Sensitive uncooled thermal imager based on liquid crystal Fabry-Perot interferometer

摘要

Microbolometers are the dominant technology for uncooled thermal imaging and recently devices based on a direct birefringence measurement of a 1 μm-thick liquid crystal (LC) transducer pixel have been shown to have comparable sensitivity to current microbolometers. A modified approach for increasing device sensitivity to the temperature-dependent indices of refeaction is use of an LC resonant cavity in an etalon structure. The measured quantity is the transmission of a visible wavelength through the etalon which requires no thermal contact with the IR absorbing cavity. In this paper a detailed device design is proposed for a LC resonant cavity between dielectric mirrors. The dielectric mirror materials beneath the cavity were chosen to be compatible with existing VLSI processing. The mirror materials above the cavity were chosen to have high transmittance for the 8-14 um LWIR band and the visible probe wavelength. The performance of this design was evaluated numerically and is shown to yield 31% change in transmitted intensity over the 200 mK temperature range considered when pixel thickness is 470 nm. For comparison, a 1 μm-thick LC pixel based on direct birefringence measurement is expected to yield a 1.6% transmission change over the same range. The etalon device represents a 19x increase in sensitivity with thinner pixels - this leads to lower pixel thermal mass and faster thermal response times.