Backward wave optical parametric oscillation in a waveguide

Backward wave oscillators represent a class of tunable sources of electromagnetic radiation that do not require a resonant cavity to satisfy the oscillation condition. In the optical regime, the Backward Wave Optical Parametric Oscillator (BWOPO) relies on a a nonlinear interaction to provide the positive feedback required for oscillation, achieved through quasi-phase matching with sub-micron periods. The unique properties of the BWOPO have so far been shown in bulk crystals only, but the absence of an optical resonator makes the BWOPO naturally suitable for integration in a waveguide format. We demonstrate the first waveguide BWOPO, showcasing an oscillation threshold nearly 20 times lower than the corresponding bulk device, and exhibiting low loss (0.2 dB/cm). The backward wave has a narrow linewidth of 21 GHz at 1514.6 nm, while the forward wave at 1688.7 nm has a broadband spectrum replicating that of the pump. A conversion efficiency of 8.4% was obtained.