This paper presents the measurement principle of the SOFO system as well as examples of application to the monitoring of three tunnels. The Luzzone dam tunnel was excavated using conventional blasting techniques; the SOFO sensors were installed in radial boreholes to evaluate the tunnel convergence and to help in the assessment of the required shotcrete thickness. The N5 tunnel is a cut and cover tunnel; the SOFO sensors were used to evaluate the shrinkage properties of concrete in the short and long term. Finally, a multi-point SOFO sensor was installed in the Mt. Terri tunnel to evaluate the rock  decompression during excavation with a tunnel-boring-machine. The sensors were installed in a borehole from an existing tunnel parallel to the new one.