  Quantum optical metrology aims to identify ultimate sensitivity bounds for
the estimation of parameters encoded into quantum states of the electromagnetic
field. In many practical applications, including imaging, microscopy, and
remote sensing, the parameter of interest is not only encoded in the quantum
state of the field, but also in its spatio-temporal distribution, i.e. in its
mode structure. In this mode-encoded parameter estimation setting, we derive an
analytical expression for the quantum Fisher information valid for arbitrary
multimode Gaussian fields. To illustrate the power of our approach, we apply
our results to the estimation of the transverse displacement of a beam and to
the temporal separation between two pulses. For these examples, we show how the
estimation sensitivity can be enhanced by adding squeezing into specific modes.
