2 μm high-power thulium- and holmium-doped fiber lasers: progress and future perspectives

Abstract

2 μm fiber lasers, owing to their unique physical and spectral properties, hold significant potential for applications in fields such as lidar, medical surgery, environmental monitoring, and nonlinear frequency conversion. This paper presents a comprehensive review of continuous-wave and pulsed 2 μm fiber lasers. First, it elucidates the spectral characteristics and energy-level transition mechanisms of the primary gain media, including thulium-doped, holmium-doped, and thulium-holmium co-doped fibers. It systematically summarizes technical approaches, efficiency optimization strategies, and specialized fiber structures for high-power operation using mainstream pumping schemes such as 790 nm, 1.55 μm, and 1.9 μm pumping. Second, it discusses key technological methods and recent research progress for two typical operational modes: continuous-wave (multi-longitudinal mode, narrow linewidth) and pulsed (Q-switched, mode-locked, gain-switched). Finally, the primary bottlenecks limiting the high-power output and beam quality optimization of 2 μm fiber lasers are analyzed. Future development directions are proposed from five perspectives: system integration and miniaturization, ultra-wide spectral tuning, artificial intelligence-based control, advancements in high-power technology, and the development of novel gain materials.

Graphical abstract

Keywords

2 μm fiber laser / Continuous-wave laser / Pulsed laser / Thulium-doped fiber / Holmium-doped fiber

1 Introduction

Compared with conventional 1 μm and 1.5 μm laser systems, the 2 μm wavelength region occupies a unique position between near- and mid-infrared laser technologies owing to its distinctive physical and spectral properties [1,2]. While 1 μm ytterbium-doped and 1.5 μm erbium-doped lasers have achieved a high level of technological maturity, further power scaling is increasingly limited by nonlinear effects and thermal management challenges [3,4]. In contrast, the longer operating wavelength of 2 μm fiber lasers provides higher thresholds for nonlinear effects such as stimulated Brillouin scattering (SBS). Additionally, the cross-relaxation (CR) mechanism in thulium-doped (Tm-doped) fibers enhances pump utilization efficiency and helps reduce thermal loading under appropriate pumping conditions [5–8]. These physical and thermal advantages establish a strong foundation for the development of high-power and high-energy fiber laser systems [2,4]. Beyond their advantages in power scaling, 2 μm lasers also exhibit several application-oriented spectral properties. The laser radiation in this wavelength band complies with human eye safety standards because it is effectively absorbed by the cornea and lens, thereby preventing laser radiation from reaching the retina and avoiding irreversible eye damage [9]. Furthermore, the 2 μm wavelength band strongly overlaps with the absorption spectrum of water molecules in biological tissues, with absorption coefficients ranging from 200 cm⁻¹ to 600 cm⁻¹, enabling improved precision in surgical procedures [10]. Moreover, this wavelength region lies within an atmospheric low-loss transmission window, characterized by long transmission distances and strong resistance to environmental interference, making it highly advantageous for long-range detection and free-space optical communication [11]. Due to the significant spectral overlap between 2 μm lasers and the characteristic absorption peaks of various gases, such lasers have also been widely employed in hazardous gas sensing and environmental monitoring [12]. Additionally, 2 μm lasers exhibit excellent nonlinear frequency conversion capabilities. Through techniques such as difference frequency generation (DFG) and optical parametric oscillation (OPO), their emission wavelength can be further extended into the 3−5 μm mid-infrared region [13,14]. Benefiting from these distinctive properties, 2 μm lasers have become an important platform bridging fundamental scientific research and industrial applications and are currently regarded as a major research focus in the laser field. Among various implementations, all-fiber 2 μm fiber lasers offer outstanding advantages, including a compact configuration, excellent heat dissipation capabilities, and high operational stability. These features make them highly competitive with other types of laser systems [2]. By combining the intrinsic benefits of the 2 μm spectral band with the advantages of fiber-based architectures, these laser systems hold irreplaceable application value in fields such as medicine [15,16], industry [17,18], national defense [19,20], and scientific research [21,22]. Moreover, laser devices operating under different regimes can meet diverse application requirements, thereby gradually establishing a comprehensive and diversified application ecosystem.

After decades of development, 2 μm fiber lasers have undergone a continuous evolution from laboratory prototypes to industrial applications. Rather than being divided solely according to chronological order, the evolution of this field can be broadly classified into three major stages based on representative technological milestones, including breakthroughs in gain media and specialty fiber technologies, output-power scaling capability, laser architecture evolution, and the expansion of practical application scenarios. The boundaries between these stages are therefore conceptual rather than strictly chronological, reflecting the dominant technological characteristics during each period. The first stage, referred to as the initial exploration stage, was marked by the pioneering work of the Hanna group, which first employed an 800 nm dye laser to pump thulium-doped fiber (TDF) and successfully achieved laser emission in the 2 μm wavelength region. This work confirmed the emission potential of thulium ions and established the theoretical and experimental foundation for subsequent research [23,24]. During this period, research primarily focused on demonstrating laser oscillation and exploring suitable gain media and pumping schemes, thereby establishing the fundamental framework for subsequent technological development. The second stage, representing a period of rapid technological development, was characterized by significant advances in pump technologies, specialty fiber fabrication, and cavity design. The widespread adoption of double-cladding fibers (DCFs), high-power laser-diode pumping, all-fiber configurations, and master oscillator power amplifier (MOPA) architectures enabled rapid power scaling from the watt level to the hundred-watt regime, while pulsed fiber lasers achieved picosecond-level performance. During this stage, many key technologies gradually matured, facilitating the transition from laboratory demonstrations to practical engineering implementation and providing the technological foundation for subsequent high-power fiber laser systems [25,26]. The third stage can be regarded as the stage of high-performance and intelligent development. Benefiting from continuous advances in nonlinear optics, ultrafast laser technology, specialty fibers, and intelligent control strategies, 2 μm fiber lasers have achieved kilowatt-level continuous-wave (CW) output and femtosecond pulse generation. Meanwhile, emerging research directions, including spatiotemporal mode-locked (ML), artificial intelligence (AI)-assisted cavity optimization, and intelligent laser control, have further expanded the performance limits and application potential of 2 μm fiber lasers, thereby accelerating their transition toward practical engineering applications and commercialization [27,28].

Combining the research background, application prospects, recent technological advances, and engineering development trends, this paper provides a systematically review of the two principal operating modes of 2 μm fiber lasers and presents a comprehensive technical framework for understanding their development. It first examines the characteristic parameters of the major gain media and the underlying physical mechanisms, highlighting the fundamental differences in performance characteristics and implementation approaches among different laser operating regimes. In addition, a comparative analysis of TDFs, holmium-doped fibers (HDFs), and thulium-holmium co-doped fibers (THCDFs) is presented with respect to wavelength coverage, power-scaling capability, conversion efficiency, beam quality, system complexity, and representative application scenarios. For CW lasers, cavity configurations are categorized into multi-longitudinal-mode (MLM) and narrow-linewidth (NLW) architectures according to the characteristics of intracavity longitudinal-mode oscillation. Particular emphasis is placed on the mainstream MOPA configuration to analyze its role in high-power scaling. The review then examines three typical pulse-generation mechanisms for pulsed lasers, namely Q-switched (QS), ML, and gain-switched (GS), while summarizing recent research advances driven by increasing performance requirements. The technical characteristics of these pulse-generation mechanisms are comparatively analyzed in terms of power scaling approaches, threshold characteristics, high-power limiting factors, and optimization strategies. Furthermore, representative applications of different laser architectures, including eye-safe lidar, precision spectroscopy, biomedical treatment, environmental monitoring, nonlinear frequency conversion, and ultrafast laser processing, are systematically discussed to clarify the relationship between laser performance and application requirements. The commercialization status and industrial prospects of representative 2 μm fiber laser systems are also reviewed, together with the key engineering challenges associated with high-power implementation, including thermal effects, upconversion losses, beam quality management, specialty fiber and passive component availability, packaging reliability, and long-term operational stability. Finally, future development trends are discussed from the perspectives of intelligent laser control and advanced fiber technologies.

2 Luminescence mechanisms of rare-earth-doped fibers in the 2 μm band and high-power fiber architectures

The output performance of 2 μm fiber lasers is largely determined by the gain media and their underlying physical mechanisms. Compared to ytterbium-doped laser systems operating at 1 μm and erbium-doped laser systems operating at 1.55 μm, the primary doped systems in the 2 μm wavelength region are predominantly based on thulium ions (Tm³⁺) and holmium ions (Ho³⁺) [29]. With the continuous advancement of high-power and ultrafast laser technologies, novel fiber structures—such as DCFs, photonic crystal fibers (PCFs), and large-mode-area (LMA) fibers—have undergone sustained optimization and development. The synergistic evolution of gain media and fiber configurations has collectively enabled continuous performance improvements in 2 μm fiber lasers, particularly in high-power operation and pulse compression. This section systematically reviews the core gain media and representative fiber structures, thereby providing the theoretical foundation for the subsequent investigation of different operating regimes.

2.1 Luminescence mechanism of Tm-doped fibers in the 2-μm band

Since the emergence of 2 μm fiber lasers, TDF has remained one of the most important and widely used gain media in this wavelength region, owing to its unique energy-level structure and highly efficient energy-conversion mechanism [5]. The primary laser transition of Tm³⁺ ions is the ³F₄ → ³H₆ transition, with an emission spectrum spanning approximately 1.6−2.2 μm. This broad emission bandwidth makes it one of the widest spectral systems among rare-earth-doped fibers [3]. The corresponding energy-level diagram and absorption/emission spectra are shown in Figs. 1a and 1b. Benefiting from its broadband emission characteristics, TDF can be employed not only in high-power CW fiber lasers but also as a fundamental platform for tunable and ultrafast ML fiber lasers. The thulium-doped system also exhibits a unique CR effect, as illustrated in Fig. 1a. An excited ion at the high-energy ³H₄ level can transfer energy to a neighboring ion in the ground-state ³H₆ level, resulting in the excitation of two ions to the ³F₄ level. Consequently, a single pump photon can effectively excite two ions involved in the laser transition [30]. This effect becomes particularly pronounced at high doping concentrations and constitutes a key mechanism for enabling high-efficiency, high-power laser output in TDF systems. However, the efficiency of the CR process is not unlimited under high-power operation. As the average output power increases, thermal accumulation within the gain fiber becomes increasingly significant due to the residual quantum defect and non-radiative relaxation processes. The resulting temperature rise can affect the energy transfer dynamics between neighboring Tm³⁺ ions, thereby reducing the efficiency of the CR process and causing the effective quantum efficiency to deviate from its ideal value. Consequently, the increased thermal loading may limit further power scaling and long-term operational stability. Therefore, optimizing the Tm³⁺ doping concentration, fiber geometry, pump configuration, and thermal management remains essential for maintaining high CR efficiency and achieve efficient high-power operation in Tm-doped fiber lasers [3–6].

The pumping schemes for TDFs are primarily designed around three characteristic absorption bands, located at 790 nm, 1.05−1.2 μm, and 1.5−1.9 μm, respectively. These wavelength regions correspond to the strong absorption peaks of Tm³⁺ ions, enabling efficient coupling of pump energy and effective population inversion at the upper laser level. The corresponding mechanisms are illustrated in Fig. 1b. Among these schemes, pumping at 790 nm is the predominant approach for high-power TDF systems. This pumping scheme utilizes the CR effect to achieve the ³H₆ → ³H₄ transition, significantly improving the optical-to-optical conversion efficiency and thereby becoming the mainstream pumping configuration for high-power 2 μm fiber lasers [31]. Pumping in the 1.5−1.9 μm wavelength range is commonly referred to as resonant pumping. This pumping scheme features a low quantum defect and superior thermal management capabilities, making it highly suitable for NLW and high-coherence laser systems. Furthermore, it can be used to extend the emission spectrum into the 2 μm wavelength region [32].

In addition, although TDF offers a broad gain bandwidth, the laser system operates as a quasi-three-level configuration when the output wavelength is shorter than 1.95 μm, resulting in ground-state reabsorption effects [33]. As the output wavelength decreases, ground-state reabsorption becomes increasingly pronounced, thereby making high-power laser generation more challenging. In ML laser systems, the broad gain bandwidth of TDF supports femtosecond pulses generation and enables the formation of various nonlinear pulse regimes, including conventional solitons, dissipative solitons, and self-similar pulses [34–36]. Additionally, thulium-doped systems exhibit excellent compatibility with high doping concentrations. Even under heavily doped conditions, favorable optical properties can still be maintained, thereby facilitating the development of compact, high-power fiber laser systems [37].

2.2 Luminescence mechanism of holmium-doped (Ho-doped) fibers in the 2-μm band

Holmium-doped fiber (HDF) plays a significant role in the development of 2 μm fiber lasers. The primary laser transition of Ho³⁺ ions is based on the ⁵I₇ → ⁵I₈ transition, with the emission wavelength primarily spanning the 2.0−2.1 μm range [38]. The associated energy-level structure and spectral characteristics are shown in Figs. 1c−1e. Consequently, for laser generation beyond 2 μm, holmium-doped fiber lasers (HDFLs) provide distinct advantages over thulium-doped fiber lasers (TDFLs). Due to the relatively narrow gain bandwidth of the holmium-doped system, HDFLs exhibit unique benefits for producing NLW, high-coherence, and wavelength-specific laser output. HDFLs are characterized by their low quantum defect [39]. As illustrated by the energy-level transition scheme in Fig. 1c, the characteristic absorption peaks of Ho³⁺ ions occur near 1.15 μm and 1.95 μm.

The most commonly used pumping scheme employs a 1.95 μm pump source, typically provided by a TDFL, thereby establishing a resonant-pumping configuration. Because the pump wavelength is close to the signal wavelength, nonradiative energy losses are significantly reduced, effectively suppressing thermal effects and enhancing optical-to-optical conversion efficiency. This feature is especially important for high-power CW lasers, particularly for NLW systems [40]. The corresponding absorption and emission spectra are shown in Fig. 1d. Another approach employs direct pumping in the 1.15 μm wavelength region to achieve 2 μm laser emission [41]. Pump sources at this wavelength are commonly based on indium gallium arsenide (InGaAs) semiconductor lasers or ytterbium-doped fiber lasers (YDFLs), whose corresponding spectral characteristics are shown in Fig. 1e. Benefiting from a relatively flat and narrow gain spectrum, combined with wavelength-selective feedback components such as fiber Bragg gratings (FBGs) and external cavities, HDFLs can achieve single-longitudinal mode or near-single-longitudinal mode oscillation, enabling laser output with linewidths at the kilohertz level or even narrower [42]. However, due to the limited gain bandwidth, holmium-doped materials are less frequently employed in ultrafast laser applications and are instead more suitable for CW laser systems operating beyond the 2 μm wavelength region.

2.3 Luminescence mechanism of thulium-holmium (Tm-Ho) co-doped fibers in the 2-μm band

In 2 μm fiber laser systems, in addition to thulium-doped and holmium-doped fibers, thulium-holmium co-doped fiber (THCDF) has become one of the most widely used composite gain media. Benefiting from efficient population inversion enabled by energy-transfer (ET) processes, this system combines the pump compatibility of TDFs with the wavelength-extension capability of HDFs, thereby enabling continuously tunable laser output over the 1.9−2.15 μm wavelength range. The corresponding energy-level transitions and spectral characteristics are illustrated in Figs. 1f−1h [43]. Figure 1f illustrates the energy-level transitions and ET processes of THCDF under 790 nm pumping. The operating mechanism can be divided into two interacting subsystems associated with Tm³⁺ and Ho³⁺ ions, respectively. Tm³⁺ ions absorb 790 nm pump photons and are excited to the ³H₄ level, followed by efficient population transfer to the ³F₄ level via the CR process. The excited Tm³⁺ ions can either directly emit fluorescence around 1.9 μm or transfer energy to Ho³⁺ ions. Subsequently, Ho³⁺ ions are excited to the ⁵I₇ level through the ET process, and laser emission near 2.1 μm is generated via the ⁵I₇ → ⁵I₈ transition.

As shown in Fig. 1g, the absorption spectrum of THCDF almost completely overlaps with that of the TDF, which is fully consistent with the energy-level transition analysis presented in Fig. 1f. Consequently, the primary pump sources for THCDFs are located in the 790 nm and 1.55 μm wavelength bands, both of which are technologically mature industrial laser sources. By comparing the emission spectra of TDF and THCDF under 790 nm pumping, as shown in Fig. 1h, it is evident that the output spectrum of the Tm-Ho co-doped system extends further into the long-wavelength region of the 2 μm band. Compared to singly Tm-doped and singly Ho-doped fibers, the Tm-Ho co-doped fiber effectively overcomes the low absorption efficiency of conventional pump sources in HDF and suppresses thermal effects and upconversion losses that occur in TDF under high-power operating conditions. Additionally, this gain medium features a broader gain bandwidth and higher optical-to-optical conversion efficiency, making it a promising platform for developing low-threshold, high-power, and widely tunable 2 μm fiber lasers.

Overall, although TDFs, holmium-doped fibers HDFs, and THCDFs all support efficient laser operation in the 2 μm wavelength region, they exhibit distinct characteristics that make them suitable for different operating regimes and application scenarios [3,5–7,44]. For a more intuitive comparison, the main characteristics of these three representative gain media are summarized in Table 1. In general, TDFs are the preferred choice for high-power and ultrafast fiber lasers owing to their broad gain bandwidth and mature pumping technologies, whereas HDFs are more suitable for NLW and high-energy applications because of their low quantum defect under in-band pumping. THCDFs provide enhanced wavelength flexibility through efficient Tm³⁺-Ho³⁺ energy transfer, although this comes at the expense of increased system complexity. Therefore, these three types of gain media should be regarded as complementary technical routes rather than competing ones, and the selection of an appropriate gain medium should be based on the desired operating wavelength, power level, efficiency, beam quality, and application requirements [3,5–7,44].

2.4 Typical fiber structures for high-power 2 μm lasers

With the advancement of 2 μm fiber lasers toward higher power, greater pulse energy, and improved operational stability, the fiber structure has become a critical factor limiting further performance enhancements. To address the nonlinear effects and optical damage issues encountered in conventional single-mode fibers during high-power operation, various novel fiber structures have been proposed and widely adopted. Among them, DCFs, PCFs, and LMA fibers represent three complementary technical approaches, each optimized for specific operating regimes and performance requirements.

DCF forms the fundamental architecture for achieving high-power laser output. It consists of a doped fiber core surrounded by a large-area inner cladding. Pump light propagates within the cladding in a multimode form and is gradually coupled into the core for absorption and energy conversion. This structure significantly improves pump-coupling efficiency and is well suited for pumping with high-power multimode laser diodes (LDs), thereby enabling laser outputs at the hundred-watt or even kilowatt level [6]. Representative DCFs employed in high-power 2 μm fiber lasers typically possess core diameters of approximately 10−25 μm, inner-cladding diameters of 125−400 μm, numerical apertures (NA) of 0.08−0.15, and effective mode-field areas generally below 500 μm² [6,45]. These characteristics provide an effective balance between pump absorption efficiency and beam quality, making DCFs well suited for high-power CW and MOPA-based 2 μm fiber laser systems.

Compared with DCFs, PCFs are characterized by periodically arranged air-hole microstructures in the cladding, which enable precise control over optical field distribution and dispersion characteristics. These fibers can simultaneously enlarge the mode-field area while maintaining single-mode transmission, and their dispersion properties can be flexibly engineered. Consequently, PCFs play a crucial role in applications such as supercontinuum generation and ultrafast pulse compression [46]. Typical PCFs designed for 2 μm fiber lasers generally feature core diameters ranging from 20 μm to 50 μm and effective mode-field areas of approximately 500−2000 μm², while maintaining effectively single-mode guidance through their microstructured cladding [45,46]. Owing to their superior dispersion engineering capability and large-mode-area single-mode transmission, PCFs are particularly suitable for ultrafast fiber lasers, nonlinear frequency conversion, and supercontinuum generation. However, their relatively complex fabrication processes and higher manufacturing costs currently limit their widespread application in high-power industrial laser systems.

Unlike PCFs, LMA fibers primarily mitigate nonlinear effects by increasing the effective mode-field area while maintaining good beam quality. They reduce optical intensity, thereby effectively suppressing nonlinear effects such as self-phase modulation (SPM) and stimulated Raman scattering (SRS), while simultaneously enhancing the optical damage threshold. These fibers typically utilize weakly guiding structures or specially designed refractive-index profiles to simultaneously achieve quasi-single-mode output and high-power transmission capability. Consequently, LMA fibers have been widely applied in high-power CW laser systems and high-energy pulsed amplification systems [47]. Representative LMA fibers generally exhibit core diameters of 25−50 μm, relatively low numerical apertures of approximately 0.03−0.07, and effective mode-field areas often exceeding 1000 μm² [45,47,48]. Such structural characteristics significantly increase the nonlinear threshold and improve thermal handling capability, making LMA fibers well suited for kilowatt-level CW fiber lasers and high-energy pulsed amplification systems. Nevertheless, maintaining stable single-mode operation becomes increasingly challenging as the core diameter continues to increase.

Overall, thulium-doped, holmium-doped, and thulium-holmium co-doped fibers, together with advanced specialty fiber structures such as DCFs, PCFs, and LMA fibers, provide complementary technological approaches for realizing high-performance 2 μm fiber lasers. Different gain media and fiber structures exhibit distinct advantages in efficient pump absorption, dispersion engineering, nonlinear suppression, power scaling, and system integration, and should therefore be selected according to the desired operating regime, output power, beam quality, nonlinear tolerance, and application requirements. Benefiting from continuous advances in gain-fiber design, specialty-fiber fabrication, and laser system engineering, 2 μm fiber lasers have achieved significant improvements in output power, operational stability, pulse energy, and beam quality, while gradually progressing toward practical industrial implementation. Nevertheless, challenges associated with material absorption losses, thermal management, nonlinear effects, and advanced fiber fabrication remain to be further addressed. Future research is expected to focus on novel gain-fiber materials, optimized specialty-fiber structures, intelligent laser-system design, and highly integrated all-fiber architectures to further improve the comprehensive performance and application potential of 2 μm fiber lasers.

3 2 μm high-power continuous wave fiber laser

2 μm CW fiber lasers represent one of the most mature and highly optimized laser platforms in this wavelength region. Due to their high average output power, excellent beam quality, and stable operational characteristics, 2 μm CW fiber laser play a central role in applications such as industrial processing, medical treatment, and coherent sensing [1,45]. Conventional free-running CW lasers typically do not incorporate dedicated longitudinal-mode selection or frequency-stabilization mechanisms. As a result, numerous longitudinal modes oscillate simultaneously within the gain bandwidth, resulting in MLM operation characterized by relatively broad linewidths, strong mode competition, and poor frequency stability. In contrast, NLW lasers utilize techniques such as narrowband filtering, specially designed cavity configurations, and longitudinal-mode selection to maintain stable single-longitudinal-mode operation. Consequently, they achieve ultra-narrow linewidth (UNLW) output while providing low noise, high coherence, and excellent wavelength stability [48]. With the rapid advancement of thulium-doped and holmium-doped gain media, specialty fiber structures, and high-efficiency pumping technologies, 2 μm CW fiber lasers have experienced significant development from laboratory demonstrations toward high-power and practical engineering applications [2,49]. This section systematically reviews the research progress of 2 μm CW fiber lasers from the perspectives of gain media, cavity configurations, power-scaling strategies, and performance metrics, focusing on both MLM and NLW fiber laser systems. Finally, recent developments in quasi-continuous-wave (QCW) fiber lasers are briefly introduced.

3.1 2 μm high-power multi-longitudinal-mode fiber lasers

2 μm MLM fiber lasers have become among the most mature CW laser sources for practical engineering applications. Their defining characteristic is that no dedicated longitudinal-mode selection structures are employed, which allows multiple longitudinal modes within the gain bandwidth to oscillate simultaneously. Consequently, these laser systems offer advantages such as relatively broad output linewidths, high optical-to-optical conversion efficiency, and simple, reliable configurations [6,50]. According to the type of gain medium employed, such lasers can generally be classified into three major technical categories: thulium-doped, holmium-doped, and thulium-holmium co-doped systems. Among these, TDFs, benefiting from mature pumping technologies and broad gain spectra, represent the mainstream solution for achieving high-power MLM output in the 1.9−2.0 μm wavelength region. HDFs exhibit larger emission cross sections in the 2.0−2.1 μm wavelength range, making them more suitable for mid- to long-wave infrared applications. Meanwhile, THCDFs utilize Tm³⁺-Ho³⁺ ET processes to simultaneously broaden the gain bandwidth and enhance pumping efficiency, thereby providing an effective approach for realizing widely tunable, high-power MLM laser operation [38,49]. This section reviews the operating mechanisms, cavity configurations, power amplification strategies, and output performance of the three types of gain media, with particular emphasis on high-power 2 μm MLM fiber lasers. The corresponding research progress and representative results are summarized in Table 2.

3.1.1 2 μm high-power multi-longitudinal-mode Tm-doped fiber laser

TDFLs are among the most widely investigated and practically implemented MLM laser sources operating in the 2 μm wavelength region. Their laser emission originates from the ³F₄ → ³H₆ transition of Tm³⁺ ions, with emission wavelengths mainly distributed over the 1.8−2.1 μm range. The output wavelength can be flexibly tuned by adjusting parameters such as the doped fiber length, cavity mirror coating characteristics, and FBG properties. Additionally, the unique CR mechanism in TDFs offers significant advantages for high-power scaling in the 2 μm wavelength region. Early MLM thulium-doped fiber lasers were constrained by the use of single-cladding fiber structures and inefficient pump coupling, resulting in output powers limited to only a few watts. In 1988, Hanna et al. first reported a TDFL pumped by a 797 nm dye laser, achieving an output power of 2.7 mW with a slope efficiency of 13% [23]. In the same year, Snitzer et al. proposed the DCF structure, fundamentally overcoming the low pump efficiency bottleneck present in early fiber laser systems [51]. With the subsequent adoption of DCF technology, pump laser could propagate within the inner cladding in a multi-mode form and be efficiently absorbed by the doped ions in the core, thereby enabling the subsequent development of high-power fiber lasers. In 1990, Hanna et al. used a 1.064 μm laser as the pump source and experimentally achieved a maximum 2 μm laser output power of 1.35 W with a slope efficiency of 37% [52]. Although the double-cladding structure had been proposed at that time, it had not yet been applied to TDF systems. Furthermore, the selected 1.064 μm pump wavelength corresponded to a relatively small absorption cross section of Tm³⁺ ions and was affected by significant excited-state absorption, thereby limiting further power scaling.

With the advancement of semiconductor laser technology and the widespread adoption of DCFs, the output power of TDFLs has increased rapidly. In 1998, Jackson and King used a 790 nm LD as the pump source and achieved a maximum laser output power of 5.4 W in the 2 μm wavelength region using a cladding-pumped configuration, with a slope efficiency of 13% and an optical-to-optical conversion efficiency of 22% [53]. In 2005, Frith et al. first reported a 793 nm LD-pumped TDFL, achieving an output power of 85 W at 2040 nm with a slope efficiency of 56% [55]. In 2007, Slobodtchikov et al. demonstrated dual-end pumping of a TDF using a 793 nm laser, achieving a CW output power of 263 W at 2050 nm with an optical-to-optical conversion efficiency of 52% and a slope efficiency of 59% [58]. The schematic diagram of the laser configuration and its output characteristics are shown in Figs. 2a and 2b. This work marked the entry of 2 μm lasers based on 793 nm pumping into the hundred-watt power regime.

With the continuous advancement of fiber fabrication technologies, all-fiber configurations have become the mainstream technical approach for high-power 2 μm fiber lasers due to their significant advantages, including a compact structure, maintenance-free operation, strong resistance to environmental perturbations, and high stability. In 2014, Hu et al. developed a high-power, all-fiber, linear-cavity TDFL using seven 790 nm LDs, each with a maximum output power of 70 W, as pump sources. The laser achieved a maximum output power of 227 W at a central wavelength of 1908 nm, corresponding to a slope efficiency of 54.3% and an optical-to-optical conversion efficiency of 51.2% [60]. In 2016, Walbaum et al. employed an all-fiber linear-cavity configuration and used four 300 W, 793 nm LDs for pumping. They achieved a multi-mode laser output power of 567 W at a central wavelength of 1970 nm, with a slope efficiency of 49.4% [62]. The schematic diagram of the laser structure and the output power characteristics are shown in Figs. 2c and 2d. In 2023, Chakravarty et al. reported an all-fiber TDFL delivering an output power of approximately 117 W at a central wavelength near 1940 nm, with a slope efficiency of about 53.8% [63]. More recently, in 2025, Sohail et al. demonstrated an all-fiber single-oscillator TDF system that achieved an output power of 203.2 W at 1.94 μm, corresponding to a slope efficiency of 61.2% [64].

Benefiting from the continuous development of DCF structures, high-power pump sources, and thermal management technologies, the output power of Tm-doped fiber lasers has gradually advanced from the watt-to-hundred-watt level toward kilowatt-class operation. An important milestone was achieved by Ehrenreich et al. in 2010, who demonstrated a 1-kW all-glass Tm: fiber laser system [65]. This work verified the feasibility of kilowatt-level operation in the 2 μm wavelength region and highlighted the critical roles of efficient cladding pumping, optimized gain-fiber design, and thermal management in overcoming power-scaling limitations. This achievement provided an important foundation for subsequent high-power Tm-doped fiber laser developments. To further overcome power-scaling limitations, researchers introduced the MOPA technique, enabling near-kilowatt-level laser output through multistage amplification. In 2016, Yin et al. reported a high-power, wavelength-tunable, all-fiber TDFL based on a MOPA architecture. The system achieved a maximum output power of 327.5 W at 1930 nm, with a slope efficiency of 57.4%. The wavelength tuning range spanned 1910−2050 nm, and the output power exceeded 270 W across the entire tuning range [66]. This work demonstrated the capability of MOPA configurations to simultaneously achieve high output power and broadband wavelength tunability, providing an effective approach for developing flexible high-power 2 μm fiber laser sources.

In 2023, Romano et al. demonstrated a MOPA-based TDFL delivering more than 937 W at 2036 nm [68]. In the same year, Ren et al. reported an all-fiber, widely tunable TDFL MOPA operating over 1943−2050 nm, delivering output powers of 1010−1039 W with slope efficiencies exceeding 51% [45]. The system configuration and output characteristics are shown in Figs. 2e and 2f. The kilowatt-level tunable output was enabled by the combination of a broadly tunable seed source, a multi-stage all-fiber amplification architecture, efficient 793 nm pump coupling, and optimized gain-fiber parameters, which maintained sufficient gain and stable power scaling over a broad wavelength range. This work provides valuable insights into the coordinated optimization of seed source, amplification architecture, pump configuration, and gain management for realizing high-power, wavelength-tunable 2 μm fiber lasers.

Subsequent studies during 2024−2026 focused on improving efficiency, operational stability, and engineering practicality, while extending kilowatt-level performance from MOPA systems to direct oscillator architectures. Optimization of the all-fiber cavity and pump-coupling scheme increased the slope efficiency to 61.2% while maintaining stable CW operation [64]. In 2026, Lee et al. demonstrated an all-fiberized CW Tm-doped fiber oscillator at 1940 nm, producing 1032 W with a slope efficiency of 58.5% under optimized thermal management [69]. A bidirectional pumping scheme theoretically reduced the maximum gain-fiber temperature by approximately 40% compared with forward pumping, while preserving near-diffraction-limited beam quality and stable output. This work represents an important milestone for kilowatt-class Tm-doped fiber oscillators and confirms that direct oscillator architectures can reach the kilowatt regime through optimized pump distribution and effective thermal management. Collectively, these results indicate that high-power multi-longitudinal-mode Tm-doped fiber lasers are progressing toward the simultaneous optimization of power scalability, conversion efficiency, and long-term reliability. Further scaling, however, remains constrained by thermal effects, transverse mode instability (TMI), and optical nonlinearities, requiring continued optimization of gain fibers, cavity designs, and thermal-management strategies.

The continuous improvements in output power, optical-to-optical conversion efficiency, and long-term operational stability have substantially expanded the application potential of high-power MLM Tm-doped fiber lasers. Benefiting from their eye-safe operating wavelength, excellent beam quality, and high CW output power, these laser systems have become attractive sources for coherent Doppler wind lidar, where higher output power significantly enhances the detection range, signal-to-noise ratio (SNR), and wind velocity measurement accuracy [70,71]. Their excellent power scalability and high average power also make them efficient pump sources for mid-infrared nonlinear frequency conversion, including OPOs and DFG, enabling coherent laser generation in the 3−5 μm spectral region [13,14]. Furthermore, owing to the strong absorption of 2 μm radiation by water and many non-metallic materials, high-power Tm-doped fiber lasers have demonstrated considerable potential in minimally invasive medical procedures, such as soft-tissue surgery and laser lithotripsy, as well as in industrial laser processing, particularly polymer welding and other thermal processing applications [72–74]. These representative applications further demonstrate that continued improvements in power scaling, conversion efficiency, and long-term reliability are essential for promoting the practical deployment of high-power 2 μm fiber laser technology in scientific, industrial, and biomedical fields.

3.1.2 2 μm high-power multi-longitudinal-mode Ho-doped fiber laser

The output wavelength of TDFLs is generally concentrated below 2.0 μm. In contrast, due to the slight redshift of the emission peak of Ho³⁺ ions, the operating wavelength can be extended beyond 2.0 μm, reaching into the atmospheric transmission window. Consequently, efficient high-power laser generation in the 2.0−2.1 μm wavelength region primarily relies on HDFLs as the core gain medium. In this spectral region, Tm³⁺ ions exhibit relatively small emission cross sections, whereas HDFLs, benefiting from their redshifted emission peaks, not only meet the requirements for atmospheric transmission applications but also effectively mitigate parasitic oscillation issues during long-wavelength tuning. As a result, HDFLs have become the mainstream solution for laser generation beyond 2.0 μm. Furthermore, the excellent nonlinear frequency-conversion potential of this wavelength range has further stimulated research and development of HDFLs. In 1989, Hanna et al. first reported a continuous-wave HDFL operating at 2.04 μm. An argon-ion laser at 457.9 nm was used as the pump source, achieving a slope efficiency of 1.7% with a pump threshold of 46 mW [75]. Due to the limitations of fiber fabrication technology at that time, combined with the use of a pump wavelength that did not correspond to the optimal absorption band, the laser’s conversion efficiency remained extremely low. Nevertheless, this work marked the formal beginning of research on HDFLs.

With the rapid advancement of fiber fabrication technologies and a deeper understanding of optimal pump wavelength schemes, HDFLs have entered a phase of accelerated development. In 2012, Hemming et al. employed resonant cladding-pumping technology to achieve a CW output power of 140 W at 2.1 μm while maintaining excellent beam quality [76]. In 2013, Hemming et al. further utilized six 1.95 μm TDFLs as pump sources, realizing a peak output power of 407 W at 2.12 μm in an all-fiber configuration [77]. Its structural schematic diagram and output characteristics are shown in Figs. 3a and 3c. In 2017, Jin et al. developed a high-power CW dual-wavelength HDFL operating at 2049 nm and 2153 nm, respectively. The system was pumped by an 1150 nm LD with a rated power of 100 W, ultimately achieving a maximum output power of 22.3 W with a slope efficiency of 23% [79].

Over the past decade, benefiting from the iterative upgrading of MOPA technology and the continuous innovation of novel structured HDFs, the development of HDFLs has become increasingly diversified, with their performance characteristics being continuously enriched and refined. In 2019, Holmen et al. reported a HDFL designed for long-wavelength applications. The laser employed a high-feedback wavelength-selective ring-cavity configuration and exhibited a tuning range from 2025 nm to 2200 nm. The maximum slope efficiency reached 58% at 2050 nm, while a slope efficiency of 27% and a single-end output power of 5.5 W were obtained at 2200 nm [80]. In 2022, Beaumont et al. fabricated a novel triple-cladding HDF and achieved a laser output power of 62 W at 2.12 μm, with a slope efficiency as high as 73% [81]. In 2024, Pokorný et al. further optimization of the linear-cavity configuration enabled an HDFL to achieve a slope efficiency of 79% at 2.1 μm, demonstrating that improvements in cavity design and pump coupling can significantly enhance the energy-conversion efficiency while maintaining stable laser operation [82]. In 2026, Švejkar et al. employed a high-power 1940 nm TDFL for core pumping and developed an all-fiber HDFL, achieving a CW output power of 95 W at 2109 nm with a slope efficiency as high as 84% [84]. Its schematic configuration and output characteristics are shown in Figs. 3b and 3d. These recent advances indicate that the development of Ho-doped fiber lasers has gradually shifted from demonstrating basic laser operation toward improving slope efficiency, wavelength extension, and all-fiber integration. The introduction of novel fiber structures, such as triple-cladding fibers, together with optimized cavity configurations and core-pumping schemes, has significantly enhanced pump absorption and energy-conversion efficiency. Nevertheless, the further power scaling of HDFLs is still constrained by the relatively narrow gain bandwidth, the limited availability of high-brightness pump sources, and thermal management issues. Therefore, continued optimization of fiber design, pump architectures, and thermal management strategies will remain essential for realizing higher-power and more efficient Ho-doped fiber laser systems.

The continuous improvements in output power, conversion efficiency, and beam quality have considerably broadened the application prospects of high-power MLM Ho-doped fiber lasers. Benefiting from their emission wavelength around 2.1 μm, which coincides with favorable atmospheric transmission windows and stronger water absorption, these laser systems have attracted increasing attention in eye-safe remote sensing and coherent Doppler wind lidar, where the longer operating wavelength provides enhanced atmospheric transmission and reduced background interference [1,85]. Moreover, their excellent beam quality, power scalability, and capability for generating high-energy laser pulses make Ho-doped fiber lasers particularly attractive for high-energy pulsed laser systems, infrared countermeasure (IRCM) technologies, and other advanced defense applications requiring long-wavelength, eye-safe laser sources [1,86]. In addition, their stable operation and high output performance have promoted their use in free-space optical communication and long-range atmospheric sensing, further expanding the practical applications of 2.1 μm fiber laser technology [1,87]. These representative applications highlight the unique advantages of Ho-doped fiber lasers in the 2.1 μm spectral region and demonstrate that continued improvements in power scaling, conversion efficiency, and operational stability will further promote their deployment in remote sensing, defense, free-space optical communication, and scientific research.

3.1.3 2 μm high-power multi-longitudinal-mode Tm-Ho co-doped fiber laser

Due to the relatively narrow absorption spectrum and specific absorption bands of HDFLs, there is currently a shortage of high-power, high-energy LDs that can directly serve as suitable pump sources. Consequently, TDFLs are typically employed for in-band pumping to achieve population inversion in HDFLs. However, this pumping scheme significantly increases system cost, complexity, and physical size [88]. Additionally, detrimental interionic interactions, such as pair-induced quenching, generate substantial thermal loads, severely limiting overall system efficiency and causing the practical efficiency of in-band-pumped HDFLs to fall below theoretical expectations [89]. Under these circumstances, THCDFs offer an effective solution to the challenges mentioned above. These fibers efficiently respond to LD pumping in the 790 nm wavelength region. The Tm³⁺ ions enhance pump efficiency through a CR mechanism, and the excitation energy is subsequently transferred from Tm³⁺ ions to Ho³⁺ ions via interionic ET processes, ultimately resulting in laser emission at the characteristic wavelengths of Ho³⁺ ions. In 1994, Oh et al. first reported a CW thulium-holmium co-doped fiber laser (THCDFL), achieving a maximum output power of 12.5 mW with a slope efficiency of 4.2%. Wavelength tuning was accomplished by varying the cavity length, resulting in a tuning range from 2.037 μm to 2.096 μm [90]. In 2002, Taniguchi et al. employed a 1212 nm third-order Stokes Raman fiber laser to pump a THCDF, achieving an output power of 450 mW with a central wavelength of 1970 nm [92]. In 2007, Jackson et al. utilized bidirectional direct pumping with 793 nm LDs in a THCDFL, achieving a maximum output power of 83 W with a slope efficiency of 42%, while maintaining excellent beam quality [93]. Its structural schematic diagram and output characteristics are shown in Figs. 4a and 4b.

Leveraging all-fiber architectures, MOPA technology, and emerging triple-clad fibers, THCDFLs have undergone further advancement. In 2014, Xue et al. reported an all-fiber wavelength-tunable THCDFL operating in the 2 μm wavelength region, with a tuning range spanning 1768−2071 nm. After amplification using a single-stage thulium-holmium co-doped fiber amplifier (THCDFA), an output power of 408 mW at 1910 nm was achieved, with a maximum slope efficiency of 42.6% [94]. In 2019, Tench et al. developed a polarization-maintaining hybrid THCDFL. Through two-stage amplification, the system achieved CW output power exceeding 25 W at a wavelength of 2051 nm, with a slope efficiency of 54.9% [95]. In 2020, Forster et al. reported a polarization-maintaining thulium-holmium co-doped triple-cladding fiber laser, achieving a laser output power of 181 W at 2.05 μm, with a slope efficiency of 34.1% and an optical-to-optical conversion efficiency of 30% [96]. In 2022, Forster et al. demonstrated a free-space THCDFL configuration. By incorporating a tunable diffraction grating, a broadly tunable laser source with a tuning range of 200 nm was realized. The system achieved an output power of 262 W at 2.1 μm with a slope efficiency of 49%. When a volume Bragg grating operating at 2.2 μm was employed, an output power of 77 W was obtained with a slope efficiency of 29% [98]. The corresponding schematic configuration and output characteristics are shown in Figs. 4c and 4d. In 2025, further optimization of the all-fiber linear-cavity configuration enabled a THCDFL to achieve a CW output power of 38.9 W at 2.08 μm with a slope efficiency of 46.84%, demonstrating that continuous improvements in fiber structure and cavity optimization remain effective for enhancing laser performance [43]. These representative studies indicate that the development of THCDFLs has gradually evolved from demonstrating wavelength tunability toward simultaneously improving output power, wavelength flexibility, and optical-to-optical conversion efficiency. The introduction of triple-cladding fibers, polarization-maintaining structures, and optimized MOPA architectures has significantly enhanced pump utilization and overall laser performance. Nevertheless, further performance improvement of THCDFLs is still constrained by the complex energy-transfer dynamics between Tm³⁺ and Ho³⁺ ions, thermal accumulation under high-power operation, and the optimization of co-doping concentrations. Therefore, continued optimization of fiber composition, cavity design, pump schemes, and thermal management strategies will remain essential for realizing higher-power, broader tunability, and more efficient thulium-holmium co-doped fiber laser systems.

Benefiting from the efficient energy transfer between Tm³⁺ and Ho³⁺ ions, MLM Tm-Ho co-doped fiber lasers provide broad wavelength tunability and broadband gain characteristics, making them attractive laser sources for a variety of wavelength-flexible applications. Owing to their wide tuning range in the 2 μm wavelength region, these laser systems are particularly attractive as widely tunable laser sources for high-resolution spectroscopy, precision optical measurements, and wavelength calibration [94,99]. Their flexible wavelength coverage also enables gas sensing and differential absorption lidar (DIAL), allowing selective detection of atmospheric molecules such as H₂O, CO₂, and CH₄ through wavelength-dependent absorption characteristics [94,100,101]. Furthermore, their broad gain bandwidth and stable CW operation make Tm-Ho co-doped fiber lasers promising sources for nonlinear frequency conversion and mid-infrared laser generation, providing versatile laser sources for environmental monitoring, molecular spectroscopy, and infrared photonics [1,99]. These representative applications demonstrate that the excellent wavelength flexibility and broadband emission characteristics of Tm-Ho co-doped fiber lasers provide unique advantages for tunable photonic systems, atmospheric sensing, and nonlinear optical technologies.

3.2 2 μm high-power narrow-linewidth fiber laser

The output of conventional CW lasers typically consists of MLM, resulting in spectral broadening. In contrast, NLW operation requires the laser to oscillate at a single stable frequency, thereby achieving an extremely narrow spectral linewidth. NLW fiber lasers, characterized by their exceptionally narrow spectral linewidth, low-noise output, and high coherence, have been extensively applied in fields such as quantum photonics, high-resolution spectroscopy, differential absorption lidar, gravitational wave detection, and coherent communications. Among these, single-frequency (SF) fiber lasers represent the core technological embodiment of the NLW approach [102–104]. The fundamental challenge in achieving SF operation stems from the fact that the gain and cavity components of conventional fiber lasers typically exhibit broad spectral responses, with bandwidths far exceeding the frequency spacing between adjacent longitudinal modes of the resonator. This readily induces intermodal beat effects, severely degrading SF output performance. To address this issue, current mainstream technical strategies can be divided into two categories. The first is the short-cavity scheme, in which the physical length of the resonator is reduced by employing distributed feedback (DFB) [105] or distributed Bragg reflector (DBR) [106] structures, thereby significantly increasing the longitudinal-mode frequency spacing and enabling single-longitudinal-mode (SLM) lasing. The second is the intracavity frequency-selection scheme, where a narrowband filtering element with a linewidth narrower than the longitudinal-mode spacing is introduced into the resonator [107]. A representative example is a dynamic filter based on a standing-wave saturable absorber (SA), which suppresses MLM oscillation through a dynamic frequency-selection mechanism. Currently, NLW fiber lasers have been successfully demonstrated in various rare-earth-doped fiber systems operating in the 2 μm band [7,108,109]. Despite these advances, further power scaling of SF fiber lasers remains fundamentally constrained by SBS, one of the dominant nonlinear effects limiting high-power NLW operation [110]. Current SBS suppression strategies can generally be classified into three categories: spectral broadening techniques (e.g., phase modulation), mode-area engineering (e.g., LMA fibers), and power-scaling architectures (e.g., multi-stage MOPA systems) [110–113]. Although these approaches have significantly improved the SBS threshold and enabled substantial power scaling, each is accompanied by inherent trade-offs associated with spectral purity, beam quality, thermal management, or system complexity [110–113]. Consequently, achieving simultaneous high output power, UNLW, and long-term operational stability remains a major challenge for 2 μm NLW fiber lasers. Against this background, this section reviews the current research status of NLW fiber lasers based on thulium-doped, holmium-doped, and thulium-holmium co-doped systems. The relevant results are summarized in Table 3.

3.2.1 2 μm high-power narrow-linewidth Tm-doped fiber laser

As the most widely used gain medium for 2 μm lasers, thulium-doped fiber has been the primary focus of NLW fiber laser research. By leveraging the fact that a short-cavity structure increases the longitudinal mode spacing—thereby enabling NLW output—the short-cavity design has become the core approach in the early development of NLW thulium-doped fiber lasers. In 2004, Agger et al. first employed a DFB short-cavity structure by inscribing a FBG in a 4.7 cm-long TDF to construct a narrowband feedback resonator. They demonstrated a single-frequency TDFL operating at 1.7 μm with an average output power of 1 mW [108]. Through the synergistic effect of the short cavity length and narrowband FBG feedback, the laser was forced to operate in a SLM state, effectively suppressing linewidth broadening induced by MLM oscillation. However, limited by early-stage fiber fabrication technology, the active fiber suffered from low gain and insufficient pump absorption efficiency, making it difficult to meet the requirements of high-power, NLW applications.

Achieving high-power, NLW laser output depends on efficient pump absorption and signal gain; however, conventional glass hosts generally cannot support high-concentration doping of thulium ions. The use of multi-component oxide glasses—such as silicate, tellurite, and germanate—as host matrices can effectively increase the doping solubility of Tm³⁺ ions. In 2007, Geng et al. developed a single-frequency DBR fiber laser based on highly thulium-doped germanate fiber, delivering an average output power of 50 mW with a slope efficiency of 35% [114]. In 2008, Zhang et al. fabricated a DFB fiber laser using a thulium-doped photosensitive aluminosilicate fiber operating at 1943 nm. Core-pumped by a 1565 nm erbium-ytterbium co-doped fiber laser, a maximum SF output power of 875 mW was achieved. Subsequently, a thulium-doped fiber amplifier (TDFA) was spliced to the rear end of the resonator, and under a total pump power of 8.1 W, the output power was further increased to 3 W [115]. In 2013, He et al. employed a self-developed, highly thulium-doped germanate fiber as the gain medium and realized a SF laser with a central wavelength of 1950 nm and a linewidth on the order of kilohertz, achieving a net gain coefficient of 2.3 dB/cm [117].

To further scale the output power of SF lasers, researchers introduced the MOPA technique to achieve efficient power amplification. In 2009, Goodno et al. adopted a MOPA architecture consisting of four cascaded TDFAs, achieving a high average power SF laser output of 608 W at 2040 nm [118]. In 2012, Shah et al. reported an NLW thulium-doped fiber MOPA system with an optimal output power of 109 W, a sub-nanometer spectral linewidth, and a slope efficiency of 46% [120]. In 2014, Lucas et al. built a MOPA configuration based on large-mode-area TDF and obtained pulsed laser output in the 2050 nm band with a pulse width of 110 ns and a peak power of 1 kW, marking the first breakthrough to the kilowatt level, corresponding to a slope efficiency of 20% [123]. The laser configuration and output characteristics are shown in Figs. 5a and 5b. In 2015, Wang et al. developed an all-fiber, high-power, SF thulium-doped MOPA laser with a single-stage main amplification configuration. Its structure and output characteristics are shown in Figs. 5c−5e. The system employed an ultra-short-cavity SF laser with an output power of 40 mW and a central wavelength of 1971 nm as the seed source. After amplification through a two-stage chain consisting of a pre-amplifier and a main amplifier, a final output power of 310 W was achieved with a slope efficiency of 56% [125].

Over the past decade, TDFLs have made significant advances in gain medium optimization, SF cavity design, and high-power output. In 2019, Roumayah et al. developed a tunable high-power SF thulium-doped fiber MOPA system aimed at atmospheric transmission applications. The system consisted of a pre-amplifier stage and a final power amplifier stage. The main cavity delivered approximately 5 mW of seed laser tunable across the 1900−2000 nm band, which was boosted to 2−3 W by the pre-amplifier and subsequently power-amplified through a TDF with a core diameter of 25 μm, ultimately reaching an output power of 100 W [127]. In 2021, Anderson et al. reported a beam-combinable all-fiber TDFA operating at 1.95 μm, delivering 1.1 kW of CW output with a slope efficiency of 50.7% and near-diffraction-limited beam quality [128]. A 5 GHz phase-modulated signal was employed to mitigate SBS during power scaling. This work marked an important milestone in kilowatt-class 2 μm NLW fiber amplification. Whereas this study primarily addressed power scaling, subsequent research increasingly emphasized spectral purity, wavelength selectivity, and operational flexibility. In 2023, Wang et al. combined an eye-shaped four-coupler dual-ring compound cavity with a SA to develop a single-frequency TDFL with a central wavelength of 2.05 μm; a measured spectral linewidth of 11.22 kHz was obtained [131]. In 2024, further optimization of the ring-cavity configuration enabled stable NLW laser operation at 1.94 μm, demonstrating continued progress in wavelength-selective control and cavity optimization for SF fiber lasers [132]. In 2025, Cai et al. proposed a bidirectional output wavelength switchable NLW thulium-doped fiber laser employing a compound ring cavity combined with a multi-objective optimization algorithm; the structure and output characteristics are shown in Figs. 5f and 5g. The device delivered SLM laser outputs at 2048.502 nm and 1942.080 nm in the clockwise and counterclockwise optical paths, respectively, featuring a high SNR, low noise, and operational stability [133]. Collectively, these studies indicate that narrow-linewidth TDFLs have evolved from an emphasis on output-power scaling toward the coordinated optimization of spectral purity, wavelength flexibility, and long-term stability. Advanced cavity architectures, together with intelligent optimization algorithms, have improved wavelength selectivity, linewidth control, and switching capability. Nevertheless, further power scaling remains constrained by SBS, thermal effects, and the intrinsic trade-off between output power and spectral linewidth. Future research should therefore focus on nonlinear-effect suppression, thermal management, advanced cavity design, and intelligent control to enable higher-power single-frequency TDFLs with UNLWs and improved operational stability.

The continuous improvements in output power, spectral purity, and frequency stability have significantly broadened the application prospects of NLW Tm-doped fiber lasers. Benefiting from their excellent temporal coherence and low phase noise, these laser systems have become indispensable master oscillators and seed sources for high-power MOPA architectures, enabling efficient power scaling while maintaining excellent spectral characteristics [134,135]. Their outstanding frequency stability and narrow spectral linewidth also make them highly attractive for high-resolution molecular spectroscopy, precision gas sensing, and frequency metrology, where accurate wavelength control and long-term operational stability are essential [126,135,136]. Furthermore, NLW Tm-doped fiber lasers have been widely employed as seed lasers for coherent beam combining (CBC) systems, in which stable phase characteristics are crucial for achieving high combining efficiency and excellent beam quality [134]. In addition, they serve as efficient pump or seed sources for holmium-doped fiber amplifiers, OPOs, and other nonlinear frequency-conversion systems, facilitating coherent laser generation in the mid-infrared spectral region [1,126]. These representative applications demonstrate the indispensable role of NLW Tm-doped fiber lasers in advanced fiber laser architectures, precision photonic instrumentation, and nonlinear optical technologies.

3.2.2 2 μm high-power narrow-linewidth HDFL and THCDFL

TDFLs are the primary laser sources for NLW emission in the 2 μm band. However, driven by the unique absorption characteristics of gases such as nitric oxide, fields including atmospheric sensing and precision medicine have generated urgent demands for NLW lasers operating at 2.1 μm and longer wavelengths [76]. Compared to HDFLs, THCDFLs benefit from an efficient Tm³⁺ → Ho³⁺ energy transfer mechanism. These lasers can be efficiently pumped by the well-established 793 nm pump source and enable Ho³⁺ ions to generate laser radiation in the 2.1−2.2 μm range. Consequently, they demonstrate superior pump efficiency and gain performance for long-wavelength NLW laser output, making them the core gain medium for laser emission in this wavelength region.

Regarding HDFLs, in 2009, Wu et al. developed a single-frequency DBR holmium-doped fiber laser, achieving an average output power of 50 mW at a central wavelength of 2053 nm [109]. In 2019, Taylor et al. employed a 1.15 μm pump source to develop a single-frequency DBR holmium-doped fiber laser operating in the 2.1 μm band [138]. In the same year, Wolf et al. developed a distributed HDF laser based on a π-phase-shifted FBG, achieving SF output at a wavelength of 2.07 μm with an output power of 53 mW and a spectral linewidth of approximately 10 kHz [139]. Its structure and output characteristics presented in Figs. 6a and 6b. In 2020, Traoré et al. reported an all-fiber single-frequency DFB laser based on a FBG at 2051 nm. The laser had an initial output power of 36 mW, an optical SNR better than 65 dB per 0.05 nm, and a full width at half maximum (FWHM) linewidth of 130 kHz measured by the heterodyne method. After power amplification through a polarization-maintaining holmium-doped fiber amplifier (HDFA), the NLW output power was scaled up to 1 W [140].

In recent years, with the continuous advancement of various frequency-selective structures, such as ring cavities, the research directions of NLW holmium-doped fiber lasers have become increasingly diverse. In 2022, Vladimirskaya et al. developed a SF ring-cavity HDFL, utilizing an 1125 nm YDFL as the pump source. The system produced a laser output of 4 mW in the 2.1 μm band and demonstrated stable operation under liquid nitrogen cryogenic conditions [141]. The laser structure and output characteristics are shown in Figs. 6c and 6d. In 2024, Wei et al. developed an all-polarization-maintaining single-frequency HDFL at 2.09 μm, pumped by a 1940 nm TDFL. A SF signal with a central wavelength of 2089.73 nm and a peak power of 57.9 mW was generated, and the output power was amplified to 1 W after passing through a HDF amplification stage [143]. In the same year, an all-fiber MOPA system further increased the output power to 15 W at 2.095 μm with a slope efficiency of 57%, demonstrating the feasibility of combining SF operation with efficient power amplification [144]. In 2026, Baer et al. presented an NLW polarization-maintaining HDF amplification system operating in the same wavelength band, achieving an output power of approximately 25 W and a polarization extinction ratio of 17 dB, with no ASE noise detected when the relative power dropped to −65 dB [145]. These representative studies indicate that recent research on narrow-linewidth HDFLs has gradually evolved from demonstrating stable SF operation toward simultaneously improving output power, amplification capability, polarization performance, and long-term operational stability. The introduction of polarization-maintaining fibers, all-fiber MOPA architectures, and optimized cavity configurations has significantly enhanced system robustness and power scalability. Nevertheless, further performance improvement of narrow-linewidth HDFLs remains limited by the relatively narrow gain bandwidth of Ho-doped fibers, thermal accumulation during power amplification, and the trade-off between output power and spectral linewidth. Future research is therefore expected to focus on high-efficiency amplification schemes, optimized cavity configurations, and polarization-maintaining architectures to further improve output power and long-term operational stability.

Regarding HTCDFLs, in 2012, Li et al. used a commercial 800 nm LD to pump a thulium-holmium co-doped tungsten-tellurite glass single-mode fiber, achieving a stable laser output of 35 mW in the 2.1 μm band [146]. In 2023, Shi et al. demonstrated the first watt-level, 2050 nm SF fiber oscillator using unpumped Tm/Ho co-doped fiber as a SA. They achieved a maximum SF output power of 1.2 W and an optical-to-optical efficiency of 20.7% with a total bidirectional pump power of 5.8 W at 1570 nm [147]. Its structure and output characteristics presented in Figs. 7a−7c. In 2024, Wei et al. fabricated a series of Tm: YAG/Ho: YAG co-derived silica gain fibers using the molten core method. By performing in-band pumping with a 1610 nm fiber laser, they successfully achieved stable single-frequency DBR laser output at 2040 nm, with a maximum output power of 306 mW and a slope efficiency of 19.3% [150]. In 2026, Yu et al. proposed a self-injection-locked, dual-wavelength switchable, SLM thulium-holmium co-doped fiber laser, achieving an UNLW output of 0.09 kHz [151]. By adjusting the intracavity polarization controller, the laser produced stable UNLW emission at 1941.76 nm and 1942.17 nm, with an optical SNR exceeding 72 dB. The laser structure and output characteristics are shown in Figs. 7d−7f. These representative studies indicate that narrow-linewidth THCDFLs have evolved from demonstrating basic SF operation toward simultaneously improving spectral purity, wavelength flexibility, output power, and operational stability. Nevertheless, further development is still limited by the complex Tm³⁺-Ho³⁺ energy-transfer dynamics, optimization of co-doping concentration, and the trade-off between output power and spectral linewidth. Future research is therefore expected to focus on optimized co-doped gain fibers, advanced cavity configurations, and intelligent control strategies for realizing high-power ultra-narrow-linewidth operation in the 2.1−2.2 μm wavelength region.

Benefiting from their excellent temporal coherence, narrow spectral linewidth, and stable emission in the 2.0−2.1 μm spectral region, NLW Ho-doped and Tm-Ho co-doped fiber lasers have emerged as promising laser sources for a broad range of precision photonic applications. Their superior spectral purity and frequency stability make them ideal master oscillators and seed sources for high-power MOPA architectures, enabling efficient power scaling while maintaining excellent coherence and beam quality [85,99]. Moreover, these laser systems are well suited for high-resolution molecular spectroscopy, precision gas sensing, and laser frequency metrology, where accurate wavelength control and long-term frequency stability are essential for resolving narrow molecular absorption features and improving measurement accuracy [1,99]. Their excellent phase stability and beam quality also make them attractive seed sources for CBC systems, providing an effective route toward coherent power scaling while preserving diffraction-limited beam quality [85,134]. Furthermore, their emission around 2.1 μm enables efficient pumping of OPOs and other nonlinear frequency-conversion systems, facilitating coherent laser generation in the mid-infrared spectral region for applications such as molecular spectroscopy, environmental monitoring, and infrared photonics [1,94]. Overall, the complementary characteristics of Ho-doped and Tm-Ho co-doped gain media provide a versatile technological platform for realizing high-coherence, high-stability, and wavelength-flexible laser sources in the 2 μm wavelength region, supporting the continued development of advanced fiber laser systems and mid-infrared photonic technologies.

Besides MLM and NLW operation, QCW fiber lasers have also attracted increasing attention in recent years owing to their capability of providing high peak power while reducing thermal loading. QCW lasers emit light intermittently in the form of high-frequency pulses. The primary distinction from CW lasers lies in the temporal characteristics of the output: QCW lasers can deliver high peak power instantaneously while effectively reducing the thermal load on the device, thereby enabling high-energy output [152–157]. In 2016, Sypin et al. developed a compact TDFL module operating in a millisecond-level pulsed mode, achieving a peak power of 500 W, an average power of 50 W, and a pulse energy of 5 J [152]. In 2022, Chen et al. pumped an intracavity TDF with a 793 nm source, obtaining pulsed laser output with a central wavelength of 1940 nm, a repetition rate of 10 Hz, a pulse width of 10 ms, a peak power of 515 W, and an average power exceeding 50 W [154]. In 2025, Lin et al. developed a high-power QCW thulium-doped fiber oscillator with a slope efficiency of 53.6%, a maximum average output power of 210 W, and a peak power generally exceeding 2 kW, with a record value of 3088.2 W at an operating wavelength of 1940 nm [156]. The system configuration and output performance are illustrated in Fig. 8.

Overall, remarkable progress has been achieved in 2 μm CW fiber lasers over the past decade. High-power, MLM fiber lasers operating in the 2 μm band have achieved kilowatt-level output and are widely deployed in industrial applications. NLW fiber lasers can stably achieve SF operation and, due to their excellent spectral purity, meet the demands of high-precision applications such as high-resolution spectroscopy, coherent laser communication, and atmospheric interferometric sensing. Compared with the technically mature single-frequency TDFLs, NLW fiber lasers based on Ho-doped and Tm-Ho co-doped fibers are still under active development, particularly in simultaneously achieving high output power, high efficiency, and UNLW operation. Future research is expected to focus on optimized gain-fiber design, optimization of the Tm-Ho co-doping ratio, advanced intracavity frequency-selection techniques, efficient nonlinear-effect suppression, intelligent control strategies, and the integration of MOPA architectures to further improve both output power and linewidth performance in the 2.1−2.2 μm spectral region. This advancement will cater to high-end applications such as gas sensing and minimally invasive surgery. In addition, recent progress in QCW operation provides a complementary approach for achieving high-peak-power laser generation with reduced thermal loading, making 2 μm QCW fiber lasers an increasingly attractive direction, particularly for medical applications.

4 2 μm high-power pulsed fiber laser

Currently, research on gain media for 2 μm pulsed fiber lasers predominantly focuses on TDFs, while investigations into holmium-doped and thulium-holmium co-doped pulsed fiber lasers remain relatively limited. High-power 2 μm pulsed fiber laser output is typically achieved through chirped pulse amplification (CPA) technology, with thulium-doped CPA systems already reaching peak powers ranging from tens to hundreds of megawatts (MW) [158]. TDFs exhibit broad and flat spectral gain characteristics, making them an ideal gain medium for generating ultrashort pulses and enabling wideband wavelength tunability. According to their pulse-generation mechanisms, 2 μm pulsed fiber lasers can generally be classified into three categories: QS, ML, and GS lasers [159]. Although all three approaches generate pulsed laser output, they differ fundamentally in pulse-formation mechanism, achievable pulse duration, repetition-rate range, pulse energy, peak-power capability, intracavity implementation, and representative applications. A comprehensive comparison of these representative pulse-generation mechanisms is summarized in Table 4. In general, QS fiber lasers generate nanosecond- to microsecond-scale pulses with relatively high pulse energies and peak powers through intracavity-loss modulation, making them suitable for laser ranging, lidar, and precision material processing [159]. ML fiber lasers produce picosecond- to femtosecond-scale ultrashort pulses at high repetition rates, enabling applications in ultrafast photonics, nonlinear optics, and optical frequency metrology [160,161]. In contrast, GS fiber lasers generate pulses through transient gain modulation, typically by directly modulating the pump source, providing a compact architecture with flexible repetition-rate control and making them attractive as seed sources for MOPA systems as well as for wavelength-tunable or high-energy nanosecond pulse generation [162,163]. The following subsections review the recent progress of these three pulse-generation mechanisms in detail. Particular emphasis is placed on thulium-doped pulsed fiber lasers, while representative developments in holmium-doped and thulium-holmium co-doped pulsed fiber lasers are also summarized, as presented in Tables 5 and 6.

4.1 2 μm high-power Q-switched pulsed fiber laser

QS technology, which modulates the quality factor (Q-factor) of the resonator, is the primary method used in fiber lasers to generate narrow pulses with durations ranging from nanoseconds to microseconds and high peak power. The Q-factor represents the ratio of energy storage capacity to loss within the resonator: a higher Q-factor indicates lower intracavity loss and facilitates the onset of laser oscillation, whereas a lower Q-factor corresponds to higher cavity loss and makes laser oscillation more difficult to establish [164]. Based on its operating principle, QS technology can be classified into active and passive QS. In 2003, El-Sherif et al. pioneered the development of a 2 μm band Q-switched TDFL, employing an acousto-optic modulator to achieve active QS operation, resulting in laser output with a peak power of 4.1 kW and a pulse width of 150 ns [165]. In 2008, Tang et al. constructed a passively QS double-clad TDFL using a 790 nm LD array as the pump source and a polycrystalline Cr:ZnSe microchip as the SA. The laser produced pulsed output in the 2 μm band with a pulse width of 120 ns, a repetition rate of 53 kHz, and a pulse energy exceeding 14 μJ [167]. In 2013, Stutzki et al. employed an 81 μm LMA fiber as the gain medium, combined with bidirectional pumping and an acousto-optic QS configuration, to realize high-performance pulsed laser output: a central wavelength of 2013.9 nm, a repetition rate of 13.9 kHz, a pulse width of 15 ns, an average power of 33 W, a pulse energy of 2.4 mJ, and a peak power of 150 kW. The structure and output characteristics of this laser are shown in Figs. 9a and 9b [169].

To increase the output power of pulsed lasers, researchers have employed the MOPA configuration to amplify the laser signal. In 2015, Ouyang et al. developed a three-stage Q-switched TDFL; when the peak power exceeded 110 W, the spectral linewidth was only 0.8 nm [172]. In 2021, He et al. employed a two-stage MOPA architecture to develop a high-power, all-polarization-maintaining, acousto-optically Q-switched TDFL at 2 μm. The laser configuration and output characteristics are shown in Figs. 9c and 9d. The device delivered a central wavelength of 2009.71 nm, a pulse width of 97 ns, a repetition rate of 20 kHz, an output power of 2.04 W, a peak power of 2.1 kW, and a pulse energy exceeding 204 μJ, with a power amplifier stage slope efficiency of 22.7% [173]. In 2025, Sójka et al. demonstrated a 1.96 μm Q-switched TDFL operating at a repetition rate of 3 kHz, delivering a pulse energy of 84 μJ, a pulse width of 272 ns, and a corresponding peak power of 309 W [174].

In recent years, extensive research on QS laser technology has focused on novel SA materials and gain fibers. In 2019, Dalloz et al. utilized dual 793 nm LD pumping to develop an actively QS thulium-holmium co-doped polarization-maintaining DCF laser, achieving an average output power of 55 W at 2.09 μm, with a pulse width of 100 ns and a repetition rate of 200 kHz. The device configuration and output performance are illustrated in Figs. 9e and 9f [175]. In 2021, Lee et al. employed a graphene oxide SA in a 2058 nm Q-switched HDFL. As the pump power increased from 759 mW to 1072 mW, the pulse width decreased from 2.01 μs to 1.56 μs, while the repetition rate rose from 45.56 kHz to 56.12 kHz. At a pump power of 1072 mW, a maximum average output power of 11.61 mW and a single pulse energy of 207.05 nJ were achieved [176]. In the same year, Forster et al. demonstrated a QS chalcogenide tellurite fiber laser operating in the 2 μm band, delivering a pulse width of 45.6 ns, a repetition rate of 63 kHz, a peak power of 15.7 kW, and an average output power of up to 48 W [177]. More recently, in 2025, further optimization of the cavity configuration and SA design enabled a QS Tm-doped fiber laser operating at 1.96 μm with improved pulse characteristics, demonstrating the continued development of compact and efficient QS fiber laser systems [174]. These representative studies indicate that the development of QS fiber lasers has gradually evolved from demonstrating high-energy nanosecond pulse generation toward simultaneously optimizing pulse energy, peak power, repetition rate, system compactness, and long-term operational stability. The introduction of advanced SAs, LMA gain fibers, and all-fiber MOPA configurations has significantly enhanced pulse performance and power scalability. Nevertheless, further performance scaling remains constrained by the damage threshold and recovery dynamics of SAs, thermal accumulation under high-power operation, and the trade-off between pulse energy and repetition rate. Future research is therefore expected to focus on high-damage-threshold modulation materials, optimized cavity designs, efficient thermal management, and intelligent pulse-control strategies to realize higher-energy, higher-peak-power, and more reliable 2 μm QS fiber laser systems.

Benefiting from their high pulse energy, high peak power, and compact all-fiber configuration, QS fiber lasers operating in the 2 μm wavelength region have attracted considerable attention in a variety of practical applications. Their nanosecond pulse duration and high peak power make them attractive light sources for laser ranging, lidar, and remote sensing, where long-distance detection and eye-safe operation are essential [85,178]. In addition, 2 μm QS fiber lasers have demonstrated great potential in precision material processing, including drilling, cutting, and micromachining of polymers, ceramics, and other infrared-absorbing materials, owing to their high pulse energy and excellent beam quality [1,178]. Benefiting from the strong water absorption around the 2 μm wavelength region, these laser systems are also widely employed in medical procedures, such as minimally invasive surgery and laser tissue ablation, where precise energy deposition and limited thermal damage are required [1,99]. Furthermore, their high pulse energy makes them efficient pump sources for OPOs and other nonlinear frequency-conversion systems, enabling coherent laser generation in the mid-infrared spectral region [1]. These representative applications demonstrate that QS fiber lasers have become important laser sources for industrial manufacturing, biomedical engineering, remote sensing, and mid-infrared photonics.

4.2 2 μm high-power mode-locked pulsed fiber laser

ML technology synchronizes the phases of different longitudinal modes within a resonator, enabling their coherent superposition to generate an ultrashort pulse train. The pulse duration can reach the picosecond or even femtosecond scale, with the repetition rate determined by the cavity length [179]. In recent years, ML fiber laser technology has advanced significantly. ML fiber lasers operating in the 2 μm wavelength band have become a key laser technology, with important applications in supercontinuum generation, biomedical imaging, and precision material processing [180]. Among various ML techniques, passive ML has become the dominant approach owing to its simple cavity configuration, compact implementation, and capability of generating stable ultrashort pulses [160,181]. To realize passive ML, several saturable absorption mechanisms have been developed, including nonlinear polarization rotation (NPR), SESAMs, CNT saturable absorbers, and nonlinear optical loop mirrors (NOLMs) [160,181,182]. Although these techniques share the same objective of initiating and stabilizing ultrashort pulse generation, they differ significantly in optical damage threshold, long-term operational stability, fabrication complexity, and suitability for high-power operation. NPR enables an all-fiber implementation without introducing additional SA materials, providing a relatively high optical damage threshold and broad operating bandwidth. However, its strong dependence on intracavity polarization states makes it susceptible to environmental perturbations, leading to relatively limited long-term stability. SESAMs provide reliable self-starting capability, excellent reproducibility, and stable pulse generation owing to mature semiconductor fabrication technology. Nevertheless, their relatively low optical damage threshold, limited operating bandwidth, and comparatively sophisticated fabrication process restrict further power scaling. CNT-based saturable absorbers offer broadband operation, simple fabrication, and low manufacturing cost, making them attractive for compact ultrafast fiber lasers. However, their thermal stability and long-term reliability under high-power operation remain inferior to those of semiconductor-based devices. By contrast, NOLMs rely on nonlinear interference in an all-fiber cavity rather than material absorption, thereby eliminating material damage limitations while providing high power-handling capability and good environmental stability, although they generally require more sophisticated cavity configurations [182–185]. Consequently, no single passive ML mechanism is universally superior. Instead, the choice should be determined by the targeted operating regime and application requirements through balancing optical damage threshold, environmental stability, fabrication complexity, pulse characteristics, and power scalability [160,185].

In 1995, Nelson et al. demonstrated the first mode-locked TDFL in the 2 μm band based on NPR, capable of delivering tunable ultrashort laser pulses with a central wavelength around 2 μm [186]. This pioneering work demonstrated the feasibility of passive ML in the 2 μm wavelength region and established NPR as one of the earliest ML techniques for 2 μm fiber lasers. Since then, research on thulium-doped, holmium-doped, and thulium-holmium co-doped ML fiber lasers in the 2 μm band has progressed continuously, with various technical approaches proposed. In 2007, Kivisto et al. developed a passively ML fiber laser using a THCDF and an antimonide-based semiconductor saturable absorber mirror (SESAM). The laser achieved wavelength tuning across the 1912−1972 nm range and produced stable soliton-like pulses with pulse widths as short as 750 fs [188]. Compared with NPR, SESAM-based mode locked provides superior self-starting capability and operational stability, but its relatively low damage threshold and fabrication complexity limit further power scaling. In 2011, Wang et al. constructed a passively ML fiber laser by combining a THCDF with an antimony-based SESAM, generating stable ML soliton pulses at a wavelength of 2.06 μm, with a pulse width of 1.1 ps and a single-pulse energy of 0.41 nJ [189]. In 2012, Chamorovskiy et al. reported the first passively mode-locked HDFL at 2085 nm. ML was achieved using a SESAM and a CNT absorber separately, generating ultrashort pulses with a pulse width of 890 fs and an average power of 46 mW. The laser configuration and corresponding output spectra for both ML elements are shown in Figs. 10a and 10b [191]. CNT saturable absorbers provide a broadband and cost-effective alternative to SESAMs, although improvements in thermal stability and long-term reliability are still required for high-power operation.

To scale the output power of 2 μm pulsed fiber lasers, researchers have adopted the MOPA architecture for power amplification. In 2012, Liu et al. developed a compact, high-power, picosecond all-fiber thulium-doped MOPA laser. The seed was passively ML using a SESAM, delivering pulses at 1962.8 nm with an average power of 15 mW and a pulse width of 1.5 ps. After two-stage double-clad TDF amplification, picosecond pulses with an average power of 20.7 W, pulse energy of 200 nJ, pulse width of 18 ps, and peak power of 11.2 kW were obtained, with a slope efficiency of 42%. The system configuration and output performance are shown in Figs. 10c and 10d [194]. In 2013, the same group further developed a 2 μm high-power all-fiber thulium-doped MOPA laser with a central wavelength of 1963 nm, delivering an average output power of 120.4 W, pulse width of 16 ps, repetition rate of 333.75 MHz, slope efficiency of 59%, and peak power of 22.5 kW [195]. In 2016, Liu et al. constructed a linearly polarized picosecond thulium-doped all-fiber MOPA laser using polarization-maintaining fibers and polarization components, achieving an average output power of 240 W at 1963 nm with a repetition rate of 127 MHz [196].

By employing novel structured gain fibers, laser gain characteristics can be effectively optimized while simultaneously enhancing both the average and peak power levels of ML pulses. In 2014, Stutzki et al. developed a high-power TDFL based on CPA, using an LMA, polarization-maintaining thulium-doped PCF with a core diameter of 50 μm as the gain medium. They achieved ultrahigh-power pulse output with an average power of 152 W and a peak power of 4 MW. The laser configuration and output characteristics are shown in Figs. 10e and 10f [197]. In 2015, Gebhardt et al. combined a free-space Kagome-type PCF pulse compressor with a large-pitch rod-type polarization-maintaining thulium-doped PCF, achieving sub-70 fs ultrashort pulse output with a peak power of up to 200 MW and an average power of 2 W [198]. In 2018, Wang et al. achieved high-power ML operation of a multimode TDFL in the 2 μm band. By employing a SA mirror for ML control, simultaneous multi-transverse-mode lasing was realized with an average output power of 10 W; the LMA fiber supported pulse energies exceeding 500 nJ and peak powers exceeding 10 kW [180]. In 2019, Zhu et al. developed a passively ML fiber laser based on a large-mode-area TDF, achieving stable ML output in the 2 μm band with a pulse width of approximately 600 fs, a repetition rate of 53.7 MHz, a maximum average power of 243 mW, and a maximum pulse energy of 4.5 nJ [200].

Although direct ML fiber oscillators have achieved remarkable progress in pulse duration, wavelength flexibility, environmental stability, and cavity design, their pulse energy and peak power remain fundamentally constrained by intracavity nonlinear effects, gain saturation, and optical damage thresholds. Consequently, CPA has become the predominant approach for scaling pulse energy and peak power while preserving ultrashort pulse durations in high-power 2 μm fiber laser systems. In a typical CPA system, ultrashort pulses generated from a ML oscillator are first temporally stretched to reduce their peak power before amplification and are subsequently recompressed to recover femtosecond pulse durations. This architecture effectively suppresses nonlinear effects during amplification and has become the core technique for realizing high-energy ultrafast fiber lasers [160,201]. In 2010, Haxsen et al. combined an LMA TDFA with a CPA architecture to achieve femtosecond pulse output, delivering an amplified power of 5.4 W and a pulse energy of 151 nJ [202]. In 2013, Wan et al. developed a high-power, polarization-maintaining femtosecond TDFL system. By leveraging CPA, they achieved an output power of 36 W, a pulse width of 760 fs, and a repetition rate of 30.84 MHz [203]. In 2018, Gaida et al. constructed an ultrafast thulium-doped fiber CPA system with a central wavelength of 1960 nm, achieving an output power of up to 1060 W, a repetition rate of 80 MHz, a pulse energy of 13.2 μJ, a pulse width of 265 fs, and a peak power as high as 50 MW [204]. In 2026, Abughazaleh et al. developed a compact, all-fiber, ML thulium-doped laser operating at 1789 nm. The cavity delivered dissipative solitons with a pulse energy of 0.7 nJ and a compressed pulse width of 270 fs; after CPA, the pulse energy increased to 41.5 nJ, and the pulse width was further compressed to 180 fs [205]. These representative studies demonstrate that femtosecond CPA systems have become the mainstream approach for realizing high-power ultrafast 2 μm fiber lasers. Recent developments have gradually shifted from simply increasing average output power toward simultaneously improving pulse energy, pulse duration, beam quality, wavelength tunability, and system compactness [160,201,204,205]. Benefiting from LMA gain fibers, polarization-maintaining architectures, optimized CPA configurations, and emerging ML mechanisms, kilowatt-level average power and sub-300 fs pulse durations have been successfully demonstrated [204,205]. Despite these remarkable advances, further performance scaling of high-power CPA systems remains constrained by several fundamental challenges. As the pulse energy and peak power continue to increase, gain narrowing, nonlinear phase accumulation, and higher-order dispersion progressively degrade pulse quality, resulting in pulse distortion and preventing transform-limited pulse compression. Meanwhile, dispersion management becomes increasingly challenging because the dispersion introduced by pulse stretchers, fiber amplifiers, and pulse compressors must be precisely matched over a broad spectral bandwidth while compensating for higher-order dispersion. Furthermore, nonlinear effects accumulated during high-power amplification, particularly SPM and SRS, limit nonlinear pulse compression efficiency and ultimately restrict further scaling of pulse energy and peak power while maintaining excellent beam quality and ultrashort pulse duration [160,206]. Future research is therefore expected to focus on advanced dispersion engineering, optimized CPA architectures, novel LMA specialty fibers, efficient suppression of nonlinear effects, and intelligent system optimization to simultaneously improve pulse quality, compression efficiency, peak power, and long-term operational stability, thereby enabling the next generation of high-power ultrafast 2 μm fiber laser systems [160,206].

In recent years, novel ML structures and techniques, such as cross-phase modulation ML, have been continuously developed, resulting in an increasingly diverse array of technical approaches for 2 μm ML pulsed fiber lasers. In 2021, Wang et al. employed an all-polarization-maintaining NOLM configuration to achieve stable noise-like mode-locked TDF laser output, exhibiting a repetition rate of 980.6 kHz, a pulse envelope tunable range of 14.1−23.6 ns, and a maximum pulse energy of up to 40.3 nJ [208]. Unlike material-based saturable absorbers, NOLMs achieve passive ML through nonlinear interference in an all-fiber cavity, making them particularly attractive for high-power ML fiber lasers owing to their excellent power-handling capability and environmental robustness. In 2022, Huang et al. utilized cross-phase modulation-induced ML technology to successfully generate both soliton and dissipative soliton pulses in an ultrafast TDFL [209]. This technique relies on the cross-phase modulation pulling effect, enabling stable ML without an external SA, with wavelength tuning ranges of 11 nm and 15 nm for solitons and dissipative solitons, respectively. In 2024, Huang et al. designed a compact self-mode-locked TDFL with a maximum pulse energy of up to 120.2 nJ [210]. In 2025, Peng et al. realized harmonic ML and noise-like pulse output in an all-polarization-maintaining TDFL through a linear-cavity interferometric nonlinear polarization evolution mechanism. In the harmonic ML state, 19th-order harmonic soliton pulses were generated with a repetition rate of 229 MHz; the noise-like pulses exhibited a central wavelength of 1962 nm, a 3 dB spectral bandwidth of 30 nm, an average output power of 284 mW, and a pulse energy of 24 nJ [211]. These results demonstrate that recent research has gradually shifted from achieving stable ML toward simultaneously improving pulse energy, repetition rate, operational stability, and cavity compactness [2,160]. These representative studies indicate that recent developments in 2 μm ML fiber lasers have evolved from conventional passive ML schemes toward diversified ultrafast pulse-generation mechanisms, including NOLM-, cross-phase-modulation-, and self-mode-locking-based architectures [2,206]. These emerging techniques significantly enhance pulse performance, wavelength tunability, environmental stability, and system integration while reducing dependence on conventional saturable absorbers [206]. Nevertheless, further performance improvement remains constrained by the trade-offs among pulse energy, repetition rate, pulse stability, and nonlinear effects, particularly under high-power operation [160,206]. Therefore, future research is expected to focus on advanced ML mechanisms, intelligent cavity optimization, novel nonlinear fibers, and high-power-compatible ultrafast architectures to realize higher-energy, shorter-pulse, and more stable 2 μm ML fiber laser systems [2,206].

Benefiting from their ultrashort pulse duration, high peak power, excellent temporal coherence, and compact all-fiber configuration, ML fiber lasers operating in the 2 μm wavelength region have become indispensable light sources for a wide range of scientific and technological applications. Their femtosecond and picosecond pulse generation capabilities make them particularly attractive for ultrafast laser micromachining, enabling high-precision material processing with minimal thermal damage [136,182]. In addition, the broad optical bandwidth and high temporal coherence of ML fiber lasers have promoted their applications in optical frequency combs, precision spectroscopy, and optical frequency metrology, providing powerful tools for precision measurements and molecular spectroscopy [1,182]. Owing to their high peak power, these laser systems are also widely employed in nonlinear optics, including supercontinuum generation, OPO, and mid-infrared frequency conversion, significantly extending coherent laser emission toward longer wavelengths [1]. Furthermore, 2 μm ML fiber lasers have demonstrated considerable potential in biomedical imaging, nonlinear microscopy, and time-resolved spectroscopy, where ultrashort pulse duration and high peak intensity are essential for improving imaging contrast and temporal resolution [212,213]. These representative applications highlight the unique advantages of ML fiber lasers as versatile ultrafast laser sources, supporting the continued development of precision photonics, nonlinear optics, biomedical engineering, and advanced ultrafast laser technologies.

4.3 2 μm high-power gain-switched pulsed fiber laser

The generation mechanism of GS pulses is similar to that of QS pulses. The essential difference lies in the fact that GS generates laser pulses by modulating pump pulses to excite the gain medium, whereas QS achieves pulse modulation via intracavity modulators or SAs [162]. Compared to QS technology, GS requires no additional intracavity functional components, resulting in a simpler and more compact optical configuration. This method overcomes the limitations imposed by the damage thresholds of components in QS lasers, which restrict the achievement of high peak power output. Furthermore, output characteristics such as pulse width and pulse energy can be flexibly adjusted solely by optimizing the pump pulse parameters without modifying the resonant cavity structure, demonstrating excellent tunability. Unlike QS lasers, pulse generation in GS fiber lasers is governed by the transient dynamics of gain buildup and depletion rather than intracavity loss modulation [162]. Consequently, finite gain depletion and residual population inversion often lead to pulse tailing and asymmetric temporal profiles, particularly under high pump powers or long pump durations. In addition, gain saturation, amplified spontaneous emission (ASE), and pulse build-up dynamics may introduce pulse broadening, timing jitter, and even multiple-pulse generation, thereby degrading pulse quality and limiting energy scaling [2]. To mitigate these effects, recent studies have focused on high-speed pump modulation, optimized cavity configurations, polarization-maintaining architectures, and hybrid pulse-generation schemes to suppress pulse tailing while improving pulse stability and energy scalability. In 2000, Dickinson et al. pumped a TDFL using a 790 nm Ti: sapphire laser and achieved tunable pulsed laser output at 2 μm. The laser configuration and output characteristics are shown in Figs. 11a and 11b [214]. In 2007, Jiang and Tayebati first realized stable short-pulse emission from a 2 μm gain-switched TDFL. Using a 1.55 μm pump source, the system delivered 10 ns pulses with kilowatt-level peak power, a slope efficiency of 50%, and a maximum repetition frequency of 500 kHz [215]. In 2009, Wu et al. reported the first gain-switched HDFL operating at 2.106 μm. A 1.909 μm gain-switched TDFL served as the pump source. At a repetition frequency of 80 kHz, pulses with an energy of 3.2 μJ and a pulse duration of 150 ns were obtained, with a slope efficiency of 44% [216]. In 2013, Yang et al. first reported a synchronous gain-switched and mode-locked THCDFL. The central wavelength was 1958 nm, the repetition frequency of the GS pulse envelope was 20 kHz, and the repetition frequency of the ML sub-pulses reached 14.8 MHz [220].

Integrating all-fiber architectures with MOPA technology has become a primary research focus for achieving high-power output in GS pulsed fiber lasers. In 2015, Li et al. reported a GS fiber laser amplification system based on a MOPA configuration, operating at a wavelength of 2050 nm. The seed source delivered pulses with a polarization extinction ratio exceeding 16 dB and a maximum output power of 470 mW. After amplification through two-stage double-clad TDFAs at a repetition rate of 40 kHz, the average power was boosted to 40.5 W, with a maximum pulse energy of 1 mJ, a pulse width of 100 ns, and a corresponding peak power of 10 kW [124]. In 2019, Romano et al. reported a nanosecond-level kilowatt-class pulsed TDFL based on a MOPA configuration, operating at a wavelength of 1952 nm, with a tunable repetition rate range of 10−500 kHz and a pulse width of 6–21 ns [224]. In 2021, Liu et al. reported a watt-level 2 μm high-repetition-rate GS thulium-doped fiber laser. Employing a 1.6 μm noise-like pulse pump source, GS pulses in the 1940 nm band were obtained, with a minimum pulse width of 14.7 ns and a maximum output power of 2.44 W [225]. In 2022, Xiao et al. demonstrated a gain-switched TDFL employing a MOPA configuration, achieving a maximum output power of 1.687 W and a slope efficiency of 19.7%, with pulse output at a pulse width of 425 ns and a repetition rate of 100 kHz [226].

Leveraging novel hybrid architectures, such as hybrid pumping and all-fiber gain-switched and mode-locked, research on 2 μm GS pulsed lasers has achieved significant advancements. In 2024, Ren et al. developed an all-fiber hybrid-pumped gain-switched TDFL operating at a wavelength of 2042 nm [227]. By adopting a hybrid pumping scheme, the laser delivered pulses with an energy of 38 μJ and a pulse width of 103 ns at a repetition rate of 25 kHz, and an output spectral SNR of 47 dB at a repetition rate of 50 kHz. The schematic configuration and output characteristics are shown in Figs. 11c−11e. In 2025, Varsha and Das reported an all-fiber gain-switched and mode-locked TDFL operating at 1925 nm. Using a 1570 nm ML laser with a pulse width of 340 ps and a repetition rate of 6.54 MHz as the pump source, GS pulses with a maximum repetition rate of 25.8 kHz were obtained [228]. These representative studies indicate that recent developments in 2 μm GS fiber lasers have evolved from conventional directly modulated pulse generation toward hybrid pulse-generation architectures integrating gain switching with ML and advanced pumping schemes [2,162]. These emerging approaches significantly improve pulse quality, pulse-energy stability, operational flexibility, and system integration while maintaining the intrinsic advantages of GS operation. These representative studies indicate that recent developments in 2 μm GS fiber lasers have gradually evolved from conventional directly modulated pulse generation toward hybrid pulse-generation architectures integrating gain switching with ML, MOPA configurations, and advanced pumping schemes. These approaches significantly improve pulse quality, pulse-energy stability, operational flexibility, and system integration while preserving the intrinsic advantages of GS operation. Despite these advances, further performance improvement remains constrained by pulse tailing, gain saturation, pulse-energy/repetition-rate trade-offs, thermal effects, and the increasing complexity of hybrid cavity architectures. Future research is therefore expected to focus on high-speed pump modulation, optimized hybrid pumping strategies, intelligent pulse-shaping techniques, highly integrated all-fiber architectures, and compact high-performance seed sources for next-generation MOPA systems [2,206].

Benefiting from their electronically controllable pulse generation, flexible repetition rate, and compact all-fiber configuration, GS fiber lasers operating in the 2 μm wavelength region have attracted increasing attention as versatile pulsed laser sources. Their excellent pulse-to-pulse stability and precise electronic synchronization make them particularly suitable as seed sources for MOPA systems, enabling stable pulse amplification while maintaining desirable temporal characteristics [178,229]. In addition, GS fiber lasers have demonstrated significant potential in laser ranging, optical time-domain reflectometry (OTDR), and distributed fiber sensing, where flexible pulse repetition rates and stable pulse timing are essential for accurate signal acquisition and long-distance detection [85,178]. Their reliable pulse generation and straightforward electronic control also make them attractive for precision spectroscopy, gas sensing, and laser diagnostics, providing stable pulsed illumination with controllable temporal characteristics [1,85]. Furthermore, GS fiber lasers can serve as efficient pump or seed sources for subsequent pulse compression, nonlinear frequency conversion, and ultrafast amplification systems, providing an effective route toward the generation of high-peak-power ultrashort pulses [1]. These representative applications demonstrate that GS fiber lasers have become important pulsed laser sources for precision sensing, advanced fiber laser systems, and nonlinear photonic technologies.

In summary, QS, ML, and GS fiber lasers each exhibit distinct advantages in pulse generation and application scenarios. QS fiber lasers are well suited for generating high-energy nanosecond pulses, ML fiber lasers have become the dominant platform for ultrashort pulse generation, whereas GS fiber lasers provide a compact and electronically controllable solution for flexible pulse generation and high-performance MOPA seed sources. Recent advances in specialty gain fibers, all-fiber architectures, CPA technology, hybrid pulse-generation schemes, and intelligent cavity optimization have significantly promoted the performance of 2 μm pulsed fiber lasers. Future development is expected to focus on simultaneously improving output power, pulse energy, pulse duration, beam quality, temporal stability, system integration, and intelligent pulse control, thereby further expanding the application potential of 2 μm pulsed fiber lasers in ultrafast photonics, precision sensing, advanced manufacturing, biomedical engineering, and nonlinear mid-infrared photonics.

5 Challenges and outlooks

Owing to their unique spectral characteristics, 2 μm band thulium- and holmium-doped fiber lasers hold broad application prospects in fields such as biomedicine [15,16], industrial processing [17,18], environmental monitoring [19,20], and national defense and security [21,22]. Although significant progress has been made in 2 μm fiber laser technology, numerous technical challenges remain to be overcome in the ongoing advancement toward higher output power, improved beam quality, and long-term stable operation. First, nonlinear effects and TMI arising during power scaling constitute the core bottlenecks limiting high-power CW laser output. As the output power increases, the optical power density inside the fiber rises significantly, and nonlinear effects such as SRS, SBS, and SPM become progressively more severe. These effects not only deplete the fundamental laser energy but also severely degrade the laser’s spectral purity and beam quality [230,231]. Meanwhile, conventional LMA thulium- and holmium-doped fibers, while enlarging the mode area, inevitably introduce multi-transverse-mode propagation, which readily induces TMI. This leads to continuous beam quality degradation with increasing power and can cause issues such as mode hopping and output power fluctuations [49]. Although novel fiber structures, such as tapered gain fibers, can alleviate these conflicts to some extent, the high-precision coordinated control of the refractive index profile, doping concentration, and fiber drawing process remains a core challenge for their engineering application [4]. Second, thermal management issues and intrinsic defects of fiber materials severely constrain the operational stability and service lifetime of lasers. The 2 μm laser system exhibits a significant quantum defect effect, generating substantial waste heat during the pumping process. This creates a large temperature gradient in the fiber core region, thereby inducing refractive index profile distortion, thermal lensing effects, and, in severe cases, thermal damage to the fiber [8]. In this context, the doping concentration and uniformity of thulium- and holmium-doped fibers, as well as the thermal conductivity and photodarkening characteristics of the silica host, directly determine the thermal load capacity of the fiber under high-power operation. Furthermore, issues such as fiber facet damage, thermal failure of the coating, and thermal stress in the packaging structure caused by high-power operation impose stringent requirements on the long-term stable operation of the laser system. The performance scaling of 2 μm fiber lasers operating in specialized regimes, such as NLW and ultrashort-pulse modes, faces numerous technical challenges. 2 μm NLW fiber lasers serve as core light sources for applications including coherent detection and lidar; however, SBS and mode competition within the fiber create an inherent trade-off between linewidth narrowing and output power scaling. Achieving synergistic optimization of both through specialty fiber design and active noise suppression techniques remains a key research focus that urgently needs to be addressed [118]. 2 μm ultrashort-pulse fiber lasers, on the other hand, confront multiple trade-off challenges involving dispersion management, nonlinear effect suppression, and gain bandwidth matching. Further increasing the average output power and single-pulse energy while preserving pulse quality constitutes a critical research direction for their high-power development [204].

Beyond the aforementioned technical challenges, the commercialization and large-scale industrial deployment of 2 μm fiber lasers also depend on the maturity of the supporting industrial ecosystem. Compared with the well-established 1 μm ytterbium-doped fiber laser industry, commercial 2 μm fiber laser technology remains at a relatively early stage despite the remarkable progress achieved in recent years. Nevertheless, driven by increasing demands in eye-safe lidar, biomedical engineering, precision spectroscopy, polymer processing, and free-space optical communication, several leading manufacturers, including IPG Photonics, Thorlabs, and AdValue Photonics, have introduced commercial 2 μm fiber laser systems for scientific, medical, and industrial applications. Current commercial products cover CW, pulsed, and ultrafast operating regimes, with operating wavelengths spanning approximately 1.7−2.1 μm and output powers ranging from tens of watts to the kilowatt level, depending on the laser architecture and application requirements. Commercial ultrafast systems are capable of generating sub-100 fs pulses for applications such as ultrafast spectroscopy and nonlinear optics, whereas high-power CW systems have been developed for industrial processing and biomedical applications. These representative products demonstrate that 2 μm fiber laser technology has gradually evolved from laboratory demonstrations toward practical deployment in polymer welding, minimally invasive laser surgery, environmental sensing, spectroscopy, nonlinear frequency conversion, and scientific instrumentation [232–234]. Despite these advances, several factors continue to limit large-scale industrialization. Compared with mature 1 μm fiber laser technology, the industrial ecosystem for 2 μm photonic components remains relatively underdeveloped. The limited availability of high-performance passive components, including FBGs, wavelength-division multiplexers (WDMs), optical isolators, and pump combiners, together with the relatively high cost of specialty fibers, thermal management, nonlinear effects, packaging reliability, and long-term operational stability, continue to present significant engineering challenges [2,4].

Considering the current technical bottlenecks of 2 μm CW and pulsed fiber lasers, along with the practical application demands across various industries, this paper summarizes and identifies five major future development directions for this type of laser, as illustrated in Fig. 12 [235–239]. Each future direction discussed below is intended to address one or more of the key technical challenges summarized above. Among them, achieving continuously tunable output over an ultra-broad wavelength range is expected to become one of the most important research priorities, as it can overcome the limited wavelength flexibility of existing 2 μm fiber lasers while alleviating the inherent trade-off among output power, spectral purity, and tuning range in NLW laser systems. Continuous ultra-broadband wavelength tuning is also a key technological advantage that enables 2 μm fiber lasers to satisfy the growing demands of multi-scenario precision sensing and spectroscopic applications [66]. However, current mainstream commercial and laboratory-scale tunable 2 μm fiber lasers still suffer from numerous shortcomings, generally characterized by narrow effective tuning ranges, poor output stability at the band edges, and low power uniformity. These limitations make it difficult to meet the demanding requirements of cutting-edge applications that require a broad operational spectral span, such as remote sensing of atmospheric multi-component trace gases, pumping of mid-infrared supercontinuum light sources, and multi-band precision spectroscopic analysis [4]. Future advances are expected to exploit the intrinsically broad gain bandwidth of Tm- and Ho-doped gain media together with adaptive resonator-loss modulation, high-precision tunable filtering, and gain-equalization techniques. These approaches can effectively suppress gain non-uniformity, longitudinal-mode instability, and spectral distortion over wide tuning ranges, thereby extending the continuous tuning bandwidth while simultaneously improving the output stability of 2 μm fiber lasers.

Second, further increasing the output power of 2 μm CW and pulsed fiber lasers remains one of the most important future research directions. Although the output power of CW fiber lasers has reached the kilowatt level and the peak power of pulsed fiber lasers has exceeded the hundred-megawatt level after years of technological development [240], these performance levels remain insufficient for many demanding applications, including directed-energy systems, long-distance space laser communications, and high-energy pump sources. Consequently, there is still strong demand for further power scaling in both scientific research and industrial applications [241,242]. At present, further power scaling is primarily constrained by nonlinear effects (particularly SBS and SRS), TMI, thermal accumulation under high-power pumping, and the limited damage threshold of conventional fiber architectures. To overcome these limitations, future research is expected to follow two complementary technological routes. The first is to continuously extend the power capability of existing fiber-laser architectures through optimized double-clad gain-fiber designs, improved multi-beam pump coupling schemes, and intracavity nonlinear-suppression techniques [230]. The second is to develop and engineer novel fiber platforms, including hollow-core PCFs and triple-clad specialty doped fibers, which provide new opportunities for power scaling through innovative gain-media designs and optical-waveguide mechanisms [98]. The combination of these strategies is expected to alleviate the limitations imposed by nonlinear effects, SBS, TMI, and thermal accumulation, thereby enabling further power scaling toward next-generation multi-kilowatt CW and high-energy pulsed 2 μm fiber laser systems.

Third, system integration and miniaturization have become important future research directions for expanding the application scenarios of 2 μm fiber lasers and accelerating their transition from laboratory demonstrations to practical engineering deployment. Although some 2 μm fiber lasers are already commercially available, most prototypes still face challenges such as discrete optical paths, cumbersome system architectures, and excessively large overall footprints. Additionally, insufficient resistance to vibration and temperature variations, along with poor environmental robustness, significantly limit their large-scale application in harsh operating conditions, including airborne, spaceborne, and field-portable detection systems [243]. To address these engineering challenges, future research is expected to advance along two complementary technological routes. The first is to optimize laser architectures through all-fiber integration, compact all-polarization-maintaining configurations, and advanced packaging technologies, thereby simplifying optical layouts while improving long-term operational stability and environmental adaptability. The second is to accelerate the development and practical implementation of integrated photonic technologies, including silicon photonics and thin-film lithium niobate platforms, together with on-chip filtering, wavelength tuning, and pump-coupling devices. These technologies provide new opportunities for realizing compact, highly integrated, and manufacturable 2 μm fiber laser systems, thereby further enhancing their industrialization potential [244,245]. The combination of advanced laser architectures and emerging integrated photonic platforms is expected to significantly improve the environmental robustness, thermal stability, and engineering reliability of 2 μm fiber lasers, thereby accelerating their commercialization and large-scale deployment in aerospace, precision sensing, industrial processing, and portable photonic systems.

Fourth, the development of novel gain materials has become an essential future research direction for overcoming the limitations of conventional silica-based gain fibers and supporting the continued performance evolution of 2 μm fiber lasers. Although Tm- and Ho-doped silica fibers remain the dominant gain media, they are inherently limited by the physicochemical properties of the silica matrix and rare-earth doping mechanisms. As a result, they generally exhibit relatively low thermal conductivity, limited nonlinear thresholds, restricted gain bandwidth, and increased susceptibility to photodarkening under high-power operation. These limitations have become increasingly significant in emerging applications requiring ultra-high-power amplification, ultra-broadband wavelength tuning, and ultrafast pulse generation [5]. To address these challenges, future research is expected to advance along two complementary technological routes. The first is to develop novel gain-media platforms, including bismuthate- and fluoride-glass fibers, together with advanced rare-earth doping strategies such as Tm-Ho co-doping and multi-ion co-doping. The second is to improve fiber fabrication technologies through approaches such as vapor-phase doping and in-situ doping, enabling more precise control of dopant distribution and glass composition [246]. These developments can improve thermal conductivity, increase nonlinear thresholds, broaden the effective gain bandwidth, and enhance resistance to photodarkening, thereby mitigating thermal degradation and spectral deterioration under high-power operation. The combination of advanced gain materials and improved fabrication technologies is expected to provide new opportunities for further increasing output power, extending the operational spectral range, and optimizing pulse characteristics, thereby supporting the continued development of high-performance 2 μm fiber laser systems.

Finally, AI-enabled intelligent regulation has emerged as an important future research direction for improving the operational stability, adaptability, and autonomy of advanced 2 μm fiber lasers. As laser architectures become increasingly sophisticated, conventional manual optimization of pump power, polarization state, intracavity filtering, and other operating parameters is no longer sufficient to compensate for dynamic disturbances such as environmental temperature variations, pump fluctuations, and device aging. Consequently, the long-term stability, operational efficiency, and adaptive capability of high-performance 2 μm fiber lasers remain significantly constrained. To address these challenges, future research is expected to advance along two complementary technological routes. The first is AI-assisted intelligent operation control, in which machine learning algorithms are integrated with fiber laser platforms to enable real-time monitoring, prediction, and closed-loop optimization of pump conditions, cavity states, dispersion characteristics, and thermal dynamics. Reinforcement learning (RL), deep neural networks (DNNs), Bayesian optimization (BO), and genetic algorithms (GAs) have demonstrated considerable potential for cavity optimization, laser-state recognition, operating-condition prediction, and autonomous parameter optimization, thereby significantly improving the stability and adaptability of advanced fiber lasers [247–251]. The second route is AI-assisted inverse photonic design, which applies intelligent optimization algorithms to the structural design of gain fibers and photonic devices. By simultaneously optimizing parameters such as fiber cross-sectional geometry, dopant distribution, and waveguide refractive-index profiles according to predefined performance objectives, AI-assisted inverse design provides new opportunities for suppressing higher-order mode oscillation, mitigating nonlinear effects, and increasing the TMI threshold at the device-design stage [252–255]. The combination of AI-enabled intelligent operation control and inverse photonic design is expected to establish an integrated optimization framework spanning laser operation and device design. By reducing reliance on empirical trial-and-error optimization, these technologies will further enhance the stability, environmental adaptability, and engineering reliability of 2 μm fiber lasers, thereby accelerating the development of next-generation intelligent laser systems with high precision, strong adaptability, and autonomous operation.

In recent years, 2 μm fiber lasers have been widely applied in industrial and scientific research fields owing to their unique advantages over other laser technologies. This paper systematically reviews the latest research progress of 2 μm fiber lasers. Firstly, it summarizes the characteristics of rare-earth-doped silica fibers operating in the 2 μm band as high-power laser gain media as well as fiber structural features. Then it emphatically discusses the implementation schemes of CW and pulsed 2 μm fiber lasers, and explores the application prospects of Tm³⁺-doped and Ho³⁺-doped fiber lasers. Finally, this paper comprehensively sorts out the state-of-the-art advances, existing challenges and future development trends in the field of 2 μm band fiber lasers, providing valuable references for researchers, engineers and relevant practitioners in this field.

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