Contents I. Introduction
Long period gratings (LPGs) have been studied for many years in applications such as band rejection filters [1] and sensors of temperature, strain, refractive index (RI), and curvature [2]–[4]. Application of LPGs for relative humidity measurement require to recoat the fiber with an hygroscopic overlayer since cladding modes have negligible interaction with water vapor [5], LPGs in microstructured fiber may offer a new approach for humidity measurement [6] but it can be a difficult task to keep air flowing through the fiber holes. Optical fiber have also been used as water level sensors by measurement of the buoyant force acting on the fiber holder [7] or by direct interaction of the liquid with the guided mode in polished fibers [8], these techniques can be straightforward adapted to Bragg or long period gratings. Recently researchers have begun to investigate the recording of these gratings in polymeric optical fibers [9]. The motivation here is to be able to utilize the different material properties of polymers compared to the traditional silica fiber material. As an example relevant to strain monitoring applications, polymeric fiber can have a much smaller Young's modulus and much higher failure strain than silica fiber [10]. Unlike silica fibers, polymer fibers can absorb water which modifies the properties of the waveguide and therefore have potential applications as distributed water level sensors; moreover, water diffusion needs to be investigated in order to prevent unwanted effects of vapor condensation in other sensing applications of polymer optical fibers. Candidate LPGs have previously been produced in microstructured polymer optical fiber (mPOF) by mechanical deformation coupled with heating [9]. In this paper, we describe for the first time to our knowledge, the UV photoinscription of LPGs in mPOF and study the sensitivity to water absorption.
