Presented in this work is a new class of optical fiber intensity sensors that do not require the use of external structures to impose perturbations on fibers. Instead, the topological structures are formed directly onto the fiber cladding. The geometry of the features is defined photolithographically and the structures are formed by chemical etching. The etched fiber sensing elements have been fabricated and tested as strain sensors. Depending on the amount of the cladding material removed and the symmetry of the etched structure, the measured gauge factor of the etched fiber strain sensors may be larger than 100. The responses of the optical fiber strain gauges are linear, repeatable, have a high cyclic endurance and no measurable hysteresis. These sensors can be used to monitor a strain as large as 0.4%.

Optical-fiber strain gauges with asymmetric etched structures have been analyzed, fabricated, and tested. These sensors are very sensitive with a gauge factor as high as 170 and a flat frequency response to at least 2.7 kHz. The gauge factor depends on the asymmetry of the etched structures and the number of etched sections. To understand the physical principles involved, researchers have used structural analysis programs based on a finite-element method to analyze fibers with asymmetric etched structures under tensile stress. The results show that lateral bends are induced on the etched fibers when they are stretched axially. To relate the lateral bending to the optical attenuation, we have also employed a ray-tracing technique to investigate the dependence of the attenuation on the structural deformation. Based on the structural analysis and the ray-tracing study parameters affecting the sensitivity have been studied. These results agree with the results of experimental investigations.

When a fiber with topological structures built directly on the cladding is compressed longitudinally, lateral bends are induced. The lateral bends lead to mode coupling. We have built several etched two-mode fiber modal coupling elements to take advantage of this effect of bending. The resulting modal coupling elements are compact. More importantly, the percentage of power converting from one mode to another is variable by varying the axial compression. In this paper, we present a theoretical model describing the mode conversion in the etched fiber elements. Also discussed are the fabrication and testing of the modal coupling elements and the experimental confirmation of the theoretical model.

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