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. However, such sensors have high. 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. End facet machining of optical bers has enabled the miniatur-ization.