Ultra-stable Mid-IR quantum cascade laser for high-resolution spectroscopy and metrology

We report the phase-locking of a quantum cascade laser (QCL) at 10 μm to the secondary frequency standard of this spectral region, a CO₂ laser stabilized on a saturated absorption line of OsO4. The stability and accuracy of the standard are transferred to the QCL resulting in a line width of the order of 10 Hz. The locked QCL is then used to perform high-resolution linear and saturated absorption spectroscopy of species of interest for applications in precision measurements.