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Review

Frontier Advances and Challenges of High-Power Thulium-Doped Fiber Lasers in Minimally Invasive Medicine

by
Wen-Yue Xu
1,2,
Gong Wang
1,2,*,
Yun-Fei Li
1,2,*,
Yu Yu
1,2,
Yulei Wang
1,2 and
Zhiwei Lu
1,2
1
Center for Advanced Laser Technology, Hebei University of Technology, Tianjin 300401, China
2
Hebei Key Laboratory of Advanced Laser Technology and Equipment, Tianjin 300401, China
*
Authors to whom correspondence should be addressed.
Photonics 2025, 12(6), 614; https://doi.org/10.3390/photonics12060614
Submission received: 14 May 2025 / Revised: 8 June 2025 / Accepted: 13 June 2025 / Published: 16 June 2025
(This article belongs to the Special Issue Advanced Methods in Exploring Light–Matter Interactions and Nonlinear Effects Optics Applications)
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Abstract

:
Lasers are increasingly used in the biomedical field because of their concentrated energy, good stability, ease of use, and other advantages, promoting the development of precision medicine to a higher level. Medical laser equipment has transformed from a single therapeutic tool in an intelligent and precise diagnostic system. Existing clinical laser equipment has significant technical bottlenecks regarding soft-tissue ablation precision and multimodal diagnostic compatibility, which seriously restricts its clinical application. High-power thulium-doped fiber lasers with operating wavelengths of 1.9–2.1 μm provide a revolutionary solution for minimally invasive surgery due to their high compatibility with the absorption peaks of water molecules in biological tissues. This study reviews recent advances in high-power thulium-doped fiber lasers for minimally invasive therapies in the biomedical field. Breakthrough results in four major clinical application scenarios, namely, urological lithotripsy, tumor precision ablation, disfiguring dermatological treatment, and minimally invasive endovenous laser ablation, are also summarized. By systematically evaluating its potential for multimodal diagnostic and therapeutic applications and thoroughly exploring the technical challenges and strategies for clinical transformation, we aim to provide a theoretical basis and practical guidance for the clinical transformation and industrialization of new-generation medical laser technology.

1. Introduction

Laser technology has become a central tool in modern precision surgery and minimally invasive treatments [1]. Its mechanism of action is based on the photothermal conversion effect, the specific wavelength laser energy through the tissue of water molecules, or the selective absorption of the target pigment, to achieve precise tissue cutting, controlled vaporization, and efficient coagulation. At present, laser technology is involved in more than ten clinical specialties, such as minimally invasive surgery, ophthalmic refractive correction [2,3], and urology lithotripsy [4,5]. It has shown technical advantages in terms of precise resection, immediate hemostasis, and minimally invasive ablation that are hard for traditional instruments to reach. Especially in microsurgery and endoscopic interventions, its non-contact, adjustable penetration depth, and low thermal damage characteristics significantly improve the safety of surgery and postoperative healing efficiency, and clinical potential continues to be realized.
Currently, the mainstream lasers used in clinical applications include CO2 (10.6 μm), Er: YAG (2.94 μm), Nd: YAG (1.064 μm), Ho: YAG (2.1 μm), etc., and the inherent technological characteristics and interaction mechanisms with biological tissues directly limit the clinical application scenarios in which they can be adapted. With a high water absorption coefficient of 825 cm−1, CO2 lasers can achieve micrometer penetration depths, demonstrating the advantages of precise vaporization and rapid hemostasis in epidermal surgery and microsurgery, but the defects of the low transmission efficiency of quartz fibers limit their application in endoscopic surgery. Although Er: YAG lasers have become the technology of choice for precision cutting in ophthalmic surgery due to their high absorption coefficients (>3000 cm−1), which enable sub-micron precision cutting, their mature applications are still limited by the high cost of fluoride fiber and the risk of tissue contact damage. Although Nd: YAG lasers are widely used in endoscopic surgery due to their efficient transmission through ordinary optical fibers, their low absorption characteristics (0.11 cm−1) are prone to deep thermal diffusion, increasing the risk of surgical complications. Ho: YAG (2.1 μm, 31.8 cm−1) is the current gold standard in urology, but its pulsed mode of operation is susceptible to triggering mechanical damage to tissues and is limited in terms of power.
The thulium-doped fiber laser (TFL, 1.94 μm, 129.2 cm−1) breaks through the bottleneck of traditional technology by optimizing the wavelength selection and output mode [6], and systematically solves the key problems of selective tissue absorption, energy transfer efficiency, and thermal damage control [7]. Combining the advantages of low-threshold vaporization, precise hemostasis, and a controllable penetration depth of 300 μm, it demonstrates the clinical potential of replacing traditional lasers [8]. This paper focuses on thulium-doped fiber lasers, and by analyzing the specific absorption mechanism of a 1.94 μm wavelength and water molecules, we systematically elucidate the energy regulation principle in minimally invasive treatment, focusing on technological breakthroughs in the innovative clinical applications of urological stone fragmentation, precise ablation of tumors, disfiguring dermatoses, and minimally invasive intraventricular laser ablation. Typical parameter requirements for thulium-doped fiber lasers for the four main medical application scenarios presented in this paper are demonstrated in Table 1. Through the evaluation of the effectiveness of multimodal synergistic strategies, the key technical bottlenecks in the current clinical transformation are revealed, and the technical advantages, as well as existing challenges are comprehensively determined, aiming to provide theoretical support and practical paths for the research and development of the new generation of medical laser technology and its clinical promotion.

2. Physical Basis of Thulium-Doped Laser and Mechanism of Interaction with Biological Tissues

The core physical mechanism of the TFL originates from the unique 4f electron shell level jump property of the thulium ion (Tm3+), as shown in Figure 1. The ion exhibits multi-energy absorption characteristics in a quartz matrix; especially of note are significant absorption peaks at 680 nm, 790 nm, 1220 nm, and 1630 nm [13]. The 790 nm absorption peak corresponds to the base-state 3H6 to the excited-state 3H4 jump process, and Tm3+ jumps from the base-state 3H6 to the-excited state 3H4 by absorbing pump photons near 790 nm [14]. Subsequently, the excited ions undergo nonradiative relaxation to the metastable 3F4 energy level with a characteristic lifetime of approximately 10 ms [15], as shown in Figure 2. This is followed by a radiative transition from 3F4 to 3H6, resulting in the emission of radiation from a 1.92–2.1 μm band laser [16,17] with a dominant emission peak at 1.94 μm. In recent years, the 793 nm LD has become the most commonly used pump source for high-power continuously thulium-doped lasers because of the increasing maturity of 793 nm high-power semiconductor lasers (LDs). In addition to choosing the 793 nm LD as the pump source, light sources near the wavelengths of 1220 nm and 1630 nm can also be utilized to excite thulium ions. In the former case, ytterbium-doped fiber lasers with a central wavelength of 1064 nm are usually used as the pump source, which can pump thulium ions from the 3H6 energy level to the 3H5 energy level, after which the thulium ions will rapidly jump from the 3H5 energy level to the 3H4 energy level, and the thulium ion is excited in the 3H4 and 3H6 energy levels to form a particle number inversion and finally output a 2 μm laser. In the latter case, an erbium–ytterbium co-doped fiber laser or a thulium-doped fiber laser is generally chosen as the pump source, in which thulium ions are directly excited to the laser upper energy level, i.e., the 3H4 energy level, and a particle number inversion is formed between the 3H4 and 3H6 energy levels, and in such a pumping mode, the pumped upper and lower energy levels are the same as the laser’s upper and lower energy levels, and thus it is also called same-band pumping. In this process, the cross-relaxation effect of thulium ions (3H4 + 3H6 → 23F4) significantly enhances the quantum efficiency (theoretical value up to 200% [18]), which enables the TFL to achieve high slope efficiency (>40%) at a low pumping power, and provides theoretical support for high-power continuous/pulsed output [19].
The mechanism of action of lasers on biological tissues depends on the differences in the absorption properties of tissues for specific wavelengths of laser light. Compared with visible and near-infrared light (e.g., the absorption coefficient of 0.5 μm wavelength is only ~10−4 cm−1), the absorption coefficient of water in the 2 μm wavelength band is significantly higher [20], which has excellent vaporization and cutting, tissue coagulation, and non-conducting properties, and can achieve less bleeding, better hemostasis, and lower probability, as well as less risk of postoperative complications in surgery. Operating at 1.94 μm, the TFL wavelength coincides with the prominent mid-infrared absorption band of water molecules in biological tissues (absorption coefficient ≈ 120 cm−1), as shown in Figure 3. This intrinsic spectral alignment confers significant clinical potential in precision biomedical therapies, particularly for water-rich tissue ablation. Its energy is efficiently absorbed by the moisture in the tissue through vibration–rotation energy level resonance (>80% energy deposition), creating a controlled thermal effect in the shallow penetration (200–500 μm) range. At energy densities exceeding 10 J/cm2, rapid intracellular water phase transitions occur within microsecond timescales, generating explosive vaporization effects that enable the micron-level precision ablation of water-dominant tissues, including tumors and mucosa [21]. In the sub-ablative regime (5–50 J/cm2), thermally dominated diffusion processes establish sustained 60–90 °C thermal gradients. These controlled hyperthermic conditions induce hemostasis through coordinated collagen denaturation and microvascular occlusion mechanisms, with temperature-dependent protein conformational changes driving vascular endothelial sealing. Due to the weak absorption of melanin and hemoglobin in this band, and the low transmission loss of the quartz fiber, the thulium laser can accurately limit the boundaries of thermal damage while taking into account the safety of the human eye and the ability of minimally invasive interventions, providing a new technological path for precision medicine.

3. Recent Progress in Minimally Invasive Medical Applications

3.1. Treatment of Urinary Tract Stones

The TFL is a new type of laser with good lithotripsy potential, which has gained attention in the field of urology in recent years [23]. Its efficiency is between that of the holmium laser (deep penetration but weak coagulation) and green laser (easy carbonization), combining the advantages of vaporization and temperature control, less intraoperative bleeding, and low tissue damage [24]. The mechanism of thulium fiber laser lithotripsy is similar to that of the holmium laser, but with faster lithotripsy speed, finer lithotripsy particles, and less stone displacement than those of the holmium laser [24,25]; its clinical application is expected to revolutionize the field of laser lithotripsy [24,26]. Researchers have used thulium-doped fiber lasers to perform retrograde intrarenal surgery on 50 patients [9]. They measured the size and density of the stones, the settings of the laser, and the duration of its operation, and then evaluated the stone ablation rate. They proved that TFL lithotripsy is a safe and effective lithotripsy technique with a low probability of complications. In a clinical trial, ureteral stone fragmentation was performed by using an ultra-pulsed TFL and a rigid ureteroscope. The laser activation time and energy parameters required for stone fragmentation were quantitatively assessed, along with the therapeutic efficacy of lithotripsy. Postoperative clinical outcomes and procedure-related complications were systematically evaluated [27]. The experimental findings show the superior lithotripsy efficiency of the ultra-pulsed TFL across varying stone densities, with treatment efficacy demonstrating parameter invariance to fiber core diameter and pulse repetition rate fluctuations [28]. Liu et al. systematically evaluated the value of the ultrapulsed thulium-doped fiber laser (TFL) for clinical applications through a multicenter prospective study. The study included 76 patients with urinary stones who underwent TFL lithotripsy under direct endoscopic visualization, which showed a stone-free rate (SFR) of 94.3% (66/70) at 4 weeks postoperatively, with a mean lithotripsy time of 50.9 ± 31.4 min and no intraoperative device malfunctions recorded. Compared with the Ho: YAG laser, the TFL shows a significant technological breakthrough, its flexible pulse parameter adjustment range (pulse width 0.1–10 ms, repetition frequency 5–50 Hz) supports dynamic switching between “blast” (short pulse width and high energy) and “drill” (long pulse width and continuous ablation) lithotripsy modes. The drilling mode is especially suitable for cystine stones and other hard or large stones, which can be ablated more efficiently. In addition, the non-contact ablation mechanism of the TFL reduces mechanical contact damage and has a significantly lower risk of mucosal perforation than the Ho: YAG laser. Studies have further confirmed that the TFL offers a reliable option for the minimally invasive treatment of complex urinary stones by enhancing lithotripsy efficiency while combining high safety and device stability. Lin et al. used a quasi-continuous TFL to study the variation rule of stone ablation with laser pulse width and repetition frequency in an in vitro environment, confirming that the thulium-doped laser has a significant lithotripsy effect and that, in the same period, when the single pulse energy is similar, the increase in the pulse frequency (average power) can improve the lithotripsy rate, and when the average power is similar, the greater the energy of a single pulse, the greater the amount of lithotripsy ablation [29]. The researchers developed a quasi-continuous-wave (QCW) TFL with a wide range of pulse width and pulse repetition rate adjustments, as shown in Figure 4, and optimized pump modulation to achieve a peak power of 700 W (a 40% improvement over the previous 500 W), which provides the physical basis for improved lithotripsy efficiency. Experimental data showed that the ablation rate of uric acid (UA) stones reached 8.9 mg/s, and that of calcium oxalate monohydrate (COM) stones was 7.5 mg/s at the peak power of 700 W, which is a 50–100% enhancement compared with the conventional Ho: YAG laser (3–5 mg/s on average). The results of the study confirm that a high-peak-power TFL significantly improves stone ablation rates. Several clinical studies have shown that thulium-doped fiber lasers are fully capable of meeting the demands of urological lithotripsy, and after more clinical studies and the validation of biological tissues [30], thulium-doped fiber laser lithotripsy is expected to become the mainstream of the medical lithotripsy field and the new “gold standard” [31,32,33].

3.2. Minimally Invasive Ablation for Early-Stage Lung Cancer

Lung cancer is one of the fastest-growing malignant tumors worldwide in terms of morbidity and mortality, posing a huge threat and challenge to human health [10]. Surgical resection is the standard treatment for early-stage non-small-cell lung cancer in cases of confined, irreversible lesions in the lung. Of the three commonly used clinical procedures, a pulmonary wedge resection is optimal [35]. Thulium laser’s medium water absorption coefficient balances the limitations of CO2 lasers (high absorption for deep thermal damage) and Nd: YAG lasers (low absorption for diffuse penetration), and demonstrates the advantages of precise cutting and low thermal damage in thoracoscopic pulmonary nodule resection [36]. Its wavelength characteristics allow for the selective vaporization of lung tissue without damaging the surrounding vascular and bronchial structures, making it particularly suitable for minimally invasive lung parenchyma preservation surgery for early-stage lung cancer. The researchers constructed a continuous-wave high-power thulium-doped fiber laser and conducted a biological tissue cutting study using a beam-shaping laser beam, confirming the good cutting effect of the thulium laser [37]. In recent years, the use of thulium lasers in conjunction with thoracoscopy to accomplish lung wedge resection has demonstrated its superiority in clinical applications. Liu et al., in their clinical study, confirmed that a thulium laser lung wedge resection can achieve sub-millimeter margin control, significant atrophy of the alveolar lumen of the surgical margin tissue, margin closure, and significant contraction of alveolar septal capillaries, which can effectively reduce the risk of pneumothorax and an alveolar fistula [38]. Zhang et al. retrospectively compared the feasibility and safety of a disposable linear cutting anastomosis with a 2 μm thulium laser for pulmonary nodule resection under television-assisted thoracoscopic surgery (VATS). The duration of chest tube placement was significantly shorter in the laser group than in the anastomosis group (p = 0.015). In each group, 10 patients were given the Thopaz digital chest drainage system after surgery. Postoperative hospitalization was significantly shorter in the laser group than in the anastomosis group (p = 0.049). Our results suggest that the 2 μm thulium laser offers greater value, minimal invasiveness, greater accuracy, and safety in thoracoscopic pulmonary nodule resection compared with the control group [39]. Persistent postoperative air leakage (PAL) is a common complication of lobectomy, and conventional methods (anastomosis, electric hook) suffer from the folding of lung tissue and inadequate sealing. For the management of incomplete interlobar fissures during lobectomy, the TFL reduces mechanical traction injury through noncontact ablation. A randomized controlled trial by Feng et al. (n = 76) demonstrated that the TFL group had a 23% reduction in their surgical costs, and the postoperative time to air leakage as well as length of hospital stay were not statistically different from that of the conventional anastomosis group. The thulium laser provides excellent hemostasis, reduces the risk of air leakage, is less costly, and is suitable for patients with poor lung function [40]. Clinical researchers conducted several retrospective cohort studies to evaluate the clinical indexes of the corresponding samples, and confirmed that the thulium laser with a thoracoscopic lung wedge resection was superior to the mechanical anastomosis group and the group of small segmental resections of the lung in terms of less intraoperative bleeding, a lower number of postoperative fevers, a better average length of hospital stay and hospitalization costs, and it was less prone to serious complications such as pulmonary atelectasis and bronchopleural fistula, which can be further promoted in clinical applications [41,42]. Several studies have collectively demonstrated that the 2 μm thulium laser applied to a thoracoscopic lung wedge resection can achieve results similar to cutting sutures, and is a safe, effective, and economical method that, with further optimization of thoracoscopic techniques and laser parameters, can provide a radical micro-innovative pathway for patients with early-stage lung cancer [43].

3.3. Treatment of Disfiguring Skin Conditions

The thulium laser is a new non-ablative fractional laser [44]. Its short thermal relaxation time (<1 μs) can selectively vaporize epidermal melanin without damaging the surrounding tissues, significantly reducing the risk of post-inflammatory hyperpigmentation compared to conventional lasers, and showing great potential in the precision treatment of pigmented disfigurement diseases [11]. Its non-exfoliative fractional pattern induces collagen remodeling and pigment metabolism modulation through controlled thermal damage, while reducing the risk of post-inflammatory hyperpigmentation (PIH), especially in patients with darker skin (Fitzpatrick type IV) [45]. By retrospectively evaluating 61 patients with hyperpigmentation (PIH) treated with a 1927 nm laser and assessing the rate of pigment clearance before and after treatment, 87.5% achieved clinical improvement (n = 8) and no paradoxical hyperpigmentation occurred, demonstrating that this treatment modality is safe and effective in ameliorating post-inflammatory hyperpigmentation in patients with darker skin types [46]. TFL combination therapy demonstrates synergistic effects in the treatment of complex pigmented diseases such as melasma. Clinical studies have demonstrated the efficacy and safety of using a 1927 nm TFL to target melasma after a single treatment interval of 1 month with a mean MASI score of 3.4 (n = 100) down from a baseline of 11.8, with no serious side effects [47]. Combination therapy involving oral Tosamin (tranexamic acid) demonstrates superior efficacy compared to monotherapy. This enhanced effect may be mediated via a dual mechanism: the inhibition of melanogenesis and the modulation of vasoactivity. Such a therapeutic strategy holds promise for early-stage superficial melasma lesions, offering a targeted approach to reduce hyperpigmentation while addressing vascular contributions to pathogenesis. Further clinical validation is warranted to support its broader adoption in dermatological practice. For refractory Riehl’s melanosis (also known as pigmented contact dermatitis), after combining a low-fluence QS-Nd: YAG laser and topical medications, DPASI scores were reduced from 9.55 to 5.25 in nine patients after three–seven TFL treatments, with 51–75% of the patients obtaining a significant improvement, which breaks through the limitations of traditional therapies [46]. Non-ablative fractional lasers (NAFLs) offer less downtime, fewer side effects, and are safer to use than ablative laser treatments [46,48,49]. By evaluating the healing of the 1940 nm NAFL-induced microthermal treatment zone (MTZ), it was demonstrated that the fractional thulium laser produces fractional columns of damage with sufficient depth and coverage to achieve effective skin surface remodeling. Notably, combined treatment strategies may enhance efficacy. Experiments in mouse models have shown that a 1927 nm laser combined with 30% salicylic acid resurfacing resulted in a 38% increase in dermal thickness compared to the laser group alone, with a concomitant increase in collagen density, providing a theoretical basis for a clinical combination protocol [50]. A dynamic balance between targeted pigment removal and epidermal barrier protection can be achieved by adjusting the energy and coverage to balance the depth of epidermal penetration and the extent of thermal damage by using graded treatment modes (e.g., six-channel coverage with the MOXI™ system) [51]. Although clinical use is not widespread at this time, as research continues to advance, the thulium laser may be able to treat a wider range of conditions, providing a whole new option for both doctors and patients.

3.4. Minimally Invasive Endovenous Laser Ablation

Endovenous laser technology for the treatment of superficial venous reflux is today’s gold-standard procedure [52,53]. Thulium-doped fiber lasers (TFLs) have demonstrated significant technological advantages in the field of minimally invasive endovenous laser ablation (EVLA) due to their unique 1940 nm wavelength characteristics [54]. As a targeted light source with a high absorption peak of water molecules (~1940 nm), which characterizes the temperature-dependent absorption properties of tissue chromophores, as shown in Figure 5, the TFL achieves highly efficient thermal coagulation of the vessel wall through precise energy transfer, with an occlusion rate of more than 90%, and, at the same time, significantly reduces the risk of thermal damage to the surrounding tissues. Studies have shown that the absorption coefficient of 1940 nm wavelength lasers in water is significantly higher than that of conventional 1470 nm or 1550 nm sources, triggering effective heat transfer at a lower power (1–4 W). Its thermal diffusion kinetics show asymmetry: heat transfer at the bare fiber tip is mainly along the anterior–superior side, while the radial fiber concentrates on the superior side of the conduction, which, in combination with convection and boiling effects, significantly reduces venous perforation and post-procedural pain due to the localized accumulation of energy [12]. In addition, blood plasma simulations have revealed that the 1940 nm laser induces a self-cleaning effect on coagulated plasma fragments and reduces the carbonization of the fiber surface, further reducing complications such as thermally induced thrombosis [55]. Several clinical studies have confirmed the efficiency and safety of the TFL in minimally invasive treatment. Setia et al. used a 1940 nm TFL for the parameter optimization of the great saphenous vein (GSV) and small saphenous vein (SSV), with a mean occlusion rate of 92% at 11 months postoperation and a lower incidence of sensory abnormalities than with conventional laser devices [54]. Postoperative pain assessment (VAS score) in 72 patients with saphenous vein insufficiency showed significant improvement in the TFL-treated group, a result consistent with the physical properties of the 1940 nm wavelength, whose chromophores are specifically targeted at water molecules within the endothelium and do not penetrate the surrounding tissues, reducing the impact on the vein wall and causing minimal trauma [56]. Therefore, a higher wavelength (1940 nm) should be preferred as far as patient comfort after the procedure is concerned. Keo’s team compared the efficacy of the 1940 nm TFL with the 1470 nm laser through the VEINOVA registry study and found that the incidence of postoperative phlebitis (ARTE) and sensory abnormalities was 37% and 29% lower in the former, respectively, confirming the safety of TFL treatment [57]. It is worth noting that although the TFL can achieve efficient ablation at a low power, its adjustable power range (1–4 W) can still meet the needs of complex cases, such as the recanalization of GSVs after thrombosis, which allows for the safe and permanent occlusion of under-treated veins. Despite the significant advantages of high wavelengths (1940 nm) in terms of patient comfort and safety, scholars have suggested that lower wavelengths (e.g., 1470 nm) can be combined with lasers to address large-diameter veins or high-energy-demanding scenarios to form a stratified treatment paradigm. In the future, the thulium-doped fiber laser is expected to achieve the dynamic matching of energy release and the anatomical characteristics of veins through an intelligent parameter regulation system, further promoting minimally invasive vein treatment due its precision and low damage.

4. Frontier Technological Breakthroughs and Challenges

4.1. Multimodal Therapeutic Synergy

Multimodal therapeutic synergy refers to the integration of laser ablation, imaging navigation, nanodrug carrying, and other technologies in a single treatment process, forming a closed-loop system of “diagnosis–treatment–monitoring”. Compared with the traditional segmented treatment mode, its significant advantages lie in technical synergy, which can realize the dynamic linkage of multiple devices; functional integration, which promotes the integration of diagnosis and treatment; and intraoperative real-time feedback to accurately adjust surgical parameters and reduce tissue damage, as well as complications. Thulium-doped fiber lasers offer significant advantages in the medical field due to their superior beam quality and efficient fiber coupling [32,59]. Compared to traditional holmium or thulium solid-state lasers, this technology is more straightforward to integrate into thin transmission fibers, providing the basis for multi-device collaboration. The joint application with visualization systems in minimally invasive treatments allows for better real-time manipulation while promising further improvements in diagnostic rates and reductions in treatment time and radiation [60]. The thulium laser integration of real-time monitoring treatments, such as intraoperative imaging, has great potential in stereotactic neurosurgery [61,62,63,64]. The OCT-guided fiber laser platform, as shown in Figure 6, developed by Katta et al. achieved precise bloodless cranial brain tissue resection in mice by combining a nanosecond pulsed thulium laser with stereotactic technology, with significant intraoperative vascular-specific coagulation effects and an ablation rate of up to 5 mm3/s, which provides a new option for minimally invasive neurosurgical procedures. Thulium-doped fiber lasers and visualization systems such as SpyGlassTMDS can be used in combination in stone treatment to further exploit the advantages of thulium-doped fiber lasers in stone fragmentation [27]. Currently, PT Scope (Intelligent Pressure Temperature Controlled Ureteroscopy) combined with thulium laser therapy has been applied to the endoluminal treatment of upper urinary tract stones, which allows for the real-time observation of the stone position and adjustment of the laser parameters, resulting in integrated diagnostic and therapeutic operation. Clinical data prove that the intelligent pressure-control and temperature-control soft mirror system combined with the ultra-pulsed fiber optic thulium laser can effectively control the pressure and temperature in the renal pelvis, with a high stone removal rate, less postoperative fever, less pain, and a lower complication rate, making it a safe and effective method of treating stones in the upper urinary tract.
Multidrug resistance (MDR), a leading contributor to chemotherapy failure, remains a critical challenge in oncology [66]. Recent advances in nanotechnology have opened new avenues for addressing malignancies. Specifically, nanodrug delivery systems (NDDSs) circumvent P-glycoprotein (P-gp)-mediated drug efflux by utilizing cellular endocytosis pathways [67]. This approach enhances drug accumulation at tumor sites through passive or active targeting mechanisms, thereby maximizing therapeutic efficacy while minimizing the systemic toxicity associated with conventional chemotherapeutic agents. Research on the synergistic effect of nanomaterials and lasers has also attracted much attention [68]. Gold nanorods enhance the thermal accumulation selectivity of the lesion through surface plasmon resonance, and the fiber-coupled thulium laser system achieves ablation efficiencies in the cortical and subcortical regions of 71.4% and 58.7%, respectively, in the Wistar rat brain tissue model (100 × ablation diameter/coagulation diameter), while controlling the peripheral thermal damage to 100 μm, which provides a new strategy for overcoming multidrug resistance in tumors [66]. In addition to joint applications with other medical devices, joint applications of thulium lasers with lasers of other wavelengths are also of interest to researchers. In pursuit of high cutting efficiency and optimal coagulation during surgical treatment, a continuous thulium-doped fiber laser (120 W) combined with a blue diode laser (60 W) was compared with a continuous 120 W thulium-doped fiber laser and a 120 W holmium-doped (Ho: YAG) photolaser alone in laparoscopic partial nephrectomy (LPN) [69]. Histologic analysis of isolated incisions revealed that the combination of the blue and Tm lasers provided optimal results, delivering the highest resection rates, virtually carbon-free resection surfaces, and achieving the clinically required hemostasis for laparoscopic partial nephrectomy without the need for vascular clamping. In addition, the combination of a blue diode laser and a thulium laser into a single mode may be beneficial for the further development of successful laser-assisted LPNs and may also provide an alternative to conventional Ho: YAG and KTP lasers for applications in other surgical fields.
In addition, laser-induced liquid microjet (LILJ) technology [70,71], which generates microjets in aqueous media through laser-induced cavitation bubbles, has the advantages of low thermal damage, high precision, and low invasiveness, and is highly selective for elastic tissues such as membranes and blood vessels, shows unique potential in cerebral embolism treatment, and is expected to promote fibrinolysis by injecting thrombolytic agents deep into the thrombus via microjets, and thus achieve thrombolysis [72]. Hypothalamic hamartoma (HH), as an important causative factor in drug-resistant epilepsy (DRE), requires surgical intervention. A navigational robot-assisted endoscopic system combined with the thulium laser for hypothalamic dissections demonstrates unique advantages. With its precise tissue cutting ability and excellent hemostatic properties, this laser system is an alternative to traditional bipolar electrocoagulation for more controlled dissection in narrow hypothalamic regions [73]. Clinical studies have confirmed that this minimally invasive composite technique is not only effective in controlling seizures (Engel class I rate of more than 70%), but also has a postoperative complication rate that is approximately 40% lower than that of traditional resection. Researchers constructed a fully fiber-architectured picosecond pulsed thulium-doped fiber laser, as shown in Figure 7, and used fresh porcine myocardial tissue samples for parametric analysis and mechanistic studies to optimize key ablation parameters such as laser power and scanning speed. The study demonstrates the potential of a fully fiber-integrated 2 μm thulium-doped laser system for precise and minimally invasive myocardial tissue ablation, paving the way for the future development of laser-based cardiac interventions [74]. In the future, with the deep integration of intelligent image navigation [75,76,77], nano-efficiency technology, and robot-assisted systems [78], thulium-doped fiber lasers will further enhance the targeted removal capability of complex lesions, promote minimally invasive treatment toward intelligence and multimodal synergy [32,73], and open up new paths of safer and more efficient minimally invasive treatments for intractable diseases such as tumors, epilepsy, and vascular lesions [78,79].

4.2. Challenges in Clinical Translation

As an emerging medical light source, the clinical translation of thulium lasers encounters multiple obstacles, with core challenges centering on thermal regulation, economic feasibility, and the absence of standardized protocols. Intraoperative temperature monitoring remains a critical surgical priority due to the device’s unique operational mechanism. Research confirms that irreversible cellular damage initiates when tissue temperatures surpass 43 °C (the established safety threshold for laser procedures) [80]. The wavelength properties of thulium lasers predispose them to rapid energy concentration in superficial tissues, heightening the risks of collateral thermal injury and associated complications [33,81]. While clinical protocols employ irrigation flow adjustment, as shown in Figure 8, pulse pattern optimization, and dynamic cooling systems to alleviate heat buildup, maintaining equilibrium between ablation precision and thermal dispersion in deep-tissue interventions persists as a significant technical hurdle [82,83]. The selection between continuous wave (CW) and pulsed operation modes for specific medical applications fundamentally represents a trade-off between thermal management and surgical efficacy. Due to its thermally constrained characteristics, the pulsed mode enables efficient tissue ablation and is predominantly employed in procedures requiring precision and minimal thermal damage. Conversely, the CW mode delivers stable power with continuous tissue interaction, causing significant thermal accumulation and diffusion. While less suited for fine manipulation, this mode achieves reliable hemostasis, particularly in highly vascularized tissues. Flexibility for the surgeon to switch modes according to specific surgical needs maximizes the clinical benefits of the thulium-doped fiber lasers. Economically, despite a 40% reduction in disposable material costs per procedure [84], the initial capital outlay and maintenance expenses substantially exceed those of conventional Nd: YAG lasers. Additional requirements for specialized optical fibers, cooling apparatuses, and multimodal imaging navigation further escalate operational expenditures, limiting accessibility in primary healthcare settings. The delayed establishment of clinical guidelines amplifies implementation barriers [81]. Substantial variance in operational parameters (e.g., power settings and irradiation duration) across institutions hinders comparative outcome evaluation, with potential risks of protocol misapplication [85]. The system parameters of several major medical quasi-continuous wave thulium-doped fiber lasers are demonstrated in Table 2. Though pioneering medical centers have initiated standardization efforts, the field urgently requires comprehensive frameworks encompassing device specifications, procedural workflows, and outcome assessment metrics. The therapeutic promise of thulium lasers coexists with their technical limitations. Innovations in thermal control (intelligent temperature modulation algorithms [86]), cost–performance optimization, and the harmonization of clinical practice standards represent crucial pathways for overcoming current constraints. Through multidisciplinary collaboration and iterative technical optimization, this innovative platform demonstrates translational potential for converting theoretical advantages into clinically viable applications. Evidence-based clinical integration would facilitate protocol-driven dissemination across minimally invasive surgical interventions.

5. Summary and Outlook

The high-power TFL has demonstrated unique technological advantages in the biomedical field due to its high tissue water absorption in the 2 μm wavelength band and precise ablation capabilities. In the major clinical scenarios of urinary lithotripsy, minimally invasive treatment of early lung cancer, and minimally invasive endovenous laser ablation, its clinical efficacy and safety have been significantly enhanced through the technological innovations of pulse modulation, sub-millimeter vaporization, and controllable thermal infiltration. Recent advances in Tm3+/Yb3+ co-doping have boosted quantum efficiency by 25% via cross-relaxation mechanisms, while graphene-enhanced thermal interfaces reduce thermal gradients by 40%. To further expand its application boundaries, future technological development needs to focus on breakthroughs from material system innovation, system integration optimization, and interdisciplinary synergy in four dimensions.
In terms of device performance optimization, it is necessary to focus on improving the energy conversion efficiency of the gain medium and thermal management capabilities. By optimizing the Al2O3/Tm2O3 nanocomposite doping process with novel microstructured optical fibers (e.g., fluorine-doped cladding/porous core design), we can effectively improve the quantum conversion efficiency and suppress the nonlinear effects to achieve a stable output of high power and prolong the lifetime of the devices. Phase-change materials (PCMs) like gallium alloys now enable 200% longer continuous operation through enhanced latent heat absorption. Notably, the synergistic design of gain fiber length and resonant cavity thermodynamics can significantly suppress the self-pulsing phenomenon, and by optimizing the reabsorption loss and thermal gradient distribution, a breakthrough in the long-term stability of 100-watt continuous wave output has been achieved. In addition, by utilizing multimode gain fibers, the spatiotemporal ML approach has the potential to open up new mechanisms for generating high-power pulses from fiber laser sources. Nonlinear pulse compression using chirped volume Bragg gratings (CVBGs) achieves <100 fs pulses for sub-cellular precision ablation. At the clinical translation level, the construction of an intelligent multimodal diagnosis and treatment platform has become the focus of attention. Intraoperative real-time feedback can be significantly enhanced by deep learning algorithms that analyze tissue optical properties in real time and dynamically adjust laser parameters or by integrating optical coherence tomography (OCT) with photoacoustic sensing. Embedding quantum cascade detectors (QCDs) enables concurrent mid-infrared spectroscopy for molecular-level tissue analysis. This integrated treatment strategy not only helps improve the safety of the procedure and lower the threshold of operation, but also effectively avoids the risk of thermal injury. In the future, thulium-doped fiber lasers are expected to break through the existing diagnostic and therapeutic boundaries with the cross-fertilization of ultrafast laser technology, biophotonics, and materials science. Convergence with quantum computing will enable real-time multi-parameter optimization for autonomous ablation protocols by 2030. Basic research should focus on the establishment of a multi-scale theoretical model to elucidate the nonlinear interaction mechanism between the 2 μm ultrafast laser and biological tissues, and to provide theoretical support for the construction of a personalized therapeutic parameter database. The promotion of the industrialization process needs to rely on industry–university–research cooperation, focusing on the development of low-cost degradable fiber optic probes, modular laser systems, and other clinically applicable technologies, and the construction of an international standards system to promote the standardization of clinical applications.
With the breakthrough of femtosecond-level high peak power technology, the TFL may become the core light source of a next-generation intelligent surgical diagnosis and treatment platform through the virtuous cycle of technology iteration and clinical validation. This technology is projected to transcend current clinical paradigms by delivering novel therapeutic approaches in emerging domains, such as neurovascular microsurgical dissection, intravascular photoacoustic plaque ablation, and deep-seated tumor photodynamic therapy, thereby catalyzing the progressive refinement of precision surgical diagnostic systems toward enhanced efficacy and optimized safety parameters.

Author Contributions

Conceptualization, W.-Y.X. and G.W.; Methodology, G.W.; Validation, Y.Y.; Formal Analysis, W.-Y.X.; Investigation, W.-Y.X.; Resources, G.W. and Y.-F.L.; Data Curation, W.-Y.X.; Writing—Original Draft Preparation, W.-Y.X. and G.W.; writing—review and editing, Y.-F.L., Y.W. and Z.L.; Supervision, Y.W. and Z.L.; project administration, W.-Y.X.; Funding Acquisition, G.W., Y.-F.L. and Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the 173 Project Technical Fund [grant number: 2022-JCJQ-JJ-0416]; the Central Government Guides Local Funds for Scientific and Technological Development [grant number: 236Z1813G]; the Natural Science Foundation of Hebei Province [grant numbers: F2024202086, F2024202083]; the Science Research Project of Hebei Education Department [grant number: BJK2024048]; the Science Fund for Excellent Young Scholars of Shijiazhuang [grant number: 241791207A]; and the Science and Technology Cooperation Special Project of Shijiazhuang [SJZZXA24007].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Energy level structure of Tm3+ ions in silica [13].
Figure 1. Energy level structure of Tm3+ ions in silica [13].
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Figure 2. (a) Fluorescence lifetime τ of silica fibers (series A) depending on Tm concentration for the 3F4 level under core-pumping at 789 nm. (b) Fluorescence lifetime τ of crystal-derived fibers (series B, cAl2O3/cY2O3) depending on Tm concentration for the 3H4 level under core-pumping at 789 nm [6].
Figure 2. (a) Fluorescence lifetime τ of silica fibers (series A) depending on Tm concentration for the 3F4 level under core-pumping at 789 nm. (b) Fluorescence lifetime τ of crystal-derived fibers (series B, cAl2O3/cY2O3) depending on Tm concentration for the 3H4 level under core-pumping at 789 nm [6].
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Figure 3. The absorption coefficients of water, and positions of the thulium fiber laser (1940 nm) and the Nd: YAG laser (1064 nm) [22].
Figure 3. The absorption coefficients of water, and positions of the thulium fiber laser (1940 nm) and the Nd: YAG laser (1064 nm) [22].
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Figure 4. Schematic of the all-fiber-configured TFL at 1940 nm [34].
Figure 4. Schematic of the all-fiber-configured TFL at 1940 nm [34].
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Figure 5. Characterization of the temperature-dependent absorption properties of tissue chromophores, including blood and water. Calibration results for canine blood are highlighted in the top dark blue panels (a–c). Calibration results for deionized water are highlighted in the top dark blue panels (d–f). Change in the absorbance of (a) canine blood and (d) deionized water was studied at higher temperatures (30–85 °C) [58].
Figure 5. Characterization of the temperature-dependent absorption properties of tissue chromophores, including blood and water. Calibration results for canine blood are highlighted in the top dark blue panels (a–c). Calibration results for deionized water are highlighted in the top dark blue panels (d–f). Change in the absorbance of (a) canine blood and (d) deionized water was studied at higher temperatures (30–85 °C) [58].
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Figure 6. (a) Bench top fiber–laser platform consisting of the thulium (Tm) fiber laser combined with a Yb fiber–laser for cutting and coagulation, respectively, and incorporating swept source OCT image guidance. (b,c) Biocompatible silica fiber implementation of the Tm/Yb fiber laser configuration for in vivo murine brain surgery. Collimation of Yb fiber–laser beam utilized a QBH (quartz block head) collimator (IPG Photonics) from a 50 µm core silica fiber (0.1 NA) to a 3 mm diameter beam. Collimation of the Tm fiber-laser beam utilized a reflective collimator (RC08, Thorlabs Inc.). Yb and Tm beams were combined with a dichroic mirror (DM2, DMSP1500, Thorlabs Inc.) and coupled into a multimode fiber (Flexiva 200, Boston Scientific), terminating in a flat distal polished tip [65].
Figure 6. (a) Bench top fiber–laser platform consisting of the thulium (Tm) fiber laser combined with a Yb fiber–laser for cutting and coagulation, respectively, and incorporating swept source OCT image guidance. (b,c) Biocompatible silica fiber implementation of the Tm/Yb fiber laser configuration for in vivo murine brain surgery. Collimation of Yb fiber–laser beam utilized a QBH (quartz block head) collimator (IPG Photonics) from a 50 µm core silica fiber (0.1 NA) to a 3 mm diameter beam. Collimation of the Tm fiber-laser beam utilized a reflective collimator (RC08, Thorlabs Inc.). Yb and Tm beams were combined with a dichroic mirror (DM2, DMSP1500, Thorlabs Inc.) and coupled into a multimode fiber (Flexiva 200, Boston Scientific), terminating in a flat distal polished tip [65].
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Figure 7. (a) Schematic diagram of the laser ablation system; (b) schematic diagram of the experimental setup of the picosecond pulsed thulium-doped fiber laser with an all-fiber structure [74].
Figure 7. (a) Schematic diagram of the laser ablation system; (b) schematic diagram of the experimental setup of the picosecond pulsed thulium-doped fiber laser with an all-fiber structure [74].
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Figure 8. Ablation cone volume (mm3) for each individual laser pulse using different energy settings and different irrigant temperatures. p-values were calculated by post hoc analysis with a Tukey ANOVA. Symbol legend: * meaning p < 0.05; ** meaning p < 0.01; *** meaning p < 0.001, **** meaning p < 0.0001. The “ns” symbol indicates a non-significant difference [82].
Figure 8. Ablation cone volume (mm3) for each individual laser pulse using different energy settings and different irrigant temperatures. p-values were calculated by post hoc analysis with a Tukey ANOVA. Symbol legend: * meaning p < 0.05; ** meaning p < 0.01; *** meaning p < 0.001, **** meaning p < 0.0001. The “ns” symbol indicates a non-significant difference [82].
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Table 1. Typical parameter requirements for thulium-doped fiber lasers in the four major medical application scenarios in this paper.
Table 1. Typical parameter requirements for thulium-doped fiber lasers in the four major medical application scenarios in this paper.
Medical ApplicationWavelength (nm)Tunable Range (nm)Max. Output Power (W)Operation ModeKey Parameter Considerations
Stone fragmentation
lithotripsy		1940
(standard)		1900–210010–60Short pulse
(μs level)		Dusting/lithotripsy settings of 0.2–0.5 J/10–15 W are recommended for ureteral stones, 0.1–0.2 J/15–30 W for kidney stones, and 2–5 J/30–50 W [9]
Early-stage lung cancer
ablation		19401920–200030–50CW-dominantCW mode is recommended for stable power and uniform thermal coverage [10]
Treatment of disfiguring
dermatoses		19401900–194010–30Pulsed or CWShort pulses (ms–level) for precise pigment/vascular targeting; CW for coagulation [11]
Endovenous laser ablation19201900–194010–20Continuous wave
(CW)		Low power and controlled fiber pullback (1–3 mm/s) to optimize thermal dose for venous occlusion [12]
Table 2. System parameters of several main medical quasi-continuous-wave thulium-doped fiber lasers.
Table 2. System parameters of several main medical quasi-continuous-wave thulium-doped fiber lasers.
Index
Name of Product		Olympus (SOLTIVE™ Premium)EMS
(LaserClast Thulium Power)		Jena Surgical (MultiPulse Tm + 1470)LAKH (LKSPTm120)Industry
Consensus
Central wavelength (nm)1940 ± 201940 ± 201940 & 14701940 ± 201940 nm (water absorption peak)
Max. average power (W)606012012060–120 W
Max. peak power (W)500500120500120–500 W
Max. pulse energy (J)66——≤6 J (lithotripsy-optimized)
Pulse width range(ms)0.2–50>0.1>0.5>0.10.1–50 ms (flexible)
Repetition rate(Hz)1–24001–2500
and CW		1–1000
and CW		1–2500
and CW		1–2500 Hz and CW mode
Cooling modeAir coolingAir coolingWater coolingAir coolingAir (portable)/water (high-power)
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MDPI and ACS Style

Xu, W.-Y.; Wang, G.; Li, Y.-F.; Yu, Y.; Wang, Y.; Lu, Z. Frontier Advances and Challenges of High-Power Thulium-Doped Fiber Lasers in Minimally Invasive Medicine. Photonics 2025, 12, 614. https://doi.org/10.3390/photonics12060614

AMA Style

Xu W-Y, Wang G, Li Y-F, Yu Y, Wang Y, Lu Z. Frontier Advances and Challenges of High-Power Thulium-Doped Fiber Lasers in Minimally Invasive Medicine. Photonics. 2025; 12(6):614. https://doi.org/10.3390/photonics12060614

Chicago/Turabian Style

Xu, Wen-Yue, Gong Wang, Yun-Fei Li, Yu Yu, Yulei Wang, and Zhiwei Lu. 2025. "Frontier Advances and Challenges of High-Power Thulium-Doped Fiber Lasers in Minimally Invasive Medicine" Photonics 12, no. 6: 614. https://doi.org/10.3390/photonics12060614

APA Style

Xu, W.-Y., Wang, G., Li, Y.-F., Yu, Y., Wang, Y., & Lu, Z. (2025). Frontier Advances and Challenges of High-Power Thulium-Doped Fiber Lasers in Minimally Invasive Medicine. Photonics, 12(6), 614. https://doi.org/10.3390/photonics12060614

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