US 3759603 A
An acousto-optical light deflector employs a crystal as a sound medium which is energized by way of a piezo-electric trandsducer with ultrasonic waves to deflect a light beam incident approximately parallel with the sound wave fronts as a function of the ultrasonic frequency. The deflector also comprises a control apparatus for supplying the piezo-electric transducer with a controllable variable frequency and is charactrized by the provision of several piezo-electric transducers which are designed for consecutive frequency ranges and which are juxtaposed on the sound medium.
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Description (OCR text may contain errors)
United States Eschler ACOUSTO-OPTICAL LIGHT DEFLECTOR HAVING INCREASED BAND WIDTH AND SHORT ACCESS TIME Sept. 18, 1973 11/1971 Pennington et al..... 350/161 7/1956 Arenberg 350/161 Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGraw Attorney-Carlton Hill et al.
[ 5 7] ABSTRACT An acousto-optical light deflector employs a crystal as a sound medium which is energized by way of a piezoelectric trandsducer with ultrasonic waves to deflect a light beam incident approximately parallel with the sound wave fronts as a function of the ultrasonic frequency. The deflector also comprises a control apparatus for supplying the piezo-electric transducer with a controllable variable frequency and is charactrized by the provision of several piezo-electric transducers which are designed for consecutive frequency ranges and which are juxtaposed on the sound medium.
10 Claims, 5 Drawing Figures 3/1972 Anderson et a1. 350/161 PATEHTEB SEP 1 8 m3 sum 1 OF 2 Inlllllll l. l
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INVENTOR Han 5 f5 6/) /er BY ACOUSTO-OPTICAL LIGHT DEFLECTOR HAVING INCREASED BAND WIDTH AND SHORT ACCESS TIME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a light deflector having several transducers for consecutive frequency ranges arranged in juxtaposition, and further relates to an acousto-optical light deflector comprising a crystal which is utilized as a sound medium and which is energized by way of a piezo-electric converter with ultrasonic waves to deflect a light ray incident approximately parallel with the wave fronts as a function of the ultrasonic frequency, and also to control apparatus for supplying the piezo-electric transducer with a controllable variable frequency.
2. Description of the Prior Art The principle of acousto-optical light deflection has been known for a long period of time. It is based on the light diffraction by ultrasonic waves. When an ultrasonic wave is transmitted through a medium, for example, a crystal, whereby pressure fluctuations are produced in the crystal, a light ray incident in the direction of the wave front is diffracted in a manner similar to that accomplished by a diffraction grating.
The angle of diffraction therefore depends on the distance of the pressure maxima, that is, however, on the wave length and/or the frequency of the ultrasonic wave. If the direction of incidence of the light ray against the wave front is inclined by a small angle, a Bragg reflection of the light ray can be observed at the wave fronts. In order for a Bragg reflection to occur, however, the angle of incidence must suffice for the Bragg condition. This principle and its advantages, as well as various applications were described in 1966 by E. I. Gordon in the article A Review of Acousto- Optical Deflection and Modulation Devices in the publication Applied Optics, Vol. 5, No. 10, page 1629 et seq., October 1966.
In the article Television Display Using Acoustic Deflection and Modulation of Coherent Light by A. Korpel, R. Adler, P. Desmares and W. Watson and published in Applied Optics, Vol. 5, No. 10, page 1667 et seq., October 1966, the authors describe how a larger number of deflection directions can be obtained. As it is known, the Bragg reflection requires the acoustical wave fronts to be symmetrical with respect to the incident and diffracted light ray. If the Bragg angle is to be modified, the acoustic wave front must change its direction. This is accomplished by a special arrangement and electronic circuitry of the phased array, whose combined wave fronts change their direction when the frequency is modified. In that way, a change of the ultrasonic frequency from 19 to 35 MHz and a light ray deflection changing in proportion thereto is attainable.
In the periodical Japan J. Apl. Phys. 8," page 81 l, 1969, N. Uchida and H. Ivaski report on an additional structure regarding a two-dimensional acoustooptical light deflector wherein a sound frequency modification was achieved between 48 and 63 MHz through the utilization of a special design.
The frequency bandwidth and the so-called capacity speed product (CSP) connected therewith of acoustooptical light deflectors (the number of resolvable spots per switching time) are limited by the varying sonic radiation output of the piezoelectric transducers at dif- 2 ferent frequencies and the solid direction of incidence (Bragg condition). Therefore, the bandwidth of the known acousto-optical light deflectors has been limited to a maximum of about one octave.
Acousto-optical light deflectors are utilized where a rapid light deflection is important. In order to scan a major surface with a light ray, perhaps for profile measurement, the deflectability of the light ray should be provided up to large angles of deflection, which corresponds to an effective approachability of the deflection crystal having a large bandwidth.
SUMMARY OF THE INVENTION The primary object of the present invention is to provide an increase in the number of deflection angles and to increase for that purpose the bandwidth of an acousto-optical deflector to a range encompassing more than one octave.
According to the invention, the foregoing objective is achieved through the provision of several piezoelectric transducers which are designed for consecutive frequency ranges and juxtaposed on a sound medium, the transducers increasing the sound frequency bandwidth to values up to 300 MHZ and maintaining the deflection efficiency effectively constant over a larger range than heretofore known.
The crystal plates used as piezo-electric transducers are preferably arranged at an angle on the sound medium so that the deflection of the incident light ray can remain stable.
The piezo-electric transducers may also be advantageously arranged in a partial area of a phased array, that is, instead of being tilted with respect to each other, they may be arranged juxtaposed and parallel and electrically series connected to a variable oscillator.
It is advantageous to construct the transducers from such different crystal materials so that the amplitudes radiated by the transducers are aligned with each other.
Moreover, the transducers are preferably dimensioned in such a manner that the frequencies coincide, for which in case of two adjacent frequencies, the deflection efficiency drops to half the maximum value. The degree of deflection effect is then actually constant over a major frequency range.
A control apparatus may advantageously be provided with an oscillator of a fixed frequency and with an oscillator of variable frequency, as well as with a mixing device in which the fixed and the variable frequencies are superposed in order to obtain the control frequencies. Another possibility resides in the control apparatus being provided with a number of oscillators whose frequencies are invariable and connected by way of electronic switches and a bus bar arrangement with one or several transducers. In order to avoid interfering higher waves, the entire frequency range is advantageously divided into octaves by supplying the frequencies of different octaves to separate bus bars which are provided with low pass filters.
The transducers should be connected with the crystal utilized as a sonic medium, preferably by cold pressing, in vacuum, under the interposition of compounds having a low melting point, such as indium, thalium, etc.
BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention, its organization, construction and operation will best be understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. I is a pictorial and schematic representation of an acousto-optical light deflector having increased bandwidth;
FIG. 2 is a graphical illustration of the total deflection efficiency effect of the multiple transducer deflector;
FIG. 3 illustrates a phased array design of an acoustooptical deflector;
FIG. 4 is a block diagram illustration of an oscillator which is synchronizable throughout the entire frequency range; and
FIG. 5 is a block diagram representation for a digitalized control of the transducers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a deflector crystal 1 is illustrated as a sound medium. The deflector crystal 1 carries three acoustic transducers 2, 3 and 4 with different frequency ranges of the sound radiation, namely, fl to f2, f2 to f3, and f3 to f4. The three transducers 2, 3 and 4 may be connected in parallel and connected to a high frequency oscillator 5. Each transducer actually represents a band-pass filter and accommodates an electrical output only in its corresponding frequency band. The transducers deliver the absorbed energy to the deflection crystal in the form of ultrasonic waves, whereby compressions 6 are produced in the deflection crystal. The distance of the illustrated compression lines corresponds to the wave lengths of the ultrasonic waves. If a laser beam enters the deflection crystal 1 from a fixed predetermined direction, it is deflected at the wave fronts 6 according to the Bragg condition. The incident laser beam 7 is deflected, depending on the ultrasonic frequency present, into the direction 8, 9 or 10. The number of possible directions of deflection of the laser beam is a function of the number of acousto-optical transducers and their usable frequency bandwidths.
It may also be advantageous to traverse the light beam as rapidly as possible over different directions of deflection. The scale for the efficiency of the light deflector is the so-called capacity speed product CSP. It is only a function of the bandwidth of the deflector and the expression Af/2 applied. With the structure described, bandwidths between lOO and 300 MHz and capacity speed products of about 2 X 10' seconds are possible.
FIG. 2 is a graphical illustration of the degree of deflection effect 1; with respect to the frequency f. The degree of effect is understood to mean the relation of deflected to incidence luminous intensity. FIG. 2 illustrates that each of the three transducers 2, 3 and 4 has a respective degree of effect of 1) l, n 2 and/or 7 3, which has a maximum value at the corresponding central frequencies fl, f2 and f3. The degrees of effect drop off on both sides of these central frequencies. The central frequencies fl, f2 and [3 are spaced such that the degrees of effect bisect where they have dropped by three decibels. From these three curves, a total degree of effect of n, 1; l 17 2 1 3, as represented in the drawing. The degree of effect of a transducer is a function of the data of the sound medium, the light wavelength of the incident beam, the dimensions of the transducer and the sound or sonic performance. At suf- 4 ficiently high sonic performance, percent of the radiated light can be deflected, because no performance is lost under the alternating effect of the light waves with the sonic field.
The manner of operation of a phased array design is illustrated in FIG. 3. Here again, the crystal 1 is employed as a sonic medium and carries three transducers 2, 3 and 4 which are juxtaposed parallel with each other. The transducers are electrically seriesconnected with a variable oscillator 5, so that at a time when the transducer 3 causes at a certain distance a compression 36 in the crystal, compressions 37 and 38 shifted by A /2 are generated by the transducers 2 and 4. The compressions 36, 37 and 38 can be consolidated into a single compression line 39 extending obliquely in the crystal. In this way compressions extending obliquely and shifted by A are produced in the crystal, whose oblique position depends on the oscillator frequency.
FIG. 4 illustrates how a variable control of the transducers is made possible with a fixed and a variable oscillator. In FIG. 4, an oscillator 10 has a fixed frequency fa and a variable oscillator l 1 provides frequencies of fb and fa, whereby the frequency fb is greater than the frequency fa. Both frequencies are superposed in a mixer 12 and supplied to the transducers by way of a low pass filter 13 for frequencies which are less than fc-fa and by way of a broad band amplifier 14. The low pass filter 13 is utilized to eliminate the upper frequency waves from influencing the deflection of the luminous beam.
FIG. 5 illustrates a block circuit diagram for a digitalized approach for energizing the transducers. Here, the oscillators 15, 16 and 17 have separate fixed frequenciesfl5,fl6 and fl7, respectively, and are consolidated by way of a bus bar 22 as a first group of oscillators, and the oscillators 18, 19, 20 and 21 have fixed frequencies fl8,f19, j20 and 121, respectively, which are consolidated by means of a bus bar 23 as a second group of oscillators. One frequency octave is contained in each group. A desired frequency can be switched to a bus bar from one of the switching inputs 24 or 25 by way of an appropriate switch gate 26. Therefore, higher waves are created which are prevented from traversing the low pass filters 27 and 28. The selected frequency fl5,fl6 or f21 passes to the transducers by way of a wide band amplifier 29.
The foregoing has described acousto-optical light deflectors and means for operating such deflectors with increased bandwidth. Although the foregoing description has been made by reference to certain illustrative embodiments, many changes and modifications thereof may becomeapparent to those skilled in the art without departing from the spirit and scope of my invention. Therefore, it will be appreciated that I'intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.
1. An acousto-optical light deflector comprising a crystal employed as a sonic medium, a plurality of piezoelectric transducers carried on said crystal, said piezo-electric transducers comprising crystal plates carried on said crystal sound medium and angularly disposed with respect to one another, each of said transducers having an individual frequency range adjacent to the frequency ranges of the other said transducers and energizable with ultrasonic energy to produce sonic wave fronts in said crystal as a function of the energizing frequency for deflecting a light beam incident approximately parallel with said sonic wave fronts, and control apparatus for supplying said piezo-electric transducers with a controllable variable frequency.
2. An acousto-optical light deflector according to claim 1, wherein said piezo-electric transducers are carried by said crystal juxtaposed and parallel and are electrically connected to each other and to said control apparatus.
3. An acousto-optical light deflector according to claim 1, wherein said piezo-electric transducers are constructed from different crystal materials so that the sonic amplitudes radiated by the transducers are aligned with each other.
4. An acousto-optical light deflector according to claim 1, wherein said transducers are constructed such that the frequencies of adjacent transducers coincide in partial frequency ranges below their half power points.
5. An acousto-optical light deflector according to claim 1, wherein said control apparatus includes a fixed frequency oscillator, a variable frequency oscillator and a mixer connected between said oscillators and said transducers for superposing said fixed and variable frequencies to provide a control frequency.
6. An acousto-optical light deflector according to claim 1, wherein said control apparatus includes a plurality of fixed frequency oscillators each having a different frequency, a bus bar connected to said transducers, and a plurality of electronic switches connected between said oscillators and said bus bar for selectively connecting said oscillators to said transducers.
7. An acousto optical light deflector according to claim 6, wherein the total frequency range is divided into octaves and said bus bar is provided as a plurality of buses each associated with a separate octave, and a plurality of low pass filters each interposed between a separate bus and said transducers.
8. An acousto-optical light deflector according to claim 1, wherein said transducers are connected with said crystal by cold pressing in a vacuum and comprising a low melting point compound interposed between said transducers and said crystal.
9. An acousto-optical light deflector according to claim 8, wherein said low melting compound comprises indium.
10. An acousto-optical light deflector according to claim 8, wherein said low melting compound comprises thalium.