New sensing mechanisms using an optical time domain reflectometry with fiber Bragg gratings

A new optical time domain reflectometry (OTDR) sensing mechanism combined with fiber Bragg gratings (FBGs) is presented. The intensity-based optical fiber sensor is implemented between a pair of FBGs in an OTDR system. Based on the ratio from two reflecting intensities of the reference and the sensing FBGs, the appropriate measurable parameters associated with the intensity variations can be detected. By applying the Fresnel reflection signals caused by FBGs as the measuring indicator, the dynamic range and signal-to-noise ratio of an OTDR FBGs system are improved in comparison to the traditional method. Besides, by adapting the system using a double-light-source scheme, the spatial resolution limited by the ghost zone in a traditional OTDR system is also resolved. The sensors in an OTDR FBGs system could be configured for multiplexing and immunity to the variations from the light source and the lead-in/out fibers caused by the external loads. The new method using single-light-source and double-light-source schemes has been validated by fiber bending loss and refractive index measurements.