Functional-film-modified SPR optical fiber sensor for dual-parameter detection in marine applications

Abstract

The coating of semiconductor films with high Refractive Index (RI) can redshift the working band of the Ag-based Surface Plasmon Resonance (SPR) sensor. By utilizing semiconductor materials with distinct optical properties for coating, decoupled dual-parameter measurements can be achieved, enabling independent and distinguishable operating wavelengths for the two channels. This approach broadens the sensing bandwidth and enhances the detection range and sensitivity. This paper presents an SPR sensor based on no core fiber (NCF), which can measure salinity and temperature simultaneously in a complex Marine environment. The Ag/MgF₂ composite film is used as the salinity sensing channel, where MgF₂ provides excellent chemical stability to protect the Ag film and improve the sensitivity of the salinity channel. The Ag/TiO₂/PDMS composite film, employed as a temperature sensing channel, significantly enhances the linearity and sensitivity of temperature measurements after thickness optimization. The high RI of TiO₂ induces a substantial redshift in the operational wavelength, effectively separating it from the salinity sensing wavelength at around 700 nm, while simultaneously broadening the overall sensing bandwidth and expanding the temperature measurement range. Experimental results indicate that the maximum salinity sensitivity reaches 7 nm/% in the short-wavelength region, while the maximum temperature sensitivity is 4.5 nm/°C in the long-wavelength region. This study provides an innovative approach to the multi-functionalization and high sensitivity of SPR sensors through advanced film structure design, demonstrating significant potential for applications in marine monitoring and related fields.