Quantum Well and Quantum Dot broadband Optical Devices for Optical Coherence Tomography Applications

In this thesis quantum well (QW) and quantum dot (QD) based devices are investigated with the aim of obtaining broad bandwidth light sources for optical coherence tomography (OCT) applications. QD based structures have many possible advantages for broadband applications due to their inhomogeneous broadening. However, more investigation is required in order to fulfill this potential. Firstly, in chapter one, an introduction to the fundamental principles of semiconductor heterostructures is provided followed by basic concepts of OCT. The experimental techniques used in this thesis are outlined and briefly discussed. Brief reviews of the gain measurement techniques which have been used throughout this thesis are presented. Free carrier effects have been highlighted as a source of line-width broadening in QD structures. However, to date the effects of free carriers have mostly been experimentally determined at comparatively high carrier densities. In chapter 2 I develop a model for the gain and spontaneous emission spectra of QD active elements and show that not only are free carrier effects important at high QD occupancies, but also at much lower carrier densities where QD lasers would normally operate. Furthermore, it is shown that the choice of carrier distribution function is far less important than was previously thought in describing the experimentally observed gain and spontaneous emission spectra. The literature has suggested that incorporating QW layers in hybrid QW/QD structures changes the behaviour of the QDs. Optical pumping of the QD active element by emission from the QW active element is investigated experimentally in chapter 3. Analysis of a QD laser, a hybrid QW/QD super luminescence diode (SLD) and mesa diodes with different active element designs show that emission from the quantum well layer does indeed modify the QD spontaneous emission, suggesting optical pumping of the QD states and the prospect for enhanced gain from the QD ground-state. Finally in chapter 4, different configurations of swept light sources (SLSs) are implemented with the aim of obtaining broader spectral bandwidth. It is demonstrated that increasing the gain of the QD-SOA is important in enhancing the sweep range. The use of complimentary SOAs is then explored. InP QW-SOAs and GaAs based QD-SOAs have overlapping gain and SE spectra which is utilised in a swept source laser (SSL) and filtered ASE configuration SLS. The results suggest that such sources may be able to achieve ~220nm sweep bandwidths. Chapter 5 summarizes the whole thesis and provides an overview of future work.