Investigation of ethanol and methanol water mixtures in the visible wavelength area using fibre-optic evanescent field absorption sensors based on a u-shaped, a coil-shaped and a meander-shaped probe

An optical fibre evanescent field sensor for methanol (CH₃OH) and ethanol (C₂H₅OH) solvents based on deformed optical fibre cores is reported. The sensitivity of an existing U-bend probe is increased by extending the exposed core to a coil shape of one and a half turns and by forming the core to a meander- shape with three bends. Using the visible wavelength area (VIS) and methanol/ethanol concentrations from 0% to 10% it is shown that the sensitivity of the meander shape sensor enables the determination of methanol concentrations of better than 0.5% and of ethanol concentrations of better than 0.2% at 650 nm. This facilitates the use of low cost optical sources and receivers provided for this wavelength range as well as the utilization of polymer optical fibres to guide the light to and from the sensor which results in low cost applications compared to all silica or polymer clad silica fibre solutions in the near-infrared (NIR) or ultra-violet (UV) wavelength range.