# High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer
## 3 Laser Performance
The linear dependence of \(1/\eta_{d}\) and cavity length is illustrated in Figure 9. The intercept of the linear fitted line is proportional to the internal quantum efficiency, and the internal loss can be deduced from the slope of the fitted line. With higher carrier injection efficiency benefiting from the built-in electric field of the composition-graded AlGaAsSb layer, the lasers exhibit 83% internal quantum efficiency and 3.9 cm\({}^{-1}\) internal loss even at 40 \({}^{\circ}\)C. A low and almost temperature-independent internal loss of 4-5 cm\({}^{-1}\) is observed as the heatsink temperature rises from 5 \({}^{\circ}\)C to 45 \({}^{\circ}\)C, which indicates that free carrier absorption is effectively reduced in our lasers. A 270 nm thick Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) waveguide layer and a 210 nm thick composition-graded AlGaAsSb layer on both sides of the active region lower the optical confinement factor of cladding layers. The temperature insensitivity of internal loss also suggests low free carrier absorption at a higher current injection level, which maintains high efficiency at a high operation current. The characteristic temperatures \(T_{0}\) and \(T_{1}\), signifying the exponential temperature dependence of the threshold current and efficiency, are determined by using the following expressions [33]:  
\[\ln I_{th}(T)=\frac{1}{T_{0}}\times T+\ln I_{th0} \tag{6}\]  
Figure 7.0  
where \(I_{th0}\) and \(\eta_{d0}\) are the ideal threshold current and slope efficiency at 0 K.  
Figure 7.1 Values of \(1/\eta_{d}\) with different cavity lengths. When lasers are in CW operation at 40 \({}^{\circ}\)C, the calculated internal efficiency assuming a cavity length approaching 0 is as high as 83%, and the internal loss is \(\sim\)3.9 cm\({}^{-1}\). Internal losses are maintained within 4–5 cm\({}^{-1}\) when the heatsink temperature ranges from 5 \({}^{\circ}\)C to 45 \({}^{\circ}\)C.  
An uncoated single emitter with a 3 mm cavity length was c-mounted on a copper heatsink for power-current performance characterization. The heatsink temperature was controlled and varied from 5 \({}^{\circ}\)C to 45 \({}^{\circ}\)C with a thermoelectric cooler (TEC). The data used for the threshold current and slope efficiency calculations were extracted from the linear interval of the power-current relation near the lasering threshold to minimize the error induced by active region heating. Figure 10 plots the temperature dependence of \(\ln{I_{th}}\) and \(\ln{\eta_{d}}\) when lasers were characterized in CW operation. \(T_{0}\) and \(T_{1}\) deduced from the slopes of linear fitted lines are \(\sim\)61 K and \(\sim\)213 K, respectively, while \(T_{0}\) and \(T_{1}\) deduced in QCW operation are \(\sim\)67 K and \(\sim\)224 K, respectively. The increase in the characteristic temperature is the consequence of heat removal, and an even higher characteristic temperature is expected when the experiment is conducted in PW operation. Sufficient carrier confinement to hinder carrier leakage caused by thermal carrier emission is essential for high-efficiency laser operation, and a high characteristic temperature confirms our design is an efficient structure.