A widely tunable 10-μm quantum cascade laser phase-locked to a state-of-the-art mid-infrared reference for precision molecular spectroscopy

Abstract

We report the coherent phase-locking of a quantum cascade laser (QCL) at 10-µm to the secondary frequency standard of this spectral region, a CO 2 laser stabilized on a saturated absorption line of OsO 4 . The stability and accuracy of the standard are transferred to the QCL resulting in a line width of the order of 10 Hz, and leading to our knowledge to the narrowest QCL to date. The locked QCL is then used to perform absorption spectroscopy spanning 6 GHz of NH 3 and methyltrioxorhenium, two species of interest for applications in precision measurements.With their rich internal structure, molecules can play a decisive role in precision tests of fundamental physics. They are being used to test fundamental symmetries such as parity 1-3 or parity and time reversal 4 , to measure absolute values of fundamental constants 5-7 and their possible temporal variation [8][9][10] . Many of these experiments can be cast as measurements of resonance frequencies of molecular transitions, for which ultra-stable and accurate sources in the mid-infrared (mid-IR) are highly desirable, since most rovibrational transitions are to be found in that region.Our group has a long-standing interest in performing spectroscopic precision measurements on molecules at extreme resolutions around 10 µm 1,9,11 . We are currently working on two such measurements: the determination of the Boltzmann constant, k B , by Doppler spectroscopy of ammonia 6,12 and the first observation of parity violation by Ramsey interferometry of a beam of chiral molecules 3,13 . For these experiments, we currently use spectrometers based on custom built ultra-stable CO 2 lasers. We obtain the required metrological frequency stability and accuracy -10 Hz line width, 1 Hz stability at 1 s, accuracy of a few tens of hertz 14,15 -by stabilizing these lasers to saturated absorption lines of molecules such as OsO 4 . CO 2 lasers have a major shortcoming: a lack of tunability. They emit at CO 2 molecular resonances. An emission line is found every 30 to 50 GHz in the 9-11 µm wavelength range, and each line is tunable over about 100 MHz. Although, as in our spectrometers, this range can be extended a few gigahertz using electrooptical modulators (EOMs), this is done at the expense of power (EOMs at these wavelengths have an efficiency of 10 −4 ) and necessitates subsequent spectral filtering. Overcoming these difficulties without the loss of stability is key to enabling precision measurements in the mid-IR.One solution would be to use frequency combreferenced continuous-wave (cw) [16][17][18] or femtosecond mid-IR sources. These are based on frequency mixing in nonlinear crystals and provide absolute-frequency referencing, reasonable line widths and tunability, but are very complex and often exhibit limited power. By comparison, cw quantum cascade lasers (QCLs) are a new mature and robust technology that offer broad and continuous tuning over several hundred gigahertz at 100 mW-level powers. Several can be combined giving access to the whole mid-IR region. Recent studies of...

An experimental study of noise in mid-infrared quantum cascade lasers of different designs

“…Novel applications of QCLs in very high-resolution spectroscopy and optical metrology have emerged in the last years, in particular in combination with optical frequency combs [2][3][4][5], which are generally more demanding in terms of low frequency-noise and nar-row spectral linewidth. QCLs have the potential to achieve very narrow linewidths with an intrinsic value of a few hundreds hertz only [6,7] resulting from their close-to-zero Henry's linewidth enhancement factor [8]. However, a narrow linewidth is generally not achieved in practice in freerunning QCLs as a result of the presence of undesired noise that compromises their spectral properties.…”

“…Hence, the char-acterization and understanding of the origin of frequency noise spectrum of free-running QCLs gained a significant interest in the recent years. A few research groups reported experimental frequency noise spectra for QCLs produced by different manufacturers and operated at temperatures ranging from cryogenic [7,9] up to room temperature [6,10,11]. Significantly different levels of noise have been observed in the considered QCLs, leading to linewidths in the range of sub-100 kHz [6] to many megahertz [7].…”

“…So far, some isolated devices have shown low-noise operation, but for uncontrolled and unknown reasons (see for instance the laser used in Ref. [6]). The approach reported here to investigate noise in QCLs was based on the combination of an experimental study realized over a large set of QCLs with some modeling and statistical data analysis.…”

Absorption-line-shape recovery beyond the detection-bandwidth limit: Application to the precision spectroscopic measurement of the Boltzmann constant

“…Climate modeling and global change research programs are setting unprecedented accuracy targets in gas-sensing missions for atmospheric CO 2 and other greenhouse gases [1]. The desired uncertainty can only be reached by making high quality measurements of spectroscopic parameters (including pressure broadening coefficients and line intensity factors) using well-designed and characterized experiments [2][3][4][5]. Similarly, the interpretation of spectra of astrophysical and planetary interest needs a precise knowledge of line-shape parametres and current databases require further improvements [6,7].…”

Quantum cascade laser-based mid-IR frequency metrology system with ultra-narrow linewidth and 1 × 10^−13-level frequency instability

“…The free-running linewidth on the timescale of 1 s is typically on the order of 1 MHz and has been the subject of several studies [2,11]. Linewidth narrowing, alongside absolute frequency stabilization and finally absolute frequency measurement, are therefore important tasks for rendering QCLs usable for high-resolution spectroscopy.It has already been shown that the QCL linewidth can be dramatically reduced, by locking to a reference cavity [10], by phase-locking to a narrow-linewidth CO 2 reference laser, reaching less than 10 Hz relative linewidth [9], or by phase-locking to the DFG wave generated from two near-IR cw lasers [5]. This latter approach however limits the spectral coverage of the QCL, since it relies on particular reference lasers.…”