Design and fabrication of a tapered fiber bundle for a pump combiner with a uniform splicing region

Abstract

A process of fabricating tapered fiber bundle (TFB) consisting of a set of input pump fibers, stacked inside a low-OH silica capillary, has been optimized to combine multiple multimode pump powers into different output fibers having cladding diameters of 400, 250, and 125 µm. Instead of individual input and output fiber processing, here pump combiner fabrication is based on the processing of the TFB to match its diameter with the output fiber to eliminate any perturbation in light guidance. Fabrication of the TFB includes multiple fusing steps and tapering with brightness conservation, followed by reduction of the outer silica layer through chemical etching to obtain the desired outer diameter. To reduce pump power leakage, the optimum fluorine cladding layer thickness outside the pump fiber cores and outer silica layer thickness have been estimated through numerical simulations using a finite element solver. The influence of fabrication parameters such as interstitial gaps between the input pump fibers, taper ratio, fluorine cladding, outer silica layer thickness, and non-uniformity of splicing region has been characterized in terms of transmission efficiency per port, as well as cumulative power handling along with the operating temperature at maximum power.