High Power 2 Micron Thulium Doped Fiber Lasers And Their Dynamics

In 1960,the first solid state laser was successfully invented.Soon after that(in 1961),the concept of fiber laser was proposed and implemented.In recent years,fiber lasers have attracted increasing attention and been advanced rapidly due to their advantages of compact structure,high conversion efficiency,good beam quality,and good heat dissipation.A large variety of fiber lasers have been reported,such as high power,narrow linewidth,short pulse,infrared,just to name a few.Tm-doped fiber lasers generate laser emission in the 1900-2100 nm wavelength range.Owing to their characteristics,such as eye-safe,etc.,Tm-doped fiber lasers can be applied in extensive areas,such as industrial machining,medical surgery,remote detection,atmospheric lidar measurement,scientific experiments,etc.Tm-doped fiber lasers are shaping up as an important area of research.In recent years,high power 2?m Tm-doped fiber lasers have developed rapidly.For continuous wave operation,the Tm-doped fiber laser has achieved more than 1kW of output power.For pulsed operation,the Tm-doped fiber laser with picosecond/femtosecond pulse has achieved more than 100 W of output power but their pulse energy is low.Now the development of high power 2?m Tm-doped fiber lasers is restricted by the thermal effect and the nonlinear effect.The objective of our work is to solve these problems and to achieve the Tm-doped fiber lasers that can output very high power or very high pulse energy by optimizing the pump scheme and increasing the fiber mode area.The primary contents and results of our research include:1.We propose a cascaded tandem pumping technique(~1900nm?~1940nm?~2020nm)and show its high power and high efficient operation in the 2?m wavelength region,opening up a new way to scale the output power of the 2?m Tm-doped fiber laser to new levels(e.g.10kW).The spectral conversion from the ~1900nm range(from 1890 nm to 1910nm)to the ~1940nm range(from 1930 nm to 1950nm)comprises the first-stage tandem pumping,while the spectral conversion from the ~1940nm range to the ~2020nm range(from 2000 nm to 2020nm)consists of the second-stage tandem pumping.In each stage tandem pumping,the quantum defect(thus heat load)is very small.The total heat loading is distributed among two stages,spreading waste heat over a longer distance,greatly diminishing the thermal problem.Combining this cascaded tandem pumping technique to the conventional cladding pumping technique can potentially scale the output power of 2?m Tm-doped fiber lasers to an unprecedented level.2.To show the power scaling capability and efficiency of the cascaded tandem pumping technique,a numerical model was built to analyze the lasing and thermal characteristics under different pump transitions.When the fiber core and coating are not burned,the maximum output power in the ~793nm?~2020nm transition is 220W;the maximum output power in the ~1900nm?~2020nm transition is 4338W;In our cascaded tandem pumping(~1900nm?~1940nm?~2020nm),the maximum output power is 5877 W.3.Using a 1942 nm Tm-doped fiber laser as the pump source with the co-(counter-)propagating configuration,the 2020 nm Tm-doped fiber laser generates 34.68W(35.15W)of output power with 84.4%(86.3%)optical-to-optical efficiency and 91.7%(92.4%)slope efficiency,with respect to launched pump power.It provides the highest slope efficiency reported for 2?m Tm-doped fiber lasers.All these show the cascaded tandem pumping technique's high power and high efficient operation in the 2?m wavelength region.4.We develop dual-wavelength 2?m Tm-doped fiber lasers with coupled cavity configurations based on fiber Bragg gratings and study their output performance and emission dynamics.Efficient cavity-coupling management leads to different dual-wavelength operation regimes.Over 3 W of output power is realized at both 1942 nm and 2020 nm,and a power ratio of the 1942 nm to the 2020 nm laser emissions can be tuned from 24.9% to 237.6%.5.We develop spatiotemporal soliton mode-locking in 2?m(anomalous dispersion)step-index multimode thulium fibers for the first time.Through carefully tuning the coupling among transverse modes and exploiting modal energy transfer,spatiotemporal soliton mode-locking is experimentally achieved,during which both the temporal and spatial shapes(pulse duration and beam size)of the multimode soliton can be self-adjusted and consequently become localized.Phenomena such as soliton attraction,spectral self-arrangement and energy concentration to the fundamental mode are investigated,revealing underlying mechanisms and unique nonlinear dynamics.Strong spatiotemporal coupling is found to be beneficial for both stabilizing the multimode solitons and scaling their pulse energy.As a preliminary example,our multimode soliton fiber laser achieves >10W of average power and >550nJ of pulse energy in the picosecond regime.