Mach–Zehnder interferometer based on graded-index core mismatch with a customizable interrogation method for protein and refractive index sensing

A Mach–Zehnder interferometer (MZI) refractive index (RI) sensor based on the core mismatch between a graded-index multimode fiber (GIMMF) and a single-mode fiber (SMF) is proposed. The device consists of an SMF segment sandwiched between two short GIMMF sections, which act as partial mode couplers between the core and the cladding modes. By tailoring the GIMMF length, the interferometer allows flexible interrogation, operating either through wavelength shifts combined with intensity variations or through intensity changes alone. Numerical simulations confirmed that mode expansion and reimaging in the GIMMF govern cladding mode excitation, while experimental validation demonstrated RI measurements in calibrated oils and protein sensing with casein and gelatin solutions. The sensor achieved maximum absolute sensitivity values of 163.69 nm/RIU for wavelength-based interrogation and 76.31 dBm/RIU for intensity-based detection. Furthermore, coating the sensing region with green-synthesized silver nanoparticles (AgNPs) enhanced the evanescent-field interaction, yielding up to nine and 7.5-fold improvements in wavelength and intensity sensitivities, respectively. These results highlight a robust and customizable optical fiber sensor with strong potential for biochemical analysis and refractive index detection.