In a paper of ours published in 2014, we have used etched Bragg gratings as liquid level sensors. Etching has been realized in HF acid with an etching rate of 1.63 μm/min and the Bragg wavelength has been continuously monitored during the procedure.
The etched Bragg gratings have shown good response upon being immersed in water and olive oil having refractive indices lower and higher than the fiber core respectively. This response is caused by the different ambient refractive index experienced by the Bragg gratings in the different environmants. A theoretical analysis has also been realized which has shown that the effective refractive index and consequently the sensitivity of the FBG, as refractive index sensor, increase exponentially by decreasing the cladding thickness.
“Theoretical and experimental study of refractive index sensors based on etched fiber Bragg gratings” G. Tsigaridas, D. Polyzos, A. Ioannou, M. Fakis and P. Persephonis Sensors and Actuators A. Physical 209 (2014) 9-15