Abstract
The modern world heavily relies on data collected by a myriad of sensors, with gas sensing technologies playing a pivotal role in numerous critical applications, spanning environmental monitoring, safety, security, oil and gas exploration, and biomedical analysis. These technologies are tasked with the detection of various gases, across a broad concentration spectrum, while providing real-time, in-situ capabilities, precise gas identification, and quantification.The burgeoning market for gas sensors offers diverse solutions categorized into non-optical and optical methods. The former includes chromatography, semiconductor, and electrochemical spectroscopy, while the latter encompasses direct absorption techniques like tunable diode spectroscopy and indirect methods such as Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS). Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) operates on the principle of indirect absorption spectroscopy through the photoacoustic effect. In this technique, a quartz tuning fork (QTF) serves as the transducer, converting acoustic waves into electrical signals. The light source, typically a laser emitting at a specific wavelength, is absorbed by the gas sample. The excited molecules undergo non-radiative processes, causing localized heating and a corresponding pressure increase in the gas. If the light source intensity is modulated, it results in periodic thermal energy generation and the emergence of a pressure wave, i.e., a sound wave, with the same modulation frequency. Quartz, being piezoelectric, generates electrical charges proportional to the sound wave's intensity when the sound wave induces an antisymmetric prong vibration in the QTF plane. The QTF is often coupled with a micro-resonator, such as an organ pipe resonator with one or two tubes, to probe the sound wave.The laser source is a critical component of the QEPAS setup, determining the targeted gas molecule based on its emitted wavelength. Key laser source properties include power (directly proportional to QEPAS signal), laser line width, and tunability. Diode lasers are favored for their compactness and the ability to tune the emission wavelength by varying temperature and current. Distributed Feedback Diode Lasers (DFB-LDs) are particularly valuable for gas sensing due to the introduction of periodic element (grating ) into the structure allowing to have single-mode operation. These grating forms a one-dimensional interference grating (Bragg scattering) that provides optical feedback for the laser. DFB-LDs are known for their high wavelength stability and narrow linewidth, making them suitable for clean single-mode operation. Their laser light precisely matches gas absorption lines, with oscillation linewidths narrower than the absorption lines.The primary objective of this work is to design and realize custom sources for QEPAS, specifically GaSb-based DFB laser diodes with the grating positioned atop the laser ridge, emitting at 2.4 µm. To enhance the coupling between the lasing mode and the grating, the goal was to reduce the thickness of the laser structure layers, including the upper cladding and top contact layers.In summary, this thesis was focused on development compact and sensitive QEPAS sensors for toxic gas detection and the development of novel custom DFB laser diodes for such applications.