Quantum Cascade Laser: a promising candidate toward an ultra-stable laser in the mid-IR L. Tombez, J. Di Francesco, S. Schilt, G. Di Domenico, D. Hofstetter and P. Thomann LTF Laboratoire Temps-Fréquence Institut de Physique, Université de Neuchâtel Neuchâtel, Switzerland
[email protected] Abstract—The frequency noise properties of free-running singlemode mid-IR Quantum Cascade Lasers (QCLs) operated near room-temperature in CW mode and emitting at 4.6 µm are presented. The frequency noise level measured in this paper is globally a factor of 100 lower than recently reported for a similar QCL but operated at cryogenic temperature. The results are very promising for the future realization of a QCL-based ultra-stable laser in the mid-IR.
I.
INTRODUCTION
Narrow-linewidth lasers exhibiting a high spectral purity have important applications in various fields, such as highresolution spectroscopy, coherent optical communications or time and frequency metrology. In the latter field, such lasers can be used as ultra-stable reference for low phase noise microwave generation when frequency-stabilized to a highfinesse optical cavity for linewidth narrowing. Indeed, the relative stability of the ultra-stable optical reference can be transferred to the microwave domain using an optical frequency comb (OFC). Such optical references were demonstrated based on external cavity and fiber lasers in the near-infrared (NIR) [1], but also in the mid-infrared (MIR) with a 3.39-µm HeNe laser stabilized to a CH4 transition [2]. In this latter case, the relative stability of the HeNe/CH4 laser was transferred using a MIR comb obtained by difference frequency generation (DFG) between two spectral parts of a NIR comb. As a result of the DFG process, the generated MIR comb has a null carrier-envelope offset (CEO) frequency, independently of the value of the NIR comb CEO. In other words, the translational degree of freedom of the NIR comb can stay free without affecting the generated MIR comb. Therefore, the CEO does not need to be stabilized nor to be measured, which relaxes the need for an octave spanning comb and the associated f-to-2f interferometer. With the objective to implement a similar optical-tomicrowave transfer based on a MIR quantum cascade laser (QCL), we studied the frequency-noise characteristics of QCLs, aiming at assessing their suitability as a potential ultrastable frequency reference. Indeed, efficient laser linewidth narrowing by frequency-stabilization to a high-finesse cavity with a reasonable feedback-loop bandwidth can be achieved
only if the frequency noise of the free-running laser is sufficiently low. An estimate of feedback loop bandwidth required for the realization of an ultra-narrow linewidth QCL will be assessed from our experimental results. II.
EXPERIMENTAL SETUP
We investigated the frequency-noise properties of freerunning commercial single-mode DFB-QCLs (Alpes Lasers SA, Switzerland) operated in CW mode near room temperature and emitting at 4.6 µm. The frequency noise measurement was performed by converting the laser frequency fluctuations into intensity fluctuations using the side of a molecular absorption in standard single-pass direct absorption spectroscopy. The emission wavelength of the laser was tuned to the centre of the linear range in the flank of the 250-MHz wide R(14) transition in the fundamental vibration band of carbon-monoxide (CO). The laser was driven (350 mA at 10V) by a home-made ultra-low noise current source (
