Showing papers on "Wavefront sensor published in 2009"

Quadriwave lateral shearing interferometry for quantitative phase microscopy of living cells

Abstract: Phase imaging with a high-resolution wavefront sensor is considered. This is based on a quadriwave lateral shearing interferometer mounted on a non-modified transmission white-light microscope. The measurement technology is explained both in the scope of wave optics and geometrical optics in order to discuss its implementation on a conventional microscope. In particular we consider the effect of a non spatially coherent source on the phase-image signal-to-noise ratio. Precise measurements of the phase-shift introduced by microscopic beads or giant unilamellar vesicles validate the principle and show the accuracy of the methods. Diffraction limited images of living COS-7 cells are then presented, with a particular focus on the membrane and organelle dynamics.

Coronagraphic Low Order Wavefront Sensor: Principle and Application to a Phase-Induced Amplitude Coronagraph

Abstract: High contrast coronagraphic imaging of the immediate surrounding of stars requires exquisite control of low-order wavefront aberrations, such as tip-tilt (pointing) and focus. We propose an accurate, efficient and easy to implement technique to measure such aberrations in coronagraphs which use a focal plane mask to block starlight. The Coronagraphic Low Order Wavefront Sensor (CLOWFS) produces a defocused image of a reflective focal plane ring to measure low order aberrations. Even for small levels of wavefront aberration, the proposed scheme produces large intensity signals which can be easily measured, and therefore does not require highly accurate calibration of either the detector or optical elements. The CLOWFS achieves nearly optimal sensitivity and is immune from non-common path errors. This technique is especially well suited for high performance low inner working angle (IWA) coronagraphs. On phase-induced amplitude apodization (PIAA) type coronagraphs, it can unambiguously recover aberrations which originate from either side of the beam shaping introduced by the PIAA optics. We show that the proposed CLOWFS can measure sub-milliarcsecond telescope pointing errors several orders of magnitude faster than would be possible in the coronagraphic science focal plane alone, and can also accurately calibrate residual coronagraphic leaks due to residual low order aberrations. We have demonstrated 1e-3 lambda/D pointing stability in a laboratory demonstration of the CLOWFS on a PIAA type coronagraph.

Binocular adaptive optics visual simulator.

Abstract: A binocular adaptive optics visual simulator is presented. The instrument allows for measuring and manipulating ocular aberrations of the two eyes simultaneously, while the subject performs visual testing under binocular vision. An important feature of the apparatus consists on the use of a single correcting device and wavefront sensor. Aberrations are controlled by means of a liquid-crystal-on-silicon spatial light modulator, where the two pupils of the subject are projected. Aberrations from the two eyes are measured with a single Hartmann-Shack sensor. As an example of the potential of the apparatus for the study of the impact of the eye's aberrations on binocular vision, results of contrast sensitivity after addition of spherical aberration are presented for one subject. Different binocular combinations of spherical aberration were explored. Results suggest complex binocular interactions in the presence of monochromatic aberrations. The technique and the instrument might contribute to the better understanding of binocular vision and to the search for optimized ophthalmic corrections.

Self-coherent camera as a focal plane wavefront sensor: simulations

Abstract: Direct detection of exoplanets requires high dynamic range imaging. Coronagraphs could be the solution, but their performance in space is limited by wavefront errors (manufacturing errors on optics, temperature variations, etc.), which create quasi-static stellar speckles in the final image. Several solutions have been suggested for tackling this speckle noise. Differential imaging techniques substract a reference image to the coronagraphic residue in a post-processing imaging. Other techniques attempt to actively correct wavefront errors using a deformable mirror. In that case, wavefront aberrations have to be measured in the science image to extremely high accuracy. We propose the self-coherent camera sequentially used as a focal-plane wavefront sensor for active correction and differential imaging. For both uses, stellar speckles are spatially encoded in the science image so that differential aberrations are strongly minimized. The encoding is based on the principle of light incoherence between the hosting star and its environment. In this paper, we first discuss one intrinsic limitation of deformable mirrors. Then, several parameters of the self-coherent camera are studied in detail. We also propose an easy and robust design to associate the self-coherent camera with a coronagraph that uses a Lyot stop. Finally, we discuss the case of the association with a four-quadrant phase mask and numerically demonstrate that such a device enables the detection of Earth-like planets under realistic conditions. The parametric study of the technique lets us believe it can be implemented quite easily in future instruments dedicated to direct imaging of exoplanets.

High Contrast Imaging and Wavefront Control with a PIAA Coronagraph: Laboratory System Validation

Abstract: The Phase-Induced Amplitude Apodization (PIAA) coronagraph is a high performance coronagraph concept able to work at small angular separation with little loss in throughput. We present results obtained with a laboratory PIAA system including active wavefront control. The system has a 94.3% throughput (excluding coating losses) and operates in air with monochromatic light. Our testbed achieved a 2.27e-7 raw contrast between 1.65 lambda/D (inner working angle of the coronagraph configuration tested) and 4.4 lambda/D (outer working angle). Through careful calibration, we were able to separate this residual light into a dynamic coherent component (turbulence, vibrations) at 4.5e-8 contrast and a static incoherent component (ghosts and/or polarization missmatch) at 1.6e-7 contrast. Pointing errors are controlled at the 1e-3 lambda/D level using a dedicated low order wavefront sensor. While not sufficient for direct imaging of Earth-like planets from space, the 2.27e-7 raw contrast achieved already exceeds requirements for a ground-based Extreme Adaptive Optics system aimed at direct detection of more massive exoplanets. We show that over a 4hr long period, averaged wavefront errors have been controlled to the 3.5e-9 contrast level. This result is particularly encouraging for ground based Extreme-AO systems relying on long term stability and absence of static wavefront errors to recover planets much fainter than the fast boiling speckle halo.

Deep turbulence effects compensation experiments with a cascaded adaptive optics system using a 3.63 m telescope

Abstract: Compensation of extended (deep) turbulence effects is one of the most challenging problems in adaptive optics (AO). In the AO approach described, the deep turbulence wave propagation regime was achieved by imaging stars at low elevation angles when image quality improvement with conventional AO was poor. These experiments were conducted at the U.S. Air Force Maui Optical and Supercomputing Site (AMOS) by using the 3.63 m telescope located on Haleakala, Maui. To enhance compensation performance we used a cascaded AO system composed of a conventional AO system based on a Shack-Hartmann wavefront sensor and a deformable mirror with 941 actuators, and an AO system based on stochastic parallel gradient descent optimization with four deformable mirrors (75 control channels). This first-time field demonstration of a cascaded AO system achieved considerably improved performance of wavefront phase aberration compensation. Image quality was improved in a repeatable way in the presence of stressing atmospheric conditions obtained by using stars at elevation angles as low as 15 degrees.

Frequency-resolved high-harmonic wavefront characterization

Abstract: We introduce and demonstrate a novel concept of frequency-resolved wavefront characterization. Our approach is particularly suitable for high-harmonic, extreme-UV (XUV) and soft X-ray radiation. The concept is based on an analysis of radiation diffracted from a slit scanned in front of a flat-field XUV spectrometer. With the spectrally resolved signal spread across one axis and the spatially resolved diffraction pattern in the other dimension, we reconstruct the wavefront. While demonstrated for high harmonics, the method is not restricted in wavelength.

Optimizing vision correction procedures

Abstract: In one embodiment, a wavefront sensor is combined with a slit lamp eye examination device so that real time aberration values of an eye being examined can be viewed during a slit lamp eye examination session.

Linear-quadratic-Gaussian control for adaptive optics systems using a hybrid model

Abstract: This paper presents a linear-quadratic-Gaussian (LQG) design based on the equivalent discrete-time model of an adaptive optics (AO) system. The design model incorporates deformable mirror dynamics, an asynchronous wavefront sensor and zero-order hold operation, and a continuous-time model of the incident wavefront. Using the structure of the discrete-time model, the dimensions of the Riccati equations to be solved are reduced. The LQG controller is shown to improve AO system performance under several conditions.

Fast-convergent algorithm for speckle-based phase retrieval and a design for dynamic wavefront sensing

Abstract: Wavefront reconstruction is carried out using sequentially recorded speckle patterns and an iterative phase retrieval method based on wave propagation. A novel fast-convergent algorithm that maintains the propagation distance in the iteration step equal to the distance between measurement planes is demonstrated. Employing the new algorithm, influences of the number of measurement planes, number of iterations, and uncertainties in the detector's transverse and axial positions on the rate of phase convergence are analyzed experimentally. A conceptual design for a dynamic wavefront sensor using arrays of beam splitters and detectors for parallel speckle recording is described.

Numerical correction of aberrations via phase retrieval with speckle illumination

Abstract: What we believe to be a novel technique for wavefront aberration measurement using speckle patterns is presented. The aberration correction is done numerically. A tilted lens is illuminated with a partially developed speckle field, and the transmitted light intensity is sampled at axially displaced planes. The speckle intensity patterns are then sent to a phase-retrieval algorithm to reconstruct the complete wavefront. The nature of the wavefront aberration is determined through Zernike polynomials. Numerical correction of the perturbed wavefront is performed based on rms error and the Strehl ratio. Restoration of the wavefront from a phase object with high spatial frequency content shows the effectiveness of our method.

Quantitative differential interference contrast (dic) microscopy and photography based on wavefront sensors

Abstract: A wavefront microscope or camera utilizes a wavefront sensor to measure the local intensity and phase gradient of the wavefront and output image maps based on the intensity and phase gradient. A wavefront sensor provides a metal film having patterned structured two dimensional (2D) apertures that convert a phase gradient of a wavefront into a measurable form onto a photodetector array. A computer is used to analyze the data by separating signals projected and recorded on the array from the different apertures, predict a center of each projection, and sum signals for each projection to display the intensity while determining a center position change/offset from the predicted center to display the phase gradient of the wavefront.

High-precision open-loop adaptive optics system based on LC-SLM.

Abstract: Used as a wavefront corrector, a liquid crystal spatial modulator (LC-SLM) has good repeatability and linearity, which are essential for open-loop adaptive optics, and the open-loop optical system can increase the light energy efficiency by a factor of two for the LC-SLM and improve the system bandwidth. In order to test the performance of the LC-SLM in open-loop correction, an indoor closed-loop configuration optical system is constructed on the open-loop control method. With this method, it is demonstrated that the residual error after open-loop correction could be smaller than 0.08lambda (RMS: root mean square value) if the initial wavefront aberration is below 2.5lambda (RMS), and the repeatability error of open-loop correction is smaller than 0.01lambda (RMS).

Adaptive thresholding and dynamic windowing method for automatic centroid detection of digital Shack-Hartmann wavefront sensor

Abstract: A Shack-Hartmann wavefront sensor (SWHS) splits the incident wavefront into many subsections and transfers the distorted wavefront detection into the centroid measurement. The accuracy of the centroid measurement determines the accuracy of the SWHS. Many methods have been presented to improve the accuracy of the wavefront centroid measurement. However, most of these methods are discussed from the point of view of optics, based on the assumption that the spot intensity of the SHWS has a Gaussian distribution, which is not applicable to the digital SHWS. In this paper, we present a centroid measurement algorithm based on the adaptive thresholding and dynamic windowing method by utilizing image processing techniques for practical application of the digital SHWS in surface profile measurement. The method can detect the centroid of each focal spot precisely and robustly by eliminating the influence of various noises, such as diffraction of the digital SHWS, unevenness and instability of the light source, as well as deviation between the centroid of the focal spot and the center of the detection area. The experimental results demonstrate that the algorithm has better precision, repeatability, and stability compared with other commonly used centroid methods, such as the statistical averaging, thresholding, and windowing algorithms.

New methods and techniques for sensing the wave aberrations of human eyes.

Abstract: During the past decade, there has been a remarkable expansion of the application of wavefront-related technologies to the human eye. The ability to measure the wavefront aberrations (WA) of an individual eye has greatly improved our understanding on the optical properties of the human eye. The development of wavefront sensors has further generated an intensive effort to revise methods to correct vision. Wavefront sensors have offered the promise of a new generation of visual correction methods that can correct high order aberrations beyond defocus and astigmatism, that is, the wavefront-guided excimer laser platforms and adaptive optics, thus improving visual performance or fundus imaging at unprecedented spatial resolution. On the other hand, current wavefront technologies suffer from some inaccuracies that may limit a wider expansion in the clinical environment. Several innovative approaches have been developed to overcome the limits of standard wavefront sensing techniques. Curvature sensing, pyramid sensing and interferometry currently represent the most reliable methods to revise and improve the measurement and reconstruction of the WA of human eyes. This review describes advantages and disadvantages of current wavefront sensing technologies and provides recent knowledge on innovative methods for sensing the WA of human eyes. In the near future, we expect to benefit from these new wavefront sensor elements, including their application in the personalised correction of optical aberrations and adaptive optics imaging of the eye.

Dynamic wavefront aberrations and visual acuity in normal and dry eyes.

Abstract: Purpose: The aim was to study the dynamic properties of wavefront aberrations and visual acuity in normal and dry eyes. Methods: Thirty dry-eye patients and 27 normal subjects participated in this study. Multi-file mode of a Hartmann-Shack wavefront sensor was used to measure dynamic wavefront aberrations for a period of 45 seconds. Dynamic measurements of visual acuity (VA) were made for 150 seconds using a multi-functional VA tester. Standard deviation of the measurements (RMS or VA) over the testing period was used to estimate instability of the dynamic wavefront aberration and VA. Results: For most subjects, both wavefront aberration and VA changed over time and the instability varied substantially among individuals. Blink-dependent fluctuation in wavefront aberration or VA was observed for some dry-eye subjects. On average, the dry-eye group had greater instability than the normal group in either the higher order wavefront aberrations (t = 2.09, p = 0.03, for OD; t = 3.76, p = 0.001, for OS) or the VA (t = 2.09, p = 0.02, for OD; t = 204, p = 0.03, for OS). Instability of VA in the dry-eye group was significantly correlated with blink rate (r = 0.28, p = 0.02). Conclusion: Dynamic changes in wavefront aberrations and VA are highly individual dependent, while the dry eye tends to be less stable than the normal eye. The results suggest that tear-film fluctuation might play a role in determining dynamic wavefront aberration and VA, however, contributions from other factors should not be overlooked. For dry eye, dynamic change in VA depends on blink rate.

High-accuracy wavefront control for retinal imaging with Adaptive-Influence-Matrix Adaptive Optics

Abstract: We present an iterative technique for improving adaptive optics (AO) wavefront correction for retinal imaging, called the Adaptive-Influence-Matrix (AIM) method. This method is based on the fact that the deflection-to-voltage relation of common deformable mirrors used in AO are nonlinear, and the fact that in general the wavefront errors of the eye can be considered to be composed of a static, non-zero wavefront error (such as the defocus and astigmatism), and a time-varying wavefront error. The aberrated wavefront is first corrected with a generic influence matrix, providing a mirror compensation figure for the static wavefront error. Then a new influence matrix that is more accurate for the specific static wavefront error is calibrated based on the mirror compensation figure. Experimental results show that with the AIM method the AO wavefront correction accuracy can be improved significantly in comparison to the generic AO correction. The AIM method is most useful in AO modalities where there are large static contributions to the wavefront aberrations.

Differential modal Zernike wavefront sensor employing a computer-generated hologram: a proposal

Abstract: The process of Zernike mode detection with a Shack-Hartmann wavefront sensor is computationally extensive. A holographic modal wavefront sensor has therefore evolved to process the data optically by use of the concept of equal and opposite phase bias. Recently, a multiplexed computer-generated hologram (CGH) technique was developed in which the output is in the form of bright dots that specify the presence and strength of a specific Zernike mode. We propose a wavefront sensor using the concept of phase biasing in the latter technique such that the output is a pair of bright dots for each mode to be sensed. A normalized difference signal between the intensities of the two dots is proportional to the amplitude of the sensed Zernike mode. In our method the number of holograms to be multiplexed is decreased, thereby reducing the modal cross talk significantly. We validated the proposed method through simulation studies for several cases. The simulation results demonstrate simultaneous wavefront detection of lower-order Zernike modes with a resolution better than λ/50 for the wide measurement range of ±3.5λ with much reduced cross talk at high speed.

A simple and robust method to extend the dynamic range of an aberrometer

Abstract: We present an algorithm to extend significantly the dynamic range of a Shack-Hartmann wavefront sensor With this method, the recorded Shack-Hartmann spots are not constrained to stay in the field of view of their lenslet The proposed algorithm is computationally effective, robust to a high level of noise on the measured centroid positions and also to missing centroid values The principle is closely related to the description of wavefronts using Zernike polynomials, which makes optimization for a given sensor and application achievable thanks to numerical simulation These features make it useful for the measurements of highly aberrated eyes

Measuring directionality of the retinal reflection with a Shack-Hartmann wavefront sensor

Abstract: The directional sensitivity of the retina, known as the Stiles-Crawford effect (SCE), originates from the waveguide property of photoreceptors. This effect has been extensively studied in normal and pathologic eyes using highly customized optical instrumentation. Here we investigate a new approach based on a Shack-Hartmann wavefront sensor (SHWS), a technology that has been traditionally employed for measuring wave aberrations (phase) of the eye and is available in clinics. Using a modified research-grade SHWS, we demonstrate in five healthy subjects and at four retinal eccentricities that intensity information can be readily extracted from the SHWS measurement and the spatial distribution of which is consistent with that produced by the optical SCE. The technique is found sufficiently sensitive even at near-infrared wavelengths where the optical SCE is faint. We demonstrate that the optical SCE signal is confined to the core of the SHWS spots with the tails being diffuse and non-directional, suggesting cones fail to recapture light that is multiply scattered in the retina. The high sensitivity of the SHWS to the optical SCE raises concern as to how this effect, intrinsic to the retina, may impact the SHWS measurement of ocular aberrations.

Laser guide stars for extremely large telescopes: efficient Shack–Hartmann wavefront sensor design using the weighted centre–of–gravity algorithm

Abstract: Over the last few years increasing consideration has been given to the study of laser guide stars (LGS) for the measurement of the disturbance introduced by the atmosphere in optical and near-infrared (near-IR) astronomical observations from the ground. A possible method for the generation of a LGS is the excitation of the sodium layer in the upper atmosphere at approximately 90 km of altitude. Since the sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture. The spot elongation strongly limits the performance of the most common wavefront sensors. The centroiding accuracy in a Shack–Hartmann wavefront sensor, for instance, decreases proportionally to the elongation (in a photon noise dominated regime). To compensate for this effect, a straightforward solution is to increase the laser power, i.e. to increase the number of detected photons per subaperture. The scope of the work presented in this paper is twofold: an analysis of the performance of the weighted centre of gravity algorithm for centroiding with elongated spots and the determination of the required number of photons to achieve a certain average wavefront error over the telescope aperture.

Image Pickup Apparatus and Method for Manufacturing the Same

Abstract: An image pickup apparatus and manufacturing method is disclosed. The image pickup apparatus comprises an optical system, an optical wavefront modulator that modulates an optical transfer function, an aperture adjacent to the optical wavefront modulator, and an image pickup device for detecting an object image passing through the optical system and the optical wavefront modulator. A product of a diameter of the aperture at a stop position multiplied by a distance between the aperture and the optical wavefront modulator is less than 2.

Wavefront Control System for Phase Compensation in Hard X-ray Optics

Abstract: A highly precise adaptive optical system that can be used in the hard X-ray region was developed. To achieve highly precise control of the wavefront shape, we discussed an optical system with a bendable mirror of deformation accuracy better than 0.4 nm RMS. Using the system, we demonstrated the controllability of the wavefront of a 15 nm hard X-ray nanobeam. The intensity profile of the wavefront-modified beam was in good agreement with the wave-optically calculated profile.

Wavefront sensing through measurements of binary aberration modes

Abstract: A wavefront sensing technique is proposed based on the principle of aberration-mode filtering and detection. The mathematical foundation of the method is provided by a series of orthogonal and binary functions, for the optical aperture, derived from the Walsh series. It is shown that the expansion of a wavefront using these basis functions is explicitly related to the expansion of the optical field itself on the same basis. This permits the determination of the coefficients associated with the binary aberration modes through simple intensity measurements with the help of a phase-only spatial light modulator and a single-mode optical fiber. These coefficients can be independently acquired in sets that characterize wavefronts in various spatial resolutions. A numerical simulation and practical implementation of the technique are also discussed.

Experimental assessment of the matched filter for laser guide star wavefront sensing

Abstract: Laser guide star wavefront sensing comes with several limitations. When imaged with a Shack-Hartmann wavefront sensor, the laser guide star is seen as extended sources elongated in the directions given by the lenslet locations and the laser axis. A test bed has been built in the Adaptive Optics Laboratory of the University of Victoria that reproduces this effect as seen on extremely large telescopes. A new wavefront sensing algorithm, the matched filter, has been implemented and its performance assessed with the test bed. Its ability to mitigate laser guide star aberrations by tracking the sodium layer fluctuations in a closed loop adaptive optics system is shown.

Photo-thermal measurement of absorptance losses, temperature induced wavefront deformation and compaction in DUV-optics.

Abstract: A measurement system for quantitative registration of transient and irreversible lens effects in DUV optics induced by absorbed UV laser radiation was developed. It is based upon a strongly improved Hartmann-Shack wavefront sensor with an extreme sensitivity of approximately lambda/10000 rms @ 193nm, accomplishing precise on-line monitoring of wavefront deformations of a collimated test laser beam transmitted through the laser-irradiated site of a sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test laser is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. This transient wavefront distortion yields a quantitative measure of the absorption losses in the sample. In the case of fused silica, an additional permanent change indicates irreversible material compaction. Results for both fused silica and CaF(2) are presented and compared.

The plenoptic camera as a wavefront sensor for the European Solar Telescope (EST)

Abstract: The plenoptic wavefront sensor combines measurements at pupil and image planes in order to obtain wavefront information from different points of view simultaneously, being capable to sample the volume above the telescope to extract the tomographic information of the atmospheric turbulence. After describing the working principle, a laboratory setup has been used for the verification of the capability of measuring the pupil plane wavefront. A comparative discussion with respect to other wavefront sensors is also included.

High Sensitivity Wavefront Sensing with a non-linear Curvature Wavefront Sensor

Abstract: A new wavefront sensing approach, derived from the successful curvature wavefront sensing concept but using a non-linear phase retrieval wavefront reconstruction scheme, is described. The non-linear curvature wavefront sensor (nlCWFS) approaches the theoretical sensitivity limit imposed by fundamental physics by taking full advantage of wavefront spatial coherence in the pupil plane. Interference speckles formed by natural starlight encode wavefront aberrations with the sensitivity set by the telescope's diffraction limit lambda/D rather than the seeing limit of more conventional linear WFSs. Closed-loop adaptive optics simulations show that with a nlCWFS, a 100 nm RMS wavefront error can be reached on a 8-m telescope on a mV = 13 natural guide star. The nlCWFS technique is best suited for high precision adaptive optics on bright natural guide stars. It is therefore an attractive technique to consider for direct imaging of exoplanets and disks around nearby stars, where achieved performance is set by wavefront control accuracy, and exquisite control of low order aberrations is essential for high contrast coronagraphic imaging. Performance gains derived from simulations are shown, and approaches for high speed reconstruction algorithms are briefly discussed.

Temporal wavefront stability of an ultrafast high-power laser beam

Abstract: We measured the temporal dynamics of wavefront aberrations in a beam produced by a commercial ultrafast high-power laser with a research-prototype real-time Hartmann-Shack wavefront sensor. Measurements were performed at two different temporal rates for a 7 mm diameter. Results showed that changes in the wavefront aberrations were always lower than 1%. The main contribution to the total root-mean-square (RMS) wavefront error was due to the effects of low order aberrations (defocus and astigmatism), which persisted even after cavity realignment. The potential improvement in the beam quality after correction of the different aberration modes was also shown. Real-time measurements of laser aberrations while modifying cavity parameters might be a useful tool to improve the beam quality.