CONICET | Buscador de Institutos y Recursos Humanos

We have experimentally quantified the entropy of isotropic optical turbulence at the laboratory. The issue here is the characterization of the long-range correlations in the wandering of a thin Gaussian laser beam over a screen after propagating through a turbulent medium [1]. To fulfill this goal, a laboratory-controlled experiment was conducted in which coordinate fluctuations of the laser beam were recorded at a sufficiently high sampling rate for a wide range of turbulent conditions by using a device called turbulator [2]. Horizontal and vertical displacements of the laser beam centroid were subsequently analyzed by implementing the symbolic technique based on ordinal patterns to estimate the well-known permutation entropy [3]. We show that entropy estimation at different time scales characterizes an interplay between different dynamical behaviors. More specifically, crossovers between different scaling regimes are observed. A transition from an extremely persistent behavior (due to oversampling) contaminated with electronic noise to long-range correlations (characterized by a Hurst exponent H ~ 5/6), as sampling rate decreases is observed for all turbulence strengths. Besides we are able to quantify the amount of electronic noise as a function of the turbulence strength. These experimental observations are in very good agreement with numerical simulations of a noisy fractional Brownian motions with a well-defined crossover between different scaling regimes. Finally, we contrast our results with those obtained by using detrendred fluctuation analysis [1].[1] L. Zunino, D. Gulich, G. Funes, and D. G. Perez, Opt. Lett. 40, 3145 (2015).[2] A. Fuchs, J. Vernin, and M. Tallon, Appl. Opt. 35, 1751 (1996). O. Keskin, L. Jolissaint,and C. Bradley, Appl. Opt. 45, 4888 (2006).[3] C. Bandt, B. Pompe, Phys. Rev. Lett. 88 (2002) 174102.