TIME DOMAIN CHARACTERIZATION OF MONOLITHIC QUANTUM DOT PASSIVELY MODE-LOCKED LASERS
Passively mode-locked lasers based on InAs/GaAs quantum dots have benefited from the unique properties pertaining to this material system, leading to the demonstration of wide mode-locking operational maps, and reconfigurable repetition rates, as well as low rms timing jitter. Applications of these passively mode-locked lasers include optical clock distribution, the generation of RF signals and high bit rate optical time division multiplexing. In addition to their utility for terrestrial applications, quantum dot mode-locked lasers have the strong potential to support applications in intra-satellite data transmission. Owing to their compact size and low power consumption properties, coupled with the potential to achieve enormous aggregate bandwidth from a single transmitter, desirable size, weight and power (SWaP) metrics can be achieved while simultaneously increasing the capacity. Supporting applications in space data transmission architectures requires a strong understanding of the evolution of the device characteristics over a broad range of operating conditions. The temperature-dependent operation of a passively mode-locked laser typically relies on the mutual interdependence of the saturable absorber and amplifying gain section in a two-section device, and therefore it is not readily apparent how these devices will perform over broad temperature excursions. In this dissertation, a detailed study is presented on a series of quantum dot passively mode-locked lasers with variable absorber to gain-section length ratios. Inputs into an analytical model used for predicting regions of mode-locking stability for a given cavity geometry, are derived from measurements of modal gain and absorption on a multi-section single pass emitter. The effects of temperature on the operational range of pulses emitted from the quantum dot ground and excited states are experimentally examined on a set of two-section mode-locked lasers having variable absorber lengths. A comparison is drawn between the experimental observations and the analytical model predictions. It is found that the model correctly predicts the temperature of maximum operability in each of the devices studied for a variety of absorber voltages. Prediction of the regimes of excited-state operation from the quantum dots is also included and experimentally verified. The quality of pulse generation from pure ground-state operation, pure excited-state operation and a simultaneous lasing of ground and excited states is examined. For the first time, the unsaturated absorption is identified as a key parameter that strongly influences the range of biasing conditions that produce stable mode-locked pulses. This is shown to be directly responsible for improvement in mode-locking characteristics at elevated temperature; a previously observed effect that was not well understood. Finally, while the range of pulsed operation from a semiconductor mode-locked laser can be determined using a digital sampling oscilloscope or an auto-correlator, true verification of mode-locking stability requires simultaneous measurements of the temporal and frequency domains. In this dissertation we examine device characteristics with a Frequency Resolved Optical Gating (FROG) pulse measurement system. This allows for direct measurement of pulse asymmetry and chirp. This measurement technique is used to examine the evolution of device characteristics with increasing temperature, whereby the time bandwidth product over temperature is studied. Additionally, FROG is used to examine a regime of operation where non-linear double pulsing occurs (two pulses per round trip). It is shown for the first time that the observed double pulsing is in fact a stable effect, thus mode-locked operation at twice the fundamental repetition rate can be reliably achieved by simply electrically biasing the device in the appropriate manner. This data set offers valuable insight into to design of future mode-locked laser devices for maximum optical pulse quality over a large range of temperature and biasing conditions. Furthermore, the results are promising for the development of temperature-insensitive pulsed sources for uncooled applications such as data multiplexing and optical clocking; this is particularly attractive for space applications as active cooling consumes a large portion of the power budget.
Semiconductor Laser, Mode-Locked Laser, Quantum Dot
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