Laser filaments generated and transmitted in highly turbulent air

Two statistical regimes in the transition to filamentation

We experimentally investigate fluctuations in the spectrum of ultrashort laser pulses propagating in air, close to the critical power for filamentation. Increasing the laser peak power broadens the spectrum while the beam approaches the filamentation regime. We identify two regimes for this transition: In the center of the spectrum, the output spectral intensity increases continuously. In contrast, on the edges of the spectrum the transition implies a bimodal probability distribution function for intermediate incident pulse energies, where a high-intensity mode appears and grows at the expense of the original low-intensity mode. We argue that this dual behavior prevents the definition of a univoquial threshold for filamentation, shedding a new light on the long-standing lack of explicit definition of the boundary of the filamentation regime.

Beam Wander Restrained by Nonlinearity of Femtosecond Laser Filament in Air

The filamentation process under atmospheric turbulence is critical to its remote-sensing application. The effects of turbulence intensity and location on the spatial distribution of femtosecond laser filaments in the air were studied. The experimental results show that the nonlinear effect of the filament can restrain the beam wander. When the turbulence intensity was 3.31×10−13 cm−2/3, the mean deviation of the wander of the filament center was only 27% of that of the linear transmitted beam. The change in turbulence location would lead to a change in the standard deviation of the beam centroid drift. Results also show that the filament length would be shortened, and that the filament would end up earlier in a turbulent environment. Since the filamentation-based LIDAR has been highly expected as an evolution multitrace pollutant remote-sensing technique, the study promotes our understanding of how turbulence influences filamentation and advances atmospheric remote sensing by applying a filament.

Nonlinear localization of high energy long wave laser pulses in fully correlated 3D turbulence

We study the interplay between three-dimensional (3D) fully correlated optical turbulence and nonlinearity in time and 3D space resolved long-wavelength infrared pulsed beam propagation. Here the average self-trapped beam waist exceeds the inner scale in contrast to near-infrared filaments, and we find that their nonlinear self-channeling remains robust even in the presence of strong turbulence. More surprisingly, our simulation results invite a conjecture that in regimes where diffraction and nonlinearity are roughly balanced, turbulence can result in a tighter localization of the nonlinear beam core.

Filament-induced breakdown spectroscopy with structured beams

Filament-induced ablation represents an attractive scheme for long-range material identification via optical spectroscopy. However, the delivery of laser energy to the target can be severely hindered by the stochastic nature of multiple-filamentation, ionization of ambient gas, and atmospheric turbulence. In order to mitigate some of these adverse effects, we examine the utility of beam shaping for femtosecond filament-induced breakdown spectroscopy with Gaussian and structured (Laguerre-Gaussian, Airy, and Bessel-Gaussian) beams in the nonlinear regime. Interaction of filaments with copper, zinc, and brass targets was studied by recording axially-resolved broadband emission from the filament-induced plasma. The laser-solid coupling efficacy was assessed by inferring thermodynamic parameters such as excitation temperature and electron density. While under our experimental conditions the ablation rate with Gaussian- and Laguerre-Gaussian beams is found to be similar, the Airy and Bessel-Gaussian beams offer the advantage of longitudinally extended working zones. These results provide insights into potential benefits of structuring ultrafast laser beams for standoff sensing applications.

Engineering equations for the filamentation collapse distance in lossy, turbulent, nonlinear media

The propagation of high peak-power laser beams in real atmospheres has been an active research area for a couple of decades. Atmospheric turbulence and loss will induce decreases in the filamentation self-focusing collapse distance as the refractive index structure parameter and volume extinction coefficient, respectively, increase. This paper provides a validated analytical method for predicting the filamentation onset distance in lossy, turbulent, nonlinear media. It is based on a modification of Petrishchev's and Marburger theories. It postulates that the ratio of the peak power to critical power at range in turbulence is modified by a multiplicative, rather than additive, gain factor. Specifically, this new approach modifies the Petrishchev's turbulence equation to create the required multiplicative factor. This is necessary to create the shortened filamentation onset distance that occurs when a laser beam propagates through the cited nonlinear medium. This equation then is used with the Marburger distance and the Karr et al loss equations to yield the filamentation onset distance estimate in lossy, turbulent, nonlinear environment. Theory validation is done against two independent sets of computer simulation results. One comes from the NRL's HELCAP software and the other from MZA's Wave Train modeling software package. This paper also shows that once the zero-turbulence onset distance is set based on link loss, the addition of turbulence creates essentially the same PDFs at similar median distances for each loss case. This result had not been previously reported. This is the first quantitative comparison between closed form equations and computer simulation results characterizing filament generation in a lossy, turbulent, nonlinear medium.

Engineering equation for filamentation self-focusing collapse distance in atmospheric turbulence

The propagation of high peak-power laser beams in real atmospheres will be affected by both linear and nonlinear effects contained therein. Atmospheric turbulence usually will induce decreases in the filamentation self-focusing collapse distance for refractive index structure parameter increases. This paper provided the first validated analytical equation for predicting the nonlinear self-focusing collapse distance based on a modification of Petrishchev's and Marburger's theories. It shows that the estimate of the peak power to critical power at range in turbulence is modified to be the product of the transmitted peak power to critical power ratio times a multiplicative factor derived from Petrishchev's turbulence equations. This estimate is used in the Marburger distance equation to yield a predicted self-focusing collapse distance. This approach was compared to previous NRL's HELCAP computer simulation results and showed good agreement. The HELCAP simulations capability has shown good agreement between its results and a previously published laboratory-scale experiment. The analytical approach in this paper may provide a guide for further numerical simulations, more formal theoretical developments and field experiments.

Short-pulse lasers for weather control

Filamentation of ultra-short TW-class lasers recently opened new perspectives in atmospheric research. Laser filaments are self-sustained light structures of 0.1-1 mm in diameter, spanning over hundreds of meters in length, and producing a low density plasma (10¹⁵-10¹⁷ cm^-3) along their path. They stem from the dynamic balance between Kerr self-focusing and defocusing by the self-generated plasma and/or non-linear polarization saturation. While non-linearly propagating in air, these filamentary structures produce a coherent supercontinuum (from 230 nm to 4 µm, for a 800 nm laser wavelength) by self-phase modulation (SPM), which can be used for remote 3D-monitoring of atmospheric components by Lidar (Light Detection and Ranging). However, due to their high intensity (10¹³-10¹⁴ W cm^-2), they also modify the chemical composition of the air via photo-ionization and photo-dissociation of the molecules and aerosols present in the laser path. These unique properties were recently exploited for investigating the capability of modulating some key atmospheric processes, like lightning from thunderclouds, water vapor condensation, fog formation and dissipation, and light scattering (albedo) from high altitude clouds for radiative forcing management. Here we review recent spectacular advances in this context, achieved both in the laboratory and in the field, reveal their underlying mechanisms, and discuss the applicability of using these new non-linear photonic catalysts for real scale weather control.

Fluorescence emission induced by the femtosecond filament transmitting through the butane/air flame

We measure the backward fluorescence spectra generated by the femtosecond filament transmitting through the butane/air flame, and study the fluorescence emission from combustion intermediates (CN, CH and C₂ radicals), air (mainly N₂ and N₂⁺). It is found that the fluorescence emission from combustion intermediates, N₂ and N₂⁺ shows difference when the femtosecond filament transmits through different parts of the butane/air flame, and we attempt to analyze it in this paper. This study demonstrates that the filament-induced fluorescence technique can be utilized to sense the combustion intermediates.

Impulsive rotational Raman scattering of N2 by a remote "air laser" in femtosecond laser filament

We report on experimental realization of impulsive rotational Raman scattering from neutral nitrogen molecules in a femtosecond laser filament using an intense self-induced white-light seeding "air laser" generated during the filamentation of an 800 nm Ti:sapphire laser in nitrogen gas. The impulsive rotational Raman fingerprint signals are observed with a maximum conversion efficiency of ∼0.8%. Our observation provides a promising way of remote identification and location of chemical species in the atmosphere by a rotational Raman scattering of molecules.

Sensing combustion intermediates by femtosecond filament excitation

Simultaneous monitoring of multiple combustion intermediates using femtosecond filament-induced nonlinear spectroscopy is demonstrated. Clean fluorescence emissions from free radicals CH, CN, NH, OH, and C(2), as well as atomic C and H, are observed when a femtosecond filament is formed in the laminar ethanol/air flame on an alcohol burner. The fluorescence signals of these species are found to vary as functions of the position of interaction of the filament with the flame along the vertical axis of the central combusting flow, opening up a possibility for remote combustion diagnostic in engines by the excitation of femtosecond laser filament.

Femtosecond laser filamentation for atmospheric sensing

Powerful femtosecond laser pulses propagating in transparent materials result in the formation of self-guided structures called filaments. Such filamentation in air can be controlled to occur at a distance as far as a few kilometers, making it ideally suited for remote sensing of pollutants in the atmosphere. On the one hand, the high intensity inside the filaments can induce the fragmentation of all matters in the path of filaments, resulting in the emission of characteristic fluorescence spectra (fingerprints) from the excited fragments, which can be used for the identification of various substances including chemical and biological species. On the other hand, along with the femtosecond laser filamentation, white-light supercontinuum emission in the infrared to UV range is generated, which can be used as an ideal light source for absorption Lidar. In this paper, we present an overview of recent progress concerning remote sensing of the atmosphere using femtosecond laser filamentation.

Effect of an energy reservoir on the atmospheric propagation of laser-plasma filaments

The ability to select and stabilize a single filament during propagation of an ultrashort, high-intensity laser pulse in air makes it possible to examine the longitudinal structure of the plasma channel left in its wake. We present the first detailed measurements and numerical 3-D simulations of the longitudinal plasma density variation in a laser-plasma filament after it passes through an iris that blocks the surrounding energy reservoir. Since no compensation is available from the surrounding background energy, filament propagation is terminated after a few centimeters. For this experiment, simulations indicate that filament propagation is terminated by plasma defocusing and ionization loss, which reduces the pulse power below the effective self-focusing power. With no blockage, a plasma filament length of over a few meters was observed.