Effect of Thermal Defocusing on Backward Stimulated Raman Scattering in CH4

The conversion efficiency of stimulated Raman scattering （SRS） in CH4 is studied by using a single longitudinal mode second-harmonic Nd：YAG laser （532 nm, linewidth 0.003 cm^-1, pulse-width （FWHM） 6.5 ns）. Due to the heat release from vibrationally excited particles, SRS processes often suffer from the thermal defocusing effect （TDE）. In view of 6.5 ns laser pulse width is much shorter than the vibrational relaxation time of CH4 molecules, TDE can only affect the SRS processes afterwards. In the cases of low laser repetition, TDE will be not serious, because it will be removed by the thermal diffusion in Raman medium before the next pulse arrives. At the laser repetition rate 2 Hz, CH4 pressure 1.1 MPa and pump laser energy 95 mJ, the quantum conversion efficiency of backward first-Stokes （BS1） has attained 73%. This represents the highest first-stokes conversion efficiency in CH4. Furthermore, due to the relaxation oscillation, the BS1 pulses are narrowed to about 1.2 ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Its beam quality is also much better and is only slightly affected by TDE. This reason is that BS1 represents a wave-front-reversed replica of the pump beam, which can compensate the thermal distortions in Raman amplify process. Under the same conditions, but pump laser repetition rate as 10 Hz, the conversion efficiency of BS1 goes down to 36% due to TDE. From this study, we expect that a well-behaved 630 nm Raman laser may be designed by using a closed CH4/He circulating-cooling system, which may have some important applications.

Spectral attenuation of a 400-nm laser pulse propagating through a plasma filament induced by an intense femtosecond laser pulse

The spectral attenuation of a 400-nm probe laser propagating through a femtosecond plasma in air is studied.Defocusing effect of the low-density plasma is an obvious effect by examining the far-field patterns of the 400-nm pulse.Besides,the energy of 400-nm pulse drops after interaction with the plasma,which is found to be another effect leading to the attenuation.To reveal the physical origin behind the energy loss,we measure fluorescence emissions of the interaction area.The fluorescence is hardly detected with the weak 400-nm laser pulse,and the line spectra from the plasma filament induced by the 800-nm pump pulse are clearly shown.However,when the 400-nm pulse propagates through the plasma filament,the fluorescence at 391 nm from the first negative band system of N2+is enhanced,while that from the second positive band of neutral N2 at 337 nm remains constant.Efficient near-resonant absorption of the 400-nm pulse by the first negative band system occurs inside the plasma,which results in the enhanced fluorescence.Furthermore,the spectral attenuation of the 400-nm probe laser is measured as a function of the pump–probe time delay as well as the pump-pulse energy.