In 1980, the first laboratory on Adaptive Optics in China was established in Institute of Optics and Electronics, Chinese Academy of Sciences. Several adaptive optical systems had been set up and applied in Inertial Confinement Fusion (ICF) and retinal high-resolution imaging. In 1985, the first adaptive optical system for ICF equipment was set up in the world. Another 45 element adaptive optical system was first built for correcting the static and dynamic wavefront aberrations existed in the large-aperture Nd: glass laser for inertial confinement fusion in 2001. Two set adaptive optical system with 19-element and 37-element deformable mirror had been developed for human retina imaging in 2000 and 2002 respectively. In this paper, the function and performance of these adaptive optical systems are described and the experiment results are presented.

A system that is able to obtain in-vivo human retinal images using en-face optical coherence tomography is presented. The system also includes an adaptive optics closed-loop system that uses a Shack-Hartmann wavefront sensor and a 37 OKO membrane deformable mirror to correct for ocular aberrations. The system has been used to produce en-face images of retinal pigment epithelium and the nerve fiber layer collected from several volunteers.

We investigate a novel camera that incorporates adaptive optics (AO) and optical coherence tomography (OCT) to determine if it can achieve the necessary 3-D resolution, sensitivity, and speed for imaging individual cells in the living human retina. An AO spectral-domain OCT system was constructed that is based on a free-space Michelson interferometer design. The OCT sub-system consists of a broadband superluminescent diode whose beam passes through an astigmatic lens to form a line illumination pattern on the retina, which is then imaged onto the slit of an imaging spectrometer. The detector of the spectrometer is a scientific-grade areal CCD. Conventional flood illumination, also with AO, was integrated into the camera and provided confirmation of the focus position in the retina. Short bursts of narrow B-scans (100x560 microns) of the living retina were subsequently acquired at 500 Hz during dynamic compensation that corrected the most significant ocular aberrations across a dilated 6 mm pupil. Camera sensitivity (up to 94 dB) was sufficient for observing reflections from essentially all neural layers of the retina. The 3-D resolution of the B-scans (3.0x3.0x5.7 microns) is the highest reported to date in the living human eye. It was sufficient to observe the interface between the inner and outer segments of individual photoreceptor cells, resolved in both lateral and axial dimensions. The waveguiding nature of the photoreceptors is suggestive at multiple reflective sites. Micro-movements of the retina during short burst imaging allow averaging to reduce speckle contrast, but they appear insufficient for significant speckle reduction.

High order aberrations in human eye can deteriorate visual acuity and contrast sensitivity. Such aberrations can not be corrected with traditional low-order (defocus and astigmatism) spectacles or contact lenses. A state-of-the-art adaptive optics vision correction system was developed using Ophthonix's Z-View diffractive wavefront sensor and a commercial miniature deformable mirror. While being measured and corrected by this system, the patient can also view a Snellen chart or a Contrast Sensitivity chart through the system in order to experience the vision benefits both in visual acuity and contrast sensitivity. Preliminary study has shown the potential that this system could be used in a doctor's office to provide patients with a subjective feel of the objective high order prescription measured on Z-View.

The spatial resolution of retinal images is limited by the presence of static and time-varying aberrations present within the eye. An updated High Resolution Adaptive Optics Fundus Imager (HRAOFI) has been built based on the development from the first prototype unit. This entirely new unit was designed and fabricated to increase opto-mechanical integration and ease-of-use through a new user interface. Improved camera systems for the Shack-Hartmann sensor and for the scene image were implemented to enhance the image quality and the frequency of the Adaptive Optics (AO) control loop. An optimized illumination system that uses specific wavelength bands was applied to increase the specificity of the images. Sample images of clinical trials of retinas, taken with and without the system, are shown. Data on the performance of this system will be presented, demonstrating the ability to calculate near diffraction-limited images.

The problem of correct measurement of human eye aberrations is very important with the rising widespread of a surgical procedure for reducing refractive error in the eye, so called, LASIK (laser-assisted in situ keratomileusis). In this paper we show capabilities to measure aberrations by means of the aberrometer built in our lab together with Active Optics Ltd. We discuss the calibration of the aberrometer and show invalidity to use for the ophthalmic calibration purposes the analytical equation based on thin lens formula. We show that proper analytical equation suitable for calibration should have dependence on the square of the distance increment and we illustrate this both by experiment and by Zemax Ray tracing modeling. Also the error caused by inhomogeneous intensity distribution of the beam imaged onto the aberrometer's Shack-Hartmann sensor is discussed.

In 1999, the first Hartmann-Shack wave-front sensor for the human eye aberration measurement in China was established. The H-S sensor was successfully improved and applied to the clinic diagnosis. In this paper, the principle and the method of measuring wave aberrations of the human eye are given. The accuracy of the Hartman-Shack sensor is measured and analyzed. The measurement results of the wave-front aberrations of the real eyes using the sensor are demonstrated.

The accurate measurement of the double-pass ocular wave front has been shown to have a broad range of applications from LASIK surgery to adaptively corrected retinal imaging. The ocular wave front can be accurately described by a small number of Zernike circle polynomials. The modal wave front sensor was first proposed by Neil et al. and allows the coefficients of the individual Zernike modes to be measured directly. Typically the aberrations measured with the modal sensor are smaller than those seen in the ocular wave front. In this work, we investigated a technique for adapting a modal phase mask for the sensing of the ocular wave front. This involved extending the dynamic range of the sensor by increasing the pinhole size to 2.4mm and optimising the mask bias to 0.75λ. This was found to decrease the RMS error by up to a factor of three for eye-like aberrations with amplitudes up to 0.2μm. For aberrations taken from a sample of real-eye measurements a 20% decrease in the RMS error was observed.

A novel wavefront sensor (Z-View^TM) using a two dimensional diffractive grating has been developed at Ophthonix, Inc. Based on the Talbot self-imaging theory, a CMOS camera is placed behind the grating to capture the first Talbot image of the aberrated wavefront. This captured Talbot image is analyzed to recover the wavefront aberration. The diffractive grating wavefront sensor has been used in Ophthonix's Z-View Aberrometer, an objective refractive vision assessment system which is now commercially used in optometrist's offices/clinics across the United States of America. Coupled with a deformable mirror and other auxiliary optics systems, Z-View wavefront sensor forms the A-View adaptive optic vision correction system at Ophthonix. This A-View system is used to study the effect of complete wavefront correction in human vision, and has potential application in prescribing Ophthonix's wavefront-guided iZon^TM lenses. In this paper, the wavefront sensing principle of this novel diffractive wavefront sensor and its applications will be discussed.

Kestrel Corporation has previously demonstrated that the Distorted Grating Wavefront Sensor (DGWFS) can successfully reconstruct wavefronts in severely scintillated conditions, and has an ongoing experiment investigating aberrations in the eye using a DGWFS. Existing aberrometers cannot accurately reconstruct wavefronts when large amounts of scattering or scintillation are present and so cannot be used with subjects who have conditions such as cataracts (opacification of the ocular lens). Consequently a large proportion of the population cannot utilize today's diagnostic aberrometers and so do not benefit from otherwise available treatments. As previously reported, a DGWFS has been integrated into an Shack-Hartmann based aberrometer provided by the International Laser Center, Moscow State University, however several issues became apparent regarding data collection from the human eye. Results from laboratory experiments intended to investigate and resolve these data collection issues will be discussed.

Images of retinal cell were collected in 60 male normal volunteers with normal vision using adaptive optic system. All have their right eye checked. Study was focused on the imaging of the outer layer cell at the fovea. And another, 12 specimen of human retina were studied under the differential interference contrast microscopy, focus was placed on different layer of the photoreceptor at the fovea. High-resolution images of the retinal cells were obtained by adaptive optic system in all the volunteers. In pathological study, we also get clear imaged of different layer of the photoreceptor. Based on histopathological support and others, it is concluded that the outer layer cell taken by the adaptive optic system is the outer and inner segment of the cone.

Wavefront aberrations of human eye are expected to change when the eye accommodates to targets from infinite distance to clear visual threshold distance (25 centimeters distance from target to eye). It is significant to measure and analyze the wavefront aberrations in the accommodated human eye, which helps to evaluate visual performance and has clinical value. The previous research on the effect of accommodation on the wavefront aberration all adopted subjective measurement technique. In this paper, we firstly adopt Hartmann-Shack wavefront sensor technique, which possesses advantages in comparison with the psychophysical subjective technique such as larger sampling points. Data from 20 eyes are reported in this study. Ages of all subjects range from 19 to 38 and defocus of them range from -6D to +3D. Every pupil size is greater than 5mm for whole eye measurements. No one in this experiment has a record of ocular disease. Root-mean-square (RMS) and peak-to-valley (PV) values of wavefront aberration have been evaluated. In a comparison between the clear visual threshold condition and the infinite distance condition, the subject DQ has the larger change in RMS wavefront error, from 0.85 μm at infinite distance to 0.43 μm at clear visual threshold distance. The RMS difference is about 0.42 μm which is 50% in proportion to the RMS value at infinite distance. While the subject JL has the smaller change of 23% in RMS wavefront error, just from 0.66 μm to 0.51 μm between two different accommodative conditions. Defocus and astigmatism have been excluded in this paper. It is found that accommodation influences wavefront aberrations of whole eye and the situation varies substantially from individual to individual. We have to consider not only the wavefront aberration at the infinite distance but also that at the clear visual threshold distance during clinical ocular therapy.

In order to understand the relative contribution of the wave-front aberrations of the cornea and the crystalline lens to the retinal image quality in the human eye, we have measured the wave-front aberrations of the anterior corneal surface, the posterior corneal surface and the complete eye with a corneal topographic system (Orbscan) and a Hartmann-Shack wave-front sensor. The 20 subjects selected to participate in the study are all no eye diseases, covering a range of age from 18 to 25. All the subjects have refractive errors of defocus varying from 0.5 D to 5 D and astigmatism varying from 0.1 D to 1.5D. Using the Orbscan, we obtained the discrete set of corneal elevation data in radial distribution over the pupil plane for the anterior and the posterior corneal surfaces directly, and the data are then transformed into wave-front aberrations of both the corneal surfaces. The wave-front aberrations of the two surfaces are then used to acquire the aberrations in whole cornea. The aberration contribution of the crystalline lens is obtained by subtracting the aberrations in the cornea from that in the complete eye. It is shown that the combination of the aberrations between the crystalline lens and the cornea could be either a compensatory or an additive process. The effect of the combination between the anterior and the posterior corneal surface is also complicated, and the aberration compensation, as well as aberration addition can be observed. It is shown from statistics point of view that the anterior corneal surface contributes more lower-order aberrations (astigmatism) to the complete eye, while the posterior corneal surface and the crystalline lens play a more important role in contributing higher-order aberrations.

In this paper we are presenting our latest results in the field of correction and formation high power pulsed laser radiation by means of adaptive optics. Our adaptive optical system consists of wavefront corrector (multy element bimorph mirror), control unit for such a mirror, PC and Shack-Hartmann wavefront sensor. The system is intended to compensate for slowly changing aberrations (frequency of corrected aberrations is about 5 Hz) with rather high amplitude (up to 20 μ). The efficiency of adaptive system was tested in various laboratories and institutes throughout the world on both femto second and high energy nano second lasers. It allowed to improve Strehl ratio up to 0.8.

The generation of energetic protons using a polyimide tape of 7.5 μm thickness was carried out with laser pulses of 30 mJ energy and 80 fs duration. A deformable mirror system with a genetic algorithm (GA) was developed to optimize the laser-focusing spot. The fitness values used in the GA were measured from the focusing intensities under the low-gain condition of the power amplifier, or from x-ray in situ signals emitted from the target. Although we obtained a diffraction-limited size of 2 μm (full width at half maximum) using the former value, a precise optimization using the latter value was essential to accelerate protons whose flux was 10⁶/MeV/shot to a maximum energy of 1.1±0.3 MeV with laser pulses of only 30 mJ energy since a laser spot that is too tight may be sensitive to wave-front distortion caused by residual thermal lenses of the power amplifier.

SLODAR (slope detection and ranging) is a technique we have developed to monitor the vertical profile of atmospheric phase distortions, for application to astronomical adaptive optics systems. The technique uses the correlation between slope measurements made using a Shack-Hartmann wavefront sensor observing a binary star. In this paper we describe the principle of SLODAR and then describe our work on using a system for the measurement of horizontal turbulence profiles for application to free space optical communications. This work has also been presented elsewhere1

A numerical model of a typical adaptive optics system is described in the report. The developed computer application corresponding to the model includes all the main elements of a real system, namely, computer codes simulating radiation propagation in a turbulent atmosphere with thermal blooming and codes simulating the elements of adaptive optics system (a model of a Shack-Hartmann wavefront sensor, two algorithms of dislocation localization, and a model of adaptive mirror with continuous surface). In the report we include solutions to the some of adaptive optics problems obtained with the model: realization of amplitude-phase control in two-mirror adaptive system, the method to improve the stability of correction for thermal blooming, evaluation of a Shack-Hartmann sensor performance, and some others. These results demonstrate that the set of the developed models is a powerful tool for simulations in the field of adaptive optics.

An intra-cavity deformable membrane mirror (DMM) has been used to optimise the brightness of a 15W, diode-pumped, grazing incidence Nd:GdVO₄ laser. In one configuration an order of magnitude improvement of laser beam quality was recorded with negligible drop in output power. Local and global optimum-locating algorithms have been developed to enable automatic optimisation of the laser quality, and have been tested in both intra- and extra-cavity configurations. A novel laser brightness sensor based on second-harmonic-generation has also been developed to assess the progress of the laser towards optimisation. A tip & tilt mirror was also incorporated in the laser resonator cavity and initial tailoring of the algorithm procedure was performed in order to enhance the optimisation capabilities.

The paper presents the results of experimental demonstration of phase conjugation of weak laser radiation by means of two-stage dynamic holography in optically addressed liquid crystal spatial light modulator, implementing the TV-relay of the interferometery data for the record of hologram-corrector.

The influence on outcoupled mode by introducing intracavity tilt-perturbation in confocal unstable resonator is analyzed. The intracavity mode properties and Zernike-aberration coefficient of intrcavity mirror's maladjustment are calculated theoretically. The experimental results about the relations of intracavity mirror maladjustment and the properties of mode aberration are presented by adopting Hartmann-Shack wavefront sensor. The results show that the intracavity perturbation of the concave mirror has more remarkable effect on outcoupled beam-quality than that of the convex mirror. For large Fresnel-number resonator, the tilt angle of intracavity mirror has a close linear relationship with extracavity Zernike tilt coefficient. The ratio of tilt aberration coefficient approaches to the magnification of unstable resonator if equivalent perturbation is applied to concave mirror and convex mirror respectively. Furthermore, astigmatism and defocus aberration also increase with the augment of tilt aberration of beam mode. So intracavity phase-corrected elements used in unstable resonator should be close to the concave mirror. Based these results, a set of automatic control system of intracavity tilt aberration is established and the aberration-corrected results are presented and analyzed in detail.

The phase changing rates of laser beam in single mode polarization maintaining fiber (PMF), Yb-doped rectangular inner cladding dual-clad fiber (DCF) and Yb-doped large-mode-area dual-clad fiber (LMF) are investigated by means of interference under situations with different exterior disturbance, for situations that all the fibers are used as transmission medium only, as well as amplifying medium with the last two. In this paper, phase fluctuation characteristics of the beam transmitted through the fibers with no amplification are presented. The results show that the phase fluctuation frequencies of the laser beam transmitted through these three kinds of fibers are mainly in the range of 100~200Hz, 100~300Hz and 80~400Hz respectively in normal laboratory environment. The phase fluctuation frequencies of the laser beam transmitted through the DCF, in cases of when the environment temperature is changing, a constant pressure is applied to the fiber and when vibration is applied, are mainly in the range of 100~600Hz, 100Hz~300Hz and 100~500Hz respectively. In the same cases, for the LMF, the phase fluctuation frequencies are mainly in the range of 150~600Hz, 100~400Hz and 100~1000Hz, respectively. It indicates that for realizing phase stabilization, the bandwidth of any potential phase control system has to be in the range of a few kilohertz.

Intracavity adaptive optics has been successfully used in solid state lasers to improve output laser beam quality. However, In order to utilize this technology to improve the output laser beam quality successfully, the distribution characteristics of phase aberration in the laser resonator should firstly be known. Thus, a set of Hartmann- Shack wave front sensor (HSWFS) to measure the time and space characteristics of phase aberration in a diode-side-pumped Nd:YAG laser was set up. In this paper, the HSWFS is briefly introduced. The experimental results for measurement of the phase aberration in a diode side-pumped Nd:YAG laser are presented. The experimental results show that, the main phase aberration in the resonator is generated by the Nd:YAG rod. The phase aberration induced by thermal deformation in the cavity mirrors is minor. The temporal behavior of phase aberration in the resonator and from cavity mirrors under light heating was also obtained.

Hartmann-Shack wavefront sensors[1] are widely used in adaptive optical systems. It can measure the spatial-temporal errors of dynamic wavefront. Not only the phase but also the amplitude of a wavefront can be measured. Unlike an interferometer, it is not necessary to have a real-time reference beam, so it can work in a disturbing environment. Besides used in adaptive optics systems, Hartmann-Shack wavefront sensors also become a powerful tool in two fields: light beam diagnosis and optical testing of optical components and systems. We have developed a serial of Hartmann-Shack wavefront sensors used in these two fields. In this presentation, various applications of Hartmann-Shack wavefront sensors in these fields will be reported.

Generalized Phase Diversity (GPD) is a phase retrieval algorithm which requires a pair of intensity images. These are created by applying equal and opposite diversity phase to the input wavefront. Unlike traditional phase diversity methods GPD is not limited to the use of defocus as the applied diversity phase. The conditions that a suitable diversity function must satisfy for use in a null sensor were presented at the 4th IWAOIM. Following our recent development of a small angle solution to the inverse problem, in this paper the GPD method will be extended to use as a full wavefront sensor. This method has a wide range of applications, including laser beam shaping, analysis of segmented optics, and metrology. Results will be presented to show the versatility and accuracy of this novel wavefront sensing method.

There are two main ways to address the wavefront sensing problem with modal wavefront sensors. First, the slope approximation that estimates a series of local first derivatives of the wavefront, as in the Hartmann-Shack sensor. Secondly, the curvature approximation, that estimates a series of second derivatives of the wavefront in different areas, as in the curvature sensor. It has been demonstrated that optical differentiation can be used as a useful first derivative wavefront sensor. Here we present a complete review of this new sensor along with a novel procedure to estimate the curvature of the wavefront phase using optical differentiation. This sensor consists of a telescopic system located in one of the arms of an interferometer and a phase step in the other arm. A variable amplitude transmission mask is placed at the focal plane of the telescopic system to perform the second derivative of the incoming field. A detailed description of the set-up and the mask is presented. The main advantages of this sensor are high resolution and easily adjusting of the sampling of the wavefront so allowing its use in high resolution wavefront sensing.

Light waves passing through the turbulent flow are aberrated by refractive index fluctuations in the flow. Refractive index fluctuations arise from density fluctuations driven by temperature and pressure variations in turbulent flow. Hartmann-Shack wavefront sensor can directly sense these aberrations and also can reconstruct the 3-dimensional structure of the refractive index distribution in a turbulent flow field with tomographic reconstruction technique. A novel method for measuring the 3-dimensional structure of the refractive index distribution in a turbulent flow field combining Hartmann-Shack wavefront sensor with tomographic reconstruction is proposed in this paper. Hartmann-Shack wavefront sensor is used for measurement of an optical wave front after passing through a flow field and the refractive index distribution is reconstructed by computed tomography technique. The principle of the tomographic reconstruction of the flow field based on the Hartmann-Shack wavefront sensor is described briefly. The static symmetric and dynamic asymmetric experimental results are presented. The experimental results indicate that Hartmann-Shack wave-front sensor combining with tomographic reconstruction technique has a potential application for material and flow field investigation regions.

An optical testbed has been developed for the comparative analysis of wavefront sensors based on a modified Mach Zender interferometer design. This system provides simultaneous measurements of the wavefront sensors on the same camera by using a common aberrator. The initial application for this testbed was to evaluate a Shack-Hartmann and Phase Diversity wavefront sensors referenced to a Mach-Zender interferometer. This testbed has the added benefit of being able to train the deformable mirror against the spatial light modulator and evaluate its ability to compensate the spatial light modulator. In the paper we present some results from the wavefront sensors along with preliminary results from the wavefront corrective elements in the optical testbed.

Wavefront sensing using a Shack-Hartmann sensor has been widely used for estimating wavefront errors or distortions. The sensor combines the local slopes, which are estimated from the centroids of each lenslet images, to give the overall wavefront reconstruction. It was previously shown that the pupil-plane irradiance profile effects on the centroid estimation. Furthermore, a previous study reported that the reconstructed wavefront from a planar wavefront with a Gaussian pupil irradiance profile contain large focus and spherical aberration terms when there is a focus error. However, it has not been reported yet how serious the pupil irradiance profiles, which can be occurred in practical applications, effects on the sensing errors. This paper considered two cases when the irradiance profiles are not uniform: 1) when the light source is Gaussian and 2) when there is a partial interference due to a double reflection by a beam splitting element. The images formed by a Shack-Hartmann sensor were simulated through fast Fourier transform and were then supposed to be detected by a noiseless CCD camera. The simulations found that sensing errors, due to the Gaussian irradiance profile and the partial interference, were found to be smaller than λ/50 which can be ignored in most practical cases where the reference and test beams have the same irradiance profiles.

This paper will present some of the recent work undertaken to extend the use of the wavefront sensor to provide both surface profile measurements and thickness measurements simultaneously using a single instrument. Some theoretical studies of the effect of thin film structures on wavefront shape will be presented along with discussion on how such knowledge can be used to gain reliable measurements of thickness and surface profile. Our experimental methods will be described with the inclusion of experimental results from a number of different sample thicknesses and materials. In addition, some initial data will be presented to illustrate how the technique can be extended to carry out surface profiling measurements on some etched and periodic structures. Finally some suggestions for future work and optimisation will be made to conclude.

We have established a new type of Hartmann-Shack (H-S) wavefront sensor which based on a micro-grating array. A H-S wavefront sensor is frequently used in an Adaptive Optics (AO) system to detect the aberrations for the system. Besides this situation, the H-S wavefront sensor is also adopted to measure those static or dynamic aberrations which exist in other kind of systems. In any one of these H-S wavefront sensors, a lenslet array seems so indispensable. But, it is difficult for a lenslet array to coincide with the photoelectric detector, some times a relay system will be necessary. And the variance of the focal length of the lenslet array will bring error to the measurement. To conquer these deficiencies, a new type of H-S wavefront sensor based on a micro-grating array other than a lenslet array has been created recently. This new kind of H-S wavefront sensor substitutes the lenslet array for a micro-grating array and a lens. On this new H-S wavefront sensor, some excellent experimental results have been obtained.

A novel PSD-based Hartmann-Shack wavefront sensor (HSWFS) prototype has been developed, which employs a 4X4 PSD array as the detector to measure the displacements of the sub-aperture spots. Compared with the conventional CCD-based HSWFS, it can operate at very high sampling rate, and it only has very short readout delay time. Our system can measure wavefront at frame rate up to 5KHz, and the detected wavefront error is less than λ/50 (λ=632.8nm). In this paper the experimental results are given. The measurement error of the PSD-based HSWFS for a given aberrated plate is compared with the measure result of the Zygo interferometer.

Phase diversity measurements using diffractive encoding systems offer a means for simultaneous measurement of the angular dependence of the turbulence induced in the wavefront distortion and either the angular dependence of scintillation induced during atmospheric propagation or the turbulence-degraded point spread function. We will describe experiments designed to measure the wavefront distortion and angular decorrelation of the atmospheric transfer functions, and discuss the observational strategy for measurement of atmospheric properties under a range of atmospheric conditions and propagation distances. By reconstructing the laser wavefront and comparing the calculated and measured images we will also aim to investigate the effect of strong scintillation on phase diversity wavefront reconstruction techniques. Laboratory tests of the equipment and preliminary measurements will be described, as well as some theory and modeling.

An experimental setup for phase extraction of 2-D phase distributions is presented. The system uses a common-path interferometer consisting of two windows in the input plane and a translating grating as its pupil. In the output, interference of the fields associated with replicated windows is achieved by a proper choice of the windows spacing with respect to the grating period. Because in this type of interferometer a grating is placed as a spatial filter, the phase changes which are needed for phase-shifting interferometry can be easily performed with translations of the grating driven by a linear actuator. Some experimental results as well as applications on Optical Tomography are shown.

A large adaptive deformable mirror with high actuator density is presented. The DM consists of a thin continuous membrane which acts as the correcting element. A grid of low voltage electro-magnetical push-pull actuators, - located in an actuator plate -, impose out-of-plane displacements in the mirror's membrane. To provide a stable and stiff reference plane for the actuators, a mechanically stable and thermally decoupled honeycomb support structure is added. The design is suited for mirrors up to several hundred mm with an actuator pitch of a few mm. One of the key elements in the design is the actuator grid. Each actuator consists of a closed magnetic circuit in which a strong permanent magnet (PM) attracts a ferromagnetic core. Movement of this core is provided by a low stiffness elastic guiding. A coil surrounds the PM. Both the coil and the PM are connected to the fixed world. By applying a current through the coil, the magnetic force acting on the core can be influenced. This force variation will lead to translation of the ferromagnetic core. This movement is transferred to the reflective mirror surface in a piston-free manner. The design allows for a long total stroke and a large inter actuator stroke. The actuators are produced in arrays which make the design modular and easily extendable. The first actuators and an actuator grid are produced and tested in a dedicated test set-up. This paper describes how relevant actuator properties, such as stiffness and efficiency, can be influenced by the design. The power dissipation in the actuator grid is optimized to a few milliwatts per actuator, thereby avoiding active cooling.

Water-cooled bimorph mirrors for beam correction and formation in high power CW lasers were developed and investigated. Different type of substrate has been considered to thermally stabilize of the mirror surface shape. Silicon bimorph mirror was tested in ceramic Nd:YAG CW laser.

This paper describes the recent progress made in the development of deformable bimorph mirrors as part of an adaptive optics toolkit - a set of low cost plug and play components aimed at non specialist applications engineers and scientists. The paper addresses the design issues, the manufacturability and the reliability, and discusses the compromises that arise due to cost and performance considerations. Current mirror designs have 55 or 31 electrodes, a deformation stroke of up to 80μm across a 40mm diameter, and a resonant frequency of up to 5kHz for a 25mm diameter device.

Recently, a number of research groups around the world have developed ophthalmic instruments capable of in vivo diffraction limited imaging of the human retina. Adaptive optics was used in these systems to compensate for the optical aberrations of the eye and provide high contrast, high resolution images. Such compensation uses a wavefront sensor and a wavefront corrector (usually a deformable mirror) coordinated in a closed- loop control system that continuously works to counteract aberrations. While those experiments produced promising results, the deformable mirrors have had insufficient range of motion to permit full correction of the large amplitude aberrations of the eye expected in a normal population of human subjects. Other retinal imaging systems developed to date with MEMS (micro-electromechanical systems) DMs suffer similar limitations. This paper describes the design, manufacture and testing of a 6um stroke polysilicon surface micromachined deformable mirror that, coupled with an new optical method to double the effective stroke of the MEMS-DM, will permit diffraction-limited retinal imaging through dilated pupils in at least 90% of the human population. A novel optical design using spherical mirrors provides a double pass of the wavefront over the deformable mirror such that a 6um mirror displacement results in 12um of wavefront compensation which could correct for 24um of wavefront error. Details of this design are discussed. Testing of the effective wavefront modification was performed using a commercial wavefront sensor. Results are presented demonstrating improvement in the amplitude of wavefront control using an existing high degree of freedom MEMS deformable mirror.

A two-voltage-driving technique is applied to build liquid crystal (LC) lens and LC microlens. One bias voltage is fixed and the other one varies to control the focal length of the LC lens or the LC microlens. The range of the variable focus is wide, and in the entire focus range the optical quality is preserved. The application of the LC lens as a focusing lens for cameras is demonstrated.

Studies on the liquid crystal lens with curved electrode are reported. The lens power is dependent on the applied voltage, and the lens size is nearly that of the curved electrode and therefore can be changed arbitrarily. The influences of material property and lens geometry on the properties of the LC lens are studied numerically. The properties of the lens of different geometries are investigated experimentally.

We present and demonstrate a technique for producing a high-speed variable focus lens using a fixed birefringent lens and a ferroelectric liquid crystal cell as a polarization switch. A calcite lenses with ordinary and extraordinary focal lengths of 109mm and 88mm respectively, was used to demonstrate focus switching at frequencies of up to 3kHz. Two identical lenses and a single liquid crystal were also used to demonstrate zoom.

The purpose of this study is to analyze the transient fluid-membrane interaction phenomenon of a micro liquid lens with the multi-fluid entrances (single and two). To understand the behavior of the lens, finite element analysis is used. Based on the results of transient analysis, the focal length of single and two entrances liquid lens is almost the same, but the two entrances design can stabilize quickly. This characteristic can shorten the response time when the liquid lens is used in high-speed focus variation system such as optical communications in micro optical electro mechanical system (MOEMS). The transient deformation measurement system of micro lens transient is not easy to be achieved; a measurement system for a 20mm diameter liquid filled lens prototype is indicated and tested for the formation of image. For the finite element analysis, finite element models constructed by the ANSYS Fluid Structure Interaction (FSI) module are employed to find the transient fluid-membrane interaction phenomenon.

In this study, the nonlinear effect of the shape of a membrane on optical parameters of the lens is discussed. To understand the effect of membranes with different shape under different liquid pressure, nonlinear finite element analysis is used to find the deformed shape of the membrane. After the deformed shape of the membrane is found, according to the shape of the membrane, the optical parameters (such as focal length, resolution and spot diagram) of the lens are studied. The parameters which considered in this study are the thickness of the membrane, the liquid pressure and the dimension of the lens. The results show that the bending of the membrane and the boundary conditions of the membrane are two important parameters to optical parameters of the lens.

We use a simplified mechanical/electrostatic model to describe the coupling between mirror and actuator. A WYKO interferometer is used to characterize the electromechanical performance of a MEMS deformable mirror (Boston Micromachines, Inc). We measured the voltage vs. deflection curves for the sample actuator with and without energizing the local adjacent neighbor actuators. This characterization results generated a quadratic and a linear equations to predict required voltage for actuators under different deflection profiles. We incorporated the MEMS mirror into a simple adaptive optics (AO) testbed. The system includes a near infrared superluminescent diode, a MEMS deformable mirror (DM), and a Shack Hartmann wavefront sensor (SHWS). The real time measurements provided by the SHWS (wavefront slopes) were the input to an integral controller. The controller was calibrated in situ by the typical method of determining an influence matrix which poke each actuator separately and measuring the resulting wavefront slopes at each lenslet. The control software then use the error signal between the current SHS positions and the desired positions, applied the characteristic model of the mirror, and determined the appropriate voltage to apply to each actuator, given the desired deflection for the surrounding actuators. The system was able to provide real time aberration compensation at loop gains of 0.3. A set of Zernike polynomial shapes were produced by DM under different loop gains to test the ability of control. A large proportion of the final wavefront shape could be achieved in a single iteration with a loop gain 1.0.

In this paper we present results using a compact, portable adaptive optics system. Such compact systems are possible thanks to the use of new technologies based on Micro-Electro-Machined deformable mirrors and liquid crystals devices among other possible technologies. In the paper we will illustrate our experience with such devices; we will address the pros and cons of such approach, some experimental results and new trends for future tests.

In most adaptive optics systems, there are two elements that control wavefront correction. These are a tip/tilt platform and a deformable mirror. The tip/tilt platform can correct the lower order aberrations like piston, tip and tilt. The deformable mirror can correct the higher order aberrations like defocus, astigmatism, etc. By mounting the deformable mirror onto the tip/tilt platform, one corrective element is now used in the system, rather than two. This is made possible by the use of a lightweight MEMS deformable mirror, as traditional deformable mirrors tend to be quite large and bulky. Other advantages are that there is less overall optics and a simpler alignment process needed with this configuration.

Many adaptive optics systems rely on a wavefront sensor (WFS) to sense the aberrations in an incoming wavefront. The required corrections are determined and applied by the wavefront corrector - often a deformable mirror (DM). We wish to develop a wavefront sensor-less correcting system, as derived from the original adaptive optics system of Muller and Buffington[1]. In this experiment we apply commands to a corrective element with adjustable segments in an attempt to maximise a metric which correlates to image quality. We employ search algorithms to find the optimal combination of actuator voltages on a DM to maximise a certain sharpness metric. The "sharpness" is based on intensity measurements taken with a CCD camera. It has shown [2] that sharpness maximisation, using the Simplex algorithm[3], can minimise the aberrations and restore the Airy rings of an imaged point source. The results are repeatable and so-called "blind" correction of the aberrations is achieved. This paper demonstrates that the technique can be applied to extended objects which have been aberated using a Hamamatsu SLM to induce aberrations. The correction achieved using various search algorithms are evaluated and presented.

The original algorithm of phase reconstruction is proposed and its characteristics are estimated. This algorithm requires detection of dislocation coordinates so it should be used together with the algorithm of dislocation localization. In the paper the comparison is performed of the proposed algorithm with the well known Fried's algorithm. After that the phase reconstruction technique is included into the model of a typical adaptive optics system.

Phase diversity is a phase-retrieval algorithm that uses a pair of defocused intensity images taken symmetrically about the wavefront to be determined. Generalised phase diversity is a phase-retrieval algorithm that uses diversity functions other than defocus. The approach adopted assumes that unknown phase changes satisfy the small-angle approximation over spatial regions that can be selected by choice of the diversity function. For smooth functions, and for discontinuous functions with only small discontinuities, this leads to a very simple analytic solution. Computer simulations were used to validate this method for the retrieved phase.

The paper discusses the influence of the Hartmann-(Shack) wavefront sensor geometry on the total error of modal wavefront reconstruction. A mathematical model is proposed which describes modal wavefront reconstruction based on Hartmann or Hartmann-Shack sensor in terms of linear operators. The modal covers the most general case and is not limited by the orthogonality of decomposition basis or by the method chosen for decomposition. The total reconstruction error is calculated for any given statistics of the wavefront to be measured. Based on this estimate, total reconstruction error is calculated for regular and randomised Hartmann masks. The calculations demonstrate that use of random masks with non-regular Fourier spectra for Zernike wavefront reconstruction for atmospheric turbulence allows to double the number of decomposition modes with the same total error.

A number of reflective wavefront correctors used in adaptive optics are based on the use of piezoelectric effect, either in piston, tip/tilt or curvature devices. The relation between the voltage applied to drive these devices and the mechanical response always presents hysteresis to some extent. In this work we study the performance of Preisach's classical and non-linear models of hysteresis on a bimorph mirror, which is a curvature device, but both models can also be applied to piston and tip/tilt devices. Bimorph mirrors with PZT actuators and a passive glass substrate were tested in an adaptive optics test-bed (AOTB) using a Shack-Hartmann wavefront sensor. First- and second-order reversal curves were sampled uniformly in Preisach space, and interpolation algorithms were implemented to test Preisach's classical and non-linear forward models respectively. Then, arbitrary voltage configuration sequences were applied to the mirror and the responses were recorded. Finally, the inversion of the models was implemented and included in the AOTB linear control algorithm to test the closed-loop performance. We found that both hysteresis models provide a similar improvement in the open-loop error. The improvement estimation depends on the particular sequence applied, the number of samples of the Preisach function and noise among other factors. Finally, we present data showing that the hysteretic behavior in a multi-electrode mirror is, within experimental error, independent of the electrode geometry, area and location.

Addressing of massive arrays of piezoelectric actuators is usually achieved by using separate high-voltage output drivers, one per channel. This approach applied to high-order adaptive optics systems results in complex, expensive and vulnerable to handling abuse driver electronics, hardly scalable to 10³-10⁴ actuators. To reduce the number of identical electronic units and simplify the control, we propose sequential multiplexing of piezoelectric actuators. The relatively large capacitance inherent in mirror piezo-actuators allows for storage of charge (high voltage) on a disconnected actuator retaining its displacement, while other actuators are addressed. As a demonstrator a 12-channel piezoelectric deformable mirror driven by a single high-voltage amplifier has been characterized experimentally. The multiplexing of actuators was accomplished by miniature optical switches. Temporal stability of ~λ/100 was demonstrated at multiplexing frequency of 700 Hz with a full-range ~2 µm inter-actuator stroke. The developed approach can be scaled to higher-order deformable mirrors.

The Durham University Centre for Advanced Instrumentation (formerly the Astronomical Instrumentation Group) has been involved in the development of both research and facility class astronomical adaptive optics systems and their associated control systems for over 20 years. The centre also has interests in commercial and non-astronomical research AO systems, including the areas of novel wavefront correctors and AO control systems. Firstly this paper summarises Durham's history in AO control, then we seek to provide an overview of why the group is moving to FPGA technology for control systems in the context of new work. We then present a collaboration between Sira Technologies Ltd. and Durham, aimed at exploiting FPGA technology to provide high speed, low cost AO control for commercial applications, we examine the results so far achieved, and finally we present our Integrated Wavefront Sensor product concept.

Confocal techniques allow the user to achieve optically sectioned images with significantly enhanced axial and improved lateral resolution compared to widefield methods. Unfortunately, as one images more deeply within a sample, sample induced aberrations lead to a significant reduction in image resolution and contrast. Using adaptive optic techniques, we report on the effectiveness of a number of algorithms for removing sample induced aberrations. The viability and efficiency at a number of fitness parameters used in the optimisation routines is also considered.

Currently, in most adaptive optical systems, the control loop between the wavefront sensor and the deformable mirror involves intense mathematical calculations, both during calibration and operation of the system. Although thorough research has been done to optimise the control loop, some issues like error propagation and system bandwidth will always be ultimately limited by the coupling between the mirror and the wavefront sensor. Closed-loop by direct feedback from the wavefront sensor to the deformable mirror was proposed by F. Roddier in his well-quoted curvature wavefront sensing paper. However, due to the natural properties of the defocused-image, this direct feed-back method is limited to bimorph mirror applications only. Recently, M.A.A Neil et al proposed a new modal wavefront sensor (MWFS), which can detect several Zernike modes by a simple intensity subtraction operation. One drawback of this method is that it can only handle a limited number of modes. However, in this paper, we refine this method to detect the orthogonal modes of a deformable mirror instead of Zernike modes in a to-be corrected wavefront. Since the number of actuators of a deformable mirror limits the number of mirror modes, the drawback is minimised in this application. Considering the mirror modes can be directly transformed to the deformable mirror control command set by a proper gain coefficient, it is reasonable to construct a direct-feed back adaptive optical system with the modal wavefront sensing. We will report our first stage investigation on direct feedback adaptive optical system which is to understand the response of MWFS to mirror modes.

High-resolution imaging can be achieved by optical aperture synthesis (OAS). Such an imaging process is subject to aberrations introduced by instrumental defects and/or turbulent media. Redundant spacings calibration (RSC) is a snapshot calibration technique that can be used to calibrate OAS arrays without use of assumptions about the object being imaged. Here we investigate the analogies between RSC and adaptive optics in passive imaging applications.

A new type of space telescope program is developing technology which is to be used in the design of a 4-meter segmented mirror telescope. One acceptable configuration being considered for the primary mirror is a lightweight honeycomb sandwich structure segmented mirror. There can be optically-significant distortions when the primary mirror is exposed to typical thermal environment. This paper discusses the surface distortions subjected to thermal distributions. Finite element analyses were performed to predict the optical surface distortions of the 4-meter primary mirror due to the effects of thermal variations. The temperature patterns were described by a least-squares fit to a polynomial expression, and the polynomial was then used to predict temperature patterns. The finite softwares were also used to analyze cases of axial gradients, radial gradients and the local thermal gradients etc. The result from the finite element analysis is analyzed, and is also presented.