Sidelobe suppression in an acousto-optic filter with a raised-cosine interaction strength

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Abstract
We have demonstrated a 10-dB reduction in the intensity sidelobes of an apodized-interaction-strength guided-wave acousto-optic filter with a center passband of 1530 nm. Acoustic-intensity weighting was achieved by launching a surface acoustic wave (SAW) beam in a straight acoustic waveguide, and gradually transferring this SAW intensity to the active device, and back out, by evanescent-wave coupling across a 20 μm barrier over a 19-mm interaction length. The intensity sidelobe was −5 dB for an unapodized filter with abrupt onset and cutoff of the interaction, but sidelobes were reduced to at most −15 dB for a SAW intensity with raised-cosine weighting. A conversion efficiency of 97% was achieved with 80 mW input power for the 2.0-nm-wide apodized filter.