Sub-Terahertz Emission in Quantum Dot Semiconductor Lasers: a Delayed Differential Equation Approach

Optical devices based on Quantum Dot (QD) structures have gained growing attention in recent years due to the expected superior properties with respect to their bulk counterparts, thus increasing the need for efficient modeling tools to analyze and predict their performances. Mode locked QD lasers are a common example of devices relying on zero-dimensional confinement with a large number of applications, ranging from the biomedical world to communication systems, with pulse repetition frequencies reaching tens or even hundreds of gigahertz. They can also be employed in the realization of compact RF signal sources generating ultra-short optical pulses at high repetition rates, providing appealing features in terms of size, power consumption and bandwidth from a sub-terahertz transmission system perspective. This represents a key research topic in Telecommunications, given the growing interest in high frequency bandwidth and 5G or 6G systems. In this Thesis, a numerical analysis of quantum dot based edge emitting and ring lasers sources for RF applications is carried out with an efficient and reliable Multi-Section Delayed Differential Equation (MS-DDE) model. Passive and harmonic mode locking techniques can be used to generate high-quality ultra-short pulses with high repetition rates from different types of lasers, and such model allows to perform simulations of edge-emitting and ring devices by partitioning the total cavity length into slices, where a set of rate equations are solved through a DDE approach. A pre-existing version of the MATLAB program implementing the MS-DDE model has been rewritten in order to improve its computational efficiency, with simulation times reduced of almost one order of magnitude. Most efforts have then been aimed at the identification of a working set of parameters allowing to achieve a stable harmonic mode locking regime in the sub-terahertz range, subsequently characterizing the final devices with some key performance indicators extracted from the simulations. This fulfills the initial goal of providing a practical tool for the feasibility assessment of QD based laser sources working in passive or harmonic mode locking regimes: the model can be exploited in the early stages of a design process for these types of devices, providing accurate results with a reduced time overhead when compared to other approaches that makes it suitable for extensive parametric simulations as a function of external bias and/or structural parameters.