Pulsations in AlxGa1−xAs buried heterostructure lasers caused by the heating of defects

Abstract

The onset of pulsations in buried heterostructure lasers was investigated with a two-pulse method. First, an above threshold pulse was applied to reversibly convert the laser from a nonpulsating to a pulsating state. The pulsating state persists for several hundred nsec. A second pulse, applied 50 nsec or more after the first pulse, was then used to study the pulsations and other associated changes in the laser. In this way, a laser which initially does not even exhibit observable relaxation oscillations was changed into a laser which has sustained pulsations with a period of 1.8 nsec. These studies provide evidence that pulsations are caused by defects locally heated by absorbed laser light. Measurements of spontaneous emission intensity, laser threshold, and quantum efficiency showed that the loss in the laser cavity increased when the laser was converted into the pulsating state. The minimum loss increase observed to cause pulsations was about 13–19 cm−1. The characteristic time for the increase in loss to rise and fall was 100–200 nsec. These changes were nonexponential with both long- and short-lived components. Both the time scale and the nonexponential behavior are expected for the increased absorption loss associated with the heating of nonradiative defects. Calculations of defect heating predicted average temperature rises of 4–12 K, depending on defect size. It is estimated that a temperature rise of 7 K could halve the defect size necessary for pulsations. The increased loss correlated with the laser intensity during the first pulse, indicating that the heating is due primarily to the absorption of laser light, rather than to injected current. Local heating will promote pulsations by increasing the optical absorption and the change in gain with carrier density.