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During the past several decades, several papers have recognized the fundamental and technological limitations to uncooled photodetector performance. Various types of uncooled photodetectors have been developed and their performance has steadily improved. Photoconductors and PEM detectors were the first uncooled devices used for fast detection of long-wavelength radiation without cooling. The present generation of uncooled LWIR devices consists of photovoltaic devices based on Hg1−xCdxTe multilayer heterostructures. The problems of poor quantum efficiency and large series resistance have been solved through the adoption of sophisticated heterostructure 3D architectures in combination with reduced physical volume of an active element by using integrated optical concentrators and optical resonance. Uncooled photodetectors are commercially available and manufactured in significant quantities, mostly as single-element devices. They have found important applications in IR systems that require fast response. Initially, they were mostly CO2 laser receivers. Examples of present applications are IR spectrophotometers (dispersive, Fourier, and laser), sensitive gas analyzers, nondestructive materials testing, plasma physics, laser metrology and technology, alerters, wide-bandwidth optical communication systems, laser rangefinders, anti-collision systems, smart munition sensors, lidars, and many others. The new applications include various quantum cascade laser-based systems, especially gas analyzers. Progress in near room temperature FPAs is relatively slow. One possible reason is the more difficult implementation of optical concentrators and low resistance input readout circuits. Recently, Peltier-cooled MWIR focal plane arrays with respectable NETD were demonstrated.
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