In nonlinear fiber optics dispersive waves occur when high power optical pulses propagate in the presence of higher order dispersion. Traditionally one considers an optical soliton propagating in the anomalous dispersion regime at a detuning δω from the zero dispersion wavelength. Assuming a constant third order dispersion it is easy to show that a signal detuned by -2δω from the zero dispersion wavelength in the normal dispersion regime will propagate in phase with the soliton. This phasematching allows for a coherent transfer of energy between the soliton and a dispersive wave. Recently, we have shown that the frequency domain nonlinear mechanism responsible for the transfer of power from the pump to the dispersive wave is cascaded four wave mixing. This theory naturally predicts several of the key features of dispersive wave generation including the phasematched frequency shift of the dispersive wave, the spectral recoil of the pump, and the need for the pump spectrum to bridge the gap between the pump and the dispersive wave before energy can be transferred. It also predicts a previously unreported feature of dispersive wave emission: the possibility of a pump in the normal dispersion regime emitting a dispersive wave in the anomalous.
Fig 1: Cascaded four wave mixing is the nonlinear mechanism that transfers power from an anomalous pump to a dispersive wave in the normal dispersion regime, and also from a normal pump to a dispersive wave in the anomalous dispersion regime.
We have experimentally demonstrated the equivalence between the cascaded four wave mixing approach and a normal time domain analysis of dispersive wave emission using quasi-CW bichromatic pumps. This allows us to explicitly demonstrate the cascaded nature of the interaction. Fig 2 shows experimental spectra of a strong bichromatic pump, centered around 1585 nm in the anomalous dispersion regime, cascading to form a dispersive wave in the normal (at 1490 nm), and an isolated 700 fs pulse, centered at 1568 nm in the normal, producing a dispersive wave in the anomalous (at 1690nm).
Fig 2: (a) A bichromatic pump in the anomalous dispersion regime produced a strong dispersive wave in the normal dispersion regime. (b) 700 fs pulse in the normal producing a dispersive wave in the anomalous: experiment (blue), numerical simulation (red).
For more details on this work please see:
K. E. Webb, Y. Q. Xu, M. Erkintalo, and S. G. Murdoch, “Generalized dispersive wave emission in nonlinear fiber optics,” Opt. Lett. 38, 151-153 (2013)
Y. Q. Xu, M. Erkintalo, G. Genty, and S. G. Murdoch, “Cascaded Bragg scattering in fiber optics,” Opt. Lett. 38, 142-144 (2013)
M. Erkintalo, Y. Q. Xu, S. G. Murdoch, J. M. Dudley, and G. Genty, “Cascaded Phase Matching and Nonlinear Symmetry Breaking in Fiber Frequency Combs” Phys. Rev. Lett. 109, 223904 (2012)