Investigation On The Properties Of Type-â…  Frequency Doubling Of Ultra-short Pulse

Frequency doubling of ultra-short pulses is an effective method to obtain short-wavelength ultra-short pulses. Conversion efficiency and pulse wave-forms will be affected by many factors due to the wide spectrum range and high intensity density of ultra-short pulses. To increase conversion efficiency and improve beam quality, it is necessary to study various physical factors involved.Properties of frequency doubling of femtosecond pulses in type-I phase-matched BBO crystal are investigated in this thesis. First, a physical model on the frequency doubling of ultra-short pulses in type-I phase-matching case has been built up on the basis of Maxwell's equations, where various physical factors such as spatial walk-off, diffraction, group velocity mismatch(GVM), dispersion (GVD) and third order nonlinear effect are considered. Physical and optical properties of BBO and general numerical solving methods are introduced, these coupling equations are normalized and a final numerical simulation program is programmed. Second, the frequency doubling process of femtosecond pulses in type-I phase-matching case is simulated, where the influence on conversion efficiency, pulse waveforms and spatial near-field distribution (in the exit plane of crystal) of harmonic pulses with different physic factors by pulse width, energy and beam radius is studied in detail in high conversion efficiency case. At last, two methods of initial angular mismatching and pulse chirping are used to compensate phase matches in frequency doubling processes, which really increases conversion efficiency. Based on phase mismatch compensation, a scheme of group velocity compensation in frequency tripling is designed by compensation principle of birefringence and polarization rotation. It is found that different factors have different decent influences on frequency doubling processes; where the pulse spatial distributions are quite similar to temporal waveforms. The chirping method is better in compensation for phase mismatch comparatively, thus can provide a practical reference in application.