Liquid surface tension and refractive index sensor based on a tilted fiber Bragg grating

A refractive index (RI) and liquid surface tension (ST) (at the level of 10−3 N) measurement system has been proposed by using a 1.80-cm-long, 8°-tilted fiber Bragg grating (TFBG). When the TFBG is pulled up from the liquid into the air at a fixed height (corresponding to a fixed contact angle between the fiber and the liquid), the ST acting upon the TFBG depends on the length of the TFBG covered with the liquid film along the fiber axis direction. The liquid film covering the grating results in the shifts of the resonant cladding modes of the TFBG. For the liquid with an RI in the range from 1.3341 to 1.4415, its RI and ST can be obtained through the measurements of shifts in the resonant wavelengths of the cutoff mode and changes in the intensities of the resonant wavelength dips in the TFBG spectrum by using the known ST and RI of water as a reference.