|dc.description.abstract||As a major component of optical transmitters, directly-modulated semiconductor lasers are widely used in today’s fiber optical link systems by taking its advantage of their low cost, compact size and low power consumption. In this work, techniques to improve the high frequency modulation characteristics of semiconductor lasers with a low-dimensional active region medium, specifically quantum dots (QDs), are studied. These techniques include a p-doped active region in single-section QD lasers, the gain-lever effect in two-section lasers and the injection-locking technique.
Firstly, the modulation performances of p-doped InAs/GaAs QD lasers were studied. Contrary to the theoretical predictions, the modulation efficiency and the highest relaxation frequency of 1.2-mm cavity length lasers decrease monotonically with the p-doping level from 0.54 GHz/mA1/2 and 5.3 GHz (un-doped dots), to 0.46 GHz/mA1/2 and 3.6 GHz (40 holes/dot). Although the maximum ground state gain of the p-doped lasers is increases with p-type concentration, the undesired increase in internal losses induces stronger gain saturation and gain compression, thus degrading the high-speed performance. The degradation of the modulation performance of the p-doped device is also attributed to a higher gain compression factor due to the carrier heating effect..
Secondly, the gain-lever effect is studied in two-section QD lasers in order to enhance the modulation efficiency and 3-dB bandwidth. An 8-dB modulation efficiency enhancement is achieved using the p-doped QD laser. Due to the stronger gain saturation with carrier density, it is found that un-doped QD devices show a more significant gain-lever effect over p-doped devices. A 20 dB enhancement of the modulation efficiency is demonstrated by the un-doped QD laser. A new modulation response equation is derived under the high photon density approximation, and a 1.7X 3-dB bandwidth improvement is theoretically predicted by the new model and realized in an un-doped QD gain-lever laser under extreme asymmetric pumping conditions. It is also demonstrated for the first time that the 3-dB bandwidth in gain-lever laser can be 3X higher than the relaxation frequency instead of 1.55X in typical single-section lasers.
Finally, injection locking in QDash lasers was analyzed. By varying the power injection ratio and detuning, the modulation bandwidth of a 0.5-mm QDash Fabry-Perot slave laser by 4 times. By analyzing the curve fitted data, it was observed that the inverted gain-lever modulation response equation can approximate the injection-locking system in the Period 1, non-linear regime. Based on the gain-lever model, an analytical expression for the relaxation frequency of an injection-locked laser is derived, and the maximum achievable 3-dB bandwidth is predicted and verified experimentally.||en_US