Fiber Bragg grating (FBG) is an optical fiber device that selectively reflects only those light signals with the wavelength matched to the grating period of refractive index modulation. When two FBGs with identical index modulation period are concatenated in series, the signal light reflected by the FBGs may interfere with itself by the process of multiple reflections between adjacent FBGs, and the reflection spectrum may behave in a complicated manner with spectrum completely different from that of a single FBG. This paper analyzes the optical spectrum reflected by the concatenated FBGs both experimentally and theoretically, and demonstrates how the spectrum varies depending on which is larger between coherency length of the light signal and separation distance of the concatenated FBGs.
