Supercontinuum generation by higher-order mode excitation in a photonic crystal fiber

We describe an experiment in which a train of femtosecond pulses is coupled into a photonic crystal fiber (PCF) by means of an offset pumping technique that can selectively excite either the mode LP01 or LP11 or LP21. The PCF presents a wide range of wavelengths in which the fundamental mode experiences normal dispersion, whereas LP11 and LP21 propagate in the anomalous dispersion regime, generating a supercontinuum based on the soliton fission mechanism. We find that the existence of a cutoff wavelength for the higher-order modes makes the spectral broadening asymmetrical. This latter effect is particularly dramatic in the case of the LP21 mode, in which, by using a pump wavelength slightly below cut-off, the spectral broadening occurs only on the blue side of the pump wavelength. Our experimental results are successfully compared to numerical solutions of the nonlinear Schrodinger equation. (c) 2008 Optical Society of America.