  We develop general tools to characterise and efficiently compute relevant
observables of multimode $N$-photon states generated in non-linear decays in
one-dimensional waveguides. We then consider optical interferometry in a
Mach-Zender interferometer where a $d$-mode photonic state enters in each arm
of the interferometer. We derive a simple expression for the Quantum Fisher
Information in terms of the average photon number in each mode, and show that
it can be saturated by number-resolved photon measurements that do not
distinguish between the different $d$ modes.
