Study On The Key Technologies Of High Efficiency Fiber Combiners

Fiber lasers have been developed quickly in these years and applied broadly in industrial manufacture and national security due to their essential characteristics such as high conversion efficiency, excellent beam quality, convenient heat management and compact configuration. Fiber laser is one of the important development orientations of high power lasers. Fiber combiners are the core components of high power fiber lasers which can supply higher pump power for the fiber laser through combining several high power pump sources. More importantly, fiber combiners can combine several moderate-power fiber lasers to obtain higher output power laser, and the output power limit due to nonlinear effect, facet fracture and thermal lens can be solved. Grasping the key technology of fiber combiner and fabricating high-efficiency high-power fiber power combiner can support strong technical and fiber component aid for high power fiber lasers and it has become a key problem to be solved in the fiber laser field. The design and fabrication of fiber combiners are studied in theory and experiment in this thesis as follows.The history of various kinds of fiber combiners is comprehensively reviewed especially their rapid development in recent years and the key role in the major breakthrough of fiber lasers. The important meaning of fiber combiners in the development of fiber lasers is illuminated and thus the key techniques of fiber combiners are proposed to investigate expecially the technique of realizing high beam quality of high power fiber laser output from a fiber combiner.The mode field distribution of a fiber taper is introduced in detail and the transformation process of core mode to cladding mode is demontrated. Using beam propagation method, the evolution of mode field along the tapered fiber bundle is analyzed. Two basic rules of fabricating high-efficiency fiber combiners, adiabatic tapering and brightness conservation are illustrated and the influence of these two rules on the transmission efficiency of fiber combiner is simulated.A detailed description of the fabrication process and the key technologies of fiber combiners are given. Through long-term exploration and attempt, the key fabrication technologies of fiber combiner is grasped, including close bundle of fibers, low-loss fused tapering with random diameters and lengths after bundle, the cut of tapered fiber bundle and the low-loss splice of tapered fiber bundle and the output fiber. These techniques can assist the fabrication of various kinds of fiber combiners.A theoretical model of pump combiner transmission loss is built based on the two basic fabrication rules and the influences of taper ratio, taper waist length, fiber bundle deformation on the beam propagation and loss are analyzed. The results indicate that when the brightness conservation is satisfied, longer taper length and shorter taper waist will lead to high transmission efficiency. Based on the key fabrication techniques of pump combiners, various types of pump combiners are fabricated and they both perform high efficiency and high power output. Higher than 400 W output power lasers are obtained using two 7×1 pump combiners with input fibers of 105/125 μm and the transmission efficiencies are higher than 98%. 3.81 k W laser is output from a 7×1 pump combiner with input fibers of 220/242 μm and the transmission efficiency is 98.4%.The transmission and evolution model of the mode of power combiner is built and the influences of duty ratio of the tapered fiber bundle, fabrication method, fiber core diameter of the output fiber and the number of input fibers on the beam quality of the output laser are quantitatively analyzed. The results demonstrate that: when the duty ratio of tapered fiber bundle is as small as possible, the input fibers are arrayed in parallel, the core diameter of the output fiber and the number of the input fibers are smaller, the beam quality of the output laser of the power combiner is better.Based on the theoretical analyses and simulation results, various N×1 power combiner with output fibers of 100 μm and 50 μm core diamters are fabricated and the transmission efficiencies are all higher than 99%. The 7×1 power combiner with output fiber core of 100 μm can support 500 W fiber laser per input port. The measured transmission efficiencies and beam quality of the fabricated various power combiners verifies the theoretical conclusions. 3×1 power combiner for broadband power combination is designed and fabricated and an incoherent combining of 3 supercontinuum sources is realized. The output power is 202 W and the total transmission efficiency is 96.6%. This power combiner introduces a new method to obtain high power supercontinuum sources.