Recent Advances in Tunable External Cavity Diode Lasers

Featured Application

Tunable ultra-narrow linewidth external cavity diode lasers can be applied in the field of laser radar, coherent communication, atomic physics.

Abstract

A narrow linewidth tunable laser source is a critical component in various fields, including laser radar, quantum information, coherent communication, and precise measurement. Tunable external cavity diode lasers (ECDLs) demonstrate excellent performance, such as narrow linewidth, wide tunable range, and low threshold current, making them increasingly versatile and widely applicable. This article provides an overview of the fundamental structures and recent advancements in external cavity semiconductor lasers. In particular, we discuss external cavity semiconductor lasers based on quantum well and quantum dot gain chips. The structure of the gain chip significantly influences laser’s performance. External cavity quantum well laser has a narrower linewidth, higher power, and better mode stability. Conversely, external cavity quantum dot laser provides a wider tunable range and a remarkably lower threshold current. Furthermore, dual-wavelength external cavity tunable diode lasers are gaining importance in applications such as optical switching and terahertz radiation generation. With the continuous optimization of chips and external cavity structures, external cavity diode lasers are increasingly recognized as promising light sources with narrow linewidth and wide tunability, opening up broader application prospects.

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 principle of 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/cm² [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. For ECDLs, the performances are influenced by many factors, such as cavity length and reflectivity. We have summarized the performances of external cavity semiconductor lasers with different structures, focusing on key parameters such as tunable range, output power, and threshold current density.

2. Basic Theory of External Cavity Diode Lasers

The external cavity diode laser is generally composed of a gain chip, a collimator, and an optical feedback element. Various external cavity configurations have been developed for ECDLs, among which the planar mirror external cavity represents the most fundamental and straightforward structure. The basic configuration is illustrated in Figure 1. The right facet of the laser is combined with the reflective mirror to form a compound reflector. The effective reflectivity is r_eff. The ECDLs take the semiconductor lasers as the active region. The optical feedback element feeds the light back into the gain chip to achieve mode selection and narrow the linewidth. The external optical feedback has great influence on the performance of ECDLs and its theoretical foundations have been extensively studied [17,18].

For the external cavity structure, as shown in Figure 1, the theoretical formula for the spectral linewidth of ECDL has been investigated. The linewidth is described by the following formula [19,20]:

$$∆\nu ={\displaystyle \frac{{\alpha }^2{\beta }^2{R}_{sp}\xi }{4\pi S}}+{\displaystyle \frac{R_{sp}\xi }{4\pi \left(S+S_e\right)}}$$

(1)

where α is the linewidth enhancement factor, β is the field amplitude, and $\beta =∆\tilde{\Omega }\left(0\right)/∆{\Omega }_0\left(0\right)$, which is derived from the longitudinal mode phase condition of the entire cavity. S and S_e are the number of photons in the gain medium and the external cavity, respectively, and while R_sp represents the spontaneous radiation, which can be expressed as:

$$R_{sp}={\displaystyle \frac{{\nu }_g^{2}h\nu g\left(\nu \right)a_{m}S}{2P_{out}}}$$

(2)

where h is Planck’s constant, ν is the frequency, and a_m is the threshold loss of the laser and is inversely proportional to the reflectance of r₁ and r_eff. P_out is the output power and ξ is the external cavity length factor, which can be expressed as:

$$\xi ={\displaystyle \frac{d/{\nu }_g}{d/{\nu }_g+l_{eff}/{\nu }_{gc}}}$$

(3)

where, l_eff represents the effective length of the external cavity, v_g is the group velocity, and v_gc is the group velocity of external cavity.

Power is another important performance parameter for external cavity semiconductor lasers, and the power can be calculated using the carrier and photon rate equations [21]. The result shows that the power is inversely proportional to the linewidth. The formula is as follows:

$$P_{out}\propto K{N}_{ph}={\displaystyle \frac{{\eta }_i\Gamma {\tau }_{ph}}{qd}}\left[j-j_{th}\left(K\right)\right]{\displaystyle \frac{K}{1-K{\alpha }_m/\left({\alpha }_i+{\alpha }_m\right)}}$$

(4)

where j is the current density, η_i is the internal cavity efficiency, q is the elementary charge of an electron, τ is the carrier lifetime, Γ is the confinement factor, τ_ph is the photon lifetime, α_m is the mirror loss, α_i is the absorption coefficient of external cavity, N_ph is the photon density, and K is the feedback strength.

The above formulas indicate that the linewidth and power of the external cavity semiconductor laser are affected by multiple factors. By selecting an appropriate external cavity structure and optimizing the laser parameters, the output performance can be significantly enhanced. For example, the linewidth can be effectively narrowed by appropriately increasing the reflectivity of r_eff [22].

3. Research Progress of ECDL

3.1. External Cavity Quantum Well Laser

Quantum well semiconductor lasers offer several advantages, including narrow linewidth, high conversion efficiency, and excellent stability. When utilized as a gain chip in external cavity semiconductor lasers, quantum well lasers can be integrated into various external cavity structures. By employing different semiconductor structures as gain chips, it is possible to design ECDLs capable of operating across a broad wavelength spectrum, ranging from ultraviolet to far-infrared [23,24,25,26].

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. Lars Hildebrandt 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 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 $A=\lambda /∆\lambda =mN$, 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. Specifically, it has been demonstrated that the polarization direction of the beam can be converted from p-polarization to s-polarization by rotating the half-wave plate placed between the lens and grating. At the same time, the first-order diffraction efficiency of grating increased from 25% to 55%.

Ding Ding et al. observed that the laser polarization direction affected the grating’s mode selection efficiency due to its polarization sensitivity [33]. The relative positions of the chip and the grating in the experiment are shown in Figure 3a and Figure 3b, respectively. For configuration A, the grating lines were oriented perpendicularly to the p-n junction of the gain chip. In contrast, for configuration B, the gain device aligned the grating lines parallel to the p-n junction. The measured linewidth for configurations A and B were 0.28 nm and 0.1 nm at an injection current of 1100 mA, respectively. This difference can be attributed to the feedback wavelength range of the grating, which is proportional to the emitter size. The emitter size for configuration A and B were 15 μm and 1 μm, respectively. The experimental results showed that configuration B exhibited superior spectral characteristics compared to configuration A.

Mode stability is a critically important performance attribute of external cavity diode lasers, particularly in achieving wide continuous mode-hop-free tuning. Ensuring mode stability is challenging due to several influencing factors, including competition among the internal diode cavity mode, mechanical vibrations, and temperature changes. For the Littrow configuration, research indicates that mode stability is influenced by key design parameters, including cavity length, the alignment and placement of the diffraction grating, and the positioning of a temperature sensor [34]. By optimizing geometric and optical design configurations, continuous wavelength tuning without mode hopping can be achieved. For instance, rotating the grating around a specific pivot point has enabled a mode-hop-free tuning range of 35 GHz at 780 nm. Moreover, long-term frequency stability of the laser has been shown to correlate strongly with the stability of the intracavity temperature. Improved frequency stability can be achieved by positioning the temperature sensor close to both the thermoelectric cooling device (TEC) and the external cavity support device.

For ECDLs, the focal length of the collimating lens significantly impacts the output light performance, such as tuning range. To investigate this effect, five different structures with varying lens focal lengths were studied [35]. The results demonstrated that both the tuning range and power initially increased and then decreased as the cavity length increased. Among the five structures, the ECDL with a collimating lens focal length of 4.51 mm exhibited superior performance. This configuration achieved a maximum output power of 42.36 mW and a broad tuning range of 152.3 nm, spanning from 1458.2 nm to 1610.5 nm.

The Littman configuration is another widely used external cavity structure, as shown in Figure 2b. In this setup, the first order diffracted light from the grating is entirely reflected by an external cavity mirror and retraced its path back to the grating. Subsequently, the first-order diffracted light from the reflected beam returns to the laser chip, while the zero-order diffraction beam serves as the output light. Compared to the Littrow configuration, the Littman structure achieves a narrower spectral linewidth by incorporating a reflecting mirror. In 2000, Jin Jie et al. presented a narrow-band tunable external cavity semiconductor laser utilizing the Littman configuration [36]. Utilizing a commercial diode laser with a center wavelength of 803 nm, the system demonstrated a tuning range of 10 nm. Additionally, the study highlighted that the laser beam maintained exceptional directional stability throughout the tuning process.

The Littman ECDL is tunable by rotating the cavity mirror. The angle of the grating during tuning is determined, which makes the direction of the output light also determined. For conventional Littman ECDL, it is difficult to achieve wide-range mode-hop-free tuning. To overcome this limitation and enable broad, continuous mode-hop-free tuning, a straightforward approach involves matching the distance between the aspheric lens and the beam waist position to the external cavity length [37]. Using this method, a wide mode-hop-free continuous tuning range of 140 nm (from 1480 nm to 1620 nm) has been achieved.

Based on the Littman external cavity structure, the researchers added other optical elements to suppress mode competition, thereby achieving further linewidth narrowing. Di Zhang et al. added an etalon to Littman external cavity structure [20]. The etalon could suppress the neighbor modes in lasing mode acted as a filter. The theoretical and experimental results showed that increasing the reflectance of the standard could increase the wavelength accuracy but decrease the tunable range. The optimal range of the etalon’s reflectivity was identified, with results showing that a reflectivity of 0.65 minimizes the output linewidth to as low as 50 kHz under the combined effects of the etalon and the Littman external cavity structure.

Researchers have also thought to optimize the spectral characteristics of the Littman external cavity structure by implementing design modifications. For example, replacing the flat mirror with a curved mirror featuring a defined radius of curvature has been proposed [38]. Naoaki Kato et al. simulated the spectral-broadness based on Littman external-cavity diode laser with dynamic-curvature end mirror [39]. The configuration is shown in Figure 4. The simulation showed that the spectral broadening can be tuned effectively by varying the radius of curvature of the mirror and adjusting the injection current. The tuning range reached 250 kHz for the single longitudinal-mode operation.

3.1.2. F-P Cavity Configuration

F-P cavity is another common external cavity structure. The FP external cavity structure has better mode stability, but it is limited by the tuning method, and the tunable range is not very large [40]. Xiao Xiao et al. proposed an innovative external cavity configuration by inserting an all-dielectric thin film Fabry–Perot filter (AFPF) into the external cavity [41]. The AFPF had good model selection, like a solid Fabry–Perot etalon. The planar mirror acted as an external cavity mirror. When the AFPF was rotated around a pivot point, a linear interrelationship was attained between its peak transmittance wavelength and the cosine value of the incident angle of the laser beam. The ECDL achieved a mode-hop-free tuning range over 40 nm range around 1550 nm. The ECDL can be applied in various fields, including environmental monitoring, atomics, and so on.

A well-designed external cavity structure is highly effective in narrowing the line width of a laser. Another effective way is using the DFB laser as a gain chip to obtain the narrow linewidth output. DFB diode laser and F-P cavity were combined to achieve an ultra-narrow-linewidth [42]. This design offers two significant advantages: first, FP cavity and DFB laser can achieve double mode selection. Second, the F-P cavity simplified the coupling of light into the cavity and made it insensitive to the misalignment due to mechanical vibrations or temperature changes. Consequently, the laser achieved an ultra-narrow line width and excellent mode stability. The measured white frequency noise levels reached 5 Hz²/Hz at a Fourier frequency as low as 20 kHz, which was over 5 orders of magnitude lower than that of the same solitary diode laser operating without resonant optical feedback.

The tuning method also has great influence on the mode stability of output laser. Yongxiang Zheng et al. demonstrated a simple and stable laser frequency stabilization method by utilizing piezoelectric ceramic actuators to control a Fabry–Perot cavity [43]. To mitigate piezoelectric drift, a duplex symmetrical structure was adopted for the piezoelectric actuators, utilizing their inherent tendency to drift in opposite direction. Therefore, the external cavity diode laser can achieve stable output. A tuning range of 14.7 GHz was obtained. This laser frequency stabilization system was significantly simplified.

3.1.3. Other External Cavity Configuration

In addition to employing gratings as optical feedback elements, researchers have developed special external cavity structures to enhance laser performance. While planar mirrors are commonly utilized in external cavities, their broad spectral bandwidth often limits their effectiveness in longitudinal mode selection. In general, the external cavity structure will be a planar mirror combined with other optical elements to achieve better mode selection. For example, planar mirrors can be used as feedback elements and combined with etalon to fabricate external cavity semiconductor laser [44,45].

Ning Wang et al. proposed an external cavity structure comprising two identical lenses, an etalon, and an end mirror [46]. The design incorporated a fixed etalon and a tunable filter to form a double mode selection mechanism. Two lenses were employed to collimate the beam emitted from the gain chip and to focus the light onto the mirror, respectively. Such an external cavity system has high tolerance to the translation and angular misalignment of the end mirror and the lenses. The laser system demonstrated a linewidth of 7 kHz, an output power of 14 dBm, and a tuning range of more than 40 nm.

High power lasers play a crucial role in a wide range of applications. However, the output power of a single laser diode is inherently limited. The researchers propose to use laser diode array combined with a suitable external cavity structure to achieve higher output power while maintaining a narrow line width and tunability. An external-cavity oscillator was employed as the seed source, with its output coupled into a tapered semiconductor optical amplifier by aspheric lenses [47]. A cylindrical lens was used to compress the slow-axis far-field divergence angle of the amplified light. Finally, an output power of 4.5 W at 670.8 nm was achieved.

The ECDL can be tuned by adjusting optical elements such as the diffraction grating and mirror. During the tuning process, a wavelength shift exceeding a quarter of the laser wavelength often leads to mode hopping, which is difficult to avoid. A novel method of realizing mode-hop-free tuning by synchronous adjustment of optical elements has been proposed [48]. This approach has the potential to significantly enhance tuning speed. The external cavity is shown in Figure 5, including a periscope, etalon, and narrow band pass filter all mounted on a rotating stage. The etalon and narrow bandpass filter are used for mode selection and mode locking, while wavelength tuning was achieved by rotating the periscope. Experimental results demonstrated a mode-hop-free continuous tuning range of 1.7 THz at a central wavelength of 652 nm.

3.2. 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 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 AsH₃ 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 AsH₃ flow rates of 3.9 × 10⁻⁶ mol/min (structure A) and 2.0 × 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/cm² [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_0.15Ga_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]. Feng Gao et al. conducted a detailed study on external cavity InAs/InP QD lasers with lasing wavelength around 1.5 μm [57]. The results showed that reasonable selection of coatings could effectively increase the range of the laser and reduce the threshold current. The tuning spectra of QD EC laser with gain chip with no facet coating, anti-reflection (AR) coating, and anti-reflection/high-reflection (HR) coatings were measured. Blue shift can be seen when comparing the EC laser with AR coating to that with no facet coating. In the short wavelength region, there was a larger density of states, and it was easier for carriers to fill high energy levels. The tunable range of the long wavelength region can be increased by coating the QD laser with high reflective coating. The threshold current of the EC lasers was compared with different facet coatings. HR/AR coating could effectively reduce cavity surface loss and obtain the minimum threshold current. These results demonstrated that implementing AR/HR facet coatings can significantly enhance the performance of QD EC lasers by optimizing their tunable range and minimizing threshold currents.

In 2021, Yan Wang et al. also studied the effects of facet coatings on the EC laser [58]. They fabricated EC semiconductor lasers incorporating InAs/InP quantum dots and reached conclusions consistent with previous research. Their results demonstrated that without facet coatings, the EC laser exhibited a limited tuning bandwidth of only 45 nm. However, by applying AR/HR facet coatings, the tuning range was significantly expanded to 92 nm, highlighting the critical role of facet coatings in enhancing the tunability and overall performance of EC lasers.

External cavity quantum dot lasers exhibit outstanding performances.

Table 1. Recent achievements in external cavity-coupled quantum dot laser.

Author	Quantum Dot	Tunable Range	Output Power	Linewidth	Year	Ref
H. Li et al.	InAs	1095–1245 nm	18 mW	<3 nm	2000	[12]
Yu. A. et al.	InAs	1125–1280 nm	200 mW	30 kHz	2008	[59]
X. Q. Lv et al.	InAs	1038.3–1246 nm	200 mW	~1 nm	2010	[54]
Ksenia A. Fedorova et al.	InAs/GaAs	1122–1324 nm	480 mW	0.13 nm	2010	[60]
Naokatsu Yamamoto et al.	InAs/InGaAs	1265–1321 nm	3 mW	210 kHz	2011	[61]
P. Chen et al.	InAs/InP	98 nm	7 mW		2011	[62]
D. I. Nikitichev et al.	InAs/GaAs	1182.5–1319 nm	870 mW	~1 nm	2012	[63]
Feng Gao et al.	InAs/InP	1457–1561 nm	34 mW		2015	[57]
Xueqin Lv et al.	InAs	1084.7–1234.3 nm	223 mW	2 nm	2018	[64]
Yunyun Ding et al.	InAs/InP	1442–1582 nm	20 mW		2018	[65]
Huihong Yuan et al.	InAs/InP	1518–1750 nm			2018	[66]
YanWang et al.	InAs/InP	1414–1506 nm	6.5mW		2021	[58]
Meng Zhang et al.	carbon	583–594 nm			2023	[67]
Xuyang Lin et al.	ZnSe/ZnS	417–424 nm		0.2 nm	2023	[68]

summarizes the research advancements in external cavity quantum dot lasers over recent years, highlighting significant progress in this field.

3.3. 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.

Dual-wavelength external cavity semiconductor lasers utilizing two gratings represent a common external cavity configuration. Mingjun Chi 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. The tunable external cavity was formed between the diffraction grating and the front facet of the superluminescent diode. Finally, a tunable spectral separation of the dual-wavelength ranging from +6.42 nm (2.63 THz) to −16.94 nm (7.1 THz) was observed. Yuhuan Lu et al. demonstrated high-power tunable dual-wavelength composite external cavity configuration, utilizing a holographic grating and a volume Bragg grating [67]. The frequency difference between the two wavelengths can be tuned from 0.41 THz to 3.89 THz. Additionally, the intensities of the two corresponding wavelengths remain nearly equal, with a minimal power difference of just 0.10%.

Quantum dot lasers can also be served as gain chips in dual-wavelength external cavity lasers. Yuan HH et al. demonstrated a tunable double-mode external cavity InAs/InP quantum dot laser employing two diffraction gratings [75]. Dual-mode operation was accomplished by adjusting just one grating while keeping the other fixed. In the external cavity structure, an attenuation slice was placed in the external optical feedback paths to equalize the intensity of the dual modes to decrease the mode competition. Furthermore, the tuning range spanned both the ground and excited states of the quantum dot assembly, demonstrating that both states can be effectively utilized for dual-mode operation. This approach highlights the versatility of quantum dot lasers in enabling wide tuning range and stable dual-wavelength performance in external cavity systems.

Dual-wavelength output can also be achieved through a distributed feedback laser diode with a single external-cavity, as shown in Figure 6c [76]. A volume Bragg grating was served as the output coupler for the external-cavity DFB laser, enabling the emission of an additional stable wavelength beam. The wavelength tuning of the DFB laser was achieved through temperature modulation. Meanwhile, the linewidth of the laser could be further narrowed by feeding the laser output back into the active region of DFB using the volume grating. As the operating temperature of the DFB laser increased from 16.1 °C to 29.8 °C, the tunable spectral separation between the two wavelengths varied from 1.08 nm (0.5 THz) to 1.92 nm (0.88 THz). This demonstrates the capability of the system for precise dual-wavelength tuning, enhanced stability, and improved spectral performance.

Figure 6. (a) Experimental setup of the ECDL with a dual-period holograph grating [72]; ©Elsevier. Copyright 2006 Optics Communications. (b) Experimental setup of the dual-wavelength ECDL with a diffraction grating and a volume Bragg grating [74]; ©AIP Publishing. Copyright 2016 Applied Physics Letters. (c) Experimental setup of the dual-wavelength external-cavity laser with a DFB laser and a volume Bragg grating [76]. ©JSAP. Copyright 2018 Japanese Journal of Applied Physics.

Figure 6. (a) Experimental setup of the ECDL with a dual-period holograph grating [72]; ©Elsevier. Copyright 2006 Optics Communications. (b) Experimental setup of the dual-wavelength ECDL with a diffraction grating and a volume Bragg grating [74]; ©AIP Publishing. Copyright 2016 Applied Physics Letters. (c) Experimental setup of the dual-wavelength external-cavity laser with a DFB laser and a volume Bragg grating [76]. ©JSAP. Copyright 2018 Japanese Journal of Applied Physics.

4. Prospects

High power external cavity diode lasers have important applications in precision machining and cutting. The output power of the external cavity semiconductor laser is mainly determined by the structure of the gain chip and the fabrication process. The power of a single laser diode is limited. Quantum dot lasers are limited by the self-assembly preparation method, and the power is very small. It is difficult to obtain small-sized quantum dot structures and achieve output in the visible light band for QD laser. Researchers try to use optical amplifiers to achieve high power output, but the tunable range is very small [77]. It is expected that the power of the laser can be further improved by optimizing the fabrication process of the laser diode.

Mode-hop-free tuning is also one of the key research contents at present. Although it has been found that the mode-free tuning of ECDL can be achieved by selecting an appropriate tuning pivot point, the tuning range is still limited. Another approach is to synchronously tune the optical components in some external cavity configurations. However, this method is limited by the external cavity structure and needs further study. For some commercial lasers, the laser, power, and cooling devices will be tightly integrated together, which is beneficial to avoid mode hopping due to mechanical vibration and temperature drift.

As a high-efficiency light source, the external cavity diode laser will also be deeply integrated into the intelligent control system. By embedding sensor and control system, the real-time monitoring and accurate control of the laser working state can be realized, which can effectively improve the operation reliability and stability of the laser.

5. Conclusions

External cavity semiconductor lasers are characterized by their simple structure and the ability to achieve tunability across different wavelength bands by employing various laser light sources. In this paper, the focus is on three types of external cavity semiconductor lasers. Quantum well lasers, benefiting from mature fabrication technologies, enable external cavity designs that can achieve ultra-narrow linewidth and higher output power. External cavity quantum dot lasers offer a wider tuning range, attributed to the size fluctuation of quantum dots. Meanwhile, external cavity quantum dot lasers have a lower threshold current due to the ground state of QD being more easily saturated. Dual-wavelength tunable lasers, as a significant category of external cavity semiconductor lasers, are also addressed. The relationship between external cavity structure and parameters and the performance of external cavity diode lasers is also discussed. Through the rational design of external cavity structures, it is possible to achieve tunable dual-wavelength output. At present, the external cavity semiconductor laser has excellent performance. With the continuous optimization of performance, it will have better application prospects in the future.