Investigation On Frequency Doubling Of Angular Dispersive Ultrashort Pulses

Optical frequency conversion generates new frequency, which is one of the important means of modern laser technology, frequency doubling is a basic method to obtain short wavelength ultrashort pulses. The width of ultrashort pulses is very narrow and the frequency spectrum is wide, so phase matching and group velocity matching must be met at the same time, otherwise group velocity mismatch can broaden the pulse width and decrease the conversion efficiency of second harmonic. Grating is used to compensate group velocity mismatch which can improve the conversion efficiency.Firstly the influence of group velocity mismatch is introduced in this paper; on the basis of phase matching, angular dispersion technique is illustrated; the relation among angular dispersion, group velocity mismatch and spatial walk-off angle is discussed; in addition, the relation between angular dispersion and pulse front tilted is proved; how Gaussian beam is changed after passing the grating and the len is given; physical properties and optical properties of negative uniaxial BBO crystal are introduced. In order to clearly understand angular dispersion on the process of frequency doubling, collinear phase matching coupled wave equations are shown from Maxwell equations, a systematic analysis is given about the physical process for ultrashort pulses type I frequency doubling, the solutions of coupled wave equations are given by small signal approximation. When phase matching and group velocity matching are met, the effects of spatial walk-off, group velocity dispersion and diffraction on second harmonic are discussed in detail. Through numerical simulation, spatial walk-off, pulse width and intensity distribution of second harmonic are analyzed with different crystal lengths, waist radius, pulse widths. It is found that angular dispersion can compensate group velocity mismatch, crystal length is no longer limited by group velocity walk-off length as common frequency doubling, then diffraction and dispersion effects play key roles in pulse width broadening of second harmonic.