The assumption that a tapered fiber core diameter shrinks down as the same ratio to the cladding diameter was
examined by observing the refractive index profiles variation along the fiber. How small the cladding diameter be and the
core would still exist was also examined. Refractive index profiling method based on quantitative phase microscopy
(QPM) gives the ability to mapping the two dimensional refractive index profiles, and the tapering variation can be
obtained from this approach. A small amount of core germanium diffusion was observed, which suggests the non-ideal
fact of the core/cladding diameter ratio. The core can still be resolved when the cladding diameter is down to ~10μm,
proving the existence of the core down to this scale.