Showing papers on "Adaptive optics published in 2007"

Adaptive optics in microscopy

Abstract: The imaging properties of optical microscopes are often compromised by aberrations that reduce image resolution and contrast. Adaptive optics technology has been employed in various systems to correct these aberrations and restore performance. This has required various departures from the traditional adaptive optics schemes that are used in astronomy. This review discusses the sources of aberrations, their effects and their correction with adaptive optics, particularly in confocal and two-photon microscopes. Different methods of wavefront sensing, indirect aberration measurement and aberration correction devices are discussed. Applications of adaptive optics in the related areas of optical data storage, optical tweezers and micro/nanofabrication are also reviewed.

Image based adaptive optics through optimisation of low spatial frequencies.

Abstract: We present a wave front sensorless adaptive optics scheme for an incoherent imaging system. Aberration correction is performed through the optimisation of an image quality metric based upon the low spatial frequency content of the image. A sequence of images is acquired, each with a different aberration bias applied and the correction aberration is estimated from the information in this image sequence. It is shown, by representing aberrations as an expansion in Lukosz modes, that the effects of different modes can be separated. The optimisation of each mode becomes independent and can be performed as the maximization of a quadratic function, requiring only three image measurements per mode. This efficient correction scheme is demonstrated experimentally in an incoherent transmission microscope. We show that the sensitivity to different aberration magnitudes can be tuned by changing the range of spatial frequencies used in the metric.We also explain how the optimization scheme is related to other methods that use image sharpness metrics.

Wavefront sensorless adaptive optics for large aberrations.

Abstract: In some adaptive optics systems the aberration is determined not by using a wavefront sensor but by sequential optimization of the adaptive correction element. Efficient schemes for the control of such systems are essential if they are to be effective. A scheme is introduced that permits the efficient measurement of large amplitude wavefront aberrations that are represented by an appropriate series of modes. This scheme uses an optimization metric based on the root-mean-square spot radius (or focal spot second moment) and an aberration expansion using polynomials suited to the representation of lateral aberrations. Experimental correction of N aberration modes is demonstrated with a minimum of N+1 photodetector measurements. The geometrical optics basis means that the scheme can be extended to arbitrarily large aberrations.

Large-field-of-view, modular, stabilized, adaptive-optics-based scanning laser ophthalmoscope

Abstract: We describe the design and performance of an adaptive optics retinal imager that is optimized for use during dynamic correction for eye movements. The system incorporates a retinal tracker and stabilizer, a wide field line scan Scanning Laser Ophthalmocsope (SLO), and a high resolution MEMS based adaptive optics SLO. The detection system incorporates selection and positioning of confocal apertures, allowing measurement of images arising from different portions of the double pass retinal point spread function (psf). System performance was excellent. The adaptive optics increased the brightness and contrast for small confocal apertures by more than 2x, and decreased the brightness of images obtained with displaced apertures, confirming the ability of the adaptive optics system to improve the pointspread function. The retinal image was stabilized to within 18 microns 90% of the time. Stabilization was sufficient for cross-correlation techniques to automatically align the images.

Broadband wavefront correction algorithm for high-contrast imaging systems

Abstract: Great strides have been made in recent years toward the goal of high-contrast imaging with a sensitivity adequate to detect earth-like planets around nearby stars. It appears that the hardware − optics, coronagraph masks, deformable mirrors, illumination systems, thermal control systems − are up to the task of obtaining the required 10-10 contrast. But in broadband light (e.g., 10% bandpass) the wavefront control algorithms have been a limiting factor. In this paper we describe a general correction methodology that works in broadband light with one or multiple deformable mirrors by conjugating the electric field in a predefined region in the image where terrestrial planets would be found. We describe the linearized approach and demonstrate its effectiveness through laboratory experiments. This paper presents results from the Jet Propulsion Laboratory High Contrast Imaging Testbed (HCIT) for both narrow-band light (2%) and broadband light (10%) correction.

Temporal Evolution of Coronagraphic Dynamic Range and Constraints on Companions to Vega

Abstract: The major obstacle to the direct detection of companions to nearby stars is the overwhelming brightness of the host star. Current instruments employing the combination of adaptive optics (AO) and coronagraphy can typically detect objects within 2'' of the star that are ~104-105 times fainter. Correlated speckle noise is one of the biggest obstacles limiting such high-contrast imaging. We have obtained a series of 284 8 s, AO-corrected, coronagraphically occulted H-band images of the star Vega at the 3.63 m AEOS telescope located on Haleakala, Hawaii. This data set is unique for studying the temporal behavior of speckle noise and represents the first time such a study on highly corrected coronagraphic AO images has been carried out in a quantitative way. We find the speckle pattern to be highly stable in both position and time in our data. This is due to the fact that the AO system corrects disturbances to the stellar wave front at the level where the instrumental wave front errors dominate. Because of this, we find that our detection limit is not significantly improved simply with increased exposure time alone. However, we are able to improve our dynamic range by 1.5-2 mag through subtraction of static/quasi-static speckles in two rotating frames: the telescope pupil frame and the deformable mirror frame. The highly stable nature of speckles will exist for any program using coronagraphy and high-order AO. Furthermore, from our data, we are able to constrain the mass of any purported companion to Vega to be less than ~45MJ at 8 AU and less than ~30MJ at 16 AU, radii not previously probed at these sensitivities.

PROPER: an optical propagation library for IDL

Abstract: PROPER is a library of IDL (Interactive Data Language) routines for simulating optical propagation in the near and far fields using Fourier-based Fresnel and angular spectrum methods. The goal of PROPER is to provide a free, easy-to-use, and versatile means for simulating systems that require diffraction-based rather than geometrical analyses, such as spatial filtering systems with intermediate optics (e.g. a stellar coronagraph for extrasolar planet imaging). It has routines for creating complex apertures and obscurations, wavefront errors (defined by Zernikes, power spectra, or user-supplied maps), amplitude modulators (e.g. coronagraphic occulters), simple lenses, and deformable mirrors. The routines automatically select which propagator (near or far-field) is best at each surface based on analytically propagating a Gaussian pilot beam. The library includes a comprehensive manual and is distributed as IDL source code.

Calibration and precompensation of noncommon path aberrations for extreme adaptive optics.

Abstract: Noncommon path aberrations (NCPAs) are one of the main limitations of an extreme adaptive optics (AO) system. NCPAs prevent extreme AO systems from achieving their ultimate performance. These static aberrations are unseen by the wavefront sensor and therefore are not corrected in closed loop. We present experimental results validating what we believe to be new procedures of measurement and precompensation of the NCPAs on the AO bench at ONERA (Office National d'Etudes et de Recherches Aerospatiales). The measurement procedure is based on refined algorithms of phase diversity. The precompensation procedure makes use of a pseudo-closed-loop scheme to overcome the AO wavefront-sensor-model uncertainties. Strehl ratio obtained in the images reaches 98.7% at 632.8 nm. This result allows us to be confident of achieving the challenging performance required for direct observation of extrasolar planets.

Adaptive optics-optical coherence tomography: optimizing visualization of microscopic retinal structures in three dimensions.

Abstract: Adaptive optics-optical coherence tomography (AO-OCT) permits improved imaging of microscopic retinal structures by combining the high lateral resolution of AO with the high axial resolution of OCT, resulting in the narrowest three-dimensional (3D) point-spread function (PSF) of all in vivo retinal imaging techniques. Owing to the high volumetric resolution of AO-OCT systems, it is now possible, for the first time, to acquire images of 3D cellular structures in the living retina. Thus, with AO-OCT, those retinal structures that are not visible with AO or OCT alone (e.g., bundles of retinal nerve fiber layers, 3D mosaic of photoreceptors, 3D structure of microvasculature, and detailed structure of retinal disruptions) can be visualized. Our current AO-OCT instrumentation uses spectrometer-based Fourier-domain OCT technology and two-deformable-mirror-based AO wavefront correction. We describe image processing methods that help to remove motion artifacts observed in volumetric data, followed by innovative data visualization techniques [including two-dimensional (2D) and 3D representations]. Finally, examples of microscopic retinal structures that are acquired with the University of California Davis AO-OCT system are presented.

Fourier transform wavefront control with adaptive prediction of the atmosphere

Abstract: Predictive Fourier control is a temporal power spectral density-based adaptive method for adaptive optics that predicts the atmosphere under the assumption of frozen flow. The predictive controller is based on Kalman filtering and a Fourier decomposition of atmospheric turbulence using the Fourier transform reconstructor. It provides a stable way to compensate for arbitrary numbers of atmospheric layers. For each Fourier mode, efficient and accurate algorithms estimate the necessary atmospheric parameters from closed-loop telemetry and determine the predictive filter, adjusting as conditions change. This prediction improves atmospheric rejection, leading to significant improvements in system performance. For a 48×48 actuator system operating at 2 kHz, five-layer prediction for all modes is achievable in under 2×109 floating-point operations/s.

Adaptive optics for enhanced signal in CARS microscopy.

Abstract: We report the use of adaptive optics with coherent anti-Stokes Raman scattering (CARS) microscopy for label-free deep tissue imaging based on molecular vibrational spectroscopy. The setup employs a deformable membrane mirror and a random search optimization algorithm to improve signal intensity and image quality at large sample depths. We demonstrate the ability to correct for both system and sample-induced aberrations in test samples as well as in muscle tissue in order to enhance the CARS signal. The combined system and sample-induced aberration correction increased the signal by an average factor of ~3x for the test samples at a depth of 700 µm and ~6x for muscle tissue at a depth of 260 µm. The enhanced signal and higher penetration depth offered by adaptive optics will augment CARS microscopy as an in vivo and in situ biomedical imaging modality.

Stabilized retinal imaging with adaptive optics

Abstract: A system provides an optical image of an object. A first module tracks a reference feature of the object. A second module includes a source for an imaging beam, a scanning device to move the imaging beam along a portion of the object and a detection device receives a signal associated with an image of the portion of the object. The first module controls the position of the imaging beam relative to the reference feature to correct for the motion of the object. A third module detects a distortion of the object and compensates for the distortion.

High-resolution adaptive optics scanning laser ophthalmoscope with multiple deformable mirrors

Abstract: An adaptive optics scanning laser ophthalmoscopes is introduced to produce non-invasive views of the human retina. The use of dual deformable mirrors improved the dynamic range for correction of the wavefront aberrations compared with the use of the MEMS mirror alone, and improved the quality of the wavefront correction compared with the use of the bimorph mirror alone. The large-stroke bimorph deformable mirror improved the capability for axial sectioning with the confocal imaging system by providing an easier way to move the focus axially through different layers of the retina.

Effects of Zernike wavefront aberrations on visual acuity measured using electromagnetic adaptive optics technology.

Abstract: Purpose This study measured the changes in visual acuity induced by individual Zernike ocular aberrations of various root-mean-square (RMS) magnitudes. Methods A crx1 Adaptive Optics Visual Simulator (Imagine Eyes) was used to modify the wavefront aberrations in nine eyes. After measuring ocular aberrations, the device was programmed to compensate for the eye's wavefront error up to the 4th order and successively apply different individual Zernike aberrations using a 5-mm pupil. The generated aberrations included defocus, astigmatism, coma, trefoil, and spherical aberration at a level of 0.1, 0.3, and 0.9 microm. Monocular visual acuity was assessed using computer-generated Landolt-C optotypes. Results Correction of the patients' aberrations improved visual acuity by a mean of 1 line (-0.1 logMAR) compared to best sphero-cylinder correction. Aberrations of 0.1 microm RMS resulted in a limited decrease in visual acuity (mean +0.05 logMAR), whereas aberrations of 0.3 microm RMS induced significant visual acuity losses with a mean reduction of 1.5 lines (+0.15 logMAR). Larger aberrations of 0.9 microm RMS resulted in greater visual acuity losses that were more pronounced with spherical aberration (+0.64 logMAR) and defocus (+0.62 logMAR), whereas trefoil (+0.22 logMAR) was found to be better tolerated. Conclusions The electromagnetic adaptive optics visual simulator effectively corrected and generated wavefront aberrations up to the 4th order. Custom wavefront correction significantly improved visual acuity compared to best-spectacle correction. Symmetric aberrations (eg, defocus and spherical aberration) were more detrimental to visual performance.

Development of Deformable Mirror Composed of Piezoelectric Thin Films for Adaptive Optics

Abstract: In this paper, we report a piezoelectric deformable mirror composed of piezoelectric thin films for low-voltage adaptive optics (AO). A 2-mum-thick piezoelectric Pb(Zr,Ti)O3 (PZT) film was deposited on a Pt-coated silicon-on-insulator (SOI) substrate, and a diaphragm structure of 15 mm in diameter was fabricated by etching a Si handle wafer. A 19-element unimorph actuator array was produced on the PZT films with an Al reflective layer over the backside of the diaphragm. Measurements of the displacement profile using a laser Doppler vibrometer demonstrated that a large displacement of approximately 1 mum was obtained by applying a voltage of 10 Vpp on one actuator. To examine the application feasibility of the deformable mirror to AO, we reproduced low-order Zernike modes by calculating the voltage on each individual electrode using an influence function matrix. The measurements demonstrated that the deformable mirror could produce the Zernike modes up to the seventh term. Considering the low-voltage actuation as well as the capability for miniaturization of the electrode size, deformable mirrors (DMs) actuated by PZT films are desirable for low-cost AO

Use of adaptive optics to determine the optimal ocular spherical aberration

Abstract: METHODS: An adaptive optics vision simulator consisting of a wavefront sensor, a 97-segmented deformable mirror to induce and correct aberrations of the eye, and a visual testing path was constructed for this study. The deformable mirror allows the effective ocular wavefront aberration to be manipulated and the resulting visual performance to be measured simultaneously. Subjective measurements of contrast sensitivity at 15 cycles per degree were performed with a 4.8 mm pupil in 5 subjects with different levels of naturally occurring SA. Contrast sensitivity was measured when SA values of 0.09 mm, 0.0 mm, 0.09 mm, and 0.182 mm were induced when the other natural aberrations of the eye were present, when the aberrations were corrected, and at defocus values of G0.25 diopter (D) and G0.50 D. RESULTS: Subjects experienced peak contrast sensitivity performance with varying levels of SA when their natural aberrations were present; however, average contrast performance peaked at 0 mm of SA. When all higher-order aberrations were corrected, all 5 subjects’ peak performance occurred at 0 m mo f SA. CONCLUSIONS: The adaptive optics vision simulator reduced the root-mean-square wavefront aberration of the eye by upto a factor of4 and allowed noninvasive testing ofthe visual performance resulting from any ocular wavefront aberration introduced by customized correction procedures. This study showed that, on average, contrast performance peaked when SA was completely corrected.

LCOS spatial light modulator controlled by 12-bit signals for optical phase-only modulation

Abstract: We developed a liquid-crystal-on-silicon (LCOS) spatial light modulator (SLM) for phase-only modulation. The SLM was designed mainly for wavefront control in adaptive optics, optical manipulation, laser processing, etc. A dielectric multilayer mirror was incorporated into the device to enhance the reflectivity. The number of pixels was 792 x 612 and their size was 20 x 20 microns square. The range of the phase modulation exceeded one wavelength, and the light-utilization efficiency for monochromatic light was approximately 90%. The silicon backplane of the SLM was mechanically weak and its surface was not flat. The poor flatness degraded the output wavefront from the SLM. The device was driven by electronics composed of a digital-visual-interface (DVI) receiver, a field programmable gate array, and 12-bit digital-to-analog converters (DACs). The converted analog voltage signals from the DACs were transmitted to the pixels of the SLM and created phase changes. The driver had several kinds of control modes for the device, according to the level of flatness compensation. In one of the modes, the driver received 12-bit data and transferred them directly to the DACs. This 12-bit control mode enabled highly flexible control of the device characteristics. In the presentation, we report details of the device and experimental results on compensation of distortion in the output wavefront from the device.

Improved lateral resolution in optical coherence tomography by digital focusing using two-dimensional numerical diffraction method.

Abstract: This paper proposes a non-iterative, two-dimensional numerical method to eliminate the compromise between the lateral resolution and wide depth measurement range in optical coherence tomography (OCT). A two-dimensional scalar diffraction model was developed to simulate the wave propagation process from out-of-focus scatters within the short coherence gate of the OCT system. High-resolution details can be recovered from outside of the depth-of-focus region with minimum loss of lateral resolution. Experiments were performed to demonstrate the effectiveness of the proposed method.

Wavefront sensing with random amplitude mask and phase retrieval.

Abstract: A light beam with an ideal wavefront that is transmitted or reflected from an object is modified by different characteristics of the object such as shape, refractive index, density, or temperature. Wavefront sensing therefore yields valuable information about the system or the changes happening to the system. A new method for wavefront sensing using a random amplitude mask and a phase retrieval method based on the Rayleigh-Sommerfeld wave propagation equation is described. The proposed method has many potential applications ranging from phase contrast imaging and measurement of lens aberration to shape measurement of three-dimensional objects.

Compact multimodal adaptive-optics spectral-domain optical coherence tomography instrument for retinal imaging.

Abstract: We have developed a compact, multimodal instrument for simultaneous acquisition of en face quasi-confocal fundus images and adaptive-optics (AO) spectral-domain optical coherence tomography (SDOCT) cross-sectional images. The optical system including all AO and SDOCT components occupies a 60×60 cm breadboard that can be readily transported for clinical applications. The AO component combines a Hartmann-Shack wavefront sensor and a microelectromechanical systems-based deformable mirror to sense and correct ocular aberrations at 15 Hz with a maximum stroke of 4 μm. A broadband superluminescent diode source provides 4 μm depth resolution for SDOCT imaging. In human volunteer testing, we observed up to an 8 dB increase in OCT signal and a corresponding lateral resolution of <10 μm as a result of AO correction.

Open-loop control of a MEMS deformable mirror for large-amplitude wavefront control

Abstract: A method is introduced for predicting control voltages that will generate a prescribed surface shape on a MEMS deformable mirror. The algorithm is based upon an analytical elastic model of the mirror membrane and an empirical electromechanical model of its actuators. It is computationally simple and inherently fast. Shapes at the limit of achievable mirror spatial frequencies with up to 1.5 μm amplitudes have been achieved with less than 15 nm rms error.

Exploiting the spatiotemporal correlation in adaptive optics using data-driven H2-optimal control

Abstract: A recently proposed data-driven H2-optimal control approach is demonstrated on a laboratory setup. Most adaptive optics (AO) systems are based on a control law that neglects the temporal evolution of the wavefront. The proposed control approach is able to exploit the spatiotemporal correlation in the wavefront without assuming any form of decoupling. By analyzing the dynamic behavior of the wavefront sensor (WFS), it is shown that if the wavefront correction device can be considered static, the transfer function from control input to WFS output reduces to a two-tap impulse response and an integer number of samples delay. Considering this model structure, a data-driven identification procedure is developed to estimate the relevant parameters from measurement data. The specific structure allows for an analytical expression of the optimal controller in terms of the system matrices of the minimum-phase spectral factor of the atmospheric disturbance model. The performance of the optimal controller is compared with that of the standard AO control law. An analysis of the dominant error sources shows that optimal control may reduce the temporal error.

Wavefront reconstruction of an optical vortex by a Hartmann-Shack sensor.

Abstract: Reconstruction the phase front of a vortex laser beam is conducted by use of a Hartmann-Shack wavefront sensor. The vortex beam in the form of the Laguerre-Gaussian LG01 mode is generated with the help of a spiral phase plate. The new reconstruction technique based on measured wavefront gradients allows one to restore the singular phase surface with good accuracy, whereas the conventional least-squares approach fails.

Diffractive Transmission Grating Tuned by Dielectric Elastomer Actuator

Abstract: A technology for the implementation of electrically tunable optical elements is presented. We illustrate, on the example of a transmission grating (TG), that dielectric elastomer actuators can be used to continuously adjust the properties of diffractive optical elements. The demonstrated TG operates with high transmission, low optical distortion ( 93 kW/cm2), is polarization-independent, and achieves a very large tuning range (7.5% grating period compression). Extending the presented technology to other optical elements, including lenses, phase shifters, or intensity filters, is expected to result in a wide range of electrically tunable optical devices.

Detection of phase singularities with a Shack-Hartmann wavefront sensor

Abstract: While adaptive optical systems are able to remove moderate wavefront distortions in scintillated optical beams, phase singularities that appear in strongly scintillated beams can severely degrade the performance of such an adaptive optical system. Therefore the detection of these phase singularities is an important aspect of strong-scintillation adaptive optics. We investigate the detection of phase singularities with the aid of a Shack-Hartmann wavefront sensor and show that, in spite of some systematic deficiencies inherent to the Shack-Hartmann wavefront sensor, it can be used for the reliable detection of phase singularities, irrespective of their morphologies. We provide full analytical results, together with numerical simulations of the detection process.

Aero-Optical Environment Around a Conformal-Window Turret

Abstract: This paper presents the aero-optical environment around a generic conformal-window turret formed from a hemisphere on a short cylindrical base. A suite of optical instruments consisting of a Malley probe, a conventional two-dimensional Shack-Hartmann wave-front sensor, and a new high-bandwidth, lower-resolution Hartmann wave-front sensor were used to measure the aberrations on the wave front of a laser beam emanating from the turret at various angles in both the forward and aft direction in the turret's zenith plane. The measurements were made over a range of Mach numbers from 0.35 to 0.45. Complementary steady- and unsteady-pressure measurements over a slightly larger range of Mach numbers were also made, along with a surface-flow-visualization study of the complex flowfield over and around the turret. The use of the suite of sensors allowed for the recognition and separation of the aberrating optical environment into components associated with stationary disturbances and convecting disturbances at the frequency of the turret's separated wake and at order-of-magnitude-higher frequencies associated with structures that form in the separated shear layers, respectively. The optical data separated in this way are valuable because of the implications for adaptive optics.

Polychromatic compensation of propagated aberrations for high-contrast imaging

Abstract: We describe the influence of Fresnel propagation in apodized-pupil coronagraphs and introduce a methodology to compensate for propagated wavefront aberrations. We start from the Fresnel integral and derive an analytical closed form for the propagated field at an arbitrary distance from a pupil. In a second part, we show that the propagation of the coronagraphic term can be neglected with sufficiently oversized optics. Then we derive a λ-Fourier expansion of the aberration at an arbitrary plane in the optical train of a telescope. Finally, we present a series of wavefront actuators, based on multiple deformable mirrors, that possess adequate chromatic behavior to correct for the dominant terms of this expansion.

Liquid crystal Adaptive Optics Visual Simulator: Application to testing and design of ophthalmic optical elements

Abstract: The concept of Adaptive Optics Visual Simulation applies to the use of an Adaptive Optics system to manipulate ocular aberrations in order to perform visual testing through a modified optics. It can be of interest both to study the visual system and to design new ophthalmic optical elements. In this work, we describe an apparatus based on a liquid crystal programmable phase modulator and explore its capabilities as a tool in the early stages of the design of ophthalmic optical elements with increased depth of field for presbyopic subjects. To illustrate the potential of the instrument, we analyze the performance of two phase profiles obtained by a hybrid optimization procedure. The liquid crystal Adaptive Optics Visual Simulator can be used to experimentally record the point spread function for different vergences in order to objectively measure depth of focus, to perform different psychophysical experiments through the phase profile in order to measure its impact on visual performance, and to study the interaction with the eye's particular aberrations. This approach could save several steps in current procedures of ophthalmic optical design and eventually lead to improved solutions.

Modelling the application of adaptive optics to wide-field microscope live imaging.

Abstract: Wide-field fluorescence microscopy is an essential tool in modern cell biology. Unfortunately the image quality of fluorescence microscopes is often significantly degraded due to aberrations that occur under normal imaging conditions. In this article, we examine the use of adaptive optics technology to dynamically correct these problems to achieve close to ideal diffraction limited performance. Simultaneously, this technology also allows ultra-rapid focusing without having to move either the stage or the objective lens. We perform optical simulations to demonstrate the degree of correction that can be achieved.

Maximum-likelihood methods in wavefront sensing: stochastic models and likelihood functions.

Abstract: Maximum-likelihood (ML) estimation in wavefront sensing requires careful attention to all noise sources and all factors that influence the sensor data. We present detailed probability density functions for the output of the image detector in a wavefront sensor, conditional not only on wavefront parameters but also on various nuisance parameters. Practical ways of dealing with nuisance parameters are described, and final expressions for likelihoods and Fisher information matrices are derived. The theory is illustrated by discussing Shack-Hartmann sensors, and computational requirements are discussed. Simulation results show that ML estimation can significantly increase the dynamic range of a Shack-Hartmann sensor with four detectors and that it can reduce the residual wavefront error when compared with traditional methods.