Off-axis laser-radiation detection based on intensity interferometry: evaluation of signal and noise

We present a method for detecting indirect, off-axis light radiated by a laser beam propagating in the atmosphere, in the presence of (primarily solar) background. Unlike in most existent methods, where laser light detection is based on its monochromaticity or intensity, the proposed approach uses a high degree of temporal coherence of laser radiation as the discriminating factor against potentially strong but very low-coherence background. The method also relies on intensity interferometry, rather than amplitude interferometry approaches frequently found in the literature. The analytic developments of this paper revolve around the evaluation of two quantities-the intensity correlation signal and its fluctuations (or noise)-envisaged as measures of the proposed coherence signature, designed to apply to both stationary and pulsed radiation. Reliable evaluation of the noise, due to strong statistical fluctuations of the high-temporal-coherence scattered field, is essential, as well as challenging, because of the presence of higher, up to the fourth order, moments of the measured optical intensities. We calculated full effects of statistical fluctuations of the laser- and background-related radiation and established the optimal detector parameters maximizing the obtained signal-to-noise value. We show that the signal-to-noise ratio may be on the order of 10 for a single recorded pulse and, in practically relevant case of a train of pulses, increases as the square-root of the number of pulses in the sequence. While the proposed approach has direct applications in the development of laser warning systems, closely related techniques should be applicable in active imaging, particularly in LiDAR systems operating in the presence of strong background.