  In this paper, we introduce Holographic Spectral Multiplexing (HSM) as a
novel technique to enable multiple-input multiple-output (MIMO) communication
in optical networks. HSM uses the spectral space of ultrashort laser pulses to
create line codes in the form of 2D holograms. The pulse processing is
performed in the temporal Fourier domain by spatially dispersing the pulse
frequency components in a spectral processing device (SPD). The 2D holograms
are composed of the patterns of intensity disparities that an SLM inscribes on
the spectrally-decomposed Fourier plane of the pulse. The holographic line
codes defined in this way transform the ultrashort laser pulses into
high-entropy data symbols, hence, enhance the communication's spectral
efficiency. Unlike conventional optical multiplexing schemes (e.g., TDM, WDM,
or SDM), HSM does not physically or abstractly separate the communication
propagation space into subchannels. Rather, HSM realizes a MIMO communication
paradigm by allowing the photonic waves under the pulse envelope to propagate
in the same space so they scatter and interfere by chromatic dispersion. This
allows HSM to form beams between the pixels of SLM at the sender and receiver
sides and optimize the beam to adapt to channel scattering situations. In this
way, HSM delivers a rate gain that in the best case exponentially increases the
information rate of communication.
