This thesis is concerned with the study of two key technologies for enabling wavelength division multiplexed optical communication systems. The first is gain equalisation of the erbium-doped fibre amplifier and the second is the routing of optical channels through the network by means of all-fibre add-drop multiplexer configurations. A laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Asymmetric and symmetric perturbations can give accurate mapping of power-evolution and coupler waist shape.