High precision fiber taper fabrication using the immersion depth control in chemical etching

High precision control is highly desirable when using the selective chemical etching technique to fabricate tapered fibers for many practical applications. So far, various methods have been proposed on this topic. In this paper, we proposed a novel and effective method to make tapered fibers in different shapes and sizes based on automatic control of the immersion depth in chemical etching. We adopted the diluted Hydrofluoric acid as etching solution in our preliminary experiment, and common selective chemical etching scheme was also implemented in our experiment in which the buffered hydrofluoric acid solution is used. We found out in our study that the etching process can be further controlled by controlling the evaporation of the etching solution. Under near-saturation condition, the ammonium fluoride (NH₄F) in the etching solution tends to crystallize as the water evaporates. The evaporation of the water and the crystallization of the ammonium fluoride cause the immersion depth of the etched fiber to decrease in certain rate, which leads to different etching time on different parts of the etched fiber. This fact enables the etched fiber to have a very smooth tapered part. By controlling the changing rate of immersion depth and other etching conditions, we can finely control the shape and size of etched fibers.