Long base inclinometer with optical measurement

A tiltmeter to measure a tilt variation of a structure from a given starting position, comprising:

• two sealed pots, each sealed pot comprising a closed high end and a low end, and each sealed pot containing an identical volume and an identical height of an identical first liquid, and an identical second liquid on top of the first liquid to prevent evaporation of the first liquid, the second liquid being less dense in volume than the first liquid;
• a tube configured to connect the two sealed pots at one side of their low ends and to enable the liquid to flow between the two pots in accordance with a principle of communicating vessels, the tube being distinct from the two sealed pots to constitute a separate component of the tiltmeter;
• two fiber optical interferometers, one fiber optical interferometer per sealed pot, each fiber optical interferometer measures a variation Δz in height of a liquid level of the liquid in the associated pot, each fiber optical interferometer comprises:
• a light source, of central wavelength λ0, to generate a light beam toward the associated pot;
• a reflecting surface positioned in a volume of the second liquid inside the associated pot;
• an optical measuring fiber to transport and guide the light beam toward the associated pot, a first end of the optical measuring fiber, fixed at a level of a high end of the associated pot and disposed facing the reflecting surface, forming with the reflecting surface an external Fabry-Pérot optical cavity;
• a detector to detect an interferometric signal generated by interference between a reference light beam and a measurement light beam, the interferometric signal comprises information on a movement of the liquid level relative to the first end of the optical measuring fiber, the reference light beam corresponds to a fraction of the light beam from the light source reflected by the first end of the optical measuring fiber, and the measurement light beam corresponds to a fraction of the light beam from the light source reflected by the reflecting surface and following an optical path in the Fabry-Pérot optical cavity, variation of the liquid level relative to the first end of the optical measuring fiber is a function of the liquid level height variation in the associated pot;
• and
• wherein the fiber optical interferometers are coupled to a common light source.