Abstracts Communications and the Arts

GaN-based distributed feedback laser diodes and their applications

by Steffan Gwyn

In recent years, there has been an increasing push to develop single frequency laser diodes (LDs) operating in the visible region of the electromagnetic spectrum, arising from the various potential applications, most notably in atomic cooling, visible light communication (VLC), medical diagnostics, and spectroscopy. This thesis investigates Gallium Nitride (GaN) based laterally coupled distributed feedback (LC-DFB) LDs, and their properties. The design, fabrication, and experimental characterisation of GaN LC-DFBs is discussed, and several parameters important in quantifying device quality, including spectral characteristics, dynamic response, and spectral linewidth, are measured. The results for DFBs are directly compared to commercially-available Fabry-Pérot (FP) LDs, with DFBs demonstrating improved modulation characteristics and spectral precision compared to FPs, while emitting light at similar optical powers. Recent fabrication improvements has also improved threshold conditions of GaN DFBs, exhibiting similar threshold currents to commercially-available FPs. Additionally, research was conducted into one potential commercial application for GaN LC-DFBs, that being filtered optical communications. Through the implementation of optical filters, it is demonstrated that solar rejection can be achieved, and multi-gigabit data transmission is reported with an interfered signal. These results suggest the strong potential for terrestrial free-space communication using GaN DFBs in a wavelength division multiplexing (WDM) system, which can greatly increase data transmission rates in a communication system, as well as an underwater system with assets communicating using GaN-based DFBs. This thesis presents the highest modulation bandwidth and data transmission rates achieved in a directly modulated GaN DFB system, as well as the lowest threshold currents reported. The results here highlight the potential of GaN DFBs for several applications, and the need for further development of this technology in order to realise commercially viable systems using GaN DFBs.