A Sensor for Simultaneous Measurement of Displacement and Temperature Based on the Fabry-Pérot Effect of a Fiber Bragg Grating

A temperature-insensitive sensor for micro displacement measurement is proposed based on a novel packaging of an FBG. In this paper, an apodized fiber Bragg grating (FBG) was glued at the specific position on the inner surface of a thin-walled ring. When the ring is deformed, the FBG could be split into two segments of FBGs in the two identical but oppositely directed chirp gradients. In this way, an effective Fabry-Pérot cavity could be induced into the FBG and the resonant peaks could be observed in the reflection spectrum of the FBG. The wavelength separation of the resonant peaks changes linearly with the change of displacement, while it is insensitive to temperature variation. The sensitivity of the wavelength separation versus displacement could be achieved as high as 117 pm mm-1 in the present experiment. Further improvement would be obtained by decreasing the radius or increasing the thickness of the ring. In addition, the ambient temperature could be obtained through a simple calculation, results in a temperature sensitivity of 28.67 pm °C-1. Owing to its compact and temperature independent advantages, the novel displacement sensor has potential application prospect in displacement and pressure measurements, especially in civil engineering structures as the long gauge sensor.