Design and fabrication of a multimode optical fiber surface plasmon resonance urea biosensor††This work was supported by the National Natural Science Foundation of China (Grant No. 61674004).

Optical fiber surface plasmon resonance (SPR) sensors are widely used in concentration measurement. Conventional approaches usually exploit wet electroless plating to deposit silver film on the surface of the core of optical fiber to produce surface plasmon resonance effect. The inability to accurately control and measure the thickness of metal growth is a drawback of chemical methods to prepare optical fiber SPR sensors. Here, we propose an indirect method to measure the thickness of metal film and design an experiment to control the thickness of metal film by using gradient concentration solution. We also discuss the design and fabrication of a multimode optical fiber SPR urea biosensor. In addition, we build a reflective fiber optic sensor system. We test the performance of different thicknesses of the sensor probe in solution. The best sensitivity is about 31.7 nm/M from 0 to 1 M.