## 3 Optical Properties
### Edge Emitting Lasers
In the temperature range of 120-200 K, the temperature dependence of the threshold current shows an N-shape [71], which can be explained by the presence of QDs in a joint QWD/QD array in the laser-active region (Section 3.1), when temperature increases above 100-150 K, the carriers start to evaporate from QWDs and are captured to QDs having deeper localization potential. In other words, QDs cause parasitic radiative recombination. Thus, additional current is required to compensate for this carrier loss, which leads to a \"hill\" in the temperature dependence of threshold current. Figure 11: Below-threshold electroluminescence (**a**) and lasing (**b**) spectra of the lasers with different numbers of QWD layers. Figure 12: The dependences of modal gain on the injection current density for devices with different numbers of QWD layers. We note that stacking as many as 10 layers of QWD without any strain compensation, does not deteriorate the device performance (Table 2) as could be expected for QWs. To estimate the relative effect of non-radiative recombination centers on device performance, we studied the dependence of integrated electroluminescence intensity on injection current in lasers based on 1 and 10 QWD layers. The current component associated with the radiative recombination is characterized by a linear dependence of the integrated intensity on the injection current, whereas the non-radiative component is characterized by the quadratic dependence (Figure 14). At low injection levels, the non-radiative recombination determines the device characteristics. As the injection current increases, non-radiative recombination saturates and radiative recombination becomes the dominant one. The injection current, at which the behavior changes, depends upon the non-radiative centers' density. In Figure 14, the crossover point is the intersection of the linear (radiative, \(P\)\(\sim\)\(J\)) and quadratic (non-radiative, \(P\)\(\sim\)\(J^{2}\)) components. In both lasers, this current is quite similar (5 and 7 Acm\({}^{-2}\) for the laser based on 1 and 10 QWD layers respectively). Thus, stacking up to 10 QWD layers does not result in a significant increase in dislocation density, which agrees with low threshold current density in that laser. For achieving high-power operation, we have designed QWD lasers based on the CLOC (coupled large optical cavity) waveguide [72] and the active area based on 2 QWD layers. As seen from Figure 15, the devices based on 2 QWD layers provide the highest internal quantum efficiency, low internal loss, and low threshold current. The CLOC structures consist of a single-mode narrow passive waveguide optically coupled to a broadened active multimode waveguide. Due to the reduced optical confinement factor and increased optical losses, a selected parasitic transverse mode is eliminated from the lasing, ensuring transverse single-mode lasing. The active region consisting of two layers of InGaAs QWDs with 40 nm GaAs spacer was placed in the center of 1.3 \(\upmu\)m undoped GaAs waveguide separated  
Figure 14: Dependencies of integral electroluminescence intensity on the current density for lasers based on (**a**) 1 QWD layer, and (**b**) 10 QWD layers. Figure 13: Temperature dependence of the threshold current of QWD-based laser. from the 220 nm GaAs passive waveguide by the 250 nm Al\({}_{0.25}\)Ga\({}_{0.75}\)As layer. The waveguide was designed to provide the elimination of the second-order mode. The thickness of the \(p\)-Al\({}_{0.25}\)Ga\({}_{0.75}\)As cladding layer was reduced down to 450 nm in order to improve heat dissipation from the active region. The laser wafer was grown with MOCVD, processed into 100 \(\upmu\)m broad-area lasers in an abovementioned manner. The laser facets were covered by anti-reflecting (AR) and high-reflecting (HR) coatings and mounted \(p\)-side down on submounts. The broad area lasers with the cavity length of 4 mm showed CW output power of 14.2 W at room temperature (Figure 15a). Maximal wall-plug efficiency of 51.8% is achieved at 4.3 A.