Independent acoustooptic modulation of the two wavelengths of a bichromatic light beam

When a single acoustooptic modulator is used to independently modulate the two nearly equal optical wavelengths of a bichromatic laser beam, each with a given acoustic frequency bandwidth, both the acoustic frequencies as well as the transducer width must be large. A general expression for this bandwidth, valid for both isotropic and anisotropic acoustooptic interactions and whatever the acoustic power and the bandwidth definition are, is derived. In uniaxial crystals, a special configuration of anisotropic interaction is shown to lead to a small acoustic frequency bandwidth with a reasonable transducer width at moderate central frequency. A practical 6-MHz bandpass modulator using the slow shear wave in paratellurite is designed, and experimental results for independent modulation of 0.5145- and 0.4880-μm wavelengths of an Ar laser are reported.