Precise measurement of liquid-level by fiber loop ring-down technique incorporating an etched fiber

A method for precise measurement of the liquid-level by using a fiber loop ring-down technique incorporating a short section of etched fiber is proposed and experimentally demonstrated. The change of liquid-level is obtained by recording the ring-down time τ of light in the fiber loop. The experimental results show that the liquid-level has a well defined linear relationship with the inverse of τ over the range of 1 cm; the relative standard deviation of τ is about 0.21% and correspondingly the least detectable liquid-level change is about 4 µm by using an etched fiber with diameter of 25.1 µm. Moreover, this method has low temperature cross-sensitivity when the refractive index of liquid is higher than that of the fiber cladding, so it can be employed for liquid-level measurement with high resolution.