Quantum cascade laser spectrometer for frequency metrology and high accuracy molecular spectroscopy around 10 μm

Summary form only given. Quantum cascade lasers (QCLs) are an emerging technology [1] suitable for high-resolution spectrocopy and frequency metrology [2] with an incomparable frequency tunability in the mid-infrared range. We are currently developing a new compact, widely tunable QCL-based spectrometer. Such an instrument will broaden the scope of our experimental setups dedicated to molecular spectroscopy-based precision measurements.Eventually, we want QCLs to reach the state-of-the-art metrological stability and accuracy of our existing stabilized CO2 lasers (~10 Hz width, 0.1 Hz frequency instability for 100 s integration time). As recently demonstrated with the CO2 laser, we will lock the QCL to a frequency comb itself referenced via an optical fibre to the atomic foutain clocks in Paris. Stabilizing the laser this way not only provides us with the ultimate frequency accuracy and stability, it also frees us from having to lock the QCL to a molecular transition, making it possible to have a stabilized QCL at any desired wavelength. The use of QCLs will allow the study of any species showing absorption between 3 and 25 μm [1]. We are currently concentrating on QCLs at ~10 μm, which allows us to test them against our CO2 laser. Our first characterization of a free-running continuous-wave mode near-room-temperature distributed-feedback 10.3 μm QCL looks promising. The beat-note with our CO2 laser shows a record ~200 kHz linewidth (see Figure 1). The low level of amplitude and frequency noise, measured using an NH3 linear absorption line as frequency discriminator, should enable spectroscopy with unprecedented levels of precision. Narrowing of the QCL linewidth was achieved by straightforwardly phase-locking the beat-note between the QCL and CO2 laser on a radiofrequency reference. The great stability of the CO2 laser was transferred to the QCL resulting in an expected linewidth of a few tens of hertz.In the future, frequency locking to absolute frequency references consisting of a sub-Doppler molecular lines or to ultra-stable Fabry-Perot cavities, and finally phase-locking to a frequency comb will be investigated. This work will result in a major technological leap that will benefit two of our main projects respectively dedicated to a precise determination of the Boltzmann constant by laser spectroscopy of a molecular gas [3] and to the first observation of parity violation in chiral molecules by ultra-high resolution molecular jet-spectroscopy [4].