Tunable carbon monoxide overtone laser sideband system for precision spectroscopy from 26 to 41 µm

We report a source of tunable laser radiation for high-precision molecular spectroscopy in the 2.6–$4.1-\mathit{\mu }\mathrm{m}$ spectral region. Laser light from a CO overtone laser is mixed with microwaves, generating tunable sidebands of $\sim 1 \mathrm{mW}$ of power. We achieve very high absolute frequency accuracy by frequency-offset locking the CO laser to a ${\mathrm{CO}}_2$ laser secondary frequency standard. The uncertainty of the laser frequency is less than 30 kHz $\left(\mathrm{\Delta }\nu /\nu =3\times {10}^{-10}\right)$, and the laser linewidth is of the order of 100 kHz. This tunable and ultrastable laser system is suitable for very accurate molecular spectroscopy and metrology in a most interesting wavelength region. We demonstrate an application of the system to saturated-absorption spectroscopy of a rovibrational transition of carbonyl sulfide.