| Author: | Li, Yujian |
| Title: | Interferometric fiber sensors based on the microstructure fibers |
| Advisors: | Yu, Changyuan (EEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Subject: | Optical fiber detectors Fiber optics Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xxvi, 179 pages : color illustrations |
| Language: | English |
| Abstract: | In this thesis, the fiber interferometric sensors based on micro-structure fiber are studied for different applications. First, the fundamental knowledge about optical fiber, including history, structure, modes in fiber, and categories are introduced in detail to give a basic understanding of optical fiber. Next, the working mechanism, advantages, and classification of the optical fiber sensors are presented one by one to get e general view of this study topic. Then, based on the principles of optical interference, the working principles and common fiber interferometer structures are induced and introduced clearly to provide essential knowledge for the following sub-chapter. Also, the Optical Vernier Effect (OVE) is introduced to improve the sensing performance of the single interferometric sensor. For the sensing device part, four different interferometric structures are proposed and demonstrated availability. The optimal fabrication process and parameters are presented in detail after several experiments. The experimental results of the spectrum and sensitivities all coincided with theoretical analysis. The first sensor is proposed for lateral stress measurement based on the Sagnac interferometer (SI) structure. The six-hole high birefringence fiber is used to detect the lateral stress change based on the stress-caused birefringence effect. The sensitivity is about 1.72 nm/kPa. Because there are no doped particles in the fiber, the sensor is temperature insensitive. In addition, employing single wavelength laser injection and light intensity demodulation method, this SI sensor can monitor the heartbeat and breath rate. The second sensor is an axial strain sensor based on the Mach-Zehnder interferometer (MZI) structure. In this structure, the twin-core single hole is used to provide a transmitting channel for the MZI between the core light and the hole light. Based on the elastic-optical and tensile deformation effects, this structure can measure the axial strain with a sensitivity of 0.59 pm/µε. To enhance the sensitivity, two sensor structures are cascaded in series to motivate the OVE. The cascaded structure can amplify sensitivity about 30 times. Furthermore, this cascaded structure enables simultaneous sensing of axial strain and temperature within the acceptable error range. Additionally, a third sensor is introduced for gas pressure measurement, utilizing a Fabry-Perot interferometer (FPI) structure. The main detection element of this sensor is single-core side-hole fiber with several micrometers, leveraging the refractive index change of the air hole induced by gas pressure. Due to the OVE, the proposed sensor has an ultra-high sensitivity of 183 nm/MPa in 0~100 kPa. This sensor is also sensitive to temperature change with a sensitivity of 440 pm/°C. Then, the temperature cross-sensitivity can be calculated as -2.4 kPa/°C, which is relatively low. The last sensor is proposed for the lateral stress measurement based on the Michelson interferometer (MI) structure. Different from the traditional MI structure, a section of Panda PMF is used in one of the two arms to motivate the OVE. The normalization sensitivity of it is about 3.76 nm/(kPa·m). Using FBG as a temperature compensation element, temperature fluctuation is monitored and demodulated to achieve no temperature cross-sensitivity usage. The four sensor structures are all investigated having promising applications in different fields to provide convenience for human beings. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/13706