Vertical Coupling Effect on Gain Bandwidth of Chirped InAs/InP Quantum Dot Structures

Quantum dot (QD) devices are usually desired to have a broadband gain spectrum. An alternative solution to achieve a broadband gain in QD devices is to use multiple layers with different QD heights, which are stacked vertically, i.e. a chirped QD structure in the active region. In the chirped stacked QD structure, the vertical strain and electron coupling effect have a significant impact on the optical transition property and thus optical gain bandwidth. However, previous studies on the vertical coupling effect have mainly focused on uniformly stacked QD structures, and the chirped QD structures have not been investigated carefully. This work presents a detailed analysis of the vertical coupling effect in chirped QD structures (i.e. ascending and descending chirped structure) and its impact on the optical gain bandwidth of the active region. It is found that the descending chirped structure leads to a wider gain bandwidth, in particular at high current injection. A Fabry-Perot mode-locked laser with the descending chirped structure presents a better performance in pulse width and frequency comb lines compared to the ascending chirped structure.