The fabrication and characterization of fiber Bragg gratings in highly birefringent photonic crystal fibers for sensing applications

The combination of the functionalities of Fiber Bragg Gratings (FBGs) and Photonic Crystal Fibers (PCFs) has unveiled new potential for FBG based sensors. The fabrication of FBGs in PCFs has been reported in literature. However, using dedicated PCFs to improve the sensitivity of FBG-based sensors has received only limited attention. In this report we therefore show how to eliminate some of the drawbacks of FBGs in conventional step-index fibers for sensor applications by exploiting the design flexibility of PCFs. The added value of PCFs stems from the ability to design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to strain but not to temperature. For this purpose we use a PCF with a birefringence on the order of 10^-3, which is one order of magnitude larger than for conventional birefringent fibers. The two FBG reflection peaks are therefore significantly separated from each other, e.g. 2 nm, which makes these FBGs suited for sensing purposes since both peaks can be unambiguously and accurately identified. As a conclusion we summarize the advantages and disadvantages of our approach to design and fabricate selective FBG-based sensors.