# High-Power, High-Efficiency GaSb-Based Laser with Compositionally Linearly Graded AlGaAsSb Layer
## 3 Laser Performance
The low turn-on voltage demonstrates that the adoption of the composition-graded AlGaAsSb layer effectively reduces the voltage penalty and improves the performance of GaSb-based lasers. The threshold current is 230 mA, corresponding to a threshold current density of 121 A/cm\({}^{2}\). Excellent threshold parameters, e.g., a low turn-on voltage and a low threshold current density, ensure the high-efficiency operation of our lasers. Figure 6: Energy band diagram and refractive index profile of InGaSb/AlGaAsSb diode lasers. The potential barrier peak of the Al\({}_{0.5}\)Ga\({}_{0.5}\)As\({}_{0.04}\)Sb\({}_{0.96}\)/Al\({}_{0.25}\)Ga\({}_{0.75}\)As\({}_{0.02}\)Sb\({}_{0.98}\) heterojunction is flattened after the introduction of the composition-graded AlGaAsSb layer, and a low series resistance of 0.144 \(\Omega\) is obtained. The slope efficiency decreases from \(\sim\)0.32 W/A with the increasing operation current, which is caused by heat accumulation in the active region when lasers are characterized in CW operation. As the actual temperature of the active region rises, the threshold current for lasering rises and the internal quantum efficiency is reduced, resulting in a low power conversion efficiency [30]. Due to the optimized mounting process, the heat generated in the lasers is dispersed quickly, ensuring a high slope efficiency in a wide operating current range and delaying output power saturation caused by thermal rollover. As a result, a high output power of 1.8 W is achieved at 8 A, while a high PCE of 28% is obtained at 1.1 A. Though PCE decreases with a higher current, it remains more than 12% at 8 A. The wavelength and FWHM of the laser operating at 3A are 2006 nm and 6.15 nm, respectively, since no additional longitudinal mode selection structure is adopted in our FP laser. A 0.95 cm laser bar containing 19,500 \(\upmu\)m wide emitters was 5% anti-reflection- and 95% high-reflection-coated and then mounted on a microchannel heatsink. Since the emitters in the laser bar are in parallel, the laser bar exhibits low-voltage, high-current, and high-power output performance. Nineteen emitters operating in parallel generate massive heat, and the heat distribution causes horizontal and perpendicular thermal stress in the laser bar and a consequent \"smile\" [31]. A microchannel heatsink with low thermal resistance and high heat flux is utilized to relieve heat accumulation and thermal stress. The laser bar with a 40% fill factor is characterized in CW operation at 15 \({}^{\circ}\)C. The CW performance is plotted in Figure 8. A record-high CW output power of 18 W is achieved at 75 A.  
Figure 8: CW performance of 0.95 cm laser bar with 40% fill factor measured at 15 \({}^{\circ}\)C. High output power of 18 W is obtained at 75 A.  
Figure 7: CW PIV characteristic of laser with 1 mm cavity length and 200 \(\upmu\)m wide ridge measured at 15 \({}^{\circ}\)C. High output power of 1.8 W and high maximum PCE of 28% is obtained. Wavelength and FWHM of laser operating at 3 A are 2006 nm and 6.15 nm, respectively. As expressed in Equation (5) [32], internal loss and internal quantum efficiency were determined with 1 mm, 1.5 mm, and 2 mm long uncoated lasers. \[\frac{1}{\eta_{d}}=\frac{q}{\hbar\omega\eta_{i}}\left(1+\frac{\alpha_{i}}{\ln \left(\frac{1}{R_{1}R_{2}}\right)}2L\right) \tag{5}\]  
where \(\eta_{d}\) is the total slope efficiency, which is 2 times the slope efficiency measured from a single facet, \(q\) is the electric charge of a single electron, \(\hbar\omega\) is the energy of a single emitted photon, \(\alpha_{i}\) is the internal loss of the laser, \(L\) is the cavity length of the laser, and \(R_{1}\) and \(R_{2}\) are the reflectivity of each facet, which is \(\sim\)35% for uncoated GaSb-based lasers. In order to eliminate the interference of heat accumulation with the laser efficiency, the characterization of emitters was conducted in QCW operation, and \(\eta_{d}\) was calculated according to the linear interval of the power-current relation near the lasering threshold.