  Two discrete-time interference channel models are developed for information
transmission over a single span of optical fiber using wavelength-division
multiplexing (WDM) and lumped amplification. The models are derived from the
nonlinear Schroedinger (NLS) equation by including the nonlinear phenomena of
self-phase modulation (SPM) and cross-phase modulation (XPM) but ignoring
four-wave mixing (FWM), polarization effects and group velocity dispersion
(GVD) within WDM bands. The first model also ignores GVD across WDM bands,
referred to as group velocity mistmatch (GVM). For the case of two users, a new
technique called "interference focusing" is proposed where each carrier
achieves the capacity pre-log 1, thereby doubling the pre-log of 1/2 achieved
by using conventional methods. For three users, interference focusing is also
useful under certain conditions. The second model captures GVM and the effect
of filtering at the receivers in addition to SPM and XPM. In a 3-user system,
it is shown that all users can achieve the maximum pre-log factor 1
simultaneously by using interference focusing, a time-limited pulse and a bank
of filters at the receivers.
