Electrooptic tunable filters for infrared wavelengths

This paper describes the performance characteristics of an electronically tunable infrared filter based on the electrooptic effect. Like the acoustooptic tunable filter, this device operates by selectively coupling principal polarizations in a birefringent crystal at a Phase-matched wavelength by means of a spatially periodic refractive index perturbation. Instead of a traveling acoustic wave, however, the electrooptic tunable filter employs a temporally static electric field. The main advantages of this filter are its very low power consumption and its versatility of passband programming by virtue of separately addressable voltages under microprocessor control. Two experimental embodiments of the filter are described, one using longitudinal fields, collinear with the light beam, the other using transverse fields in a "thick waveguide" configuration. Experimental results of filter characteristics in the wavelength range
2-10 \mu
m are presented. We also present examples of passband synthesis that are achieved by apodizing the amplitude of the applied electric field perturbations. These include passband broadening and sidelobe suppression. The experimental transfer characteristics exhibit good agreement with the theoretical computer plots. Typical driver voltages are ∼ 500 V.