Study Of High Power Efcient Fiber Lasers At2Micron

Laser sources at2μm have many applications including basic science, industry,military,andmedicaltreatment. Fiberlaserspossessexcellentproperties,butachievinghigher power and efciency2μm fber laser is still a challenge. To present a betterunderstanding, two kinds of2μm fber laser are investigated in detail by theoreticalmodels.Firstly, we have derived a model to provide a unique insight into the tempera-ture dependent efects in Tm-doped silica fber lasers with consideration of importantenergy transfer processes and full spectra of laser transition. A good agreement wasachieved between simulations and experiments, even in the kilowatt power domain.On the basis of the model, the investigation shows that the thermal distribution of ionsover the Stark sub-levels contributes a higher threshold and a lower laser power asthe temperature increase. Fortunately, this degradation of laser performance can belargely weakened when higher pump intensity is employed. The simulations also showthat lower output coupling and longer operational wavelength can mitigate the outputpower variation arising from the temperature. The model was further used to analyzethedependenceoflaserspectrumonthefberlengthandpumppowerwhentemperaturechanges. In addition, the laser output power and temperature distribution in kW-levelare also studied.Secondly, a theoretical model to describe Holmium-doped fber lasers pumped byLaser-diodes around1.125μm is presented. With considerations of important energytransfer processes, such as laser re-absorption, cross-relaxation (CR), energy transferupconversion (ETU), and excited state absorption (ESA), good agreement is achievedbetween the simulations and experimental measurements. The ESA process is found playinganimportantroletothefberlaserperformance,andtheCRandETUprocessesmake a negative contribution to the lasers. On the basis of the model, optimizationof the lasers threshold and slope efciency as well as output power on the length ofactive media, the output coupling, pump wavelength, and Ho ions concentration areinvestigated.In addition, a high power passively mode-locked thulium-doped fber laser is de-veloped with semiconductor saturable absorber mirror (SESAM). Pumped by793nmlaser diodes, the fber laser produces stable noise-like pulses at a repetition frequencyof17.58MHz. Themaximumaverageoutputpowerof2.02Wwithsinglepulseenergyup to115nJ. Such high noise-like energy is realized by a large mode area (LMA) fbermakes this system favorable for high power mid-infrared supercontinuum generation.