Title:  Applications of nonlinear dynamics in semiconductor lasers with time-delayed feedback in microwave photonics

Committee:

Dr. David Citrin, ECE, Chair, Advisor

Dr. Alexandre Locquet, ECE, Co-Advisor

Dr. Paul Voss, ECE

Dr. Jennifer Hasler, ECE

Dr. Doug Yoder, ECE

Dr. Nico Declercq, ME

Abstract: The main objective of this research is to investigate the rich nonlinear dynamics of a semiconductor laser diode (LD) subjected to time-delayed optoelectronic (OE) feedback, emphasizing applications in microwave photonics and communications. A semiconductor LD based OE feedback constitutes an oscillator that produces self-sustained optical output modulation through the intrinsic nonlinearities of the system without needing any external modulators. To explore the wide variety of dynamics in the optical intensity, the LD needs to be perturbed out of the steady-state free-running behavior, so the photodetected optical signal is appropriately amplified prior to feeding it back into the LD injection terminal. The complex dynamics of such an oscillator have been studied theoretically and experimentally in recent decades. In this work, however, we report several novel dynamical effects by re\"{e}xamining this rich nonlinear system with state-of-the-art experiments and supported that by comprehensive modeling. In particular, we have identified operating conditions that exhibit high-order locking between LD relaxation oscillations with harmonics of the feedback delay frequency for a OE feedback with large delay. We also observe that this system exhibits a stepwise change in LD oscillation frequency as the feedback level is varied. Further, upon varying the injection current near threshold, we also can generate a periodic pulse train with repetition rate at the feedback delay frequency arising from gain-switching between the on and off states of LD. This pulse train grows into pulse clusters as we increase the current. In addition, driving an LD at very high currents and strong feedback results in square-wave pulses whose repetition rate is determined by the feedback delay of the OE loop. The square-waves at a fixed current have been shown to exhibit a double-peaked optical spectrum that depends on the feedback level. These interesting discoveries advance the understanding of the nonlinear OE oscillator and could find applications in communications, sensing, measurement, and spectroscopy.