A non-destructive technique for characterising couplers by means of a local perturbation is described. A CO2 laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Asymmetric and symmetric perturbations give respectively, accurate mapping of power-evolution and coupler-waist shape. Using this technique, both the information of the power distribution and coupling profile along the coupler waist are obtained. The method is studied theoretically and verified experimentally by characterising different types of fused fibre-couplers namely: half-cycle, full-cycle and non-uniform half-cycle couplers. Add-drop multiplexers based on the inscription of Bragg gratings in the waist of fibre couplers have attracted some attention lately. The correct mapping of the power evolution along the coupler length gives the exact positions in the coupler waist where Bragg gratings should be written in order to obtain optimum add-drop operation. Recently, it was shown that non-uniform coupler structures based on two highly coupled lateral regions and a weakly coupled central region, can provide an ideal add-drop multiplexing operation. These couplers are characterised using the described coupler characterisation technique and the weakly coupled region, where the grating should be written, is clearly identified. This non-destructive method for characterising fibre couplers can be used as a tool for accessing the uniformity of the fabricated couplers waist or the influence of the tapered transition regions in the coupler performance. Additionally it can be used to optimise the performance of add-drop multiplexers based on the inscription of Bragg gratings in the waist of fibre-couplers.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.