  Capacity is the eternal pursuit for communication systems due to the
overwhelming demand of bandwidth hungry applications. As the backbone
infrastructure of modern communication networks, the optical fiber transmission
system undergoes a significant capacity growth over decades by exploiting
available physical dimensions (time, frequency, quadrature, polarization and
space) of the optical carrier for multiplexing. For each dimension, stringent
orthogonality must be guaranteed for perfect separation of independent
multiplexed signals. To catch up with the ever-increasing capacity requirement,
it is therefore interesting and important to develop new multiplexing
methodologies relaxing the orthogonal constraint thus achieving better spectral
efficiency and more flexibility of frequency reuse. Inspired by the idea of
non-orthogonal multiple access (NOMA) scheme, here we propose a digital domain
power division multiplexed (PDM) transmission technology which is fully
compatible with current dual polarization (DP) coherent optical communication
system. The coherent optical orthogonal frequency division multiplexing
(CO-OFDM) modulation has been employed owing to its great superiority on high
spectral efficiency, flexible coding, ease of channel estimation and robustness
against fiber dispersion. And a PDM-DP-CO-OFDM has been theoretically and
experimentally demonstrated with 100Gb/s wavelength division multiplexing (WDM)
transmission over 1440km standard single mode fibers (SSMFs). Two baseband
quadrature phase shift keying (QPSK) OFDM signals are overlaid together with
different power levels. After IQ modulation, polarization multiplexing and long
distance fiber transmission, the PDM-DP-CO-OFDM signal has been successfully
recovered in the typical polarization diversity coherent receiver by successive
interference cancellation (SIC) algorithm.
