Simultaneous measurement of liquid level and surrounding refractive index using tilted fiber Bragg grating

In this paper, we present a novel fiber optic sensor for the simultaneous measurement of liquid level and surrounding refractive index based on tilted fiber Bragg grating (TFBG). The transmission loss and the bandwidth of cladding modes are two measures of the liquid level, while their resonance wavelengths are employed to determine the surrounding refractive index. A liquid level sensitivity of over 0.6 dB/mm is achieved with good linearity. Temperature independent surrounding refractive index sensing is possible using the Bragg mode of TFBG as a temperature reference. What is more, a scheme is proposed to realize a large measurement range of liquid level by connecting a certain number of TFBGs in series or parallel. The scheme is verified by an experiment in which three TFBGs are connected in parallel.