On-Chip Optical Stabilization of High-Speed Mode-locked Quantum Dot Lasers for Next Generation Optical Networks

by Abhijeet Ardey

Institution: University of Central Florida
Degree: PhD
Year: 2014
Keywords: Dissertations, Academic  – Sciences; Sciences  – Dissertations, Academic; Mode locking; optical pulse generation; quantum dot; four wave mixing; optical injection locking; timing jitter
Record ID: 2029697
Full text PDF: http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/6240


Monolithic passively mode-locked colliding pulse semiconductor lasers generating pico- to sub-picosecond terahertz optical pulse trains are promising sources for future applications in ultra-high speed data transmission systems and optical measurements. However, in the absence of external synchronization, these passively mode-locked lasers suffer from large amplitude and timing jitter instabilities resulting in broad comb linewidths, which precludes many applications in the field of coherent communications and signal processing where a much narrower frequency line set is needed. In this dissertation, a novel quantum dot based coupled cavity laser is presented, where for the first time, four-wave mixing (FWM) in the monolithically integrated saturable absorber is used to injection lock a monolithic colliding pulse mode-locked (CPM) laser with a mode-locked high-Q ring laser. Starting with a passively mode-locked master ring laser, a stable 30 GHz optical pulse train is generated with more than 10 dB reduction in the RF noise level at 20 MHz offset and close to 3-times reduction in the average optical linewidth of the injection locked CPM slave laser. The FWM process is subsequently verified experimentally and conclusively shown to be the primary mechanism responsible for the observed injection locking. Other linear scattering effects are found to be negligible, as predicted in the orthogonal waveguide configuration. The novel injection locking technique is further exploited by employing optical hybrid mode-locking and increasing the Q of the master ring cavity, to realize an improved stabilization architecture. Dramatic reduction is shown with more than 14-times reduction in the photodetected beat linewidth and almost 5-times reduction in the optical linewidth of the injection locked slave laser with generation of close to transform limited pulses at ~ 30 GHz. These results demonstrate the effectiveness of the novel injection locking technique for an all-on-chip stability transfer and provides a new way of stabilizing monolithic optical pulse sources for applications in future high speed optical networks.