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A Method to Estimate Internal Stray Lighting of Optical Trains of Optoelectronic Systems

Keywords:

A.V. Makarenko, A.V. Pravdivtsev, A.N. Udin


Modern optoelectronic systems for target recognition usually operate in MWIR (3–5 micron) and/or LWIR (8–14 micron) ranges. When photon or quantum detectors are used, they are cooled to cryogenic temperatures (typically 30–80 K). It allows reaching of BLIP mode (background-limited infrared photodetector), when the threshold flux equivalent to detector’s noise is limited by fluctuations of noise radiation, which also includes the stray lighting of the optoelectronic system’s optical train. A method which allows simulating internal stray lighting in optical trains of optoelectronic systems of 3–5 and 8–14 micron infrared ranges is presented. The method is based on a general mathematical model, tuned for specific layout and analyzed optical train parameters (there is a possibility of direct import from popular CAD systems). The model accounts emission and multiple reflections from lenses, cells and other structural parts of optical trains, deals with real materials and coatings. It is also possible to flexibly specify the detector’s configuration and parameters. Estimation of flux on the detector is performed by direct calculation of the real ray path from specified sources through specified propagation conditions, including optical system’s aberrations. The core of the developed model is the ZEMAX CAD program, which was chosen among specialized programs (ASAP, LightTools, TracePro etc.) due to its optimality by criterion of efficiency analysis and optical system synthesis. This allows to create integrated simulation-modeling systems for development of optoelectronic IR systems on the base of one CAD platform and, in turn, reduces terms and costs of development, refinement and use of modeling benches. The developed method of analysis of stray lighting, generated by optical trains of IR-systems, allows to get absolute and relative values of noise flux reaching the detector, estimate spatial distribution of the flux on detector’s surface and to analyze the contribution of optical train elements in overall noise flux. The system of strays flux parameters estimates, formed by the model, allows adequately and constructively solve not only direct, but also the inverse problem, i.e. to optimize characteristics and configuration of the lens’s cells and other construction elements of the optical train to reduce noise light level. Model’s correctness and adequacy check is performed in the article for both overall stray flux and its spatial distribution. It is shown that relative bias comparative to precision analytical calculation in all cases is less then 1.14%. Ways of improving the proposed model are check for model’s adequacy model with real experiments and increasing information performance. In the first place this concerns estimating of strays flux statistical parameters, account light scattering in lenses, lens’s cells and other construction elements of optical train. The second part is to provide the capability of multispectral optical system analysis and account for strong temperature gradients in optical trains.
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