Optical fiber sensor based on tapered long period gratings

• Main Author: Ng, Henry Choong Leng.
• Other Authors: Tjin Swee Chuan
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: http://hdl.handle.net/10356/54503
• Institution: Nanyang Technological University

Long Period Gratings (LPGs) have demonstrated great potential in environmental sensing applications specifically due to their high intrinsic sensitivities towards physical properties like temperature, surrounding refractive index (SRI), and strain etc. At present, water salinity sensing performed by electrical-based sensors is usually inadequate due to the sensors’ inherent susceptibility to electromagnetic interference. LPGs offer an attractive solution over the electrical-based sensors due to their advantages of immunity to electromagnetic interference, compactness, and intrinsically high sensitivities to changes in ambient SRI and temperature conditions. As a result, LPGs demonstrate great potential for administration in water quality monitoring and control. Although standard LPGs are traditionally known to exhibit high sensitivity characteristics, we propose the combination of LPGs with a fiber taper technique to enhance the sensitivity characteristics of the resultant fiber device. A special type of single-mode fiber, the W-type fiber, with a doubly clad profile was utilized in the project. The fiber was subjected to a tapering procedure to produce 3 configurations of tapered fiber segments with different waist diameters. LPGs of 375 µm were written in each of the tapered fiber segments to produce tapered LPGs which were investigated for their SRI and temperature sensitivity characteristics. We noted a moderate SRI sensitivity of 349.5 nm/ RIU, and a reasonably high temperature sensitivity of 56.87 pm/ oC for a particular configuration of the tapered LPGs. The sensitivity performances were benchmarked against other fiber devices reported in the works of other research groups. Additionally, the OptiGrating software was also utilized to simulate the spectral behaviours of the tapered LPGs in an attempt to establish the dependence of the fiber devices on the ambient property changes. The research conducted provided a valuable addition to the ultimate aim of deploying the tapered LPGs onto real-life implementations for evaluating the SRI and temperature conditions of aquatic environments.