Our laboratories have developed a measurement system called SOFO, based on low-coherence interferometry in singlemode optical fibers and allowing the measurement of deformations of the order of 1/100 mm. This system is especially useful for the long-term monitoring of civil structures such as bridges, tunnels, dams and geostructures. The SOFO system requires the installation of two fibers in the structure to be monitored. The first fiber should be in mechanical contact with the structure in its active region and follow the structure deformation in both elongation and shortening. The second fiber has to be installed freely in a pipe near the first one. This fiber acts as a reference and compensates for the temperature dependence of the index of refraction in the measurement fiber. This contribution presents the design process as well as the lab and field tests of a sensor responding to these requirements and adapted to the installation in concrete structures. The active region can be between 25 cm and 8 m in length, while the passive region can reach at least 20 m. While the reference is free, the measurement fiber (installed in the same pipe) is pre-stressed between two glue-points at each end of the active region. The glue was chosen in order to avoid any creeping problems even at temperatures up to 160 degree(s)C and elongation up to 2%. The sensor was tested in laboratory and field conditions. The lab tests included survival to concreting, high temperatures, freezing, thermal cycling, vibrations, cracking and corrosion; response to elongation and compression, measurement range and creeping of the glue points at high temperatures and high tensions. The field tests included installation of a number of these sensors in a bridge deck and in a tunnel vault. In these applications we tested the ease of use, the rapidity of installation and the survival rate.