Direct measurement of core profile diffusion and ellipticity in fused-taper fibre couplers using atomic force microscopy

The geometrical shape and refractive index profile of the cores of fused-taper fibre couplers made by the heating, fusing and tapering technique have been measured to an accuracy of better than 10 nm. This approach involves precision cleaving, hydrofluoric acid etching and atomic force microscope (AFM) measurement. Results reveal that the core profile and cross-sectional geometry along the taper differ significantly from the normal assumption of scaled circular cores