Fig. 1. (a) Schematic diagram of the sensing structure, (b) splicing diagram of air holes introduced between PCFs, (c) optical microscope image of the collapsed area and air holes between two PCFs, (d) surface microscopic image of the grapefruit-shaped PCF end

Fig. 2. Structure and preparation process of the sensor

Fig. 3. Schematic diagram of the sensing system

Fig. 4. Effect of different sputtering time on the sensor

Fig. 5. SPR transmission spectra of the sensor and relationship between wavelength shift and refractive index

Fig. 6. Comparison of spectral resonance peak depths of four sensors with different sensing structures

Fig. 7. Sensor response to temperature and linear fitting