Phase correction of laser beam passed through turbid medium Ilya Galaktionov, Julia Sheldakova, Alexis Kudryashov Moscow State University of Mechanical Engineering Moscow, Russia ilya_galaktionov@activeoptics.ru Model of laser beam propagation in turbid medium with variable parameters was suggested. Phase and intensity distribution of scattered light with different wavelengths and various whether conditions (particles sizes, concentrations, refractive indices and medium depths) were obtained. Keywords—turbid medium; light scattering; light propagation model; adaptive optics. I. INTRODUCTION Turbid medium is a medium with optical inhomogeneity. Such a medium contains particles causes scattering and absorption of light. Both scattering and absorption remove energy from a beam causing attenuation [1]. For particles lager or compared to wavelength (as an aerosol particles in our case) Mie theory of scattering is used. Moreover, laser beam obtains additional phase distortion because of scattering. In this work we implemented numerical simulation of laser beam propagation to estimate wavefront aberrations (type, amplitude, dependencies of particles concentration, radius and refractive index). II. MODEL The aim of this work was investigation of “beam – medium” interaction and analysis of obtained intensity and phase distribution of scattered light. We used Monte-Carlo simulation to model light propagation through turbid medium [2]. Laser beam shape was represented as a large number (10 9 ) of photon packets equally distributed over the initial beam aperture (similarly to uniform intensity distribution). Initial position, transport free path, scattering and azimuthal angle were sampled for each photon packet. All of photon packets contained information about optical path length, position and angle of emergence, number of scattering events and mean free path length at the end of the medium III. MODELING RESULTS Initial beam aperture was assumed to be 5 millimeters, sensor aperture is 6 millimeters. Radiation wavelength was equal to 1.3 microns (according to atmospheric transparency window) [3]. Number of photons packets was about 10 8 . As an example of turbid medium we used moderate fog. For such conditions the range of visibility is about 500 meters, mean particles size is 50 microns whereas the concentration is about 50 - 70 particles per cubic centimeters. Refractive index of particles was 1.55 and refractive index of the medium was assumed 1.00 [4]. In this case we considered turbid medium layer to be 10 x 10 x 10 meters. As a result we obtained intensity and phase distributions and were able to draw interferograms. Forward scattered light fraction is 75%, whereas ballistic photons packets (passed through medium without being scattered) fraction is 14%. It is well seen that such phase distortions can be compensated for by means of deformable mirror with high spatial resolution (for example stacked actuator mirror). Fig. 1. Phase disctibution of scattered light. REFERENCES [1] H.C. van de Hulst, Dover Publications Inc., New York, 1981. [2] L. Wang, S. Jacques, “Monte Carlo Modeling of Light Transport in Multi-layered Tissues in Standard C,” University of Texas M. D. Anderson Cancer Center, November 1995. [3] C. Bohren, D. Huffman, Absorption and Scattering of Light by Small Particles, A Wiley-Interscience Publication, New York, 1985. [4] E.J. McCartney, Optics of the Atmosphere, John Wiley & Sons, New York, 1976.