Continuous-wave stimulated Raman gain spectroscopy with cavity ringdown detection

Cavity ringdown (CRD) spectroscopy is extended for the first time from its conventional optical-absorption mode of operation into the regime of coherent Raman spectroscopy. Continuous-wave (cw) stimulated Raman gain (SRG) spectra of the 2916.5-cm-1 ν1 rovibrational fundamental Raman band of methane (CH4) gas are measured, using tunable cw laser light at ∼1544 nm to probe ringdown decay from a rapidly swept optical cavity that is itself inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.5 nm. The resulting change of ringdown decay rate is dependent on pump laser irradiance and is associated with Raman gain. Remarkably, such SRG-CRD resonances display ringdown times that are longer than in the off-resonance case, contrasting with the usual reduction of ringdown time associated with absorption and other loss processes.