High power lasers often suffer from phase aberrations due to spatially non-uniform temperature distributions in optical elements. Adaptive optics has been established to provide engineering control over phase aberrations, but it has been mostly applied to astronomy and has been too costly to use in commercial lasers. The goal of our research has been to develop low-cost adaptive optics systems and components for laser aberration compensation. We present here a summary of our adaptive optics work and our vision of the future of adaptive optics.

Manufacturing of optical quality micromachined deformable mirrors for use in adaptive optic (AO) correction is described. Several non-standard manufacturing techniques have been developed to improve optical quality of surface micromachined mirrors. Two challenges to manufacturing optical quality micromachined mirrors are reducing surface roughness and increasing reflectivity. A chemo-mechanical polishing process has been used to improve surface quality of the mirrors, and a gold coating process has been developed to improve the reflectivity without introducing a significant amount of stress in the mirror membrane. Surface reflectivity and topography measurements of optically flat and smooth mirrors are presented. Based on these results, a new 1024 actuator mirror has been designed and is currently being fabricated. Design considerations and performance expectations for this mirror will be presented.

Two approaches toward the fabrication of a spatial light modulator are presented. The first approach uses a pixelated nitride membrane that is suspended by a grid structure over an electrode array. Deformation of each membrane segment is achieved by means of electrostatic attraction. By using a continuous reflective surface, we achieve a 100% optical fill factor. Since we can coat with any metal or combination of materials, these devices can, in principle, handle high optical loads over a wide spectral range. We have tried different spacer materials and dimensions with this approach. In the second approach, the membrane rests on a viscoelastic carrier layer. During fabrication we use the thin nitride membrane to ensure uniformity of the elastic layer after which it is used as an etch stop for the bulk silicon etchant. This type of device is more robust, can use smaller pixel sizes, but has less sensitivity at low voltages. To achieve an optimal, but simple fabrication procedure, the membranes and electronics are fabricated in different processes, ensuring a higher optical quality of the membrane and increasing yield at lower costs. Applications lie in the field of projection displays, optical lithography, optical communication networks, etc.

Micromachined electrostatic membrane mirrors have been in use for several years in active and adaptive optics applications. We introduce a deformable mirror with small static aberrations and resistant to damage during electrostatic snap-down in an architecture designed so that a multi-layer dielectric coatings that can be applied before the mirror is released. We also make laboratory performance measurements and produce a closed loop control algorithm.

Deformable mirrors have been fabricated using microelectromechanical system (MEMS) technology. The mirrors have been integrated into an optical test bed capable of generating static and dynamic aberrations in the beam path. It was found that the DM could be used to improve optical system resolution in the presence of static aberrations. Strehl ratio was measured for the optical system under four test conditions. A Strehl ratio of 0.81 was obtained for the case in which an introduced aberration was compensated by the DM, compared to a Strehl ratio of 0.45 for case in which the aberration was uncompensated and the DM was removed from the optical path. A parallel stochastic gradient descent approach was used for control.

The idea of using liquid crystal as adaptive optics components has been proposed by several authors. In recent years a vigorous research effort has been carried out, and it is still flourishing, in several countries. Mainly the research and experimental work has been concentrated in US, U.K. and Russia. There are several reasons why liquid crystal may represent a valid alternative to the traditional deformable mirror technology that has been used for the past two decades or so. The main attractiveness of LC is resides in the cost. Current deformable mirror technology has a range of price going from $2K to $15K per channel. LC technology promises to be at least a couple of orders of magnitude cheaper. Other reasons are connected with reliability, low power consumption and with a huge technological momentum based on a wide variety of industrial applications. In this paper I present some of the experimental results of a 5 years, on going, research effort at the Air Force Research Lab. Most of the work has been on the development of suitable devices with extremely high optical quality, individually addressable pixels, fast switching time. The bulk of the work has been concentrated in the arena of the untwisted nematic material. However new devices are now under development using dual-frequency nematic material and high tilt angle ferroelectric material.

Phase multiplexing for recording holograms is usually made with the help of digital Hadamard matrices, which have the particularity to be mutually orthogonal. We propose a phase coding for multiplexed holographic memories where the reference uses orthogonal Zernike polynomials generated with the help of a deformable membrane mirror. This membrane mirror is made of silicon nitride and coated with aluminum. It is fifteen millimeters large, less than a micrometer thick and electrostatically activated by thirty-seven electrodes disposed hexagonally. The first fifteen Zernike modes can be efficiently generated by this configuration of electrodes, which allows writing holograms for each of these modes without cross talk. The Zernike modes are measured with the help of a Shack-Hartmann sensor. The pages of data are composed by an LCD spatial light modulator (LCTV with 640x480 pixels) and undergo an optical Fourier transform before being holographically recorded in a photorefractive crystal (Iron doped lithium-niobate) with the phase coded reference beam. The deformable membrane mirror has a frequency response of up to 1 kHz, and therefore the pages of data can be retrieved up to this speed, if the receiving CCD camera is fast enough. Theoretical considerations and experimental results will be presented.

We describe the performance of a bulk micromachined deformable mirror coated with a dielectric stack for adaptive optics applications with high power lasers. A reflectance of greater than 99.9% was measured and the mirror had a residual static aberration of less than 90 nm rms primarily in astigmatism. A thermally induced distortion of 71 nm RMS was observed for an incident intensity of 300 W/cm² and an average power of 57 W. The multi-layer coated deformable mirror survived over half a billion cycles without degradation and survived for 30 hours with 36 W cw 1064 nm laser light. In addition, the thermally induced distortion with 22 W of average laser power (350 W/cm²) was reduced from 88 nm to 31 nm rms.

We present a 1xN switch for single mode fiber optical communication systems, which is composed of an array of fibers, an achromatic lens, and an adaptive membrane mirror. The working principle of the optical switch is as follows: the center fiber of the array delivers the input signal, this signal is collimated by the lens, back reflected on the membrane mirror and refocused by the lens to an other fiber. The addressing of the receiving fiber is made by lateral displacement of the lens. However, using the achromatic lens under off-axis conditions introduces aberrations, which cause coupling losses to the receiving single-mode fibers. The deformable membrane mirror is used to adaptively correct these aberrations. The optimization of the coupling efficiency is made with the help of a genetic algorithm. For each position of the lens, the optimized voltages on the electrodes of the membrane mirror can be stored during the calibration procedure and afterwards recalled during operation of the switch. A demonstrator has been set up with a commercially available linear array of 32 single-mode fibers disposed in V-grooves, an achromatic lens mounted on a two-dimensional translation stage, and a membrane mirror made of silicon nitride coated with aluminum and electro-statically activated by thirty-seven electrodes. To demonstrate the capabilities of the aberration correction we used the first fiber in the array as input fiber and optimized the coupling efficiency to all the other fibers in the array. We obtained insertion losses of less than 3 dB and a cross talk below 30 dB. These results prove the feasibility to build a switch with a two-dimensional array of more than 1000 addressable fibers.

A sub-millisecond wavefront sensor based on a non-linear Zernike filter is described. The sensor's key element is an optically addressed spatial light modulator (OASLM). The OASLM is composed of ferroelectric liquid crystal and an amorphous silicon carbide photo-conducting film. The OASLM is placed into the focal plane of a Fourier filtering system. The sensor operates with circularly polarized input light. For the high intensity (zero order) spectral component the deviation of the ferroelectric liquid crystal director by (pi) /4 (45 degree(s)) results in a (pi) /2 phase shift. This provides wavefront phase distortion visualization similar to a conventional Zernike filter. The registered characteristic response time of the sensor is near 0.2 ms for light intensity on the order of 100 nW.

In this paper we demonstrate a closed-loop adaptive-optics system that uses a twisted nematic liquid-crystal television (LCTV) as an adaptive Shack-Hartmann wave front sensor (SHWS). This system writes a dynamic lenslet array onto the LCTV so that each subaperture generates a focal spot at the focal length of the lenslet array. The focal spots of the lenslet array are detected by a video CCD camera. The focal spots move around if turbulence exists in the system, therefore the locations of the distorted focal spots are computed using a centroid algorithm and used to correct the local tilted wave fronts. Using the centroid shift data from all of the subapertures of the lenslet array, the incident wave front can be calculated and the phase can be reconstructed. In this experiment we assume that the slope of the disturbed wave front in a subaperture is the spatially averaged wave front tilt, and a correcting LCTV uses a simplifying linear approximation to generate a compensating tilt at each subaperture.

We report on an integrated Hartmann wavefront sensor (WFS) using passive-pixel architecture and pixels clustered as position-sensitive detectors for dynamic wavefront analysis. This approach substitutes a conventional imager, such as a CCD or CMOS imager, by a customized detector, thus improving the overall speed performance. CMOS (complementary-metal- oxide-semiconductor) technology enables on-chip integration of several analog and digital circuitry. The sensor performance depends on the feature size of the technology, noise levels, photosensitive elements employed, architecture chosen and reconstruction algorithm.

Three kinds of real-time wave-front reconstruction algorithms used in adaptive optics (AO) systems, the direct- gradient algorithm, the mirror-eigen modal algorithm and the sensor-eigen modal algorithm, were analyzed in this paper. The direct-gradient reconstruction algorithm was compared with other two modal reconstruction algorithms in experiments. The mirror-eigen modal algorithm was based on the modal functions gotten by orthogonalizing the coupling matrix between actuator influence functions of deformable mirror. The sensor-eigen functions were deduced by orthogonalizing the correlation matrix between Hartmann- Shack wave-front sensor and deformable mirror. The functions were used as basic modes in a novel modal reconstruction algorithm. All these three algorithms were tested on a 61- element AO system in atmosphere turbulence. The experiment data proved that the performances of AO system could be improved by using the two modal algorithms compare to those using the direct-gradient algorithm in the same work condition.

In this paper, we present the results of the phase diversity algorithm applied to simulated and laboratory data. We show that the exact amount of defocus distance does not need to be known exactly for phase diversity algorithm. We determine, through simulation, the optimum diversity distance. We compare the aberrations recovered with the phase diversity algorithm and those measured with a Fizeau interferometer using a HeNe laser. The two aberration sets agree with a Strehl of over 0.9. The contrast of the recovered object is found to be 10 times that of the raw image.

We discussed wave phase front estimation method, basing on the distributions of field intensity near aperture plain. Estimation algorithm was proposed.

Influence of tip-tilts and general defocus aberrations on performance of adaptive systems with liquid-crystal phase modulator is analyzed. Main results concern the system with lateral shearing interferometer as phase visualization element. Some general estimation for performance degradation is obtained for this system.

This research involves the design and implementation of a complete line-addressable control system for a 32 X 32 electrostatic piston-actuated micromirror array device. Line addressing reduces the number of control lines from N² to 2N making it possible to design larger arrays and arrays with smaller element sizes. The system utilizes the electromechanical bistability of individual elements to hold arbitrary bistable phase patterns, a technique previously used on tilt arrays. The control system applies pulse width modulated (PWM) signals to the rows and columns of the device to generate a static phase pattern across the array. Three modes of operation are considered and built into the system. The first is the traditional signal scheme which requires the array to be reset before a new pattern can be applied. The second is an original scheme that allows dynamic switching between bistable patterns. The third and final mode considered is an effective voltage ramp across the device by operating above mechanical cutoff. Device characterization and control system testing are conducted on samples from two different foundry processes. The test results showed that the control system was successfully integrated, however individual bistable control was not successfully demonstrated on the micromirror arrays tested. The inability to demonstrate bistable control is attributed to flaws in the device and variations in snap-down voltage with the application of PWM signals below mechanical cutoff. Methods to correct these flaws for a future redesigned line- addressable device are proposed.

Two methods, the spectral and the covariance, are investigated and compared for generating phase screens for modeling the propagation of light through turbulent atmosphere. The spectral method uses the Kolmogorov spectrum which does not define a spectrum below a specified low spatial frequency cutoff. The difficulties of defining the spectrum below the cutoff and the corresponding difficulties this presents for modeling are discussed. Phase screens are shown using the spectral method and have the correct spectrum. However, the phase screens lose accuracy for truncated low spatial frequencies because of a Fourier transform operation. Phase screens are shown that are produced using the covariance method and Kolmogorov structure function for turbulence in the atmosphere. This method avoids taking a Fourier transform but cannot easily model low spatial frequencies below the cutoff for the Kolmogorov spectrum. We show that because the largest powers are in the low spatial frequencies, according to the Kolmogorov spectrum, that truncating the largest eigenvalues has the effect of reducing the low spatial frequencies. The covariance method take longer to compute and is the subject of ongoing investigation.

Well-characterized test conditions are essential for validating the engineering design of an adaptive optical system. A technique for fabricating high-resolution, well- characterized pseudo-random phase plates that addresses this need is described. Among other uses, these phase plates can be used to test adaptive optics systems under controlled conditions. Machining a surface whose relief height is proportional to the desired phase forms a pixellated phase plate. Using Lexitek's Near-Index-Match^TM approach, a sandwich of two materials is formed that produces the desired phase. Phase plates with 20 micron pixels have been fabricated using a 4096 X 4096 pixel grid. Results are presented.

We present a study and preliminary experimental results on the possibility of using an adaptive optics system for reduction of geometrical fluctuations of input laser beam in long baseline interferometric detectors of gravitational waves. Presently used completely passive systems are expected to reduce fluctuations only at a level that, due to coupling of geometrical fluctuations with interferometer asymmetries, impose requirements on interferometer operation which are at the limit of present technology. Active pre-stabilization could reduce fluctuations and relax these requirements, allowing a safer and more robust interferometer operation on the planned time-scale of years of continue data acquisition.

We present performance results from an experimental adaptive system based on a recently proposed adaptive optics control technique -- the decoupled stochastic parallel gradient descent (D-SPGD) optimization technique. The system consists of an opto-electronic Zernike wave front sensor implemented with a 128x128 element LC phase modulator from Boulder Nonlinear Systems and a 127 element LC phase modulator from Meadowlark Optics used as a wave front corrector. Results demonstrate that adaptive wave front correction using the D-SPGD optimization technique can provide efficient compensation of phase distortions.

Focal plane filters are being used to implement wave front sensors in high-resolution adaptive optics systems. We present experimental results obtained with a closed-loop adaptive optics system that uses a differential phase- contrast focal plane filter. The differential filter in our system is implemented using an electrically-addressed liquid-crystal spatial light modulator (LCSLM) operated in a phase-mostly mode. Two additional phase-mostly LCSLM devices are used in our experiment. One is used as a phase screen to introduce simulated turbulence into the propagating input beam, and the other is used as the wave front corrector driven by closed-loop feedback signals from the phase- contrast wave front sensor. This paper describes the system implementation and presents some preliminary experimental results.

This paper describes design and development of a microelectromechanical, micromachined spatial light modulator ((mu) SLM) integrated with complementary metal- oxide semiconductor (CMOS) electronics, for control of optical phase in phase-only optical correlators. The (mu) SLM will consist of a large array of piston-motion MEMS mirror segments (pixels) each of which capable of altering the phase of reflected light by up to one wavelength for infrared (1.5 micrometers ) illumination. Results of a proof-of- concept study are presented along with an electromechanical model and details of the fabrication process for the (mu) SLM.

An adaptive system based on a model-free optimization technique (stochastic parallel gradient descent optimization) is used to correct the aberration in a model of the human eye. The system is composed of a digital camera and a 37 electrode MEMS mirror connected to a computer. Static as well as slowly changing dynamic aberrations are corrected with the system.

In many instances, mechanical vibrations, not atmospherics, are the dominant contributors to time varying optical tilts affecting both astronomical and terrestrial observations. We used a pair of inexpensive micromachined accelerometers placed on the secondary mirror mount of a 12' telescope, inferring angular deviations from twice temporally integrated acceleration signals. We then applied this result with appropriate gain to a feed-forward tip/tilt mirror correction loop with good results.

There is a considerable and growing interest in the use of thin membrane mirrors for optical and infrared systems in space. The possibility of very large, monolithic light-collecting apertures with extremely low areal densities is strong motivation for developing the various technologies that might allow the realization of such systems. During the course of working toward space deployable, large mirror systems, we are also aware of possible ground-based applications of membrane optical elements. This paper briefly discusses some of our recent work and progress with membrane mirror elements, and possible future directions.

The Directed Energy Directorate is developing a large space-based optical membrane telescope. The goal is to develop technologies that will enable 20-meter, or greater, diameter telescopes, with areal densities of less than 1 kilogram per square meter. The challenges include the development of a new material process that dramatically improves the optical quality of available films, choosing a process that is conceivably scalable to these larger diameters, and designing new structural concepts to meet surface accuracy requirements and areal density restrictions. A significant part of the realization of these goals relies on the development of a stress-coated net-shape film. A stress-coated net-shape film is a bilaminate system comprised of a pre-shaped polymer substrate coated with a compressive dielectric coating. This article is restricted to a discussion of surface data information on a 40-centimeter diameter, 10 tm thick, uncoated net-shape film. Passively forming these films to a near final shape (i.e. net-shape) will reduce the force, power, and range burden of the actuation system required to acquire and maintain the optical figure. Additionally, passively maintaining the form of these film structures will reduce the stiffness requirements of the supporting structure. The union of the polymer substrate and dielectric coating is still under development and will be reported on at a later date.

Among the trends of the space-telescope engineering, a considerable attention is paid nowadays to telescopes with linear and nonlinear correction of image distortions and to the lightweight large-clear-aperture primary mirrors (PM). These studies, on the one side, provide a deeper insight into operational characteristics of these telescopes and, on the other, allow us to reveal their limitations and drawbacks, which stimulates us to consider alternative ways of getting images of high quality and thus to create a basis for designing high-resolution telescopes of the next generation. An alternative to the analog process of correction is evidently a digital process. In this paper, we briefly discuss advantages and drawbacks of telescopes with the nonlinear-optical (analog) correction and analyze a possible design of a space telescope with a 'pure' digital correction of image distortions, caused by the lightweight membrane primary mirror of poor quality.

'Dynamic holography is demonstrated as a technique for high- dynamic-range, multi-function laser wavefront control. In this paper, we describe three variations for hologram generation and display. These include all-optical holography for severe aberration compensation, computer-processed holography for high-optical-efficiency severe aberration compensation and computer-generated holography for multi- function laser wavefront control including dynamic tip, tilt, focus and aberration control. A prototype hologram display system operates with total optical efficiencies up to 93% and with refresh rates on the order of 10 Hz. The prototype system has resolution sufficient to introduce about 200 waves of diffractive wavefront control at 532 nm optical wavelength.'

It was found out, that in optically addressed liquid crystal spatial light modulator with the nematic liquid crystal and photoconductor, comprised by p-i-n diode on the base of amorphous hydrogenated silicon, it is possible to record the dynamic diffraction gratings with diffraction efficiency over 50%. The gratings were recorded with the use of S- effect. Possible reasons of the discovered effect are considered in the paper.

In recent experiments of M. Gruneisen et al., the possibility was shown to record the so-called 'blazed' dynamic holograms in optically addressed liquid crystal spatial light modulator (OA LC SLMs). In such holograms the phase retardation distribution across the holographic fringe has a saw-like shape (asymmetrical triangle). Such holograms can provide a very high (in a limit - nearly 100%) diffraction efficiency. In previous experiments such 'blazed' holograms were recorded with the use of digital computer methods. We propose a novel, analogous an completely optical approach to the record of such holograms. One can record a saw-like profile of phase retardation in an OA SLM, placed into the nonlinear interferometer. In such interferometer the probe wave reads out the phase retardation in SLM, interferes with reference wave, and the phase retardation in the said SLM is controlled by their interference pattern. The results of the direct numerical simulation of the system show that by this method can be formed the saw-like grating, providing nearly 100% diffraction efficiency. Presented is the analysis of the system dynamics and stability.

This paper discusses the novel adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time - the frequency of corrected aberrations is less than 25 (30) Hz. The amplitude of such aberrations - about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations - up to 15th Zernike polynomial (excluding tip-tilt).

Bimorph mirrors for laser beam correction and formation were developed and investigated. Different types of substrates and active piezoceramics materials were considered to fabricate temperature independent shape of the mirror surface and to maximize the sensitivity of the mirror. High reflectivity coatings for different wavelengths were studied.

Shack-Hartmann wave-front sensor (SHWS) was designed to measure both intensity distribution and phase distortion of optical fields in real time and high accuracy. It can be widely used not only in measuring, diagnostic, but also in adaptive optical systems to compensate for phase distortions. Various parameters such as peak-to-valley, root-mean square, Zernike coefficients, beam quality (M²) could be calculated with the help of such a sensor. The results of wavefront measurements by using our SHWS are reported in this paper.

When adaptive optical system is applied for laser beam control it is very important to know the beam parameters such as beam width, divergence angle, beam quality factor M² etc. before and after correction. That is why the sensor making such estimations should be included in any laser adaptive optical system. This paper describes the sensor design, possibilities, the principals of measurements and its place in the whole adaptive optical system.