Metrological-grade tunable coherent source in the mid-infrared for molecular precision spectroscopy

Summary form only given. Molecular precision spectroscopy opens new perspectives on tests of fundamental laws of physics and of fundamental constants variation [1]. The mid infrared (IR) is a key region for molecular spectroscopy, and the efforts towards the development of versatile, spectrally pure, and tunable coherent sources in this region have been constantly growing in the last decade. A convenient way to generate a mid-IR tunable source for precision spectroscopy is difference-frequency generation (DFG) in a nonlinear crystal starting from two visible or near-infrared sources. Cs-traceability can be achieved by locking the pump and signal lasers to an optical frequency comb referenced to the primary frequency standard.Here we present our approach for the development of a versatile mid-IR source of metrological grade [2]. We demonstrate its robustness by measuring a vibrational transition around 6 μm on a highly-excited, metastable state of CO at the kHz level. We obtain a Doppler-broadened absorption profile with a FWHM of ~1 MHz after a tenminutes scan, whereas 10-hours averaging yields an uncertainty on the line center of this vibrational transition of 3 kHz, four orders of magnitude better than the value available in the literature [3]. Our source (Fig.1) is based on the combination of three building blocks: i) the frequency reference is delivered from the Italian national laboratory for metrological research (INRIM) through a stable and ever-growing fibre network that links many laboratories thousands of km apart; ii) the mid-IR radiation is produced by a quantum cascade laser (QCL) whose output, at 10 mW level, is almost entirely used for spectroscopy and not for the locking scheme; iii) a novel OP-GaP crystal is used as the nonlinear medium for the DFG, allowing to cover, in principle, the entire 1-10 micron range.