# Recent Advances in Tunable External Cavity Diode Lasers
## 3 Research Progress of ECDL
### External Cavity Quantum Well Laser
#### 3.1.1 Grating-Feedback Configuration  
External cavity quantum well lasers have demonstrated excellent performance. Most tunable ECDL designs typically employ grating feedback external cavity configuration, such as Littrow and Littman configurations [27]. As shown in Figure 2, the laser's output beam is diffracted by the grating. The Littrow configuration is shown in Figure 2a. In Littrow ECDLs, the first-order diffracted light from the grating is directed back along the incident light's original path, while the zero-order diffraction serves as the output beam. The grating is used as the reflector and output mirror of the external cavity, and the output wavelength can be tuned by rotating the grating. Littrow external cavity semiconductor laser is widely used due to its simple structure. [PERSON] et al. prepared an external cavity semiconductor laser in a Littrow configuration, which was employed to detect the saturated absorption spectrum of atomic indium at 410 nm [28]. The gain chip in this setup was a commercial GaN laser diode, achieving a tuning range of 4 nm, from 406.8 to 410.7 nm. The performance of the gain chip significantly impacts the overall performance of the external cavity semiconductor laser. By optimizing the gain chip structure, the performance of external cavity semiconductor lasers can be greatly enhanced. InGaAsP-InP, multiple quantum wells laser with a bent ridge waveguide were fabricated using low-pressure metal-organic chemical vapor deposition (LP-MOCVD) [29]. When employed as a gain chip, these lasers demonstrated a tunable range from 1494 to1667 nm, effectively covering four communication windows. Additionally, the maximum output power reached 81 mW. In the external cavity structure, the grating serves as a reflection element. According to Formulas (1), (2), and (4), the laser linewidth and output power can be optimized by adjusting the grating parameters, including the reflectivity and resolution [21]. First, the grating reflectivity determines the intensity of feedback light. Research showed that increasing the first-order diffraction efficiency from 10% to 50% reduces the linewidth from 0.18 nm to 0.08 nm while simultaneously expanding the tuning range from 5.0 nm  
Figure 2: External cavity semiconductor configurations with diffraction grating. (**a**) Littrow configuration (**b**) Littman configuration. to 11 nm [30]. However, a higher grating reflectivity leads to a decrease in output power; for instance, the output power drops from 750 mW to 400 mW as reflectivity increases. Second, the linewidth of the laser is also influenced by the grating's ability to suppress competition between neighboring modes, a property quantified as grating resolution. Grating resolution is defined as \(A=\lambda/\Delta\lambda=mN\), where \(m\) is the diffraction order and \(N\) is the number of grating lines. According to this formula, the grating resolution is directly proportional to the number of grating lines. Comparisons of external cavity semiconductor lasers with different grating parameters reveal that the linewidth decreases by 20% when the reflectivity increases from 0.2 to 0.6 while maintaining constant resolution. Furthermore, doubling reduces the linewidth by 40% [31]. In summary, greater grating reflectivity and resolution lead to narrower linewidths. Experimental results showed that adjusting the grating parameters in the same external cavity structure can narrow the linewidth from 560 kHz to 320 kHz, highlighting the critical role of grating optimization in external cavity semiconductor laser performance. The external cavity (EC) and the internal cavity of the ECDL system could be regarded as Fabry-Perot (FP) cavities. When a half-wave plate was induced in the Littrow configuration to control the polarization state of the laser source, the first-order diffraction efficiency of grating was also affected [32]. This approach offers an additional method for tuning the diffraction efficiency of the grating.