Suppression of cryocooler-induced microphonics in infrared imagers

Abstract

Sophisticated portable infrared imagers developed for surveillance, reconnaissance, targeting and navigation play a vital role in the modern tactics used for carrying out military and antiterrorist operations. Their superior performance relies on novel optronic technologies and maintaining the infrared focal plane arrays at cryogenic temperatures using close cycle mechanical Stirling cryogenic coolers. Such coolers are well known sources of high-frequency interference contaminating the image signal and causing essential degradation in the overall imagery performance. This phenomenon is generally referred to as microphonics, which mostly manifests itself at the resonant frequencies of sensitive mechanical components of infrared packages. The ruggedizing of these components involves stiffening and dampening which are, evidently, not the best solutions to implement inside, say, an evacuated envelope, where the issues of added heat load, thermal mass, ageing, vacuum contamination due to outgassing are of the primary concern. The paper examines the idea of using externally mounted wideband dynamic absorbers for suppressing the above resonant responses and associated microphonic noise. The suggested analytical model relies on a set of complex frequency response functions experimentally captured on the original infrared package. The optimised dynamic absorber yields about a tenfold suppression of the resonant responses and a threefold attenuation of the overall RMS level of microphonic noise. The results of the analytical prediction are in fair agreement with experimental data.