Measuring and modeling the influence of atmospheric turbulence on a 2-µm laser beam

Within the scope of NATO SET-304 research task group, a field trial was conducted in October/2022 at the ONERA campus located in Le Fauga, Toulouse, France. Part of this trial was devoted to the study of the influence of atmospheric turbulence on laser beam propagation. In this paper, measurement results obtained during the trial for a laser beam degraded by optical turbulence are presented along with the results of analytical expressions and a numerical model. The beam of a 50-Hz pulsed 2-µm laser source (owned and operated by FFI, Norway) was focused on a target screen located approximately 1 km away from the laser emitter. Throughout the trial under various atmospheric turbulence strengths (C_n^2 varying between 5×10-16 m-2/3 and 3×10-13 m-2/3), the scattered laser radiation was captured with an infrared camera (owned and operated by TÜBİTAK BİLGEM/İLTAREN, Türkiye) looking at the nearby target screen in an oblique direction. Also, at different times of the trial, a point receiver setup consisting of two photo-diodes was positioned in the propagation path to directly capture intensity fluctuations of the incident laser beam for further investigation of the turbulence effects. The infrared camera was triggered by a 100-Hz trigger signal that enabled successive recordings of the laser beam profiles and instantaneous background. Several post-processing techniques (background subtraction, 2X soft aperture etc.) are performed on the recordings to correctly evaluate the effect of atmospheric turbulence on laser beams such as beam spreading, wandering, and degradation. Calculated beam metrics such as short- and long-term beam radii and beam wander variance are compared with the results obtained from analytical expressions and it is observed that measurement results match well with theoretical expectations. Furthermore, propagation scenarios encountered in the trial are simulated using a wave-optics-based split-step phase screen model with/without considering temporal dynamics. Numerical model results also show good agreement with the measurement results.