DE2253913A1 - ACOUSTIC LIGHT DEFLECTOR - Google Patents

Description

RCA 65 117 November 3, 1972

U.S.Ser.No. 196.046 74 57-72 Dr.v.B / E

Filed: November 5, 1971

RCA Corporation

New York N.Y. (V.St.A.)

Acoustic light deflector

The present invention relates to an acoustic device for deflecting a light beam with an acousto-optical medium which is traversed by the light beam Beam path is arranged, and with a plurality of electromechanical transducers that are in contact with a surface of the medium.

A known acoustic light deflection device contains an acousto-optical medium, such as glass or lead molybdate, which is arranged in the beam path of a light beam generated by a laser. On one side of the acousto-optical Medium an electromechanical transducer is arranged in order to generate a mechanical tension wave, which is propagates through the medium and creates a kind of i? on diffraction grating in this. The light beam is through this diffraction grating diffracted, and the angle between the undiffracted beam and the diffracted or output beam depends on the period of the mechanical stress wave (acoustic wave), i.e. ultimately on the frequency of the electromechanical

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Converter supplied electrical signal.

The disadvantage of the known acoustic light deflection devices is their very limited deflection range. the The amount of light diffracted by the incoming light bundle is only maximal if the incoming light bundle is exactly at the Bragg angle for the direction of propagation of the acoustic oscillation forming the diffraction grating in the acoustic-optical medium. When the frequency of sound is increased in order to increase the deflection angle of the diffracted light, the proportion of the diffracted light increases Since the incoming light would have to be incident at a different angle at larger deflection angles, in order to meet the Bragg condition to suffice. The angular range into which a light beam can be diffracted (deflected) is therefore with the known ones Devices limited to a range of about 5 milliradians if reasonable diffraction efficiency can be assured target.

However, it is already known that the deflection range can be increased by changing the direction of propagation of the sound energy in the acousto-optical medium with the frequency of the electrical signal supplied to the transducer. A device in which the direction of propagation of the sound wave is changed so that it satisfies the Bragg condition for the various deflection directions is known from US Pat. No. 3,502,879. This deflection device contains several transducers which are arranged in a row and are in contact with a surface of the acoustic-optical medium. When the signal frequency changes, the direction of propagation of the acoustic oscillations can be changed by a predetermined amount by changing the phase relationships of the electrical signals fed to the various transducers.

The object of the present invention is to-3098 19/0872

basic, acoustic light deflection devices of the type specified above with regard to the achievable deflection range to improve.

This task is performed by an acoustic device of the aforementioned type solved / characterized is that the transducers have corresponding conductive thin-film electrodes, which are arranged in a first row and with a Surface of the medium are in "intimate" contact, furthermore a piezoelectric crystal plate, one side of which is connected to the thin-film electrodes, and further a second Series of electrodes connected to a side of the crystal plate opposite to the said side, and that the electrodes of the first row are staggered with respect to those of the second row and are arranged in an overlapping manner are. ·

Further developments and refinements of the invention are characterized in the subclaims and explained below.

The concept of the invention and its refinements and developments will now be explained using exemplary embodiments explained in more detail with reference to the drawing; show it:

Fig. 1 is a schematic representation of an acoustic Light deflection device with an enlarged deflection area compared to corresponding known devices, according to a Embodiment of the invention;

2 shows an enlarged sectional view of part of the transducer of the device according to FIGS

3 and 4 are diagrams to which reference is made to explain the mode of operation of the device according to FIG will be.

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The device shown in Fig. 1 as an embodiment of the invention contains an acousto-optical Medium 10, which may be made of hater, glass, quartz, or any other suitable photoelastic material that for the light to be deflected is permeable and an effective propagation of acoustic oscillations or voltage waves guaranteed. One end of the medium 10 is provided with an electromechanical transducer arrangement 12 for sound vibrations to generate, which pass through the medium 10 to an acoustic closure 13. The transducer assembly 12 is made a first row of discrete electrodes 14, 15, 16 and 17, a piezoelectric plate 20 and a second row discrete electrodes 22, 23 and 24. The end electrodes 14 and 17 are connected by leads 26 and 27 with a variable frequency electrical voltage source 28 connected.

The first row of electrodes 14, 15, 16 and 16 is relative to the second row of electrodes 22, 23 and 24 staggered and arranged to overlap. This results in a series arrangement of effectively discrete transducer elements, each consisting of overlapping parts of electrodes of the first and second row as well as the one in between Volume of the piezoelectric plate 20 made of a crystal, for example. E.g. the overlapping Parts of the electrodes 14 and 22 form a first transducer element 30. The overlapping parts of the electrodes 50 and 22 form a second transducer element 31. When the end electrodes 14 and 17 are connected to the voltage source 28, every second Transducer element 30 in one spatial phase and the transducer elements 31 in between in the opposite phase excited. At a certain instantaneous value of the alternating voltage, for example, the electric field in the transducer elements 30 be directed downward in the drawing, while then at the same time the electric field in the intermediate Converter elements 31 is directed upwards. The sound waves 38 therefore propagate through the medium 10 in one direction

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from whose angle depends on the frequency of that of the voltage source 28 generated voltage depends.

During the production of the multi-element transducer arrangement arranged at one end of the acousto-optical medium 10 12 one can proceed as follows: A large plate 20 is made from a piece of a piezoelectric Crystal, e.g. lithium niobate, with a smoothly lapped surface that fits on a smoothly lapped end face of the acousto-optical medium. The medium 10 and the wafer 20 become then with aligned masks defining the desired first row of electrodes 14-17 in a vacuum chamber arranged. The associated faces are both coated with gold and chrome to a thickness of a few thousand angstrom units metallized and then coated with an indium layer of approximately the same thickness. Now the medium becomes 10 and that out the piezoelectric crystal existing platelets 20 connected to each other by the surfaces coated with indium in a vacuum for a few minutes with a force of about 280

2
compresses up to 350 kp / cm. In this way, the first row of electrodes of the multi-element transducer arrangement 12 and a good sound-transmitting connection between the piezoelectric material and the acoustic medium are formed. The piezoelectric material attached to the medium can then be brought to a desired thickness according to the intended sound oscillation frequencies by grinding and lapping. For frequencies in the order of magnitude of 150 MHz, the thickness can be about 25 μm.

After the final thickness of the piezoelectric plate is reached, the second set of electrodes is made 22, 23 and 24 in a staggered arrangement with respect to the first row of electrodes, as shown, by means of metallization formed on the exposed surface. Wires are then attached to the end electrodes by soldering, silver paste or pressure connection attached, which form the line in 26 and 27.

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The dimensions of the multi-element transducer arrangement are dimensioned in such a way that synchronism zv / ischen the angle of the direction of propagation of the first order bundle consisting of the sound waves 38 in the medium 10 and the angle between the incoming light bundle and the sound wave bundle, which is necessary for the fulfillment of the Bragg Condition and effective deflection in a large range of light beam deflection / angles is required, is guaranteed. At the beginning, a Bragg reference frequency f is set at which the synchronization with the SiaLl wave bundle should begin. The set-up angle α = γ- f is determined by this Bragg reference frequency v / λ, where λ is the wavelength of the light and ν is the speed of sound in the acoustic-optical medium. The thickness of the transducer or crystal plate is chosen so that its resonance is at a frequency f. Which is greater than f. The dimension d (Fig. 2) of each pair of transducer elements is given by the formula

f -f

given, where λ = v / f is the sound wavelength in the acousto-optical Medium is at the reference frequency.

The deflection angle range U in radians is given by the formula for dispersion-free conditions

given, where w is the length of the row of transducer elements. The smaller the length w, the greater the

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Angular range and the smaller is the efficiency of light diffraction. It is advisable to choose \ i so that an efficiency of about 50% results. For a high-frequency control power in the order of magnitude of 0.5 to 1 watt in the frequency range between 60 and 500 MHz, w can be in the range from 6 to 12 mm.

In the operation of the device according to Fig. 1, an incoming light beam, for example from one not shown, is allowed Laser, fall along a beam path 34 through the acousto-optical medium 10, from which the light beam normally emerges along a "zero order" beam path 36. if the voltage source 68 sends a signal of a predetermined frequency via the lines 26 and 27 to the electrodes 14 and 17 of the transducer anordnuxj 12 supplies, the transducer arrangement generates acoustic signals Vibrations which propagate through the acousto-optical medium 10 to the end 13. Those passing through the medium 10 Sound vibrations 38 are shown in Fig. 1 by parallel lines, the wave fronts to less mechanical tension correspond to a predetermined point in time. The illustrated Schallschwingunqen have such a direction of propagation that the propagating wave fronts run parallel to a straight reference line 40.

In the illustration in FIG. 1, the beam path 34 of the incoming light beam forms the angle α with the reference straight line 40, with the. Angle α is the Bragg angle, at which results in a maximum of first order diffraction. The first-order diffracted output light bundle runs along a Stralileng ^ nges 42, which forms an angle β with the reference straight line forms. The angle 3 is equal to the Bragg angle ά. So long so the converter arrangement 20 through the voltage source 28 is not is excited, the incoming light beam 34 runs along the - ~ rahlenganges 36 zeroth order. However, if the handleranorar-m-r 20 with a voltage of a predetermined frequency of the

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Voltage source 28 is excited, this is along the beam path 34 incoming light bundles on the beam path 36 zeroth Order in the first-order beam path 42 and further higher-order output beam paths shown in the drawing are not shown, flexed. The tilting of the direction of sound propagation in the acoustic-optical medium IO is achieved by causes the cancellation and amplification of the sound waves emitted by the individual transducer elements 30, 31.

In the following, reference is now made to the diagram according to FIG. 3, in which the directions of the sound waves propagating in the acousto-optical medium 10 are indicated in polar coordinates. The amplitude and direction of the sound waves 38 in FIG. 1 are denoted in FIG. 3 by the same reference symbol. The first-order bundle consisting of the sound waves 38 forms an angle φ in FIG. 3. with the reference direction 44 of the zero order sound wave bundle. A second, unused first order sound wave bundle is shown at 46. The incoming light bundle running in the beam path 34 is captured by the first-order sound wave bundle consisting of the sound waves 38, a diffracted first-order output light bundle running along the beam path 42 being created. The incoming light bundle in the beam path 34 forms the angle α _χ with the reference straight line 40, which represents the direction of the wave fronts of the sound waves 38. The beam path 42 of the diffracted output light bundle forms the angle 3 with the reference straight line 40, where S _{1 is} equal to the Bragg angle α. is. The diagram according to FIG. 3 shows the mode of operation of the light deflection device according to FIG. 1 in the event that the voltage source 28 supplies a voltage of a predetermined frequency f., Which generates a deflection angle e ^.

The diagram according to FIG. 4 corresponds to that of FIG. 3 and shows the mode of operation of the light deflection device according to FIG Fig. 1, when the voltage source 28 has a voltage of higher fre-

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quenz f ₂ supplies in order to deflect the light bundle arriving along the beam _{path 34 by a larger angle e 2.} The frequency of the voltage supplied by the voltage source 28 can be varied within an octave, for example between 100 and 20 MHz. In FIG. 4, the beam path 34 of the incoming light bundle forms the same angle C with a base reference line as in FIG. 3. In FIG. 4, the higher frequency f _{2 of} the voltage from the voltage source 28 results in the first order sound wave bundle 38 ' Forms an angle Θ in the medium 10 with the reference straight line 44, which is smaller than the angle φ resulting at the frequency f, in FIG. 3. Because of the other direction of the sound wave bundle 38 ', the wave fronts parallel to a reference line 40' form an angle α with the beam path 34 of the incoming light bundle, which is greater than the angle a ₁ in FIG. 3. This larger angle a 2 in FIG to bend a larger deflection angle e ₂ than the deflection angle e _x in FIG. 3. The angles shown in FIGS. 3 and 4 are greatly exaggerated for the sake of explanation, but the range of deflection angles of the output _{light bundles between angles e 1} and e 2 is much larger in the present device than in a light deflection device without the multi-element transducer arrangement 12 .

The multi-element transducer arrangement 12 is constructed in such a way that the sound wave bundle of the first order which propagates through the medium 10, in each case follows directions which correspond to those directions which are necessary for the fulfillment of the Bragg condition and thus for an effective deflection of the light bundle in one Output light beam angle range between ej and e _{2 are required.} The synchronism or synchronism is ensured by the construction described, which causes the sound wave diffraction angle φ to change at the same speed as the Bragg angle α in the

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light diffraction with good efficiency results. The construction is very effective because the zero order bundle and the even higher order bundles are suppressed and about 82% of the energy of the incoming bundle on the bundles first order ". This means that the used First order beam contains about 41% of the incoming light energy. The construction described also has the Advantage that the intensity of the light beam of the first order remains constant when the deflection angle by changing the frequency the voltage source 28 is changed.

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Claims (3)

Claims che 1! Acoustic device for deflecting a light beam with an acousto-optical medium which is arranged in the beam path traversed by the light beam, and with several electromechanical transducers that are in contact with a surface of the medium, characterized in that, that the transducers (30, 31) corresponding conductive thin-film electrodes (14 to 17), which are in a first Are arranged in a row and are in intimate contact with a surface of the medium (10), a piezoelectric crystal plate (20), one side of which is connected to the thin-film electrodes, and furthermore a second row of electrodes (22 to 24), which are connected to a side of the crystal plate opposite to said side and that the electrodes (14 to 17) of the first row are staggered with respect to the electrodes (22 to 24) of the second row and they are arranged in an overlapping manner. 2. Device according to claim 1, characterized marked,. that the overlapping parts of the Electrodes (14 to 17; 22 to 24) of the first and second rows as well as the piezoelectric crystal lying between them effectively a plurality of individual transducer elements (30,31) form and that an arrangement (26, 27) is provided to the electrodes (14, 17) at each end with a frequency-variable voltage source; (28) connect so that one on the other transducer element (30) in a spatial phase and the intermediate transducer elements (31) in the opposite phase are excited. 3. Device according to claim 2, characterized marked that the transducer elements (30, 31) are dimensioned in such a way that the direction of propagation of a sound bundle first order in the medium (10) with the fre- 309 8 19/087 2 sequence of the voltage source (28) changes in such a way that it continuously coincides with the direction that is extended in a The range of light deflection angles ensures that the Bragg condition is met and that the light beam is deflected efficiently. 309819/0872 Blank page