  We analyze a quantum optical memory based on the off-resonant Raman
interaction of a single broadband photon, copropagating with a classical
control pulse, with an atomic ensemble. The conditions under which the memory
can perform optimally are found, by means of a `universal' mode decomposition.
This enables the memory efficiency to be specified in terms of a single
parameter, and the control field pulse shape to be determined via a simple
nonlinear scaling. We apply the same decomposition to determine the optimal
configurations for read-out.
