Anomalous sidelobes and birefringence apodization in acousto-optic tunable filters

Integrated-optic acousto-optic filters have large asymmetric sidelobes that have not been explained in previous publications. We show that this is caused by waveguide birefringence nonuniformity. For the devices described here, the dominant cause of the nonuniformity is a thermal gradient associated with the generation and absorption of the surface acoustic wave. When the thermal birefringence gradient is included in the coupled-mode model, asymmetric sidelobes in good agreement with observations are predicted. In addition, we show that waveguide inhomogeneities also lead to birefringence nonuniformity and can be used to tailor the birefringence to supress sidelobes in a two-section acousto-optic filter.