Optical turbulence induces distortions in amplitude and phase in any beam propagating through it, resulting in beam spreading, beam wandering, and irradiance fluctuations among other effects. Due to the dynamic nature of these effects, the complex field reconstruction of a perturbed beam presents a great experimental challenge. Interferometric wavefront reconstruction techniques require very sophisticated assemblies prone to alignment errors due to their high sensitivity to environmental disturbances. This hinders its experimental implementation. New complex phase retrieval methods overcome most of the limitations of interferometric methods: they are suitable for amplitude or phase objects (or both) and their reconstruction algorithms-based on propagation equations-make unnecessary any a-priori knowledge of the beam to be reconstructed. We propose an experimental implementation of a complex phase retrieval technique for the characterization of Gaussian beams propagating through turbulence. This technique is based on binary amplitude modulation using a digital micro-mirror device (DMD) which has proven to be suitable for dynamic applications. To our knowledge, this is the first experimental high-speed complex wavefront reconstruction of optical beams-by binary amplitude modulation-through controlled real turbulence. This experiment represents the first step in our research focused on understanding optical turbulence from an experimental point of view.
In the last decade, a nascent trend of characterizing turbulence from observing features of distant targets through ground-layer turbulence have been relentless growing. Either from observing regular geometrical features of buildings or arrays of LEDs, it is possible to retrieve the structure constant of the refractive index fluctuations. On the other hand, because of the lack of a definitive theoretical model describing anisotropic or inhomogeneous turbulence, most experimental observations have been reduced to mere descriptions in the event of deviations from expected Obukhov-Kolmogorov predictions. Our group has been able to retrieve power-spectrum exponents, without a prior knowledge of a subjacent model, and henceforth determine anisotropic behavior in controlled optical turbulence; furthermore, under convective turbulence, an exponent can be obtained from time series of the occurrence of power drops in optical communication links: extreme events.In this manuscript, we present a technique identifying as extreme events suden changes in morphological characteristics of an array of point sources observed through real controlled anisotropic turbulence assisted by a deep-learnig ad-hoc. This approach provides an effective approach to reduce high-volume data from imaging targets into a real-time stream of parameters to fully characterize optical turbulence.
We have previously reported [D. G. Perez et al., Imaging Appl. Opt. 2019 (pcAOP), invited talk] that image-plane scintillation index can provide a universal law to extract C n 2 —independent of the target shape and light source. Also, we observed σ l 2 > 1 on the image plane occurs at any turbulence strength; particularly, as we move from weak to strong optical turbulence the number of pixels with scintillation greater than one increases in the neighborhood of high-contrast regions (edges). As the number of pixels with large scintillation values is observed with higher frequency in targets illuminated with incoherent light, whether wandering plays a mayor influence or not in image-plane scintillation regardless of the turbulent regime is still unknown. In this communication, we have devised a new experiment capable of differentiating the contributions of amplitude and phase to the pixel scintillation. Moreover, structured targets were introduced to distinguish anisotropic regimes, a property that was largely undetected in our original setup.
The Obukhov-Kolmogorov (OK) theory has provided us with the very well- known –11/3-power spectrum; widely assumed as a valid model for the refractive index fluctuations in the whole atmosphere. Within its reach, the strength of the fluctuations are conveyed by the structure constant, C n 2 . Yet, the OK model presupposes the existence of statistical isotropy and homogeneity–both for the velocity and refractive index fluctuations. In this work, we show that this is a rare occurrence in turbulent airflow above the ocean. Therefore, we need to re-evaluate the physical meaning of C n 2 measurements.
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