A unitary operator representing the exponential of the phase differenc
e between two modes of the electromagnetic field is introduced. The ei
genvalue spectrum has a discrete character that is analyzed. This oper
ator is related with a suitable polar decomposition of the Stokes para
meters of the field. The cases of weakly and highly excited states are
considered, discussing to what extent it is possible to talk about th
e phase for a single-mode field.