  We determine the ultimate classical information capacity of a linear
time-invariant bosonic channel with additive phase-insensitive Gaussian noise.
This channel can model fiber-optic communication at power levels below the
threshold for significant nonlinear effects. We provide a general
continuous-time result that gives the ultimate capacity for such a channel
operating in the quasimonochromatic regime under an average power constraint.
This ultimate capacity is compared with corresponding results for heterodyne
and homodyne detection over the same channel.
