Enhancement of the efficiency of generating radiation at shifted wavelengths by transforming a single-mode femtosecond laser beam into multimode one
D.V. Apeksimov, P.A. Babushkin, Yu.E. Geints, A.M. Kabanov, V.K. Oshlakov, E.E. Khoroshaeva
V.E. Zuev Institute of Atmospheric Optics of Siberian Branch of the Russian Academy of Science, Tomsk, Russia
Keywords: femtosecond laser pulse, turbulence, laser filamentation, plasma, aerosol, two-photon-excited fluorescence, angular distribution, THz radiation generation
Abstract
The results of complex studies of filamentation of high-power femtosecond laser pulses in experiments where a localized air layer with a randomly inhomogeneous refractive index (artificial turbulent layer) is created at the beginning of the propagation path are discussed. It is shown that the technique of forced chaotic modulation (power stochastization) of a radiation beam, which initiates inhomogeneities in the transverse energy structure of laser radiation, causes the splitting of an original beam into many high-intensity light subbeams (channels) due to strong optical nonlinearity of the air medium. These channels are characterized by high intensity and stability along significant distances. The segmentation of the radiation by a turbulent layer multiply increases the number of high-intensity optical channels, which arise during propagation of a 2.5-cm beam of a Ti:Sapphire laser with a wavelength of 800 nm and power of up to 150 GW in air. The characteristic intensity of these optical channels is high enough for two-photon absorption in a volume of colored (rhodamine or uranine with a concentration of 0.4 g/l) water aerosol microparticles, created at the end of a 100-m optical path, which, in turn, almost doubles the fluorescence signal from the particles recorded by the lidar scheme. In addition, it was determined that a localized turbulent layer created at the beginning of an optical path enables a multiple enhancement of the efficiency of low-frequency (THz) electromagnetic radiation generation from laser filamentation region.
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