Theoretical and experimental research of lossy mode resonance-based high-sensitivity optical fiber refractive index sensors

A high-sensitivity refractive index sensor based on lossy mode resonance (LMR) was achieved by coating TiO2/PSS nanofilm on the core of cladding-removed multimode fiber using a layer-by-layer method. The influence of nanofilm thickness, resonance orders, and surrounding refractive index on mode extinction coefficients, sensitivity, and nonlinearity was analyzed based on mode-coupling theory using the finite element method for the first time, to the best of our knowledge. The sensitivity can theoretically reach more than 63,823 nm/RIU in the RI range of 1.383–1.403 in the midinfrared band. In addition, the sensor with sensitivity up to 6,754 nm/RIU in the RI range of 1.355–1.375 in the visible band was obtained in the experiment, which is highest among TiO2-coated LMR-based sensors. In addition, the reason for spectrum broadening in LMR-based multimode fiber sensors was also explored with a combination of finite element and transfer matrix methods, which proved that the spectrum broadening is due to the difference of the resonance wavelength of different modes, and the key rule to narrow the spectrum is to reduce the transmission modes in a fiber waveguide. This paper pointed out the design rules of LMR-based optical fiber sensors after systematical analyzation, which provides guidance for future research.