The propagation of laser radiation through optically inhomogeneous media causes beam broadening and energy redistribution. A turbid medium, such as atmospheric aerosols (fog, haze, clouds, or dust storms), contains suspended particles that induce multiple scattering. The initial beam energy is not lost but converted into a diffuse glow, which hinders imaging and focusing. Therefore, it is crucial to understand these effects and increase the power transmitted through such media.
Adaptive optics can mitigate scattering. While classical wavefront correction works in media with smooth refractive index changes, high concentrations of random inhomogeneities, like in biological tissues, disrupt coherence and require more sophisticated techniques. This study operates in the "crossover mode," where the transmitted radiation retains partial coherence, allowing the use of classical adaptive algorithms despite the presence of diffusers.
Numerical calculations and experimental measurements analyzed the transformation of a 0.65 μm laser beam through a layer of suspension. Polystyrene microspheres (1 μm diameter, refractive index 1.582) served as diffusers in a medium with an average refractive index of 1.33. Concentrations ranged from 10⁵ to 10⁶ mm⁻³, modeling optical conditions from weak haze to dense fog, with visibility from several kilometers down to 300–500 meters.
The beam broadening was quantified using the Full Width at Half Maximum (FWHM), with the initial beam width normalized to 1.0. Simulations were performed using the Monte Carlo method. At a scatterer concentration of N = 1.5 × 10⁵ mm⁻³, slight beam widening occurred, with FWHM increasing to 1.06. Increasing the concentration to N = 3.8 × 10⁵ mm⁻³ resulted in more pronounced broadening, reaching an FWHM of 1.24. At the maximum concentration of N = 7.5 × 10⁵ mm⁻³, the broadening was most significant, with FWHM increasing to 1.6 and the beam profile becoming highly diffuse. The results clearly demonstrate that the total beam width at half maximum increases with the concentration of scatterers in a turbid medium.
