Quantum cascade lasers for frequency comb-based mid-infrared applications
Authors:   , Komagata Kenichi N. 
Published: November 2023

The mid-infrared (MIR) part of the electromagnetic spectrum is characterized by strong molecular interactions with light and low scattering losses. Thus, it is a strategic wavelength range for sensing, free-space telecommunications and high-precision spectroscopy. Optical frequency combs (OFCs or FCs) excel in these applications, making their generation in the MIR highly relevant. Quan- tum cascade lasers (QCLs) are a possible source of MIR OFCs, and present the unique merits of high emission power, simple operation, and compatibility with mass-production. However, QCL OFCs have not yet been employed in a wide range of MIR applications despite their potential. In fact, there is a lack of established methods for their coherent control such as frequency sta- bilization: techniques from other OFC technologies could not be adapted in a straightforward manner. So far, they have been used almost exclusively for dual comb spectroscopy (DCS), where active stabilization was not imperative. In the absence of other simple OFC sources in this spectral range, they have been producing excellent results in this configuration. But enabling QCL FCs to drive the manifold of comb applications will require advanced stabilization techniques that ideally do not undermine simplicity and compactness. In this thesis, I investigated frequency stabilization, calibration, and the ap- plication of QCLs with the goal of demonstrating their fitness as versatile MIR FC sources. In a first approach, I tackled the issue of low duty-cycle impaired by the computational correction necessary for free running DCS. I developed a scheme to mutually lock two QCL FCs and to allow real-time processing of the multiheterodyne beat. One challenge for QCL comb stabilization schemes is the moderate frequency response of the usual control method based on electrical feedback. In a second approach, I characterized the influence of near-infrared light on a QCL FC and developed a locking scheme employing a standard telecommunication laser, improving the tight-locking ability to a satisfactory level for most applications. Moreover, I demonstrate high spectral purity across the bandwidth of the comb and reveal the comb coherence at new time-scales. In a third experiment, I performed the full frequency stabilization of a MIR QCL FC. I then implemented comb-calibrated spectroscopy and retrieved the center frequency of several molecular transitions. The accurate agreement between the retrieved and literature values proves that QCL FCs are suited for frequency metrology applications. In a final experiment, I tackled the issue of frequency calibration in fast and high-resolution DCS. I designed and tested a scheme that allowed the critical combination of high frequency accuracy, acquisition speed, resolution, and sensitivity. It was found that the particularly high repetition rate of QCLs is actually an advantage for direct comb spectroscopy. All these results can already be transferred to applications. For example, the tight-locking capability will allow the use of enhancement cavities for increased sensitivity. Moreover, frequency calibration in fast and high resolution DCS al- lows QCLs to be employed for high precision studies of the physics of molecules. However, the reported results represent only the tip of the iceberg in the po- tential of QCL OFCs to harness the prospects of the MIR. While they merely scratches the surface, they establish concretely that QCLs are fit in terms of frequency stabilization and calibration as versatile MIR FC sources. Further development towards photonic integration, pulse emission, and low power con- sumption promise to make QCLs ubiquitous in the society of the future.