Acousto-optic tunable filter using double interaction for sidelobe reduction.

We present a bifrequency acousto-optic tunable filter (AOTF) for applications using polychromatic laser beams. The acousto-optic device is based on two successive anisotropic interactions in paratellurite: the first takes place with an acoustic shear wave tilted at 10 degrees from the [110] axis, while the latter is provided by the acoustic shear wave propagating collinear to the [110] axis. It is then demonstrated that the sidelobes of the impulse response to optical wavelengths of such a bifrequency AOTF are greatly reduced. General expressions of operating frequencies, spectral bandwidths, and acoustic powers are derived. Numerical computations have been drawn for paratellurite. The bifrequency AOTF has been tested using a multiline beam radiated by an argon laser. According to the spectral power distribution of this polychromatic laser beam, the most critical configurations of wavelength filtration have been considered. Experimental results confirm the theoretical predictions.