A wave-optics-based numerical simulation analysis of the impact of speckle-beacon size on the performance of an adaptive optics (AO) system operating in volume atmospheric turbulence is presented. For clarity, the speckle beacon was represented by a laser beam of a super-Gaussian profile, scattered off an extended flat target with Lambertian surface roughness. The control loop of the AO system included a high-resolution scintillation-resistant multi-aperture phase contrast wavefront sensor (MAPCO WFS), an ideal (infinite-resolution) wavefront corrector, and phase-conjugation (PC) type controllers utilizing either conventional PC or advanced speckle-average (SA) PC control algorithms. The results obtained show that the use of the advanced control algorithms makes it possible to partially mitigate the target-induced speckle effects and turbulence-induced wavefront aberrations for extended beacons, the size of which is comparable to or even exceeds the diffraction-limited beam spot size of the corresponding laser beam projection system.
The capabilities of neuromorphic (event) based sensors for atmospheric turbulence characterization and refractive index structure parameter ( C2n ) sensing are investigated. The experimental setup used a system that consisted of a telescope with an attached neuromorphic camera that was imaging features of a corner of an installation on the roof of a building in 7 km distance. Synchronously with recording of the event stream from neuromorphic sensor the refractive index structure parameter was measured with a commercial scintillometer along the same propagation path. A processing technique was developed to compare the distribution-width of events generated by the edges of the imaged corner within a given time-span to the measured strength of turbulence from the scintillometer. Spatio-temporal analysis was applied to show the possibility to detect influence of wind flow in the of recorded event stream data.
A deep machine learning-based electro-optics system (TurbNet sensor) was developed to measure atmospheric turbulence refractive index structure parameter (C2n) at a high temporal resolution by processing short-exposure intensity scintillation patterns. The TurbNet sensor was composed of a remotely located LED beacon, an optical receiver telescope with a CCD camera for capturing short exposure pupil-plane intensity scintillation patterns, and a Jetson Xavier Nx embedded AIcomputing platform to implement the deep neural network (DNN)-based processing of LED beam scintillation images. Performance of the TurbNet sensor was evaluated over a 7 km atmospheric propagation path.
An electro-optics system (TurbNet sensor) composed of a remotely located laser beacon, optical receiver telescope with a CCD camera capturing short-exposure intensity scintillation patterns, and deep neural network (DNN)-based processor implemented on Jetson Xavier Nx embedded AI-computing platform was developed and utilized for real-time sensing of the atmospheric turbulence refractive index structure parameter (2Cn) over a 7 km propagation path at high temporal resolution.
Probability density functions of Gaussian laser beam irradiance measured after propagation over 7km atmospheric path in comparison with various theoretical models are presented. The initial laser beam diameter at the e-2 intensity level was about 6 mm, the receiving aperture size was 14.4 cm. The experimental observations were performed in a wide range of turbulence strengths. The cases of weak, moderate, and strong intensity fluctuation regimes have been analyzed. Different receiving aperture radiuses were considered. The chi-square metric was used to estimate the agreement between the experimental and different theoretical statistics. The fractional gamma distribution has shown the best results for probability density distributions of apertures with sizes about 1 cm and 4 cm under strong turbulence conditions. The aperture averaging effect results in excluding near-zero irradiance values, which are typically observed on-axis at strong turbulence and high value of scintillation index which qualitatively transforms the observed statistics, so that experimental probability density functions can be well approximated by the fractional gamma distribution. With the increase of the aperture size, a further transformation of the statistics was observed. The statistics of experimental data for moderate and weak fluctuation regimes approached the lognormal and gamma distributions.
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