Suppression of cryocooler induced microphonics in infrared imagers

Abstract

Commercially available inexpensive and rugged miniature Stirling cryocoolers are frequently used in sophisticated infrared imagers. Such coolers are well known sources of high-frequency interference contaminating the image signal and causing essential degradation in the overall imager 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 for implementation 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 microphonic noise. The analytical model suggested relies on a set of complex frequency response functions experimentally measured on the original infrared package. The optimised dynamic absorber yields about a tenfold reduction in the amplification ratio at typical resonant frequencies and a threefold reduction in the overall rms level of microphonic noise. The results of the analytical calculation are in fair agreement with experimental data.