Delayed transmission/reflection ratiometric reflectometry

A new Time Division Multiplexing (TDM) optical fiber interrogation system for distributed Fiber Bragg Gratings (FBGs), designated as Delayed Transmission/Reflection Ratiometric Reflectometry (DT3R), is presented. The Bragg wavelength shift of FBG, which varies according to temperature or strain, is estimated as the ratio of transmittance and reflectance of a slope filter, to which the reflected light from the FBGs is input. To obtain both simultaneously using a single detector, a definite propagation delay is given to either transmission or reflection path, both of which are added to each other and detected using a common photodiode. The reflection impulse response of the FBGs is analyzed using OTDR based on Pseudo-random Noise code (PN) correlation scheme. The hardware structure of an interrogator is simple, resulting in low cost. The FBG topology is a "bus": multiple FBGs branch from the through line started from the interrogator. Each FBG is distinguished by its time of flight between the interrogator and the FBG. Because of these schemes, the FBGs can all be identical. A breakdown of any FBGs has no effect on the other sensors; moreover, "hot swapping" is possible. Experimental evaluation results are presented herein.