When
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Title
Application Space and System Design for the Extended Shortwave Infrared Band
Abstract
The extended shortwave infrared (eSWIR) band, spanning the wavelengths 2 - 2.5um, is defined by one of the clearest atmospheric transmission windows anywhere in the infrared (IR) spectrum, but lies in a signal valley between the peak of the solar spectrum in the Visible (Vis, 0.4 - 0.7um) band and that of the typical thermal spectrum in the longwave infrared (LWIR, 8 - 12um). Though the technology to detect and image in the eSWIR has become commercially available in the last decade, the application space for eSWIR remains an open question, driven by the paradoxical value proposition of sensing in the band: the eSWIR gains benefit over the competing IR bands at long range, but there are few targets with sufficient eSWIR signal to be imaged at long range in the first place. This work explores two regions of the application space for the eSWIR: laser-illuminated active imaging and long-range jet seeking. Both performance and system design requirements for each application are investigated. In the active imaging application space, the importance of a number of focal plane array (FPA) parameters in eSWIR active system design is then investigated and compared to their importance in eSWIR passive system design. FPA noise sources are found to be especially significant to active systems relative to their passive counterparts, with dark current and read noise found to be the likely limiting noise sources for CW and LRG active systems, respectively. The range performance of continuous wave (CW) and laser range-gated (LRG) eSWIR active systems in degraded visual environments (DVEs) are then compared to that of analogous systems in the shortwave infrared (SWIR, 1.0 - 1.7um) band. Theoretical modeling indicates an eSWIR system of equivalent power to have a small advantage in dust, fog, and rain, and a substantial one in smoke due primarily to differences in particle size distributions. A preliminary field test in white smoke supports the models qualitatively. In the emissive imaging space, a theoretical rule of thumb is derived for the minimum target temperature required to achieve a threshold contrast-to-noise ratio (CNR) against a terrestrial background as a function of an eSWIR imaging system’s f/#, pixel pitch, integration time, and throughput. Experimental data indicate the rule to be a robust predictor, and analysis of the rule provides limits on the tradespaces for eSWIR system design, including requirements for focal plane array (FPA) cooling, f/#, pixel pitch, and integration time. The performance of eSWIR in long-range jet-seeking applications bands is then investigated and compared to the midwave infrared (MWIR, 3.0 - 5.0um) band in multiple atmospheric and illumination conditions and for forward and rear aspects of the aircraft. The eSWIR is found to have equivalent or superior detection range on all aspects of jet aircraft for all atmospheric and illumination conditions. A field test provides support for this conclusion.
Please email Jini at jini@optics.arizona.edu or Angus at angushendrick@arizona.edu for a Zoom link.