An Interferometric Dual Microfiber Refractive Index Sensor via Tapering of Femtosecond Laser Perforated Optical Fiber

We propose and demonstrate a dual microfiber (DMF) interferometric sensor here. The DMF can be fabricated from standard single-mode optical fibers by using a two-step process. First, we punch a hole through the fiber side-on via femtosecond laser machining, and then, we thermally taper the perforated fiber region, resulting in two parallel microfibers. We systematically investigated the refractive index (RI) sensing performance from both theoretical and experimental aspects. The simulation results show that a DMF sensor with waist widths of 2.6 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.7~\mu $ </tex-math></inline-formula>m achieves a high sensitivity of 11000 nm/RIU in the low RI range of 1.3330–1.3360, which is much higher than that of conventional optical fiber RI sensors. We further experimentally examined the sensing capability of the DMF sensor, achieving a high RI sensitivity of 10442.4 nm/RIU within the RI range of 1.3330–1.3360, which agrees well with theoretical results. We also investigated the RI and temperature cross-sensitivity of the DMF sensor, achieving a result of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 1.48\times 10^{-4}$ </tex-math></inline-formula> RIU/°C. The DMF sensor offers advantages such as ease of fabrication, ultrahigh RI sensitivity, and compact size, demonstrating the significant potential for applications in clinical testing, biomolecular diagnostics, and food safety.