

FIG. 1 shows the first embodiment of the standing 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 substrates 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-transmissive base material, and interdigital electrodes are formed on the surface of the substrate by vapor-deposited thin films of metal as an acoustic wave generator.

As shown in FIG. 1, the substrate 1a is a piezoelectric substrate, and an interdigital finger electrode formed on one surface of the substrate by a metal thin film deposition technique and a semiconductor fine processing technique is used as the acoustic wave generator 2a. It is arranged.

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 between the two phase modulation elements composed of the combination of the substrate 1c and the acoustic wave generator 2b, the substrate 1a, 1c
It can be said that the structure is such that the base material 1b having the acoustic characteristics equivalent to that is sandwiched. 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 base material 1b,
SAW can also be generated on the front and back surfaces of the substrate 1b by the acoustic wave generators 2a and 2b.

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.

Furthermore, the location of the acoustic wave generators 2a, 2b is
Set the light transmissive area in the center of the base material so as to face each other,
The interval is determined in consideration of the size of the incident light beam to be transmitted. The practical maximum value of this interval is a SAW frequency of 200 MHz using a piezoelectric substrate such as lithium niobate as the base material.
Assuming that the propagation loss is about 0.3 dB / cm, it is considered to be about 10 cm when considering the maximum diameter of the substrate that can be used at present and the manufacturing equipment such as mask aligner. This value is much larger than the shape of the optical diffraction grating that is currently engraved on the glass material and is sufficiently practical.

Next, the state of SAW propagation and the incidence and phase modulation of light will be described with reference to FIG. 2, which is a sectional view taken along a plane including the SAW traveling direction axis and the incident optical axis of the present embodiment.

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

As shown in Fig. 2, SAW1, SAW1 ', S
Name them AW2 and SAW2 ′. Since SAW1 and SAW1 'are SAWs emitted from the same acoustic wave generator 2a, they completely maintain the same phase in space. Similarly, it is clear that SAW2 and SAW2 'also have the same spatial phase. Therefore, the acoustic waves (SAW) emitted from the acoustic wave generators 2a and 2b travel in spatially opposite directions, and are seen from the optical axis direction in the light transmission region located almost in the center of the substrate. In this case, it is necessary to adjust the arrangement so that the directions of the phase planes (lattice directions) intersect in the same direction.

There are several possible phase adjustment methods, but the simplest and surest method is to adjust the arrangement of the generation mechanism of each acoustic wave generator and match the SAW phase plane.

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.

FIG. 3 shows a block diagram of the second embodiment. In this embodiment, a double-sided phase modulation element 6 having acoustic wave generators 2a and 2b on both sides is adhered with homogeneous substrates 1a and 1c.
With two acoustic wave generators 2a, 2b and three light-transmitting base materials 1a, 1b, 1c, the operation of generating SAW on the four sides is the same as that of the first embodiment shown in FIG. Is. However, the technology used in the manufacturing process is different, and in the first embodiment, the positions of the acoustic wave generators 2a and 2b are matched at the stage of combining the two phase modulation elements, and the SAW traveling direction and the phase plane (lattice direction) are matched. ) Adjustment assembly technology is required,
On the other hand, in this embodiment, when manufacturing the double-sided phase modulation element 6, the double-sided mask alignment technique for matching the spatial phase 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 the present embodiment is considered to be effective for simplifying the manufacturing process and making the quality of finished products uniform.

The embodiments of the method for generating SAW, which is the basis of the present invention, have been described above. However, in the practical application of the present invention, the invention is also shown in the above-mentioned cited invention "a light diffraction 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. 4 shows an example of use in a light deflecting device as a simple application example of the present invention.

Two sets of SAW generated on the facing surfaces of the substrates 1a and 1b and the facing surfaces of the substrates 1b and 1c by the sinusoidal electric signal of frequency f ₀ have a spatial period d corresponding to the lattice constant, and a velocity v
And proceed to cross each other 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 just 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. The value of α 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 amount of change 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.

Further, as described in the above [Action] part, the two sets of SAWs traveling in an intersecting manner can be considered as standing waves of wavelength d from the optical axis direction, and these standing waves are As a whole, it repeatedly appears and disappears at twice the frequency of SAW.

Therefore, the diffracted bright spot also repeats blinking in the same cycle as the standing wave generation cycle.

〔The invention's effect〕

As described above, according to the present invention, two or more acoustic wave generators are sandwiched between light transmissive substrates, and one acoustic wave generator produces two substrate surfaces. Generate SAWs with the same frequency and phase and place half of the acoustic wave generators at opposite positions across the light transmission region, and the SAWs generated from them are spatially the same in the light transmission region. Since the phase plane (lattice direction) crosses and passes each other, it is possible to realize highly efficient phase modulation, which was not possible with conventional devices. In addition, standing wave-like phases that periodically occur and disappear due to intersecting SAWs are realized. The grid could be formed. If the plane wave light spatially and temporally modulated by this phase grating is converted into a diffraction image with a lens, for example, it will flash at high speed at twice the frequency of SAW (several hundred MHz to several GHz). The returned diffraction bright spot is obtained. Since this blinking cycle is synchronized with the SAW cycle,
It can be used as a high-speed optical chopper. Moreover, this device is characterized in that it can handle light (luminous flux) having a wide area, and can be used as a phase grating with a variable grating constant in combination with the frequency variability of SAW.

Furthermore, a method of using it as a light deflecting device or utilizing diffracted light that repeats high-speed blinking as a strobe light source is also conceivable.

[Brief description of drawings]

FIG. 1 shows a configuration of a standing 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 and a plurality of SAWs and incident light in the first embodiment of FIG. FIG. 3 shows the configuration of the second embodiment of the present invention. FIG. 4 shows a light deflection state 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, 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 transparent 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. A first surface acoustic wave, which is interposed in at least one of the opposing optical planes and has a predetermined wavelength and a predetermined phase in a predetermined traveling direction,
Of the acoustic wave generator and at least one of the optical planes of the plurality of base materials facing each other, having the same wavelength as the predetermined wavelength in the direction opposite to the predetermined traveling direction, and one or A second acoustic wave generator for generating two or more second surface acoustic waves having the same phase on an optical plane, and light transmitted by a grating formed from the first and second surface acoustic waves. Standing waveform surface acoustic wave light modulator that modulates.