Infrared precision spectroscopy using femtosecond-laser-based optical frequency-comb synthesizers

Femtosecond-laser-based optical frequency comb synthesizers (OFSs) can play a fundamental role in precision spectroscopy and laser frequency control in the infrared (IR), as has happened in the visible and near-IR regions of the spectrum. Increasing demand for precise frequency measurements in the IR has resulted in a number of ideas for IR frequency synthesis (IFS) with visible combs. IR frequency synthesis and measurement require narrow linewidth, well-controlled and often widely tunable laser sources as well as high-accuracy frequency references. In this paper we review the state of the art of IR metrology following the advent of the OFS. In particular, we describe our system which uses a nonlinear difference-frequency-generated (DFG) IR source combined with an OFS to develop a widely tunable IR laser source with very narrow linewidth and with a frequency directly traceable to the Cs primary standard. The frequencies of saturated absorption spectra belonging to the intense molecular ro-vibrational transitions in this region can be measured with our OFS-referenced DFG source. An unprecedented dense grid of secondary frequency standards can be created with this DFG-OFS combination.