AFM observation of surface topography of fibre Bragg gratings fabricated ingermanium-boron codoped fibres and hydrogen-loaded fibres.

摘要

This paper reports the measurement of the surface topology of optical fibrescontaining a fibre Bragg grating (FBG) using an atomic force microscope (AFM).The AFM observation was made on FBGs fabricated via the phase mask technique ingermanium–boron codoped optical fibres, in hydrogen-loaded germanium–boroncodoped fibres and in standard telecommunications optical fibres. The surfaceimages reveal that a spatial corrugation pattern was induced by the UV-irradiation, with a period that is half of the period of the phase mask. ThisUV-induced surface structure was found only on the side of the fibre facingtowards the incident UV-irradiation and did not appear on the rear surface. TheAFM probe scanned a 10×10 μm2 surface area at seven sites along the 6.0 mmlength of fibre that was exposed to the UV-irradiation. The amplitude of thespatial corrugation pattern observed on the AFM image was quantified for eachsite. It was found that the amplitude in a range of 0.7–3.2 nm was a function ofUV-laser intensity distribution and the type of fibre. Hydrogen loaded opticalfibres exhibited a corrugation with an amplitude twice as large as that observedin the Ge–B doped fibres that were not hydrogen-loaded. This correlates with theincrease in photosensitivity produced by the hydrogen loading. A similar UV-induced spatial corrugation was also observed on standard telecom fibres, butwithout inducing the refractive index change in the fibre core. The observationof surface topology provides an insight into the structural changes inducedduring FBG fabrication. UV-induced densification and laser ablation couldaccount for the formation of the surfac