Browsing by Author "He, Jiaji"
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- Acoustic Waveguides and Sensors for High Temperature and Gamma Radiation EnvironmentHe, Jiaji (Virginia Tech, 2021-01-12)Sensing in harsh environments is always in great need. Although many sensors and sensing systems are reported, such as optical fiber sensors and acoustic sensors, they all have drawbacks. In this dissertation, fused quartz and sapphire acoustic waveguides and sensors are developed for high temperature and heavy gamma radiation. The periodic structure, acoustic fiber Bragg grating (AFBG), is the core sensor structure in this dissertation. To better analyze the propagation of acoustic waves, the acoustic coupled more analysis is proposed. It could solve for the reflection spectrum of the AFBG with at most 2.1% error. For the waveguide, the fused quartz "suspended core" waveguide is designed. It achieved strong acoustic energy confinement so surface perturbations no longer affected the wave propagation. Single crystal sapphire fiber features low acoustic loss, and survivability under high temperature. It is also chosen as an acoustic waveguide. AFBGs are fabricated in both waveguides. The fused quartz suspended core AFBG is shown to sense temperature up to 1000 C and to have stable reading at 700 C for 14 days. The sapphire AFBG as a temperature sensor works up to 1500 C and also provides continuous stable reading at 1100 C for 12 days. Both waveguides with AFBGs are then tested under long-term gamma radiation. Despite some fluctuations from radiation-related causes, the readings of both sensors generally remain stable. Given the experimental observations, the fused quartz AFBG waveguide and the sapphire AFBG waveguide are shown to work well in high temperature and gamma radiations.
- Coupled Mode Analysis for 3D Stress-Free Elastic Acoustic WaveguideHe, Jiaji; Homa, Daniel S.; Pickrell, Gary R.; Wang, Anbo (IEEE, 2019)Acoustic sensors and acoustic measurements receive much attention in various applications. Because waveguides are commonly used in sensor design, theoretical means to study acoustic propagation and interaction in waveguides are necessary. However, current methods for elastic wave coupling, including the transfer matrix method and coupled mode theory in planar 2D waveguides, are not satisfactory. In this work, a coupled mode analysis for acoustic waves in 3D stress-free elastic waveguides is proposed. Similar to the coupled mode theory in optical waveguides, the analysis is presented by the evolution of modal amplitudes. It can solve various modal conversion and scattering problems in elastic waveguides with small changes of cross sections and stress-free boundaries. To demonstrate the practicability, the coupled mode analysis is used to calculate the reflection spectrum of the newly proposed structure, the acoustic fiber Bragg grating. In a notch-based grating fabricated on a thin cylindrical waveguide, the results from coupled mode analysis are in good agreement with those from the transfer matrix method, which has been already validated experimentally. The coupled mode analysis is a promising method to solve various scattering problems.