Spatial-to-spectral phase coupling mechanisms in bulk continuum generation

We study the coherence properties of continuum generation in YAG crystals seeded by 180-fs pulses at 1035nm when the driving beam exhibits small fluctuations of the spatial phase. The relative stability of the continuum spectral phase is first assessed as a function of the driver wavefront aberrations. Furthermore, we evidence and quantify a coupling mechanism between these fluctuations and the spectral phase of the continuum. The coupling coefficients increase with the spectral broadening and are also unexpectedly large (up to tens of rad/rad at ≃750nm). Experimental evidence supports that longitudinal shifts of the position of the filament within the crystal are responsible for such strong effects.