In recent years, thulium-doped fiber lasers have attracted more and more attention due to their advantages such as compact structure, good beam quality, and high quantum efficiency. Among them, high-power continuous thulium-doped fiber lasers have important applications in many fields such as medical care, military security, space communications, air pollution detection, and material processing. In the past nearly 20 years, high-power continuous thulium-doped fiber lasers have developed rapidly, and the current maximum output power has reached the kilowatt level. Next, let’s take a look at the power improvement path and development trends of thulium-doped fiber lasers from the aspects of oscillators and amplification systems.
The pump source of early thulium-doped fiber lasers generally used low-power 1064 nm YAG laser or 790 nm dye laser. Due to the low power of the pump source and the limitations of the backward doped fiber preparation process at the time, the output power of thulium-doped fiber lasers was only in the watt level. With the introduction of double-cladding pump technology and the increasing maturity of high-power semiconductor laser technology, the output power of thulium-doped fiber lasers is also constantly increasing.
In 1998, Jackson et al. from the University of Manchester in the UK used a 790 nm semiconductor laser as a pump source and used cladding pumping technology to build a spatially structured continuously tunable thulium-doped fiber laser with a maximum output power of 5.4 W. In 2007, a thulium-doped germanate fiber laser was developed. The experimental device is shown in Figure 1. Under single-end pumping mode, a continuous laser output of 64 W was obtained at 1900 nm. In order to obtain higher output power, the researchers used double-end pumping and used a 40 cm long gain fiber, and finally obtained a 1900 nm continuous laser output of 104 W.
In 2009, Harbin Institute of Technology developed a thulium-doped fiber laser with an all-fiber linear cavity structure. It consists of a reflective fiber Bragg grating and the Fresnel reflection formed by the thulium-doped fiber end face to form a resonant cavity. It is pumped by 793 nm LD. Finally, an output power of 39.4 W was obtained. In addition, they also compared the output power and spectral characteristics obtained when FBG and dichroic mirrors were used as high-reflection couplers respectively, and found that the slope efficiency of the all-fiber structure was lower and the threshold power was higher. Compared with the spatial structure, the all-fiber structure was initially limited by the performance of the optical fiber device and the quality of splicing, and its advantages were not obvious. With the continuous improvement of optical fiber device preparation technology and splicing level, all-fiber structures have gradually shown huge advantages.
In the same year, a high-power thulium-doped fiber laser based on a spatial structure used a 793 nm LD to pump a thulium-doped fiber with a core diameter of 25 μm and a numerical aperture (NA) of 0.08, and achieved a single-mode laser output of 300 W. Later, with a similar structure, a large-mode field fiber with a core diameter of 40 μm and a numerical aperture of 0.2 was used to obtain a 2040 nm multi-mode laser output of 885 W, which is the maximum output power obtained by a single thulium-doped fiber oscillator.
In 2014, Tsinghua University reported a high-power thulium-doped fiber laser with an all-fiber linear cavity structure, consisting of a fiber Bragg grating and a 3 m-long gain fiber. Seven 790 nm LDs with a maximum output power of 70 W were used as pump sources. Finally, an output power of 227 W was obtained. In the same year, the National University of Defense Technology used two high-power 1173 nm Raman fiber lasers (RFL) as pump sources to build a high-efficiency narrow linewidth thulium-doped fiber laser with an all-fiber straight cavity structure, and finally achieved an output of 96 W. power. This was the first reported thulium-doped fiber laser with a pump wavelength near 1200 nm and an output power in the order of hundreds of watts. It also provided a very promising pumping solution for increasing the output power of thulium-doped fiber lasers.
In 2015, Huazhong University of Science and Technology used self-made thulium-doped double-clad silica fiber to build a thulium-doped fiber laser with an all-fiber linear cavity structure. It used three high-power 793 nm LDs for pumping and obtained an output power of 121 W. This It is the first time to use domestic thulium-doped optical fiber to obtain output power of hundreds of watts at a wavelength of 1915 nm. In addition, experiments found that increasing the inner cladding diameter of the gain fiber can achieve better heat dissipation, which also provides ideas for thermal management and power improvement of thulium-doped fiber lasers.