JPH01144024A - Multiple reflection type surface acoustic wave optical diffraction element - Google Patents

Abstract

PURPOSE:To enhance deflection efficiency by providing light reflecting layers oppositely on the front and rear faces of a piezo-electric substrate in such a manner that incident light passes >=2 times the surface acoustic wave (SAW) excitation region on the front layer of the substrate and constructing the title element in such a manner that the light is multiple-reflected within the substrate. CONSTITUTION:The reflecting layers 5, 6 for light are respectively provided to the front and rear faces of the piezo-electric substrate 2 and the incident light is guided between these layers and is thereby multiple-reflected, by which the number of interactions of the light and the SAW is increased. The quantity of the + or - primary diffracted light 11 to be used as deflecting light is, therefore, considerably increased. The sufficient quantity of the deflection light is thereby obtd. even if input electric power is decreased and the stable deflection operation is enabled with small driving power. The multiple-reflection type surface acoustic wave optical diffraction element which has the large quantity of the deflection light and is practicable is thus obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical deflection device that deflects the traveling direction of light using ultrasonic waves, and in particular, it relates to an optical deflection device that deflects the traveling direction of light using ultrasonic waves. The present invention relates to a multiple reflection type surface acoustic wave optical diffraction element that is used as a grating and can adjust the direction of light diffraction by changing the lattice constant.

[Conventional technology]

Conventional techniques for deflecting light at high speed include, for example, a method of rotating a polygon mirror at the speed of light; a method of vibrating a piezoelectric element equipped with a reflecting mirror with a high frequency signal; and a method of using surface acoustic waves (S A W, 5u
There is a method using diffracted light by rface acoustic have. Among these, the former 2
One method uses mechanical motion to polarize light by changing the incident angle of the light by rotating or vibrating a reflecting mirror.
The upper limit of its operating speed is currently about several hundred kHz for periodic operation. - On the other hand, the latter method using SAW uses a SAW generated on the solid surface as a sinusoidal diffraction grating, and although there is a limitation that only the ±1st-order diffracted light is deflected, by changing the grating constant, the light can be The diffraction angle, ie the deflection angle, can be controlled. This lattice constant is SAW
It is determined depending on the propagation speed of the SAW and the high-frequency signal manually applied to the electrode that generates the SAW. By changing the input frequency, it is possible to deflect the light to any position or stop it, and it is also possible to obtain the maximum deflection angle. The operating time can also be shortened to about several μs. One example of a method of an optical deflector using such a SAW is the invention "Surface Acoustic Wave Variable Diffraction Grating (Japanese Patent Application No. 73416/1989)" by the same applicant and inventor.

In this invention, as a means to increase diffraction efficiency, a large high-frequency power is input, and by creating a structure that efficiently dissipates the unnecessary heat generated at that time into the outside air, the lattice constant is stabilized, that is, the deflection angle is achieved stabilization.

[Problem that the invention seeks to solve]

However, there is a limit to the input high-frequency power, and if the electric field generated between the interdigital electrodes becomes large, the piezoelectric substrate will be destroyed.

For example, it is known that the breakdown electric field of lithium niobate crystal, which is often used as a substrate for SAW devices, is about 1Oν/μm. In this way, the heat dissipation efficiency is improved,
Even if the amount of diffracted light is increased by increasing the input power, there is a drawback that the upper limit is determined by the characteristics of the piezoelectric substrate used.

Furthermore, the optical diffraction in the above invention (surface acoustic wave tunable diffraction grating) utilizes an effect called Ramanners diffraction, and when compared with Bragg diffraction, which is often used in optical switches, light diffraction that causes a diffraction phenomenon is On the one hand, the interaction time (distance) between light and ultrasonic waves is short, and the diffraction efficiency is close to that of the Bragg angle IJ.
It also has the disadvantage of being only a few tenths (several percent) of 'r (80%).

[Means for solving problems]

Therefore, in the present invention, in order to solve the above-mentioned problems, a method is adopted in which the incident light is confined for a long time in the interaction region with the ultrasonic wave. In which scheme, the transmission vIi loss of the SAW amplitude is, for example, for the above-mentioned lithium niobate crystal, -
The SA is as small as about 0.3 dB/cm, which is sufficient to cause light diffraction over a long area in the SAW propagation direction.
It can be seen that W is occurring. Therefore, so that the incident light passes through the SAWMJJ vibration region on the surface layer of the substrate more than once, light reflecting layers are provided on the front and back surfaces of the piezoelectric substrate, and a structure is adopted in which the light is multiple-reflected inside the substrate. Improves deflection efficiency.

[Effect]

As shown in Fig. 1, which shows the relationship between the frequency f of the electrical signal input to the interdigital electrode 1, the propagation velocity of the SAW of the piezoelectric substrate 2, and the spatial period d of the SAW, the interdigital When the electrode interval of the electrode worker is d, when an impulse-like electric signal is applied to this electrode, a SAW of wavelength d is generated on the piezoelectric substrate.

This SAW propagates to the left and right of the electrode at a speed of V, and therefore the time. (= d/v) later, the phase of the SAW and the position of the interdigital electrodes are again in the same positional relationship as at the time when the impulse signal was applied. here,
Furthermore, if an impulse-like electrical signal is added, this SAW will propagate with further increasing amplitude. Therefore, the period La, that is, the frequency fo (=I/16-=V/d
) is continuously applied to the interdigital electrodes, the amplitude of the SAW is increased in the same phase, and is emitted onto the piezoelectric substrate 2 as a strong traveling wave.

Such a state is called a resonant state.
A similar effect can be obtained even if a sine wave signal of 0 is used, and a sine wave signal that is easily generated is usually used. As the frequency of the electrical signal changes from fo, the resonance state collapses and the strength of the SAW decreases. In the current SAW element, the frequency bandwidth of the range where the SAW intensity is 110% is the resonant frequency f0
It is about 5 to 10% of that.

FIG. 2 shows how light is deflected when a SAW is used as a phase grating.

A piezoelectric group Fi2 is generated by a sinusoidal electrical signal of frequency f0.
The SAW generated on the surface has a spatial period d corresponding to the lattice constant, and moves at a speed V in the direction of the arrow.

The piezoelectric substrate 2 is transparent to light, and when incident light from the left direction in the figure passes through the piezoelectric substrate 2, the incident light is transmitted to the surface of the piezoelectric substrate 2 by the SAW. The piezoelectric substrate 2 undergoes phase modulation due to the sinusoidal shape, unevenness, and changes in the density just below the surface of the piezoelectric substrate 2, that is, changes in the refractive index.

This phase modulation is periodic due to the repetition of the spatial period d, so when these lights are focused on the focal plane 4 of the lens by the lens 3, they form a diffraction image, just like those transmitted through a normal phase grating. arise. By the way, if the incident light is monochromatic light with wavelength λ, the diffraction image will have ±1 determined by the lattice constant d.
The following diffraction bright spots are generated. The generation position of this diffraction bright spot is a distance α apart from the optical axis on the focal plane 4, and the direction is the same as the propagation direction of the SAW. a=Fλ/d=foFλ/v −−−−−−−
---It is expressed as (1). Here, if the frequency of the sinusoidal electric signal changes by ±Δf/2 around r, the displacement Δα of the ±1st-order diffraction bright spot on the focal plane 4 is Δ
α=Δ IF λ/V ・・・−−−−
・−・・−−−−−−・−・ (2) As is clear from this equation, the 5AWO propagation speed is slow (, the focal length of lens 3 is long, and the wavelength λ of the light is long) The amount of displacement Δα
takes a large value.

What has been described above is the effect when the incident light passes through the SAW excitation region only once, but when multiple reflections of light occur inside the piezoelectric substrate, this is the effect of the SAW excitation region in Figure 2.
This is equivalent to a state in which many AW elements are stacked one on top of the other, and the diffracted light diffracted from each piezoelectric substrate at that time will not be separated and will form a single bright spot if the piezoelectric substrate is sufficiently thin. It can be deflected.

[First Example] FIGS. 3(a) and 3(b) show an example of the configuration of a multiple reflection type surface acoustic wave optical diffraction element according to the present invention. In this example, the back surface of the piezoelectric substrate 2 is A first light-reflecting layer 5 is provided on the surface, and a second light-reflecting layer 6 and interdigital electrodes 1 for SAW generation are provided on the surface, and these are placed on a substrate 7 made of a poor thermal conductor, and piezoelectric On the end face of the substrate 2, there is a sound absorbing material 8 that prevents reflection of the SAW and absorbs it and converts it into heat, and a heat radiator 9 is brought into close contact with the sound absorbing material 8 to improve heat radiation efficiency. The cross-sectional view shows the method of light incidence and multiple reflections.・Figure 3 (
As shown in b), the light incident on the point a on the surface of the piezoelectric substrate 2 travels from the first light reflective layer 5 to the second light reflective layer 6 to the first
After repeating reflection several times in the order of light reflecting layer 5, the light is separated into main axis light IO and ±1st order diffracted light 11, which are emitted from point a° on the surface of piezoelectric substrate 2. At this time, the emitted light undergoes phase modulation when reflected at points a, a'' and the second light reflection layer 6, but at points a and a', the periodic refractive index change of the medium due to the SAW The wavefront of the light is subjected to phase modulation, and in addition to the effects at points a and a' in the first antireflection film, it is also subjected to phase modulation due to the periodic amplitude of the reflective layer.
As a result, the amount of deflected light can be much increased compared to a single transmission type optical diffraction element.

The piezoelectric substrate 2 and interdigital electrodes 1 that constitute these SAW elements. First light reflective layer5. The second light-reflecting layer 6 can be made of conventionally used materials, ie, piezoelectric materials such as lithium niobate crystals, and electrode materials such as aluminum and gold.

In addition, the electrode width and intersecting interval of the interdigital electrodes are several μm.
Although the thickness is as small as approximately 105 m, these processes can be realized using photolithographic microfabrication technology.

[Second Embodiment] In the embodiment shown in FIG. 3, the light incident part a had a structure in which the surface of the piezoelectric substrate was directly exposed to the air. However, −
Generally speaking, piezoelectric media have a high refractive index; for example, a lithium niobate crystal has an ordinary refractive index n0 of 2.286 and an extraordinary refractive index n0 of 2.286 for light with a wavelength of 0.633 μm.
2, and the reflectance at normal incidence is about 15 for n.
%.

Therefore, when the embodiment shown in FIG. 3 is used in an apparatus, it is necessary to block the reflected light from the light incident part a using a shielding plate or the like. Therefore, if an anti-reflection film is applied to the light incident area a using a material such as silicon oxide that has little effect on 5AWO propagation, most of the unnecessary reflected light can enter the inside of the substrate, and the amount of deflected light can also be increased. can be increased.

Of course, by applying an anti-reflection film to the light emitting portion a', it is possible to expect a further increase in the amount of deflected light.

[Third Example] FIG. 4 shows the measurement results of the relationship between the amount of diffracted light and the SAW frequency in the first example shown in FIG. 3.

In the figure, the vertical axis represents the diffracted light ff1 (uW), and the horizontal axis represents S.
The light incident angle, which indicates the AW frequency (MHz), is 25° in (a), 30° in (b), and 35° in (C).

(d) The figure is 40″.

The solid line indicates the first embodiment (MRT: Multiple Re
The dotted line indicates the amount of diffracted light of the first embodiment shown in FIG.
When light is reflected only once by the reflective layer 5 (SRT
:Sing-Ie Reflection Type
) shows the amount of diffracted light.

? IRT provides 5 to 10 times as much diffracted light as SRT at each light incident angle. However, as is clear from Figure 4, in MRT, the SAW frequency band where the amount of diffracted light increases is narrow, so it is difficult to deflect the light and continuously sweep a wide H range in space (Figure 4). Therefore, the frequency at which the amount of diffracted light takes the maximum value appears multiple times, so by switching these frequencies, it is possible to deflect the light to several points in space in terms of g number).

Therefore, the third embodiment aims to obtain high deflection efficiency in a wide SAW frequency band by using two-dimensional SAW propagation. Here, the two-dimensional propagation of the SAW means that the SAW propagation direction changes depending on the SAW frequency (wavelength).

Various methods for two-dimensional propagation of SAW have already been proposed, and in the third embodiment, curved interdigital electrodes 12 are used as shown in FIG. This interdigital electrode 12 is characterized in that the electrode interval changes continuously, and a part of the electrode resonates depending on the power frequency applied to the electrode.

Therefore, the SAW with the added frequency propagates in the normal direction of that part.

Now, MI? The reason why the amount of diffracted light increases at T is that, as described in the first embodiment, diffracted light is generated each time the light passes through the SAW region many times, and these are superimposed.

However, in order for this effect to occur, it is necessary that the respective diffracted lights are superimposed with their phases aligned, and at the SAW frequency where the amount of diffracted light takes a maximum value in the third embodiment shown in FIG. This condition is met.

In addition, at the next frequency where the amount of diffracted light takes a maximum value and the frequency where the amount of light takes a minimum value, the respective diffracted lights are superimposed with opposite phases.

For example, in the first embodiment, SA where the amount of diffracted light takes a minimum value
In order to increase the amount of diffracted light even at the W frequency, it is necessary to change the optical path that the diffracted light follows within the substrate from that in the first embodiment so that each diffracted light is superimposed in the same phase. This is because the phase difference between each diffracted beam is determined by the difference in optical path length that the diffracted beams follow within the substrate.

The direction in which the diffracted light is generated is the wave number vector of the principal axis light.

, the wave number vector of ±1st-order diffracted light is /, the wave number vector of SAW is /KsAw, /to = /Ko±/Ks, ・−・−・−・−
・・・－・・・Determined by (3).

Therefore, if the wave number vector of the SAW, that is, the SAW direction hearsay direction is set so that each diffracted light generated in the direction determined by the above equation (3) is superimposed with the same phase, a wide SAW High deflection efficiency can be obtained in the frequency band.

In addition, as a demonstration example that the deflection efficiency can be increased over a wide band according to this embodiment, when the sound absorbing material 8 shown in FIG. 3 is not applied in the first embodiment, the interdigitated electrodes are SA! which propagates in a direction non-parallel to the 5AW (call this 130) emitted from 1! W (this is called +Sl) is generated by the reflection of 30, but this S,, S, generates diffracted light at different diffraction bright spots, and the light intensity of the two diffracted lights takes maximum values at different SAW frequencies. It has been observed that

〔Effect of the invention〕

As described above, in the present invention, light reflecting layers are provided on each of the front and back surfaces of the piezoelectric substrate, and incident light is guided between the layers and subjected to multiple reflections, thereby increasing the number of interactions between light and the SAW. As a result, it has become possible to significantly increase the amount of ±1st-order diffracted light used as polarized light.

Therefore, even if the input power is lower than that of conventional elements, the same or higher amount of deflected light can be obtained, making it possible to perform stable deflection operation with small driving power, and to use practical multiplexing with a large amount of deflected light. A reflective surface acoustic wave optical diffraction element has become possible.

[Brief explanation of the drawing]

The first shows the relationship between the frequency r of the electrical signal, the SAW propagation velocity V of the piezoelectric substrate, and the spatial period d of the SAW. Figure 2 shows how light is deflected when the SAW is used as a phase grating. 3 shows the configuration of the first embodiment of the multiple reflection type surface acoustic wave optical diffraction element according to the present invention. FIG. 4 shows the amount of diffracted light and the SAW frequency in the first embodiment shown in FIG. FIG. 5, which shows the experimental results, shows the structure of a third embodiment of the multiple reflection type surface acoustic wave optical diffraction element according to the present invention. In the figure, ■ is an interdigital electrode, 2 is a piezoelectric substrate, 3 is a lens, 4 is a focal plane, 5 is a first light reflective layer, 6 is a second light reflective layer, 7 is a substrate, and 8 is a sound absorbing layer. 9 is a radiator, 10 is an O-order first light, 11 is a ±1st-order diffracted light, and 12 is a curved interdigital electrode. Patent Applicant Anri Co., Ltd. Agent Patent Attorney Ryu Koike No. 1 Figure 1... Interdigital electrode 2... Piezoelectric substrate d... Electrode spacing of interdigital electrode No. 4 FREQUENCY (Mllz) (a) FRhiOUhiNCY(Mllz) (b) --MIIl,
TIPLESINGl,l FREQUENCY(Mllz) (C) (C1)

Claims (1)

[Claims] a piezoelectric substrate (2) having optical transparency;
2) a first light reflecting layer (5) provided on the first surface;
At least one layer is provided on the surface of the piezoelectric substrate (2) and generates a surface acoustic wave forming an optical diffraction grating on the surface.
a second interdigital electrode (1) provided on the second surface of the piezoelectric substrate (2) and forming a multiple light reflection space between the first light reflection layer (5) and the first light reflection layer (5); a light reflecting layer (6);
generated at the interdigital electrode (1) and generated at the piezoelectric substrate (2).
) that absorbs surface acoustic waves propagating on the surface of the sound absorbing material (8) and converts it into heat; A multiple reflection type surface acoustic wave optical diffraction element comprising a radiator (9) made of a good heat conductor for dissipating the heat to the outside air.