An overview of modeling, analysis and demonstrations of optical wavefront control implemented with a reconfigurable diffractive optic is presented. This includes, analytic modeling of the wavelength-dependent diffraction efficiency and wavefront errors associated with modulo-Nλ₀ optical path control and analysis of the effects of discrete-element addressing and fill factor on diffraction efficiency. More than 200 waves of 2-D wavefront control are demonstrated with a 640x480-element optical path modulator. Wavefront steering together with field-angle-dependent aberration compensation is demonstrated as a technique for acquiring extended field-of-regard mosaic images. The effects of diffraction phenomena on image quality are analyzed.

A scalable wavefront control approach based upon proven liquid crystal (LC) spatial light modulator (SLM) technology was extended for potential use in high-energy near-infrared laser applications. With use of an ultra-low absorption transparent conductor in the LC SLM and materials with better physical properties, the laser power handling capability of the device was improved. The experimental results are reported regarding a LC SLM functioning as a wavefront control device under illumination of a kilo-watt laser source. Compared to conventional deformable mirrors, this non-mechanical wavefront control approach offers substantial improvements in speed (bandwidth), resolution, power consumption and system weight/volume, and the zero-coupling between pixels enables a fast feed-forward wavefront correction scheme.

We have developed a hybrid liquid-crystal spatial light modulator (LC-SLM) in which a reflection-type optically addressed (OA) LC-SLM is combined with a liquid crystal display (LCD) via coupling optics. The LCD is controlled by 8-bit video signals. This construction allows us to eliminate diffraction artifacts due to the pixellated structure of the LCD from the modulated light of the hybrid LC-SLM and enables the hybrid device to be electrically addressed. Nematic liquid crystal molecules in the OA LC-SLM are homogeneously aligned to create pure phase modulation having a variation of one wavelength. These features make the hybrid LC-SLM suitable for wavefront control. Wavefront control with large phase stroke and high stability is desirable to realize high-quality adaptive optics, high-quality optical manipulation, and so forth. Therefore, we experimentally investigated the stroke and stability of the phase modulation of the hybrid LC-SLM. We used the wrapped-phase representation to expand the phase stroke virtually. The results show that the hybrid LC-SLM could produce a phase stroke of more than 20 wavelengths and a phase instability of less than 0.001 wavelengths. We also conducted an experiment to compensate for the nonlinearity of the phase modulation. The results suggest that nonlinearity could be suppressed to less than 1%, and that approximately 200 gray levels over one wavelength of phase variation were available, even after compensation.

Multi-segment liquid crystal phased arrays have been demonstrated as adaptive optics elements for correction of atmospheric turbulence. High speed dual-frequency nematic liquid crystal has sufficient bandwidth to keep up with moderate atmospheric Greenwood frequencies. However the segmented piston correction only spatial nature of the devices requires novel approaches to control algorithms especially when used with Shack-Hartmann wave front sensors. In this presentation we explore approaches and their effects on closed loop Strehl ratios. A Zernike modal based approach has produced the best results. The presentation will contain results from experiments with a Meadowlark optics liquid crystal device.

This paper reports on the measurement and reconstruction using two algorithms of an Adaptive Tilt Mirror's (ATM) reflected wavefront using a Shack-Hartmann (SH) based wavefront sensor. The ATM consists of a deformable mirror mounted onto a fast steering mirror platform. Reconstruction of the wavefront was performed using Finite Difference and Finite Element reconstruction algorithms for comparison. The SH wavefront sensor with high frame-rate readout camera and the two types of software reconstructors provide a visualization of the ATM's surface while being moved physically on the fast steering mirror platform at a rate of 30 Hz.

Presented in this paper is the development of a 4096 element continuous membrane deformable mirror under development for the Gemini Planet Imaging instrument designed for extra solar planet detection. This deformable mirror will enable the next generation of adaptive optics ("Extreme" AO) capable of achieving contrasts of up to 10⁸, required to detect these planets that are obscured by the brightness of its parent star. This surface micromachined MEMS deformable mirror will have an active aperture of 25.2mm consisting of thin silicon membrane mirror supported by an array of 64x64 electrostatic actuators exhibiting no hysteresis and sub-nanometer repeatability. This deformable mirror will be capable of 4μm of stroke, have a surface finish of <10nm RMS with a fill factor of 99.8%, and be capable of frame rates in excess of 2.5kHz. This development effort combines new design features, fabrication processes and packaging methods with those developed for commercially available 1024 and 140 element MEMS deformable mirrors to achieve unprecedented performance and reliability.

Small MEMS (Micro-Electro-Mechanical Systems) deformable mirror (DM) technology is of great interest to the adaptive optics (AO) community. These new, MEMS-DM's are being considered for many conventional AO applications since they posses some advantages over conventional DM's. The MEMS-DM technology is driven by the expectation of achieving improved performance with lower costs, low electrical power, high number of actuators, high production rates, and large reductions in structural mass and volume. In addition to the imaging community, the directed energy community is also interested in taking advantages of some of the characteristics which MEMS-DM's offer. The Air Force Research Laboratory has undertaken the challenge of developing and analyzing several continuous face-sheet MEMS-DM's designs. This paper proposes a simple controls loop computer model for a typical continuous reflective face-sheet MEMS-DM. There are a variety of MEMS-DM's which are being developed for the Air Force Research Laboratory. Data from a typical continuous reflective face-sheet MEMS-DM has been acquired and analyzed. From this data a model for a typical MEMS-DM device could be developed and verified. This MEMS-DM's data can be compared to previous design models which were developed in previous articles. In addition a feedback, closed adaptive optics loop can be designed and analyzed. The research goal is to get a realistic idea of the behavior and performance of these new, innovative devices.

Multi-Conjugate Adaptive-Optical (MCAO) systems have been proposed as a means of compensating both intensity and phase aberrations in a beam propagating through strong-scintillation environments. Progress made on implementing a MCAO system at the Starfire Optical Range (SOR), Air Force Research Laboratory, Kirtland AFB, is discussed. As a preliminary step toward controlling a two deformable mirror (DM) system, the First-stage Intensity Redistribution Experiment (FIRE) examines one aspect of an MCAO system-control and compensation of wavefront intensity. Two wavefront sensors (WFS) and a single DM are employed for this experiment. One WFS is placed conjugate to the DM while the second WFS is located at a distance which produces a desired Fresnel number for the propagation between the WFSs. The WFS measurements are input to a Gerchberg-Saxton based control algorithm in order to determine the DM commands. The phase pattern introduced by the DM is chosen so propagation along the path between the two WFSs produces a desired intensity profile at the second WFS. The second WFS is also used to determine the accuracy of the intensity redistribution and measure its effects on the wavefront phase. In the next phase of MCAO development, a second DM will be added conjugate to the second WFS in order to correct the remaining phase aberrations. This paper presents the setup and operation for FIRE along with initial laboratory results.

Adaptive optics systems are commonly added onto conventional astronomical telescopes to improve the wavefront quality in the presence of atmospheric turbulence. Recent successes in the development of carbon fiber reinforced polymer telescopes have significantly reduced the weight of meter class telescopes making them portable, however, most adaptive optics systems continue to be constructed on large optical benches. The Navy Prototype Optical Interferometer is developing several 1.4 m portable telescope with internal wavefront correction. As part of this upgrade, a prototype 0.4 m aperture telescope has been constructed and a light weight, compact adaptive optics system is being developed. We present in this paper the design of an adaptive optics system for the lightweight telescope. The key to this system is the incorporation of a compact wavefront correction device and a novel collimation optic within the base of the telescope.

Many high-resolution optical methods are affected by the presence of optical aberrations. These include microscopy, three-dimensional optical data storage and optical micromachining. We investigate the use of adaptive aberration correction applied to these techniques. In particular, it is shown how a deformable membrane mirror can be used to correct the aberrations when focusing deep into a multilayer optical data storage medium for both recording and read-out of data. Aberration correction for optical micromaching deep inside a substrate is also demonstrated.

To compensate for large phase errors at high bandwidth, a dual deformable mirror (DM) architecture is introduced. One DM (the tweeter) handles the high spatial resolution errors with a small stroke, high bandwidth, capability while the other DM (the woofer) corrects the larger but more slowly varying phase errors with a larger stroke capability. An offload control architecture is shown to be very effective with the Kolmogorov turbulence spectrum. The architecture is also shown to be effective when used with a self-referencing interferomenter (SRI) wavefront sensor and an exponential control law for phase unwrapping. Performance is verified in simulation and in an advanced wavefront control testbed facility at the Air Force Research Laboratory.

When compared to a Shack-Hartmann sensor, a CMOS image sharpness sensor has the advantage of reduced complexity in a closed-loop adaptive optics system. It also has the potential to be implemented as a smart sensor using VLSI technology. In this paper, we present a novel adaptive optics testbed that uses a CMOS sharpness imager built in the New Mexico State University (NMSU) Electro-Optics Research Laboratory (EORL). The adaptive optics testbed, which includes a CMOS image quality metric sensor and a 37-channel deformable mirror, has the capability to rapidly compensate higher-order phase aberrations. An experimental performance comparison of the pinhole image sharpness feedback method and the CMOS imager is presented. The experimental data shows that the CMOS sharpness imager works well in a closed-loop adaptive optics system. Its overall performance is better than that of the pinhole method, and it has a fast response time.

This paper discusses the application of adaptive control methods in the Atmospheric Simulation and Adaptiveoptics Laboratory Testbed at the Starfire Optical Range at the Air Force Research Laboratory, Kirtland AFB. Adaptive compensation is useful in adaptive optics applications where the wavefronts vary significantly from one frame to the next or where wind velocities and the strength of atmospheric turbulence change rapidly, rendering classical fixed-gain reconstruction algorithms far from optimal. The experimental results illustrate the capability of the adaptive control scheme to increase Strehl ratios and reduce jitter.

The self-referencing interferometer (SRI) wavefront sensor (WFS) is being developed for applications requiring laser propagation in strong scintillation. Because it directly measures the optical field of the wavefront, the SRI WFS is less effected by scintillation than conventional WFSs. This feature also means the phase determined from the WFS measurements is limited to the range -π to π, due to the use of the arctangent function. If a segmented wavefront corrector is used, this constraint is not a problem. However, if a continuous facesheet deformable mirror is used, the resulting phase should be unwrapped in order to minimize fitting error. There are a couple of places in the adaptiveoptical (AO) closed-loop control process where an unwrapping algorithm can be inserted. Simulations of these configurations have shown that how and where the unwrapping is carried out affects overall AO performance and loop stability. This paper presents an overview of the unwrapping options and the associated issues. A laboratory demonstration of two control loop configurations was carried out to test the validity of the simulation results. These experiments and their outcome are discussed.

We demonstrate, for the first time to our knowledge, successful beam control of a fiber optic phased array containing a large number of polarization maintaining fibers. As many as forty-eight fibers have been coherently combined via individual all-fiber phase modulators. The residual phase error is less than 1/30th of a wave. Results with both near-field interferometric control and target-in-the-loop control have been obtained. Experimental results are compared with numerical simulations and excellent agreement has been achieved. We investigated propagation of this phased array output through a turbulent atmosphere, and used the all-fiber phase modulators for the compensation of turbulence effects on the array output. This work paves the way towards scaling such fiber optic phased arrays to very high fiber count. Eventually thousand of fibers can be controlled via such a scheme.

For fast wavefront correction based on coherence-gated wavefront sensing (CGWS) photon and speckle noise determine how well the wavefront can be corrected and, thus, how much resolution, signal and contrast of multi-photon microscopy can be improved. We investigate numerically the performance of a virtual Shack-Hartmann sensor (vSHS), which can be used to reconstruct the wavefront in CGWS, taking into account photon noise and speckle noise.

Interferometric techniques are attractive in wavefront sensing because they give a direct measure of the phase, which means they are useful for use with a piston-only wavefront corrector (such as a liquid crystal spatial light modulator, or some MEMS mirrors). We describe a novel method of implementing a common-path phase-shifting point diffraction interferometric wavefront sensor. The sensor simultaneously gives two phase-shifted outputs which can be used to drive a phase-only wavefront corrector. The device can also give a null output which can be used to calibrate any scintillation.

Based on the Talbot self-imaging principle, a diffraction-based wavefront sensor, the Z-View^TM wavefront sensor, has been developed at Ophthonix Inc. According to the Talbot effect, a periodic grating can be self-imaged at certain distances behind the grating, commonly known as Talbot distances, without the aid of any imaging device. The fidelity of the Talbot image to the grating pattern is affected by the wavefront aberration in the illumination beam. Therefore, the wavefront distortion can be retrieved through numerical analysis of the Talbot image. Unlike the well-known Shack-Hartmann wavefront sensor, where a group of pixels on the camera is responsible for only one wavefront data point, each camera pixel in the Z-View wavefront sensor has a corresponding wavefront data. The Z-View wavefront sensor measures the wavefront at 1024 x 1048 data points, and can achieve a dynamic range of wavefront curvature of 20 diopters. The Z-View wavefront sensor has been successfully used for wavefront sensing in ophthalmic aberrometry, adaptive optics, and lensometry at Ophthonix.

Over the last few years the Starfire Optical Range, Air Force Research Laboratory, Kirtland AFB, has been developing the self-referencing interferometer (SRI) wave front sensor (WFS). The objective of this project has been to demonstrate and evaluate the performance advantages the SRI WFS provides over conventional WFSs, particularly in applications requiring laser propagation in strong scintillation. The initial SRI prototypes relied on a temporal phase shifting approach to produce and capture the required interference images for wave front reconstruction. This approach simplified the initial development by minimizing issues related to detector calibration and the co-alignment of beams. In this paper we discuss the next step in our SRI development efforts and present the design of a spatial phase shifting SRI WFS. The design allows all four interference images- with respective phase shifts of 0, π/2, π, and 3π/2 between the reference and signal beams- to be captured simultaneously on a single camera. Initial results from a laboratory demonstration of the design are shown.

High-resolution and large-range wavefront aberration sensing and analysis present the principal challenges to wavefront control. Innovative alternatives to present wavefront sensing approaches are necessary to address these challenges. Wavelet-Based Wavefront Control is being developed to sense wavefront aberration using wavelet-based phase determination and to cancel phase distortion by means of adaptive wavelet echo cancellation. This approach has the potential of overcoming many hardware limitations while increasing sensitivity to wavefront distortion. Wavefront sensing of a distorted beam is achieved by an interferometer generating an interferogram. The phase properties of the Morlet wavelet transform are used to compute the phase of the 1-D signal taken from the 2-D interferogram. Assembled corrected 1-D signals can then be assembled as a corrected 2-D wavefront. For distortion cancellation, the phase determined above has two parts: signal and distortion. The signal needs to be reproduced while suppressing the distortion in a manner analogous to echo cancellation. The distorted signal can be decomposed into high and low subbands. High frequencies will be applied to cancel random noise in the distorted signal. Distorted fringes are broadened. Further subband decompositions will be carried out and a series of thresholds applied to produce the corrected 1-D signal.

One of the simplest implementations of adaptive optics requires an adaptive correction element and a single pinhole photodetector. Aberration measurement is performed by the sequential application of chosen aberrations using the correction element and appropriate processing of the corresponding photodetector intensity measurements in order to maximise the detector signal. These wave front sensorless adaptive optics systems have been demonstrated in many applications, which have included confocal microscopy, intra-cavity aberration correction for lasers, fibre coupling and optical trapping. The maximisation procedure, the choice of the applied aberrations and the processing of the intensity measurements must be optimised if the system is to work efficiently. In many practical systems aberrations can be accurately represented by a small number of orthogonal modes. We present a model of such systems and show that they have properties that facilitate algorithm design, in particular a well defined maximum and spherical symmetry. Through mathematical reasoning these properties can be used to calculate optimum parameters, rather than obtaining them in an empirical manner. The model leads to a direct maximisation algorithm that has much better convergence properties than search algorithms and permits the measurement of N modes with only N+1 intensity measurements. We also describe a general scheme for such wave front sensorless algorithms and relate various methods to this general scheme.

Phase-only liquid-crystal spatial light modulators can provide a powerful means of wavefront control. With high resolution and diffractive (modulo 2 π) operation, they can accurately represent phase maps with a large dynamic range. Because of this, they are an excellent means of producing electrically controllable, dynamic, and repeatable aberrations. However, proper calibration is critical in achieving accurate phase maps. Consequently, several calibration methods from previous literature were considered. With simplicity and accuracy in mind, we selected one method for each type of necessary calibration and augmented one of the methods with a new step. Then using our preferred calibration methods, we evaluated the performance of the spatial light modulator in the laboratory. We studied various phase maps using interferometry and atmospheric aberrations using a Shack- Hartmann wavefront sensor. All of these measurements show good agreement with theoretical expectations.

We have developed a novel system that emulates the optical effects of bulk atmospheric turbulence in a dynamic, repeatable, and accurate way without moving parts. Such turbulence-emulating systems (TES) are necessary for testing laser systems including laser weapons, free-space optical communications, and atmospheric imaging systems. Most current TESs utilize the layered turbulence model with static phase plates or diffractive optics acting as the turbulent layers. Until now, the only way to emulate bulk turbulence in a laboratory has been by creating real turbulence with a heating element and a fan contained in a miniature wind tunnel. In contrast, the TES that we developed uses phase retrieval-based wavefront control to shape a laser beam into a turbulence-distorted beam. Several important properties of the measured irradiance patterns have shown good agreement with the theoretical expectations.

The production of atmospheric-like turbulence is important to evaluate adaptive optics performance. Several techniques are commonly used to produce atmospheric-like turbulence in the laboratory, such as heating airflow, using diffractive and refractive optical components, and the atmospheric turbulence phase plate; however, the atmospheric turbulence phase plate has the advantage of low cost and easy control of the turbulence property. The properties of phase plates have been studied by many authors; however, a practical methodology to characterize the turbulence strength of the phase plate has not been discussed. The purpose of this paper is to propose a practical methodology to characterize the turbulence strength of the phase plate. In order to develop the methodology, a theoretical model of the optical system with the phase plate is derived first and the turbulence strength metric, which is in terms of the incident beam size versus D/r₀, is obtained based on a comparison of theoretical and experimental data.

A one meter diameter prototype lightweight composite mirror is being developed. The mandrel on which the composite mirror will be laid up is an ultra low expansion quartz glass, TSG, whose thermal expansion coefficient, 10^-7/°C or less, is similar to that for the composite material. A precise layup procedure for the composite material is required to obtain this coefficient of expansion. The mandrel surface will be superpolished to 6-8 Å rms or better, resulting in ten times less scattered light in the visible region than displayed by a typical astronomical mirror. It has been shown experimentally that mandrel microroughnesses of this order can be successfully transferred to a replicated composite mirror faceplate. The composite faceplate is one to three millimeters thick, very tough, and unlike thin glass adaptive optic faceplates does not fracture easily. Actuators designed for atmospheric correction of the thinner adaptive optic faceplates have a response time of 1/2 msec, which is fast enough to correct for all atmospheric distortion. The thicker faceplates are used for active mirrors and are mounted differently. Controlled by stepper motors they can be used for computer controlled remote focusing over long distances or fine tuning of mirror tip-tilt as well as for gravitational sag of other mirrors in the optical train. All mirror actuators operate in the 30-70 v range.

The portability of meter-class telescopes has been limited by the weight of the mirror, tube assembly and the mount required to provide pointing and tracking. The novel lightweight carbon fiber reinforced polymer telescopes being developed for array population at the Naval Prototype Optical Interferometer are orders of magnitude lighter than traditional telescopes. When combined with a lightweight carbon fiber mount, these telescopes will be easily transportable from one telescope station to another to change the interferometer baseline. The mount for a lightweight telescope is currently under development at Composite Mirror Applications, Inc. This paper reports on the design constraints of the mount, the scalability to larger aperture telescopes and the integration of sensors to measure the performance characteristics of this system during operation.

The task of delivering sufficient level of airborne laser energy to ground based targets is of high interest. To overcome the degradation in beam quality induced by atmospheric turbulence, it is necessary to measure and compensate for the phase distortions in the wavefront. Since, in general, there will not be a cooperative beacon present, an artificial laser beacon is used for this purpose. In many cases of practical interest, beacons created by scattering light from a surface in the scene are anisoplanatic, and as a result provide poor beam compensation results when conventional adaptive optics systems are used. In this paper we present three approaches for beacon creation in a down-looking scenario. In the first approach we probe whole volume of the atmosphere between transmitter and the target. In this case the beacon is created by scattering an initially focused beam from the surface of the target. The second approach describes generation of an uncompensated Rayleigh beacon at some intermediate distance between the transmitter and the target. This method allows compensation for only part of the atmospheric path, which in some cases provides sufficient performance. Lastly, we present a novel technique of "bootstrap" beacon generation that allows achieving dynamic wavefront compensation. In this approach a series of compensated beacons is created along the optical path, with the goal of providing a physically smaller beacon at the target plane. The performance of these techniques is evaluated by using the average Strehl ratio and the radially averaged intensity of the beam falling on the target plane. Simulation results show that under most turbulence conditions of practical interest the novel "bootstrap" technique provides better power in the bucket in comparison with the other two techniques.

High contrast imaging from space must overcome photon noise of the diffracted star light and scattered light from optical components defects. The very high contrast required (up to 10^-10 for terrestrial planets) puts severe requirements on the wavefront control system, as the achievable contrast is limited by the quality of the wavefront. The "Peak-a-boo" correction algorithm, presented here, is a closed loop correction method for the shaped pupil coronagraph to minimize the energy in a pre-defined region in the image where terrestrial planets would be found. The reconstruction part uses three intensity measurements in the image plane with a pinhole added to the shaped pupil for diversity. This method has been shown in simulations to converge to the nominal contrast in 2-3 iterations. In addition, the "peak-a-boo" has shown to be effective in broadband conditions.

The broadband performance of the high-contrast imaging testbed (HCIT) at JPL is investigated through optical modeling and simulations. The analytical tool is an optical simulation algorithm developed by combining the HCIT's optical model with a speckle-nulling algorithm that operates directly on coronagraphic images, an algorithm identical to the one currently being used on the HCIT to actively suppress scattered light via a deformable mirror. It is capable of performing full three-dimensional end-to-end near-field diffraction analysis on the HCIT's optical system. By conducting speckle-nulling optimization, we clarify the HCIT's capability and limitations in terms of its broadband contrast performance under various realistic conditions. Considered cases include non-ideal occulting masks, such as a mask with optical density and wavelength dependent parasitic phase-delay errors (i.e., a not band-limited occulting mask) and the one with an optical-density profile corresponding to a measured, non-standard profile, as well as the independently measured phase errors of all optics. Most of the information gathered on the HCIT's optical components through measurement and characterization over the last several years at JPL has been used in this analysis to make the predictions as accurate as possible. The best contrast values predicted so far by our simulations obtainable on the HCIT illuminated with a broadband light having a bandwidth of 80nm and centered at 800nm wavelength are C_m=1.1x10^-8 (mean) and C₄=4.9x10^-8 (at 4λ/D), respectively. In this paper we report our preliminary findings about the broadband light performance of the HCIT.

We present a clinically deployable adaptive optics scanning laser ophthalmoscope (AOSLO) that features micro-electro-mechanical (MEMS) deformable mirror (DM) based adaptive optics (AO) and low coherent light sources. With the miniaturized optical aperture of a μDMS-Multi^TM MEMS DM (Boston Micromachines Corporation, Watertown, MA), we were able to develop a compact and robust AOSLO optical system that occupies a 50 cm X 50 cm area on a mobile optical table. We introduced low coherent light sources, which are superluminescent laser diodes (SLD) at 680 nm with 9 nm bandwidth and 840 nm with 50 nm bandwidth, in confocal scanning ophthalmoscopy to eliminate interference artifacts in the images. We selected a photo multiplier tube (PMT) for photon signal detection and designed low noise video signal conditioning circuits. We employed an acoustic-optical (AOM) spatial light modulator to modulate the light beam so that we could avoid unnecessary exposure to the retina or project a specific stimulus pattern onto the retina. The MEMS DM based AO system demonstrated robust performance. The use of low coherent light sources effectively mitigated the interference artifacts in the images and yielded high-fidelity retinal images of contiguous cone mosaic. We imaged patients with inherited retinal degenerations including cone-rod dystrophy (CRD) and retinitis pigmentosa (RP). We have produced high-fidelity, real-time, microscopic views of the living human retina for healthy and diseased eyes.