In the field of optical propagation through the atmosphere a knowledge of optical turbulence strength and other key statistical parameters is crucial for performance prediction and system design. This work presents techniques for reliably estimating the most essential parameters of optical turbulence, namely r₀, the Fried coherence length, f_G, the Greenwood frequency, and l₀, the inner scale of turbulence from Shack-Hartmann wavefront sensor measurements. The earliest approaches for estimating r0 were based on MTF measurements.1 The MTF approach requires accurate calibration and stability of the system MTF which is often problematic. Astronomers have used differential motion and scintillation to measure seeing conditions.^2-5 Others have used the slope structure function estimated from a Hartmann wavefront sensor principally for r0 estimation.⁶ We have shown that the slope discrepancy⁷ or rotational component of the slopes can be used effectively in turbulence estimates.⁸ The techniques we describe here can be used to estimate r₀, f_G, and l₀. The inner scale estimate is based on the assumption of the Hill spectrum for refractive index fluctuations.^9-11 A high resolution, high frame rate, mobile sensor has been developed to utilize these estimation techniques. Section 2 describes the estimation techniques. Results from field measurement campaigns will be presented in Section 3.

The Large Synoptic Survey Telescope will map out the dark matter in the universe and is scheduled to see "first light" in 2014. This telescope will require active correction of its mirrors to remove the aberrations that arise from changing gravitational force vectors and from thermal drifts in the telescope during observational runs. In this article we present a comprehensive evaluation of a Shack-Hartmann wavefront sensor and reconstruction algorithm which is capable of meeting the unique challenges associated with this wide field-of-view survey telescope. The advantages of this technique over other potential wavefront sensing technologies are discussed and the potential problems encountered with this approach are analyzed and solutions to these problems presented.

In earlier work we have shown that pupil plane branch points carry information about the conditions of the atmospheric turbulence. Experiments in the Atmospheric Simulation and Adaptive-optic Laboratory Test-bed (ASALT) at the Air Force Research Laboratory, Directed Energy Directorate's Starfire Optical Range have shown that branch points can provide the number and velocity of turbulence layers. Here we demonstrate that these measurements can further be used to estimate the turbulence layers' altitude and strength. This work is the culmination of research demonstrating that a methodology exists for identification of the number, altitude, strength, and velocity of atmospheric turbulence layers.

We propose a sensor that measures the number, strength, altitude and velocity of atmospheric turbulence layers. Recent research has shown that pupil plane branch points contain four independent and measureable parameters and that these four parameters can be used to estimate four independent turbulence layer parameters--number, strength, altitude and velocity--for each atmospheric turbulence layer. Here, we summarized previous results and then demonstrate how these results allow for construction of a turbulence layer sensor.

The ASALT lab has been investigating the use of a segmented MEMS DM in adaptive optics systems. Here, we investigate the fitting error for a segmented deformable mirror with flat subaperture segments. This investigation is done in the regime where the hidden phase is significant. Data from both simulation and theory are presented giving initial estimates of the magnitude of the error.

Micro-Electro-Machined Systems (MEMS) have been increasingly used as mirrors in place of conventional continuous face sheet deformable mirrors (DM) in adaptive optics (AO) systems. Here we study the diffraction effects introduced into the optical path when a segmented MEMS DM is used to correct for the wavefront aberrations. Diffraction effects are monitored through the intermediate focus plane prior to the wavefront sensor. Low pass spatial filter is used at that plane in order to investigate how the masking of various diffraction orders affects the phase. Measured phase and focal image plane data for various turbulence conditions are presented and analyzed.

Phase compensation instability (PCI) is the time-dependent development of spatial perturbations that occur within thermally bloomed high-energy laser (HEL) beams. These types of spatial perturbations act as local hot spots that create small negative lenses within the HEL beam. Closed-loop adaptive optics (AO) corrects for these spatial perturbations by applying small positive-lens phase compensations, which only increases the strength of the local hot spots and leads to runaway in the adaptive-optics servo. This study uses a straightforward wave-optics code to model horizontal propagation with the effects of thermal blooming for a focused Gaussian beam. The strength of the thermal blooming effects is characterized using the classic dimensionless distortion number. A nominal AO system is used to mitigate phase distortions accumulated from thermal blooming. Parameters within the AO system, such as the number of actuators on the deformable mirror and the resolution of the wavefront sensor, are varied to determine the impact of spatial resolution in the development of the PCI. A discussion is given on the potential use of control theory to diminish the effects of the PCI.

We will develop and then compare object spectrum phasor reconstruction results for several speckle imaging approaches. Each phasor reconstruction algorithm results from minimizing a very naturally defined weighted-least-squares error function. Once we pick a phasor-based error function, the remaining steps in our algorithms are developed by setting the error function variation, with respect to each phasor element, to zero. The resulting coupled nonlinear equations for the minimum error phasor array are then solved iteratively. In the applications, we will compare and contrast three implementations: 1) Knox-Thompson; 2) Bispectrum, using only two bispectrum planes; 3) Bispectrum, using four bispectrum planes. In each application of the three approaches, we first calculate the modulus of the object spectrum using a Wiener- Helstrom filter to remove the speckle transfer function. The methods then differ only in their object spectrum phasor reconstructions. In the simulations, we will implement all three methods on a simple object at low photon-per-frame light levels. Next, we will apply the methods to a complex extended object.

A field test experiment on a range tracking telescope at the U. S. Army's White Sands Missile Range is exploring the use of recently developed adaptive control methods to minimize track loop jitter. Gimbal and platform vibration are the main sources of jitter in the experiments, although atmospheric turbulence also is a factor. In initial experiments, the adaptive controller reduced the track loop jitter significantly in frequency ranges beyond the bandwidth of the existing track loop. This paper presents some of the initial experimental results along with analysis of the performance of the adaptive control loop. The paper also describes the adaptive control scheme, its implementation on the WSMR telescope and the system identification required for adaptive control.

Polymer membrane deformable mirrors offer a low-cost alternative to conventional technology in a wide variety of adaptive optics or laser beam shaping applications. In this paper we evaluate the suitability of two different kinds of polymer membrane deformable mirrors for laser machining. We present results showing that a 12.5-mm diameter nitrocellulose membrane fails near 400 microns of motion. We present results from a demonstration of a high peak power beam shaping and show a new compact laser beam shaping system using a polymer membrane deformable mirror. We evaluate the effect of Q-switched laser radiation on polymer membranes at 355nm and 1060nm.

The Naval Research Laboratory has developed a new method for generating atmospheric turbulence and a testbed that simulates its aberrations far more inexpensively and with greater fidelity using a Liquid Crystal (LC) Spatial Light Modulator (SLM) than many other methods. This system allows the simulation of atmospheric seeing conditions ranging from very poor to very good and different algorithms may be easily employed on the device for comparison. These simulations can be dynamically generated and modified very quickly and easily. In addition, many models for simulating turbulence often neglect temporal transitions along with different seeing conditions. Using the statistically independent set of Karhunen-Loeve polynomials in conjunction with Kolmogorov statistics in this model provides an accurate spatial and temporal model for simulating turbulence. An added benefit to using a LC SLM is its low cost; and multiple devices can be used to simulate multiple layers of turbulence in a laboratory environment. Current testing with using multiple LC SLMs is under investigation at the Naval Research Laboratory and the Naval Postgraduate School.

Iris AO has been developing a 489-actuator, 163 piston-tip-tilt segment, deformable mirror system controlled with a personal computer. The system includes the MEMS-based DM, drive electronics, and a precision factory-calibrated position controller. The position controller implements both position limiting to keep DM segments within the safe operating region and calculates the actuator voltages that correspond to desired DM piston, tip, and tilt positions. This paper describes recent speed enhancements and benchmarking results for the 489-actuator deformable mirror system. Benchmarking showed an execution time of 157.5 μs from the start of the DM piston/tip/tilt (PTT) position controller operation to when the last bit was output from the computer interface card to the DM drive electronics. Initial testing of an asynchronous write operation for the computer interface card shows that the PTT controller function can return within 5 μs of a data transfer, thereby shortening the processor time required for a DM to an estimated 74.4 μs. All aspects that give rise to latencies and bandwidth are presented herein, namely: 1) PTT controller safe-operating-point limiting and voltage calculations; 2) computer interface and DAC latencies; 3) drive electronics bandwidth, and 4) DM bandwidth.

We introduce and demonstrate a quasi-ternary nonmechanical beam steering design based on Polarization Gratings (PGs). That uses a single wave plate and N PGs to generate 2(^N+1)-1 steering angles. When compared to conventional binary (2^N) or ternary (3^N) liquid crystal PG steering designs, this technique uses fewer elements arranged in a simpler configuration to obtain the same number of steering angles. This advantageous property can be achieved by selecting proper diffraction angles and alignment of the PGs. Due to fewer elements per stage, losses due to electrode absorption and Fresnel reflections are reduced, thereby increasing the overall steering efficiency. Using this approach, we demonstrate a four-stage (N = 4) quasi-ternary beam steering device that achieves 52° Field Of Regard (FOR) with 1.7° resolution (31 steering angles) at 1550 nm wavelength.

The ASALT lab has been investigating the use of a segmented MEMS DM in adaptive optics systems. One of the anticipated benefits of a segmented device is that in monochromatic light the throw is essentially infinite due to the modulo 2π nature of the device. Earlier work demonstrated how this modulo 2π behavior interacts unexpectedly with a standard proportional integral controller. Here we present experimental data on this effect to include the testbed on which the data was taken and the methodology used to measure the effect.

The concept of the laser guide star was proposed by Foy and Labeyrie, 1985. The Laser Guide Stars (LGS) depend on the abundance and distribution of sodium in the mesosphere. The mesopheric sodium often appears to consist of more than one layer, each of which exhibits time variation in density and altitude. The non-zero thickness and finite range of the layers results in elongation of the LGS defocus on extremely large telescopes such as TMT and VLT. Na variability will be examined for determination, analysis and impact on Adaptive Optics aberration correction.

It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band of wave length 0.9 to 1.7 μm. Numerous studies of these phenomena have demonstrated that the irradiance shows significant temporal and spatial variations in the night sky. Changes in weather patterns, seasons, sun angle, moonlight, etc have the propensity to alter the SWIR air glow irradiance pattern. By performing multiple SWIR measurements a mosaic representation of the celestial hemisphere was constructed and used to investigate these variations over time and space. The experimental setup consisted of two sensors, an InGaAs SWIR detector and a visible astronomical camera, co-located and bore sighted on an AZ-EL gimbal. This gimbal was programmed to view most of the sky using forty five discrete azimuth and elevation locations. The dwell time at each location was 30 seconds with a total cycle time of less than 30 minutes. The visible astronomical camera collected image data simultaneous with the SWIR camera in order to distinguish SWIR patterns from clouds. Data was reduced through batch processing producing polar representations of the sky irradiance as a function of azimuth, elevation, and time. These spatiotemporal variations in the irradiance, both short and long term, can be used to validate and calibrate physical models of atmospheric chemistry and turbulence. In this paper we describe our experimental setup and present some results of our measurements made over several months in a rural marine environment on the Islands of Kauai and Maui Hawaii.

It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band between 0.9 and 1.7 μm wave length. This has been demonstrated as an effective illumination source for night time imaging applications. It addition it has been shown that observation of the spatial and temporal variations of the illumination can be used to characterize atmospheric tidal wave actions in the sky glow region. These spatiotemporal variations manifest themselves as traveling wave patterns whose period and velocity are related to the wind velocity at 85 km as well as the turbulence induced by atmospheric vertical instabilities. Ground to space observation systems especially those employing adaptive optics are adversely affected by high altitude turbulence and winds. In this paper we propose the use of sky glow observations to predict and characterize image system degradation due to upper atmosphere turbulence.

Adaptive optics (AO) is a technology which improves the performance of optical systems by reducing the effects of rapidly changing optical distortion. Wave front active control by combining wave front sensor and MEMS deformable mirrors which made of polyimide thin film actuated by electrostatic force is one possible solution. Wave front sensor detects the image and aberration could be described with Zernike polynomials, and the distorted wave-front is corrected by deformable mirror. Combining these two technologies, we fabricate a large-scale MEMS deformable mirror with a 20mm diameter circular opening and 67 hexagonal actuation electrodes in this thesis. Moreover, we use commercial software, Ansys, to simulate the deformation behavior of the membrane with different electrodes applied and give some device parameter tuning for versatile application. We measure the maximum stoke is 39 um as 195 volts applied to 67 electrodes. Due to the large-scale of our thin membrane, resonant frequency is around 8 Hz. Besides, we also discuss some possible ways to improve device characteristics and we think deformable mirror has a good potential for wave front active control based on our experiment results.