Integrated Optical Waveguide Bragg Grating Sensing Based on Cladding Thickness Engineering and SPR-Assisted Signal Recovery

Abstract

Flexible design freedom and multifunctional integration capability are key drivers for the practical deployment of on-chip sensing systems. This paper describes an on-chip waveguide Bragg grating sensor system fabricated via femtosecond laser direct writing, integrating cladding thickness engineering and surface plasmon resonance (SPR) enhancement strategies. The study investigates the impact of cladding thickness on modal distribution and sensing sensitivity. Experimental results demonstrate that with a cladding thickness of 50 μm, the sensitivity reaches 537.7 nm/RIU, which is comparable to that of conventional tilted fiber Bragg gratings. To mitigate the weak evanescent field penetration typical of thick cladding, this work introduces a gold-film-assisted surface plasmon resonance strategy to strengthen interfacial electromagnetic coupling. The SPR structure improves resonance detectability and enables reliable refractive index measurement for thick-cladding waveguide Bragg gratings with weak environmental interaction. SPR resonances were observed for the investigated inscription depths, indicating that an RI-dependent spectral feature can be retained over the tested range of waveguide positions. These results demonstrate the potential of the proposed platform for integrated refractive-index sensing. With appropriate surface functionalization and the incorporation of a selective recognition layer, the platform may be further investigated for biochemical sensing applications.