A Fabry-Perot (FP) interferometer or etalon is an optical device that can detect, in a digital way, temperature, pressure, or mechanical vibration via the change in optical path length between two surfaces with high reflectivity. In this paper we report some studies of a fiber Fabry-Perot etalon (FFPE). Unlike a conventional FPE consisting of bulky structures, FFPE does not require exact parallelism between two reflection surfaces. It is small, flexible, and can be very long, yielding high measurement sensitivity. Single-mode, multimode, and Selfoc fibers have been studied.