Showing papers on "Wavefront published in 2001"

History and Principles of Shack-Hartmann Wavefront Sensing

Abstract: developed out of a need to solve a problem. The problem was posed, in the late 1960s, to the Optical Sciences Center (OSC) at the University of Arizona by the US Air Force. They wanted to improve the images of satellites taken from earth. The earth's atmosphere limits the image quality and exposure time of stars and satellites taken with telescopes over 5 inches in diameter at low altitudes and 10 to 12 inches in diameter at high altitudes. Dr. Aden Mienel was director of the OSC at that time. He came up with the idea of enhancing images of satellites by measuring the Optical Transfer Function (OTF) of the atmosphere and dividing the OTF of the image by the OTF of the atmosphere. The trick was to measure the OTF of the atmosphere at the same time the image was taken and to control the exposure time so as to capture a snapshot of the atmospheric aberrations rather than to average over time. The measured wavefront error in the atmosphere should not change more than ␭/10 over the exposure time. The exposure time for a low earth orbit satellite imaged from a mountaintop was determined to be about 1/60 second. Mienel was an astronomer and had used the standard Hartmann test (Fig 1), where large wooden or cardboard panels were placed over the aperture of a large telescope. The panels had an array of holes that would allow pencils of rays from stars to be traced through the telescope system. A photographic plate was placed inside and outside of focus, with a sufficient separation, so the pencil of rays would be separated from each other. Each hole in the panel would produce its own blurry image of the star. By taking two images a known distance apart and measuring the centroid of the images, one can trace the rays through the focal plane. Hartmann used these ray traces to calculate figures of merit for large telescopes. The data can also be used to make ray intercept curves (H'-tan U'). When Mienel could not cover the aperture while taking an image of the satellite, he came up with the idea of inserting a beam splitter in collimated space behind the eyepiece and placing a plate with holes in it at the image of the pupil. Each hole would pass a pencil of rays to a vidicon tube (this was before …

Chapter 4 - Singular optics

Abstract: Singular optics is a branch of modern physical optics that involves a wide class of effects associated with the phase singularities in wave fields and with the topology of wave fronts. Optical singularities (optical vortices) exhibit some fundamental features absent in the "usual" fields with smooth wave fronts. Namely, optical vortices possess orbital angular momentum, topological charge for helical wave front of beams with well-defined direction of propagation. As a result, an interesting spatial evolution can be generated such as optical vortices "nucleation" and "annihilation" by pairs with participation of phase saddles, often called "optical chemistry." To study the structure of the circular edge dislocation, an isolated dark (zero-amplitude) ring is created within a light beam, with an analytical description of the field. A screw wave-front dislocation has a feature that the spatial structure of the wave front has the form of a helicoid around the dislocation axis. The chapter also describes reflection, refraction, interference and diffraction of OVs. Both frequency up- and down-conversion processes possess essential peculiarities for light beams with OVs. The chapter discusses the topology of wave fronts and vortex trajectories. Gouy phase shift in singular optics is also described in the chapter.

Age-related changes in monochromatic wave aberrations of the human eye.

Abstract: PURPOSE To investigate the relations between age and the optical aberrations of the whole eye The eye’s optical quality, as measured by the modulation transfer function (MTF), degrades with age, but the MTF does not provide a means to assess the contributions of individual aberrations, such as coma, spherical aberration, and other higher order aberrations to changes in optical quality The method used in this study provides measures of individual aberrations and overall optical quality METHODS Wave aberrations in 38 subjects were measured psychophysically using a spatially resolved refractometer Data were fit with Zernike polynomials up to the seventh order to provide estimates of 35 individual aberration terms MTFs and root mean square (RMS) wavefront errors were calculated Subjects ranged in age from 229 to 645 years, with spherical equivalent corrections ranging from 105 to 260 D RESULTS Overall RMS wavefront error (excluding tilts, astigmatism, and defocus) was significantly positively correlated with age (r 5 033, P 5 0042) RMS error for the highest order aberrations measured (fifth through seventh order) showed a strong positive correlation with age (r 5 057, P 5 00002) Image quality, as quantified by the MTF, also degraded with age CONCLUSIONS Wave aberrations of the eye increase with age This increase is consistent with the loss of contrast sensitivity with age observed by other investigators (Invest Ophthalmol Vis Sci 2001;42:1390 ‐1395)

Aberrations and visual performance following standard laser vision correction.

Abstract: 6 pages, 5 figures.-- PMID: 11583239 [PubMed].-- Presented at the 2nd International Congress of Wavefront Sensing and Aberration-free Refractive Correction, in Monterey, CA, on February 9-10, 2001.

Closed-loop adaptive optics in the human eye.

Abstract: We have developed a prototype apparatus for real-time closed-loop measurement and correction of aberrations in the human eye. The apparatus uses infrared light to measure the wave-front aberration at 25 Hz with a Hartmann–Shack sensor. Defocus is removed by a motorized optometer, and higher-order aberrations are corrected by a membrane deformable mirror. The device was first tested with an artificial eye. Correction of static aberrations takes approximately five iterations, making the system capable of following aberration changes at 5 Hz. This capability allows one to track most of the aberration dynamics in the eye. Results in living eyes showed effective closed-loop correction of aberrations, with a residual uncorrected wave front of 0.1 μm for a 4.3-mm pupil diameter. Retinal images of a point source in different subjects with and without adaptive correction of aberrations were estimated in real time. The results demonstrate real-time closed-loop correction of aberration in the living eye. An application of this device is as electro-optic “spectacles” to improve vision.

Apparatus and method for objective measurement of optical systems using wavefront analysis

Abstract: An apparatus for determining aberrations of an eye includes a patient head rest allowing for positioning adjustment. The patient head rest is operable with an optical table having a base. The base includes a probe beam generating apparatus, probe beam directing optics which itself comprises a beam splitter; a mirror; and a lens. The probe beam directing optics is capable of directing a probe beam toward an eye of a patient positioned on the patient head rest. Video image components are provided and comprise a light source, a mirror; and a video camera. The video image components are capable of generating an image of an eye of a patient positioned on the patient head rest. Eye fixation components generate a target that the eye of a patient positioned on the patient head rest can view. The eye fixation components comprise a fixation target, a light source, a lens, and a mirror. Wavefront directing and analyzing components measure a wavefront emanating from the eye of a patient positioned on the patient head rest and determine aberrations of the eye that range from at least about + or −1 diopters to at least about + or −6 diopters. The wavefront directing and analyzing components include a lens; a mirror, a microlens array, a camera, and a data processor.

Wavefront coding phase contrast imaging systems

Abstract: The present invention provides extended depth of field or focus to conventional Phase Contrast imaging systems. This is accomplished by including a Wavefront Coding mask in the system to apply phase variations to the wavefront transmitted by the Phase Object being imaged. The phase variations induced by the Wavefront Coding mask code the wavefront and cause the optical transfer function to remain essentially constant within some range away from the in-focus position. This provides a coded image at the detector. Post processing decodes this coded image, resulting in an in-focus image over an increased depth of field.

Planar laser illumination and imaging (PLIIM) system employing wavefront control methods for reducing the power of speckle-pattern noise digital images acquired by said system

Abstract: A planar laser illumination and imaging (PLIIM) based system employing wavefront control methods for reducing the power of speckle-pattern noise within digital images acquired by the system. The system comprises a housing having a first light transmission aperture, a second light transmission aperture, and a third light transmission aperture, wherein the first and second light transmission apertures are optically isolated from said third light transmission aperture spatially aligned on opposite sides thereof. A pair of planar laser illumination arrays (PLIAs) are mounted within the housing, for producing and projecting a first and second planar laser illumination beams (PLIBS) through the first and second light transmission apertures respectively, thereby producing a composite planar laser illumination beam (PLIB) outside of the housing, and illuminating an object therewith as the object is transported past the first, second and third light transmission apertures. An image formation and detection (IFD) module is mounted within the housing, and has a linear image detection array and imaging forming optics for providing the linear image detection array with a field of view (FOV) which is projected through the third light transmission aperture, and along which digital images of illuminated portions of the object can be detected. Notably, digital images contain speckle-pattern noise, and the PLIB and FOV are arranged in a coplanar relationship along the working range of the PLIIM based system so that the composite PLIB illuminates primarily within the FOV of the IFD module. A wavefront control mechanism is mounted within said housing, for controlling one or more characteristics of the wavefront of the composite PLIB so as to reduce either the spatial and/or temporal coherence of the composite PLIB prior to its illumination of a selected portion of the object, so that time-varying different speckle-noise patterns are present in the digital images detected at the linear image detection array. These time-varying different speckle patterns are temporally averaged at the linear image detection array during the photo-integration time period thereof so that the power of speckle-pattern noise at the linear image detection array is substantially reduced.

Evaluating the effect of transmissive optic thermal lensing on laser beam quality with a shack-hartmann wave-front sensor.

Abstract: We examine wave-front distortion caused by high-power lasers on transmissive optics using a Shack-Hartmann wave-front sensor. The coupling coefficient for a thermally aberrated Gaussian beam to the TEM(00) mode of a cavity was determined as a function of magnitude of the thermally induced aberration. One wave of thermally induced phase aberration between the Gaussian intensity peak and the 1/e(2) radius of the intensity profile reduces the power-coupling coefficient to the TEM(00) mode of the cavity to 4.5% with no compensation. With optimal focus compensation the power coupling is increased to 79%. The theoretical shape of the thermally induced optical phase aberration is compared with measurements made in a neutral-density filter glass, Faraday glass, and lithium niobate. The agreement between the theoretical and the measured thermal aberration profiles is within the rms wave-front measurement sensitivity of the Shack-Hartmann wave-front sensor, which is a few nanometers.

Method and apparatus for reducing laser speckle

Abstract: A method and apparatus for reducing laser speckle in a laser illuminated system where a line illumination produced by a laser is scanned across a diffuse service is preferably incorporated in a display apparatus where a two dimentional image is produced on a display screen. The display apparatus includes a ligh modulator, a wavefront modulator, and a projection/scanning optical arrangement. The light modulator modulates a laser illumination to form a line image made up of a linear array of pixels. The wavefront modulator varies a spatial phase across a width of the line image thus forming a phase modulated wavefront. The projecting/scanning optical arrangement projects the line image onto the display screen and scans the line image over the display screen. The phase modulated wavefront produces multiple speckle patterns that are averaged as the line image is scanned and, thus, produces the reduced laser speckle.

Wavefront healing: a banana–doughnut perspective

Abstract: SUMMARY Wavefront healing is a ubiquitous diffraction phenomenon that affects cross-correlation traveltime measurements, whenever the scale of the 3-D variations in wave speed is comparable to the characteristic wavelength of the waves. We conduct a theoretical and numerical analysis of this finite-frequency phenomenon, using a 3-D pseudospectral code to compute and measure synthetic pressure-response waveforms and ‘ground truth’ cross-correlation traveltimes at various distances behind a smooth, spherical anomaly in an otherwise homogeneous acoustic medium. Wavefront healing is ignored in traveltime tomographic inversions based upon linearized geometrical ray theory, in as much as it is strictly an infinite-frequency approximation. In contrast, a 3-D banana‐doughnut

Wavefront coding optics

Abstract: Improved Wavefront Coding Optics, which apply a phase profile to the wavefront of light from an object to be imaged, retain their insensitivity to focus related aberration, while increasing the heights of the resulting MTFs and reducing the noise in the final images. Such improved Wavefront Coding Optics have the characteristic that the central portion of the applied phase profile is essentially flat (or constant), while a peripheral region of the phase profile around the central region alternately has positive and negative phase regions relative to the central region.

Apparatus for wavefront detection

Abstract: An apparatus for wavefront detection includes a wavefront source for the production of a wavefront, an optical system transforming the wavefront, a diffraction grating through which the transformed wavefront passes, and a spatially resolving detector following the diffraction grating. The wavefront source has a two-dimensional structure.

Wavefront coded imaging systems

Abstract: The present invention provides improved Wavefront Coding imaging apparatus (100, 800, 1100) and methods composed of optics (102, 802, 1102), wavefront coding (104, 806, 1110), detection (106), and processing (112, 810, 1112) of the detected image The optics are constructed and arranged to have the characteristic that the transverse ray intercept curves form substantially straight, sloped lines The wavefront coding corrects for known or unknown amounts of 'misfocus-like' aberrations by altering the optical transfer function of the imaging apparatus in such a way that the altered optical transfer function is substantially insensitive to aberrations Post processing then removes the effect of the coding, except for the invariance with regard to aberrations, producing clear images

Customized corneal ablation : the quest for super vision

Abstract: An introduction to treating refractive errors with a sophisticated technology - the use of computerized corneal topography and wave front sensing. Using these techniques, many scientists believe that vision can be improved to even the 20/8 level, which is three to four lines better than 20/20 on the visual acuity chart. This type of super or hyper vision is possible with newer laser techniques using wavefront aberration testing coupled with customized spot excimer laser treatments.

Computer-generated holograms of three-dimensional realistic objects recorded without wave interference

Abstract: We propose a method of synthesizing computer-generated holograms of real-life three-dimensional (3-D) objects. An ordinary digital camera illuminated by incoherent white light records several projections of the 3-D object from different points of view. The recorded data are numerically processed to yield a two-dimensional complex function, which is then encoded as a computer-generated hologram. When this hologram is illuminated by a plane wave, a 3-D real image of the object is reconstructed.

Aberration compensation in digital holographic reconstruction of microscopic objects

Abstract: We present a digital method for the compensation of the aberrations appearing when a lensless hologram of a microscopic object is reconstructed. The digital hologram of the microscopic object is recorded on a relatively low resolution device (CCD sensor). An expansion of the hologram by interpolation of the recorded intensity is performed in order to increase the number of pixels. For digital reconstruction of the wavefronts the expanded hologram is multiplied by the reference wave followed by simulation of the diffraction using the Rayleigh-Sommerfeld propagation relation. Experimental results are presented.

Implementing the fast marching eikonal solver: spherical versus Cartesian coordinates

Abstract: Spherical coordinates are a natural orthogonal system for describing wavefronts emanating from a point source. A regular grid distribution in the Cartesian-coordinate system tends to undersample the wavefront description near the source (at the highest wavefront curvature) and oversample it away from the source. Spherical coordinates, in general, provide a more balanced grid distribution for characterizing point-source wavefronts. Our numerical implementation confirms that the recently introduced fast marching algorithm is both a highly efficient and an unconditionally stable eikonal solver. However, its first-order approximation of traveltime derivatives can induce relatively large traveltime errors for waves propagating in a diagonal direction with respect to the coordinate system. Examples, including the IFP Marmousi and the SEG/EAGE 3D salt-dome models, show that a spherical-coordinate implementation of the method results in far fewer errors in traveltime calculation than the conventional Cartesian-coordinate implementation, and with practically no loss in computational advantages.

Submicron focusing of hard x rays with reflecting surfaces at the ESRF

Abstract: We describe Kirkpatrick-Baez (KB) reflecting mirror systems that have been developed at the European Synchrotron Radiation Facility (ESRF). They are intended to be used mainly in the hard x-ray domain from 10 KeV to 30 KeV for microfluorescence, microdiffraction and projection microscopy applications. At 19 KeV a full width at half maximum (FWHM) spot size of 200x600 nanometers has been measured and with an estimated irradiance gain of 3.5x105. The alignment and bending processes of the system are automated based on the wavefront information obtained by sequentially scanning slits and reading a position-sensitive device located in the focal plane. The sub-microradian sensitivity of this method allows us to predict the spot size and ot provide a metrology map of the surfaces for future improvements of the performances. A novel device based on specular reflection by a micromachined platinum mirror has been used to determine the spot size with an equivalent slit size of less than 100 nanometers. Projection phase images of submicron structures are presented which clearly show both the high potential and also the present limitations of the system. First microfluorescence images obtained at 20.6 KeV are shown. Finally, a roadmap towards diffraction-limited performance with metal and multilayer surfaces is presented.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Deformable magnetic mirror for adaptive optics: technological aspects

Abstract: Ground-based telescopes suffer from atmospheric turbulences which perturb the quality of the light arriving from space. Astronomers use adaptive optics in order to correct the wavefront of oncoming light. In this context, a prototype of electromagnetic miniature (O 50 mm) deformable mirror is developed, using available microtechnologies. The mirror is composed of a thin polymer membrane (2–5 μm) covered with a matrix of permanent magnets, and of an array of planar microcoils on a O 50 mm substrate. The paper presents the various technologies used to build the actuator (membranes, coils, magnets). The mechanical behaviour of the mirror is tested. Deformations of up to 20 μm are achieved with currents in the range 1–3 A (6.6 μm/A). Resonance occurs at 485 Hz, with good linearity up to 200 Hz.

Multilayer reflective mirrors for EUV, wavefront-aberration-correction methods for same, and EUV optical systems comprising same

Abstract: Multilayer mirrors are disclosed for use especially in “Extreme Ultraviolet” (“soft X-ray,” or “EUV”) optical systems. Each multilayer mirror includes a stack of alternating layers of a first material and a second material, respectively, to form an EUV-reflective surface. The first material has a refractive index substantially the same as a vacuum, and the second material has a refractive index that differs sufficiently from the refractive index of the first material to render the mirror reflective to EUV radiation. The wavefront profile of EUV light reflected from the surface is corrected by removing (“machining” away) at least one surficial layer of the stack in selected region(s) of the surface of the stack. Machining can be performed such that machined regions have smooth tapered edges rather than abrupt edges. The stack can include first and second layer groups that allow the unit of machining to be very small, thereby improving the accuracy with which wavefront-aberration correction can be conducted. Also disclosed are various at-wavelength techniques for measuring reflected-wavelength profiles of the mirror. The mirror surface can include a cover layer of a durable material having high transparency and that reduces variations in reflectivity of the surface caused by machining the selected regions.

Small-phase solution to the phase-retrieval problem.

Abstract: A solution to the phase-retrieval problem when the unknown phase is small is presented. The solution specifies the even and odd parts of the unknown phase in two separate equations. The odd part requires a single intensity measurement, and the even part requires two measurements. Phase diversity is used for the second measurement, and computer simulations are given.

High resolution, multispectral, wide field of view retinal imager

Abstract: An ophthalmic instrument (for obtaining high resolution, wide field of area multi-spectral retinal images) including a fundus retinal imager, (which includes optics for illuminating and imaging the retina of the eye); apparatus for generating a reference beam coupled to the fundus optics to form a reference area on the retina; a wavefront sensor optically coupled to the fundus optics for measuring the wavefront produced by optical aberrations within the eye and the imager optics; wavefront compensation optics coupled to the fundus optics for correcting large, low order aberrations in the wavefront; a high resolution detector optically coupled to the imager optics and the wavefront compensation optics; and a computer (which is connected to the wavefront sensor, the wavefront compensation optics, and the high resolution camera) including an algorithm for correcting, small, high order aberrations on the wavefront and residual low order aberrations.

Laser eye surgery system using wavefront sensor analysis to control digital micromirror device (dmd) mirror patterns

Abstract: A system (10) and method for performing corneal ablation or reshaping with a laser (22) in order to correct aberrations in the optical system of the eye (40) utilizes a wavefront sensor which defines a wavefront correction for the eye (40) and then, based upon that defined wavefront correction, drives a digital micromirror device (DMD) which modulates a laser beam to the eye (40) to perform the correction. As the DMD (26) is a 2-D array of individually controlled mirrors, and the wavefront sensor analysis can provide a sequence of two dimensional arrays of values which together define the wavefront correction for the eye (40), the combination of the two produces a method for correcting the corneal surface. The system (10) may be operated in either of two manners to achieve optimum refractive corrections: (1) off-line measurement of the eye optical system via the wavefront sensor followed by DMD-based laser refractive surgery, or (2) realtime measurement of the eye optical system via the wavefront sensor which directs a DMD-based laser refractive surgery system.

Geometry of phase and polarization singularities illustrated by edge diffraction and the tides

Abstract: In complex scalar fields, singularities of the phase (optical vortices, wavefront dislocations) are lines in space, or points in the plane, where the wave amplitude vanishes. Phase singularities are illustrated by zeros in edge diffraction and amphidromies in the heights of the tides. In complex vector waves, there are two sorts of polarization singularity. The polarization is purely circular on lines in space or points in the plane (C singularities); these singularities have index +/- 1/2. The polarization is purely linear on lines in space for general vector fields, and surfaces in space or lines in the plane for transverse fields (L singularities); these singularities have index +/- 1. Polarization singularities (C points and L lines) are illustrated in the pattern of tidal currents.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nonlinear Hyperbolic Waves in Multidimensions

Abstract: In this book, the authors have discussed the propagation of a curved nonlinear wavefront for the ordinary fluid dynamics(ie without considering magneto-fluids) according to two fundamental physical processes:i) at different points of the wavefront,it travels with different speeds depending on the local amplitude leading to a longitudinal stretching of rays; and ii) a lateral deviation of rays is produced due to non-uniform distribution of the amplitude of the wave front.