Limits on the power and spectral efficiency of direct detection systems with optical amplifiers

Extending some old results on the capacity of incoherent communication channels, it is shown that the capacity of a fiber optic channel with optical amplifier repeaters and square law detection is bounded by C/spl les/(W/2)log/sub 2/(1+(2Pr/N/sub o/W)), where W is the optical bandwidth, P, is the average signal power at the receiver input, and N/sub o/ is the amplified-spontaneous emission noise spectral density in a single polarization referred to the receiver input. The implications of this result on maximum distance between amplifiers, and on maximum spectral efficiency that may be achieved with alternative modulation and coding techniques, is discussed.