Fiber Bragg Grating Inscription in a Thin-Core Fiber for Displacement Measurement

A compact power-referenced fiber displacement sensor is proposed and experimentally demonstrated. It comprises a simple structure in which a section of a photosensitive thin-core fiber (TCF) containing a fiber Bragg grating (FBG) is spliced into another single-mode fiber. Both the core mode and cladding mode of TCF are coupled at the core-mismatch junction and appear as two well-defined resonance bands in reflection spectrum from the downstream FBG. The cladding modes exhibit strong bending dependence due to bending-induced loss in the recoupling from the cladding modes to the core mode. The cladding modes in reflection are collected to measure the bending of fiber. Meanwhile, the intensity of the core mode is virtually unaffected by the bending of fiber and, thus, can be used as a power reference to cancel out any light source fluctuations. In addition, temperature change just makes the reflection spectrum shift and does not change its intensity, making it a good candidate for measuring displacement and temperature simultaneously.