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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. [ABSTRACT FROM AUTHOR]

A sensitivity-improved fiber-optic ultrasonic sensor is proposed and employed for ultrasonic imaging of seismic physical models (SPMs). The sensor consists of a single mode fiber (SMF) embedded into the hollow-core-fiber (HCF) bubble. Using gas pressure assisted arc discharge, the diameter of the bubble can be larger than $430 \,{u}\text{m}$ with a wall thickness of approximately $0.48 \,{u}\text{m}$. Besides, by using the coating technique, the optical reflectivity and extinction ratio of the ultra-thin bubble are further improved. Fiber tails of SMF and the HCF are partly bonded to form an open-cavity sensor. When applied with ultrasonic waves (UWs), the compact bubble can balance the difference of air pressure between inside and outside. Eventually, a side-band filter technique is used for UWs interrogation. Compared with enclosed uncoating fiber bubble, the sensor shows an enhanced response to pulsed ultrasound. The SPMs of rugged topography and reservoir type are both designed and the proposed sensor is used for ultrasonic imaging of the two specific models. [ABSTRACT FROM AUTHOR]