Single Peak Fiber Bragg Grating Sensors in Tapered Multimode Polymer Optical Fibers

This research demonstrates how low loss commercially available multi-mode (MM), perfluorinated polymer optical fibers (POFs) can be used for robust and reliable fiber Bragg grating (FBG) based sensing. A single peak FBG reflection spectrum is achieved by tapering the fiber to a diameter just small enough to make the fiber effectively single-moded and then inscribing the FBG in the waist section of the taper. Here we use plane-by-plane, direct writing inscription with a femtosecond laser. In particular we used the GigaPOF-50SR MM fiber and tapered it by a factor of 0.24, which was found to be the critical ratio that provided a single grating reflection peak. The GigaPOF-50SR fiber was chosen because it allows operation at 1550 nm and has a suitable small 50 mu m core diameter, which minimizes the required taper ratio and thus the insertion loss of the taper. The FBG sensor was found to have a strain and relative humidity sensitivity of 20 nm/%strain and similar to 6.7 pm/%RH, respectively. The grating proved to be largely insensitive to temperature. This approach to single-mode POF sensor fabrication will enable multi-point strain sensing at 1550 nm over 50 m long fiber. It combines the main advantages of single mode POFs (robust FBG sensing) and MM POFs (low transmission loss) and demonstrates that tapered MM POFs constitute an efficient solution for the development of robust and reliable long distance POF FBG sensor networks.