A. V. Larichev

High-quality correction of wavefront distortions using low-power phase conjugation in azo dye containing LC polymer

Abstract 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 ∼10 2 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.

Polygon pattern formation in a nonlinear optical system with 2D feedback

Abstract 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.

Filamentation of femtosecond laser pulses governed by variable wavefront distortions via a deformable mirror

Filamentation of focused UV and IR femtosecond laser pulses and plasma channel formation governed by variable wavefront distortions was experimentally and numerically studied. A deformable mirror was used to control the plasma channel length by introducing a spherical aberration into the initial transverse spatial distribution of a femtosecond laser pulse. An at least double increase of the plasma channel length was observed with increasing deformation of the mirror. Numerical calculations show that the hat-like phase shape of the aberration ensures that the energy of the initial laser pulse remains confined for a longer distance within the limited transverse size of the filament.

Experimental observation of space-time chaos in a nonlinear optical system with 2D feedback

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.

Deconvolution of color retinal images with wavefront sensing

The spatial resolution of the retinal images cannot approach a diffraction limit due to the high-order aberrations of the human eye. We present a technique, which allows restoring fine details on the retinal images using information about OTF (optical transfer function) of the eye obtained by the Shack-Hartman wavefront sensor. The precision of wavefront measurements greatly enhanced by reference source scanning on the retina. A closed loop adaptive system based on the bimorph mirror suppresses low-order aberrations. The residual errors are removed by the image deconvolution. The finite depth of retina layers of the human eye significantly reduce resolution of color retinal images as far as it introduces additional defocusing depending on the wave- length of the reflecting light. We present a novel technique of color retinal image deconvolution. The key feature of the algorithm is in use of information on retina structure. This permits calculating of optical transfer functions for each of the retina layers. Significant improvement of image quality was obtained. The processing time was about a few dozens of seconds for contemporary PC computers and image size 2000*2000 pixels.

LCLV-based system for high resolution wavefront correction: phase knife as a feedback intensity producer

Abstract 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.

Usage of wavefront sensor for estimation of atmospheric turbulence parameters

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.

Advanced phase knife technique

Abstract 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.

Test of the stochastic parallel gradient descent algorithm in laboratory experiments

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.

Low-cost, High Resolution Scanning Laser Ophthalmoscope for the Clinical Environment

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.