In recent years, lasers have achieved significant advancements in the field of biomedicine, particularly in minimally invasive surgeries, owing to their excellent monochromaticity, collimation, and high energy density. The interaction of lasers with biological tissues primarily relies on laser-induced thermal effects, which vary depending on the wavelength and energy of the laser.

Water molecules are a major component of biological tissues, and their absorption coefficients for lasers of different wavelengths are a critical factor in laser-induced thermal effects. The absorption coefficient of water molecules increases with wavelength: it is as low as 10−4 cm−1 in the visible light band, but can reach up to 600 cm−1 in the 2 μm band, enabling shallow penetration depth in biological tissues and excellent thermal coagulation and hemostasis effects. Meanwhile, studies have shown that the damage threshold of 2 μm laser to the human eye is 8 orders of magnitude higher than that of 0.69 μm and 1.069 μm lasers, and 3 orders of magnitude higher than that of 1.5 μm laser, demonstrating superior eye safety. Therefore, in clinical applications, 2 μm laser transmitted through low-loss optical fibers and combined with endoscopes can achieve high surgical precision and good safety.

Common 2 μm medical lasers include holmium-doped and thulium-doped lasers, which can be further classified into solid-state lasers and fiber lasers based on the gain medium used. Compared to traditional solid-state lasers, fiber lasers offer advantages such as higher electro-optical conversion efficiency, greater stability, better beam quality, and ease of integration, making them a key development direction for future medical lasers. Thulium-doped lasers, being closer to the 2 μm water absorption peak than holmium-doped lasers, enable more efficient tissue vaporization and cutting with higher precision. Consequently, they outperform holmium-doped lasers in both tissue ablation and lithotripsy procedures, garnering greater preference among researchers and medical professionals.

Current Development Status of Biomedical Thulium-Doped Fiber Lasers

Thulium-doped lasers primarily include thulium-doped solid-state lasers and thulium-doped fiber lasers. The gain media for thulium-doped solid-state lasers mainly include thulium-doped yttrium aluminum garnet (Tm:YAG), thulium-doped yttrium aluminum perovskite (Tm:YAP), and thulium-doped yttrium lithium fluoride (Tm:YLF). Traditional solid-state lasers require coupling the laser generated by stimulated emission of the gain medium into a transmission fiber through a lens, but this coupling process involves significant loss.

In contrast, the all-fiber structured thulium-doped fiber laser, a special third-generation solid-state laser, uses thulium-doped fiber as the gain medium. The pump light is transmitted through the fiber without the need for spatial structure coupling, and the laser remains confined within the fiber core, resulting in low slope efficiency and transmission loss. Therefore, fiber lasers offer superior beam quality and collimation, higher energy density, and feature compact structures and easy integration, making them an ideal choice for medical lasers.

Since the 1980s, alongside the growing application demands for thulium-doped fiber lasers, research on these lasers has continuously advanced both domestically and internationally. In the past decade, significant breakthroughs have been achieved in both continuous-wave (CW) and pulsed laser systems. 

In the biomedical field, thulium-doped lasers have found extensive applications. Thulium-doped laser technology can serve as an alternative to conventional tissue suturing and is used in various medical procedures, including: Stone fragmentation (lithotripsy)、Varicose vein closure、Minimally invasive laryngeal surgery、Resection of oral squamous cell carcinoma.These applications leverage the laser’s precise tissue ablation capabilities at the 2 μm wavelength, combining high efficiency with minimal thermal damage to surrounding tissues.

Biomedical Applications of Thulium-Doped Lasers

Thulium-Doped Laser Tissue Ablation

Water is a major component of biological tissues, and due to the high absorption efficiency of water molecules for thulium-doped laser energy, thulium-doped lasers are extremely widely used in human tissue ablation. When thulium-doped laser acts on biological tissues with high water content, water molecules absorb laser energy and rapidly heat up.

Studies by Markolf H. Niemz et al. have shown that the normal human body temperature is 37°C. When tissue temperature reaches 45–50°C, it causes tissue necrosis and forms a thermal damage zone. When the temperature exceeds 60°C, the thermal denaturation of proteins in the tissue leads to tissue coagulation. When the tissue temperature reaches 100°C or higher, water molecules vaporize, leading to tissue carbonization and ablation. The various thermal effect zones caused by laser-tissue interaction are shown in Figure 1.

Figure 1. Various Thermal Effect Zones Caused by Laser-Tissue Interaction

Research on the application of thulium-doped lasers in tissue ablation dates back to the 1990s. Studies have investigated the interaction between 2μm lasers and biological tissues under varying conditions such as laser power, irradiation time, spot radius, and cutting speed by adjusting laser parameters, aiming to observe the effects of laser thermal effects on biological tissues. Experimental results show that within a certain range, increasing the laser energy absorbed by biological tissues—such as by enhancing laser power or irradiation time—can achieve faster ablation rates.

Thulium-Doped Laser Lithotripsy

Urinary stones are common urological diseases in modern humans, with diverse formation mechanisms and compositions. Clinically, calcium oxalate monohydrate (COM) stones and uric acid (UA) stones are most prevalent. In addition to the tissue thermal ablation mechanism mentioned earlier, thulium-doped laser lithotripsy also employs a "micro-explosion" mechanism. This mechanism refers to the process where water molecules within the voids of the stone absorb laser energy and vaporize upon laser exposure, creating localized high pressure. The differing thermal transport coefficients between water molecules and the stone itself also induce pressure changes within the stone, causing fragile areas to fracture and achieving stone fragmentation.

Thulium-doped fiber lasers have gradually become the mainstay in clinical lithotripsy applications due to their advantages of low cost, easy integration, high energy density, multiple adjustable parameters, and compatibility with small-core-diameter fiber transmission.

Changjin 25/400 Thulium - Doped Optical Fiber

Compared with foreign countries, the development of high-power thulium-doped fiber lasers in China has been relatively slow, and their core component—thulium-doped fibers—has long relied on imports. Therefore, in-depth research on key parameters is a critical pathway to develop high-performance thulium-doped silica fibers that can replace imported products, thereby further enabling the stable operation of high-power thulium-doped fiber lasers. Through years of technical accumulation and innovative R&D, Changjin Laser has successfully developed high-performance thulium-doped fibers, with performance and consistency reaching the level of imported fibers and high customer satisfaction.

The technical parameters of Changjin's developed 25/400 double-clad thulium-doped silica fiber are shown in the figure below. It can be seen that the thulium-doped fiber developed by Changjin has excellent laser performance and the potential to be applied in high-power fiber laser systems. Based on this large-mode-area thulium-doped fiber, a 530 W all-fiber structured thulium-doped fiber laser has been achieved.

Figure 2. Parameters of Changjin 25/400 Double-Clad Thulium-Doped Silica Fiber

530 W Bidirectional Pumping All-Fiber Structured Thulium-Doped Amplifier

In 2020, in collaboration with Huazhong University of Science and Technology, using the large-mode-area thulium-doped fiber provided by Changjin Laser, a thulium-doped fiber continuous-wave laser with a single-stage MOPA (Master Oscillator Power Amplifier) amplification structure was successfully built. After amplification, the seed source achieved a maximum output power of 530 W at a central wavelength of 1980.89 nm, with a corresponding slope efficiency of 50%. The specific experimental setup of the 530 W all-fiber structured MOPA system is shown in Figure 3, which includes two parts: the seed source and the amplification stage.

The seed source also employs an all-fiber structured oscillator. The amplification stage adopts a bidirectional pumping scheme. To ensure full absorption of pump light and provide sufficient gain, 8 meters of the aforementioned 25/400 double-clad thulium-doped fiber is used as the gain medium in the amplification stage. The dimensions and numerical apertures (NA) of the pigtails at the combining ends of the two combiners are matched with those of the active fiber.

Figure 3. Structural Diagram of 530 W Bidirectional Pumping Thulium-Doped All-Fiber MOPA System 

As shown in Figure 4(a), the graph depicts the relationship between the output power of the thulium-doped fiber amplifier and the back-reflected power measured at Port 1 as a function of pump power. The output laser power increases linearly, with no observed power drop throughout the process. When the pump power reaches 979 W, the output power of the thulium-doped fiber amplifier reaches 530 W, with a corresponding slope efficiency of 50%.

The optical spectrum measured at an output power of 500 W is shown in Figure 4(b), where the signal-to-noise ratio of the output laser is 27 dB. Scanning the laser spectrum in the range of 1950–2000 nm reveals no obvious spontaneous emission or nonlinear effects. As shown in the inset of Figure 4(b), this indicates that the output laser power is only limited by the pump power. This result represents the highest output power currently achieved by domestic 2 µm band fiber lasers and also verifies the reliability of domestically produced thulium-doped silica fibers in high-power systems.

Figure 4. (a) Variation of output power from the amplification stage and backward reflected light with pump power; (b) Optical spectrum corresponding to an output power of 500 W

With the popularization of thulium-doped fiber lasers in biomedical applications, the demand for high-performance thulium-doped fibers will continue to grow. Changjin Laser is committed to aligning with market needs, sustaining investments in R&D and production of thulium-doped fibers, and providing robust technical support for the development of China's biomedical thulium-doped fiber laser industry.