Dynamic control of mode field diameter and effective area by germanium doping of hexagonal photonic crystal fibers

We demonstrate dynamic control of the effective area (A _eff) of photonic crystal fibers (PCFs) in the range of 18.1–8.22 μm² and the mode field diameter in the range of 4.78–3.42 μm. This control was realized by altering their structural properties and varying the germanium (Ge) doping rate, which changed the refractive index difference (Δn _Ge) between 1.0 and 3.0% relative to the refractive index of the silica cladding. This was achieved by adjusting the Ge doping rate in the core and changing the radius (d _core) of the doped region, i.e., by changing the equivalent refractive index, using numerical calculations. Numerical results were verified by comparison with experimental results for a fabricated Gedoped PCF obtained by far-field scanning based on the ITU-T Petermann II definition. The proposed approach will simultaneously decrease A_eff and achieves high light confinement and high nonlinearity in PCFs. It enables architectonics/controllability of highly nonlinear PCFs with passive optical devices in photonic networks and life science applications.