Dissertations / Theses on the topic 'Wavefront correction'

Medisinsk ultralydavbildning er et relativt rimelig verktøy som er i utstrakte bruk på dagens sykehus og tildels også legekontor. En underliggende antakelse ved dagens avbildningsteknikker er at vevet som skal avbildes i grove trekk er homogent. Det vil i praksis si at de akustiske egenskapene varierer lite. I tilfeller der denne forutsetningen ikke holder vil resultatet bli betraktlig reduksjon av bildekvaliteten. Prosjektet har fokusert på hvordan man best mulig kan korrigere for denne kvalitetsforringelsen. Arbeidet har resultert i et styrket teoretisk rammeverk for modellering, programvare for numerisk simulering. Rammeverket gir en felles forankring for tidligere publiserte metoder som "time-reversal mirror", "beamsum-correlation" og "speckle brightness", og gir derfor en utvidet forståelse av disse metodene. Videre har en ny metode blitt utviklet basert på egenfunksjonsanalyse av et stokastisk tilbakespredt lydfelt. Denne metoden vil potensielt kunne håndtere sterk spredning fra områder utenfor hovedaksen til ultralydstrålen på en bedre måte enn tidligere metoder. Arbeidet er utført ved Institutt for matematiske fag, NTNU, med professor Harald Krogstad, Institutt for matematiske fag, som hovedveileder og professor Bjørn Angelsen, Institutt for sirkulasjon og bildediagnostikk, som medveileder.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.
Includes bibliographical references (p. 239-243).
As the first generation of laser interferometric gravitational wave detectors near operation, research and development has begun on increasing the instrument's sensitivity while utilizing existing infrastructure. In the Laser Interferometer Gravitational Wave Observatory (LIGO), significant improvements are being planned for installation in 2007 to increase the sensitivity to test mass displacement, hence sensitivity to gravitational wave strain, by improved suspensions and test mass substrates, active seismic isolation, and higher input laser power. Even with the highest quality optics available today, however, finite absorption of laser power within transmissive optics, coupled with the tremendous amount of optical power circulating in various parts of the interferometer, result in critical wavefront deformations which will cripple the performance of the instrument. Discussed is a method of active wavefront correction via direct thermal actuation on optical elements of the interferometer; or, "thermally adaptive optics". A simple nichrome heating element suspended off the face of an affected optic will, through radiative heating, remove the gross axisymmetric part of the original thermal distortion. A scanning heating laser- will then be used to remove any remaining non-axisymmetric wavefront distortion, generated by inhomogeneities in the substrate's absorption, thermal conductivity, etc. This work includes a quantitative analysis of both techniques of thermal compensation, as well as the results of a proof-of-principle experiment which verified the technical feasibility of each technique.
by Ryan Christopher Lawrence.
Ph.D.

When conducting experiments on the human eye it is sometimes required to correct some form of astigmatism. A fexible and cost effective way to achieve this is using two identical cross-cylinder lenses, as described in the article ‘Adaptive Astigmatism-Correcting Device for Eyepieces' by Arines and Acosta. When rotating these lenses the strength and angle can be adjusted to correct for different types of astigmatism.The measurements were made on two identical SR = +2 D, CR = - 4 D lenseson rotating mounts. A Shack-Hartmann wavefront sensor detected the change in a beam of collimated light directed through the lenses. Two types of measurements were conducted, to record the results of just the relative angle between the lenses and to find the aberrations the lenses themselves introduce to the system in combination with rotation. The two-lens-system we created adhered well to the theory and the results obtained by Arines and Acosta, although we obtained a slight variation in our values. The minimum cylindrical strength was close to -9 D and the aberrations were very small (< 0:01 μm). The reasons for these systematic errors are discussed but for the purposes of aiding in visual optics research this solution is well suited for its ease of use, and cost effectiveness.

Because atmospheric turbulence causes distortions in stellar wavefronts, passive ground based telescopes, no matter how large, are limited to the resolution limit of a 0.1-0.2m aperture when imaging in the visible. If the new class of large aperture (10 m) telescopes is to reach its resolution potential, adaptive optics must be employed to compensate for the atmospheric wavefront distortions. Vital to an adaptive optics system is the ability to accurately sense the distorted wavefront. Two new methods for wavefront sensing show great promise for the field of adaptive optics. A reflective hybrid of the traditional Shack-Hartmann wavefront sensor has produced near diffraction limited imaging with the Multiple Mirror Telescope, a hexagonal array of six, 1.83 m mirrors. It is also directly applicable to filled aperture telescopes. Another wavefront sensor, the stellar phase shifting interferometer, has produced for the first time ever direct phase map measurements of atmospherically distorted wavefronts. The ability to directly measure the phase of the wavefront at each detector pixel paves the way for a new generation of light efficient and accurate wavefront sensors for adaptive optics.

Just over two decades ago the first planet outside our solar system was found, and thousands more have been discovered since. Nearly all these exoplanets were indirectly detected by sensing changes in their host stars' light. However, exoplanets must be directly imaged to determine their atmospheric compositions and the orbital parameters unavailable from only indirect detections. The main challenge of direct imaging is to observe stellar companions much fainter than the star and at small angular separations. Coronagraphy is one method of suppressing stellar diffraction to provide high star-to-planet contrast, but coronagraphs are extremely sensitive to quasi-static aberrations in the optical system. Active correction of the stellar wavefront is performed with deformable mirrors to recover high-contrast regions in the image. Estimation and control of the stellar electric field is performed iteratively in the camera's focal plane to avoid non-common path aberrations arising from a separate pupil sensor. Estimation can thus be quite time consuming because it requires several high-contrast intensity images per correction iteration.

This thesis focuses on efficient focal plane wavefront correction (FPWC) for coronagraphy. Time is a precious commodity for a space telescope, so there is a strong incentive to reduce the total exposure time required for focal plane wavefront estimation. Much of our work emphasizes faster, more robust estimation via Kalman filtering, which optimally combines prior data with new measurements. The other main contribution of this thesis is a paradigm shift in the use of estimation images. Time for FPWC has generally been considered to be lost overhead, but we demonstrate that estimation images can be used for the detection and characterization of exoplanets and disks. These science targets are incoherent with their host stars, so we developed and implemented an iterated extended Kalman filter (IEKF) for simultaneous estimation of the stellar electric field and the incoherent signal. From simulations and testbed experiments, we report the increased FPWC speed enabled by Kalman filtering and the use of the IEKF for exoplanet detection during FPWC. We discuss the relevance and future directions of this work for planned or proposed coronagraph missions.

Deformable mirror is a widely used wavefront corrector in adaptive optics system, especially in astronomical, image and laser optics. A new structure of DM-3D DM is proposed, which has removable actuators and can correct different aberrations with different actuator arrangements. A 3D DM consists of several reflection mirrors. Every mirror has a single actuator and is independent of each other. Two kinds of actuator arrangement algorithm are compared: random disturbance algorithm (RDA) and global arrangement algorithm (GAA). Correction effects of these two algorithms and comparison are analyzed through numerical simulation. The simulation results show that 3D DM with removable actuators can obviously improve the correction effects.

To improve optical measurements, which are degraded by optical distortions, wavefront correction systems can be used. Generally, these systems evaluate a guide star in transmission. The guide star emits wellknown wavefronts, which sample the distortion by propagating through it. The system is able to directly measure the distortion and correct it. There are setups, where it is not possible to generate a guide star behind the distortion. Here, we consider a liquid jet with a radially open surface. A Mach–Zehnder interferometer is presented where both beams are stabilized through a fluctuating liquid jet surface with the Fresnel guide star (FGS) technique. The wavefront correction system estimates the beam path behind the surface by evaluating the incident beam angle and reflected beam angle of the Fresnel reflex with an observer to control the incident angle for the desired beam path. With this approach, only one optical access through the phase boundary is needed for the measurement, which can be traversed over a range of 250 μm with a significantly increased rate of valid signals. The experiment demonstrates the potential of the FGS technique for measurements through fluctuating phase boundaries, such as film flows or jets.

For more than a century it has been known that the eye is not a perfect optical system, but rather a system that suffers from aberrations beyond conventional prescriptive descriptions of defocus and astigmatism. Whereas traditional refraction attempts to describe the error of the eye with only two parameters, namely sphere and cylinder, measurements of wavefront aberrations depict the optical error with many more parameters. What remains questionable is the impact these additional parameters have on visual function. Some authors have argued that higher-order aberrations have a considerable effect on visual function and in certain cases this effect is significant enough to induce amblyopia. This has been referred to as ‘higher-order aberration-associated amblyopia’. In such cases, correction of higher-order aberrations would not restore visual function. Others have reported that patients with binocular asymmetric aberrations display an associated unilateral decrease in visual acuity and, if the decline in acuity results from the aberrations alone, such subjects may have been erroneously diagnosed as amblyopes. In these cases, correction of higher-order aberrations would restore visual function. This refractive entity has been termed ‘aberropia’. In order to investigate these hypotheses, the distribution of higher-order aberrations in strabismic, anisometropic and idiopathic amblyopes, and in a group of visual normals, was analysed both before and after wavefront-guided laser refractive correction. The results show: (i) there is no significant asymmetry in higher-order aberrations between amblyopic and fixing eyes prior to laser refractive treatment; (ii) the mean magnitude of higher-order aberrations is similar within the amblyopic and visually normal populations; (iii) a significant improvement in visual acuity can be realised for adult amblyopic patients utilising wavefront-guided laser refractive surgery and a modest increase in contrast sensitivity was observed for the amblyopic eye of anisometropes following treatment (iv) an overall trend towards increased higher-order aberrations following wavefront-guided laser refractive treatment was observed for both visually normal and amblyopic eyes. In conclusion, while the data do not provide any direct evidence for the concepts of either ‘aberropia’ or ‘higher-order aberration-associated amblyopia’, it is clear that gains in visual acuity and contrast sensitivity may be realised following laser refractive treatment of the amblyopic adult eye. Possible mechanisms by which these gains are realised are discussed.

L’imagerie d’exoplanètes est limitée par deux obstacles intrinsèques : le faible écart angulaire entre planète et étoile, et le très faible flux lumineux en provenance de la planète par rapport à la lumière de l’étoile. Le premier obstacle est surmonté par l’utilisation de très grands télescopes, de la classe des dix mètres de diamètre, et éventuellement depuis le sol de systèmes d’optique adaptative, qui permettent d’atteindre de hautes résolutions angulaires. Le deuxième obstacle est surmonté par l’utilisation de coronographes. Les coronographes sont des instruments conçus pour filtrer la lumière de l’étoile tout en laissant passer la lumière de l’environnement circumstellaire. Cependant, toute aberration optique en amont du coronographe engendre des fuites de lumière stellaire à travers le coronographe. Ces fuites se traduisent par un fouillis de tavelures dans les images scientifiques, tavelures qui cachent d’éventuelles planètes. Il est donc nécessaire de mesurer et de corriger les aberrations quasi-statiques à l’origine des tavelures. Cette thèse présente des contributions théoriques, numériques et expérimentales à la mesure et à la correction des aberrations des imageurs coronographiques. La première partie décrit le contexte et présente la méthode de la diversité de phase coronographique, un formalisme qui considère l’analyse de surface d’onde post-coronographique comme un problème inverse posé dans un cadre bayésien. La deuxième partie concerne l’imagerie depuis le sol. Elle présente tout d’abord une expression analytique permettant de modéliser l’imagerie coronographique en présence de turbulence, puis l’extension de la méthode de diversité de phase coronographique à la mesure depuis les télescopes au sol donc en présence de turbulence résiduelle, et enfin une validation en laboratoire de cette méthode étendue. La troisième partie est consacrée aux futurs imageurs spatiaux à très hauts contrastes pour lesquels il faut corriger non pas seulement la phase mais tout le champ complexe. Elle présente la validation en laboratoire de la mesure d’un champ complexe d’aberrations par diversité de phase coronographique, ainsi que des premiers résultats d’extinction de la lumière en plan focal par une méthode non linéaire, le non-linear dark hole
Exoplanet imaging has two intrinsic limitations, namely the small angular separation between the star and the planet, and the very low light flux from the planet compared to the starlight. The first limitation is overcome by using very large telescopes of the ten-metre diameter class, and, for ground-based telescopes, adaptive optics systems, which allow high angular resolution imaging. The second limitation is overcome by using a coronagraph. Coronagraphs are optical devices which filter the starlight while granting passage to the light coming from the stellar environment. However, any optical aberration upstream of the coronagraph causes some of the starlight to leak through the coronagraph. This unfiltered starlight in turn causes speckles in the scientific images, and the light of the planets that could be there is lost among the speckles. Consequently, measurement and correction of the quasi-static aberration which generate the speckles are necessary for the exoplanet imagers to achieve their full potential. This thesis introduces theoretical, numerical, and experimental contributions to the topic of measurement and correction of the aberrations in coronagraphic imagers. The first part describes the context and introduces coronagraphic phase diversity, which is a Bayesian inverse problem formalism for post-coronagraphic wave-front sensing. The second part is focused on ground-based imaging. It introduces an analytic expression for coronagraphic imaging through turbulence, the extension of coronagraphic phase diversity to on-sky measurement through residual turbulence, and a laboratory validation of the extended method. The third part is concerned with future high-contrast space-based imagers, which will require not only phase correction, but a full complex wave-front correction. It presents the laboratory validation of coronagraphic phase diversity as a post-coronagraphic complex wave-front sensor, and first results of active contrast enhancement in the focal plane through thecreation of a non-linear dark hole

The aberration in the center position of wavefront can be corrected well when the deformable mirrors (DM) used in high-resolution adaptive optics system of telescope. However, for the defocus and spherical aberration of telescope, the four corners of the wavefront cannot be corrected well. A novel correction method with different levels and regions of deformable mirror is proposed to solve this problem. The control elements of wavefront in four corners are divided. And every four or five DM units in one corner is in a group. Compared with conventional correction method, the location-grouping method showed significant advantages in correction of different order aberrations.

A new type of adaptive optics (AO) based on the principles of digital holography (DH) is proposed and developed for the use in wide-field and confocal retinal imaging. Digital holographic adaptive optics (DHAO) dispenses with the wavefront sensor and wavefront corrector of the conventional AO system. DH is an emergent imaging technology that gives direct numerical access to the phase of the optical field, thus allowing precise control and manipulation of the optical field. Incorporation of DH in an ophthalmic imaging system can lead to versatile imaging capabilities at substantially reduced complexity and cost of the instrument. A typical conventional AO system includes several critical hardware pieces: spatial light modulator, lenslet array, and a second CCD camera in addition to the camera for imaging. The proposed DHAO system replaces these hardware components with numerical processing for wavefront measurement and compensation of aberration through the principles of DH. We first design an image plane DHAO system which is basically simulating the process the conventional AO system and replacing the hardware pieces and complicated control procedures by DH and related numerical processing. In this original DHAO system, CCD is put at the image plane of the pupil plane of the eye lens. The image of the aberration is obtained by a digital hologram or guide star hologram. The full optical field is captured by a second digital hologram. Because CCD is not at the conjugate plane of the sample, a numerical propagation is necessary to find the image of the sample after the numerical aberration compensation at the CCD plane. The theory, simulations and experiments using an eye model have clearly demonstrated the effectiveness of the DHAO. This original DHAO system is described in Chapter 2. Different from the conventional AO system, DHAO is a coherent imaging modality which gives more access to the optical field and allows more freedom in the optical system design. In fact, CCD does not have to be put at the image plane of the CCD. This idea was first explored by testing a Fourier transform DHAO system (FTDHAO). In the FTDHAO, the CCD can directly record the amplitude point spread function (PSF) of the system, making it easier to determine the correct guide star hologram. CCD is also at the image plane of the target. The signal becomes stronger than the image plane DHAO system, especially for the phase aberration sensing. Also, the numerical propagation is not necessary. In the FTDHAO imaging system, the phase aberration at the eye pupil can be retrieved by an inverse Fourier transform (FT) of the guide star hologram and the complex amplitude of the full field optical field at the eye pupil can be obtained by an inverse FT of the full field hologram. The correction takes place at the eye pupil, instead of the CCD plane. Taking FT of the corrected field at the eye pupil, the corrected image can be obtained. The theory, simulations, and experiments on FTDHAO are detailed in chapter 3. The successful demonstration of FTDHAO encourages us to test the feasibility of putting CCD at an arbitrary diffraction plane in the DHAO system. Through theoretical formulation by use of paraxial optical theory, we developed a correction method by correlation for the general optical system to perform the DHAO. In this method, a global quadratic phase term has to be removed before the correction operation. In the formulation, it is quite surprising to find that the defocus term can be eliminated in the correlation operation. The detailed formulations, related simulations, and experimental demonstrations are presented in Chapter 4. To apply the DHAO to the confocal retinal imaging system, we first transformed the conventional line-scanning confocal imaging system into a digital form. That means each line scan is turned into a digital hologram. The complex amplitude of the optical field from each slice of the sample and aberration of the optical system can be retrieved by digital holographic process. In Chapter 5, we report our experiments on this digital line-scanning confocal imaging system. This digital line-scanning confocal image absorbs the merits of the conventional line-scanning confocal imaging system and DH. High-contrast intensity images with low coherent noise, and the optical sectioning capability are made available due to the confocality. Phase profiles of the samples become accessible thanks to DH. The quantitative phase map is even better than that from the wide field DH. We then explore the possibility of applying DHAO to this newly developed digital line-scanning confocal imaging system. Since optical field of each line scan can be achieved by the DH, the aberration contained in this field can be eliminated if we are able to obtain the phase aberration. We have demonstrated that the phase aberration can be obtained by a guide star hologram in the wide field DHAO systems. We then apply this technique to acquire the aberration at the eye pupil, remove this aberration from the optical fields of the line scans and recover the confocal image. To circumvent the effect of phase aberration on the line illumination, a small collimated laser beam is shone on the cylindrical lens. Thus the image is solely blurred by the second passage through the aberrator. This way, we can clearly demonstrate the effect of DHAO on the digital line-scanning confocal image system. Simulations and experiments are presented in chapter 6, which clearly demonstrates the validity of this idea. Since line-scanning confocal imaging system using spatially coherent light sources has proven an effective imaging tool for retinal imaging, the presented digital adaptive optics line-scanning confocal imaging system is quite promising to become a compact digital adaptive optics laser scanning confocal ophthalmoscope.

Thesis (M.A.)--Boston University
A retrospective chart review performed from December 2007 to September 2012 identified 3,223 patients that underwent LASIK treatment with the STAR S4 IRTM Excimer Laser. In this group, 109 patients (3.4%) required a retreatment. All charts were reviewed for pre-operative age, gender, initial manifest refraction spherical equivalent (MRSE), total astigmatism, and method of primary LASIK treatment (conventional versus wavefront-guided) to identify risk factors that may lead to retreatment. A second chart review from December 2007 to January 2013 identified 120 patients who had a retreatment. A comparative analysis on the final post-operative visual acuity and MRSE was performed on this group to evaluate the efficacy of conventional versus wavefront-guided retreatment. Increased incidence rates of retreatment post- LASIK were associated with pre-operative age greater than 40 years (p < 0.001), initial MRSE greater than -5.0 diopters (D) (p < 0.004), hyperopia (p <0.031), and astigmatism greater than -1 D (p < 0.001). There was a 12.3% incidence rate of epithelial ingrowth post-retreatment, and a 1.7% development of clinically significant epithelial ingrowth, which necessitated flap lift and scrapping. There was no statistically significant difference in visual acuity and MRSE post- retreatment with either conventional or wavefront-guided retreatment for residual hyperopic or myopic refractions. All secondary retreatments were in the wavefront-guided retreatment groups (myopic p = 0.16 and hyperopic p = 0.01). Ablation depth was significantly different between myopic conventional and wavefront-guided (p = 0.01) and hyperopic conventional and wavefront-guided (p = 0.04). While no statistically significant difference was found between final outcome vision between conventional and wavefront-guided treatments, conventional treatment ablating less corneal stroma and resulting in fewer complications and additional retreatments provides a strong argument for retreatment with conventional over wavefront-guided.

Las aberraciones ópticas determinan la formación de imágenes en el ojo, tanto en el proceso de la visión como en las observaciones oftalmoscópicas del fondo de ojo. La corrección total de estas aberraciones permitiría una resolución limitada sólo por la difracción en las pupilas utilizadas. Las aberraciones del ojo difieren de un sujeto a otro y no responden a modelos sencillos. En este trabajo se propone el uso de técnicas de Óptica Adaptativa para el desarrollo de un sistema experimental para la medida y corrección de las aberraciones estáticas del ojo. Estas técnicas pueden ser igualmente útiles para obtener imágenes de alta resolución de la retina, utilizarse en el diseño de lentes oftálmicas, etc. Para la medida de la función aberración de onda, se han utilizado dos métodos no invasivos aplicables al ojo humano: La Recuperación de Fase a partir de dos imágenes de Doble Paso, y el Sensor de Hartmann-Shack. Para la corrección de la aberración se ha utilizado un Modulador Espacial de Cristal Líquido.Se han desarrollado los procedimientos de control y de calibrado de estos métodos, y se estudia la viabilidad de aplicación para el ojo. Finalmente, se han realizado medidas de la aberración, mediante ambos métodos, y su posterior corrección mediante el modulador espacial de cristal líquido, en un ojo artificial y en sujetos reales.
The image formation properties of the eye are determined by the aberrations of the optics. The complete correction of the aberrations would allow diffraction-limited resolution. The aberrations of the eye are not easily modeled and are different for each subject.This thesis proposes the use of adaptive optics techniques to measure and correct the static aberrations of the eye. The principles and methods developed are useful in specific applications, i.e., high-resolution retinal imaging, ophthalmic lens design, etc.Two non-invasive methods have been used to measure the wave aberration function: Phase Retrieval Techniques from two double-pass retinal images; and the Hartmann-Shack sensor. A Liquid Crystal Spatial Light Modulator was used to adaptively correct the wave front aberration of the eye.This thesis also includes guidelines to calibrate and control the proposed techniques.Finally, experimental explorations of these methods are reported. Several results are presented, including the measure and the subsequent compensation of the wave aberration for artificial and human eyes.

One of the highest resolution astronomical images ever taken in the visible were obtained by combining the techniques of adaptive optics and lucky imaging. The Adaptive Optics Lucky Imager (AOLI), being developed at Cambridge as part of a European collaboration, combines these two techniques in a dedicated instrument for the first time. The instrument is designed initially for use on the 4.2m William Herschel Telescope (WHT) on the Canary Island of La Palma. This thesis describes the development of AOLI, in particular the adaptive optics system and a new type of wavefront sensor, the non-linear curvature wavefront sensor (nlCWFS), being used within the instrument. The development of the nlCWFS has been the focus of my work, bringing the technique from a theoretical concept to physical realisation at the WHT in September 2013. The non-linear curvature wavefront sensor is based on the technique employed in the conventional curvature wavefront sensor where two image planes are located equidistant either side of a pupil plane. Two pairs of images are employed in the nlCWFS providing increased sensitivity to both high- and low- order wavefront distortions. This sensitivity is the reason the nlCWFS was selected for use with AOLI as it will provide significant sky-coverage using natural guide stars alone, mitigating the need for laser guide stars. This thesis is structured into three main sections; the first introduces the non-linear curvature wavefront sensor, the relevant background and a discussion of simulations undertaken to investigate intrinsic effects. The iterative reconstruction algorithm required for wavefront reconstruction is also introduced. The second section discusses the practical implementation of the nlCWFS using two demonstration systems as the precursor to the optical design used at the WHT and includes details of subsequent design changes. The final section discusses data from both the WHT and a laboratory setup developed at Cambridge following the observing run. The long-term goal for AOLI is to undertake science observations on the 10.4m Gran Telescopio Canarias, the world's largest optical telescope. The combination of AO and lucky imaging, when used on this telescope, will provide resolutions a factor of two higher than ever before achieved at visible wavelengths. This offers the opportunity to probe the Cosmos in unprecedented detail and has the potential to significantly advance our understanding of the Universe.

Thesis (Ph.D.)--Kent State University, 2005.
Title from PDF t.p. (viewed Sept. 14, 2006). Advisor: Philip J. Bos. Keywords: liquid crystal; diffractive optical element; optical phased array; spatial light modulator; high resolution wavefront control; aberration correction. Includes bibliographical references (p. 206-213).

Through the application of adaptive correction, both the permanent and changing aberrations of the eye that affect vision acuity are compensated for in order to provide diffraction-limited imaging. This thesis investigates the aspects of establishing an adaptive optics system for the correction of ocular wavefront errors using a twistednematic liquid-crystal on silicon (TN-LCoS) chip as the wavefront corrector in conjunction with a Hartmann-Shack device as the wavefront sensor. TN-LCoS chips are originally designed to be used as mass-manufactured, high resolution image generating elements in video projectors. With higher density of correcting elements than the conventional form of ocular wavefront correctors (different forms of deformable mirrors), LCoS chips are emerging as potential solutions for realising cost-effective AO systems. Through the application of phase-wrapping, the resolution and stroke of the LCoS chip are fully exploited to operate the AO system as a reliable aberrometer and wavefront corrector. One of the set-backs when using these LCoS chips is that they can generate a high proportion of unwanted beams, i.e., beams other than the first diffraction order. In particular, chips with TN alignment of the LC molecules will show a strong zeroth-order contribution. That affects both the chips’ true wavefront generation capabilities as well as the fidelity of the Hartmann-Shack wavefront sensor, creating an unpredictable correction scheme that does not achieve diffraction-limited performance. In this thesis the ability of the TN-LCoS chip to act as a light modulator has been thoroughly investigated. Based on this, modifications to the experimental adaptive optics set-up and the closed-loop control algorithm have been implemented to accommodate for the chip’s limitations. Experiments that were carried out include the generation of Zernike wavefronts, measurements of Zernike coefficients of elements with known and unknown optical properties, as well as open- and closed-loop corrections of static and dynamic aberrations. Parameters such as experimental and simulated point spread functions (PSFs), and residual wavefront RMS values in both the pupil and retinal planes were correlated. The closed-loop wavefront corrections with volunteers’ eyes have shown narrowed point spread functions (PSFs) and higher Strehl ratios (SRs). The system established has full functionality for ocular wavefront correction, permitting future extension of usage in retinal imaging systems or for psychological experiments.

A method is proposed which uses a lower-frequency transmit to create a known harmonic acoustical source in tissue suitable for wavefront correction without a priori assumptions of the target or requiring a transponder. The measurement and imaging steps of this method were implemented on the Duke phased array system with a two-dimensional (2-D) array. The method was tested with multiple electronic aberrators [0.39π to 1.16π radians root-mean-square (rms) at 4.17 MHz] and with a physical aberrator 0.17π radians rms at 4.17 MHz) in a variety of imaging situations. Corrections were quantified in terms of peak beam amplitude compared to the unaberrated case, with restoration between 0.6 and 36.6 dB of peak amplitude with a single correction. Standard phantom images before and after correction were obtained and showed both visible improvement and 14 dB contrast improvement after correction. This method, when combined with previous phase correction methods, may be an important step that leads to improved clinical images.
Dissertation

This thesis is about light. Specifically it explores a new way sensing the spatial distribution of amplitude and phase across the wavefront of a propagating laser. It uses spatial light modulators to tag spatially distinct regions of the beam, a single diode to collect the resulting light and digitally enhanced heterodyne interferometry to decode the phase and amplitude information across the wavefront. It also demonstrates how using these methods can be used to maximise the transmission of light through a cavity and shows how minor aberrations in the beam can be corrected in real time. Finally it demonstrate the preferential transmission of higher order modes. Wavefront sensing is becoming increasingly important as the demands on modern interferometers increase. Land based systems such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) use it to maximise the amount of power in the arm cavities during operation and reduce noise, while space based missions such as the Laser Interferometer Space Antenna (LISA) will use it to align distant partner satellites and ensure that the maximum amount of signal is exchanged. Conventionally wavefront sensing is accomplished using either Hartmann Sensors or multi-element diodes. These are well proven and very effective techniques but bring with them a number of well understood limitations. Critically, while they can map a wavefront in detail, they are strictly sensors and can do nothing to correct it. Our new technique is based on a single-element photo-diode and the spatial modulation of the local oscillator beam. We encode orthogonal codes spatially onto this light and use these to separate the phases and amplitudes of different parts of the signal beam in post processing. This technique shifts complexity from the optical hardware into deterministic digital signal processing. Notably, the use of a single analogue channel (photo-diode, connections and analogue to digital converter) avoids some low-frequency error sources. The technique can also sense the wavefront phase at many points, limited only by the number of actuators on the spatial light modulator in contrast to the standard 4 points from a quadrant photo-diode. For ground-based systems, our technique could be used to identify and eliminate higher-order modes, while, for space-based systems, it provides a measure of wavefront tilt which is less susceptible to low frequency noise. In the future it may be possible to couple the technique with an artificial intelligence engine to automate more of the beam alignment process in arrangements involving multiple cavities, preferentially select (or reject) specific higher order modes and start to reduce the burgeoning requirements for human control of these complex instruments.

Trabalho Final do Mestrado Integrado em Medicina apresentado à Faculdade de Medicina
Objetivo: Comparar os resultados entre queratectomia fotorefrativa (PRK) com perfis Custom-Q ou Wavefront-optimized (WFO) relativamente à asfericidade e aberrações esféricas, 6 meses após a cirurgia. Local: Centro de Referência Terciário (Centro Hospitalar e Universitário da Universidade de Coimbra, Coimbra, Portugal). Participantes e Métodos: Neste estudo retrospetivo foram incluídos 53 olhos (39 doentes) com miopia e/ou astigmatismo submetidos a cirurgia refrativa com PRK (Allegretto WAVE Eye-Q Excimer Laser System, Alcon). Trinta e quatro olhos foram tratados com o procedimento Custom-Q e 19 olhos com o procedimento Wavefront-optimized. Foram incluídos doentes com um seguimento mínimo de 6 meses; idade acima de 21 anos; erro refrativo estável por 2 anos; equivalente esférico inferior a 5.50 dioptrias (D); percentagem de tecido alterado inferior a 40% e curvatura final da córnea esperada acima de 35 dioptrias. Foram excluídos olhos com outras patologias oftalmológicas. A asfericidade basal e pós-operatória e as aberrações ópticas foram avaliadas com Pentacam (Oculus Optikgeräte, Wetzlar, Germany). Resultados: Os dois grupos eram semelhantes quanto aos dados demográficos e dados refrativos pré-operatórios (p≥ 0.05). O equivalente esférico no pós-operatório foi inferior a 0.50D em 100% dos olhos no grupo Custom-Q e 78.90% dos olhos no grupo Wavefront-optimized, e foi inferior a 0.25D em 97.06% e 73.70% dos olhos, respetivamente. A variação do valor Q foi de 0.60 ± 0.35 (intervalo -0.07-1.24) no grupo Custom-Q e 0.65 ± 0.40 (intervalo -0.05-1.40) no grupo Wavefront-optimized (p=0.61). A variação do valor Q não foi influenciada pelo perfil de ablação (B=0.04, p=0.49, 95%CI [-0.08,0.17]) e o equivalente esférico foi um forte preditor (B=-0.30, p<0.01, 95%CI[-0.39,-0.21]). Verificou-se uma diferença significativa nas aberrações de alta ordem em cada grupo (p<0.01) apesar de não se ter observado uma diferença significativa entre os grupos (p=0.48). Discussão e conclusão: Na nossa amostra, a ablação com o perfil Custom-Q não foi significativamente diferente do perfil Wavefront-optimized relativamente à asfericidade pós-operatória. Apesar do aumento nas aberrações de alta ordem, ambas as técnicas foram eficazes e seguras para a correção de miopia e/ou astigmatismo até -5.50D.
Purpose: To compare the results between photorefractive keratectomy (PRK) with Custom-Q or with Wavefront-optimized (WFO) profiles in terms of asphericity and spherical aberrations, 6 months post-operative. Setting: Tertiary referral center (Centro Hospitalar e Universitário da Universidade de Coimbra, Coimbra, Portugal). Patients and Methods: Fifty-three eyes (39 patients) were enrolled on this retrospective case series, including patients with myopia and/or astigmatism, submitted to refractive surgery with PRK (Allegretto WAVE Eye-Q Excimer Laser System, Alcon), in a Custom-Q ablation (34 eyes) or Wavefront-optimized procedure (19 eyes). We included patients with a minimum follow-up of 6 months; age over 21 years; stable refractive error for 2 years; spherical equivalent (SE) inferior to 5.50 diopters (D); percentage of altered tissue under 40% and expected final corneal curvature above 35 D. Eyes with other ophthalmological pathologies were excluded. Baseline and post-operative asphericity and optical aberrations were evaluated with Pentacam (Oculus Optikgeräte, Wetzlar, Germany). Results: The demographic and preoperative refractive data was similar between groups (all p≥0.05). Post-operative spherical equivalent in the Custom-Q and Wavefront-optimized groups was within 0.50D in 100% and 78.90% of eyes, respectively and within 0.25D in 97.06% and 73.70% of eyes, respectively. Variation of Q-value was 0.60 ± 0.35 (range -0.07-1.24) for Custom Q group, and 0.65 ± 0.40 (range -0.05-1.40) in the Wavefront-optimized group (p=0.61). In a multivariate linear regression model, variation of Q-value was not influenced by the ablation profile (B=0.04, p=0.49, 95%CI [-0.08,0.17]). SE was a strong predictor (B=-0.30, p<0.01, 95%CI[-0.39,-0.21]). There was a significant increase in RMS higher-order aberrations (p<0.01 for both groups) and no difference between groups (p=0.48). Discussion and Conclusion: In our sample, Custom-Q ablation was not significantly different from Wavefront-optimized ablation regarding post-operative asphericity. Although the increase in higher-order aberrations, both techniques were effective and safe for myopic and/or astigmatic correction up to -5.50D SE.

Trabalho Final do Mestrado Integrado em Medicina apresentado à Faculdade de Medicina
Objetivo: Comparar os resultados entre queratectomia fotorefrativa (PRK) com perfis Custom-Q ou Wavefront-optimized (WFO) relativamente à asfericidade e aberrações esféricas, 6 meses após a cirurgia. Local: Centro de Referência Terciário (Centro Hospitalar e Universitário da Universidade de Coimbra, Coimbra, Portugal). Participantes e Métodos: Neste estudo retrospetivo foram incluídos 53 olhos (39 doentes) com miopia e/ou astigmatismo submetidos a cirurgia refrativa com PRK (Allegretto WAVE Eye-Q Excimer Laser System, Alcon). Trinta e quatro olhos foram tratados com o procedimento Custom-Q e 19 olhos com o procedimento Wavefront-optimized. Foram incluídos doentes com um seguimento mínimo de 6 meses; idade acima de 21 anos; erro refrativo estável por 2 anos; equivalente esférico inferior a 5.50 dioptrias (D); percentagem de tecido alterado inferior a 40% e curvatura final da córnea esperada acima de 35 dioptrias. Foram excluídos olhos com outras patologias oftalmológicas. A asfericidade basal e pós-operatória e as aberrações ópticas foram avaliadas com Pentacam (Oculus Optikgeräte, Wetzlar, Germany). Resultados: Os dois grupos eram semelhantes quanto aos dados demográficos e dados refrativos pré-operatórios (p≥ 0.05). O equivalente esférico no pós-operatório foi inferior a 0.50D em 100% dos olhos no grupo Custom-Q e 78.90% dos olhos no grupo Wavefront-optimized, e foi inferior a 0.25D em 97.06% e 73.70% dos olhos, respetivamente. A variação do valor Q foi de 0.60 ± 0.35 (intervalo -0.07-1.24) no grupo Custom-Q e 0.65 ± 0.40 (intervalo -0.05-1.40) no grupo Wavefront-optimized (p=0.61). A variação do valor Q não foi influenciada pelo perfil de ablação (B=0.04, p=0.49, 95%CI [-0.08,0.17]) e o equivalente esférico foi um forte preditor (B=-0.30, p<0.01, 95%CI[-0.39,-0.21]). Verificou-se uma diferença significativa nas aberrações de alta ordem em cada grupo (p<0.01) apesar de não se ter observado uma diferença significativa entre os grupos (p=0.48). Discussão e conclusão: Na nossa amostra, a ablação com o perfil Custom-Q não foi significativamente diferente do perfil Wavefront-optimized relativamente à asfericidade pós-operatória. Apesar do aumento nas aberrações de alta ordem, ambas as técnicas foram eficazes e seguras para a correção de miopia e/ou astigmatismo até -5.50D.
Purpose: To compare the results between photorefractive keratectomy (PRK) with Custom-Q or with Wavefront-optimized (WFO) profiles in terms of asphericity and spherical aberrations, 6 months post-operative. Setting: Tertiary referral center (Centro Hospitalar e Universitário da Universidade de Coimbra, Coimbra, Portugal). Patients and Methods: Fifty-three eyes (39 patients) were enrolled on this retrospective case series, including patients with myopia and/or astigmatism, submitted to refractive surgery with PRK (Allegretto WAVE Eye-Q Excimer Laser System, Alcon), in a Custom-Q ablation (34 eyes) or Wavefront-optimized procedure (19 eyes). We included patients with a minimum follow-up of 6 months; age over 21 years; stable refractive error for 2 years; spherical equivalent (SE) inferior to 5.50 diopters (D); percentage of altered tissue under 40% and expected final corneal curvature above 35 D. Eyes with other ophthalmological pathologies were excluded. Baseline and post-operative asphericity and optical aberrations were evaluated with Pentacam (Oculus Optikgeräte, Wetzlar, Germany). Results: The demographic and preoperative refractive data was similar between groups (all p≥0.05). Post-operative spherical equivalent in the Custom-Q and Wavefront-optimized groups was within 0.50D in 100% and 78.90% of eyes, respectively and within 0.25D in 97.06% and 73.70% of eyes, respectively. Variation of Q-value was 0.60 ± 0.35 (range -0.07-1.24) for Custom Q group, and 0.65 ± 0.40 (range -0.05-1.40) in the Wavefront-optimized group (p=0.61). In a multivariate linear regression model, variation of Q-value was not influenced by the ablation profile (B=0.04, p=0.49, 95%CI [-0.08,0.17]). SE was a strong predictor (B=-0.30, p<0.01, 95%CI[-0.39,-0.21]). There was a significant increase in RMS higher-order aberrations (p<0.01 for both groups) and no difference between groups (p=0.48). Discussion and Conclusion: In our sample, Custom-Q ablation was not significantly different from Wavefront-optimized ablation regarding post-operative asphericity. Although the increase in higher-order aberrations, both techniques were effective and safe for myopic and/or astigmatic correction up to -5.50D SE.

博士
國立臺灣大學
電信工程學研究所
91
The wavefront distortion effects caused by heterogeneous tissue layers and the consequential image quality degradation have been extensively studied and many correction algorithms have been developed. In this dissertation a model is introduced that incorporates the cumulative wavefront distortion effects caused by spatial heterogeneities along the path of propagation, and a corresponding model-based wavefront distortion-correction method is presented. In the proposed model, a distributed heterogeneous medium is lumped into a series of parallel phase screens. The distortion effects can be compensated — without a priori knowledge of the distorting structure — by backpropagation of received wavefronts through hypothetical multiple phase screens located between the imaging system and targets, while each point-wise time shift is adjusted iteratively to maximize a specified image quality factor at the final layer. Theoretical analyses indicate that the mean speckle brightness decreases monotonically with the root-mean-square value of distributed phase distortions, and therefore the speckle brightness can be used as an image quality factor. Experimental 1-D array data with simulated distortion effects based on a real 2-D abdominal-tissue map are used to evaluate the performance of the proposed method and two main existing aberration-correction techniques: time-shift compensation (TSC) and backpropagation followed by time-shift compensation (BP+TSC). The simulated characteristics of wavefront distortion and relative performance of existing correction techniques are similar to reports based on abdominal-wall data and breast data. Numerical results also show that the proposed method provides better compensation for wavefront distortion. Intuitively, satisfactory compensation can be obtained if the received wavefront was large enough and the exact distorting structure was known. In clinical situation it is questionable whether the compensation based on exact knowledge of the distorting structure is optimal because a finite aperture is always used. Influences of aperture size on wavefront distortion correction are investigated in this dissertation both theoretically and numerically. Numerical simulations were performed using the above mentioned distortion correction methods, i.e., TSC, BP+TSC and the multilayer phase screen compensation (MPSC) method proposed by the author. Performances are evaluated by errors between the corrected wavefronts and the undistorted one. In addition, point spread functions were calculated to evaluate the relative image quality. Theoretical analysis shows error will decrease with aperture size when exact phase compensation (EPC) is applied, although finite errors will always exist along the edges of the corrected wavefront. Numerical results show that the quality of wavefront with EPC is essentially limited by the aperture size, while the correction methods considered are relatively robust against the aperture size. It also shows that for low aberration, results with MPSC and EPC are comparable; for high aberration, however, MPSC significantly outperforms EPC in suppression of error and sidelobes. This study suggests that for most medical ultrasound imaging systems, the exact structure of the distorting medium may not be necessary to be known a priori for optimal distortion correction because of the limitation imposed by finite aperture size.