Determining Bulk Aerosol Absorption from Off-Axis Backscattering Using Rayleigh Beacon Laser Pulses

Aerosol absorption and scattering can play a key role in degraded high energy laser performance in the form of thermal blooming and beam attenuation. Aerosol absorption properties are not completely understood and thus affects how we are able to quantify expected high energy laser weapon performance. The Air Force Institute of Technology Center for Directed Energy (AFIT CDE) developed both Laser Environmental Effects Definition and Reference (LEEDR) and the High Energy Laser End-to-End Operational Simulation (HELEEOS) code to characterize atmospheric radiative transfer effects and evaluate expected directed energy weapon system performance. These packages enable modeling total irradiance at given off-axis locations through an off-axis scattering algorithm, which uses scattering phase functions to predict the amount of radiation scattered from the beam toward a particular observation location. The phase functions from off-axis high energy laser irradiance measurements taken at several different angles is compared to the predicted scattering phase function shapes. Laser energy measurements were conducted using a side telescope and ultrafast laser, located at John Brian Observatory in Yellow Springs, Ohio. The off-axis irradiance was measured using a Mini-SWIR JSX snapshot camera, calibrated to capture scattered irradiance in the visible spectrum. Aerosol characterization information was gathered using a MAGIC200 Condensation Particle Counter (CPC), a Scanning Mobility particle Sizer (SMPS), and a Continuous Light Absorption Photometer (CLAP). The differences in the measured versus predicted phase function shapes elucidate bulk aerosol absorption properties.