Quasi-phase-matched parametric downconversion via cascaded optical nonlinearities in an aperiodically poled MgO:LiNbO3 superlattice was studied in theory and experiment. Coupling equations reveal that multiple-wavelength parametric downconversion in a wide infrared spectrum range can occur in a single crystal. Enhancement of the conversion efficiency and output stability through coupling of two nonlinear processes is demonstrated. The result also reveals that cascaded parametric downconversion process can be used to efficiently downconvert the fundamental wavelength to longer wavelength of infrared region.
Bandwidth enhancement in difference frequency generation (DFG) has been investigated using domain-shifted quasi-phase-matching (QPM) structure of electrically poled ferroelectric crystals. The exact periodic QPM condition is relaxed by extending the periodic structure to a more flexible domain-shifted structure. The dependence of bandwidth enhancement and flatness on the positions and number of shifted domains are systematically studied for optimum output efficiency. The results presented here are useful for the fabrication of wide-bandwidth wavelength conversion devices based on the DFG process.