A multi-aperture laser transmissometer system for long-path aerosol extinction rate measurement

We present the theory, design, simulation and experimental evaluations of a new laser transmissometer system for aerosol extinction rate measurement over long paths. The transmitter emits an ON/OFF modulated Gaussian beam that doesn’t require strict collimation. The receiver uses multiple point detectors to sample the sub-aperture irradiance of the arriving beam. The sparse detector arrangement makes our transmissometer system immune to turbulence induced beam distortion and beam wander caused by the atmospheric channel. Turbulence effects often cause spatial discrepancies in beam propagation and lead to miscalculation of true power loss when using the conventional approach of measuring the total beam power directly with a large aperture optical concentrator. Our transmissometer system, on the other hand, combines the readouts from distributed detectors to rule out turbulence induced temporal power fluctuations. As a result, we show through both simulation and field experiments that our transmissometer systems works accurately with turbulence strength C_n^2 up to 10-12 m-2/3 over a typical 1.0 km atmospheric channel. In application, our turbulence and weather resistant laser transmissometer system has significant advantages for the measurement and study of aerosol concentration, absorption and scattering properties, which are crucial for directed energy (DE) systems, ground level free space optical (FSO) communication systems, environmental monitoring, and weather forecasting.