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We have experimentally demonstrated an ultra-high sensitivity gold-based fiber refractive index (RI) sensor whose main structure is composed of multimode fiber (MMF) and photonic crystal fiber (PCF). The gold film is deposited on V-shaped PCF by magnetron sputtering, and sensing experiments are performed based on the principle of surface plasmon resonance (SPR). Numerical simulation results indicate that the cladding mode of the V-shaped PCF is more capable of stimulating the SPR effect than the core mode. The experimental results show that the RI measurement range of the sensor is 1.333–1.421, with a maximum sensitivity of 10015 nm/RIU. In addition to RI sensing, sensing probes can be coated with polydimethylsiloxane (PDMS) on a gold film for temperature sensing. For temperature detection, the range is from 10 to 100 °C and the maximum sensitivity is 3.5 nm/℃. Besides high sensitivity in RI measurement, the proposed sensor also has good sensing performance in temperature sensing. With the advantages of high sensitivity, good stability, and easy preparation, this sensor has become an important reference in the field of high-performance sensing. • This work proposes a refractive index sensor with ultra-high sensitivity, with a maximum sensitivity of 10015 nm/RIU. • The sensor uses PCF with a large cladding area as the substrate and is easy to prepare. • The sensor selects a stable gold film, which is not easily oxidized, as the plasma material. • The maximum sensitivity of this sensor used in the field of temperature sensing is 3.5 nm/°C. [ABSTRACT FROM AUTHOR]

• Metal oxides with high RI will improve the sensing performance of fiber sensors. • Compared to silver-based sensors, the maximum sensitivity of TiO 2 composite film sensors has increased by 16.92 %. • Compared to silver-based sensors, the maximum sensitivity of ZnO composite film sensors has increased by 4.62 %. • The maximum temperature sensitivity of the two composite membrane sensors is 4.9 nm/℃ and 4.3 nm/℃, respectively. With the widespread application of fiber surface plasmon resonance (SPR) sensors in the field of biochemical sensing, especially in the detection of large molecules such as proteins, the sensitivity of traditional fiber SPR sensors has made it difficult to meet the requirements of detection accuracy in practical applications. This article proposes and proves an effective method for selecting the optimal metal oxide material to improve the sensing performance of SPR sensors. Metal oxides (TiO 2 and ZnO) can improve the sensitivity by enhancing the evanescent field of fiber SPR phenomenon. Through simulation, it was found that when the metal oxide film of the composite film sensor maintains the same thickness, the metal oxide with a high effective refractive index (RI) has a better improvement effect on the sensitivity of the SPR sensor. Experimental results show that the maximum RI sensitivity of TiO 2 composite film sensors and ZnO composite film sensors in the RI range of 1.333–1.420 is 15,200 nm/RIU and 13,600 nm/RIU, respectively. Compared to silver-based sensors, RI sensitivity has increased by 16.92 % and 4.62 %, respectively. To further verify the enhancement effect of different metal oxides on fiber sensors, another layer of PDMS film is deposited on TiO 2 and ZnO films. Within the temperature sensing range, the maximum temperature sensitivity of these two composite film temperature sensors is 4.9 nm/°C and 4.3 nm/°C, respectively. Overall, metal oxides with high RI can improve the sensing performance of fiber optic sensors. [ABSTRACT FROM AUTHOR]

• This work explains that TiO 2 and ZnO can significantly modulate the resonance wavelength range of SPR sensors. • It is experimentally demonstrated that the two oxide films have the effect of enhancing the sensitivity of the SPR sensor. • The dual-channel sensor prepared using the cascade method of two oxide films has a wide sensing bandwidth. • The performance of the proposed dual-channel sensor is highly stable and not easily oxidized. In order to broaden the bandwidth of SPR dual-channel sensors, this work proposes to tune the resonance wavelength range of the sensors using a cascade of ZnO and TiO 2 films. The two oxides are transversely deposited on Ag-based no-core fiber to form a dual-channel sensor that can be used for refractive index (RI) and temperature measurements. Among them, Ag/ZnO is used for RI sensing and Ag/TiO 2 /PDMS is used for temperature sensing. The simulation and experimental results show that ZnO and TiO 2 have the effect of broadening the resonance wavelength range by 44 nm and 72 nm, respectively. In addition, when the two oxides are added to the Ag-based sensor, the SPR valley is red-shifted, and the sensitivity is enhanced. The experimental results also show that the two oxides make the silver film less susceptible to oxidation and enhance the stability of the sensor. Numerical results show that the maximum RI sensitivity of the sensor is 4630 nm/RIU, and the temperature sensitivity is 5.8 nm/°C. The range of the object to be measured is 1.333–1.385, 0–50 °C. This ability to broaden the sensor bandwidth provides a new direction for designing and applying high-performance multi-channel SPR sensors. [ABSTRACT FROM AUTHOR]

• This work utilizes TiO 2 film to tune the sensing bandwidth and the resonance wavelength range to the near-infrared band. • A dual-channel sensor with wide bandwidth is proposed. • The experimental results show that the RI range of the sensor is 1.333–1.385, and the temperature range is 10–80 ℃. • The sensor has simple structure and good repeatability. Facing the diversity and complexity of the types of parameters to be measured, multi-channel surface plasmon resonance (SPR) fiber sensors provide an excellent solution. However, the sensing bandwidth is a key factor affecting the sensor performance. In this paper, we explore a method to modulate the sensing bandwidth using the TiO 2 film. A dual-channel SPR sensor with a wide sensing bandwidth is proposed. In this sensor, no-core fiber (NCF) is used as the sensing probe, and multimode fibers are used for the transmission of optical signals. The sensor has a cascaded composite film structure, the region coated with Ag film as the refractive index sensing region and the region coated with Ag/TiO 2 /PDMS as the temperature sensing region. The experimental results show that the sensing bandwidth of the sensor varies for different TiO 2 coating times. It indicates that the TiO 2 film can regulate the sensing bandwidth flexibly. Therefore, to achieve the optimal sensing effect, the TiO 2 coating time can be flexibly adjusted according to the RI characteristics of the environment. The maximum sensitivity of this dual-channel sensor is 4.5 nm/°C and 3151 nm/RU when the temperature range is between 10 and 80 °C and the RI range is between 1.333 and 1.385, respectively. In addition, we discuss the repeatability and stability of this dual-channel sensor. [ABSTRACT FROM AUTHOR]