Spatiotemporal coherence of white light beam trapped within dark spatial soliton

We study experimentally and theoretically temporal and spatial coherence of white light beam through its interaction with a coherent dark spatial soliton in LiNbO3:Fe noninstantaneous nonlinear self-defocusing crystal. Experimental observations show that during this process a portion of the white light beam is trapped within the dark notch of the dark spatial soliton, thus forming a sharp intensity spike. Moreover, numerical simulations show that in this region the coherence length of white light beam increases from ∼11 to 1200 μm after 6 mm of propagation for almost all frequencies. We find that the intensity spike of each frequency of white light increases with the increase of frequency. While the spatial coherence length of each frequency of white light decreases with the increase of frequency.