Adaptive Optics for Industry and Medicine

The following sections are included:

• Sponsors

• Acknowledgements

• Introduction

• Contents

We have developed a closed-loop adaptive optics system based largely on off-the-shelf components. A bimorph-type mirror using low-voltage PZT material is controlled from a 200MHz Pentium PC, using locally written software. The Shack-Hartmann wavefront sensor uses an array of computer-generated zone plates rather than a lenslet array, to allow flexibility in the layout of the sensing apertures. The system currently operates at a frequency of 11 Hz in closed-loop mode, achieving full correction in two cycles. Strehl ratios as high as 0.8 have been measured for the output beam.

The basic arrangement and simulation results of a wavefront correction system for correcting static and dynamic wavefront errors in a new inertial confinement fusion system are reported.

Most large laser systems at Lawrence Livermore National Laboratory require adaptive optics to correct for internal aberrations in these high-power systems. Many of these systems, including those being developed for Inertial Confinement Fusion and Laser Isotope Separation, already contain adaptive optics based on conventional deformable mirror technology. Increasing requirements for laser system applications are currently driving wavefront control technology toward increased spatial frequency capacity, as well as reduced system costs. We will present recent progress in the utilization of liquid crystal spatial light modulator technology appropriate for high-resolution wavefront control of high-power laser systems.

The CO₂ laser has an increasing influence on small and middle-scale companies in the producing industry as well as on large companies like the car industry as a versatile tool, which is free of wear. It is mainly used for two manufacturing processes: Laser beam welding and laser beam cutting. In the following we want to introduce the capability of the adaptive optics of the Diehl's company integrated in laser machines in order to realise a high quality laser beam processing. As a first example a system technique for the compensation of beam path differences in laser beam cutting machines is described, secondly a processing head for laser beam welding combined with a novel sensor for a focus position control is introduced.

A 17-electrode bimorph piezoelectric deformable mirror is used to compensate for the thermal lensing and serious growth defects of a 40-mm diameter Ti:sapphire-crystal, which is used as amplifying medium in the 8-TW TiS-laser ATLAS 10. It was demonstrated that the peak fluence in the near and far field could be reduced by a factor of 3 and at the same time the focusability was enhanced from 3.5 to 2 times diffraction limited.

The Diehl Stiftung & Co. develops and manufactures adaptive mirrors which are used in the beam delivery system of CO₂ laser machines to improve the processing results. With help of two adaptive mirrors the focus diameter and the focus position of the laser beam can be controlled independently. This offers a method to compensate the varying beam path lengths in laser machines with flying optics, as well as a capability to adapt the focus parameters to the processing demands in laser cutting, welding, and scanning systems.

A general description of a Hartmann-Shack sensor to measure the aberrations of the human eye is presented. Different factors affecting its performance are reviewed: the size of the microlenses, the number Zernike modes required to describe the ocular aberrations, the statistical accuracy and the exposure time.

The following sections are included:

• Introduction

• Experimental set-up

• Results

• Discussion

• Acknowledgements

• References

This paper describes a new adaptive optics instrument and associated diagnostic system for volumetric, in vivo imaging of the human lens and visual acuity characterization. The system is designed to allow one to capture simultaneous, in-focus images of the human lens at multiple "image planes." Based on the adaptation of a deformable grating originally developed for atmospheric turbulence measurements, the instrument will demonstrate an improvement over current techniques for imaging cortical, nuclear and posterior subcapsular cataracts. The system will characterize the human lens optically and will automatically produce an estimate of visual function as affected by the measured abnormalities in the lens. The process that Kestrel and DERA Malvern will use to demonstrate the key techniques for simultaneously acquiring in vivo lens imagery at multiple focus planes employs a surrogate lens. Eventually the camera could be considered as a replacement for most standard slit lamp instruments allowing them to be converted into a 3-D imaging system.

We evaluated the potential of using a liquid crystal spatial light modulator with 127 elements in an hexagonal array to correct focussing errors of the human eye. Refractive errors were induced by placing ophthalmic lenses of known power in front of the eye. A HEX-127 (Meadowlark, Inc.) spatial light modulator (SLM) was imaged in the entrance pupil of the eye and programmed to generate refractive power of the same magnitude as the ophthalmic lens but of the opposite sign. When the induced error was no greater than 1.5 diopters, the SLM satisfactorily corrected the induced error and the measured visual acuity was comparable to that when no refractive errors were present. Computations indicated that the SLM failed to correct refractive errors greater than 1.5 diopters because insufficient density of control cells under-sampled the desired wavefront. Consequently, the SLM diffracted significant amounts of energy into the higher orders, resulting in a multi-modal point spread function. The net effect was multiple duplication of the visual target in the retinal image with the overlapping of the duplicates leading to deterioration of visual acuity.

Spatial light modulators (SLMs) have not yet provided diffraction-limited imaging through the human eye's ocular media. To guide future improvements in SLM designs that might enable such imaging, we have modeled the performance of hexagonally- and square-packed configurations of segmented SLMs in conjunction with measured wave aberration data of normal human eyes. The model included the effects of pupil size, number and arrangement of actuators, and mono- and poly-chromatic light. Results indicate that diffraction-limited performance should be attainable for medium sized pupils with existing, commercial SLMs. The required segment density was found to be substantially higher at the pupil's edge than at its center. In polychromatic light, SLM performance at large pupil sizes was found to be limited more by phase wrapping than material dispersion of the corrector. Both, however, were substantially less degrading than the naturally occurring longitudinal chromatic aberration of the human eye.

An optical system used to measure aberrations in human eyes is proposed. A monochromatic light spot illuminates the retina and a Shack-Hartmann wave-front sensor analyses the backscattered light. The wave-front analysis is based on a modal wavefront estimation, using the Zernike polynomials. An adjustable diaphragm allows to change the diameter of the incoming probe-beam. With this experiment, it is shown that one can obtain in vivo a relatively repeatable measurement of the astigmatism and defocus of the eye.

Multi-photon scanning confocal microscopy with extremely short optical pulses (10fs) requires reflective optics, and ultimately requires control of the spatial wavefront in order to achieve diffraction-limited performance. We have demonstrated the use of a deformable mirror to adaptively optimize the focusing of a l0fs pulse in a scanning confocal microscope. We optimize the second harmonic generation or the two photon fluorescence signal from our sample using a feedback loop based on a genetic algorithm. To improve the speed of the convergence of our algorithm we use a Zernike orthogonal basis to control the deformable mirror.

A confocal microscope offers superior imaging compared to a conventional optical microscope. However most confocal microscopes operate in reflection because of the imaging difficulties caused by aberrations induced by the specimen itself. Work at the University of Sydney has focussed recently on overcoming these problems and constructing a practical confocal transmission microscope. We have demonstrated image improvement by removing image motion in real time using a tip-tilt mirror, and in post-processing of CCD images. Other low order aberration correction requires a wavefront sensing technique appropriate to the confocal situation and a device to provide the correction, both of which are also being investigated.

Tactical airborne electro-optical systems are severely constrained by weight, volume, power, and cost. Micro-electrical-mechanical (MEM) adaptive optics provide a solution that addresses the engineering realities without compromising spatial and temporal compensation requirements. Through modeling and analysis, we determined that substantial benefits could be gained for laser designators, ladar, countermeasures, and missile seekers. The development potential exists for improving seeker imagery, extending countermeasures keepout range, improving the range for ladar detection and identification, and compensating for supersonic and hypersonic aircraft boundary layers. Innovative concepts are required for atmospheric path and boundary layer compensation. We have developed designs that perform these tasks using high speed scene-based wavefront sensing, IR aerosol laser guide stars, and extended-object wavefront beacons.

The Active Micro Mirror (AMM) is a new kind of micromachined deformable mirror. It is a silicon mirror which can be electrostatically actuated by electrodes located on each side of the mirror. In this article we describe the characteristics and the results of modelisation and testing of our deformable micro mirror.

New deformable mirror components and designs developed at Trex Enterprises (formerly ThermoTrex) are reviewed. These include long stroke actuators, zero holding-power actuators, wide operating temperature actuators, and simply replaceable facesheets. Systems built with each of these features are shown.

The bend and polish method of manufacture for aspheric optical surfaces, as originally proposed and used by Bernhard Schmidt [1] for the "adaptive" correction of the low order spherical aberration of a spherical mirror in his astrographic camera, and more recently used to manufacture the facets of the Keck 10m telescope primary mirror [2] and other astronomical components [3], has the advantage of producing smoothly continuous aspheric surfaces with fine surface texture. In the classical application, when the method is used to manufacture Schmidt corrector plates, the assumption is that a simply supported circular plate deformed by atmospheric pressure will assume a paraboloic shape. This is an approximation which is adequate for some applications, but needs refinement if it is to be of more general use. This paper describes the precise modelling of the classical case and gives the general requirements and results of rigorous modelling where the aberration correction requirements are more subtle.

We describe the recent progress and intrinsic limitations of the technology of micromachined membrane deformable mirrors (MMDM).

Liquid crystal wavefront correctors have traditionally been used as zonal wavefront correctors, whereby the influence function is usually piston-only over the area defined by the device pixels. By applying a distributed voltage across the device, a continuous LC cell may produce modal wavefront deformations. In this paper we will describe the concept of modal liquid crystal wavefront correctors, review some of the current results.

A 19-element small PZT deformable mirror with outer diameter 24mm has been developed. In this paper, the characteristics of this DM such as influence function, hysteresis and non-linearity of deformation, resonance frequency are reported. The fitting capability of the DM to the lower order Zernike aberrations is investigated experimentally. The results are reported.

The following sections are included:

• Introduction

• Current and Future Developments

• References

Aspherical surfaces will be widely used in future. Due to high fringe densities interferometrical evaluation is difficult. Adapted reference surfaces or appropriate compensating optics, refractive or CGHs can be used to reduce the fringe density.

By implementing an adaptive mirror in a Twyman Green type Interferometer it is possible to reduce the fringe density in order to allow numerical evaluation. As an example an aspherical surface was analysed where an adapted mirror was used. For the evaluation of the aspherical coefficients ray tracing is needed. A known surface was used for the calibration.

Our main activity was to study the potential of an adaptive mirror device in measuring aspherical surfaces. This was one part of our activities in the ESPRIT Project MOSIS. The used membrane mirrors were supplied by Gleb Vdovin, TU Delft, project partner in MOSIS.

In this article an efficient and straightforward method for the compensation of thermally induced birefringence in high power Nd:YAG rod lasers is discussed. The scalar aberrations of a birefringence compensated two rod system are analyzed. Resonators with a small stability range and therefore beam quality near the diffraction limit are introduced. To improve the brightness of such resonators a multi-channel deformable high reflecting mirror is investigated with the aim of using it as a resonator mirror. As the damage threshold of the multi-channel mirror is still too low, results with an adaptive resonator were obtained using a single actuator mirror as HR-mirror. The maximum average output power of the adaptive resonator is 145 W at a beam quality of M²≅2.

We present numerical calculations of the eigenmodes of laser resonators with aberrations. Due to the aberrations the diffraction losses of the resonator increase drastically if the resonator is at the boundary of the stable region. This means that is impossible to obtain near diffraction-limited beam quality if aberrations are present. A resonator with an adaptive mirror could greatly improve the beam quality of high-power solid state lasers.

The design of an adaptive wavefront control system for a high-power Nd:Glass laser will be presented. Features of this system include: an unstable resonator in confocal configuration, a multi-module slab amplifier, and real-time intracavity adaptive phase control using deformable mirrors and high-speed wavefront sensors. Experimental results demonstrate the adaptive correction of an aberrated passive resonator (no gain).

Veriable reflectivity mirror in the unstable resonator was used to improve the quality of multimode beam of the multi-kW cw CO₂ laser. Eight-fold increasing of the axial brightness of radiation and angular divergence of 1 mrad were achieved. Characteristics of the industrial 1.5 kW cw CO₂ laser beam with a temporal reflectivity controlled mirror as a output mirror of the resonator were investigated. The pulse-periodical regime of the beam generation in the range of pulse repetition up to 2 kHz was realised.

The thermally induced lens is a critical issue in high-power diode-pumped solid-state lasers. A self-adaptive scheme for the compensation of the thermal lens is presented. The requirements for such an element and its influence on the resonator are discussed. With an appropriate compensating element and the correct resonator design, constant beam parameters are expected to be achieved over a pump range of several kilowatts.

A method is presented for dynamic compensation of thermally induced birefringence and thermal focusing effects in heated laser rods, using an imaging variable radius mirror (IVRM). This compensation method performs imaging together with retracing that dynamically adapts to the pump power load. The IVRM is comprised of discrete elements, where the distance between the elements determines the amount of optical correction. The unique feature of this method is its simplicity and efficiency in correction of birefringence effects together with radial thermal lensing. The IVRM was successfully implemented in stable resonators, based on Nd:Cr:GSGG laser material, to optimally compensate for thermal lensing and thermal induced birefringence under dynamic thermal conditions.

Theoretical and experimental formation of a super-gaussian fundamental modes of the 4th and 6th orders of CW CO₂-laser with stable resonator with the help of an intracavity flexible controlled mirror is shown. We used semi-passive bimorph deformable minor with four controlling electrodes. The increase of power of fundamental modes up to 12% and the enlarging of the peak intensity in the far-field in 1.6 times in the comparison with the gaussian TEM₀₀ mode of the same resonator are observed. The theoretical calculations has shown the possibility of the formation of the super-gaussian fundamental modes at the output of YAG:Nd₃₊ laser resonator by means of intracavity flexible mirror.

The results of the research activities of the Group of Adaptive Optics from IPLIT RAN are presented. The design of the different types of the bimorph correctors for the low-order aberrations compensation is discussed. The 20 channel compact and low-cost control system capable of interfacing with personal computer is presented.

One of the fundamental sensitivity limits of Hartmann-type wavefront sensors, especially for coherent light, is the crosstalk between adjacent lenses caused by the hard-edge diffraction from the lens apertures. In this paper we describe the use of lens apodization to reduce the hard-edge diffraction and thus reduce the crosstalk between lenses. First, diffraction modeling of the crosstalk reduction between classic square aperture diffraction and gaussian diffraction is presented. We present a new technique for apodization of micro-lenses based upon evaporation of a metal through a micromachined array of apertures. A one-dimensional trace through the apodization aperture was shown to fit well to a 3^rd order super-gaussian profile. Finally, the intensity profiles from unapodized and apodized lenses are presented showing no hard-edge ringing with apodization when observed on a CCD.

Shack Hartmann technology is classically presented in a matrix approach. We describe in this paper a new method for wavefront sensing with a linear Shack-Hartmann. The idea is based on the real need of the user : a « waveline » can provide a simple way and low cost system for tilt and focus control and also a very useful information for optical adjustment; in the other hand, the same system can be used for a complete characterization of the wavefront after a radial scan of the beam. The line geometry and smart softwares are offering new powerful advantages for such sensors: speed, dynamic, accuracy…

We report on the possibilities for a novel integration approach to Hartmann-Shack wavefront sensors, which combines two consolidated technologies, i.e., planar micro-optics and microelectronics standard CMOS process. Therefore, a compact low-cost device comprising micro-lenses and silicon photodetectors can be fabricated, taking full advantage of component optimization in both technologies. Currently, expensive CCD image sensors, a powerful computer and complicated data reduction algorithms are required to generate the wavefront profile. This hinders system miniaturization and real-time results. As an alternative, an array of optical position-sensitive detectors (PSD's) can be tailored to the aimed application and generates output voltages proportional to the spots positions, thus avoiding CCD's rather complex image analysis. This paper presents the initial step for an integrated H-S sensor based on a micro-lens array to be geometrically mounted on top of a PSD array fabricated in standard CMOS process line. To date, several PSD's featuring different photosensitive characteristics and two-dimensional layouts have been fabricated at our laboratory. Results on these devices are discussed.

It is now possible to build low cost adaptive optics systems with commercial electronics and with no specialized computing coprocessors such as dedicated digital signal processing boards. The ability to change configuration of a wavefront sensor including its sensitivity, dynamic range, and also subsequent type of wavefront reconstructor have a variety of applications in adaptive optics, wavefront control, and deformable mirror calibration. We present our implementation of these methods as used in adaptive optics systems, metrology systems, and commercial wavefront sensors.

Traditionally the spatial resolution of Shack-Hartmann wavefront sensing has been limited by the size of the lenses used in the array. To see features smaller than a lens diameter the wavefront had to be magnified before entering the sensor with a lens or set of lenses. The disadvantage of this technique is that the magnifying lenses then impose their own aberrations and the field of view of the wavefront sensor is reduced. We present a technique for increasing spatial resolution using the hard aperture of the lens to limit the wavefront area being sensed. Diffraction modeling and experimental results using this technique are presented.

Optical systems are normally aligned by centering the energy distribution in various apertures. However, the use of both irradiance and phase information can in many cases greatly simplify this process, and can provide information for closed-loop alignment and control of an optical system. This can be accomplished by using a Shack-Hartmann wavefront sensor for alignment and performance testing.

While Shack-Hartmann wavefront sensors are commonly used for adaptive optics, they have many other applications. The modern Shack-Hartmann wavefront sensor is compact, rugged, and insensitive to vibration, and has fully integrated data acquisition and analysis. Furthermore, even wavefronts of broad band sources that cannot normally be tested with interferometers can be measured with Shack-Hartmann wavefront sensors. The instrument also provides information about the optical system performance, including peak-to-valley (PV) wavefront deviation, RMS wavefront error, the modulation transfer function (MTF), and the point spread function (PSF). Since the difference of two wavefronts is easily computed, the effect of individual optical elements on a complex optical system can be examined.

In this paper we will present an alignment methodology using the Shack-Hartmann wavefront sensor to setup even complex optical systems. An example of using the methodology to align a lens is presented. Alignment of the Shack-Hartmann wavefront sensor to a 24-inch telescope at Stanford University is presented. The higher-order static aberrations in the telescope are then measured with the Shack-Hartmann wavefront sensor.

A common path interferometer (CPI) provides an attractive wavefront sensing scheme due to its inherent simplicity and robustness. The working principle is, that a part of the incoming wavefront is extracted and perturbed to generate a so-called synthetic reference beam for interference with the remainder of the wavefront. Many different CPI schemes have been proposed and independently analysed. We have developed a new general approach that can be applied to CPI analysis, which simplifies the treatment and understanding of the operation of the majority of CPIs that utilise spatial frequency filtering.

Wavefront sensing for adaptive optics applications is nowadays a routine technique based primarily on two schemes: the Shack-Hartmann (SH) and the Curvature Sensing/Phase Diversity (PD). While both these schemes are well suited for applications of ground-based observations of exo-atmospheric objects, this is not necessarily true for other type of applications. For laser with very short pulse duration the bandwidth problem associated with conventional wavefront sensing is a serious short coming. In this framework our group started the a program of investigation for an all optical wavefront sensor based on moiré techniques.

The Transport of Intensity Equation is explored in various applications for sensing of wave fronts. Curvature sensing provides the phases in a plane where the intensity is well known, by solving partial differential equations. It is also possible to measure phases in two planes under simple assumptions. Solutions of differential equations depend strongly on boundary conditions that require accurate measurement. It is shown that scintillation can be corrected in such cases, even without the boundary conditions.

A Green's function solution to the Intensity Transport Equation is used to determine the wavefront of a laser-beam from measurements of the intensity profile in two planes equally spaced about the wavefront measurement plane. This paper describes the algorithm, the experimental arrangement to make the measurements and presents some preliminary results.

Optical aberrations in an actual adaptive optics system which occur after the wavefront sensing can be the limiting aberration in a system. We describe work on correcting these errors using dithering techniques in the ELECTRA adaptive optics system.

Fundamental limitation on field-of-view in a non-astronomical adaptive optical system due to its anisoplanatism is analyzed. Some possibilities of field-of view widening in adaptive optics are discussed. An estimation of possible isoplanatic patch of an adaptive system for retinoscopy is made using some heuristic approximation for irregular aberration structure function.

Laser beam adaptive control with the use of a multidither algorithm is considered under the conditions of thermal blooming. It was shown that if the algorithm is applied before the termination of transient processes in a medium, an adaptive system finds the region of the extremum but leaves it in a short while. Time required to find the extremum is defined by the value of a gradient step, lesser is the step, greater is the time, but the time interval during which the system remains in the vicinity of the extremum is inversely proportional to the value of the step. The problem becomes even more complicated if local maxima appear in the space of control coordinates.

The control over high-power laser beam based on the aperture sounding algorithm and its modification is considered in this paper using methods of numerical experiment. A domain of the problem parameters is shown to exist for which local extrema appear in the space of control coordinates. A decrease in the efficiency of correction for the beam thermal blooming due to these local extrema is estimated. The method for seeking the global (basic) maximum is proposed.

We examine the fundamental limitations of a phase-stepping wavefront sensor for use with a thermal source in an adaptive optics system. We show that - in principle - it should be possible to measure wavefronts to sufficient accuracy at the required speed provided that white-light fringes are used and the aberrations are small. The errors caused by using phase-stepping with white light fringes are determined and error-compensation schemes suggested.

We describe a simple technique for imaging multiple layers within an object field simultaneously onto a single camera. The approach uses a binary diffraction grating in which the lines are distorted such that a different level of defocus is associated with each diffraction order. The design of the gratings is discussed and their ability to image multiple object planes is validated experimentally. The grating provides a simple and robust method of obtaining the multiple images required in a phase diversity wavefront sensing system. Incorporation of a grating into such a system is described.

A low cost adaptive optics system with an open loop frame of several hundreds of frames per second, using a single processor is described. It is constructed almost entirely of commercially available components. The construction of the system, its control and performance are discussed.

We describe the results of series of experiments on the dynamic holographic correction for distortions in the wide spectral range of visible light. The hologram-corrector was recorded in optically addressed liquid crystal spatial light modulator by pulsed radiation and was read out in the scheme of one-way image correction by the incoherent wide spectral band radiation of the thermal source. The performance of the system with the holographic corrector was evaluated in standard optical terms of frequency\contrast characteristics.

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 realise 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 realisation of such an interferometer.

The following sections are included:

• Introduction

• Wavefront modulators (WFMs)

• Wavefront sensors (WFSs)

• Discussion

• Acknowledgements

• References

A method of finding a near optimum correction of a phase aberration is demonstrated. A liquid crystal spatial light modulator (SLM) is to be used as the corrective element and the phase screen applied to this is designed by an evolutionary algorithm. Evolutionary algorithms mimic biological evolution, where sets of trail solutions compete to reproduce and form the next generation. Only the best solutions survive. Each generation takes a step closer to a global optimum. This method requires no wavefront sensor, is easy to set up, and has a low cost

June 1999 on the 4.2m William Herschel Telescope (one of the Isaac Newton Group of telescopes on La Palma in the Canary Islands, Spain) saw two runs with the University of Durham ELECTRA adaptive optics system. The first seven nights were for commissioning various new features and the remainder for 'service mode' AO observing. The new features included the Durham TEIFU integral field unit which can feed WYFFOS with adaptively corrected optical spectra from 500 sky elements simultaneously. This capability will soon be enhanced to 1000 elements (hence TEIFU: Thousand Element Integral Field Unit).

In this paper we present the results of a novel compact adaptive optics system for the optimization of optical power into a single mode optical fiber. This system uses a micromachined deformable mirror for aberration correction. The stochastic parallel gradient descent algorithm is used to maximize the coupling efficiency without a direct wavefront measurement.

Side and front coupling of pump laser diodes of 1W emitted power into a double clad doped fiber using a multielectrode deformable mirror is investigated. The shape of the mirror was controlled using a simplex-based algorithm. An increase of the coupling efficiency and the fluorescence output power has been demonstrated in both cases

We present experimental results of the compensation of a laser beam transmitted through static aberrators using a Shack-Hartmann-based adaptive optics system. We investigate system performance as a function of beacon intensity, beacon scintillation, and beacon wavefront.

The implementation of a wavefront curvature sensor to measure atmospheric turbulence over horizontal paths is described. These measurements are important for defining adaptive optical systems for imaging along such paths. Currently little is known about the expected turbulence structure so the sensor is to be deployed to measure many atmospheric quantities in sufficient detail and over a long period of time so a library can be constructed giving atmospheric conditions for any particular time period. By using this information AO systems can be more accurately defined to cope with horizontal imaging situations. The sensor is a novel device using the intensity transport equation to calculate the wavefronts from information collected using a distorted grating.

A laser communications demonstration experiment (LCDE) between the Communications Research Laboratory in Tokyo and the International Space Station is being designed and implemented. To efficiently couple the downlink laser beam into a single-mode Erbium-doped preamplifier and to reduce uplink beam wander and scintillations, we plan to use low-order adaptive optics. The envisaged system dithers the wavefront actuators and evaluates the resulting variations of the received optical power. Simulation shows that, when controlling tilt, focus, and astigmatism, the power penalty due to residual wavefront error amounts to 2.3dB.

Wavefront control is a key issue for developing different systems including : lasers (for various applications : isotopic separation, femtosecond solid state lasers, microlithography…), imaging through turbulent media. ophthalmology… For these purposes, we have developed an adaptive optics system that can achieve wavefront very high precision correction.

Subsystems will be described including the Wavefront Sensor (Shack-Hartman type), the Deformable Mirror (Bimorph type) and the Real Time Computer (PC type). Emphasis will be made on results that can be obtained using such a system : ultimate quality of the corrected wavefront, measurement range (important during capture phases)…

The 'Grand Interféfomètre à Deux Télescopes' (GI2T) is a Michelson Stellar Interferometer. The interferometric signal, i.e. the fringe contrast, is directly related to the high spatial frequencies of the object being observed. Because of atmospheric turbulence, the interferogram in the image plane is spread into numerous uncorrelated fringed speckles, thus the dilution of light is decreasing dramatically the Signal-to-Noise Ratio (SNR). We plan to integrate Adaptive Optics (AO) in both interferometric arms. Operating at visible wavelengths, the correction of the atmospheric aberrations, although partial, will concentrate the light in a central core surrounded by a few bright speckles, leading to an enormous increase of the SNR. We describe here the status of the development of the wavefront analysor of the system and some simulation results will show the improvement of the sensitivity of the interferometer equipped with AO. We will also emphasize some problematics concerning the calibration of fringe contrast measured on interferograms corrected by AO.

Conventional deformable mirrors have a limited throw and to compensate the many waves of aberrations of a large aperture membrane mirror we need to use holographic correction. Optically addressed Spatial Light Modulators (SLM) have been used in laboratory demonstrations to remove up to 200 waves dynamic aberrations [1]. The next step is to use a blazed holographic dynamic correctors to more efficiently transform the aberrated beam into a well compensated one, eliminating all the diffractive orders. We are presenting here the results obtained using a combination of an optically addressed. Parallel Aligned Nematic Liquid Crystal SLM and an electrically addressed SLM. Using these SLMs in conjunction with a CCD video camera and a computer we were able to introduce and then correct for first order aberrations (tilt, defocus, astigmatism) with up to 50 waves @543nm, we also used blazed holographic correction techniques to correct for around 20 waves of aberrations on a 2" area of a membrane mirror

The following sections are included:

• Introduction

• Experimental Demonstration

• Laser Printer Design

• Discussion

• Acknowledgments

• References

The following sections are included:

• List of Delegates