Fiber Optic Liquid Level Sensor Based on Integration of Lever Principle and Optical Interferometry

We present a fiber-optic liquid level sensor that is conducted by a combination of optical interferometry and lever principle. The sensing unit is a Mach-Zehnder interferometer (MZI), which is formed by sandwiching a piece of photonic crystal fiber (PCF) between two single-mode fibers (SMFs). The measuring equipment is composed of a rotatable lever and a fixed link. The rotatable lever includes two different length arms, i.e., L₁ and L₂. Both ends of the MZI are glued on the tip of the L₂ arm and the fixed link using ethoxyline, respectively. A hanging stick, which is dipped into a liquid tank, is directly mounted on the other end of the rotatable lever. The buoyancy will increase as the stick depth of immersion into the liquid increases. The tension the MZI subjected will increase according to the proportion of L₁/L₂ on account of the lever principle. The sensitivity of the sensor could be regulated with different ratios of lever arms. In our experiment, a maximum sensitivity of 111.27 pm/mm was obtained with a 1 : 7.8 ratio of two lever arms L₁/L₂. The demonstrated liquid level sensor has the advantages of simple structure, easy fabrication, low cost, and high sensitivity.