Integrated approach to airborne laser communication

Abstract

Lasers offer tremendous advantages over RF communication systems in terms of bandwidth and security due to their ultra-high frequency and narrow spatial beamwidth. Unfortunately, atmospheric turbulence significantly increases the received power variation and bit error rate (BER) in free-space optical communication (FSOC) systems. Further, airborne optical communication systems require special considerations in size, complexity, power, and weight.If two or more laser beams are sufficiently separated so that their turbulence effects are uncorrelated (i.e. anisoplanatic), they can effectively "average out" turbulence effects. This requisite separation distance is derived for multiple geometries, turbulence conditions, and optical properties. In most cases and geometries, the angles ordered from largest to smallest are: phase uncorrelated angle (equivalent to the tilt uncorrelated angle and phase anisoplanatic angle), tilt isoplanatic angle, phase isoplanatic angle, scintillation uncorrelated angle (or scintillation anisoplanatic angle), and scintillation isoplanatic angle (θ ψ ind > θ T A > θ 0 > θ χ ind > θ χ 0 ). Conventional adaptive optics (AO) systems only correct for phase and cannot correct for strong scintillation, while multiple-transmitter systems use several transmission paths to "average out" effects of the strong scintillation by incoherently summing up the beams in the receiver.Since all three airborne geometries (air-to-air, air-to-ground, and ground-toair) are studied, a comparison of multiple-beam airborne laser communication system performance is presented for the first time. Wave optics simulations show that a combination of transmitter diversity, receiver and transmitter trackers, and adaptive thresholding can significantly reduce BER in an air-to-air FSOC system by over 10,000 times. As demonstrated in this work, two transmitters alone separated by only 31 cm (100 km path length, 1.55 µm wavelength, 4 km in altitude) provide a significant iv BER improvement over one transmitter, especially for the strong turbulence regime where the required SNR for a fixed BER is reduced by 9 dB. Including the tracking and adaptive thresholding techniques, resulted in a 13 dB overall improvement.Two beams also reduce the fade length, suggesting even greater improvement can be obtained when interleaving and forward error correction coding is implemented. Major Schmidt also knew when to hand me water and how to keep me on the road, since he had just finished his marathon.

Airborne laser sensors and integrated systems

“…Efforts were also made to develop two laser communication transceivers to be externally mounted on two T-39 test aircraft and used to demonstrate communication between the aircraft at data rates of 1 Gbps at ranges between 50 and 500 km and altitudes up to the 40,000 ft. Since airborne laser-communication systems are subjected to specific size, complexity, power, and weight requirements, the reduction of the received signal variability by implementing optimized multiple-transmitter systems to average out the effects of turbulence by including anisoplanatic conditions for the phase and amplitude effects have been developed [139]. The enhancement of ground-to-space laser communication systems by counteracting the effects of atmospheric turbulence have been made possible, thanks to the advances in adaptive optics [140,141].…”

“…The methodology to be used in laser safety assessments is prescribed by various national and international laser safety standards [126][127][128][129][130][131][132][133][134][135][136][137][138][139][140]. They provide generic suggestions on how to apply the various proposed safety area calculation routines in the case of highly dynamic platforms, such as airborne designation systems.…”