(a) Evolution of the cladding (blue triangles) and core diameters (red dots) versus the z position in the taper drawn from fiber 1. Error bars on Φ_core are ±0.1 μm (smaller than the size of the cladding marks). (b) Evolution of the ratio of the core and cladding diameter R = Φ_core/Φ_clad versus the z position in the taper in the area where the core diameter is measurable. Error bars are smaller than the size of the marks (±0.1%).