Low temperature and high refractive index sensing based on SPR in a GeO2-doped PCF

Abstract To address the demand for high-precision sensing in low-temperature environments (-50°C to 20°C) and within a high refractive index (RI) range (1.40 to 1.445) in fields such as biopharmaceuticals, this paper proposes a surface plasmon resonance (SPR) sensor based on a germanium dioxide (GeO₂)-doped silica-based photonic crystal fiber (PCF) with a doping concentration of 13%. The GeO₂ doping significantly increases the refractive index of the substrate material, optimizing its sensing sensitivity in the high RI region. Structurally, a dual-side polishing process is employed on the fiber, utilizing the exposed air holes after polishing to naturally form groove structures. This design eliminates the need for complex additional grooving procedures, reducing manufacturing difficulty. The design creates two independent sensing channels: a gold film on one side directly measures RI, while a gold film coated with the temperature-sensitive material polydimethylsiloxane (PDMS) on the other side measures temperature. This enables dual-parameter sensing (RI and temperature) with independent channels, requiring no decoupling module. The sensor's performance was systematically studied using the finite element method. Results show that within the RI range of 1.40 to 1.445, the maximum RI sensitivity reaches 122,000 nm/RIU. Within the temperature range of -50°C to 20°C, the maximum temperature sensitivity is 56 nm/°C. Compared to sensors using a standard silica substrate, this GeO₂-doped silica-based sensor demonstrates superior sensing performance in both low-temperature and high RI regimes, while also offering a simplified structure for practical fabrication.