Design of a double-pass shear mode acousto-optic modulator

The design of a compact double-pass shear mode acousto-optic modulator for high power operation in the UV is presented. Using a spherical mirror retroreflector in the second pass to correct for beam deflection, changes in beam position and angle during frequency tuning can be eliminated. The use of the shear mode acousto-optic interaction offers several key advantages, but the strain-induced birefringence creates significant levels of spectral leakage in the system. These effects and the polarization of the beams are analyzed to minimize leakage. Using a heterodyne interferometry scheme, two double-pass acousto-optic modulators with offset acoustic frequency inputs are set up to measure the leakage intensity. The designed double-pass shear mode acousto-optic modulator was tested with λ = 351.1 nm and demonstrated a peak double-pass efficiency of 42.6% and a bandwidth of 28 MHz . Over this bandwidth the spectral leakage was reduced to ∼ 0.01 % - 0.04 % relative intensity.