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A new type of noncollinear acousto-optic filter with large angular aperture is described. By an appropriate choice of acoustic wave propagation direction, the momentum mismatch caused by the angular deviation of the incident light from the matching condition in a noncollinear interaction can be compensated by the angular change of optical birefringence. A TeO2 noncollinear acousto-optic filter was constructed that had a half-power bandwidth of 40 Å at an f/4 (±7°) aperture. Tuning from 700 to 450 nm is obtained by changing the acoustic frequency from about 100 to 180 MHz. About 95% maximum transmission is obtained with an electrical power density of 1 W/cm2 applied to the transducer.

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The theory and practice of tunable acousto -optic filters are reviewed. Topics discussed include the basic principle of operation, filter characteristics and configurations. Experimental results of a noncollinear Te02 filter are also presented. IntroductionThis paper is intended to review the theory and practice of Tunable Acousto -Optic Filters (TAOF). The TAOF is an electronically tuned optical filter that operates on the principle of acousto -optic diffraction in an optically anisotropic medium. The spectral bandpass of the filter can be rapidly tuned over large optical regions by changing the frequency of the applied RF signal. The most significant feature of the TAOF is its high resolving power with a large angular aperture, which leads to various interesting applications.The concept of using acousto -optic diffraction for electronically tuned spectrometer applications is not new. It is well recognized that the principle of acousto -optic diffraction is similar to that of the transmission grating. The grating constant of the "acousto-optic" grating is equal to the acoustic wavelength.

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