Branch points are seen in many adaptive optical experiments where the sensed beam has propagated over an extended path, through sufficiently strong turbulence. It has been shown that branch points provide information on how the turbulence responsible for their formation is distributed and moving along the path. Shack-Hartmann wave front sensors have previously been somewhat limited in their ability to fully capture the branch points present within their measurements. A new technique for the detection of branch points based on the second moment statistics of the individual spots in images created with a Shack-Hartmann wave front sensor system is examined. Data collected by Small Mobile Atmospheric Sensing Hartmann (SMASH) units are used to test the method under a range of turbulence conditions. The results of the second moment technique are compared with the standard elementary circulation method.
The Reflective Atmospheric Turbulence Simulator (RATS) in the Air Force Research Laboratory’s Beam Control Laboratory is used to impart realistic distortions to a propagating wavefront for testing in the advancement of adaptive optical technologies. Here RATS is being used to simulate turbulence conditions over which the Small Mobile Atmospheric Sensing Hartmann (SMASH) system has typically operated. An optical clone of the SMASH system installed on the optical bench behind RATS measures the imparted optical disturbance and makes an estimate of the turbulence profile. The results are compared with the profile calculated based on the configuration of the RATS system.
The Reflective Atmospheric Turbulence Simulator (RATS) is an instrument used to simulate the propagation of light through atmospheric turbulence. RATS can simulate up to six layers of atmospheric turbulence using reflective phase wheels with many configurations that can simulate a broad range of atmospheric turbulence conditions. Two ShackHartmann wavefront sensors are integrated with RATS. One of these Shack-Hartmann wavefront sensors has an effective aperture size of 40cm, and the other has an effective aperture size of 6.75cm. We simulated propagation through atmospheric turbulence using RATS and made measurements of turbulence parameters using the two Shack-Hartmann wavefront sensors. In this paper, we compare the measurements of turbulence parameters from both Shack-Hartmann wavefront sensors to each other as well as to the theoretical values for atmosphere turbulence simulated by RATS.
The advancement of adaptive optics (AO) has a tradition of using benchtop optical simulators to progress control technologies from concept towards fielded systems. This paper presents a reflective atmospheric turbulence simulator (RATS) for the Air Force Research Laboratory's Beam Control Laboratory (BCL) with which the next steps in AO will be tested. The reflective nature of the system allows operation over a broad range of wavelengths. RATS consists of six moveable phase screens etched with Kolmogorov turbulence phase patterns. The configuration of the system can be varied to simulate a wide range of atmospheric turbulence conditions setting the needed parameters of the Fried coherence length, Greenwood frequency, Rytov variance and the isoplanatic angle, to meet a given scenario. Shack-Hartmann measurements of the turbulence generated by RATS are compared to the system design.
The tilted shearing interferometer (tSI) wavefront sensor (WFS) is being developed to expand characterization capabilities for optical propagation experiments at the Air Force Research Laboratory's Environmental Laser Test Facility (ELTF). The instrument utilizes the phase retrieval technique of a digital holographic WFS with laterally sheared beams rather than a local oscillator reference. This WFS provides gradients similar to a Shack-Hartmann WFS, allowing it to benefit from all of the processing developed for the Small Mobile Atmospheric Sensing Hartmann (SMASH). At the same time, the interferometric nature of the wave front sensor provides access to additional information, i.e. branch cuts. Initial development of the tilted shearing interferometer in the Air Force Research Laboratory's Beam Control Lab (BCL) is presented.
Knowledge of the atmospheric conditions along an optical path is crucial to many experiments. A technique using differential scintillations was adapted for the Small Mobile Atmospheric Sensing Hartmann (SMASH) system to estimate proles of the refractive-index structure constant, C2n(z), and the wind speed. Estimates of those parameters from data taken along a 1 km horizontal path over level ground at a height of about five feet at Kirtland AFB is presented. Five sonic anemometers, placed along the path, serve as an independent estimate of the turbulence conditions with which to evaluate SMASH's performance.
KEYWORDS: Turbulence, Beam path, Air force, Temperature metrology, Atmospheric propagation, Air temperature, Wind measurement, Profiling, Sun, Refractive index
Sonic anemometers are devices that use ultrasound to provide instantaneous wind velocity and sonic temperature measurements. One of these devices, in conjunction with other meteorological equipment, provides characterization of the local atmosphere at a fixed point. Combining multiple sonic anemometers can provide an estimate of the index of refraction structure function, C2n(z), along a beam path. This work details this process for characterization of an optical propagation path for use in the evaluation of the performance of turbulence measurement instruments. Experimental results are presented from a one-kilometer horizontal path.
An implementation of differential scintillations to characterize the C2n(z) profile along a nearly horizontal propagation path measured by a Shack-Hartmann wavefront sensor is developed and demonstrated. Measurements from a Small Mobile Atmospheric Sensing Hartmann (SMASH) instrument using an LED source to characterize 500 m, 1 km and 2 km paths at the Environmental Laser Test Facility (ELTF) are presented.
The inner scale plays a critical role in beam scintillation and branch point evolution in optical propagation through atmospheric turbulence. Understanding this parameter, in-situ, during experiments is therefore of great interest. We compare different methods of estimating the inner scale using AFRL’s Small Mobile Atmospheric Sensing Hartmann (SMASH). The investigations are conducted with data collected at Kirtland, AFB in New Mexico along multiple paths varying from weak to strong irradiance fluctuation conditions.
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