The ZnO Wavelength-Modulated Cascaded SPR Fiber Optic Sensor for High-Sensitivity Detection of RI and Temperature in the Visible-NIR Band

For spectral analysis fiber optic sensors, wider operating bandwidth is the key to increase the number and variety of sensing parameters. In this article, we explore a method of using ZnO to modulate the sensor bandwidth and successfully broaden the sensor wavelength to the near-infrared region and propose a cascaded dual-channel surface plasmon resonance (SPR) fiber sensor. In the structural design, the multimode optical fiber is used as the optical signal transmission channel, and two sections of coreless optical fiber are used as the functional area and probe substrate of the two channels, respectively. The functional film region was constructed on the substrate by deposition technique, in which the Ag-coated part was used to construct the refractive index (RI) sensing channel, and the Ag/ZnO/PDMS-coated part was used to construct the temperature sensing channel. The experimental results show that the sensor bandwidth can be flexibly controlled by the deposition time of ZnO, so the deposition time of ZnO can be flexibly adjusted according to the requirements of environmental RI testing. When the RI range is 1.333–1.415, the maximum RI sensitivity is 9370.9 nm/RIU, and when the temperature range is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0~^{\circ }$ </tex-math></inline-formula>C–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100~^{\circ }$ </tex-math></inline-formula>C, the maximum temperature sensitivity is 7.97 nm/°C. Sensor repeatability is good, stable performance. It can be used as a reference for expanding the working bandwidth and testing range of external parameters of spectral cascaded optical fiber sensors.