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Wavefront reconstruction from local slope estimates is performed by a family of variational methods with a fractional order stabilizer designed to ensure best uniform reconstruction of the front on the entire frequency spectrum. Analytical and numerical investigation of the space-frequency characteristic of the family and numerical experiments with reconstruction of regular and turbulent wavefronts have demonstrated the advantage of the proposed method over the common Simpson method.

An original design is proposed that makes it possible to create a two-channel optoelectronic instrument for diagnostics of the shape and quality of intraocular lenses. This instrument consists of a collimation-type channel and a channel based on a Shack–Hartmann wavefront sensor. The hardware characteristics of the instrument are examined, i.e., the radius of curvature of the wavefront in the plane of the lens raster, and the dynamic range to which possible measurable aberrations of the intraocular lenses are restricted. A methodology is proposed for diagnostics of the design parameters of intraocular lenses. The systematic error of the output parameter of the instrument being developed is examined within the framework of precision calculations. The calculations that were performed made it possible to transfer to the design stages of an optoelectronic instrument based on the wavefront sensor for monitoring the parameters of the shape of intraocular lenses.

The purpose of the paper is to theoretically investigate 2D wave patterns in a nonlinear optical system with diffractive feedback. We consider a delayed functional differential diffusion equation on a thin annulus with Neumann boundary conditions. To study pattern formation phenomena, we construct the Faria normal form of a Hopf bifurcation, which is necessarily degenerate because the equation exhibits O(2) symmetry. The coefficients of the normal form determine the excitation of rotating or standing waves with the prescribed spatio-temporal characteristics, but these coefficients can be expressed only implicitly. However, for sufficiently thin annuli, the equation corresponds to a limit 1D model on a circle, which was carefully analysed in our previous papers and whose normal form was explicitly calculated. 2D wave stability predictions that we make based on the normal form of the limit 1D model are in good agreement with direct numerical simulations of the 2D model. To accelerate computation, we elaborated an efficient fast finite Hankel transform algorithm for thin annular domains.

PURPOSE: The aim of this work is the determination of quantitative diagnostic criteria based on the spectral characteristics of fundus autofluorescence to detect early stages of degeneration in the retina and retinal pigment epithelium (RPE). METHODS: RPE cell suspension samples were obtained from the cadaver eyes with and without signs of age-related macular degeneration (AMD). Fluorescence analysis at an excitation wavelength of 488 nm was performed. The fluorescence lifetimes of lipofuscin-granule fluorophores were measured by counting time-correlated photon method. RESULTS: Comparative analysis of fluorescence spectra of RPE cell suspensions from the cadaver eyes with and without signs of AMD showed a significant difference in fluorescence intensity at 530–580 nm in response to fluorescence excitation at 488 nm. It was notably higher in eyes with visual pathology than in normal eyes regardless of the age of the eye donor. Measurements of fluorescence lifetimes of lipofuscin fluorophores showed that the contribution of photooxidation and photodegradation products of bisretinoids to the total fluorescence at 530–580 nm of RPE cell suspensions was greater in eyes with visual pathology than in normal eyes. CONCLUSION: Because photooxidation and photodegradation products of bisretinoids are markers of photodestructive processes, which can cause RPE cell death and initiate degenerative processes in the retina, quantitative determination of increases in these bisretinoid products in lipofuscin granules may be used to establish quantitative diagnostic criteria for degenerative processes in the retina and RPE.

A new technique is suggested for wavefront reconstruction from gradient measurements on the basis of solution of the variational problem. The spatial frequency transfer function is derived for its reconstructor. The reconstruction accuracy is studied at different spatial spectra of wavefront distortions in numerical simulation. The reconstructor sensitivity is analyzed in the case of data loss in some nodes of the measuring grid.

An experiment on the simulation of the conditions for propagation of laser pulses in dense neodymium atomic vapor is proposed for the development of a technological setup for the laser separation of the 150Nd isotope from a kilogram sample of natural mixture. The comparison of waveforms of laser pulses in the presence and in the absence of absorption shows that the absorption predominantly takes place at the leading edge of the pulse. The experimental distortion of the waveform is related to the saturation of absorption. The effect is well described using the calculations based on the formalism of the rate equations.

A modified bispectral method is proposed for solving the inverse problem of image reconstruction in ophthalmology. Modeling shows that the method produces stable results for typical symmetric aberrations of the human eye's imaging system in the presence of turbulent distortions leading to a signalternating optical transfer function.

In this paper we deal with the problem of optical sectioning. This is a post processing step while investigating of 3D translucent medical objects based on rapid refocusing of the imaging system by the adaptive optics technique. Each image, captured in focal plane, can be represented as the sum of in-focus true section and out-of-focus images of the neighboring sections of the depth that are undesirable in the subsequent reconstruction of 3D object. The problem of optical sectioning under consideration is to elaborate a robust approach capable of obtaining a stack of cross section images purified from such distortions. For a typical sectioning statement arising in ophthalmology we propose a local iterative method in Fourier spectral plane. Compared to the non-local constant parameter selection for the whole spectral domain, the method demonstrates both improved sectioning results and a good level of scalability when implemented on multi-core CPUs.

We report the results of modeling the speckle structure of an optical field, which arises due to scattering of laser radiation at the retina of the human eye. The presence of such a speckle structure in the scattered radiation leaving the eye affects the accuracy of wavefront measurements aimed at determining the overall aberrations of the eye. We compared two methods for suppressing such speckle structures based on spatial and spectral averaging of a variety of speckle field realizations. The results of both theoretical calculations and numerical simulations appeared to be in a good agreement with experimental data obtained with the eye model and in vivo.

A Tikhonov-regularized bispectral variational method is proposed for image restoration in the presence of strong phase distortions. This method combines a number of advantages of the bispectral approach, such as preservation and restoration of phase information, invariance to random shifts of the original signal, and no requirement of high-accuracy prior information about statistical properties of observed signals. In combination with the Tikhonov-regularized variational method, which is adapted to stable processing of large images, we obtain a fairly efficient image restoration method. Test results in the presence of atmospheric and underwater phase distortions reported in this article establish the advantages of the proposed method relative to the traditional recursive bispectral method.

Results of laboratory experiments on compensating distortions of the initial laser beam wavefront using the stochastic parallel gradient descent algorithm (SPGDA) are presented. It is shown that the optimum choice of the initial and iteration steps of the algorithm and of the noise threshold coefficient enables the compensation of main distortions of the wavefront for 150–200 SPGDA iterations.

The iterative reconstruction methods of the wavefront phase estimation from a set of discrete phase slope measurements have been considered. The values of the root-mean-square difference between the reconstructed and original wavefront have been received for Jacobi, Gauss-Seidel, Successive over-relaxation and Successive over-relaxation with Simpson Reconstructor methods. The method with the highest accuracy has been defined.

A method is proposed to estimate the structure constant Cn2 and the outer scale L0 of turbulent fluctuations in Hartmann sensor measurements of the wavefront parameters of a light beam transmitted through a turbulent path. The method is based on expansion of phase fluctuations within a given aperture into a series of Zernike polynomials and statistical analysis of the coefficients of this expansion. Application of the method to estimating the parameters of fluid cell turbulence yielded results which are in good agreement with estimates obtained by other methods. The paper also presents the results of modeling based on measurements of the transverse component of the wind velocity on the path determined by correlation of the local slopes at four virtual subapertures.

Measurement of the absolute value of the humidity of the cornea of the human eye and its dynamics is of paramount importance for the preservation of eyesight. In the present paper we have demonstrated that terahertz technologies can be practically applied for quantitative measurement of the physiological dynamics of tear film and sensing of corneal tissue hydration. We suggest uses of the equipment for application in clinics and a method for absolute calibration of the values for measurement. The proposed method is fundamentally different from existing and currently available methods of ophthalmological diagnosis. This suggests that the developed technique may have high diagnostic significance and can be used in the study and treatment of several diseases of the ocular surface.

A method utilizing a Hartmann sensor for obtaining estimates of the external scale L 0 of turbulent fluctuations and the structural constant C 2 during the variation of the parameters of the wavefront of a light beam that has passed through turbulence is suggested. The method is based on Zernike polynomial decomposition of phase fluctuations at the given aperture and on analysis of this expansion coefficient’s statistics. Application of the method for making an estimate of the turbulence characteristics in a water cell yielded results that are in good agreement with the estimates obtained by other methods.

The speckle structure of a light field resulting from laser radiation scattering by human eye retina is analyzed. The presence of a speckle structure in the radiation scattered by an eye substantially impedes wave-front measurements in the study of aberrations. Two methods for suppressing such a structure based on the spatial and spectral averaging of the realizations of the speckle field are considered. The results of the theoretical calculation and numerical simulation are in agreement with the experimental data.

A new method using modal phase tomography for diagnosis of human eye aberration was elaborated and investigated. The method enabled the separation of the contributions from the individual elements of the eye into summary aberrations. The forms of the radiation wavefronts from several reference sources being formed on the retina of the eye as initial data for analysis were considered. Distortions of every element of the eye were represented as a Tsernike polynomial expansion. In this work, the theoretical basis of the method, numerical investigation and experimental modeling results confirming the efficiency of the method are presented.

The work is devoted to the research and development of a new method of modal phase tomography for diagnostics of aberrations of the human eye. Implementation of the method is based on a series of eye aberration measurements taken at different angles to the optical axis by means of a wave-front sensor. The technique of restoration permits the contributions of different elements of the eye to the total aberrations to be separated. The results of numerical research and a model experiment are presented.

The use of an optical system with a phase knife is proposed to record qualitatively the shape of the optical-radiation wave front. Theoretical and numerical studies of a sensor based on a phase knife are performed, the algorithms for reconstruction of the phase function and schemes for sensor realisation are developed, and the range in which the device can be used expediently is determined. The sensor component parameters required for its realisation are determined, and the sensor efficiency in various operational modes is tested experimentally.

A compact adaptive system capable of imaging a human-eye retina with a spatial resolution as high as 6 μm and a field of view of 15° is developed. It is shown that a modal bimorph corrector with nonlocalised response functions provides the efficient suppression of dynamic aberrations of a human eye. The residual root-mean-square error in correction of aberrations of a real eye with nonparalysed accommodation lies in the range of 0.1 — 0.15 μm.

The compensation dynamics of phase distortions in optical phase conjugation (OPC) using the film of azocontaining liquid-crystalline polymer was experimentally investigated. The quality of compensation of distortions of the wave front was determined, and the effect of experimental conditions on the OPC dynamics was studied. The coefficient of reflection in OPC reached 35% for a total incident light intensity of ∼17 mW/cm2 at a wavelength of 532 nm. The response time of the OPC system was ∼1 s at a polymer temperature of 69°C. The high-quality compensation of the wave front distortions of the laser radiation was obtained. The residual error of compensation decreased by a factor of ∼100 during the transient OPC process. It is demonstrated that the spatially modulated light beam can be retrieved during repeated passages through a phase plate with strong small-scale aberrations.

The influence of speckles on the performance of a Shark—Hartmann wavefront sensor is investigated in the eye aberration studies. The dependence of the phase distortion measurement error on the characteristic speckle size is determined experimentally. Scanning of the reference source was used to suppress the speckle structure of the laser beam scattered by the retina. The technique developed by us made it possible to study the time dependence of the human eye aberrations with a resolution of 30 ms.

Dynamics of light wavefront correction using optical phase conjugation (OPC) in the film of azo dye containing liquid-crystal polymer has been investigated experimentally. The OPC reflectivity more than 35% was attained at ∼1 mW power of pump and signal CW beams at λ=532 nm. We studied the dynamics of compensation of wavefront distortions at different operation conditions. The OPC response time∼1 s was reached at the polymer temperature of 69 ° C. High-quality correction of light wavefront distortions was obtained: the residual RMS error of compensation decreased by a factor of ∼102 at the end of OPC transient process. Restoration of spatially modulated light beam on its passage through the phase plate with strong aberrations has been demonstrated.

A scheme for optical differentiation of images based on spatial filtering is proposed. A phase knife—a glass plate consisting of two halves, whose thicknesses are chosen so that the optical path difference between the light waves passed through them makes up half the wavelength—serves as a filter. The linearity of the transformation and the sensitivity of the system to noise are investigated theoretically. The scheme was experimentally implemented on the basis of a liquid-crystal light valve. The experimental results confirm that such a setup can be efficiently used for processing images with a narrow-band spatial spectrum and for visualization of phase objects.

A two-component nonlinear optical system based on a liquid crystal light valve (LCLV) with external feedback loops is experimentally investigated. A degenerate codimension-2 bifurcation point in the parameter space is experimentally located, and the nonlinear behavior of steady and drifting roll patterns in the vicinity of this point is investigated. The experimental results are found to correlate well with the nonlinear theory developed recently. The results reported represent, to our knowledge, the first experimental observation of such nonlinear effects in optics.

The dynamics of holographic recording in a film of azo-containing liquid-crystal polymer is studied both experimentally and theoretically in the presence of the orienting electric field. The possibility is demonstrated to efficiently control the optical recording regime by varying the applied field. The specificity of hologram formation in the case of different polarisation of the interacting light beams is considered. A theoretical model describing the holographic recording dynamics in a film of azo-containing nematic polymer is presented; numerical simulation of the processes of recording and read-out in anisotropic holograms is carried out as well.

A nonlinear optical system with spatially distributed feedback was studied both theoretically and experimentally. The phase-to-intensity transformation in this system was performed by a spatial filter capable of suppressing low spatial frequencies. Hard excitation of stationary spatial structures observed in this system was explained by an analysis of the phase space structure of the amplitude equations. The developed theoretical approach, which uses a step-function approximation of the steady-state solution, allows one to determine the main quantitative characteristics of the generated structures. The basic properties of the response of the system to external perturbations with various symmetries were investigated experimentally. The obtained experimental data qualitatively agree with the results of the theoretical analysis.

The photoinduced processes in azo-compound-containing side-chain polymer films with liquid-crystal properties are examined theoretically. A model is proposed whereby it is possible to consider the dynamics of the optical response of a medium taking into account the anisotropic saturation in the angular distribution of the azo-dye isomers as well as the intermolecular interaction. The influence of the liquid-crystal ordering in the polymer is taken into account by introducing a phenomenological mean-field factor. Analytical solutions describing changes in the optical properties of a polymer film during the initial illumination stages are in good agreement with experimental data.

We report the first, to our knowledge, experimental investigation of a two-component Kerr-type nonlinear optical system with diffractive feedback. In accordance with theoretical predictions, transitions between steady and drifting roll patterns were experimentally observed near a certain point of the parameter space. Temporal frequency of the drifting rolls measured as a function of control parameters agreed qualitatively with the theoretical dependence.

A nonlinear optical system with distributed feedback, based on a liquid-crystal light valve (LCLV), is considered. The use of a Fourier-filtering technique allows a feedback configuration which suppresses high-order aberrations. Predictions of the linear theoretical analysis are confirmed by computer simulations and experiments. Suppression efficiency characteristics, such as wavefront rms errors and Strehl ratios, are measured for distorted and corrected wavefronts. An outlook on the system performance improvement is discussed.

A nonlinear optical system with a spatially distributed feedback is considered. The phase—amplitude transformation is performed in this system by a phase plate located in the Fourier plane of a confocal two-lens analyser. It is shown that introduction, into the feedback loop, of a geometric field transformation such as specular reflection of the feedback distribution results in self-excitation of spatial structures in the form of one-dimensional gratings. Experimental investigations are reported of the dependences, on the parameters of the problem, of the following characteristics of the excited gratings: their amplitude, spatial frequency, and phase shift relative to the centre of the aperture. The experimental results are compared with a theoretical analysis of the problem carried out in the first approximation.

By varying an intensive parameter in a nonlinear optical system with two-dimensional feedback the wave front displays a transition from a uniform to a patterned state, and eventually to space-time chaos. A nonlocal interaction, introduced by means of a feedback rotation, is the source of the observed phenomena. The observed space-time chaotic behavior is characterized by a rapid decay of correlation. We study the possibility that the system eventually evolves from space-time chaos to a turbulent regime. \textcopyright{} 1996 The American Physical Society.

Small-scale phase inhomogeneities visualization methods based on phase knife filtering are considered. A new modification of such a technique with controllable lower threshold frequency is suggested: when transversally displaced from the center of the Fourier plane, the phase knife demonstrated some new features. Phase-to-intensity transfer function of the filtering system for arbitrary depth of harmonical phase modulation is obtained analytically, numerically and experimentally. Possibility of optical implementation of “exclusive or” (XOR) logical operation by means of 2D phase knife is shown.

We report the emergence of a space-time disordered dynamical regime from a regular, patterned state in a system formed by a Liquid Crystal Light Valve (LCLV) with nonlocal feedback. This is obtained for increasing the amplitude of the supply voltage applied to the LCLV, thus inducing a strengthening in the nonlinear coupling between light and matter in this device. Evidence of loss of correlation of the signal is given both in the temporal and in the spatial domain. A comparison is drawn with turbulence in hydrodynamics.

The development of affordable means to image the retina is an important step toward the implementation of eye disease screening programs. In this paper we present the i-RxCam, a low-cost, hand-held, retinal camera for widespread applications such as tele-retinal screening for eye diseases like diabetic retinopathy (DR), glaucoma, and age-related ocular diseases. Existing portable retinal imagers do not meet the requirements of a low-cost camera with sufficient technical capabilities (field of view, image quality, portability, battery power, and ease-of-use) to be distributed widely to low volume clinics, such as the offices of single primary care physicians serving rural communities. The i-RxCam uses a Nikon D3100 digital camera body. The camera has a CMOS sensor with 14.8 million pixels. We use a 50mm focal lens that gives a retinal field of view of 45 degrees. The internal autofocus can compensate for about 2D (diopters) of focusing error. The light source is an LED produced by Philips with a linear emitting area that is transformed using a light pipe to the optimal shape at the eye pupil, an annulus. To eliminate corneal reflex we use a polarization technique in which the light passes through a nano-wire polarizer plate. This is a novel type of polarizer featuring high polarization separation (contrast ratio of more than 1000) and very large acceptance angle (>45 degrees). The i-RxCam approach will yield a significantly more economical retinal imaging device that would allow mass screening of the at-risk population.

A bistable optical device, made of an extended system as a liquid crystal light valve (LCLV) plus a global feedback, displays a time dependent behavior whereby the fields corresponding to the two stable states fill different regions of the available space, and the boundary between the two regions moves periodically with a front velocity depending on the properties of the medium as well as on the feedback strength.

Experimental evidence is presented of polygon-type pattern formation in a 2D feedback system using a nonlinear interferometer consisting of a liquid crystal light valve (LCLV) plus a polarizer and a diffraction length. A mathematical model of this system is also presented.

Researchers have sought to gain greater insight into the mechanisms of the retina and the optic disc at high spatial resolutions that would enable the visualization of small structures such as photoreceptors and nerve fiber bundles. The sources of retinal image quality degradation are aberrations within the human eye, which limit the achievable resolution and the contrast of small image details. To overcome these fundamental limitations, researchers have been applying adaptive optics (AO) techniques to correct for the aberrations. Today, deformable mirror based adaptive optics devices have been developed to overcome the limitations of standard fundus cameras, but at prices that are typically unaffordable for most clinics. In this paper we demonstrate a clinically viable fundus camera with auto-focus and astigmatism correction that is easy to use and has improved resolution. We have shown that removal of low-order aberrations results in significantly better resolution and quality images. Additionally, through the application of image restoration and super-resolution techniques, the images present considerably improved quality. The improvements lead to enhanced visualization of retinal structures associated with pathology.

A single-pass nonlinear interferometer with feedback field rotation is considered. Increasing feedback gain results in excitation of spatial subharmonics that distort optical reverberators, which are basic output patterns of the system. Features of the period-doubling process depend on the number of the reverberator’s petals. A new spatiotemporal effect is observed: A basic structure with an odd number of petals is perturbed not by a static subharmonic but by a rotating wave.

Amorphous chalcogenide thin films are excellent materials for holographic recordings. AsSeS thin film coating is a useful optical material for it's thickness to be easily corrected with the use of exposure to light and consecutive chemical etching. Following properties allow to treat the surface of AsSeS chalcogenide films and to use them in adaptive optics systems for correction of the optical wavefront. Hereby, we characterize AsSeS film properties to be used for correction of optical aberrations of the human eye. The thickness of the film is characterized with the method of spectrodensitometry and the surface profile depth with a Hartman- Shack waveform analyzator.

The purpose of this work is studying a corneal asphericity change after a myopic refractive correction by mean of excimer lasers. As the ablation profile shape plays a key role in the post-op corneal asphericity, ablation profiles of recent lasers should be studied. The other task of this research was to analyze operation (LASIK) outcomes of one of the lasers with generic spherical ablation profile and to compare an asphericity change with theoretical predictions. The several correction methods, like custom generated aspherical profiles, may be utilized for mitigation of unwanted effects of asphericity change. Here we also present preliminary results of such correction for one of the excimer lasers.

We have developed and researched a new method of modal phase tomography for diagnostics of human eye aberrations. The technique of restoration permits separating the contributions of different elements of the eye to the total aberrations. The results of numerical researches and modeling experiment are presented.

We consider a mathematical model of an optical system where the phase modulation introduced by a nonlinear optical element involves a phase shift in an additional time-integral feedback loop, which is determined by the gain coefficient . In the case of , this model describes the dynamics of a nonlinear interferometer, where the effect of phase distortion compensation has been numerically and experimentally investigated before. In this paper, the results of 2D numerical simulations are presented for the case . It is shown that different types of small-amplitude stationary phase inhomogeneities can be fully compensated with time. The effect of the partial suppression of dynamic distortions given in the form of traveling waves and Kolmogorov-spectrum atmospheric irregularities is demonstrated; the quality of the distortion compensation is analyzed, depending on the spatiotemporal characteristics of the waves. The obtained results confirm the potential of using the optical feedback configuration with an additional integral circuit for suppression of both static and dynamic phase distortions.

We present the experimental implementation of ophthalmic diagnostic systems with adaptive optics compensation of human eye aberration. The systems feature high speed operation and utilize deformable bimorph mirrors for wavefront correction. The results of aberration measurements and correction are discussed.

A new significantly redesigned version of clinically applicable adaptive optics multispectral fundus imager is presented. Along with greatly improve adaptive system loop rate, the device performs reliably and is convenient for use in clinical practice. This new imager has allowed us to use new approaches for retina image analysis and obtain original results on the distribution of aberrations in the human eye.

The aim of this work was to determine the relative performance of a Shack-Hartmann (SH) wavefront sensor and a distorted grating wavefront curvature sensor (DGWFS) when used to measure the aberrations in the human eye. Previous work carried out by Kestrel and others suggests that the DGWFS is able to successfully reconstruct wavefronts in severely scintillated conditions in which SH sensors typically fail to give a good reconstruction. The poor performance of conventional SH sensors in scintillated conditions prevents their use in ophthalmic aberrometers with human subjects who have medical conditions such as cataracts. This limitation substantially restricts the percentage of the population that can take advantage of emerging technology enabled by having accurate aberration data for the anterior segment. The SH sensor utilized has a novel dithered reference source which mitigates scintillation problems. However, the DGWFS potentially offers a simpler, lower cost and more robust solution.

The spatial resolution of retinal images is limited by the presence of time-varying aberrations present within the eye. A low-order adaptive optics system using a Shack-Hartmann wavefront sensor and a bimorph deformable mirror is described. Using a standard Hitachi video camera and a PC running Windows 2000, control of low-order aberrations within the eye can be achieved, enabling high resolution images to be obtained. The wavefront sensor employs a novel dithered reference technique to reduce speckle recorded in the hartmannogram. As a measurement of the residual errors within the image is known, it is possible to use post-processing techniques to further improve the resolution of the image. Example images of retinas taken with the system and then post-processed are shown, demonstrating the ability to calculate near diffraction-limited images. Data on the performance of the system are presented.

We present the novel instrument combining the Shack-Hartman wavefront sensor, compensator of low order aberrations and bimorph adaptive mirror for high order aberration correction. We have tested the developed instrument in the clinical environment on human subjects. The measurement rate was 77 frames per second. The typical residual error of correction was between 0.1-0.15 microns was achieved. The wavefront can be reconstructed in form of up to 36 Zernike polynomials. Automatic low order aberration compensator allows introduction of spherical and astigmatic terms with amplitudes up to ±15D and ±6D correspondingly and accuracy of 0.05 D. An 18 electrode bimorph deformable mirror corrector makes it possible to model Zernike aberrations up to the 3-th order + Spherical aberration with RMS amplitudes up to 1 micron. The results of measurements were compared with clinical refraction and good correspondence was found.