A Novel Method for Detecting Fe²⁺ at a Micromolar Concentration Based on Multiple Self-Mixing Interference Using a...

The concentration of an electrolyte is an optical characteristic of drinking water. We propose a method based on the multiple self-mixing interference with absorption for detecting the Fe²⁺ indicator as the electrolyte sample at a micromolar concentration. The theoretical expressions were derived based on the lasing amplitude condition in the presence of the reflected lights considering the concentration of the Fe²⁺ indicator via the absorption decay according to Beer's law. The experimental setup was built to observe MSMI waveform using a green laser whose wavelength was located in the extent of the Fe²⁺ indicator's absorption spectrum. The waveforms of the multiple self-mixing interference were simulated and observed at different concentrations. The simulated and experimental waveforms both contained the main and parasitic fringes whose amplitudes varied at different concentrations with different degrees, as the reflected lights participated in the lasing gain after absorption decay by the Fe²⁺ indicator. The experimental results and the simulated results showed a nonlinear logarithmic distribution of the amplitude ratio, the defined parameter estimating the waveform variations, versus the concentration of the Fe²⁺ indicator via numerical fitting.