Acoustic wave coupling between optical fibers of different geometries

Abstract

In this study, we investigate coupling of acoustic guided waves from different types of input fibers, through a bonded coupler, to an optical fiber. These acoustic waves can then be detected with conventional fiber Bragg gratings (FBGs). The input waves are measured using a high-resolution 3D laser Doppler vibrometer, and the output waves in the optical fiber are measured using an FBG. We demonstrate that the wave coupling between two waveguides varies with the cross-sectional area and the modulus of elasticity of the fibers.

Modeling of ultrasonic coupling between optical fibers through an adhesive bond for sensing applications

“…Then the coupled energy decreases with the cross-sectional area. This result shows the same trend as the experimental results by Kim et al, 6 plotted in 3(b). These measurements were obtained by measuring the mode amplitude in the input fiber using a laser Doppler vibrometer and the mode amplitude in the output fiber using a fiber Bragg grating, as shown in 3(c).…”

“…Four geometric parameters were varied in the COMSOL model to perform parametric sweep: input fiber silica diameter, output fiber silica diameter, the thickness of the coupler adhesive, and the length of the coupler. The input fiber silica diameter was modeled as 125µm, and 200µm to 650µm in increments of 50µm, replicating the range of fiber diameters was used in the experiments of Kim et al 6 For these cases, the coating thickness of the polyimide is 12.5µm, and the coupler thickness is 12.5µm. Measurements are taken in displacements to directly compare with past experimental results, which was collected using a fiber Bragg grating (FBG) sensor in strain values.…”

“…Then the results are normalized to the default control case, in which the diameter of silica on both the input and output fibers are 125µm. The result is organized in cross-sectional area of silica instead of the diameter for direct comparison with the past experimental results 6 and are plotted as the ratio of the amplitude of the output fiber L 01 mode to that in the input fiber. It shows that the amplitude of the coupled L 01 mode increases with the cross-sectional area of silica, until it reaches a maximum at 0.2 mm 2 cross-sectional area.…”

“…The authors recently demonstrated that longitudinal modes can also be transferred between optical fibers using an adhesively bonded coupler (see Figure 1). 6,7 However, the parameters governing the coupling coefficient are not known. Safaai-Jazi 8 modeled the acoustic coupler using the coupled mode theory, the same theory used conventionally to model coupling in optical fiber couplers due to mode overlap.…”

Improved Damage Mapping with Hyperbola Approach for Guided Waves Based Structural Health Monitoring Using Fiber Bragg Grating sensors

“…The acoustic coupling relates to the transfer of the mechanical wave carried by the optical fiber from one fiber to the other using physical connection between the two fibers. This acoustic coupling may be achieved through fusion coupling [4]or through a simple adhesive bonding between two fibers [5]. This acoustic coupling has been used by the authors to measure multiple locations on the structure using a single FBG sensor [6].…”