Influence of the Cut-Off Wavelength on the Supercontinuum Generation in a Highly Non-linear Photonic Crystal Fiber

We report on the influence of the cut-off wavelength on the supercontinuum
generation in which the spectral broadening occurs only on the blue
side of the pump wavelength. As a consequence a limit to the extent of the supercontiuum
is set and thus a way for tailoring the broadened spectrum according to a
peculiar application is provided. The experiment consists of launching a train of
femtosecond pulses into a 45-cm-long span of a photonic crystal fiber (PCF) by
means of an offset pumping technique that can selectively excite higher-order
modes. The PCF presents a wide range of wavelengths in which the fundamental
mode experiences normal dispersion, whereas higher-order modes, LP11 and LP21
propagate in the anomalous dispersion regime, generating a supercontinuum based
on the soliton fission mechanism. When exciting LP21 we are able to generate an
almost purely visible supercontinuum even with pulse energies below 100 pJ. Our
experimental results are compared with the numerical solutions of the nonlinear
Schrödinger equation.