A double-die modification of micro-pulling-down method for in situ clad/core doping of fiber crystal

A novel technique for single-crystal fibers cladding immediately in the course of the growth process based on the micro-pulling-down (μ-PD) method has been proposed and verified. The principle of the double-die-μPD implies that the capillary nozzle provides capillary feeding to the growing fiber simultaneously from the inner pipe channel with undoped melt and from the outer surface with doped melt. The menisci shape, melt flow outside the nozzle, control of the doped/undoped melt ratio and process stability are discussed in terms of capillarity. Examples of practical crucible design are discussed in connection with radial dopant distribution. Mn-doped lithium niobate fiber, 0.5 and 0.7 mm in diameter, having 40-60 μm doped outer area were grown successfully.