Fiber Bragg Grating (FBG) sensors

Fiber Bragg Grating (FBG) strain sensing is used to monitor transient and permanent strain deformations of the rock volume during the experiments. The sensors utilize the reflections of a laser pulse at artificially induced grid points, the gratings, in fibre optic cables. The gratings are designed in a way that they act on one specific wavelength from the incident wavelength spectrum, while the rest of the spectrum is transmitted unaffected. In combination with an interrogator (the device that emits the laser pulse and records the reflection), the sensors can record strains with a sample rate of 1000 Hz at a resolution of 1µε with an accuracy of 0.85 µε. By processing, the resolution is increased to 0.1 µε. Ten FBG sensors are combined in one sensor chain each. Up to two chains are installed in the monitoring boreholes (in total 80 FBG sensors). Sensor chains were custom made by Marmota.