Dual-parameter Cu²⁺ concentration and temperature sensor based on a wavelength-tunable fiber laser incorporating SNMNMS interferometer and FBG

A dual-parameter copper ion concentration and temperature sensor based on the tunable-wavelength erbium fiber laser was proposed and demonstrated. Furthermore, an SNMNMS interferometer, comprising single-mode fiber (SMF), no-core fiber (NCF) and multimode fiber (MMF), was functionalized self-assembly by multi-layer chitosan (CS) and polyacrylic acid (PAA). The functionalized interferometer was connected in parallel with a fiber Bragg grating (FBG) and placed separately in the copper ion solution and on a heated plate, jointly enabling laser filtering, wavelength tuning, and dual-parameter sensing. With the temperature rising from 30 to 120 °C and the concentration increasing from 100 to 1000 mg/L, respectively, the laser wavelengths shifted from 1532.239 nm to 1533.068 nm and from 1556.507 nm to 1558.428 nm. The sensitivities for temperature and concentration sensing were 0.0092 nm/℃ and 0.0022 nm/(mg/L), with linearity coefficients of 0.998 and 0.995. The side-mode suppression ratio (SMSR) of the dual-laser outputs exceeded 27.482 dB and 27.5 dB, overcoming broadband and low extinction ratio limitations in interferometric sensors causing reduced positioning accuracy. Consequently, the dual-parameter laser sensor simultaneously monitors copper ions and temperature, ensuring the quality of copper foil during the plating process. Additionally, it enhances signal recognition accuracy, while the multilayer coating improves stability in high-concentration environments, preventing failure due to excessive adsorption.