Properties of buried heterostructure single quantum well (Al,Ga)as lasers

Abstract

Unlike conventional semiconductor lasers, single quantum well (SQW) lasers with high reflectivity end facet coatings have dramatically reduced threshold currents as a result of the smaller volume of the (active) quantum well region. A cw threshold current of 0.55 mA was obtained for a buried graded-index separate-confinement heterostructure SQW laser with facet reflectivities of ~80%, a cavity length of 120 µm, and an active region stripe width of 1 µm. This is believed to be the lowest threshold current so far reported for any semiconductor laser at room temperature. The submilliampere threshold currents of these lasers allow them to be modulated at high speed without any current prebias or feedback monitoring. The relaxation oscillation frequency for these lasers was also measured. Values of differential gain derived from these measurements demonstrated that the differential gain in the uncoated lasers is less than in the coated devices. This result was expected because of gain saturation. As predicted, SQW lasers have substantially narrower spectral linewidths than bulk double heterostructure lasers. This result is attributed to lower internal loss, linewidth enhancement factor, and spontaneous emission factor. A further major reduction (>3x) in the linewidth of these SQW lasers was observed when the facet reflectivities were enhanced. This observation is explained theoretically on the basis of the very low losses in coated SQW lasers and the value of the spontaneous emission factor.