Nonlinear dynamics of long cavity semiconductor lasers: a study of dynamical regimes and temporal events

The aim of this thesis is to study the dynamical regimes and the temporal events of two different external long-cavity multi-longitudinal mode semiconductor lasers operating in fundamental transverse mode. The dynamical regimes of both systems are characterized by analyzing laser output acquired via experiments at several operating conditions in the form of discretized time-series using several standard statistical tools in temporal and Fourier domain. An existing ordinal pattern technique, known as, permutation entropy (PE) is used to determine the presence of underlying timescales to gain insights into the physics appropriately. The ordinal analyses can be further generalized into gaining an understanding of existing characterization techniques, thereby, opening opportunities to further develop protocols for using these existing tools.

The first system comprises an optically-pumped vertical external cavity surface emitting laser (VECSEL) with a semiconductor saturable absorber mirror (SESAM) acting as an intra-cavity saturable absorber to generate picosecond regime fundamentally mode-locked (FML) pulses at a repetition rate of 200 MHz. The system operates near the current lower limit of repetition rate for FML pulse generation. The undertaken non-traditional path of investigation, systematically extends the region of observation to laser emission wavelengths outside the FML regime, under optimal conditions of SESAM temperature and pump powers. An unanticipated and diverse range of dynamical outputs is exhibited by the system, of which, FML is reported to be one of the five possible dynamics. This is novel considering the existing studies conducted to-date focus explicitly on FML only. The exhibited dynamics, comprise both pulsed and non-pulsed regimes with two additional sub-regimes exhibiting destabilized pulsed outputs. Therefore, the study reported in this thesis, not only provides a new perspective on mode-locked systems, but, also adds to the limited existing knowledge about low repetition rate VECSELs. Such classification of dynamical regimes enables one to investigate the presence of complex dynamics in such systems. Availability of higher average output power outside the FML regime and a systematic shift of the observed regimes present opportunities for a wider wavelength tunability and higher power scalability in future.

Furthermore, the time-series of the acquired FML pulses is used to explore the possibilities of pulse characterization using PE. The correlation between pulse-amplitudes are investigated using PE to relatively quantify pulse-peak jitter. Additionally, standard statistical tools along with observations obtained using PE analysis, are used to identify possible sources of jitter and its contribution towards the PE observations. PE is also demonstrated to detect timescales having a periodicity of tens of microseconds. Such long timescales typically go unnoticed due to many reasons including limitations in sampling periods of time-series, and, low resolution and storage memory of data acquisition instruments. The transition of FML to different regimes are also interrogated using PE to uncover the presence of additional signature timescales to comprehend the factors regulating the spectral stability of mode-locking. 

The second system uses an electrically pumped quantum-well laser in an external cavity configuration with variable optical feedback to form a time-delayed optical feedback system. For the specific case of long cavity configuration, this system is analyzed using two variables the optical feedback level and the laser drive current. The sensitivity of semiconductor lasers to optical feedback has been known since the 1970s due to the rich variety of dynamical outputs spanning from a steady state to varying levels of stochasticity. However, there exists a long-standing gap between experimental observations and theoretical model. The thesis uses the travelling-wave model (TWM) which has been demonstrated to predict laser output over a low to high optical feedback levels. It is shown that an existing TWM-based computational model offering optical feedback as a variable is restricted in parameter space where it produces typically expected results. The thesis presents modifications to the existing computational model to increase the robustness of the model based on critically identified issues and widen the parameter space. The new version of the model is a step towards the ultimate goal of an all-inclusive stand-alone model that predicts laser output over extended range of parameter space and similar to experimental observations.