Optical notch filter manufacture in optical fibre waveguide by plastic deformation

A method of generating an optical notch filter in an optical fibre by creating therein a resonantly coupled iterative set of axially spaced perturbations each of which is created by axially stretching the fibre to produce plastic deformation in a heat softened zone thereof which is sufficiently sharply localised to result in the formation thereat of a non-adiabatic taper which is weakly couples the zero order mode of the fibre with a higher order mode to provide a coupling coefficient small compared with 3 dB and substantially uniform over a wavelength range large compared with the FWHM bandwidth of the notch filter wherein the optical fibre is a single mode optical fibre which, prior to the creation of the spaced perturbations, is pretapered to provide, between two full-sized portions of the single mode fibre, two adiabatic tapers joined by a reduced diameter portion of the single mode fibre which zero order mode power is guided substantially exclusively by the interface between the material of the fibre and the ambient surrounding the reduced diameter portion of the fibre, and wherein the spaced perturbations are created in the reduced diameter portion.