Abstract: Quantum dot (QD) diode lasers are electrically pumped semiconductor lasers with quantum dots as the gain medium. Quantum dots (QDs) are nano-sized semiconductor objects embedded in larger band-gap semiconductor materials on a wafer. Because of strong confinement of carriers in the QDs, their energy levels are quantized and discrete. QDs have large differential gain and multiple gain-recovery times. Because of the size variation for different dots, a QD ensemble shows inhomogeneous broadening and has a broad gain bandwidth. In ultrashort-pulsed diode lasers, the pulse width is limited by both the gain bandwidth and a linewidth-broadening mechanism because of amplitude-phase coupling that occurs when the gain is due to quantum wells or other types of semiconductor material. Motivated by the unique gain properties of QDs and their broad gain bandwidth and small linewidth-broadening effect, I focused my research on passively mode-locked pulsed diode lasers with two sections. In such lasers, a picosecond bright pulse train can be obtained. Moreover, bistability in the lasing wavelength can be observed in two-section diode lasers. In this thesis, I reported the demonstration of the first dark pulse train in a passively mode-locked laser. This laser is an external-cavity, passively mode-locked QD diode laser with a saturable Bragg reflector as the absorber. I used the Haus model (a physical model to describe the pulse evolution in a mode-locked laser) and numerical simulation to show that a dark pulse is a stable solution in this laser\'s dynamics, as long as it includes gain and a saturable absorber. The multiple-gain dynamics of the QDs are the key to stable dark pulses in this laser. I also studied a nonlinear electrically pumped waveguide array with respect to waveguide coupling through evanescent wave coupling and carrier diffusion. I observed optical power oscillations in this device.
Keywords: Pure sciences, Dark pulse, Mode-locking, Optical bistability, Quantum dot semiconductors, Ultrafast lasers, Waveguide array, Quantum dots