Characteristics of femtosecond second-harmonic generation in periodically poled lithium niobate

摘要

The temporal and spectral characteristics of femtosecond second-harmonic-generation (SHG) in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) are investigated experimentally and theoretically. In the experiments, we studied SHG in PPLN waveguides with different device design and fabrication conditions by adopting ～100-fs optical pulses with different peak powers in the 1.55-μm band. Large phase modulation and spectral broadening of the fundamental wave, and a shift of the centre frequency of the second-harmonic wave were observed when the peak power of the fundamental wave was high enough. Saturation of the conversion efficiency was also observed, which depends on fundamental pulse width and peak power. These phenomena could be attributed to nonlinear phase accumulation under large group-velocity mismatch. In the theory, evolutions of pulse waveforms and optical spectra were studied for both the fundamental and second-harmonic pulses by solving the coupled-mode equations. The simulation results are in good agreement with the experimental data.