Abstract

Acoustooptic (AO) tunable filters operating in the dispersive region of the birefringent crystal exhibit improved acoustic drive power efficiency because the refractive indices and the relevant photoelastic coefficients, hence the AO figure of merit M₂, increase as the wavelength approaches the bandedge. Also, the filter bandwidth is decreased because the dispersive constant increases in that spectral region. These advantages must be traded against increased optical absorption near the bandedge. A preliminary version of such a filter was reported previously; its interaction length was short, the bandwidth was wide, and the filter was not oriented for maximum angular aperture.¹ The present filter is oriented with θ_i = 54.25°; acoustic shear waves propagate at 109.45° to the CdS c-axis. The measured filter bandwidth at 5320 Å was 1.2 Å FWHM. The angular aperture at 5320 Å was ±20° FWHM. Tuning curves and bandwidth are given for the 5260–5400 Å region. The bandedge of CdS shifts with temperature; measured filter characteristics vs temperature are presented.

© 1991 Optical Society of America