Wavelength-tunable erbium-doped fiber laser based on peanut structure and U-bent single-mode fiber coated with CS/PAA for Cu²⁺ concentration sensing

In the paper, a single wavelength-tunable erbium-doped fiber ring laser for copper ions concentration sensing was proposed and demonstrated. The Mach-Zehnder interferometer (MZI), as the frequency-selective component of the laser, comprised a fiber peanut structure (FPS) and a U-bent fiber coated with layer-by-layer self-assembled chitosan (CS) and polyacrylic acid (PAA). The functionalized MZI exhibited a blue-shift of the interference dip with increasing concentration, and the sensitivity was 0.0015 nm/(mg/L). The maximum signal-to-noise ratio (SNR) was only 7.45 dB, hindering the precise localization of sensing signals. By inserting the functionalized MZI into the laser cavity, a single-wavelength laser was achieved; and the laser wavelength shifted from 1532.21 nm to 1529.00 nm as the concentration increased from 100 to 1000 mg/L. Furthermore, the sensitivity was 0.0036 nm/(mg/L), and the maximum SNR was 27.82 dB, significantly higher than that of the MZI sensor for copper ions concentration sensing. The laser wavelength fluctuations measured were less than 0.14 nm and 0.09 nm for the copper ions concentrations of 100 mg/L and 700 mg/L, respectively, over a scan time of 20 min. The laser sensor and U-bent probe structure make it potentially great for application in plating solution tanks and other environments unsuited for sensor deployment.