## 3 Optical Properties
### Edge Emitting Lasers
The values of threshold current densities for QWD lasers are larger than those in QD lasers [1; 60; 67] but smaller than in QW ones [68]. It should be noted that for 5 and 10 QWD layers, \(J_{QWD}\) turned out to be almost the same. The dependence of the inverse differential efficiency (Figure 10c) on the cavity length was used to derive internal quantum efficiency and internal loss (Figure 10c). The internal losses increase from 0.7 to 2.4 cm\({}^{-1}\) with an increase in the number of QWD layers from 1 to 10, whereas the internal differential quantum efficiency (\(\eta_{i}\)) weakly depends on the number of QWD layers of and amounts to about 80%. It should be noted that in the case of the lasers based on 1 and 2 QWD layers, the differential efficiency dramatically deteriorates when the cavity becomes shorter than 325 \(\upmu\)m. In contrast, the lasers with 5 and 10 QWD layers do not show any drastic decrease in differential efficiency up to the shortest cavities of 125 \(\upmu\)m.  
The dependence of the modal gain on injection current (Figure 12) was calculated from the experimental dependence of threshold current density on external loss as described in detail in [69]. The maximal modal gain for the laser with 1 QWD layer of about 75 cm\({}^{-1}\) is achieved at 1900 Acm\({}^{-2}\). Higher gain values can be reached at higher currents, but the lasing switches to the electronic states of GaAs waveguide. The laser based on 2 QWD layer shows twice higher maximal modal gain (about 150 \(\,\mathrm{cm}^{-1}\)). For the lasers based on 5 and 10 QWD layers, the maximal modal gain was calculated to be 150 cm\({}^{-1}\) as limited by the minimal cavity length used in the experiments. One can expect higher \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Number of QWD** \\ **Layers** \\ \end{tabular} } & \multicolumn{2}{c}{**Threshold Current Density, A/cm\({}^{2}\)**} & \multirow{2}{*}{\begin{tabular}{c} **Lasing** \\ **Wavelength** \\ **(\(\lambda\)), nm** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Internal** \\ **Quantum** \\ **Efficiency (\(\eta_{i}\))** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Internal** \\ **Loss (\(\alpha_{i}\)),** \\ **cm\({}^{-1}\)** \\ \end{tabular} } \\ \hline
1 & 100 & 100 & 1088 & 0.79 & 0.7 \\ \hline
2 & 120 & 60 & 1092 & 0.85 & 1.0 \\ \hline
5 & 200 & 40 & 1108 & 0.82 & 1.5 \\ \hline
10 & 375 & 38 & 1117 & 0.78 & 2.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The characteristics of edge-emitting lasers based on a different number of QWD layers. The threshold current density and lasing wavelength are indicated for the cavity length 4 mm. Figure 10: The dependencies of laser characteristics on stripe length for devices with different number of QWD layers: (**a**) lasing wavelength, (**b**) threshold current density, and (**c**) inverse differential efficiency. values of the maximal modal gain in multiply stacked QWD samples, even taking into account that the modal gain does not necessarily increase proportionally to the number of QWD layers. Figure 13 shows a typical temperature dependence of the threshold current density of a QWD laser. In the temperature range of 220-320 K, the characteristic temperature \(T_{0}\) is 163 K. Typical characteristic temperatures of QD lasers with undoped active regions are 50-70 K, whereas the use of \(p\)-type modulation doping of active region allows one to increase it above 1000 K [60]. The characteristic temperature of the QW lasers typically slightly exceeds 100 K [70]. Note that QWD active region was undoped. Thus, the temperature stability of QWD lasers is comparable with the best QW lasers and exceeds that in QD lasers with the undoped active region.