Picosecond synchronously-pumped optical parametric oscillator based on chirped quasi-phase matching

We investigate and model a picosecond synchronously pumped optical parametric oscillator (OPO) based on an aperiodically poled lithium niobate (APPLN) nonlinear crystal with a chirped quasi-phase matching (QPM) grating. We observe remarkable spectral features with an asymmetric OPO spectrum consisting of a main peak with lower sidelobes. Depending on the sign of the QPM chirp rate, the sidelobes are located either on the red or on the blue side of the main peak. Meanwhile, sidebands develop in the depleted pump spectrum. We attribute these features to cascaded sum-/difference-frequency generation processes, which are quasi-phase matched at different positions in the APPLN crystal. A terahertz-generation cascading effect is also observed and characterized at high pump power.