Generating picosecond pulses from Q-switched microchip lasers
thesisposted on 28.03.2022, 23:27 by Alexander Craig Butler
Since their first demonstration in the 1960s, picosecond laser pulses have initiated and defined the area of ultrafast laser physics. Their ability to generate a burst of high-intensity, coherent light has opened up areas of science and enabled the observation of events that would otherwise remain out of the realm of human understanding. -- The particular mechanism commonly associated with picosecond pulse generation is mode-locking. These systems are typically complex, bulky and correspondingly expensive. A compact, cheap and robust option that produces comparable pulses would be of great value to the scientific and engineering communities. To this end, we examine the growing area of Q-switched microchip lasers: simple and compact devices that generate sub-nanosecond pulses by virtue of extremely reduced resonator cavity lengths. Incorporation of a passive Q-switch device, a semiconductor saturable absorber mirror or SESAM, enables truly minimal cavity lengths, and therefore minimal pulse durations, to be accessed. -- In this thesis we explore the limits of generating the shortest pulses from such microchip lasers. We develop a comprehensive numerical simulation model, based on the laser rate equations, to effectively model SESAM Q-switched microchip lasers. We incorporate additional phenomena such as two-photon absorption (TPA) in the Q-switch and SESAM etalons to derive a complete picture of the abilities of these micro-lasers. We show that TPA will increasingly affect the performance of these lasers, as shorter pulses are generated, and suspect that our model underestimates its effect on our experimental results. We examine the switching dynamics of the SESAM and describe the role of relaxation oscillations in the switching process, as well as demonstrating controlled partial switching. Scaling dependencies between laser component parameters and laser performance are drawn, providing guidelines for development of these lasers. We examine these relationships in experiment and verify the key relationship between short cavities and short pulses. We demonstrate a laser with record short pulses of 22 ps duration by extending the scaling of these lasers to the shortest demonstrated cavity length of 110 μm. -- To alleviate the low efficiency associated with lasers using thin gain media, we propose, model and develop an energy-scavenging amplification scheme. We demonstrate that a complete, amplified Q-switched microchip laser system has the potential to rival amplified mode-locked systems in generating few-picosecond, microjoule pulses, although practical validation of this will approach require careful laser engineering.