Fabrication of GaSb-based Y-junction Laser for Gas Spectroscopy
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Trace gas detection and pollution monitoring are becoming increasingly important for human safety and conservation of the environment. GaSb based semiconductor diode lasers, emitting in the mid-infrared wavelength region of 2-2.5um, are promising light source candidates for trace-gas sensing systems. In this wavelength-range, detection of gases like CO2, CO and CH4 with high selectivity and sensitivity is possible. For such systems, tunable light sources with a large tuning range, simple tuning scheme and large side-mode suppression ratio (SMSR) are in demand. The Y-junction laser is a simple geometrical structure, increasing the tuning range of the optical device, without the need for complex processing steps. Two laser branches conjoin to form a folded Mach-Zehnder configuration, providing the ability for wavelength tuning by adjusting the current ratio into the two branches, which affects the interference in the merged branch. This thesis describes the design, fabrication and testing of Y-junction laser and the related S-bend laser structure. S-bend lasers, which are an integral part of the Y-lasers, have been fabricated to investigate the relationship between curved waveguides and optical loss. The fabricated lasers show single-mode behaviour for waveguide linewidths 5um and below, with SMSR above 13dB and output power exceeding 5mW. Waveguides with larger curvatures were found to exhibit larger losses. Both shallow-etch quasi-index guided waveguides and deep-etch index-guided devices were fabricated. The quasi-index guided devices have improved performance, with lower threshold current densities and increased slope efficiency. Optimization of the ridge-waveguide etch and passivation layer etch-back is presented, laying the groundwork for improved performance in future devices. Y-junction lasers were fabricated and large wavelength tunability (19nm) and improved SMSR was demonstrated. Y-junction lasers based on the GaSb material system promises wide tuning range and simpler processing steps compared to competing structures in the mid-infrared wavelength range. Simpler processing means reduction of cost and increased yield.