Reflections near 1030Â nm from 1540Â nm fibre Bragg gratings: Evidence of a complex refractive index structure

Reflections at wavelengths other than the expected harmonics of phase-mask-written fibre Bragg gratings (FBGs) have been observed. It is argued that these are a consequence of the complex structure induced in the fibre core during fabrication. Various FBGs have been investigated, involving different formation regimes and fibre types, different writing wavelengths, exposure sources (e.g., pulsed and continuous wave) and different phase masks. The experiments showed that no specific observation could be attributed to a particular aspect of the fabrication process except the phase mask. The lack of higher diffraction order suppression of the phase mask is suggested as the origin of the complex structure in refractive index modulation that is induced within the fibre core, giving rise to more than one set of harmonic reflections that would be expected from a simple, single period grating.