Bragg grating induced cladding mode coupling caused by ultra-violet light absorption

A numerical model based on coupled-mode theory and accurate representation of the refractive index is shown to reproduce the transmission characteristics of Bragg gratings in depressed cladding fibres. It is demonstrated that a ghost dip on the short wavelength side of the Bragg reflection is caused by asymmetry from transverse absorption of ultra-violet light during writing, and cannot be eliminated by assuring zero blaze angle.