Abstract

The study of mode-locking in generating short pulses began in the 1960s. Since then, the advances have been remarkable over almost 50 years and some of the mode-locked lasers (MLLs) have been commercialized. Short pulses from sub-picoseconds to femtoseconds have been successfully demonstrated from crystal and fiber based lasers. The diverse applications of MLLs have been pushing the development of MLLs in high bit rate transmission, optical time division multiplexed transmission, optical clock recovery, ultrafast signal processing and frequency comb, etc. Semiconductor lasers have advantages of simplicity, compactness and high efficiency. They have attracted interests in the application of optical communications. Until now, semiconductor MLLs are mainly based on bulk and quantum well (QW) structures. More recently, quantum dot (QD) based MLLs have attracted more and more attentions. The main characteristic of QD is the delta-function-like density of states with electrons confined in all three dimensions. It is promising in ultrashort and ultrafast pulse generations as a result of inhomogeneous gain broadening, broad gain bandwidth and fast carrier dynamics. In passive mode-locking, a two-section structure is usually used. A saturable absorber section is essential in the lasing cavity to initiate and shape pulses, which is also the case in almost all QD MLLs. However, without the absorber, passive mode-locking can also be achieved in single-section QD cavity, which has not been well studied yet. This thesis focuses on investigating single-section InAs/InP QD MLLs. It aims at improving the laser performance by both experimental and theoretical analyses. The following works have been done in this thesis.
Firstly, as an important parameter of all semiconductor lasers, the linewidth enhancement factor is measured using two methods: the Hakki-Pauli method that is used for a laser below threshold and injection-locking technique for a laser above threshold. The results from the two methods agree with each other, and it is found that the linewidth enhancement factor of our QD lasers is much smaller than that of QW based lasers.
Secondly, the time-domain travelling-wave model is used to investigate the single-section QD MLLs. By introducing an equivalent saturable absorber, the pulse generation and evolution are successfully simulated. Furthermore, this model is improved by including the effects of group-velocity dispersion (GVD) and self-phase modulation. It is found that the GVD effect plays an important role in the pulse width evolution of our mode-locked lasers. The improved model can be widely extended to other types of semiconductor lasers and amplifiers.
Thirdly, high-repetition-rate pulse trains of up to 1 THz are generated from a QD laser combined with fiber-Bragg-grating (FBG) external cavities. The QD laser is used for multi-mode gain and several specific modes are selected by the FBGs. The pulse train is measured by using the time-domain autocorrelator, and the repetition rate is in agreement with the frequency spacing of the FBGs. Finally, tunable terahertz beat waves of up to 2.1 THz are generated also using FBG external cavities. This method may find applications for generating microwave, millimeter wave and terahertz wave.