| tunable thulium doped fiber lasers(TDFL)can be widely used in material processing,atmospheric remote sensing,laser radar,biomedicine,and other fields because their wavebands are located in the eye safety band and atmospheric window.It is currently a research hotspot in the field of laser engineering.So far,thulium doped fiber lasers have only achieved the order of 1 k W.Research suggests that the following factors mainly limit the increase in power:First,The number of devices in the 2μm band is rare and immature,Due to the unique nature of this band,it is necessary to develop more devices to meet different application needs and improve the performance and reliability of the devices;Secondly,the special energy level results of thulium ions are more likely to cause photodarkening,which severely limits the power improvement of fiber lasers.This work focuses on tunable polarization maintaining thulium-doped fiber lasers,photon darkening,and anti-resonant fiber devices in the 2μm band.The main innovative points of the thesis are as follows:First,a physical model has been established to describe the evolution process of photodarkening.Compared to existing models,this model can quantitatively analyze the relationship between pump power and photon frequency in the core,core area,core temperature,and PD loss.It is concluded that there is a positive correlation between the power of high-energy photons and photodarkening rate,and a positive correlation between temperature and saturated photodarkening loss.A photodarkening measurement system was built using a self-made gallium indium tin alloy temperature control device,further verifying the accuracy of the photon darkening mechanism and physical model in the fiber laser established in this work.The mechanism of photodarkening in fiber lasers established in this work and the accuracy of the physical model are further verified.Secondly,a fully polarized tunable thulium doped fiber ring cavity laser was constructed using all fiber and all polarization maintaining devices.Under the pumping of a 793nm semiconductor laser with a power of 16W,a tunable thulium doped fiber ring cavity laser was achieved 459m W@1993nm Laser output,wavelength tuning range1925nm～2034nm.Thirdly,a highly stable all fiber tunable polarization maintaining thulium doped fiber amplification system has been built.The maximum output power obtained is 5.65W(@1993nm),with a 3d B linewidth less than 0.5nm,M~2≈1.25,a polarization extinction ratio of 21.4d B,an instability of 0.0307d B,and a wavelength tuning range of 110nm(1925～2034nm).We have established a power spectrum simulation model for thulium doped fiber amplifiers,overcoming the limitation of traditional amplifier simulations that can only solve for changes in the optical power of a single wavelength signal.The accuracy of the simulation model was verified by comparing it with experimental results.Fourthly,in order to achieve low loss transmission of high-power 2μm laser,a new structure of hollow-core nested anti-resonant fiber was designed and two types of embedded anti-resonant fiber were optimized.In the design,the confinement loss was limited to 0.0098 d B/km and a high high-order mode suppression ratio(691)was obtained,which is crucial for maintaining the mode purity and stability of the laser signal.By correctly designing and optimizing the structure of anti-resonant fibers,losses can be effectively reduced,while maintaining the quality and stability of laser signals,further promoting the application of thulium-doped fiber lasers in the 2μm band.There are 85 pictures,10 tables,and 193 references. |
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