A fiber optic free water in fuel (WIF) sensor is proposed by utilizing a long period fiber grating (LPFG). The existence
of free water in fuel is indicated by the appearance of a characteristic loss band. The free water level in fuel can be
determined by measuring the transmissions of two characteristic loss bands.
We propose here a liquid level sensor using a long period fiber grating (LPFG) in which direct liquid level measurement is carried out by utilising an LPFG, 100mm in length and a periodicity of 1mm. The LPFG was exposed to liquids with varying levels and the wavelength shift of a selected loss band of the transmission spectra was monitored using a broadband light source and an optical spectrum analyzer. The mechanism of this LPFG sensor is based on the fact that the effective Refractive Index (RI) of a cladding mode is directly dependant on the RI of the surrounding medium, be it air, or in this case water and petrol. As the surrounding RI changes, so does the phase matching condition of the LPFG. The result for the level change of the liquid with a specific RI is both a shift in wavelength and a change in the attenuation level of the selected loss band. For the selected loss band, continuous wavelength shifts of 9.5 nm and 25 nm for 100 mm of water and petrol level change have been observed respectively, with sub-millimetre accuracy.