Design of a Cryogenic IR Detector with Integrated Optics

Design of a Cryogenic IR Detector with Integrated Optics -...

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Design of a Cryogenic IR Detector with Integrated Optics Michael Singer Semiconductor Devices, P O Box 2250, Haifa 31021, ISRAEL email : m_singer@scd.co.il Dov Oster Israeli Ministry of Defense ABSTRACT Cryogenically cooled IR detectors, which are used in applications such as situational awareness, search & track, missile launch and approach warning, typically use wide angle, single field of view optical systems. We describe a complete IR imaging optical assembly for such applications, which is mounted inside a cold shield and is maintained at a stabilized cryogenic temperature inside the dewar. A typical system houses two to four lenses and a cold filter, and weighs 5 grams or less. Despite this integration and added complexity, the resulting Detector-Dewar-Cooler Assembly (DDCA) has overall dimensions similar to those of equivalent-performing DDCAs without integrated optics. Moreover, Compact designs integrating wide-angle optics and a warm, high-magnification, telescope module for narrow FOV applications are seen as a straightforward extension of our system. We conclude with an in-depth, technical overview describing the design considerations for a typical wide-field imaging system. Keywords: IR detectors, IR imaging. 1. INTRODUCTION In the conventional approach to IR imaging, the imaging optics is located outside the detecting DDCA (Detector Dewar Cooler assembly). This is because the imaging optics typically has a relatively high thermal mass. A typical imaging optics arrangement includes a few imaging lenses and/or mirrors, as well as a focus correction system permitting the system to remain in focus at a range of ambient temperatures. Such imaging optics is associated with mechanical assemblies and electronics. With the conventional approach, an IR imaging system suffers from thermal noise reducing the system performance. On the one hand, uncooled imaging optics, located outside a DDCA, is highly sensitive to temperature changes, because the refractive index of the optics in the IR spectral range is highly dependent on its temperature. A change in the refractive index unavoidably introduces optical aberrations and focus changes, which require focal correction, and which in turn needs the use of focus control and focus adjustment mechanisms. On the other hand, imaging optics (e.g. lenses and mechanical components), as any other object, emit thermal energy in the IR, which presents a noise component in the detected IR signal thus reducing the signal to noise ratio of such systems. Some thermal noise effects associated with temperature changes in the imaging optics might be compensated by utilizing the non-uniformity correction (NUC) procedure for calibration and correction of the readout signal collected from the FPA. However, during the use of the NUC procedure for calibration of the IR imaging system, the system is put in an inoperative state (during which the system is "blind"). It is, therefore, preferable to minimize the number of NUC procedures that are required during the operation of the system.
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Design of a Cryogenic IR Detector with Integrated Optics -...

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