Spontaneous parametric down-conversion (SPDC) is one of the basic mechanism in quantum optics to provide strongly-correlated quantum light, from the process of nonlinearly splitting in a crystal an incoming photon (from the pump) into two daughter so-called signal and idler photons, which can be entangled.
SPDC induced by a wide monomode Gaussian pump is a strongly multimode beam. Almost single-pulse (actually $\approx 30$) high-multimode characterization has been made by Lantz et al.[1]