A number of distributed optical fiber temperature sensors are requested to be only activated where and when the temperature field gets near to, and exceeds a threshold value set to be the alarm value. The proposed system uses the microbending loss effect in a graded index multimode fiber with OTDR readout. The transducing mechanism is based on a continuous bimorphous metal coating that is periodically and partially weakend so that temperature increase induces a microbending at the prescribed modal beat period. The pair of metals, as well as the coating process must be selected in order to place the maximum sensitivity in the neighbourhood of the alarm temperature. The sensitivity can be adjusted to the space-time fea-tures of the events to be detected, eg, semi-local hot spots or a heat source diffusing in an unbounded volume.