By employing thin film technology to form Fabry–Perot (FP) cavities on the end-face or inside the fiber, sensitivity to different physical quantities can be achieved using different materials, and this greatly expands the application range of fiber sensing. Fabry-Perot interferometers have stimulated numerous scienti c and technical applications rang-ing from high resolution spectroscopy over metrology, optical lters, to interfaces of light and matter at the quantum limit and more.