Matrix-addressable infrared filters for the protection of highly sensitive detectors

Highly sensitive infrared detectors have to be protected against the impact of intense infrared radiation, for example from lasers or sun light. Conventional laser safety filters work only in a limited wavelength range, electro-optical shutters reduce the transmission significantly and micro-opto mechanical systems (MOEMS) show problems with picture quality due to difficulties in aligning precisely all mirrors. As in many applications the IR illumination originates from a local spot in the image field, filters consisting of many segments that can be switched individually from "transparent" to "opaque" are therefore desirable. Individual switching requires a sophisticated combination of infrared materials and an optimized geometrical design, to achieve fast switching in combination with negligible crosstalk between neighbouring segments. We have realized fast switchable matrix-addressable infrared filters in the optical range between 2 and 12 μm based on complex composite devices. Vanadium dioxide (VO₂) was used as a thermochromic layer, diamond as a heat-conducting layer, and chalcogenide glass as a heat insulating submount. Finite element calculations accompanied by an extensive series of experiments showed, that switching times less than lms can be achieved if all materials are properly designed, prepared and matched together. A first prototype with a 10×10 pixel array has been realized at Fraunhofer IAF and successfully tested. The measurements confirmed the numerical simulations and proved the great potential of these devices.