Study On Tandem Pumping Technology Of Ytterbium-doped Fiber Lasers

Investigations on high power fiber lasers (HPFLs) are intense in recent years. Butthe power of HPFLs that directly pumped by laser diodes (LDs) are limited in thekilowatt level for years due to the low brightness of diode laser, as well as the highthermal load inside the fiber. Tandem pumping, in which one or several fiber laserspump another, is regarded as an ideal scheme for much higher power scaling, for itsoutstanding characteristics, like high pump brightness and low quantum defect. To date,the most powerful fiber laser ever known is realized by tandem pumping theytterbium-doped fiber (YDF). In this thesis, we focus on the key technologies of tandempumped YDF laser.The thesis begins with numerical analysis on tandem-pumped YDF laser. Based onthe characteristics of Yb-ion, it is found that the optimal wavelength for tandempumping is near1020nm. Then by solving the laser rate equations, the signal wavelength of tandem pumped fiber laser is also determined. Then we analyze the scalabilityof tandem pumped fiber lasers, considering pump coupling, nonlinear and thermal limits.Our analysis shows that the power limit of tandem pumped YDF laser that employscommercially available fiber components is8.5kW.To obtain high power pump sources for tandem pumping, we give systematicallyinvestigation on1018nm fiber lasers and amplifiers. We theoretically analyze andexperimentally demonstrate the critical parameters of1018nm fiber laser. Main factorslike ASE feedback, reflectivity, and doped concentration are analyzed by solving thelaser rate equations which takes ASE into account. In experiment,476W1018nm fiberlaser is obtained, which we believe is the highest power ever reported. For the amplifier,we analyze the effect of ASE feedback, seed laser properties, fiber specifications andpump power on the amplifier’s output characteristics, both in theory and experiment.We achieve113W1018nm signal output from the amplifier, which is the mostpowerful fiber amplifier of1018nm reported in open detail. We also extend theconclusions derived from the investigation on1018nm laser to other special wavelengthfiber lasers, for instance,980nm and1030nm.Next, we instance fiber laser pumped by1018nm fiber laser to make analysis ofthe tandem pumped fiber laser system. We numerically calculate the change of outputpower of1018nm laser and amplifier induced by backward signal laser to determinewhich one is more suitable for tandem pumping. Comparisons between amplifierspumped at976nm and1018nm are also made according to numerical results.Experimental setup of tandem pumped fiber laser is built in the lab and both core andcladding pumped experiments are carried out. The signal laser is amplified to180W in the1018nm cladding pumped experiment, with the efficiency as high as89.6%. Tovalidate the possibility of employing1030nm fiber laser for high power tandempluming,1030nm fiber laser pumped fiber amplifier are also constructed. The outputsignal power is80W in the cladding pumped experiment, with an efficiency of67%.At last, exploration investigations of high power tandem pumping are carried out,for the purpose of improving the limits of pump brightness, nonlinear effects andthermal damage on output power. Missmatched-modefield fiber laser (MMFL) andself-organized coherent combination method are proposed to improve the beam qualityof1018nm fiber laser.85W single mode1018nm laser with an efficiency of72%isachieved from the MMFL. We experimentally demonstrate55W of spontaneouslyphase-locked two-laser array in an all-fber and all-passive confguration, with thecombining efficiency of90%. To enhance the absorption of1018nm laser in tandempumping, we propose the approach of heating the gain fiber. The possibility ispreliminarily demonstrated in experiment. In order to raise the threshold of thermaldamage of fiber, all-glass fiber is introduced for tandem pumping. We analyze the powerlimit of fiber amplifier employing all-glass gain fiber. The numerical results indicate itspotential advantages.