Refractive index optical fiber sensor based on tapered long period gratings

• Main Author: Zhang, Bo
• Other Authors: Tjin Swee Chuan
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: http://hdl.handle.net/10356/60444
• Institution: Nanyang Technological University

Adiabatically tapered long period fiber grating (LPG) is a new type of grating based sensor that shows high sensitivity to refractive index change of the ambient medium. Compared with fiber Bragg gating (FBG), the diffraction structure in LPG is inscribed by Ultraviolet (UV) laser source through an amplitude mask with a larger period. The LPG couples fundamental core mode into phase-matched co-propagating cladding modes, corresponding to each resonance dip in transmission spectra. Due to the exposed evanescent fields to the surrounding medium of fiber, the LPG can be used as photonic refractive index (RI) sensor and the sensitivity is higher than that of FBG. In this project, we demonstrated RI sensors based on LPG inscribed in photosensitive single mode fiber with single cladding and double cladding profiles. The influence of diameter reduction and fiber type on the RI sensitivity of TLPG sensors has been investigated by analyzing the transmission spectra and RI characterization diagrams from both experimental and simulated models. First, reducing the fiber diameters will increase the RI sensitivity. Second, double cladding fiber has shown more significant contributions on the incremental values and the detection limit for refractive index is 1.601 x 10-5 . Last but not least, the Boron and Germanium co-doping inner cladding of double cladding fiber lead to the opposite wavelength shifts compared to conventional LPG. In addition, the experiment and simulation results from tapered single cladding fiber with diameter of 60 µm indicate that there is a turning point in cladding modes, where the RI sensitivity is the largest. All these critical findings will have great application potentials to be used in chemical or bio-sensing areas. In the future, more precise simulation tools and wider wavelength range of light source should be implemented to optimize the stable sensor configuration by achieving the largest RI sensitivity.