Frequency stabilisation and SI tracing of mid-infrared quantum-cascade lasers for precision molecular spectroscopy

TL;DR

This paper presents a robust method for stabilizing and referencing mid-infrared quantum-cascade lasers, enabling high-precision molecular spectroscopy through linewidth narrowing, frequency stabilization, and SI-traceable absolute frequency calibration.

Contribution

It introduces a novel approach combining linewidth narrowing, frequency stabilization, and SI-traceable referencing of MIR QCLs using an optical frequency comb and remote primary standard.

Findings

Achieved a frequency instability of 5×10⁻¹³ at 1 s

Demonstrated absolute frequency calibration via remote SI-traceable standard

Implemented phase lock of QCL to a linewidth-narrowed optical frequency comb

Abstract

The advancement of technologies for the precise interrogation of molecules offers exciting possibilities for new studies in the realms of precision spectroscopy and quantum technologies. Experiments in these domains often address molecular vibrations in the mid-infrared (MIR) spectral region, thus generating the need for spectrally pure and accurate MIR laser sources. Quantum-cascade lasers (QCLs) have emerged as flexible sources of coherent radiation available over a wide range of MIR frequencies. Here, we demonstrate a robust approach for the simultaneous linewidth narrowing, frequency stabilisation, and absolute frequency referencing of MIR QCLs all of which are prerequisites for precise spectroscopic experiments. Following upconversion of its radiation to the visible domain, we implement a phase lock of the QCL to a linewidth-narrowed optical frequency comb which is referenced to a remote SI-traceable primary frequency standard via a fibre link for absolute frequency calibration. To achieve a reliable frequency counting of the beat note between the QCL and the OFC, we employ redundant tracking oscillators and demonstrate a frequency instability of $5\times10^{-13}$ at 1 s and $2\times10^{-14}$ at 1000 s integration time, limited by the accuracy of our remote reference.