Research progress of thulium-doped fiber lasers

At present, 2u0026mu;m band lasers mainly have three application requirements: 　　 One is for medical and biological research. Since water molecules have a strong absorption rate near the wavelength of 2u0026mu;m, and this wavelength laser is safe for the human eye, when this wavelength laser is used for surgery, the blood at the laser-irradiated site will quickly coagulate, and it has good hemostasis. It is expected to become a laser One of the important breakthroughs in the field of surgery.　　 The second is for lidar and telemetry research. Because the 2u0026mu;m band lasers simultaneously cover three important molecular absorption bands of 1.88u0026mu;m, 1.91u0026mu;m and 2.41u0026mu;m, it is a coherent Doppler wind radar and a water vapor parabolic differential absorption lidar system Ideal light source.　　 The third is for research on mid-infrared optoelectronic countermeasures. Since the 2u0026mu;m band laser can be directly used as the pumping source of the ZGP optical parametric oscillator, it has the characteristics of high efficiency, compact and reliable structure, so the ZGP-OPO laser pumped by the 2u0026mu;m laser can obtain 3u0026mu;m to 5u0026mu ;m and 8u0026mu;m to 12u0026mu;m band mid-infrared laser is one of the effective ways. Compared with the 2u0026mu;m solid-state laser, the thulium-doped fiber laser with the same wavelength output has attracted wide attention due to its simple structure, good beam quality and narrow line width. Its output power is different from the original single-clad millimeter. The watt level has developed to the double-clad 100 watt level. At present, the 150W thulium-doped fiber laser of German IPG has been commercialized, and it has made good progress in efficiency, miniaturization and normal temperature operation.　　1 Progress in the research of thulium-doped fiber lasers　　The host materials of thulium-doped fiber lasers mainly include quartz substrates and fluoride substrates. In the quartz matrix, all energy levels can undergo non-radiative relaxation through multiphonon radiation, which makes the threshold very high. The H4 energy level is the only energy level that can radiate laser light observed so far; while in fluorine In the compound matrix, more metastable multi-states emitting fluorescence can be provided, and laser oscillations of multiple wavelengths can be formed. However, quartz optical fiber has the advantages of easy drawing and good stability, so it is widely used.　Thulium-doped fiber laser is a quasi-three-level system at room temperature. The pumping light transfers Tm3+ from the H6 ground state to the H4 energy level, and the excited ions relax from the H4 energy level to the laser energy level F4. The laser action occurs between the F4 energy level and the laser energy level in the H6 ground state multiple state, and the output radiation Approximately 2u0026mu;m.