# Recent Advances in Tunable External Cavity Diode Lasers
## 3 Research Progress of ECDL
### Dual-Wavelength External-Cavity Diode Laser  
Dual-wavelength diode laser systems have important applications in many fields such as dual-wavelength interferometry, optical switching, and terahertz (THz) radiation generation. These applications take advantage of the adjustable nature of dual-wavelength sources. For example, modifying the frequency difference between the two emitted wavelengths enables control over the frequency of the generated terahertz radiation and allows for scanning of the vibrational modes analyzed [69]. The dual-wavelength external-cavity diode lasers show a relatively wide tuning range and relatively high output power [70; 71]. To achieve dual-wavelength output, the external cavity structure must support two mode-selection mechanisms, with at least one of them being tunable. Various external cavity designs have been developed to fulfill this requirement. The simplest is the dual-period holographic grating structure, as shown in Figure 6a [72]. The configuration is similar to the Littrow structure, incorporating two types of periodic gratings within a single holograph element. A constant-period grating and a variable-period grating are superimposed at the same position. The wavelength separation can be adjusted by simply moving the holographic element. In this setup, the wavelength reflected by the constant-period grating remains fixed, while the wavelength returned by the chirped-period grating varies with its position due to the changing in the local period sampled by the incident beam. This design allows for tunable spectral separation between the dual-wavelength outputs, ranging from 0.76 nm to 6.27 nm. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Author** & **Quantum Dot** & **Tunable Range** &
\begin{tabular}{c} **Output** \\ **Power** \\ \end{tabular} & **Linewidth** & **Year** & **Ref** \\ \hline [PERSON] et al. & InAs & 1095–1245 nm & 18 mW & \textless{}3 nm & 2000 & [12] \\ [PERSON] et al. & InAs & 1125–1280 nm & 200 mW & 30 kHz & 2008 & [59] \\ [PERSON] et al. & InAs & 1038.3–1246 nm & 200 mW & \textless{}1 nm & 2010 & [54] \\ [PERSON] et al. & InAs/GaAs & 1122–1324 nm & 480 mW & 0.13 nm & 2010 & [60] \\ [PERSON] et al. & InAs/InGaAs & 1265–1321 nm & 3 mW & 210 kHz & 2011 & [61] \\ [PERSON] et al. & InAs/InP & 98 nm & 7 mW & 2011 & [62] \\ [PERSON] et al. & InAs/GaAs & 1182.5–1319 nm & 870 mW & \textless{}1 nm & 2012 & [63] \\ [PERSON] et al. & InAs/InP & 1457–1561 nm & 34 mW & 2015 & [57] \\ [PERSON] et al. & InAs & 1084.7–1234.3 nm & 223 mW & 2 nm & 2018 & [64] \\ [PERSON] et al. & InAs/InP & 1442–1582 nm & 20 mW & & 2018 & [65] \\ [PERSON] et al. & InAs/InP & 1518–1750 nm & & & 2018 & [66] \\ [PERSON] et al. & InAs/InP & 1414–1506 nm & 6.5 mW & & 2021 & [58] \\ [PERSON] et al. & carbon & 583–594 nm & & & 2023 & [67] \\ [PERSON] et al. & ZnSe/ZnS & 417–424 nm & & 0.2 nm & 2023 & [68] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Recent achievements in external cavity-coupled quantum dot laser. Dual-wavelength external cavity semiconductor lasers utilizing two gratings represent a common external cavity configuration. [PERSON] at al. developed a tunable dual-wavelength ECDL system based on double-Littrow external-cavity feedback design [73]. To achieve a high output power, a tapered diode amplifier with a central wavelength of 801 nm was employed. The two beams can be individually tuned using diffraction gratings, enabling a wide tuning range from 0.5 to 5.0 THz. When the wavelength difference between the two beams is small, mode competition may occur, resulting in the suppression of one wavelength. A dual-wavelength external cavity semiconductor laser can also be realized using a diffraction grating in combination with a volume Bragg grating, as shown in Figure 6b [74]. The volume Bragg grating was fixed to enable the external cavity to maintain a stable reference wavelength while the diffraction grating functions as an end mirror, allowing wavelength tuning within external cavity.