Abrupt biconical fiber taper for refractive index sensing

• Main Author: Chia, Ivan Zheng Da
• Other Authors: Tjin Swee Chuan
• Format: Final Year Project
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: http://hdl.handle.net/10356/71671
• Institution: Nanyang Technological University

Refractive index sensing has been bringing tremendous convenience, enabling the identification of contaminations and change in concentration in substances. Among the different refractive index sensors, fiber-based refractive index sensors are gaining popularity due to its high sensitivity and low fabrication cost. However, majority of the fiber-based refractive index sensor are susceptible to the influence of external strain and temperature. As the technology is constantly improving, different kinds of fiber structures are used to fabricate such fiber-based sensor. A non-adiabatic tapered fiber yield the highest refractive index sensitivity among the other fiber structure. While a d-shaped fiber is less popular due to its lower refractive index sensitivity resulting from its interrogation method. The motivation behind this study, is to perform a comparison between the d-shaped fiber and the non-adiabatic tapered fiber so as to understand the sensing properties of both the fibers. The two types of fibers use for comparison are fabricated in a school laboratory. The d-shaped fiber was fabricated on a FBG grafted fiber to alter the means of interrogation from the conventional d-shape fiber. The means of fabrication was by side polishing. Different methods of securing the fiber for polishing was explored. On the other hand, mechanical tapering was used to fabricate tapered fiber of three different waist diameter of 4.7µm, 7.1µm and 12.6µm to study the properties of tapered fiber. After fabrication was done, data are collected from experiments and simulations on refractive index sensitivity, strain and temperature characteristics. The results showed that the sensitivity increased as the waist diameter of the tapered fiber reduces. Tapered fiber with a waist diameter of 4.7µm produced the highest sensitivity of 6882.35 nm/RIU but was also subjected to the highest influence in strain and temperature characterization. Over the three different tapered fiber, the tapered fiber with the smallest diameter produced the most significant wavelength shift over a range of applied strain and temperature change. Meanwhile, d-shaped fiber exhibited a lower sensitivity of 533.33 nm/RIU but showed excellent insensitivity towards strain and temperature changed. It displayed little changes on the wavelength shift when it underwent a strain and temperature change. These results had displayed the properties of both fibers and presented the pros and cons of the fibers. Furthermore, it also highlighted the potential of d-shaped fiber where it could be use on applications where sensitivity is secondary to the influence of strain and temperature. Suggestion of choosing an optimum taper profile during fabrication could likely minimise the disadvantage of the fiber and a proposed reader unit was mentioned to increase the commercial viability of the tapered fiber refractive sensor.