Novel microspherical fiber Fabry-Perot interferometer based on the Vernier effect for micro-displacement sensing

A novel optical fiber Fabry-Perot interferometer (FPI) based on the Vernier effect is proposed for micro-displacement sensing. The sensor is composed of an optical fiber microsphere and a section of multimode optical fiber with a flat end surface. The air cavity in the middle is used as the Fabry-Perot cavity. The extinction ratio of the reflection spectrum is significantly improved by the optical fiber microsphere, and the extinction ratio is similar to 12 dB. First, a single FPI is tested for micro-displacement. A sensitivity of 2.13 nm/mu m and linearity of 99.31% are obtained within the range of 0-1.872 mu m via phase demodulation. The fabricated optical fiber FPI is used as the sensing arm and cascaded with the designed analog reference arm. The micro-displacement sensitivity of the sensing arm can be improved to 28.92 nm/mu m through the Vernier effect, i.e., 13.58 times that of a single FPI. The fabricated sensor exhibits the characteristics of high sensitivity, contrast, and linearity; a compact structure; and low cost. It can be used for micro-displacement detection. In addition, the analog reference arm is unaffected by the environment, and it can freely adjust sensitivity magnification. Accordingly, it is applicable to micro-displacement detection scenarios under different sensitivity requirements.