The Fiber Bragg Grating is one of the most popular elements in field of fiber-optic sensing. For many applications where wants to measure very small temperature or strain changes, acoustic waves the sensitivity can be enhanced by using pairs of FBGs. The fiber Fabry-Perot Interferometer is such pair of FBGs. In this case the small phase shift can be detected. By coating the fiber between the gratings with an electric, magnetic or acoustic enhancing coating can be measured small changes of these fields. For sensing purposes and to evaluate small vibration or acoustic signal via an interferometric method, it is often sufficient to work with alow-finess Fabry-Perot cavity. The transmission spectrum of Fabry-Perot fiber interferometer is presented in the graph.