Abstract: Fiber-optic devices and sensors are implemented using fiber gratings. The fiber-optic device includes the fiber grating, an input mode stripper and an output mode stripper placed at both ends of the fiber grating, to admit only one mode therethrough, and a polarizer. The fiber grating of this invention is used in combination with a directional coupler instead of a mode stripper and a polarizer. The fiber-optic device outputs desired wavelengths, a switching signal or an interference signal. An external perturbation, such as a strain, a temperature, light pulse, and etc. may be applied to the fiber grating in its preparation to change a characteristic of the fiber grating. These perturbations are actively used to make fiber-optic devices and sensors.