Design and fabrication of high-performance interband cascade tunable external cavity lasers
Chryssis, Athanasios Nicholas
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The 3-4 &mum spectral region, which contains a number of important lines for chemical detection and other civilian and military applications, has proven to be one of the most challenging for semiconductor laser development. Recently alternative interband semiconductor diode laser technologies that have the potential to meet today's high power performance requirements have made groundbreaking progress. The interband cascade laser is one of the leading candidates in that particular spectral region and has recently been shown to operate continuously at room temperature, which is a significant milestone towards the widespread commercialization of the technology. In this work we are going to talk about the fundamental principles of operation of the interband cascade laser, its performance characteristics, and about the design and fabrication of an interband cascade external cavity laser that will impact the chemical sensing and free space communications industries. Our first goal for this work was to successfully develop a fabrication process for this new laser, based on a Gallium Antimonide (GaSb) material system. Based on our established Indium Phosphide (InP) process, we fine tuned our etching, dielectric deposition, metalization, annealing and mounting techniques in order to successfully produce ridge interband cascade lasers. Devices were then characterized by doing a series of length dependence experiments to determine critical physical parameters. Parameters like slope efficiency, threshold current density, internal loss, characteristic temperature, wall plug efficiency were extracted for different laser designs, as part of an optimization process of the quantum well structure of our interband cascade lasers. Lower cladding thickness, number of cascades, separate confinement heterostructure doping, heat dissipation and the confinement factor &Gamma, were found to be five of the most dominant effects affecting the device performance and high temperature operation. Based on our extensive dielectric coating experience for the near infra red, we were able to create new, broadband, anti reflection coatings specifically designed for the mid-IR. We created a simulation platform that designs double layer dielectric coatings. Based on the desired wavelength, spectral response, reflectivity and the modal nature of the laser's output beam, our program was able to predict coating designs, optimized for ultra low reflectivity. Our designs were grown in our e-beam evaporation facility, with very high control on the thicknesses and indices of refraction of the coating layers, using an in situ monitoring technique. Finally we were able to demonstrate wavelength tunability using our anti-reflection (AR) coated interband cascade lasers inside an external cavity configuration. This is the first demonstration of an interband cascade external cavity laser.Our tunable laser also demonstrated the widest to date tuning range of 300 nm (208 cm<super>-1</super>).