Study On Mode Transformation And Control Of Fiber Combiner

Fiber laser has become increasingly popular for industrial and defense applicationsdue to the advantages of high effciency, good beam quality, compactness, convenientheat management and high reliability. However, the power scaling of the fber laser islimited by the thermal damage and the nonlinear effects. The all-fiber fiber combiner,which merges the outputs of a multitude of single-mode fiber lasers, is the keycomponent for high power output. At present, multimode fiber lasers have been scaledto output power of50kW by IPG. However, the beam quality is poor and the M2factoris32.7. How to scale power while simultaneously achieving high brightness by a fbercombiner for more applications is a challenge. The mode transformation and modecontrol of the fiber combiner are investigated and analyzed in theory, simulation andexperiment in this thesis as follows.The paper begins with reviewing the history of fiber combiners and theirapplications on the fiber lasers. It puts forward that the fiber combiner is an effectiveapproach for high power output. Owing to the poor beam quality of the combiningoutput, the range of the applications are narrowed severely. Therefore, the modetransformation should be studied and the way of the mode control should be explored toimprove the beam quality.The output properties of the fiber combiner under different incidence are simulatedand studied numerically. The output field and mode properties of the single multimodefiber and the71fiber combiner are analyzed under perfect incidence, off-axisincidence and tilted incidence by the mode analysis method. The power spetral density(PSD) method is introduced, and the effect with different incidence on the intensitydistribution and phase distribution are researched. The mechanism that the beam qualityof output beam of the fiber combiner degenerates is exploredAccording to two design rules, the low loss fiber combiners with hundreds wattpower available are fabricated. Four methods of bundling fibers are brought forwardaccording to the types of the fiber combiner. A new package for combiner with excellentcooling are devised and the package is tested for the capability of4kW output powerwhen the efficiency is larger than95%。Several methods for evaluating the beam qualityare compared and analyzed. The modified beam propagation factor (BPF) and M2factorare selected as the appropriate beam quality factors for the fiber combiner. Theinfluence of the TFB duty ratio and phase errors on the beam quality are studied indetail.The model for high-gain fiber laser arrays is revised to analyze the passivecoherent combining of the tapered fused fiber bundles. The influence of the magnitudeof the phase errors on the output phase properties of the5-,10-and19-fused fiber bundle arrays are discussed. The relationship between the tolerance of the phase errorsand the number of the arrays is also caculated.A sheme for an all-fiber active coherent combining via a fiber combiner isillustrated. The feasibility of a single MMF laser cleanup is proved in experiment. Anexperimental system of the active combining and contol of the fiber combiner is built.The system carried on the mode control for the coherent output by beam cleanup systemafter phase-locking the fiber combiner. The beam cleanup of a home-made21fibercombiner are investigated in experiment under incoherent combining, coherentcombining without phase locking and coherent combining with phase locking. Theresluts proves the feasibility for high brightness output of the fiber combiner.