Highly Sensitive Ultrasonic Sensor Based on Polymer Bragg Grating and its Application for 3D Imaging of Seismic Physical Model

A miniaturepolymer Bragg grating (PBG) sensor is fabricated and employed for ultrasonic imaging of seismic physical models (SPMs). The sensing Bragg grating is inscribed into an ultraviolet (UV) glue polymer waveguide with a femtosecond laser. The uniform polymer waveguide is fabricated by sealing the UV glue into a capillary fibre through capillary effect. By using line-by-line inscription technique, the laser beam is scanned transversely to periodically modulate the refractive index of the polymer waveguide, leading to the formation of the PBGs with various grating lengths. The sensor response to ultrasonic waves are investigated experimentally. When compared to single-mode fiber Bragg grating and phase-shifted fiber Bragg grating, the PBG with the same reflectivity presents a higher response amplitude due to its lower Young’s modulus. Besides, the sensor has a good spectral stability when transferred from air to water due to the waterproof coating on the sensor end. Finally, the sensor is used to scan a large-scale 3D SPM and the structural features, such as fault, fluctuation, and depositional termination, can be distinctly reconstructed. The proposed PBG sensor provides a new technique with easy fabrication, high sensitivity, and good stability for high-fidelity ultrasonic imaging of seismicphysical models.

I. Introduction

Seismic physical model (SPM), as a scaled analog to geological structures, has become a successful tool for research of seismic wave propagation [1]–[3]. At present, one of the key issues in SPM technique is the effective detection of sound fields. It relies on the availability of highly sensitive ultrasound sensors in order to be suitable for detection to the area of interest [4]. Ultrasonic waves (UWs) in SPM have normally weak amplitude, which requires a high-sensitivity sensor [5]. Piezoelectric transducers (PZTs) have been widely used for SPM detection. However, they suffer strongly resonant effects and offer lower sensitivities at smaller sizes [6], [7]. Comparatively, optical ultrasound detection offers several advantages, such as wider detection bandwidths, higher sensitivities at smaller sizes, and resistance to electromagnetic noise [8].