JPS6232458B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an acousto-optical modulator that controls a light flux in response to an electrical signal through interaction between a coherent light flux and a controllable sound field (acousto-optic effect).

The present invention controls a pixel consisting of many points (e.g., 8) in response to a (e.g., 8-bit) digital image signal, requires a moderate signal transmission rate, and provides a hard copy of text and image information. This article relates to a high-speed computer printer/plotter suitable for production.

An example of this type of optical modulator is a frequency multiplexed Bragg acousto-optic modulator using parallel signals, which is described in USP 3744039 and Japanese Patent Publication No. 53-9856, and is capable of exposing seven points at the same time. Ta.

However, in conventional ultrasonic optical modulators, the ultrasonic input is large (in other words, the modulation voltage is high), heat is generated, the diffraction direction changes, and the modulator does not operate stably.

The purpose of the present invention is to modify a conventional ultrasonic optical modulator into an optical modulator suitable for parallel processing. 2) It has high transmittance and high extinction ratio. (3) It has low acoustic input and has a stable and clear focal point.

The main object of the present invention is particularly object (3).

As a clue to solving the object (3), it is disclosed in Japanese Patent Application Laid-Open No. 118439/1983 filed by the inventors of the present invention. This patent deals with the multiplication effect of repeated interference on the electro-optic effect resulting in Raman Nass diffraction of the light beam (the direction of incidence of the light is perpendicular to the direction of the spatial modulation of the refractive index).

However, it was completely unclear what kind of effect would be exhibited when bragging conditions with different incidence methods were satisfied.

An outline of the present invention will be explained below.

The present invention uses repeated interference to increase the length of interaction between sound waves and light waves, lowering the modulation voltage and eliminating thermal disturbances due to sound wave consumption.
Alternatively, the number of light spots that can be controlled can be increased. Although the modulation band becomes narrower by using repeated interference, it can be used sufficiently at a frequency of 10MHz.

Next, the present invention aims to establish the beam width of the incident light beam and the number of repeated reflections in order to control a large number of points without significantly impairing the multiplication effect.

Furthermore, the present invention uses thin film manufacturing techniques (sputtering, electron beam evaporation, etc.) to make the acoustic solid material sufficiently thin so that instability due to changes in the distance between the two reflective films due to thermal expansion does not increase and it works stably.
The goal is to devise ways to utilize the information.

An outline of the acousto-optical multiplex modulator of the present invention will be described below with reference to FIG. 1, which schematically represents multiple interference.

The acousto-optic solid material 3a has a high refractive index for light and a large acousto-optic constant (for example, LiTaO ₃ , LiNaO ₃ , tellurium-containing glass, etc.), and its two planes perpendicular to the Z axis are 1/100th of the wavelength of light. Polished to one or more degrees of parallelism and flatness. Moreover, on the two polished planes, each of the refractive index n
_h and _nl , made by alternately depositing 7 to 15 layers of two types of dielectric materials in which the ratio of _nh and _nl is as large as possible, each with a thickness of a quarter of the wavelength of light. Dielectric multilayer reflective films 3b ₁ and 3b ₂ are attached, respectively. The laser beam is expanded to a diameter sufficiently larger than the thickness of the acousto-optic solid material 3a (the distance between the reflective films), and the optical path difference in one round trip between the reflective films is an integral multiple of the wavelength of the light. It is incident at an angle of . A transducer (not shown) creates a pseudo-plane wave perpendicular to the reflective film, and creates a sound wave, that is, a sound field (indicated by dotted lines) that travels in a direction perpendicular to the pseudo-plane wave and perpendicular to the reflective film. , mounted on the acoustic solid material 3a.

There are also methods of making thinner acousto-optic material films using thin film manufacturing techniques.

First, a dielectric multilayer reflective film 3b ₁ is mounted on a piezoelectric substrate (not shown), an acousto-optic material film 3a is coated on the reflective film, and a dielectric multilayer reflective film 3b ₂ is further mounted on top of that. . A comb-shaped electrode transducer (not shown) is mounted on the side of the piezoelectric material on which the reflective film is mounted. The acoustic surface waves on the piezoelectric substrate emitted from the comb-shaped electrode transducer propagate faster than the bulk waves in the piezoelectric substrate, pass through the region of gradually changing thickness without significant reflection, and the acoustic surface waves in the acousto-optic material Convert to pseudo plane wave.

The frequency of these sound waves needs to be controlled by the same number of individual frequencies as N, and all of them satisfy the approximate bragging condition between the laser beam and the light beam diffracted by the sound waves of adjacent frequencies. It has a frequency interval that is divided into N parts.

The present invention was realized by focusing on the fact that the number of points to be controlled N is determined only by the incident beam width W and the effective beam width Wo by increasing the length of interaction between a sound wave and a light wave using repeated interference. The output beam width Wo is determined only by the incident angle φ _a that satisfies the approximate bragging condition of the incident beam and the effective number of repeated reflections: M, which will be explained below.

The number of points to be controlled N, the width of the incident light flux W, and the acousto-optic material 3a of the transmitted light flux t ₁ to the incident light flux i ₇ of the incident light flux
The intensity of the luminous flux transmitted inside the material 3a is reflected back and forth repeatedly within the material 3a, and the energy part is not lost as a transmitted luminous flux through the reflective film.e The transmitted luminous flux becomes ^1/2
The present inventor found the following relationship between the effective luminous flux width Wo up to _o : N=W/(4Wo).

This means that the effective luminous flux width W ₀ is determined by the angle of incidence of the incident luminous flux that satisfies the bragg condition and the effective number of repeated reflections M until it is sequentially attenuated by the reflectance γ and becomes ^1/2 . If the thickness of the plate 3a is known, the difference between the effective number of repeated reflections M and the (width) reflectance γ is M=γ/(1−γ ² ).
It was found that there is a relationship as follows, and that it is sufficient to determine the amplitude reflectance γ.

In other words, the number N to be controlled is
The plate thickness of a and the incident angle φ that satisfies the approximate Bragg condition
It is determined only by _a and the amplitude reflectance γ, which simplifies the configuration.

The operation of repeated multiple interference according to the present invention will be briefly explained below.

When no AC voltage is applied to the transducer, the light beam incident on the acousto-optic material plate 3a travels in the X-axis direction like a traveling wave while being repeatedly reflected within the material plate 3a, and the traveling wave A part of the light passes through the reflective film _3b2 and becomes the output light flux _t1 to _t0 . Due to the intensity of the emitted light flux at that time, in December 1962, LF
Johnson and D. Kahng in the Journal of Applied Physics.
It is explained in detail in the document entitled "Piezoelectric Optical-Maser Modulator" in Vol. 33, No. 12. As shown in Fig-2 in that document, the output luminous flux is 2
It has large peaks and valleys with a period of π, and as the reflectance increases, the ratio of peaks to valleys increases.

When an alternating current voltage is applied to the transducer, a sound wave is generated in the material plate 3a by the transducer, and the sound wave propagates within the material plate 3a and creates a sound field.

With a luminous flux width W, the coherent luminous fluxes i ₁ to i ₇ that have entered the material plate 3a repeatedly travel between the first dielectric multilayer reflective film 3b ₁ and the second dielectric multilayer reflective film 3b ₂ . It becomes a beam of light that undergoes reflection. Most of the luminous flux is caused by the Z component of the electric field
The light wave is expressed as a partially standing light wave between the reflective films, and therefore has a strong electric field. Moreover, a part of it becomes the transmitted light flux t ₁ to t _o . The partially standing light wave is a traveling light wave that slowly travels in the +X direction as if propagating through a waveguide between the first and second reflective films, and the effective light flux decreases by the time it is substantially attenuated. You can advance up to Width Wo. If the wave number (K _a ) of the sound field is substantially twice the wave number (K _x ) of the traveling wave (K _a =2K _x ), then
The traveling wave and the sound field interact efficiently, and the traveling wave is reflected by each sound field to generate a backward wave. However, the wave number K _x of the traveling wave is given by the relationship K _p sinφ ₀ =K _x where the wave number K _p of the light wave in space and the angle of incidence are φ ₀ . Thereafter, the backward waves interact with each of the sound fields, and the backward waves are diffracted and the diffracted light flux γ ₁
〜γ _o . As shown in Fig. 2 of the above-mentioned document, the overall intensity of the diffracted light beam has a period of 2π, and when the reflectance γ is increased, it has sharp peaks and troughs and changes in intensity greatly. Furthermore, if the reflectance γ is increased, the number of reflections increases and the effective luminous flux Wo also increases. Since the width of the region where the acoustic wave and the light wave interact is not W but Wo, the multiplication effect due to repeated interference appears as the multiplication ratio of the interaction length (Wo/W). As a result, the modulation voltage can be lowered, and the number of controllable points can also be increased by using the same modulation voltage.

The present invention will be described in detail below using examples.

FIG. 2 is a diagram schematically showing a first embodiment of the present invention in which a tellurium-containing glass plate (component name: tellurite glass) is used as an acousto-optic material plate.

In Figure 2a, 1 has a beam expander to obtain a diameter W of the incident beam of 6.3 mm, and the wavelength is
It is a He-Cd laser that emits light of 0.442μm. This output beam is tilted with respect to the normal (Z-axis) of the plane of the diffraction cell 3 , whose side view is shown in Figure 2b, at such an angle as to give a field-free angle of incidence φ _a0 of 1.346 milliradians. It enters the diffraction cell 3 . At this time, the direction of the electric deflection is within the plane of incidence (so as to couple with Ez in the film). The main body of the diffraction cell 3 has a density P of 5.06×10 ³ Kgm ^-3 acousto-optic coefficient P ₁₁ of 0.254, a refractive index n _a of 1971, and a velocity of acoustic longitudinal waves V _a of 3.33×10 ³ m
The tellurite glass plate 3a has a sec ^-1 and a thickness Z ₁ of 3 mm. The figure of merit M ₂ for the acousto-optic interaction specific to the arrangement described above is 20.9×10 ⁻¹⁵ sec ³ Kg ⁻¹ .
Dielectric multilayer reflective films 3b ₁ and 3b ₂ are applied to mutually parallel surfaces of the tellurite glass plate 3a, respectively.
Each reflective film is adjusted so that the amplitude reflectance γ is 0.96, that is, the effective number of repeated reflections M is 12. An X beam with a frequency of about 40 MHz is placed on another flat surface perpendicular to both sides of the flat surface (perpendicular to the X direction) that is parallel to the incident plane.
The width of the acoustic longitudinal wave propagating in the direction of the thickness Z ₁ is 2.5 mm.
A piezoelectric transducer 4d is formed which transmits across the entire region. 3 frequencies equally spaced 1.34MHz
a ₁ , a ₂ , a ₃ are respectively generated by oscillators 4a ₁ , 4a ₂ , 4a ₃ and controlled by gates 4b ₁ , 4b ₂ , 4b ₃ according to a digital image signal (in this case (011)), The signals are combined into one signal, amplified by a high frequency amplifier 4c, and applied to a piezoelectric transducer 4d. The components that are Bragg diffracted by each acoustic field corresponding to each of the three frequencies are focused on the lens 5a to form a light spot figure 011 equivalent to a digital image signal.
is resolved and obtained. The power required per each channel (per light spot) was 1.49 milliwatts.
This diffraction cell allows a half-power bandwidth of 21.45 MHz (although the one shown is a three-point channel), so a maximum of 16 light-point channels can be controlled. The power requirement of 24 milliwatts per channel of all 16 light points is 1/50th that of a conventional Bragg diffraction modulation of the same material and width but 10 mm long.

FIG. 3 is a diagram schematically representing a second embodiment of the present invention in which a sputtered thin film of tellurite glass is used as the acousto-optic material plate.

In Figure 3a, 1 is a He that emits a light beam with a wavelength of 0.633 μm and has a magnifying device to make the diameter W 24.7 mm.
-Ne laser. This output light flux is incident light such that the electric displacement is perpendicular to the incident light plane and the incident angle φ _a0 of no electric field with respect to the normal to the surface is 5.34 milliradian, and is incident on the surface of the LiTaO ₃ crystal 4e. The diffraction cell 3 , whose side view is shown in FIG. 3b, is stacked on a Y-cut LiTaO ₃ crystal 4e arranged so that the Y-Z plane of the crystal is parallel to the plane of incidence. A dielectric multilayer reflective film having an amplitude reflectance γ of 0.99 is deposited on the back surface of the crystal 4e. Then the thickness Z ₁ is
45.4μm, density P is 5.87Kgm ^-3 acousto-optic constant P ₂₁
0.241, refractive index n _a is 2.09, velocity of longitudinal sound wave is 3.40
A film of acousto-optic material of x10 ³ msec ^-1 is deposited. The figure of merit M ₂ characteristic of acousto-optical interactions, corresponding to the polarization of the light flux and the acoustic wave mode mentioned above, is 21
×10 ^-15 sec ³ Kg ^-1 . One end of the acousto-optic material film perpendicular to the incident plane is made to have a thickness that gradually changes. A dielectric multilayer reflective film 3b 2 having an amplitude reflectance γ of 0.99 is deposited again on the acousto-optic material film _3a . Reflective films 3b ₁ and 3b ₂ on both sides of the acousto-optic material layer 3a
constitutes a repeating interferometer in which the effective number of repeated reflections M is 50. A comb-shaped electrode 4f for transmitting an acoustic surface wave of about 120 MHz is formed on the back surface of the crystal 4e in the X-direction (in the Z-axis direction of the crystal).
The velocity of the acoustic surface wave, 3.49×10 ³ msec ^-1 , is extremely close to the velocity of the volume wave across the width of the acousto-optic material film 4e, and a considerable portion of the acoustic wave power is transferred to the quasi-plane wave in the acousto-optic material film. It is converted when passing through a portion 3c where the thickness gradually changes at one end of 3a.

Three frequencies a ₁ at regular intervals of 87.5KHz,
a ₂ and a ₃ are oscillated by oscillators 4a ₁ , 4a ₂ , and 4a ₃ respectively, and are controlled by gates 4b ₁ , 4b ₂ , and 4b ₃ according to a digital image signal (in this case (101)), and one electrical signal is generated. are synthesized, amplified by a high frequency amplifier 4c, and applied to a comb-shaped electrode 4f. lens 5a
focuses the Bragg-diffracted light beam from the diffraction cell 3 into a resolved light spot image conveying the image signal 101 on the temporary image plane 5d. The power required per one light point channel was 4.38 milliwatts. A band of 89.64MHz is allowed in this case, and a maximum of 256 light spot channels can be controlled. This required power is T=0.08m
Equivalent to W. This means that some of the acoustic input will be nullified and will flow through the substrate. 256
The power requirement for the optical spot channel is 1.12 watts, which is 1.88 times smaller than a conventional Bragg diffraction modulator of length L=10 mm.

Through a detailed description of preferred embodiments of the present invention, significant advantages of the present invention have been demonstrated, such as the power requirements are significantly reduced or the number of light spot channels that can be controlled simultaneously is increased. The product of the number of channels and the rate of increase is actually 500 to 800. A method for constructing a repetitively interferometrically multiplied Bragg diffraction modulator using thin film fabrication technology is presented.

This means that a wider range of acousto-optic materials can now be used than conventional glass manufacturing techniques allow. The reduction in film thickness due to the use of thin film manufacturing technology is related to the phase matching condition of repeated interference, especially when the effective number of repeated reflections M is large.
This resulted in extremely high thermal stability.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically representing multiple interference. FIG. 2 is a diagram schematically showing a first embodiment of the present invention in which a tellurium-containing glass plate (component name: tellurite glass) is used as an acousto-optic material plate. FIG. 3 is a diagram schematically representing a second embodiment of the present invention in which a sputtered thin film of tellurite glass is used as the acousto-optic material plate. Explanation of symbols, 1... Laser, 3 ... Diffraction cell,
3a... Acousto-optic material, 3b... Dielectric multilayer reflective film, 4a... Oscillator, 4b... Gate, 4c...
High frequency amplifier, 4d...piezoelectric transducer,
4c... Acousto-optic crystal, 4f... Comb-shaped electrode, 5a
……lens.

Claims (1)

[Claims] 1. A coherent light beam, an acousto-optic material plate having a pair of parallel planes defined by a second axis and a third axis perpendicular to the first axis, and attached to one of the planes. a first dielectric multilayer reflective film attached to the other plane, a second dielectric multilayer reflective film attached to the other plane, and the first and second dielectric multilayer reflective films attached to the acousto-optic material plate. means for making the coherent light beam incident at an angle inclined to the first axis of the plane to produce a partially standing light wave in the direction of the first axis between the films and a wave traveling in the direction of the second axis; , substantially 2 of the wave number of the traveling wave portion of the partially standing light wave in response to a set of modulation signals, each independently controllable in the direction of the second axis.
means for generating a sound field having twice the wave number; and means for forming an image of a diffracted coherent light beam generated by the action of the coherent incident light beam and the sound field in the acousto-optic material plate to produce a light point image. A multipoint acousto-optic modulator consisting of 2. The multi-point acousto-optic modulator according to claim 1, wherein the acousto-optic material is a piezoelectric material plate. 3. The means for generating a sound field includes a transducer attached to a surface perpendicular to the first axis of the acousto-optic material plate, a plurality of high-frequency oscillators, and intermittent operation of the oscillator according to a set of modulation signals. A multi-point acousto-optic modulator according to claim 1 or 2, comprising a gate circuit. 4 A coherent light beam, an acoustic wave substrate, a first dielectric multilayer reflective film formed on the acoustic wave substrate, and an acousto-optic material film formed on the first dielectric multilayer reflective film a second dielectric multilayer reflective film formed on the acousto-optic material film; a means for generating a surface wave as a sound field on an acoustic wave substrate; and a means for generating a surface wave as a sound field on an acoustic wave substrate; a means for matching as a quasi-plane wave; and a means for generating a partially standing light wave in the first axis direction between the first and second dielectric multilayer reflective films and a wave traveling in the second axis direction. means for making the coherent light beam incident at an angle inclined to a first axis, and forming a light point image by imaging the diffracted coherent light beam generated by the action of the coherent incident light beam and the sound field into the acousto-optic material plate. a multi-point acousto-optical modulator comprising means for producing. 5. The multi-point acousto-optic modulator according to claim 4, wherein the acoustic wave substrate is a piezoelectric material plate. 6. The means for generating a surface wave as a sound field includes a pair of comb-shaped electrodes formed on the acoustic wave substrate, a plurality of high-frequency oscillators, and a gate circuit that turns on and off the oscillators according to a set of modulation signals. A multi-point acousto-optical modulator according to claim 4 or 5, comprising: