Simulation of femtosecond pulse propagation in sub-micron diameter tapered fibers

Ultrashort pulse propagation and supercontinuum generation in tapered and microstructured optical fibers is usually simulated using the corrected nonlinear Schrödinger equation. One of the underlying approximations is the use of a wavelength-independent effective area or, equivalently, of a constant nonlinear coefficient γ. In very thin waveguide structures with strong light confinement, including silica wires and sub-micron tapered fibers and some microstructured fibers, the validity of such an approximation comes into question. In this paper we present an improved model in which all modal properties are fully taken into account as functions of the wavelength. We use comparative numerical simulation to identify certain regimes in which an improved model is needed for quantitatively correct results.