A Method for Improving Acoustic Switching Ratio in Opto-Acoustic Transducers
Previous theoretical and experimental work on Opto-Acoustic Transducers (OATs) has shown their switching ratios to be lower than that necessary for most imaging applications. We will show that this problem can be overcome by applying intensity-modulated light to the OAT instead of constant intensity light, as suggested previously.
The conductivity of the photoconductive layer of the OAT follows the same time variations as the intensity of the light applied to it. Under these conditions the OAT may be considered as a linear system with a time-varying component. It will be shown, using an equivalent circuit, that sinusoidally intensity-modulated light applied to the OAT gives rise to sideband signals in the illuminated region which do not exist in the dark region. Therefore, in principle, infinite switching ratios may be realized by allowing one of these sidebands to occur at the resonant frequency of the OAT.
There is an additional advantage to using intensity-modulated light. When a Fresnel zone pattern is projected onto the OAT the acoustic signals from the illuminated and dark regions, being at different frequencies, focus at different depths thus improving the signal to noise ratio at the desired focal point.
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