# Recent Advances in Tunable External Cavity Diode Lasers
## 3 Research Progress of ECDL
### External Cavity Quantum Dot Laser  
Since the first demonstration of the external cavity quantum dot laser (ECQDL), significant advancements have been made in its performance metrics, including higher photoelectric conversion efficiency and improved high-temperature stability. These improvements are largely attributed to the quantum effects of quantum dots. Moreover, lower threshold current output can be achieved due to the size fluctuation of quantum dots. The size fluctuations of self-assembled quantum dot structures are unavoidable [49]. The variation in size induces spectral broadening, resulting in a very wide half-width of the  
Figure 5: Design of the laser cavity. A periscope, etalon and narrow band pass filter was placed on a rotating stage [48]. ©OSA. Copyright 2023 Optics Express. EL spectrum, which is affected by the growth process of quantum dots. Growth interruption is an important process to promote material redistribution and atomic diffusion. Controlling the AsH\({}_{3}\) flow velocity of the growth interruption process has an important effect on the morphology and distribution of quantum dots [50]. When QDs were grown at the AsH\({}_{3}\) flow rates of 3.9 \(\times\) 10\({}^{-6}\) mol/min (structure A) and 2.0 \(\times\) 10\({}^{-5}\) mol/min, the ground state has higher optical gain at high flow rate. Meanwhile, the density of states of the quantum dots in laser is relatively low, which leads to the ground state being more easily saturated, while the excited state can be filled at a comparatively lower current density [51]. Combining quantum dot lasers with a grating-coupled external cavity system, a wide-tunable, low-threshold, external cavity semiconductor laser can be achieved by rotating the grating [52]. In 2000, a grating-coupled external cavity quantum dot laser achieved tuning over a range of 201 nm with a maximum bias current of 2.87 kA/cm\({}^{2}\)[53]. Researchers have made significant efforts to optimize the external cavity quantum dot laser's performance. One approach focuses on enhancing the chip performance by modifying the structure and growth conditions of quantum dots to improve the output light characteristics of ECQDLs. For example, a QD gain device incorporating chirped multiple QD active layers and a bent waveguide structure was developed [54]. The chirped multiple QD active region consisted of 11 QD layers and different thicknesses of In\({}_{0.15}\)Ga\({}_{0.85}\) as a strain-reducing layer. When the QD diode laser was coated with an anti-reflection film and used as a gain chip, the bent waveguide structure can effectively suppress the laser mode in ECQDL. The experimental results showed that the line width is about 1 nm. A tuning range of 201.7 nm (from 1038.3 nm to 1246 nm) was obtained. The cavity length of chips in an external cavity diode laser has a significant impact on its characteristics, such as tunable range and threshold current. To compare the influence of chip cavity length on laser performance, three coated QD lasers of 1.5, 2.0, and 3.0 mm in length were used as gain chip, respectively [55]. For better performance, a chirped multilayer quantum dot laser was chosen. The tuning range and threshold current of the external cavity laser with different chip cavity lengths were tested. The results revealed that shortening the chip cavity length optimized both the short-wavelength and long-wavelength tuning limits. Additionally, the threshold current decreased as the cavity length was reduced. Leveraging the property that both the ground and excited states of quantum dots can be populated under low current conditions, an ECQDL with an ultra-low threshold current can be achieved by appropriately reducing the cavity length. The surface coating of the chip significantly influences the optical feedback intensity and mode competition in quantum dot lasers. The effect of facet coating on the performance characteristics of tunable external-cavity QD lasers has been systematically investigated [56]. [PERSON] et al.