Abstract
The monitoring of gaseous compounds in trace concentrations is crucial across various fields, from industry to biology. Continuous improvement of gas sensors is essential to meet diverse application requirements. Optical sensors stand out for their fast and accurate results. Among these, laser-based sensors offer unprecedented sensitivity and selectivity. Over the past few decades, quantum cascade laser (QCL) technology has demonstrated significant progress that can be exploited in spectroscopy.This thesis presents advancements in both photothermal spectroscopy and QCL sources. The first part of the work is devoted to photothermal interferometry, an indirect method in which thermal effects, photoinduced in the sample by the excitation source, are detected with an interferometric readout of a probe laser. This approach enables small sensing volumes and high sensitivity which scales with optical power of the excitation source. This work makes use of the Fabry-Pérot Interferometer (FPI), since it offers compactness, due to the close mirror spacing, and high sensitivity. For real-case scenarios, two fundamental requirements must be fulfilled: spatial overlapping of the probe and excitation beams, between the mirrors of the FPI, and frequency tuning of the probe laser to the interferometric fringe. A diode laser (DL) source has been chosen as probe, for its cost-effectiveness, swift current tuning capabilities, and mature technology in the telecom region.A model for the transduction properties of a DL coupled with the FPI in presence of sample excitation was derived. The model agreed with the experimental results, tested on the case of nitric oxide detection. A novel normalization scheme and locking method were developed for a stable signal readout, compensating for system’s drifts by self-referencing the measurement to the cavity quality. Detection limits of few parts-per-million were achieved, leading to normalized noise equivalent absorptions on the order -10-6 W cm-1 Hz-1/2.The second part of this manuscript is focused on the long-wavelength (LW) QCL technology, based on the InAs/AlSb material system. The LW regime (λ > 10 μm) plays a pivotal role for the detection of targeted organic compounds since they exhibit strong absorptions between 12.5 – 15 μm. Tapered QCLs emitting near 14 µm were fabricated for this purpose. The tapered waveguide aims at improving the available optical power by enhancing the active volume in the resonant cavity. Tapered lasers with angles between 0° and 3° were fabricated and compared in terms of optical and electrical properties. A scaling of the optical power was observed with respect to the ridge device, up to a factor of 3 for the largest taper. Far-field intensity distribution was investigated to assess the beam quality factor. In most cases, a diffraction-limited beam was observed. Thin dielectric coatings were also deposited on the laser facets to improve the outcoupled optical power. Spectroscopic applications usually require single-frequency operation, which was demonstrated for all the tapered devices by fabrication of Bragg gratings in the upper cladding. Side-mode suppression ratio above 20 dB, in both pulsed and continuous wave operation, has been achieved. Finally, the potential of long-wavelength QCLs for spectroscopic applications was explored in the context of classical absorption, in combination with a multi-pass cell for the detection of benzene, acetylene and carbon dioxide.