A method and device for sensing spatial variations and/or temperature variations in the locality of a fiber optic cable 1 is disclosed, wherein a broadband light source 47 is used to shine incident light onto a series of fiber Bragg gratings contained within zones A, B and C. Each zone contains a plurality of fiber Bragg gratings, the plurality of fiber Bragg gratings in any one zone having a substantially identical grating period, and the fiber Bragg gratings in the respective zones having different grating periods. The reflected light from each fiber Bragg grating is returned back down the fiber optic cable 1 and redirected via a 2.times.1 coupler 51 to a wavelength detection system 53 and a personal computer 63. The combination of wavelength detection system 53 and personal computer 63 allow analysis of the reflected light patterns, as well as providing a user interface which enables detection of the occurrence of a spatial and/or a temperature variation. The location of the said variation along the fiber optic cable 1 is advantageously detectable in terms of the particular zone A, B or C in which the said variation has been sensed.