Mid-infrared cavity ring-down spectroscopy using DFB quantum cascade laser with optical feedback for radiocarbon detection

A linewidth reduction of a distributed feedback quantum cascade laser (DFB-QCL) based on optical feedback for a mid-infrared (MIR) cavity ring down spectroscopy (CRDS) 14C spectrometer is presented. A cat-eye reflector as well as a path-length enhancement by a Herriott cell were employed for a compact optical setup. The laser linewidth was evaluated by monitoring the beat frequency between the DFB-QCL and a MIR optical frequency comb (OFC). The linewidth reduction by optical feedback was clearly observed although slow frequency drifts caused by environmental changes were still visible. A low-bandwidth beat-note lock to the OFC was conducted for long-term stabilization as well as for precise frequency scanning and thereby 14C measurement by MIR CRDS with optical feedback was demonstrated successfully.