  Twin-beam coherence properties are analyzed both in the spatial and spectral
domains at high-gain regime including pump depletion. The increase of the size
of intensity auto- and cross-correlation areas at increasing pump power is
replaced by a decrease in the pump depletion regime. This effect is interpreted
as a progressive loss in the mode selection occurring at high-gain
amplification. The experimental determination of the number of spatio-spectral
modes from $g^{(2)} $-function measurements confirms this explanation.
