# Measuring molecular frequencies in the 1--10 {\mu}m range at 11-digits accuracy

## TL;DR

This paper introduces a high-resolution MIR spectroscopy setup combining quantum cascade lasers and difference frequency generation, achieving unprecedented 11-digit frequency accuracy across the 1--10 μm range, enabling precise molecular measurements.

## Contribution

The work presents a novel, broadly tunable MIR spectroscopy system with high power and 11-digit frequency accuracy, surpassing previous limitations in spectral range and precision.

## Key findings

- Achieved 11-digit frequency measurement accuracy in the 6 μm region.
- Demonstrated precise measurement of CO vibrational transition.
- Extended high-resolution spectroscopy capabilities across 1--10 μm range.

## Abstract

Mid infrared (MIR) photonics is a key region for molecular physics [1]. High-resolution spectroscopy in the 1--10 {\mu}m region, though, has never been fully tackled for the lack of widely-tunable and practical light sources. Indeed, all solutions proposed thus far suffer from at least one of three issues: they are feasible only in a narrow spectral range; the power available for spectroscopy is limited; the frequency accuracy is poor. Here, we present a setup for high-resolution spectroscopy that can be applied in the whole 1--10 {\mu}m range by combining the power of quantum cascade lasers (QCLs) and the accuracy achievable by difference frequency generation using an OP-GaP crystal. The frequency is measured against a primary frequency standard using the Italian metrological fibre link network. We demonstrate the performance of the setup by measuring a vibrational transition in a highly-excited metastable state of CO around 6 {\mu}m with 11 digits of precision, four orders of magnitude better than the value available in the literature [2].