CO₂Laser Tapering of Intrinsic Fabry–Perot Interferometers for Sensing

We present a highly reproducible method of fabricating a tapered intrinsic Fabry–Perot interferometer (IFPI) device with 5–6 $\mu \text{m}$ diameter at the taper waist. A femtosecond laser was applied to inscribe an IFPI with 3-cm cavity length in a single-mode fiber. A CO2 laser-heated tapering process enabled by digitally controlled mirrors and a precision motorized fiber feed system was used to create a stable heating zone with the desired temperature profile for tapering the fiber IFPI cavity. The well-engineered tapering process produced tapered IFPI devices with insertion loss less than 0.3 dB at 1550 nm. A strong evanescent field exposed by the taper Section was explored for refractive index (RI) sensing. Using swept optical frequency domination reflectometry (OFDR), the tapered IFPI fiber sensor achieved a minimal RI sensing resolution of $2\times 10^{-{5}}$ . This article demonstrates an integrated laser fabrication technique to produce tapered fiber optic devices for sensing applications.