Book Description

This dissertation investigates how to measure dynamic strain including quasi-static strain, vibration, acoustic emission, and ultrasonic waves with fiber Bragg grating based optical fiber sensors. Fiber optic sensors are inherently immune to electromagnetic interference, light weight, small size, corrosion resistance, and capable of multiplexing. With narrow linewidth tunable lasers, the strain induced spectral shift of the Bragg wavelength of the sensor can be demodulated. However, the spectrum of the uniform fiber Bragg grating can not satisfy the sensitivity, resolution, and dynamic range requirements. To address these challenges, we propose and demonstrate a sensor structure based on chirped fiber Bragg gratings combined with Fabry-Perot cavity. Taking advantage of large bandwidth provided by the chirped fiber Bragg grating and the narrow resonance peaks formed by the Fabry-Perot cavity, it can simultaneously achieve high resolution, high sensitivity, and large dynamic range measurement.The second chapter provides the theoretical analysis and numerical simulation on the spectra of chirped fiber Bragg gratings and Fabry-Perot cavities. Based on such context, we are motivated to propose a dynamic strain measurement scenario which take advantage of both high resolution and large dynamic range of the sensor. Due to the different and unique spectral intervals of the notches in the wavelength bandwidth used for measurement, the spectral notches can be unambiguously recognized in each spectral frame without the need for fringe counting. Using this principle, we demonstrated high-resolution and absolute static and dynamic strain measurement. In chapter three, we study the acoustic emission detection with the proposed sensor based on high finesse short cavity structure and explore the potential of using the narrow resonance peak as the laser locking source to reduce the laser noise while functions as ultrasound sensor. Additionally, since the Bragg wavelength is highly related to the polarization, birefringence causes polarization dependent center-wavelength shift. We propose a 90-degree rotation method for grating fabrication in the UV laser beam side exposure technique to reduce the birefringence. Therefore the sensor is insensitive to the polarization state of the laser, the ultrasound detection system can be simplified by omitting the polarization controller. Chapter four expands our work on ultrasonic sensor by using coiled fiber with low-finesse Fabry-Perot interferometer formed by two chirped fiber Bragg gratings. Our work has successfully demonstrated a strain and temperature insensitive fiber-optic ultrasonic detection by combining the coil structure, wide spectral range, and quadrature demodulation. The ultrasonic sensing scheme is immune to the laser wavelength drift, therefore no wavelength locking mechanism is needed. Future work will continue on exploring new design of the sensor structure and optimizing the measurement system to further improve the feasibility while reduce the overall cost.