Design and fabrication of high-performance interband cascade tunable external cavity lasers

Title of dissertation: DESIGN AND FABRICATION OF HIGH-PERFORMANCE INTERBAND CASCADE TUNABLE EXTERNAL CAVITY LASERS Athanasios Nicholas Chryssis, Doctor of Philosophy, 2010 Dissertation directed by: Professor Mario Dagenais, Department of Electrical and Computer Engineering The 3 4 μm 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 signi cant 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 rst 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 ne 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 e ciency, threshold current density, internal loss, characteristic temperature, wall plug e ciency were extracted for di erent 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 con nement heterostructure doping, heat dissipation and the con nement factor Γ, were found to be ve of the most dominant e ects a ecting 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 re ection coatings speci cally designed for the mid-IR. We created a simulation platform that designs double layer dielectric coatings. Based on the desired wavelength, spectral response, re ectivity and the modal nature of the laser's output beam, our program was able to predict coating designs, optimized for ultra low re ectivity. 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 antire ection (AR) coated interband cascade lasers inside an external cavity con guration. This is the rst demonstration of an interband cascade external cavity laser. Our tunable laser also demonstrated the widest to date tuning range of 300 nm (208 cm−1).