A laser of sufficient intensity traveling through air will-by itself-engineer a narrow channel over which light will propagate for tens or even hundreds of meters. Such filaments of laser light were first created at the end of the twentieth century, and investigators are now beginning to explore new applications for them. Light Filaments: Structures, challenges and applications brings together exciting results from this area of research. With the development of high repetition rate sources, new aspects of waveguiding are emerging based on the hydrodynamic perturbation created by each filamenting pulse. This book is organized from general overviews to more specialized topics and is aimed at those involved in lasers and other optical wave propagation systems. The goal of this book is to cover the multiple aspects of light filamentation from strong field ionization and molecular properties to laser development and beam shaping, and the wide range of radiation associated with it from THz, lasing in air to supercontinuum generation. The book starts with tutorial chapters about the science of filamentation, followed by in depth chapters on the latest research, technologies and applications in atmospheric studies, guiding waves, laser induced discharge and lightning. This book is the result of a Multidisciplinary University Research Initiatives (MURI) Program on light filaments from the USA Army Research Office.
Inspec keywords: fibre lasers; laser beams; photoionisation; high-speed optical techniques; light propagation
Other keywords: astrophysical fluid dynamics; Schrodinger equation; laser beams; light propagation; fibre lasers; plasma light propagation; light filaments; plasma density; high-speed optical techniques; astrophysical plasma; photoionisation
Subjects: Laser beam modulation, pulsing and switching; mode locking and tuning; Ultrafast optical techniques; General electrical engineering topics; Textbooks; Design of specific laser systems; Laser beam modulation, pulsing and switching; mode locking and tuning; Laser beam characteristics and interactions; Conference proceedings
This book is organized from general overviews to more specialized topics. The first few chapters provide a comparison of filamentation in different wavelength ranges. The bulk of the research over the last decades has concentrated on the Ti:sapphire laser systems. Chapter 1, by Aure ́lien Houard and Andre ́ Mysyrowicz of the Laboratoire d'Optique Applique ́e (LOA), presents a broad picture of the state of the art of filamentation at 800 nm, with applications to laser-induced discharges, improving the speed of trains, supersonic drag reduction, plasma antenna, coherent THz emission, and lasing in air.
Femtosecond filamentation is a spectacular phenomenon whereby a short laser pulse propagates nonlinearly through a transparent medium allowing high peak light intensities to be transferred over distances far exceeding the beam Rayleigh length, as if diffraction were suppressed. In this chapter, after a brief description of some key features of filamentation, we put the accent upon some potential applications.
This chapter starts with a discussion of the main qualitative differences between UV and mid-IR filaments: from multiphoton ionization in the UV to tunnel ionization in the near- to mid-IR. A general qualitative analysis of the properties of single filaments versus wavelength follows. Because of their long pulse duration, a quasi-steady-state theory of their propagation is possible. An eigenvalue approach leads to a steady-state field envelope that is compared to the Townes soliton. However, that solution is close enough to a Gaussian shape to justify a parametric evolution approach. After this theoretical introduction, an experimental verification at 266 nm follows. Femtosecond UV filaments were generated with frequency-tripled Ti:sapphire sources and KrF amplifiers. The source for long-pulse filaments is an oscillator-amplifier Nd-YAG Q-switched system, frequency doubled, compressed, and frequency doubled again to reach 170 ps pulses of 300 mJ energy. The sub-nanosecond duration of the UV pulse may revive the debate as whether the filament is a moving focus or self-induced waveguide. Two applications of UV filaments are presented in the last two sections. It is shown that isotopically selective laser-induced breakdown spectroscopy (LIBS) is possible by exploiting the narrow dips observed in the emission spectrum. These dips are due to reabsorption by the material in the plume created by the impact of the filament on a solid. These absorption lines are only a few-pm wide and are exactly centered at the wavelength of a transition from ground state of the material, without any Stark shift or broadening. A final application is laser-induced discharge, which is a guided discharge that follows exactly the path of the inducing UV filament. Laser-induced discharge may lead to the control of lightning, which is a topic of intense research in Europe.
In this chapter, we described the remarkable progress in development of terawatt-power CO2 lasers predominately in the USA.
We have shown that efficient and clean compression of mid-infrared pulses to the single optical cycle is straight forwardly achieved with soliton self-compression in gas-filled ARR-PCF. This is the most efficient post-compression technique suitable for high repetition rate and high average power mid-infrared pulses demonstrated today. The achieved combination of high peak and average power is ideal for nonlinear interactions such as HHG of coherent X-rays or multimodal spectroscopies. The excellent mode from the ARR-PCF and the high efficiency is also ideal for low pulse energies but high repetition rates, or applications in which near diffraction limited modes are desired. The sub-cycle soliton transient at the fibre exit can be output directly to vacuum to drive strong field processes. The UV spectrum from the DW is suitable for inducing structural changes and investigating dynamics in complex molecules.
This chapter presented a significant increase in the plasma survival time, from 0.98 ns for a single-beam filament to 1.565 ns for two filaments positioned 180mm from each other.
The study of light filamentation and the processes associated with it is currently a very active area of research. The formation of a light filament allows self-guided propagation of high-intensity laser light over long distances. Within a filament, several nonlinear processes can occur, including stimulated Raman scattering (SRS), nonlinear wave mixing and the generation of plasma. To help understand this phenomenon, careful numerical studies capable of dealing with numerous processes at different spatial, temporal and frequency scales are necessary. In this chapter, we will highlight theoretical and computational approaches to the study of coupled light filaments. The chapter primarily considers two-colour filament interactions, and we also touch on the numerical implementation of backscattering of light by stimulated Raman processes. The choices of models and methods aim at closely reflecting recent and proposed experiments.
This chapter considers the propagation of femtosecond laser pulses under tight focusing, the formation of plasma channels under these conditions, the features of the energy reservoir behaviour, post-filamentation mode, and other processes occurring under conditions of such propagation.
In this chapter, the introduction of engineered wave packets with emphasis on accelerating beams and light bullets has opened numerous opportunities in linear and nonlinear optical sciences were discussed.
In this chapter, we proposed an easy exploration of the parameter space of one of the most widespread propagation and laser-matter interaction model in the field of ultrashort laser pulse filamentation.
In this chapter, the theory of an amplification scheme based on Kerr nonlinearity is developed. It is well known that Kerr nonlinearity exhibits both temporal (modulation) and spatial (filamentation) instability. Typically, a single, intense beam propagating in a Kerr nonlinear material has small perturbations in the form of noise that become exponentially amplified. In turn, the noise drastically modifies the beam given a long enough propagation distance, and the beam can form filaments. Subsequently, conical emission ensues-the emission of broadband radiation at a frequency-dependent angle to the filament.
When the intense femtosecond laser pulse propagates in air, the laser pulse self-focuses owing to the Kerr effect, and the laser intensity increases rapidly. Then, the air is ionized, and electrons are generated. The dynamic competition of self-focusing, electron defocusing, and diffraction supports the long-distance propagation of the femtosecond laser pulse. This process is called filamentation. During filamentation, the ionization plays an important role in the formation of a plasma channel and the generation of supercontinuum. This chapter starts with the illustration of the optical field ionization models that are commonly used for the simulation of filamentation in different field conditions. Based on the ionization coupled with the nonlinear Schrodinger equation, theoretical studies on plasma filaments under different conditions are illustrated. Then several measurement methods for electron density inside filament are introduced. Based on these, characteristics of electron density under different conditions are studied.
Laser filamentation is especially appealing for atmospheric applications, due to its unique ability to propagate over long distances in the atmosphere. The high intensity conveyed remotely by filaments is particularly useful to induce non-linear laser-matter interaction for spectroscopy, like light detection and ranging (Lidar)remote sensing of traces in the air. Furthermore, as filaments can remotely trigger multiphoton photochemistry and ionization, they allow one to initiate various physical and chemical processes, including the manipulation of the transmission through fog and clouds, or the control of high-voltage discharges. In this chapter, we review such applications successively.
In this chapter, we briefly explained of some of the potential applications of filamented laser beams in no way summarizes the large body of work performed by many authors.