Study On Photon Darkening Effect In High Power Thulium-doped Fiber Laser

With the continuous growth of the output power of Thulium-Doped Fiber Laser(TDFL)and the continuous expansion of the application range,the power stability requirements of the output laser are becoming more and more strict.When TDFL works for a long time,due to the influence of Photon Darkening Effect(PD),the output power will gradually decrease,the stability will become worse,and the temperature will rise,which seriously affects the sustainable development of rare-earth doped lasers.Based on the current research on photonic darkening,this thesis studies the PD of thulium-doped fiber lasers,which can also provide a useful reference for further exploring the generation mechanism of the darkening effect of other rare-earth-doped fiber lasers.The main research contents include:(1)Firstly,the energy level structure of thulium ions is introduced,and the theoretical models of three pumping modes of thulium-doped fiber laser outputting 2μm wavelength are established.At the same time,the numerical simulation of the pumping mode at 793 nm is carried out.The simulation shows that the power distribution of the pump light gradually decreases with the fiber length,and there are optimal doping concentration thresholds and optimal fiber length thresholds,which can make the output optical power reach the theoretical maximum value,respectively.(2)Based on the current PD experimental research,a method for detecting the photonic darkening effect of thulium-doped fiber lasers is proposed,and the change in the loss of the external light source signal is used to measure the effect of photonic darkening,which improves the convenience and accuracy.sex.This device can intuitively detect the darkening degree of the thulium-doped fiber.After fitting the experimental data with the tensile exponential function,the indicators such as the additional loss and darkening rate in the equilibrium state are obtained.(3)Using the self-designed PD device,the relationship between the signal light,pump light,optical fiber concentration and other factors and the photon darkening intensity was analyzed experimentally.The experimental results show that the 1064 nm pumping method has higher darkening intensity due to the highest degree of up-conversion of thulium ions,and the darkening rate is several orders of magnitude faster than other methods.The greater the pumping power,the greater the darkening degree.In the visible light band,the wavelength of the light source is inversely related to the degree of photon darkening,and the darkening intensity is lower when the thulium ion concentration is higher in the 793 nm pumping mode.(4)The photonic darkening effect suppression experiment of thulium-doped fiber was carried out,and the thulium-doped fiber was darkened to a near-saturated state and then pumped at other wavelengths or heated at high temperature.The results show that when the high temperature reaches 650℃,the additional loss in TDF can be basically eliminated to achieve the complete bleaching effect,but heat-induced loss will be generated in the early stage of heating,and the heating rate will also affect the bleaching effect of PD.High-temperature bleaching is repeatable and bleaching.Both the preheated fiber and the preheated fiber will show some darkening resistance.In addition,when 405 nm laser was used to pump thulium-doped fiber,it was found that the 405 nm laser had a partial bleaching effect,which was not obvious but reached 17.3%.At the same time,we tried to photobleach it again with 405 nm wavelength at 300 °C.,it was found that the bleaching effect was further enhanced,and the total bleaching power reached 43%.