Study On The Propagation Property Of Single-mode-to-multi-mode Fiber Combiner

As the key component of all-fiberized incoherent beam combining,single-mode-to-multi-mode(SM-MM)fiber combiner is used for combining several single-mode or quasi-single-mode fiber laser beams into a multi-mode output fiber.Although attractive results have been obtained in previous studies on SM-MM fiber combiner,the detailed studies on the propagation of the optical filed,the effects of parameters on the combiner,and the design rules of the combiner are still quite limited.Thus,in this thesis,based on the manufacturing method of inserting input fibers into a capillary and tapering,we take 3×1 SM-MM fiber combiner for instance and study from the propagation characteristics of the SM-MM fiber combiner.The effects of the input fiber,the output fiber,and taper parameters on the combiner are analyzed,and finally the design rules for SM-MM fiber combiner are provided.The main work in this thesis is listed as follows:Firstly,we introduce the theoretical methods for analyzing a taper fiber,including the geometrical optics method and beam propagation method.The basic ideas and algorithms of the two methods are introduced.Simultaneously,we analyze and compare the differences between two methods and the range of application,and finally point out that the beam propagation method should be used for studying the propagation property of the combiner.Secondly,we establish the numerical model with infinite cladding for the situation of silica capillary whose refractive index is equal to the cladding.The evolvement of the optical field in the combiner is studied and the effects of the output fiber and taper parameters on the performance of the combiner are analyzed.It is found that the output fiber only has a negligible effect on the transfer efficiency of the combiner.To achieve high transfer efficiency,the diameter of the output fiber core should be large enough and the taper length should be long enough.Furthermore,considering of the beam quality,the taper ratio should be optimized to make the output fiber core as small as possible.Finally,we establish the numerical model with finite cladding for the situation of low-index fluorine-doped capillary whose refractive index is lower than the cladding,and investigate the effects of the finite cladding and other parameters on the property of the combiner.The result shows that because of the existence of the external medium,the finite cladding has an indispensable effect on the confinement of the optical field,and the taper ratio and transfer efficiency can be both greatly improved.Secondly,it is found that due to the single mode preservation of the optical field in the input fiber cores,high transfer efficiency can be realized by an output fiber core with NA much smaller than the finite cladding NA.This result makes it possible to increase the brightness of output beam by reducing the core NA of output fiber.Thirdly,it is found there should be an up-limitation of transfer efficiency which decreases with the increment of the taper ratio caused by the loss in the TFB.Fourthly,it is found that the taper length should be long enough to realize the adiabatic taper.The numerical results illuminate that the adiabatic taper length should reduce with the increase of the taper ratio.It is also found the increment of core-to-cladding ratio and NA of the input fiber is beneficial to improve the transfer efficiency.