The Mechanism Of Amplified Optical Signal During Femtosecond Laser Filamentation

Since the first ruby laser was invented by Maiman, researchers have been working on shortening the duration and increasing the peak power of laser pulse. Laser develop from continuous laser to nanosecond pulse, picosecond pulse and femtosecond pulse which has been very mature now, and even the duration of laser pulse will be shortened to attosecond in the future; the peak power of laser pulse is also increased from 109 W to 1012 W,1 51 0 W. With the advent of ultrashort and ultra-intense laser pulse, it was found that when laser pulse with sufficient power propagates in transparent medium, filaments are produced. With the increase of laser energy, the propagation distance of filamentation is far more than the Rayleigh length in theory, this phenomenon attracted numerous attention.Studies found that when ultrashort laser pulse interact with atom and molecule of the medium, it will be ionized and produce filamentation. Filamentation can spread very far without diffraction. Many non-linear effects are formed during filamentation, so the filament has been widely used in many fields. The fluorescence during filamentation is collected to determine material composition, but its intensity is very weak, so it is difficult for remote detection. In this paper, we will use a pump-probe system to study the amplified optical signal during filamentation whose intensity is several orders higher than the fluorescence. So the amplified optical signal can be better applied to remote sensing control or remote atmosphere detection field in the future.The main research contents of this paper are as follows:(1) We analyze the physical mechanism of filament theoretically. Then we introduce three physical models of filament: self-guiding model, moving focus model, dynamic spatial replenishment model, and describe the nonlinear effects during the filamentation.Filament lay a foundation of amplified optical signal due to theose effects.(2) We analyze the amplified optical signal at 391.4nm during filament theoretically,and design a series of experiments to verify the physical mechanism behind it.Experimental results show that: only when the spectrum of the probe pulse covers the391.4nm, the amplified optical signal can be observed; Regardless of the pump pulse and the probe pulse in the same or opposite direction, the amplified optical signal at 391.4nm can be both observed. when the pump and the probe pulse are completely separated, the amplified optical signal still exists; the amplified optical signal is linearly proportional to the power of probe pulse. All of the results indicate that the amplified optical signal at391.4nm is produced by the stimulated radiation mechanism. We can also observe that amplified optical signal gradually increased with the increase of filament length, this feature promotes the development of remote sensing technology and remote atmosphere detection technology.(3) We analyze the amplified optical signal at 394.3nm during filament theoretically,and verify the physical mechanism behind it through a series of experiments. The experimental results show that: the amplified optical signal at 394.3nm is caused by four-wave mixing mechanism. Firstly, when the probe pulse spectral is changed, the wavelength of the amplified optical signal will change, and this feature lays the theoretical foundation for the generation of tunable wavelength laser; Secondly, because of the phase mismatching, once the pump pulse and the probe pulse are separated in time domain, the amplified optical signal will disappear immediately; Finally, the amplified optical signal is linearly proportional to the square of the intensity of pump pulse. Meanwhile, it satisfies the relationship between the intensity of the four wave mixing process.