Subnanometer Resolution Displacement Sensor Based on Vernier Effect

A cascaded Fabry-Perot interferometer (FPI) for displacement sensing with the Vernier effect is proposed and experimentally demonstrated. The sensor consists of a single mode fiber (SMF) fusion spliced with a section of capillary and a hollow microsphere with a hole (HMH). Two kinds of Vernier effects are generated through three beams reflected from the three reflecting surfaces, including the SMF endface, erbium-doped fiber(EDF) endface, and inner surface of the HMH. We explore the specific relationship between the generation conditions of the Vernier effect and the enlargement factor of the envelope period. Consequently, we find the critical value of the Vernier effect generation by simulating the superimposed spectra of different distances between the SMF endface and the EDF endface. Moreover, the experimental results indicate the proposed sensor has a displacement sensitivity of 94.6 pm/nm with a subnanometer resolution of 0.21 nm.