Narrow-linewidth near-degenerate optical parametric generation achieved with quasi-group-velocity-matching in lithium niobate waveguides.

We demonstrate narrow-linewidth near-degenerate optical parametric generation in reverse-proton-exchange lithium niobate waveguides with quasi-group-velocity-matching, which is realized by using wavelength-selective directional couplers and tight-radius bends. With appropriate designs for 1.6 ps long pump pulses at 785.1 nm we obtained near-degenerate signal (idler) pulses with a time-bandwidth product as low as 1.1, compared with 10.5 for conventional devices without quasi-group-velocity-matching. This improvement in the temporal property is a result of both a pulse compression effect and a filter effect coming from our scheme of quasi-group-velocity-matching.