We report on the recent progress in the development membrane micromachined deformable mirrors (MMDM) technology. A MMDM with a membrane diameter of 50 mm developed for an astronomical application has a P-V surface error of 1.8 micrometers within light aperture of 35 mm. The mirror features a maximum surface deflection of 18 micrometers and a response time better than 2 ms. A highly reflective (R approximately 99.8%) MMDM can be continuously operated under a CW laser load of up to 50 W. A special linear configuration of MMDM was recently used to compress a femtosecond laser pulse from 150 fs to 15 fs, also circular MMDM was used to correct wavefront deformations in femtosecond lasers. We also report on the application of the MMDM in a self-referenced adaptive optical system with the loop frequency of 8 Hz and for coupling of pumping radiation in fiber lasers with considerable--up to 48%--improvements of the coupling/luminescence efficiency.

A micromachined deformable mirror ((mu) -DMs) for optical wavefront correction is described. Design and manufacturing approaches for (mu) -DMs are detailed. The (mu) -DM employs a flexible silicon membrane supported by mechanical attachments to an array of electrostatic parallel plate actuators. Devices are fabricated through surface micromachining using polycrystalline silicon thin films. (mu) -DM membranes measuring 2 mm X 2 mm X 2 micrometers , supported by 100 actuators are described. Figures of merit include stroke of 2 micrometers , resolution of 10 nm, and frequency bandwidth DC - 7 kHz. The devices are compact, inexpensive to fabricate, exhibit no hysteresis, and use only a small fraction of the power required for conventional DMs. Performance of an adaptive optics system using a (mu) - DM was characterized in a closed-loop control experiment. Significant reduction in quasi-static wavefront phase error was achieved. Advantages and limitations of (mu) -DMs are described, in relation to conventional adaptive optics systems and to emerging applications of adaptive optics, such as high resolution correction, small aperture systems, and optical communication.

We report on an on-chip projection-type spatial light modulator that consists of a continuous reflective surface formed by a thin stretched silicon nitride membrane supported by a grid structure providing an array of individually accessible deformable pixels. The curvature of each pixel can be controlled electrostatically by changing the potentials on underlying integrated electrodes. This type of modulator can be used directly as a phase modulator (possibly combined with a microlens array) and in dark schlieren projection scheme as a display device. The continuous surface of the membrane provides 100% optical fill factor and, being appropriately coated, can handle high optical loads over a wide spectral range, including the infrared region. The support grid structure and electronics can be scaled so as to achieve a wide range of resolutions with a uniform technology. Integrated control electronics and the deformable pixelated mirror structure are fabricated independently on different wafers, allowing optimized fabrication of each and ensuring higher yield and quality and lower costs.

The use of liquid crystal devices for wavefront control has been suggested and implemented by several authors. Our group has been at the forefront of the development of this technology. In this paper we report some preliminary experimental results on the use of nematic based liquid crystal devices. Several experimental efforts have been carried out in the past few months. One of the main aims was to characterize a new device that uses dual frequency nematic material.

We have designed and constructed a prototype adaptive mirror using a nematic liquid crystal as the optical phase modulating material. The mirror has 127 hexagonal elements (actuators) in a 12 mm diameter clear aperture. The liquid crystal is a dual-frequency type that can be driven both parallel to, and orthogonal to, the external driving field by changing the drive frequency. With the dual-frequency liquid crystal we have achieved 1 micron of optical phase delay with full-cycle switching times of 6 ms. The electronic driver was designed to interface with an IBM compatible PC.

Liquid crystal spatial light modulator technology appropriate for high-resolution wavefront control has recently become commercially available. Some of these devices have several hundred thousand controllable degrees of freedom, more than two orders of magnitude greater than the largest conventional deformable mirror. We will present results of experiments to characterize the optical properties of these devices and to utilize them to correct aberrations in an optical system. We will also present application scenarios for these devices in high-power laser systems.

Opto-electronic feedback systems that consist of electrically addressed high-resolution phase spatial light modulators and large-scale arrays of opto-electronic feedback circuits are analyzed. Experiments were performed with a liquid crystal television panel used as a high- resolution phase modulator, and a CCD camera used as a photo-array. By synthesizing various nonlinearities and using controllable spatial coupling one can design systems capable of high-resolution adaptive phase distortion suppression and the formation of self-organized arrays of localized laser beam structures. High-resolution phase arrays are also used here for opto-electronic image edge detection.

An adaptive laser beam focusing system using a 127 channel liquid crystal phase modulator is presented. The controller for the system is a circuit built with prototype VLSI chips that implement the stochastic parallel gradient descent algorithm. The controller is driven by a scalar laser beam quality metric and can run at the rate of 150 kHz. The system performance is characterized and a secondary control loop manipulating one of the algorithm parameters is experimentally investigated. The performance of the system is reported and performance improvements obtained by using the recent history of the beam quality metric to control an algorithm parameter is demonstrated.

Space debris removal from the earth orbit is getting substantial attention since the International Space Station (ISS) construction started in the orbit. Because of its size and length of operation period, it is considered that the ISS has high probability of being hit by debris. There is a certain range of debris size where its detection and protection are very difficult. This paper describes use of space borne laser system to protect space assets against debris. It is proposed that the system utilizes phase conjugation phenomena (Brillouin Enhanced Four Wave Mixing or BEFWM in short) to focus high power laser radiation to debris with very high accuracy. Basic study has been conducted to investigate energy efficiency, phase conjugation fidelity and phase conjugation dynamics.

Formation of the given intensity distributions of fundamental mode by means of intracavity controlled mirror is discussed. Such mirror is a water-cooled bimorph flexible one having four controlling electrodes. Analysis has confirmed the possibility to form doughnut-like and super- gaussian intensity distributions at the output of the stable resonators of industrial CW CO₂ lasers. Results of experimental formation of the super-gaussian fundamental modes of the 4th and 6th orders by means of the intracavity flexible mirror are presented. The increase of power up to 12% and the enlarging of the peak value of the far-field intensity in 1.6 times in the comparison with the traditional gaussian TEM₀₀ mode of the same resonator are observed.

We demonstrate real time severe large and small scale phase distortion correction of arbitrary coherent and incoherent images, using ferroelectric liquid crystal spatial light modulators to record the dynamic hologram of the phase distortions.

In this paper, we present experimental results of dynamic aberration correction based on gradient descend algorithms. The experimental setup included a 37-actuators piezoelectric deformable mirror to distort dynamically an input laser beam and a 37-element micromachined deformable membrane mirror to correct the resulting wavefront distortions. We generated time-varying aberrations using the first mirror and used the light power focused onto a pinhole as our optimization matrix. We programmed a computer to maximize this metric and control the shape of the micromachined deformable membrane mirror for wavefront correction. We implemented in this computer a simple gradient descend algorithm and a stochastic perturbation gradient descent algorithm. We present experimental data on the convergence and stability of this adaptive system for various conditions of dynamic turbulence.

We have developed a digital image processing system for real-time digital image processing feedback control of adaptive optics systems and simulation of optical image processing algorithms. The system uses multi-computer architecture to capture data from an imaging device such as a charge coupled device camera, process the image data, and control a spatial light-modulator, typically a liquid crystal modulator or a micro-electro mechanical system. The system is a Windows NT Pentium-based system combined with a commercial off-the-shelf peripheral component interconnect bus multi-processor system. The multi-processor is based on the Analog Devices super Harvard architecture computer (SHARC) processor, and field programmable gate arrays (FPGAs). The SHARCs provide a scalable reconfigurable C language-based digital signal processing (DSP) development environment. The FPGAs are typically used as reprogrammable interface controllers designed to integrate several off-the- shelf and custom imagers and light modulators into the system. The FPGAs can also be used in concert with the SHARCs for implementation of application-specific high-speed DSP algorithms.

The luminescence method and the method of dual-beam shear interferometry were used to perform the measurements of spatial and time characteristics of molecular gas subsonic turbulent flow fluctuations under strong vibrational non- equilibrium. Gas-discharge tubes of fast-axial-flow CO₂ laser served as the investigation object. A turbulent component of gas density fluctuations has been separated by method of frequency selection of the registered signal. The spectrum of density pulsation has been determined, as well as the amplitude dependence of small-scale inhomogeneities on energy input has been specified. A degree of cross correlation of fluctuations of wave-front phase incursion in the turbulent flow has been evaluated under non-equilibrium conditions of electric discharge.

In this paper, we describe laboratory experiments and measurements to characterize an OKO Technologies membrane mirror. The measurements are then compared to theoretical calculations.

Further results from the previously reported atmospheric turbulence chamber are discussed in this paper. The generated turbulence has been characterized by evaluating isotropy and homogeneity within the chamber by measuring temperature structure functions for various nozzle pressures and temperature differences. Measurements of higher-order intensity moments were utilized to reconstruct the probability density functions and were compared with Log- Normally Modulated Exponential, Log-Normal and Gamma distributions. The power spectrum of intensity fluctuations of a laser beam propagated through the generated turbulence provided additional characterization of the chamber regarding scale sizes and frequency cut-offs. Fried's parameter (r₀) and the Greenwood frequency of the artificially generated turbulence were computed and were shown that it is possible to simulate the real atmospheric parameters with a modified design of the present chamber.

The results are reported of theoretical and experimental studies of the dependence of diffraction efficiency of hologram-corrector upon an orientation of a diffraction grating vector and of the polarization of the reading-out radiation with respect to the normal to the smectic layers. The gratings recorded using DHF-effect and Clark-Lagerwall effect were studied. The conditions were determined when the dependence of diffraction efficiency vs. radiation polarization is weak. It was found out that in the hologram- corrector, using the polymer photoconductor, the diffraction efficiency is significantly dependent of the vector of grating orientation, while for the hologram-correctors using a-Si:C:H photoconductor this dependence is practically absent.

Large numerical aperture telescope with nonlinear optical correction for distortions, designed for the remote self- luminous object imaging, was realized in experiment and investigated. Dynamic hologram, recorded in optically addressed liquid crystal spatial light modulator, was used as the corrector. Nearly diffraction limited performance of the system was demonstrated.

Two-wavelength holography, when the hologram is recorded at one wavelength and reconstructed at some shifted wavelength, is an efficient tool for many applications. Optically addressed liquid crystal spatial light modulators are very convenient for recording thin dynamic holograms and, in particular, for the purposes of the dynamic two-wavelength holography. On such a basis one can realize the dynamic interferometer, providing the arbitrary scaling of the wave front distortions. Such an interferometer can be of use for solution of some of the tasks of the adaptive optics, namely, for simplification of the procedure of measuring of the robust wavefront distortions, for recording of the dynamic holographic correctors, working in spectral ranges, where the direct holographic record is impossible, in particular, in mid-IR range of spectrum, and for extension of the range of distortions, which can be corrected by means of the phase valve, mounted in the negative optical feedback loop. We report the experimental realization of such an interferometer.

Real-time holography compensates for severe aberrations in membrane-mirror based telescope systems. Laboratory demonstrations in both imaging and beam projection have been conducted. Prototype optically addressed liquid-crystal spatial light modulator devices, developed and adapted for this application, are demonstrated with significantly improved diffraction efficiencies.

Some last ten years the problem of compensation for dynamic distortions of primary mirrors in low-weight large-size imaging telescopes by methods of dynamic holography is actively being investigated. Today the performance of such systems has been experimentally proved. Thus, the idea to use in such a telescopic system a low-weight-film primary mirror, which can, in principle, be of large diameter in an unfolded mode, is very attractive one. In this paper the problems are considered related to integration of a thin- film primary mirror in a telescope with a non-linear corrector for an object image. Namely, there will be discussed the architecture of the telescope with the thin- film primary, optical quality of the mirror, requirements on the non-linear corrector, the telescope's field of view, and quality of the image of an object.

Thin membrane inherently require a certain minimum amount of strain to adequately perform as optical elements. This minimum strain can be established by simultaneously considering the effects of strain on the reflective surface, film thickness variations, and the corrective range of the adaptive optics (AO) scheme. To show how strain and the optimal optical surface are related, 75 and 125-micron thick polyimide films were examined under various strain conditions. Thickness variations were also mapped and correlated. The limits of the AO correction scheme set the films surface topography requirement. Our results will help to partially define an optical quality membrane, which is an important initial step toward the manufacturing of such a film.

The feasibility of forming very thin (approximately 100 um), flexible membranes into low-cost, low-mass, large diameter optical elements is being explored. While spherical or parabolic shapes are the ultimate goal for imaging and other light-gathering applications, there are potential applications for large, planar surfaces. Also, knowledge gained while working with planar membranes is being applied to concave structures. Recent efforts have concentrated on measuring and understanding the behavior of currently available materials. This paper discusses experimental results, and describes measurement techniques and membrane materials used. Highlighted are our most recent results on a 11-inch diameter membrane mirror which we measured to be flat to approximately 0.1 um rms.

The surface of an initially planar membrane, which is subsequently subjected to a pressure difference, can be manipulated into a variety of shapes. This report discusses two methods by which optically desirable deterministic shapes might be achieved. The first involves pre-straining of the membrane, a technique which has already been demonstrated to reduce the spherical aberration in such a mirror. However, near-parabolic shapes at low f-numbers appear not to be achievable with this method, i.e., using pressure differences and pre-strain alone. The second technique is a somewhat novel one involving the use of a plunger to translate the central region of the membrane along the optical axis. Preliminary results suggest that attainment of a near-parabolic shape over a substantial area of the membrane may indeed be possible with this method. The experiments described here use an aluminum coated 125 micron thick polyimide membrane with a clear aperture of 11 inches.

Calculation analysis and experimental measurements are described of the operation of optical phase modulator composed from two DHF (deformed helix ferroelectric) liquid crystal cells, mounted in series. The limit dependence of effective refractive index on molecular tilt angle in helical smectic C* phase is calculated as well as the influence of electric field on effective refractive index for different values of molecular refractive index components and tilt angle. The presence of gray scale in DHF regime is shown for crossed DHF cells and jump in change of effective refractive index was found, though the smooth deviation of average refraction index ellipsoid of single DHF layer was found at change of applied voltage from zero to untwisting value. Ferroelectric liquid crystal mixtures are developed with response times less than 0.5 ms. Developed FLC materials have very large tilt angle about 40 degrees and very short pitch of helix less than 0.2 micrometers . The model phase modulator was composed from two DHF cells mounted in series. The behavior of optical phase shift was investigated in Fizeau interferometer. The gray scale phase modulation only is obtained in accordance with calculations. The limit change of phase retardation was found 0.75 (lambda) in transmission mode between undisturbed and totally untwisted helix for cell thickness 16.5 micrometers . The strong nonlinear behavior of the phase shift versus d.c. electric field was found. The quantitative results agree with the theoretical calculations.

Influence of the input field spatial stationary modulation (roll) on the pattern formation in a nonlinear optical system has been under theoretical and numerical investigation. We show that variation of the input field modulation amplitude dramatically change the spatial dynamics of the system. If the modulation amplitude is large enough stimulated patterns appear in the system output.

Large deployable space-based optical systems will likely require complex structure position controls in conjunction with an adaptive optic to maintain optical tolerances necessary for near diffraction-limited performance. A real- time holographic (RTH) compensation system can greatly reduce the requirements and complexity of the position control system and enable the use of novel or imperfect optical components for large mirror surfaces. A hologram of the distorted primary is recorded with a local beacon at 532 nm (approximately 100 nJ/exposure) on an optically addressed spatial light modulator and transferred as a phase grating to a ferroelectric liquid crystal layer. The hologram is played back with target light containing the same optical distortion. A corrected image is obtained in the conjugate diffracted order where the phase of the optical distortion is subtracted from the distorted image. We report recent test results and analysis of a RTH-compensated deformed mirror of 0.75 m diameter. The short exposure hologram is recorded at video frequencies (30 Hz) at bandwidths up to 5 kHz. Correction for tens of waves of static and dynamic optical distortions including mechanical and thermal warp, mechanical vibration, and air turbulence are shown for monochromatic (532 nm) and broadband (532 +/- 40 nm) illuminated targets.

The Air Force Research Laboratory is developing a large space-based optical membrane telescope. When this research began a little more than three years ago, the conceptual design was based upon a totally inflatable structure. An inflatable structure has been used for space solar power collection and radio-frequency antennas. To place the development of the membrane optical telescope in perspective, a short history of past inflatables will be presented. The totally inflatable lenticular design used in a variety of space-based applications in radio and radar antennae, solar power for propulsion applications and solar shields is of particular interest. Recently, a new version of a membrane telescope has emerged. Thin membranes on the order of 10 to 100 micrometers thick will be packaged and deployed without using inflation to maintain the surface figure. The move away from a pure inflatable is driven by several factors, including wavelength-level tolerances required of optical telescopes, even when real-time holography is invoked as the adaptive optics correction technique. Issues that led us to de-emphasize an inflatable, lenticular design and concentrate on a near-net shape film using stress coatings and dual boundary edge control are discussed.

Thin flexible membranes with curvature are gaining favor as lightweight optical components. The Surveillance Technologies Branch of the Air Force Research Laboratory has demonstrated that membranes can yield a near diffraction limited image when combined with real-time holography as the wavefront correction method. Researchers at the University of Strathclyde in Glasgow, Scotland are using large membrane mirrors in a volumetric imaging project, while others at the Vavilov Laser Physics Institute in Saint Petersburg, Russia are investigating the use of real-time holography to correct membrane mirror aberrations. Existing membranes, however, have not been designed with optical imaging as the intended application. Thus, there is a need to design and construct optical quality membranes.