High Refractive Index Liquid Level Measurement via Coreless Multimode Fiber

We demonstrate a high refractive index (RI) liquid level sensor based on coreless multimode fiber (CMF) which is capable of measuring the liquid level and discriminating the high RI. The direct exposure of the CMF enables a strong interaction between the high-order modes guided in it and the surrounding medium through evanescent wave. Part of the energy leak out at the fiber/liquid interface when the RI of the liquid exceeds that of the CMF. The transmission of the single-mode fiber (SMF)-CMF-SMF structure is a function of both the liquid level and RI which experiences leakage loss with the increasing liquid level. Liquid level information can be determined by simply selecting a wavelength range with linear power variation. The proposed sensor with 4.1-cm CMF has approximately linear responses of -0.223, -0.253, -0.316, -0.392, and -0.461 dB/mm for glycerol solution concentrations of 98%, 96%, 92%, 88%, and 84% in a 2-nm wavelength range, respectively. Such a simple and low-cost sensor may be integrated as biochemical probes, indicating preferable sensing performance and practicability.