| High power double-clad fiber amplifiers have been widely used in various fields such as optical fiber communication, fiber sensors, optical spectral measurements, thus they are considered to be a main direction of fiber amplifier development. The double-clad fiber with small core area is used as governing gain medium so far. When amplifier operated at high power, small area means high power density which may cause many nonlinear effects. And nonlinear effects will have seriously influence on output characteristic that make improvement the output of the fiber amplifier much harder. Multimode fiber with large core diameter is considered the effective ways to solve this problem.In this paper, we summarize three main requirements of double-clad fiber as gain medium by studying the development and application of fiber amplifier and double-clad fiber. They are high absorption efficiency, low loss and high nonlinear threshold. In order to further improve the double-clad fiber absorption efficiency and fiber amplifier gain, a double cladding fiber structure with a large-diameter-helical core and D inner cladding shape is presented in this paper. Using the optical geometric method and optical absorption efficiency per unit length, we analysis the cladding absorption efficiency with a helical core. We find a large-diameter-helical core will give an improvement to the absorption efficiency of double-clad fiber with D shape inner cladding because of the disturbance it brings to the index of refection. By simulation and analysis of mode gain coefficient, we found that a relative doping radius range between 0.5 and 0.6 is beneficial to the selection of base model. Using loss formula of helical core, various geometric parameters are analyzed and discussed, and finally a set of suitable threaded shaped fiber helical-core parameters is given. |
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