## 3 Optical Properties
### Edge Emitting Lasers  
In this section, we describe properties of edge-emitting lasers based on QWDs. The active area of semiconductor laser diodes, as well as device design, should be optimized for certain applications. For instance, high power QW lasers usually contain one or two QWs in the active area because the devices should have low internal losses. In the case of QD lasers, more QD layers (typically 5-10) are required to avoid gain saturation and switching to excited state lasing at high injection currents [60]. QWDs are an intermediate case between QWs and QDs, so the number of QWDs layers in the active area should be optimized. The laser wafers with different numbers of QWDs in the active region (from 1 to 10) were grown by MOCVD on the GaAs substrates misoriented on 6\({}^{\circ}\) toward [111] direction. Each QWD layer was formed by the deposition of 8 ML of In\({}_{0.4}\)Ga\({}_{0.6}\)As. The QWD sheets were separated with 40 nm thick undoped GaAs spacers. The laser structures have undoped GaAs waveguides with a thickness of 0.68 \(\upmu\)m not exceeding the third mode cut-off. The active region locates in the center of the waveguide, which ensures lasing on the fundamental transverse mode. The waveguide was sandwiched between \(p\)-type and \(n\)-type Al\({}_{0.4}\)Ga\({}_{0.6}\)As claddings having the thicknesses of 0.75 \(\upmu\)m and 1.5 \(\upmu\)m, respectively. For reducing the internal loss, the claddings doping levels of \(2\times 10^{18}\) cm\({}^{-3}\) were reduced down to \(7\times 10^{17}\) cm\({}^{-3}\) in the vicinity of the waveguide. The wafers were processed into broad-area lasers with 100-\(\upmu\)m-wide shallow-mesa ridges with etching through the \(p\)-contact and partly through the \(p\)-cladding layers. The individual laser chips were mounted \(p\)-side down on copper heatsinks using an indium solder. No facet coatings were used. The dependencies of laser characteristics on stripe length for devices with a different number of QWD layers are shown in Figure 10a-c. An increase in the number of QWD layers from 1 to 10 leads to a red-shift of the lasing wavelengths (from 1088 nm to 1117 nm in laser diodes of the longest cavity length), see Table 2. The red-shift was also observed in the electroluminescence spectra (Figure 11a). Such effect was not revealed in case of stacking strain balanced QWs [7; 61; 62], where peak positions do not depend on the number of the layers. We attribute this effect, at least partly, to the elastic strain redistribution in the multilayer QWD medium, similar to the case of stacking several layers of self-organized QDs [63; 64; 65]. A decrease in the cavity length leads to a blue-shift of the laser wavelengths. With decreasing cavity length output loss, increase and higher injection currents are required to achieve lasing. Since the maximum of gain spectra shifts to shorter wavelength with increasing injection current, the lasing wavelength also experiences a red-shift. The smaller the number of QWD layers, the larger the shift. For instance, as the cavity shortens from 4 mm to 0.25 mm, the wavelength changes by 20 nm in the case of 10 layers of QWDs, while for the laser based on single QWD layer, the wavelength shift is three times stronger (60 nm). In the case of the laser based on one QWD layer, the lasing line switches from the wavelength, which corresponds to the QWD ground state, to the wavelength of GaAs waveguide when cavity length decreases down to 200 \(\upmu\)m. No lasing switching from the ground state to higher-energy states was observed for the devices with 2, 5, and 10 QWD layers. The threshold current density (\(J_{th}\)) increased from 100 to 375 Acm\({}^{-2}\) with an increase in the number of QWD layers from 1 to 10 (Figure 10b) that is typical for lasers based on both QWs and QDs [66]. Correspondingly, the threshold current density normalized to the number of the QWD layers (\(J_{QWD}\)) decreased from 100 to 38 Acm\({}^{-2}\).