Orientation-dependent fiber-optics displacement sensor by a grating inscription within four-mode fiber

We proposed and experimentally demonstrated an orientation-dependent fiber displacement sensor. The sensing device consists of a simple fiber Bragg grating (FBG) inscription within a four-mode fiber (FMF). The FBG is not used as a sensing element but as a “mirror” for multi core modes of FMF. The FMF is spliced with the single mode fiber (SMF), and bent at the section between the FBG and the splicing joint with leading-in SMF in the measurement. The reflection intensity of the coupling mode is measured for different displacement, which is highly orientation-dependent to fiber displacement. And the wavelength information of coupling mode is used to monitor the temperature change. Therefore, the simultaneous measurements of displacement and temperature can be implemented by this sensor.