|
cousto-optic devices occupy a gray area between
purely optical signal processing devices such as fiber Bragg
gratings and interferometers, and purely electronic devices
such as the familiar integrated circuit. They are useful in
optical computing, optical communications, and as wide-bandwidth
spectrum analyzers for electronic signal processing.
Acousto-optic devices work because the velocity of a sound wave is much slower than the speed of light. For example, the speed of sound in water is 1500 meters per second, or 200,000 times slower than the speed of light. Slow shear acoustic waves in tellurium dioxide crystals are even slower, only 616 meters per second. Other useful materials include lithium niobate and mercurous chloride, which has the slowest propagation velocity of any commonly-used AO material (the velocity of the Hg2Cl2 S[110] shear mode wave is an incredibly slow 347 meters/second). Because of this useful property, the wavelengths of radio frequency signals at UHF frequencies, when converted to sound waves in a crystal, become short enough that the crystal can act as a Doppler-shifting diffraction grating. A laser beam pointed at the crystal will be partially diffracted at an angle roughly proportional to the wavelength of the RF signal, and the result is a real-time Fourier transform of the radio signal.
This book describes the design considerations for acousto-optic devices, including materials, transducers, geometry, principles of operation, and engineering tradeoffs. The presentation is sufficiently detailed and clear that readers could design and build their own AO device (if they have a clean room and can lay their hands on a large chunk of ultrapure lithium niobate). Chapters include principles of acousto-optics, AO modulators, AO deflectors, AO tunable filters, transducer design, and manufacturing and testing of AO devices. The book does not discuss competing technologies such as all-electronic analog-to-digital converters.
The inherent parallelism of AO devices gives them a powerful advantage over electronic devices. However, since this book was published, electronic ADCs, which are superior in terms of accuracy and resolution, have made amazing increases in speed, with boards capable of digitizing over 100 million samples/second becoming widely available. Flash ADCs are able to digitize as fast as 1 Gsps. Within a few years, such electronic devices may catch up to or even surpass acousto-optic devices in speed. Until then, an acousto-optic Bragg cell is still the fastest way of finding a short-duration radar pulse or the faint signal from E.T. in the vast ocean of radio waves.
