OPOs for high-precision spectroscopy and frequency metrology

E. Kovalchuk, D. Dekorsy, A. Lvovsky, A. Peters

OPOs possess a number of unique advantages in comparison to other tunable continuous wave sources, such as wide emission range, high stability, narrow linewidth and high output power. A particularly valuable feature of  cw OPOs is that their tuning range covers the mid-infrared region around 3µm where fundamental vibrational modes of molecular bonds containing hydrogen occur. These transitions are normally characterized by a high dipole moment and therefore molecular gases exhibit strong absorption even if present in low concentrations. By accessing these transitions via  techniques of Doppler-free spectroscopy OPOs have a potential as an analytical spectroscopic tool of very high sensitivity.

Our goal is to develop and use an OPO suitable for this application. We employ a singly-resonant OPO with the idler in the IR range and an intracavity etalon to do Doppler-free spectroscopy of methane. To date, we have completed our first measurements. We split the 3.39-mm idler output and direct it, via two counter-propagating beams (pump and probe) into a methane cell. We use the frequency modulation method and by scanning the OPO frequency in the range of 5 MHz we obtain the dispersion form of a homogeneously broadened (300 kHz) methane line. This result is of great importance as it is the first demonstration of using OPOs for Doppler-free spectroscopy.

The next step is to further stabilize the OPO to be able to access much narrower Doppler-free lines (several kHz). An exciting opportunity for the future is to develop a new generation of frequency standards by stabilizing the cw OPO’s frequency to narrowband molecular transitions. These standards would be operating in the mid-IR range with an expected frequency instability on the order of 10 Hz or less.

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