Numerical study of pulse width dependence of SHG in QPM waveguides

Abstract

Quasiphase matched (QPM) wavelength converters have attracted much attention due to their versatile applications, such as unltrashort-pulse generation at short wavelength. The behavior of pulse propagation in QPM waveguides has been an interesting research topic around the world. In this report, numerical study of second harmonic generation (SHG) in QPM waveguides is, for the first time, performed systematically for the fundamental pump pulses from nanosecond to femtosecond. In the proposed theoretical approach, all the dispersion effects are considered. Furthermore, our simulations take into account not only the SHG effect but also the sum frequency generation (SFG) effect on the nonlinear interaction when a pulsed pump light is used. Group-velocity mismatch (GVM) and phase-velocity mismatch are also considered in the study. Therefore, the proposed simulation model is suitable for analyzing the SHG of ultrashort-pulse since the spectral full width at half maximum (FWHM) increases with the decrease of pump pulse width. It is shown that conversion efficiencies are strongly dependent on the fundamental pulse width since the walk-off effect gradually dominates with the decrease of the fundamental pulse width. Furthermore, the temporal FWHM of converted pulses is determined by the fundamental pulse width as well as the effective interaction length related to the GVM. Under the condition of large GVM, large distortion exhibits in converted pulses. The dependence of the conversion efficiency on pump energy is also studied. The results show that the conversion efficiency saturates when the pump energy increases. The simulation results provide a guideline of device design and applications of the QPM wavelength converters in ultrashort-pulse region.