Spatiotemporal thermal analysis of tapered fibers in optical cavity sensing applications at 633 nm and 1550 nm

Abstract

Fiber optic sensors with tapered silica fibers as sensing heads are attractive for various sensing applications. A mode propagating in a tapered fiber generates heat and induces temperature changes in and along the surface of the tapered fiber. The mode’s effective index also changes due to the thermo-optic coefficients of silica and analyte surrounding the tapered fiber. It is essential to analyze the spatiotemporal thermal response of tapered fibers due to the heat generated by a propagating mode to optimize the sensor’s performance. Here, we investigate the thermal response of air-clad and water-clad tapered fibers in Fabry-Pérot cavity sensors by analytically solving the heat equation in conjunction with finite element method simulations at 633 nm and 1550 nm. We find that the tapered fiber surface temperature and resonant wavelength of the cavity sensor change by ten times more at 633 nm than at 1550 nm. We also find an optimum radius of tapered fiber for sensing in aqueous solutions where thermal error becomes zero. The optimum radius is 0.35 μm at 633 nm and 0.85 μm at 1550 nm for a 25 cm long cavity made of 99.99% reflectivity mirrors with 2 cm long tapered fiber at 10 mW input power. We believe that the present work will give researchers better insight into understanding and controlling thermal-dependent properties of tapered fibers, specifically towards sensing applications and generally towards related technologies such as tapered fiber amplifiers and supercontinuum sources.