A Comb-calibrated Raman Spectrometer for High-accuracy Measurements of Quadrupole Transitions in Gases

Coherent Raman scattering is a powerful technique to probe quadrupole transitions in molecules. It has been deeply exploited in the last decades of the past century for high-resolution studies of gaseous molecular samples [1]. The invention of frequency combs at the turn of the century has brought the widespread realization of high-accuracy spectrometers to measure rovibrational transitions in gaseous molecular samples[2]–[4], yet, to date, no comb-calibrated Raman spectrometer has been realized. Here, we present a comb-calibrated Raman spectrometer for metrology of quadrupole transitions in hydrogen-like molecules, based on Stimulated Raman Scattering (SRS). In SRS, two collinear laser beams (called pump and Stokes) probe a rovibrational transition at the frequency equal to their detuning, allowing one to address low energy (mid- and far-infrared) transitions with a pair of visible or near-infrared lasers. Our spectrometer (see Fig. 1) is based on two narrow-linewidth cw lasers emitting at 740 nm (pump) and 1064 nm (Stokes), whose frequency detuning is calibrated through an Er:fibre optical frequency comb that is first spectrally broadened and then frequency doubled to overlap with the cw lasers emission. The laser beams are made to propagate in a temperature- and pressure-stabilized multipass cell with 30 m optical path and high-reflectivity mirrors. Intensity modulation of the Stokes beam at 9.7 MHz and lock-in amplification of the weak stimulated Raman loss signal on the pump beam are implemented to reach shot noise-limited detection of the vibrational mode profile. Active stabilization of the Stokes beam alignment with respect to the pump is also implemented to minimize errors in the measured vibrational frequency introduced by finite angles of non-collinearity between the propagation axes of the two lasers. Fig. 1 Schematic of the comb-calibrated srs spectrometer. Eom: electro-optic modulator; m: mirror; bc: beam combiner; bs: beam sampler; dm: dichroic mirror; bd: beam dumper; pd: photodiode; ofc: optical frequency comb; shg: second-harmonic generation.