Direct non-invasive measuring techniques of nanometric liquid level variations using extrinsic fiber Fabry-Perot interferometers

This article investigates two different non-contact non-invasive solutions for measuring nanometric-order liquid-surface displacements with Extrinsic Fiber Fabry-Perot interferometers. They are investigated for developing hydrostatic leveling sensor (HLS) systems targeting the detection of very slowly-evolving movements in geophysics and geotechnics. In the first technique, the sensing beam from the interferometer traverses a liquid of known refractive index and is reflected by a mirror submerged at the bottom of the HLS vessel. The liquid-level variation is thus sensed as a variation of the optical path length of the interrogating beam. The second solution, on the other hand, directly exploits the reflection of the sensing beam at the air-liquid interface in the absence of a reflective surface in the vessel. The subsequent variation of liquid level is then measured directly as the beam’s optical path variation in air. The common denominator of these two techniques is an Extrinsic Fabry-Perot sensor with nanometric precision operating at a wavelength of ~1310 nm. The interrogating beam suffers from high IR absorption in water, hence the latter solution is more advantageous in terms of dynamic range. In applications where liquids other than water can be employed, the use of low optical absorption liquids such as Polydimethylsiloxanic fluids is recommended at this operating wavelength. Being more viscous and less volatile than water, these fluids can significantly improve the noise floor of HLS systems, hence contributing to a larger dynamic range, lower instrumental drift and higher signal-to-noise ratio.