Equalisation of Spectral Non-Uniformities in Broad-Band Chirped Fibre Gratings

The use of chirped fibre Bragg gratings (FBGs) in a dispersion compensating role has been shown to be a successful technology with great promise for future network upgrades [1,2]. A natural consequence of using FBGs in a negative dispersion sense is that although the structure is designed to phase-match forward- and backward-propagating LP01 modes, the phase-matching condition for coupling from the forward propagating fundamental mode to a higher order cladding-mode is also met for wavelengths just below that of the fundamental Bragg reflection. Because propagation in cladding-modes is extremely lossy there is a significant out-coupling of this shorter wavelength light. The chirped nature of FBGs designed for broad-band (> 5 nm) dispersion compensation means that this cladding-mode loss is integrated along the length of the grating with the result that the reflection spectrum has a slope extending from the short wavelength edge of the useable bandwidth. In a practical transmission system this in-band variation of reflectivity is unacceptable. In this paper the authors present for the first time a demonstration of spectral equalisation of cladding-mode losses by exercising control over local apodisation along the length of a 8.5 nm 75 cm long continuously-written chirped FBG. No post-processing was used.