Bistability, High Speed Modulation, Noise and Pulsations in GaAlAs Semiconductor Lasers (2006)

This thesis describes seven different subjects relevant to semiconductor laser diodes which fall in the following three categories: Bistability and pulsations, high speed modulation and noise. Bistable semiconductor lasers based on inhomogeneous current injection, achieved with a split contact scheme, were proposed around 20 years ago. However, actual devices showed no or only a small hysteresis and they were in addition beset by pulsations for reasons not well understood at the time. In this thesis we show that lasers with an optimized design can display bistability with a giant hysteresis. Crucial to the understanding of the bistable laser is a negative differential resistance across the absorber section, reminiscent of a tunnel diode characteristic. Depending on the electrical biasing circuit this negative differential resistance leads to bistability or light-jumps and self-pulsations. A simple model based on the conventional rate equations explains the observed behavior. Investigation of the switching dynamics of this optoelectronic device reveals a delay time which is critically dependent on the trigger pulse amplitude and which is typically in the order of a few nanoseconds at a power-delay product of 100pJ. We also investigate the characteristic of this laser coupled to an external optical cavity and we demonstrate that this bistable laser can be used as a self-coupled stylus for optical disk readout. Under a different biasing condition this laser coupled to an external optical cavity can be used to generate ultrashort optical pulses through passive mode locking. Unlike previous mode locking techniques, the method presented here does not rely on absorption introduced by damaging the crystal and is consequently much more reliable. The high speed modulation behavior of semiconductor lasers is investigated theoretically and experimentally. In this thesis we derive the fundamental limits of injection lasers for pulse modulation and small signal modulation. We place emphasis on developing laser structures optimized for high frequency modulation and experiments on such structures show that they can be modulated at frequencies up to 8GHz. At these frequencies the parasitic elements can no longer be neglected and they are included in the analysis. The noise equivalent circuit of a semiconductor laser diode is derived from the rate equations including Langevin noise sources. This equivalent circuit allows a straightforward calculation of the noise and frequency modulation characteristics of a laser diode combined with electronic components. The field spectrum of injection lasers is observed also experimentally. It is a unique feature of injection lasers that their linewidth is increased through a strong amplitude phase coupling by the factor (1 + α2) where α is the linewidth enhancement factor. A model is developed which shows that the same factor α enters in the small signal modulation characteristics and a careful measurement of the small signal amplitude and phase modulation at high frequencies enables us to obtain this important factor α for the first time by a direct measurement.