Performance of lasers containing three, five and seven layers of quantum dots

The authors have analysed the performance of quantum dot lasers containing three, five and seven layers of InGaAs quantum dots. At room temperature the threshold current density of devices of length between 320 μm and 2000 μm, with uncoated facets, decreases with device length and is lower for higher layer numbers. They explain this behaviour by analysis of measured modal gain spectra, internal optical mode loss and the temperature dependence of the threshold current density. They find that increasing the number of dot layers can improve device performance because the optical mode loss does not increase while the modal gain at a particular wavelength does increase with more dot layers. However, at low temperatures <200 K they find that the three-layer structure exhibits the lowest threshold current density for devices of length 550 μm and longer. They conclude that the improved threshold current behaviour of the higher layer number devices at room temperature is not primarily due to intrinsic gain saturation but because the required modal gain can be achieved in high layer number samples with a lower quasi-Fermi level separation and hence a reduced population of higher lying energy states