Low-Level Concentration Measurement of Mercury Ions Using CLF-GIMF-Based Multimode Interference Sensing in a Fiber Optic Ring Cavity Laser

The escalating prevalence of severe illnesses due to heavy metal ion contamination, even at very low concentrations, has prompted the development of high-precision sensors in healthcare and environmental monitoring systems. In this article, a novel fiber optic transmissive sensor based on multimode interference (MMI) phenomena is proposed and experimentally validated to measure the concentration of mercury ions (Hg2+) by incorporating the sensor in a fiber ring cavity laser. The MMI sensing probe includes an 8.9 cm claddingless fiber (CLF) connected to a 4.5 cm graded index multimode fiber (GIMF) fused in between two single-mode fibers (SMFs) to form an SMF-CLF-GIMF-SMF (SCGS) combinational structure. The self-imaging capabilities of the sensor are employed to extract the low-level concentrations of Hg2+ ions from the analytical solution by measuring the shift in its output wavelength. The proposed sensor is capable of detecting various mercury ion concentrations at ultralow levels with a detection sensitivity of 0.01563 nm/nM to the extent of 0–40 nM, 0.01875 nm/nM in the interval of 40–70 nM, and 0.01226 nm/nM spans in the range of 70–100 nM and R2 are measured as 0.96849, 0.99711, and 0.96859, respectively; also the sensing resolution of
$6.2\times {10}^{-{9}} \text {M}$
is obtained. With a detection threshold of
$11.96\times {10}^{-{9}} \text {M}$
, the average figure of merit (FOM) of the sensor is estimated as 0.81373 nM−1. Accordingly, the MMI-based mercury ions sensor holds much potential for noninvasive in vivo monitoring and remote sensing in biological, chemical, and medical applications.