Threshold current calculations and optical cavity optimization for PbSe/PbSrSe multiple quantum well structures

Abstract

Threshold current is a key parameter in the design and proper operation of quantum well lasers. In this publication, threshold current analysis and calculations are done on four PbSe/Pb0.934Sr0.066 Se quantum well laser structures: SQW, SCH-SQW, MQW, and MMQW. The current work is a continuation to previous publications where energy levels, modal gain, optical confinement, and total losses were published for these four structures assuming the energy bands are non-parabolic. The threshold current as a function of total losses, cavity length, and cavity end mirror reflectivity was obtained for these structures. It is shown that the threshold current decreases with a decrease in the cavity length and then increases at a critical cavity length. The effects of non-parabolicity on the threshold current values are more obvious for short cavities and decreases with an increase in cavity. Whether the SQW or the MQW is the better structure depends on the loss level. At low loss, the SQW laser is always better because of its lower current density where only one QW has to be inverted. At high loss, the MQW is always better because the phenomena of gain saturation can be avoided by increasing the number of QW's although the injected current to achieve this maximum gain also increases. Owing to this gain saturation effect, there exists an optimum number of QW's for minimizing the threshold current for a given total loss. At this typical value, the effects of non-parabolicity on the threshold current values can be neglected without loss of accuracy. However, there is a 20% shift in the output lasing energy that cannot be neglected.