# Recent Advances in Tunable External Cavity Diode Lasers
## 1 Introduction  
Narrow-linewidth tunable laser sources play a pivotal role in a diverse array of applications, including spectroscopy, atomic physics, coherent communication, laser lidar, and precise measurement [1, 2, 3, 4]. For example, narrow linewidth lasers significantly enhance the sensitivity of detection methods by reducing noise and improving signal-to-noise ratios in spectroscopy. The demand for lasers with simpler designs, lower costs, narrower linewidths, and broader tuning ranges continues to drive innovation, enabling new applications and expanding the technology's potential across various fields. To achieve a tunable output, various laser structures have been designed, such as distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers [5, 6]. DFB and DBR lasers integrate gratings with the active regions of semiconductor lasers, enabling tunable output through the selective reflection of the Bragg gratings. Based on the principleof multilayer dielectric membrane filters, tunable vertical cavity surface emitting lasers (VCSELs) have also been successfully fabricated, offering excellent single-mode output performance [7]. However, the tuning range of these lasers is typically limited, as they are primarily adjusted through injection current or operating temperature. In contrast, external cavity diode lasers (ECDLs) provide an alternative approach to tunable output. The external cavity diode lasers employ an external optical resonant cavity to feed the emitted light back into the laser, effectively functioning as extended cavity lasers. The external cavity configuration not only suppresses the frequency drift of the semiconductor laser but also enhances the mode stability of laser output. By fine-tuning the optical elements within the resonant cavity, ECDLs can achieve a significantly broad tuning range [8; 9]. The gain chip is the most critical component of the external cavity semiconductor, as its selection significantly influences the overall performance of the laser. External cavity semiconductor lasers based on quantum well lasers can achieve stable output, characterized by a wide tunable range and a relatively narrow linewidth. For instance, an external cavity diode laser employing a ridge waveguide laser diode as the gain chip and a grating as feedback element can achieve a tuning range of 10 nm, spanning from 452 nm to 462 nm, with an output power of up to 35 mW [10]. Quantum dot (QD) lasers, another frequently employed light source, have a relatively wide half-width of the electroluminescence (EL) spectrum [11]. The external-cavity semiconductor lasers based on quantum dot lasers have an excellent performance in tuning range. A tuning range of 150 nm in a grating-coupled external cavity QD laser has been obtained with a maximum threshold current density of 1.1 kA/cm\({}^{2}\)[12]. The superior low threshold current characteristic of quantum dot lasers are primarily attributed to their low density of states and broad gain profile. The low density of states facilitates rapid filling of the quantum dot energy levels, enabling the exploitation of a broader gain profile under low operating current conditions [13; 14]. Another significant application of external cavity technology is the development of dual-wavelength operation. Generally, one or two diffractive optical elements and a semiconductor laser are used to form two resonant cavity structures [15]. After mode competition, dual-wavelength output is formed. Dual-wavelength lasers can be used to generate terahertz radiation. For example, using a quantum-dot-based gain medium integrated with a single volume Bragg grating (VBG), dual-wavelength generation has been demonstrated with a peak output power of 186 mW and a stable spectral separation of 0.62 THz [16]. In this paper, we review the recent advances of external cavity diode lasers based on different configurations. External cavity quantum well lasers, external cavity quantum dot lasers, and dual-wavelength external-cavity diode lasers are introduced.