  The characterization or subsequent use of propagating optical quantum state
requires the knowledge of its precise temporal mode. Defining this mode
structure very often relies on a detailed a priori knowledge of the used
resources, when available, and can additionally call for an involved
theoretical modeling. In contrast, here we investigate a practical method
enabling to infer the optimal temporal mode directly from experimental data
acquired via homodyne detection, without any assumptions on the state. The
approach is based on a multimode analysis using eigenfunction expansion of the
autocorrelation function. This capability is illustrated by experimental data
from Fock states and Schrodinger cat-like state preparation.
