Vernier Effect-Based Displacement Sensor Using In-Fiber Hollow Microsphere With a Hole

We demonstrate a compact subnanometer resolution displacement senor based on Vernier effect, which consists of a Section of capillary fusion spliced with a single-mode fiber (SMF) and an in-fiber hollow microsphere with a hole (HMH). Two concave erbium-doped fibers (EDFs) obtained by chemical etching are fusion spliced for the hollow microsphere, which further has a hole by vertical polishing and chemical etching. The Newton’s rings phenomenon observed on the endface of EDF is the judgment condition of the hollow microsphere’s ultrathin wall via polishing, and then of the beginning of etching. Consequently, there is no concave area around the hole on the EDF endface, which significantly enhances the endface reflectivity. Combined with the lead-in SMF endface and the inner surface of the HMH, three-beam interference is generated from beams reflected from the three mirrors, which can produce two kinds of Vernier effect. They are analyzed theoretically and, thus, distinguishable in spectral patterns. Due to the low loss of light transmission in the hollow microsphere, the wider reflection spectra of identifiable interference pattern with Vernier effect are obtained, and the maximum displacement sensitivity can be further improved to 451.5 pm/nm with a subnanometer resolution of 0.044 nm. In addition to the high robustness resulting from the in-fiber structure of the hollow microsphere, the displacement sensor shows prospects in health monitoring, surface topography, precision machining, and other fields.