Simultaneous Measurement of Liquid Level and Refractive Index Employing a Side-Polished Spiral Polymer Optical Fiber Based SPR Sensor

The work describes a surface plasmon resonance (SPR) sensor that measures the liquid level and refractive index (RI) simultaneously. The sensor is fabricated by a polymer optical fiber (POF) with a side-polished spiral structure. The POF is wound around a plastic rod with the same pitch and polished into a series of separated side-polished areas to form the sensor probe. Afterwards, the polished surfaces are coated with a gold layer to construct the SPR sensor. The proposed sensor can provide multi-point liquid-level measurement besides RI sensing. The influences of structural parameters such as the bending radius, polishing depth, and pitch on the sensing performance are experimentally studied. The experimental results indicate that the RI could be determined by employing the SPR wavelength shift, and the RI sensitivity of 1862 nm/RIU within the 1.34–1.40 range is obtained. Moreover, the SPR peak’s depth variation can be utilized to monitor the liquid level, and the sensor can provide adjustable resolution and a measurement range for liquid level sensing. With its simple fabrication, low cost, and flexible sensing ability, this sensor probe is suitable for the usage in biochemical applications.