Acoustic laser deflector quadruples resolution

Acoustic laser deflector quadruples resolution RCA scientists have developed a laserbeam deflector that employs acoustic beam steering within a crystal to provide four times the maximum dynamic bandwidth obtainable with conventional acousto-optic deflectors. Key to the development is a simplified easy to fabricate transducer that both generates and automatically flits the column of sound waves to efficiently deflect a laser beam over a broad angular range. Because the transducer can be made with economical evaporation and masking techniques commonly employed in fabricating electronic devices, the deflector (Fig.2) has potential application in high-speed capacity optical computer memories and in laser scanning for television picture projection. In acousto-optic deflectors a laser beam is diffracted as a result of the periodic disturbance created by sound waves propagating through certain crystals or liquids. The frequency of the acoustic waves determines the amount or angle of deflection. Thus, by electronically altering the input to a transducer that converts rf waves to sound waves, a laser beam can be directed to a number of different spots very quickly. But deflectors with a fLxed column of sound waves have a limited deflection range because acoustic waves diffract light efficiently over a narrow angular range only (the Bragg angle). Changing the angle at which the laser beam strikes the acoustic waves by tilting the column of sound waves has the effect of enlarging the angular deflection range. Scientists have known that a sound column could be tilted by changing the frequency of sound waves passing through an acoustic grating, but up to now have been unable to develop a practical inexpensive mechanism for doing this. Earlier techniques involved cutting steps in the acousto-optic material and bonding several transducers to it, a relatively complicated procedure requiring a great deal of handling that often resulted in damage to the acousto-optic material. RCA's solution uses a single transducer platelet bonded to a lead molybdate crystal. The interdigitated

ULTRASONICS. MAY 1973

Fig.2

An early experimental model of the laser-beam deflector

electrodes within the transducer are deposited in layers through masks by an evaporation technique. The electrode arrangement acts like an acoustic grating so that for any given rf input frequency the column of sound waves is tilted at a particular angle. Any change in the rf input frequency produces changes in the tilting angle of the sound column as well as in the frequency of the sound waves.

This automatic tilting of the sound column in combination with the frequency change in the acoustic waves allows the deflector to move the laser beam over a much wider bandwidth than is possible with conventional deflectors. Laboratory results show bandwidths of 210 MHz compared with conventional deflector bandwidths of 54 MHz.

David Sarnoff Research Centre, PHnceton, New Jersey 08540, USA

Testing ceramic composites An ultrasonic testing method has been developed which can detect interlaminar disbonds in costly ceramic electronic components. Up till now inspection methods have not been successful in detecting such disbonds. As a result uncounted failures in electronic substrates, hightemperature fixtures, ablative structures and allied components have been recorded. Key to the success of the method is the fact that the density and elastic properties of the adhesive closely match those of the base materials. Thus, the bond, if sound, looks no different to the ultrasonic detection unit than the base materials themselves. To overcome the resolution problem an ultrasonic method was set up using a mirror. Rather than focusing on the bond and looking for the reflections, the unit focuses on a reflecting

surface placed behind the test piece and looks for absence of reflection (Fig.3). A regular mirror or piece of polished stainless steel is sufficient for the test. Thus, the beam must pass through the part twice without interference in order for the area to pass inspection. A void blocking either the transmitted wave or reflected wave results in no signal and marks the area as 'questionable'. An example of its use is the detection of intedaminar disbonds in 1 x 2 x 116 in (25 x 50 x 1.6 mm) ceramic substrate mounted on a printed circuit board. The substrate consists of two layers of ceramic joined with a special adhesive. Circuitry is deposited in gold on the ceramic and since the components cost more than ~;100 each, they are too expensive for random destructive testing. However, voids cannot be tolerated in the part because they would weaken the part and

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