Nonlinear control of low-energy ultrashort laser pulse by cross-phase modulation in a photonic crystal fiber

We report on the efficient spectral control of ultrashort sub-nanojoule violet laser pulses by means of cross-phase modulation (XPM) with a pump pulse in a highly nonlinear photonic crystal fiber (PCF). Spectra of the output pulses were recorded as a function of pump-probe delay for different pump central wavelengths. For fiber lengths that are much longer than the pulse walk-off distance, we consistently observe simultaneous downshifted and upshifted bands in the output pulses, associated with the frequency shift induced by both the leading and trailing edges of the pump as it sweeps over the probe pulses. These features can only be observed with pump pulses propagating in the anomalous dispersion region of the PCF, where pump pulse compression occurs and provides the necessary pump asymmetry.