Comparison of wavelength conversions based on cascaded second-harmonic generation/difference-frequency generation under continuous-wave and pulsed pumping

摘要

The implementation of wavelength conversion around 1.55 μm in quasi-phase-matched (QPM) periodically poled LiNbO_3 (PPLN) waveguides is mostly based on the cascaded second-harmonic generation/difference-frequency generation (SHG/DFG) process. As usual, a continuous wave (CW) and a pulsed wave are injected into a PPLN waveguide. Of them, the pulsed wave is regarded as the information carrier, and the CW is taken as the control. To transfer the information of optical codes from one wavelength to another, the codes can be applied to either the signal (CW-pumped scheme) or the pump (pulse-pumped scheme). In this work, the temporal and spectral properties of wavelength conversions during pulse propagation as well as the conversion efficiency in the two pumping schemes were compared experimentally and theoretically under different conditions of input pulse width, pump power and pump central wavelength. In the experiments, we adopted an MgO-doped PPLN waveguide, and a 40-GHz tunable picosecond-pulse source. The conversion characteristics were systematically investigated when the CW and the pulsed wave were alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In the theory, we solved the coupled-mode equations and explained the physical insights for the numerical results and experimental observations. The conversion properties of the two pumping schemes were quantitatively compared. The simulated results agree well with the experimental data, and the obtained results provide some guidelines for the design and application of QPM waveguides in wavelength conversion.