Quantum cascade laser frequency stabilization at the sub-Hz level

Abstract

High-precision measurements with molecules may refine our knowledge of various fields of physics, from atmospheric and interstellar physics to the standard model or physics beyond it. Most of them can be cast as absorption frequency measurements, particularly in the mid-infrared 'molecular fingerprint' region, creating the need for narrow-linewidth lasers of well-controlled frequency. Quantum cascade lasers provide a wide spectral coverage anywhere in the mid-infrared, but show substantial free-running frequency fluctuations. Here, we demonstrate that the excellent stability and accuracy of an ultra-stable near-infrared laser, transferred from a metrological institute through a fibre link, can be copied to a quantum cascade laser using an optical frequency comb. The obtained relative stability and accuracy of 2 × 10 −15 and 10 −14 exceed those demonstrated so far with quantum cascade lasers by almost two orders of magnitude. This set-up enables us to measure molecular absorption frequencies with state-of-the-art uncertainties, confirming its potential for ultra-highprecision spectroscopy.M olecules are increasingly being used in precision tests of physics thanks to progress made in controlling molecular degrees of freedom 1,2 . They are now being used, for example, to test fundamental symmetries 3-5 and to measure fundamental constants 6-8 and their possible variation in time 9-11 . Most of these experiments are spectroscopic precision measurements and are often in the mid-infrared (MIR) domain where the molecules exhibit intense and narrow rovibrational transitions. This creates a need for efficient MIR laser sources, prompting efforts to develop ultra-stable and accurate continuous wave (c.w.) lasers as well as MIR frequency combs (refs 12 and 13 for instance). Quantum cascade lasers 14,15 (QCLs) are promising c.w. sources-they are available anywhere in the 3-25 µm MIR range, and each QCL can be tuned over several hundreds of gigahertz. However, their freerunning linewidth of tens to thousands of kilohertz makes their frequency stabilization challenging [16][17][18][19][20][21][22][23][24][25] .Common references used for frequency stabilization in the MIR region include molecular rovibrational absorption lines 3,26 . However, molecular degrees of freedom cannot be controlled as efficiently as atomic ones, leading to limited frequency reproducibility and accuracy. Attempts to develop MIR ultra-stable cavities have been made, but their performances are far from those reported in the near-infrared (NIR) or visible regions 27,28 . It is thus appealing to use the best ultra-stable lasers as frequency references. As these are predominantly in the NIR region, it is necessary to bridge the gap between the NIR and MIR domains. This is possible using an optical frequency comb (OFC). The MIR frequency is locked to a high harmonic of the OFC repetition rate using sum-or difference-frequency generation processes. This not only provides ultimate stabilities of lasers locked to state-of-the-art ultra-stable cavities 29 , but ...

Hybrid fiber links for accurate optical frequency comparison

“…Optical frequency transfer by the active noise cancellation (ANC) method [1,12] has a wide range of applications, including remote high-resolution and high-accuracy spectroscopy [13][14][15], very long baseline interferometry [16], and remote narrow-linewidth light sources [2,14,17]. However, if only a comparison of the optical frequencies is required, the recently proposed two-way method [18][19][20] offers a good alternative with the advantage of simplified experimental apparatus without active components.…”

Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials

“…These features lead to their wide adoption for spectroscopic applications in the mid-IR region. However, their free running linewidth is subject to undesired noise that makes their frequency stabilization challenging8. High finesse optical cavities have the potential for substantial reduction of laser noise910 and have been recently investigated11.…”

Terahertz Frequency Metrology for Spectroscopic Applications: a Review

“…Indeed, in the last years, OFLs have shown undoubted effectiveness in comparing remote atomic clocks beyond the ultimate limit of GPS-based comparisons, attaining precision levels not limited by the frequency transfer method [191][192][193][194][195][196], and are also envisioned as key elements for the development of a new kind of high-accuracy relativistic geodesy [195,197]. Besides optical clock comparisons, short-range OFLs have recently been exploited in order to increase the spectral performance of a local laser and to allow for precision molecular spectroscopy [198]. It has been recently demonstrated that long-haul fiber-based optical frequency dissemination is a reliable tool for remote end users to perform high-precision procedures well beyond the ultimate capabilities of GPS dissemination [199].…”