The Influence of the Gain—Carrier Density Characteristic on Q-Switching in Quantum-Dot Lasers

A comparison is made between the Q-switched performance of both quantum dot and quantum well lasers operating in the 1-mum emission range. In contrast to the 2-D system, the quantum dot devices readily produce high-power pulses, but this is associated with significant pulse broadening such that the minimum pulsewidth is in excess of 1 ns. Comparison of the experimental results with a simple rate equation model shows that the Q-switching characteristics of the dot lasers are due to their operation close to the point of total inversion of the charge carrier populations. The resulting severe gain saturation, which drives the differential gain close to zero, significantly modifies the ratio of differential absorption to differential gain, which is known to govern the Q-switching characteristics of a laser diode. Measurements on the absorption properties of the dots show that there is no change in their absorption coefficient under a reverse bias, and correspondingly, there is minimal alteration of the Q -switch pulse parameters.