N. G. Iroshnikov

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.

Nonlinear dynamics of neuromorphic optical system with spatio-temporal interactions

The simplest spatially distributed neuromorphic dynamic system with large-scale spatio- temporal interaction is investigated. The model of the system is nonlinear optical ring cavity with field rotation around optical axis and signal time-delay in the two-dimensional feedback loop. The characteristics of rotary modes of the system are determined by the parameters of nonlocal spatio-temporal interconnections. The competition of rotary modes takes place like in neural networks of WTA type.

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.

Express Analysis of Turbulence Parameters

A method utilizing a Hartmann sensor for obtaining estimates of the external scale L0 of turbulent fluctuations and the structural constant Cn2 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.

Modal tomography of aberrations of the human eye

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.

Adaptive optics in ophthalmology

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.

The impact of speckle on the measurement of eye aberrations

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.

Distortions of laser pulses in neodymium atomic vapor

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.

Estimation of atmospheric turbulence parameters with wave front sensor data

Estimates of atmospheric turbulence parameters can be calculated on the basis of data, obtained with wave front sensor. The method described is based on decomposition of phase fluctuations into Zernike series and analysis of statistics of this decomposition coefficients. Estimates of turbulence outer scale L0 and refractive index structure constant C2/n obtained in experiments with turbulence in water cell showed good agreement with previous results.

Influence of ocular aberrations on the diameter of retinal blood vessels

Lower-order and higher-order ocular aberrations influence not only visual perception but also appearance of retinal images. We suggest that the diameter of blood vessels may differ for various patterns of aberrations based on how the maps of point spread functions are orientated to these vessels. We tested our hypothesis by a star chart simulation method and by using simulating aberrations in a real image of a living retina. We observed that broadening of the blood vessels due to ocular aberrations depends not only on the relative orientation of the wavefront but also on their actual width. The change in the diameter ratio is clinically significant and is comparable to changes in the arteriovenous ratio due to various retinal and systemic pathologies. We hypothesise that an ophthalmologist when examining the fundus of an eye must pay attention to a patients aberrations.