Novel concepts in high power semiconductor lasers
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This doctoral thesis deals with innovations to the cavity optics of high power semiconductor lasers emitting light at 9xx nm. High power laser diodes are complex electronic and photonic systems. Developments in epitaxial crystal growth techniques and the quality that ensued has been the driving force in the progress of the field. Semiconductor lasers with high output powers and high efficiencies have thus become possible. Commercial single emitters each with over 10 watts output with efficiencies reaching 60% is available.Even higher output powers have been demonstrated in the lab. High power semiconductor lasers have many applications such as acting as optical pumps in other lasers, range finding, optical storage, light sources in sensors and medical tools. The demand for higher powers and efficiencies continues. Among several possibilities, one of the main limits of maximum output power is the catastrophic optical mirror damage (COMD). At high pump currents and hence output powers, facet absorption leads to temperatures high enough to damage the cavity mirrors. This thesis is focused on novel approaches to increase the COMD threshold. We demonstrate design, fabrication and characterization of the high power strained InGaAs/AlGaAs lasers emitting light at 9xx nm. To prevent facet absorption which decreases the laser efficiency especially at high injection currents, band gaps in the vicinity of the laser facet are increased using impurity-free vacancy disordering (IFVD) while preserving the band gap in the lasing region away from the facets. A record large bandgap at the facet region, relative to that of the lasing region is achieved by thermal stress management of a bilayer dielectric structure. We demonstrate excellent optical loss and optical power output with this bilayer approach. Further, positive feedback cycle during absorption at the facets is broken by keeping the facets cold, by design. Thus, in this cold window approach, we extend the passive unpumped windows to keep the heat source from the main body of the cavity away from the facets while eliminating the additional loss incurred by biasing this section to transparency. This new biased window approach leads to much cooler facet temperatures while reducing the bulk temperatures as well. Thus, we use thermore ectance spectroscopy to measure facet temperature as a function of pump and bias current. We clearly demonstrate that, for the first time, facet temperatures have been decreased below the bulk temperature without penalty on the output power.