We present the results from experimental mitigation of wavefront distortions induced by atmospheric turbulence within a 2.4 km near horizontal propagation path using an adaptive optics system based on a model-free optimization strategy. A laser source with a diffuser or a multi-mode fiber-coupled laser were used to model a partially coherent speckle beacon. Propagation path characteristics (intensity scintillations and Strehl ratio fluctuations) were determined for different turbulence conditions. The adaptive optics system comprises a micro-electromechanical mirror and a VLSI controller that implements a stochastic parallel gradient descent (SPGD) algorithm for the optimization process. Experiments performed in an adaptive receiver as well as in an adaptive transceiver configuration demonstrate improvement of the average Strehl ratio even under strong scintillation conditions.

We have previously demonstrated and reported on the use of sub-field speckle processing for the enhancement of both near and far-range surveillance imagery of people and vehicles that have been degraded by atmospheric turbulence. We have obtained near diffraction-limited imagery in many cases and have shown dramatic image quality improvement in other cases. As it is possible to perform only a limited number of experiments in a limited number of conditions, we have developed a computer simulation capability to aid in the prediction of imaging performance in a wider variation of conditions. Our simulation capability includes the ability to model extended scenes in distributed turbulence. Of great interest is the effect of the isoplanatic angle on speckle imaging performance as well as on single deformable mirror and multiconjugate adaptive optics system performance. These angles are typically quite small over horizontal and slant paths. This paper will begin to explore these issues which are important for predicting the performance of both passive and active horizontal and slant-path imaging systems.

A 40-control-channel adaptive optics system capable of minimizing the impact of propagation medium induced thermal blooming effect in the target-in-the-loop optical setting is presented. The optical system uses two adaptive mirrors: a 37-channel deformable mirror manufactured by Xinetics Inc. and a 3-channel tip-tilt/defocus adaptive mirror constructed at the Army Research Laboratory. System operation is based on phase control of the outgoing wavefront based on the stochastic parallel gradient descent optimization technique. The optimized metric depends solely on characteristics of the returned speckle field scattered by an extended target surface registered at the transmitter plane. Results demonstrate that adaptive wavefront correction using a speckle-field-based beam quality metric can improve laser beam concentration on extended objects in the presence of propagation medium thermal blooming effects.

A new Army Research Laboratory (ARL) facility, the Atmospheric Laser Optics Testbed (A_LOT), designed to support research and development for a wide range of laser communication, atmospheric optics, beam control and imaging programs is presented. A_LOT provides a research laboratory environment that includes a 2.3 km near horizontal atmospheric optical path for studying the major challenges facing applications that employ laser beam propagation or imaging through the boundary layer atmosphere. Simultaneous and continual characterization of the meteorological and atmospheric turbulence parameters with characteristics of atmospheric imaging and laser beam propagation is accomplished with a network of various sensors connected to an automated data collection and archiving system. "Local weather" control capabilities allow generation of smoke, fog or rain. Devices and experiments can be controlled and data examined remotely via internet connection and remote client software. Examples of recent laser communication and imaging results obtained using the A_LOT facilities are presented.

In this article, a simple low-cost, statistically repeatable, hot air optical turbulence generator based on the mixing of two air flows with different temperatures is described. Characterization results show that it is possible to create any turbulence strength up to C_N²Δh ≈ 6 x 10^-10 m^1/3, allowing Fried's parameter as small as r₀ ≈ 1.7 mm for one crossing through the turbulator or r₀ ≈ 1.1 mm for two crossings. Outer scale of (L₀ ≈ 133 ± 60 mm) is found to be compatible to the turbulator chamber size (170 mm), and inner scale (l₀ ≈ 7.6 mm ± 3.8 mm) compatible with usual values measured by other authors for the free atmosphere. Power spectrum analysis of the centroid of the focused image shows a perfect and accurate agreement with Kolmogorov's theory, allowing to conclude that this device can be used with confidence to emulate good and easily controllable turbulence. In particular, this turbulator will be used with the MCAO test bench developed at the University of Victoria. By allowing two passes of the optical beam through the turbulator, without overlapping, two independent turbulent layers, set at equivalent altitudes of 5 and 15 km above the telescope entrance pupil, will be generated.

Nonlinear optical processes in liquid crystals (LC) can be used for construction of all-optical spatial light modulators (SLM) where the photosensitivity and phase modulating functions are integrated into a single layer of an LC-material. Such spatial light integrated modulators (SLIMs) cost only a fraction of the conventional LC-SLM and can be used with high power laser radiation due to high transparency of LC materials and absence of light absorbing electrodes on the substrates of the LC-cell constituting the SLIM. Recent development of LC materials the photosensitivity of which is comparable to that of semiconductors has led to using SLIM in schemes of optical anti-jamming, sensor protection, and image processing. All-optical processes add remarkable versatility to the operation of SLIM harnessing the wealth inherent to light-matter interaction phenomena.

In this paper we review progress towards making a liquid crystal spatial light modulator (LC-SLM) which has all the desired specifications required for (astronomical) adaptive optics (AO). Our work at Durham is currently focused on developing modal LCs, as they have some key advantages over conventional LC-SLMs. A modal LC-SLM is a device whose optical properties more closely resemble a deformable facesheet mirror than a conventional LC-SLM which are pixelated. Therefore they have a Gaussian, rather than a piston-only, influence function.

Adaptive optics systems have seen widespread adoption in the astronomy community. However, next generation telescopes with large apertures and wide fields of view, not to mention the desire to correct for atmospheric turbulence at optical wavelengths, will require a dramatic increase in the number of actuators required for correction. Micro-Electro-Mechanical Systems (MEMS) provide a potential solution to this demand for densely packed actuator arrays. In this paper the characteristics of a 140 actuator MEMS based deformable mirror (DM) are investigated with particular emphasis on its application in astronomical adaptive optics systems. In particular the DM surface quality, actuator stroke and influence function are investigated as well as the residual error when attempting to correct Zernike modes.

It is often convenient to represent a wavefront aberration by the superposition of several aberration modes, for example, using the set of Zernike polynomials. In many practical situations the total aberration can be accurately represented by a small number of such modes. It is therefore desirable to be able to measure directly the modal content of the wavefront. The modal wavefront sensor allows us to do just this. This sensor can be applied to closed-loop aberration correction in adaptive systems and to direct absolute measurement of modal aberration coefficients. One implementation offers the possibility of an adaptive optics system where we have disposed altogether of a separate wavefront sensor. We present here extension of the modal wavefront sensing theory and explore the options for optimization of the design. We investigate the linear measurement range of the sensor and the performance in closed-loop systems.

Scene-based wave-front sensing (SBWFS) is a technique that allows an arbitrary scene to be used for wave-front sensing with adaptive optics (AO) instead of the normal point source. This makes AO feasible in a wide range of interesting scenarios. This paper first presents the basic concepts and properties of SBWFS. Then it discusses the application of this technique with AO to remote imaging, for the specific case of correction of a lightweight optic. End-to-end simulation results establish that in this case, SBWFS can perform as well as point-source AO. Design considerations such as noise propagation, number of subapertures and tracking changing image content are analyzed.

A high-resolution phase contrast technique is applied for the measurement of an atmospheric-like artificial turbulence distribution along laser beam propagation path. This technique is based on a nonlinear Zernike wave front sensor that employs an optically addressed ferroelectric liquid crystal spatial light modulator. The measurements of dynamically changing phase distortions located at different distances from the wave front sensor were obtained. The spatial resolution, dynamic range, and time response of this technique are examined and compared with a theoretical analysis.

The subapertures defined by a Shack-Hartmann wave front sensor are highly oversampled when a high-resolution pixelated liquid crystal device (LCD) is used for wave front correction. Wave front reconstruction requires that appropriate phase values be assigned to all LCD pixels within each oversampled subaperture. We have implemented a reconstructor that uses a least squares approximation to calculate the phase values for the subaperture corners based on the Shack-Hartmann data. It then uses a zonal fitting interpolation method to assign phase values to the rest of the LCD pixels without creating discontinuities between the subaperture regions. Experimental results are presented.

The circle polynomials of Zernike are a vital tool in the analysis of optical systems. Decomposition of wavefronts into Zernike polynomials can be insightful. Computation in the Zernike basis, however, is quite cumbersome and inefficient. This paper will address how rational polynomials such as Zernike, Laguerre, Legendre and Chebyshev can be represented as affine combinations of a Taylor monomial set. This paper also demonstrates the efficiency of common routines using a Taylor basis as well as implementation and optimization issues.

We have adapted a Shack-Hartmann wavefront sensor (SHWFS) to the measurement of highly aberrated large optics. The experiment uses a concave mirror operating at the radius point with a small lens to re-collimate the light onto the wavefront sensor. It is used to test large (300 mm) fused silica wafers in double pass transmission. The optic under test is placed in the intermediate path near the large return mirror. The aberrations of the large mirror, beam splitter and other optics are subtracted by recording a reference set of focal spot on the SHWFS without the wafer. The wavefront error for some of these wafers is nearly 100 waves, yet we are able to make accurate measurements with the wavefront sensor by selecting a sensor with the appropriate combination of focal length and lenslet diameter. The special sensor that we developed uses a megapixel camera with an arrangement of 100 X 100 lenslets. This sensor could achieve several hundred waves of dynamic range with better than λ/20 accuracy. Additional wafer thickness measurements that were made at NIST with the XCALIBIR interferometer corroborate the SHWFS results.

A spatial light modulator, which is capable of high-resolution wavefront compensation and high accuracy beam steering, has been demonstrated using a Liquid Crystal On Silicon (LCOS) microdisplay with 1024×768 XGA resolution. When the device is used as a wavefront corrector, about 18.7 waves (peak-to-valley at 632.8nm) of aberration in the optical system is corrected to a residual of 1/9 wave (peak to valley) or 1/30 wave rms. Measurement of the far field beam profile confirmed the strehl ratio improved from 0.006 with the wavefront correction off, to a strehl ratio of 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. We demonstrated we can steer the beam continuously in the range of ±4 mrad in X-Y plane, with a steering accuracy better than 10μrad, or about 1/10 the diffraction limited beam divergence. The quality of the steered beam remains very high during the steering as the ellipticity of beam is smaller than 1±0.04, focused beam waist is 1.3x the diffraction limited beam waist and strehl ratio remains higher than 0.66. The 1-D beam steering efficiency is 80% at the maximum steering angle of 4 mrad, which agrees very well with our Finite Difference Time Domain (FDTD) simulation result of diffraction efficiency 86% at maximum steering angle. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency.

The paper is devoted to investigating the ways of development of the lightweight mirrors with imperfect optical quality but still suitable for being used in the telescopes with the primary mirror's aberration correction. The work involves the studies aimed at creation of the lightweight membrane mirrors with satisfactory optical quality and a suitable value of the F-number of around 1. As the first step, we demonstrate the possibility to control parameters of a pre-shaped membrane mirror by means of electrostatic field. It is shown that both the spherical component of the shape of the membrane mirror and its astigmatism can be controlled using the proposed approach.

In a one-dimensional liquid crystal optical phased array (LCOPA), a liquid crystal layer is electrically addressed by an array of long, narrow electrodes. A spatially periodic voltage profile can be applied to the electrodes in order to induce a sawtooth-shaped index of refraction variation in the liquid crystal layer that will steer an optical beam in a fashion analogous to that of a blazed diffraction grating. Because of non-ideal device behavior, measured phase vs. voltage data cannot be used to predict the control voltages necessary to achieve efficient steering. This paper presents a simple application of optimization to determine the appropriate voltages for every electrode in order to optimize the steering efficiency. Experimental results show that this approach can quickly determine optimal voltages for a desired far field diffraction pattern. Steering efficiency improvements of over 100 percent are obtained as compared to open loop device calibration.

Programmable diffractive optics utilizing a high-resolution liquid-crystal phase modulator is demonstrated as a technique for large-range, two-dimensional aberration control. A high-resolution phase modulator system introduces modulo-2π phase compensation via discrete-step phase modulation and operates with 307,200 independently addressable elements, and total optical efficiencies of up to 93%. Near-diffraction-limited imaging and beam directing is demonstrated in a telescope system with large off-axis aberrations. Wavelength-agile and extended spectral bandwidth operation are also demonstrated.

Super-heterodyne array interferometry enhances the ability of a laser-based system to unambiguously measure surfaces with features that are much deeper than the optical wavelength. Super-heterodyning combines measurements of the surface made at different optical wavelengths to effectively create synthetic wavelengths that can be much larger than the optical regime. The additional information obtained from a set of measurements of the same surface taken at different wavelengths is used by a phase unwrapping algorithm to automatically reconstruct the surface phase profile without phase jumps. We describe the development of an automated heterodyne interferometry technique with scalable dynamic range for measuring the surface profile of surfaces with large deviations and step features. Combined concepts from phase shifting interferometry, super-heterodyne imaging, and multiple-wavelength holography are reviewed. A phase unwrapping algorithm is also described that is useful in the context of applications such as adaptive and deployable optics or automated inspection, that may require automatic effective wavelength selection for adaptive resolution in the measurement of surface deviation. Extension to fully automated surface measurement is discussed.

Conventional beam steering for optical sensors is mechanically complex, consisting of a large number of optical and mechanical components. Steering the field of view requires large and heavy gimbal components. One approach to non-mechanical beam steering consists of a phase-modulated optical fiber array. Agile beam steering of optical radiation using fiber arrays offer significant advantages, such as weight, stability, speed and power requirements, over conventional beam steering systems based on large optics telescopes and gimbals. This paper represents an analysis of an agile beam steering system incorporating a fiber array and phase modulator to continually track a moving object. Expressions and modeling of the far-field beam pattern and off-axis beam steering efficiency are presented.