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7 May 2012 Free-space optical communication at 1.55 μm and 4.85 μm and optical correlation through the evaporation layer
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The short-wave infrared (SWIR) and mid-wave infrared (MWIR) bands contain atmospheric transmission windows spanning approximately 1.50-1.75 μm and 4.6-4.9 μm, making lasers emitting in these ranges suited for high bandwidth covert free-space optical (FSO) communication. In addition to 1.55 μm lasers, a quantum cascade laser exhibiting room temperature emission at ~4.85 μm has been developed for FSO. Transmission coefficients and losses are simulated using MODTRAN for optical path lengths of up to 2 km to for various atmospheric conditions. By better understanding the effects of turbulence and associated refractive index structure parameter on FSO transmission, measures can be implemented to reduce the bit error rate and increase data throughput, enabling more efficient and accurate communication links. FSO beam optimization is provided through adaptive optics using a deformable mirror and Shack- Hartmann wavefront sensor, whereby wavefront distortion of a transmitted beam is measured and the wavefront is modulated in real time to compensate for the effects of turbulence to provide optimized FSO reception through the evaporation layer. Additionally, a dedicated automatic target recognition and tracking optical correlator system using advanced processing technology has been developed. Rapidly cycling data-cubes with size, shape, and orientation are employed with software algorithms for this system to isolate correlation peaks and enable tracking of targets in maritime environments with future track prediction. Using advanced techniques and compensation, limitations associated with infrared FSO transmission and reception through the evaporation layer may be overcome or circumvented to provide high bandwidth communication through turbulence or adverse weather conditions.
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John Zeller and Tariq Manzur "Free-space optical communication at 1.55 μm and 4.85 μm and optical correlation through the evaporation layer", Proc. SPIE 8388, Unattended Ground, Sea, and Air Sensor Technologies and Applications XIV, 838805 (7 May 2012);

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