Investigation On Spectra Of Nanosecond Laser Broadening In Liquid

Fresnel diffraction is able to be eliminated by using broadband laser in high power laser system, so the small-scale self-focusing is restrained. The transverse SBS is restrained and improve the near field uniform of laser beam at the same time. These are propitious to picking up energy from amplifier and enhancing laser beam quality.In this thesis, we study spectral broadening by the interaction between nanosecond high power laser pulses with liquid medium. It is studied that the parameters which contain the cell length, the peak power of input pulse, the pulse width and beam diameter have effect on the spectral broadening.First, we educe the nonlinear Schr?dinger equation from Maxwell's equations considering the nonlinear polarization density. The nonlinear adaxial wave equation has been derived using slowly varying envelope approximation and some else approximation. Then self-focusing has been discussed. We obtain the formulas of self-focusing distance, critical power for self-focusing and wavelength broadening. And we discover self-phase modulation is the main mechanism for spectral broadening. And we simply discuss the polarization of the output laser.In the experiment, we study the effect of the parameters on the spectral broadening by changing them, and compare the experimental results with the theoretical calculation. The max spectral broadening is 1.26nm in our experiment. The spectral broadening increases with the cell length increasing, the power of pulses increasing, the pulse width decreasing and the diameter decreasing. There is a critical power. When the pulse power is above it, there is spectral broadening. When the pulse power is below it, there is no spectral broadening. And the critical power, which is mainly determined by the medium, is inverse proportion to n₂ of the medium. Whenever there is spectral broadening of the output pulse, there will be an aberration relative to the input pulse.