Remote control of high-intensity laser beams in the atmosphere is an important problem of atmospheric optics. It is of special interest for atmospheric sounding, where turbulence can affect beam propagation. We experimentally study the effect of a turbulent layer produced at the beginning of a laser radiation propagation path on the characteristics of the filamentation domain and generation of high-intensity plasma-free channels for laser beams 2.5 and 5 cm diameter, including under the phase control of the transverse beam structure with a deformable mirror. In the presence of turbulence, the beginning of multiple filamentation domain approaches, however, insignificantly (<10

Laser-induced fluorescence spectroscopy (LIFS) is actively used for remote sensing of atmospheric aerosols and is currently one of the most sensitive and selective techniques for determining small concentrations of substances inside particles. The use of high-power femtosecond laser sources for LIFS-based remote sensing of aerosols contributes to the development of new-generation fluorescence atmospheric lidars since it makes it possible to overcome the energy threshold for the nonlinear-optical effects of multiphoton absorption in particles and receive the emission signal at long distances in the atmosphere. Our study is aimed at the development and experimental demonstration of the technique of nonlinear laser-induced fluorescence spectroscopy (NLIFS) based on the remote excitation of aerosol fluorescent emission stimulated by a spatially structured high-power femtosecond laser pulse. Importantly, for the first time to our knowledge, we demonstrate the advances in using stochastically structured plasma-free intense light channels (postfilaments) specially formed by the propagation of femtosecond laser radiation through a turbulent air layer to improve NLIFS efficiency. A multiple increase in the received signal of two-photon-excited fluorescence of polydisperse-dyed aqueous aerosols by the structured postfilaments is reported.

Для дистанционного зондирования атмосферы уже долгое время активно применяется метод флуоресцентной спектроскопии (LIFS), который является одним из наиболее чувствительных и селективных методов определения малых концентраций веществ. Применение мощных фемтосекундных лазерных источников для дистанционного зондирования атмосферы методом флуоресцентной спектроскопии способствует развитию лидаров нового поколения. Целью исследований было изучение возможности применения интенсивных световых каналов, сформированных турбулентным экраном для реализации метода флуоресцентной спектроскопии жидкофазного аэрозоля. Показана возможность удаленного детектирования спектров флуоресценции и увеличения регистрируемого свечение при сегментировании лазерного пучка. For atmospheric remote sensing, the method of fluorescence spectroscopy (LIFS) has been actively used for a long time, which is one of the most sensitive and selective methods for determining trace concentrations of substances. The use of powerful femtosecond laser sources for remote atmospheric sensing by fluorescence spectroscopy contributes to the development of next-generation lidars. The purpose of the research was to investigate the feasibility of using intense light channels formed by a turbulent screen for implementing the fluorescence spectroscopy method for liquid-phase aerosol. The possibility of remote detection of fluorescence spectra and increased registered luminescence due to segmentation of the laser beam was demonstrated.

The work presents the results on the influence of the temperature of a turbulent layer on the transverse energy structure of femtosecond (multi-ring and Gaussian) laser beams. Gaussian and multi-ring beam profiles exhibit a monotonous growth in the number of intense light maxima with increasing the strength turbulence of air jet and increasing laser pulse energy. The formed intense light channels (~ 1011 W/cm2) make it possible to excite two-photon fluorescence of dyes with a signal level that makes it possible to take at a distance of 100 m.

We present the fluorescence spectra of single millimeter water droplets and micron-sized dyed water aerosol (rhodamine 6G) stimulated by a high-intensity femtosecond Ti:sapphire-laser pulse (carrier wavelength 792 nm) upon its nonlinear propagation in air. The distinctive feature of our experimental measurements is that the droplet fluorescence is obtained in the area of plasma-free pulse propagation after the pulse filamentation has already been terminated (postfilamentation region). Our results significantly expand the working area of femtosecond laser-induced fluorescence spectroscopy for remote diagnostics of atmospheric aerosols.

Представляются экспериментальные результаты исследования особенностей формирования и распространения интенсивных световых каналов в условиях структурирования начального профиля пучка мощного фемтосекундного лазерного излучения в виде отдельных субапертур. Такое особым образом структурированное излучение получено путем амплитудной модуляции лазерного пучка с использованием масок. Применение амплитудных масок позволяет получить заданное число высокоинтенсивных световых каналов, и осуществить их управляемое распространение в конкретном нелинейном режиме, преимущественно на небольших (десятки метров) расстояниях в атмосфере.

В работе представлены результаты по влиянию турбулентного слоя вначале трассы на характеристики области филаментации для пучков диаметром 2,5 см и 5 см. Внесение турбулентности приводит к приближению начала области множественной филаментации (ОМФ) к источнику. Сокращение дистанции до начала и конца ОМФ, при наличии турбулентности, является несущественным (< 10%). Сформированный в начале трассы турбулентный слой приводит к кратному увеличению количества интенсивных каналов (~ 1011-1012 Вт/см2), протяженностью > 100 м.

We present the results of our experimental study of the propagation dynamics of high-power femtosecond laser radiation in air with initially imposed amplitude and/or phase modulations. Depending on the modulation type and magnitude, the laser pulse upon nonlinear propagation breaks up into several high-intensity spatially localized light channels, which may or may not contain air plasma and thus are referred to as laser filaments, post-filaments, or plasmaless channels. The pulse modulations are implemented by means of control of the phase or amplitude front using a bimorph deformable mirror or amplitude masks, respectively. We show that the distance of formation and spatial length of high-intensity light channels along a propagation path strongly depend on the shapes and spatial positions of the inhomogeneities created in the transverse phase/amplitude pulse profile, but weakly depend on their sizes.

The results of experimental studies of the features of formation and path propagation of high-intensity light channels under conditions of distortions of the initial profile of a high-power femtosecond laser radiation beam are presented. The distortions are introduced by amplitude modulation of the laser beam using masks. The use of amplitude masks allows generation of a preset number of high-intensity light channels and their controllable filamentation mainly at short (tens of meters) distances of radiation propagation in air.

В работе представлены результаты лабораторных экспериментов по управлению областью филаментации сфокусированного фемтосекундного лазерного пучка с пространственно структурированным (с помощью деформируемого зеркала) волновым фронтом. Исследовались аберрационные пучки, состоящие из когерентных вложенных друг в друга кольцевых субапертур, получаемых при специфической, определенной из теоретических оценок форме деформируемого зеркала. Это позволяет создавать в процессе распространения лазерного пучка неоднородности распределения амплитуды оптического поля, которые впоследствии могут стать затравками для формирования световых филаментов. Такой режим управления филаментацией мощного излучения не требует компенсации искажений начального профиля пучка, а, наоборот, основан на контролируемом внесении предварительно рассчитанных аберраций волнового фронта.

We present the results of the turbulent layer influence on the characteristics of the filamentation domain at the path beginning for the beams with a diameter of 2.5 and 5 cm. Addition of turbulence results in the fact that the beginning of the multiple filamentation domain (MFD) to the source coming closer. Decreasing of the distance to the beginning and end of the MFR in the presence of turbulence is inappreciable (<10%). The turbulent layer formed at the beginning of the path leads to a multiple increase in the number of the intense channels (~ 1011-1012 W/cm2) with a length >100 m.

Results of laboratory experiments on control of the filamentation domain of a focused femtosecond laser beam with a spatially structured (with a deformable mirror) wavefront are described. Aberration beams consisting of coherent interleaved ring subapertures obtained with a deformable mirror of specific form are studied. This allows producing inhomogeneities in the distribution of the optical field amplitude during laser beam propagation; these inhomogeneities can subsequently become seeds for light filaments. This approach to control high-power radiation filamentation does not require compensation for distortions of the initial beam profile; on the contrary, it is based on a controllable introduction of pre-calculated wavefront aberrations.

Представлены результаты экспериментов по возбуждению и регистрации двухфотонной флуоресценции красителей интенсивными световыми каналами, сформированными при управлении волновым фронтом фемтосекундных лазерных импульсов. Свечение красителей и прием обратного сигнала осуществлялся по лидарной схеме на удалении 100 м от источника.

The results of model experiments on recording the spectral characteristics of the emission glow of residual amounts of a substance on a blade of an aircraft turbojet engine after the mechanical impact of a foreign object are presented. The spectra were measured by laser-induced breakdown spectroscopy, which made it possible to distinguish the emission lines of the substance against the spectrum of the blade material. This nondestructive testing allows quick diagnosis of the blades at the site of the aircraft inspection before a flight and draw conclusions about possible destruction of the fuselage and the criticality of the damage.

The results of the experiments on the excitation and detection of two-photon fluorescence of the dyes by the intense light channels generated by controlling the wavefront of the femtosecond laser pulses are presented. Emission of the dyes and the detection of the backward signal were carried out according to the lidar scheme at a distance of 100 m from the source.

In laboratory conditions, the fluorescence spectra of rhodamine 6G were obtained in the domain of post-filamentation channels upon interaction with femtosecond laser radiation. A fluorescence signal was received from a distance of 7.5 m from the aerosol and the droplet according to the lidar sensing scheme.

Results of the complex experimental study of the spectral and temporal characteristics of the emission glow of control samples of several solid-state substances (imitation of topographic targets in the atmosphere) and of solid aerosols (imitation of air pollutants) under the action of femtosecond pulses of a Ti:Sa laser (at a carrier wavelength of 800 nm) and nonlinear optical effects are presented.

The evolution of the small-scale transverse structure of high-power femtosecond laser radiation propagating in air in the filamentation mode has been experimentally and theoretically studied. Experimental results were obtained using wideaperture (centimeter) collimated beams of femtosecond pulses of a titanium-sapphire laser. As a result, the features of propagation of spatially isolated high-intensity light channels, the diameter of which is several millimeters, were determined. They are formed because of Kerr self-focusing of intensity inhomogeneities in the initial transverse profile of the laser beam. It is shown that the formation of a filament (a localized light structure, the existence of which is associated with the plasma formation and the generation of conical emission) does not occur in each of these channels. The theoretical evaluation of this light channels characteristics was carried out. It is based on the diffraction-ray model of single filamentation of femtosecond laser pulses. Studying of the evolution transverse profile of a laser beam with a centimeter radius and subterawatt power along air path showed that the initial radius of intensity inhomogeneities, for which laser filamentation occur, is equal several (2.5-3.5) millimeters. The power in these inhomogeneities varies from 19 to 26 GW. Differences in the values of the radius and power of these inhomogeneities are the cause of different distances from the laser pulse source at which their self-focusing occurs.

The results are presented of experiments on controlling the characteristics of the multiple filamentation domain of femtosecond laser pulses along atmospheric paths via variations in the initial spatial focusing, beam radius, and pulse energy, as well as the optical field structure at the initial beam aperture.

The results of experimental and theoretical studies of the filamentation of femtosecond laser pulses using a bimorph deformable mirror, which allows controlling the position of the filamentation domain throughout a model path due to phase distortions of different parts of a laser beam, determining localization of filaments and high-intensity channels in the beam cross section.