Development Of Mid-infrared Medical Fiber Laser And Research On Interaction Of The Laser With Biologic Tissue

In recent years,the 2 μm laser surgery system has great advantages of less bleeding,small wound and high safety etc.compared with traditional open surgery and transurethral resection of the prostate（TURP）.Because of the spectral absorption peak of water molecules in 1.94 μm,when tissue is under the laser irradiation,the internal moisture will quickly absorb laser energy to reach the gasification conditions instantaneously and thermal decomposition.In this process,the tissue of vaporizing decomposition could take away plenty of heat and reduce the effect of heat transfer in the adjacent tissue,also reduce thermal damage around the target tissue.However,the current available light sources of thulium laser scalpel use LD end pumped Tm: YAG and Tm: YLF laser system.Compared with the solid state laser,the thulium doped fiber laser has good beam quality,convenient integrations and high compactness,and thus it has potential application value for replacing the traditional solid laser scalpel.So,it is necessary to study the thulium doped fiber laser systems interact with fresh tissue by quantitative analysis and qualitative discussion.In this,we developed a low power continuous/quasi continuous all-fiber thulium laser therapeutic instrument and a high power thulium laser scalpel experiment system based on all-fiber MOPA structure.Moreover,we focused on the interaction of thulium doped fiber laser and biological tissue.The main research achievements are shown follows:（1）Based on Tm³⁺ absorption level with 793 nm pump,the numerical model involving rate equations were constructed,and the output characteristics were simulated combined with the boundary condition.Also,the fiber length that at the expected power and the slope efficiency of output laser were calculated.The results of numerical simulation show that the output laser power of 3.4 W could be achieved under the pump power of 12 W while the length of Tm³⁺-doped fiber was 6 m.In addition,the MOPA amplifier output characteristics of thulium doped fiber laser were also simulated by adjusting the parameters of the boundary conditions.The output power of 81.75 W with a slope efficiency of 48.9% could be realized under the seed source power of 3 W when we used 10 m Tm³⁺-doped fiber and set the pump power of 160 W.（2）Firstly,based on the simulation of output power,pump parameter and gain fiber parameter of thulium doped fiber laser,the continuous laser source system structure was set up and encapsulated in the 2 μm all fiber thulium therapeutic instrument.The system could output laser with the center wavelength of 1981.53 nm and the FWHM of 0.07 nm.A maximum output power of 2.0 W in continuous mode and 1.0 W in quasi continuous mode with the duty ratio of 50% are achieved.Secondly,the single point ablation experiment of fresh pig skin by using the all-fiber thulium laser scalpel was researched.The tissue thermal penetration depth is below 2.5 mm by quantitative analysis.Compared with continuous laser ablation,the 1 Hz quasi continuous laser ablation depth of tissue will be greatly increased.Also the skin tissue can also reach minimum and maximum ablation depth and coagulation depth at this operating frequency.Finally,with the contrast experiment of 2 μm and 3 μm laser ablation skin tissue,3 μm erbium laser has more shallow penetration depth.However,we still can adjust the parameters of thulium laser to adaptive the effect of erbium laser irradiation on tissue.（3）2 μm high power thulium laser based on all-fiber MOPA structure with high power 793 nm laser diode as the pump source was researched and constructed.The seed resource could output the laser with the center wavelength of 1980.4 nm and the FWHM of 0.05 nm.A maximum output power of 78.1 W was achieved with a slope efficiency of 57% when the pump power was increased to 138.8 W.At the same time,the experimental research of 30 W level high power thulium doped fiber laser cutting fresh pig kidney was carried out.In the case of cutting effect,the width and depth of tissue coagulation reduce about 50% while the cutting speed increased from 3 mm/s to 5 mm/s.All the data show that the depth of tissue thermal effect is within 3 mm and the max depth of coagulation is about 1 mm.Furthermore,the possibility of medical development of 2 μm high power thulium doped fiber laser was verified.