A Coherence-Based Off-Axis Laser Beam Detection System

We describe a concept of a system for detecting laser radiation in the presence of other (background) radiation sources. Unlike most existent laser-warning methods, which detect laser beams based on their intensity and monochromaticity, the proposed approach uses a high degree of coherence as the distinguishing property of laser radiation. Further, in contrast to previously considered coherence detection systems, based on amplitude interferometry (AI), we propose to utilize intensity interferometry (II), pioneered by Hanbury Brown and Twiss and originally applied to thermal radiation sources, but here generalized to coexisting high- and low-coherence radiation. In this contribution we concentrate on a possible application of the proposed system in detection of off-axis laser beam radiation scattered on atmospheric medium particles (water droplets and aerosols, especially dust). A particular design, which may operate in the visible and infrared regions, utilizes a small array of photodetectors (such as PIN photodiodes) and electronic correlator circuits identifying correlations in photocurrent fluctuations of neighboring detectors. It is shown that the normalized cross-correlation coefficients will be significant, even in the presence of a strong incoherent (e.g., solar radiation) background, provided the coherent radiation degeneracy parameter (number of photoelectrons generated by the detector during the coherence-time interval) exceeds the ratio of the background to signal intensities. This condition, which should not be difficult to achieve in realistic situations, ensures that the coherent signal is larger than the shot noise due to the background.