Development of a fiber-optic interferometric inclinometer

The introduction of long-gauge fiber optic sensors has opened new opportunities for the monitoring of large structures. Using these sensors it is possible to observe the local elongation of a structure and calculate its global deformations. The use of internal sensors detecting integrated strain is however unable to detect the rigid-body motion of a structure, relative to an external reference system. This would require the installation of long deformation sensors connecting the structure with external reference points. Another possibility is to use gravity as a reference to measure vertical rotations of the structure. A gravity-referenced rotation sensor is conventionally called an inclinometer and measures the angle variations between a nominally vertical reference line in the structure and the direction of gravity. It is possible to use uni-axial inclinometers or bi-axial ones, measuring rotations in the two perpendicular vertical planes. Today's inclinometers are based on a variety of techniques and can achieve sensitivities in the micro-radian range. When using fiber optic deformations sensors in a given structure, it would however be interesting to use the same technology to obtain inclination reading [1]. This prompted us to develop a fiber optic-based inclinometer compatible with the SOFO system [2]. This contribution describes the development of this novel sensor and the first measurements obtained with it.