A DOUBLE-ELEMENT BROAD-BAND LIQUID-CRYSTAL TUNABLE FILTER - FACTORS AFFECTING CONTRAST RATIO

A double-element broad-band liquid crystal tunable filter consisting of a high-order quartz wave plate in series with a very-low-order electrooptically tunable liquid crystal phase plate and placed between crossed polarizers is described. A possible application for such a device is protection against intrusive laser radiation. The advantage of this double-element filter over a single-element low-order LC filter is that it increases the wavelength resolution of the device, which, when operated in the protect mode, will allow continued sensing of the scene. Experimental and theoretical studies were made of the contrast ratios obtainable with such a filter. It is shown that contrary to what was found for a single-element device, the contrast ratio was strongly wavelength dependent and was reduced by as much as a factor of 4 at some wavelengths. A theory based on polarization disruption of a percentage of the beam due to multiple reflections is proposed to explain the effect.