Potential of electrical-domain interference driven by modal distribution in optical fibers for strain and displacement sensing

This paper reports the sensing potential of an interference-like spectral response in the electrical domain, observed when sub-cutoff-wavelength light passes through certain fiber arrangements, like a plastic optical fiber (POF) or an air gap between silica single-mode fibers. Monitoring the photo-detected signal on an electrical spectrum analyzer reveals that interference dips move when external environmental conditions change. Measuring the frequency of the first spectral dip, results show that applying strain to the POF shifts the dip frequency, suggesting potential for strain sensing. On the other hand, adjusting the air gap between silica fibers also shifts the spectral dip, enabling displacement sensing. Our experimental study reveals that these behaviors cannot be described by standard Mach-Zehnder or typical multimodal interference, and is presumably driven by the modal distribution and the resulting measured power in the electrical domain. While the mechanism remains under study, the fast operation capability and low-cost nature of the method suggest that it can be effectively exploited for strain and displacement detection.