Focusing with an optoacoustic transducer

A prototype of an ultrasonic transducer has been developed that uses an optical input to control the ultrasonic output. This transducer is called an optoacoustic transducer (OAT) and provides an ultrasonic pattern that is spatially similar to the optical pattern used to illuminate it. When a focus-inducing optical pattern, such as a zone plate, is used, an acoustic focus is achieved. The success of this procedure depends on the use of amplitude-modulated light at the input and on filtering the received signal to eliminate the primary frequency. This provides an increase, from 30 dB to 70 dB, in the ratio of acoustic pressure in the illuminated regions to that in the dark regions. The prototype operates at 2.8 MHz and has been used to provide a good acoustic focus in water. A 3-dB beamwidth of 3.5 mm was measured at a range of 92 mm. The construction techniques and materials used are discussed.