Nonlinear Propagation of Laser Radiation in the Atmosphere
In the historical aspect, the main contribution to the foundation of nonlinear optics was done by V. E. Zuev from former USSR with his colleagues and followers (see Reference 1 and the bibliography therein). By now, a major cycle of basic investigations in the optics of high-power laser radiation in the atmosphere has been accomplished [2—4]. In these works, mechanisms of optical nonlinearity of liquid-droplet media have been studied, and efficient calculation methods and semiempirical models have been developed for the prediction of losses in energy and direction of high-power laser beams at long atmospheric paths. Physical principles for new methods of diagnosis of dispersive media have been formulated.
This research field remained actual till nowadays, growing at a new stage due to advent of high-power femtosecond lasers systems. The application of these systems in the atmosphere opens new ways of the usage of optical technologies for studying natural phenomena occurring in the inhomogeneous and irregular atmosphere.
As a rule, nonlinear optical effects deteriorate the conditions of atmospheric propagation of laser beams, and, therefore, they should be taken into account when estimating the efficiency of laser energy transfer to long distances. Thus, in the case of continuous-wave laser radiation, thermal aberrations of laser beams in air have been observed . In fogs, however, the transparency of medium increases for infrared (IR) radiation and long pulses attenuate due to evaporation of droplets . As high-power picosecond and femtosecond laser pulses propagate in air, the phenomenon of filamentation takes place, which is connected with self-focusing of laser beam into filaments, formation of plasma, and generation of broadband radiation or the so-called supercontinuum . The longer laser pulses propagate in aerosol medium, the stronger optical breakdown of air occurs near aerosol particles (see also Chapter 1 and bibliography in Reference 4). This is accompanied by additional attenuation of the radiation.
Many nonlinear optical effects have served as a basis for new methods of diagnostics of the atmosphere and water medium. The optical breakdown of aerosol media has attracted the attention as a source of emission spectrum of the component material of suspended particles [6—9]. The two-photon absorption in aerosols containing organic molecules causes their fluorescence, which serves an indicator of these molecules [10,11]. With the use of broadband supercontinuum radiation, the diagnostics of gas and aerosol media by the method of multiwave sensing has become possible .
In this chapter, we present the high-priority theoretical results obtained from the study of the interaction of laser radiation of various spectral composition and pulses of different durations with atmospheric constituents based on the framework of nonlinear atmospheric optics.