High-precision molecular interrogation by direct referencing of a quantum-cascade-laser to a near-infrared frequency comb

Abstract

This work presents a very simple yet effective way to obtain direct referencing of a quantum-cascade-laser at 4.3 μm to a near-IR frequency-comb. Precise tuning of the comb repetition-rate allows the quantum-cascade-laser to be scanned across absorption lines of a CO2 gaseous sample and line profiles to be acquired with extreme reproducibility and accuracy. By averaging over 50 acquisitions, line-centre frequencies are retrieved with an uncertainty of 30 kHz in a linear interaction regime. The extension of this methodology to other lines and molecules, by the use of widely tunable extended-cavity quantum-cascade-lasers, paves the way to a wide availability of high-quality and traceable spectroscopic data in the most crucial region for molecular detection and interrogation.

Comb-calibrated sub-Doppler spectroscopy with an external-cavity quantum cascade laser at 77 μm

“…In the latter respect QCLs were recognized early as excellent candidates for high-resolution applications because of a quantum noise limit of few hundred hertz [7]. However, their emission linewidth remained at a level of a few MHz for several years even for the narrower distributed-feedback versions [8,9], mostly due to the impact of current noise from power supplies [10]. Besides improving the noise level of current drivers, a variety of locking techniques have emerged to shrink their emission linewidth.…”

High accuracy line positions of the ν 1 fundamental band of 14 N 2 16 O

“…In the mid-IR, surveys over more than 50 cm -1 have been provided for the  3 band of N 2 O [16] and of CH 4 [17,18], as well as for CO 2 near 4.3 and 2.7 m [19,20]. A technological hurdle here originating from the lack of commercial mid-IR comb synthesizers and hence from the need to resort to nonlinear optics to get referencing of a mid-IR probe laser to a near-IR comb [21][22][23][24][25][26][27][28]. A second nontrivial requirement is a widely tunable single-mode laser, most of all beyond 5 μm, where cw sources based on difference-frequency-generation or optical parametric oscillation are not available.…”

Coherent phase lock of a 9 μm quantum cascade laser to a 2 μm thulium optical frequency comb

“…The important mid-IR spectral range is generally accessed with the help of near-IR (NIR) frequency combs or comb-referenced lasers through a variety of nonlinear mixing techniques, such as downconverting two frequency combs to a mid-IR frequency via difference frequency generation (DFG) [11], downconverting two CW lasers locked to a frequency comb via DFG [5], upconverting the QCL with a CW laser and comparing the upconverted light to a NIR frequency comb [13,14], or upconverting the mid-IR light with a frequency comb via sum frequency generation (SFG) [15,16]. These nonlinear mixing schemes carry the advantage of being independent of the carrier envelope offset frequency (f ceo ) of the comb, which cancels out in the heterodyne process and therefore needs no stabilization.…”

“…Most nonlinear referencing schemes were based on periodically poled lithium niobate crystals, whose absorption edge is situated around 5 μm [16,20]. Longer wavelengths, including most of the molecular fingerprint region, require a different referencing scheme.…”