ABSTRACT
A universally applicable approach is proposed for the fabrication of fiber-optic polymer sensors. The hollow-core fibers (HCFs) with inner diameters of 30â µm, 50â µm, and 75â µm are spliced coaxially with dual-hole fiber (DHF) or photonic crystal fiber (PCF). Owing to the sized-matched air holes within HCF and DHF/PCF, an interconnected in-fiber microchannel is constructed, which facilitates rapid and complete filling of the HCF's central hole with liquid glue. After the ultraviolet-induced polymerization, a polymer Fabry-Perot interferometer is achieved by cutting the HCF end with a desired cavity length. Besides, the interference visibility is significantly enhanced by adding a refractive-index-modulated polymer cap onto the cutting surface. Experimental results demonstrate the optimized interference spectra and the interconnection of the matched air-hole fibers. The polymer sensor exhibits a signal-to-noise ratio of 56.8â dB for detecting pulsed ultrasonic waves, which is more than twice that of a partially polymer-filled sensor. Due to the hermetically-sealed structure, the sensor probe presents constrained performance with a temperature sensitivity of 230.2 pm/°C and a humidity sensitivity of 93.7 pm/%RH, which can be further improved by releasing the polymer waveguide from fiber cladding. Based on interconnected holey fibers, the proposed approach has a uniform size-controlled polymer waveguide dimension with increased spectrum visibility, rendering it suitable for a diverse range of microstructure-matched optical fibers.
ABSTRACT
A micro ultrasonic sensor based on an advanced suspended-core fiber is proposed and employed for in-lab seismic physical modeling. A free suspended core is obtained by acid corrosion and two cascaded uniform fiber Bragg gratings (FBGs) are imprinted in the suspended-core fiber. The sensor response and stability are largely improved due to the using of dual-FBG reflectors instead of weak-reflection fiber mirrors for constructing an in-fiber interferometer. The characteristics of reflection spectra and ultrasonic response of the sensor are analyzed and demonstrated experimentally. Comparative measurements are also carried out to prove the sensor superiority over the conventional weak-reflection one. Moreover, the sensor is used for seismic physical modeling to show its ability of practicable usage. Both the crosswell seismic and surface seismic in seismic exploration are modeled respectively based on reservoir and fault models. Various reservoir velocities are measured and each is consistent with the reported results. The fault features are also well reconstructed in the form of a cross-section model image. The improved sensor approach greatly promotes the application of the suspended-core fiber for weak acoustic detection in seismic physical modeling.
