Conventional fiber interferometric sensors are usually composed of several discrete components to split,combine, or delay signals.Sometimes a polarization controller is essential for good interferences.Those discrete components make fiber sensors have a device length longer than several centimeters.However, a micro fiber sensor at the length of less than hundreds of microns is helpful to investigate the variations of measurands in a very local area and in a cramped space.In addition, this micro fiber sensor can also detect index variation under a tiny sample volume at a picoliter level, which is beneficial to cellular-dimensional volume microsensing.To make micro fiber sensor with a device length of hundreds of microns based on a short piece of fiber, the core-cladding modes interference can be a good choice.In this talk, the abrupt tapering and the core size mismatch will be introduced to understand the excitation methods of high order cladding modes based on which the intra-fiber couplers and phase shifter are thus made.An abrupt-tapered Mach-Zehnder interferometer, a Michelson interferometer, and a Sagnac interferomter at the device length of less than 180 μm, 400 μm, and 300 μm, respectively, will be introduced for advanced microsensing applications.