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.

I. Introduction

Environmental pollution has drawn considerable attention on a global scale in recent years. Heavy metal pollution is one of the environmental issues that has become more prevalent in many countries due to their propensity to accumulate in the environment and their traces of solubility in water [1]. Especially certain heavy metals, such as Mercury (Hg²⁺) and Lead (Pb²⁺), are highly toxic metal ions that are detrimental to human health [2]. As of yet, numerous Hg²⁺ detection methods have been reported, including electrochemical sensors [3], atomic fluorescence spectrometry [4], atomic absorption spectrometry [5], mass spectrometry [6], and nanoparticles [7], or functional polymers coated sensors [8]. These techniques typically require intricate, specialized laboratory equipment, time-consuming assay procedures, and expensive samples. Thus, there is a critical need for a portable sensor that is simple, quick response, and inexpensive.