Investigation of Side-Polished Panda Fibers for Strain Measurement and Surface Plasmon Resonance-Based Biochemical Sensing

Abstract

We propose and experimentally demonstrate a Sagnac interferometer for strain measurements in side-polished birefringent fibers. The interferometer is fabricated by welding a section of side-polished panda fiber to a 3 dB coupler. Experimental results show birefringence decreased as the polish depth increased, and strain sensitivity linearly increased with increasing the polish length. We find that the use of polishing birefringent fibers is a very effective method for improving strain sensitivity. A relatively small polishing length (40 mm) could effectively increase strain sensitivity from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$19.66\ \ \text{pm}/\mu\varepsilon$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$37.12\ \text{pm}/\mu\varepsilon$</tex> in the first type of sample. In addition, a thin-film gold coating is applied to the polished fiber surface to produce a biochemical sensor utilizing surface plasmon resonance (SPR). Experimental results indicate that the SPR absorption and the figure of merit (FOM) are different in the two types of samples. With the analyte refractive index (RI) varying from 1.32 to 1.40, the sensitivity increases from 800 nm/RIU to 3500 nm/RIU and the FOM increases from 14.8 to 52.2 in the first type of sample. These values of the FOM are approximately twice as high as that of an SPR fiber sensor based on a D-shaped single mode fiber in the same RI environment. These results suggest a D-shaped panda fiber could develop promising fiber devices for the internet of things sensing technology.