Abstract: An optical parametric oscillator (OPO) system comprises an optical waveguide including a hollow core containing a fluid, wherein the optical waveguide is configured to receive pump light and to convert the pump light into signal light and idler light via a third order non-linear optical effect. The OPO system further comprises an optical feedback arrangement for recycling at least a portion of the signal light and/or for recycling at least a portion of the idler light in an optical cavity that includes the optical waveguide. The OPO system may be used, in particular though not exclusively, in metrology, gas and solid-state spectroscopy, laser-assisted manufacturing, semiconductor technology, biomedicine, healthcare, and scientific laboratory use.