Backward wave optical parametric oscillation in a waveguide

Backward wave oscillators (BWO) represent a class of tunable sources of electromagnetic radiation that do not require a resonant cavity to satisfy the oscillation condition. Electronic BWOs are widely used as high-power sources of microwave radiation. In the optical regime the backward wave optical parametric oscillator (BWOPO) rely on a conceptually similar principle between counter-propagating electromagnetic waves, where a nonlinear interaction provides the positive feedback required for oscillation. The unique properties of the BWOPO have so far been shown in bulk second-order nonlinear crystals only, but the absence of an optical resonator makes the BWOPO concept naturally suitable for integration in a waveguide format. Here, 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.