Optical impairments such as dispersion and fibre nonlinearity are
compensated by generating a pre-distorted electrical signal at the
transmitter. This signal is modulated onto a carrier signal, so that it
is upconverted in frequency. This up converted signal is then used to
modulate an optical source. Generally the optical signal will have two
sidebands, one of which has the correctly pre-distorted information and
the other which is unwanted. Information in the unwanted optical sideband
is either filtered optically or electrically. In the preferred
embodiments, the transmitter uses a tunable semiconductor laser with an
integrated electroabsorption modulator to modulate the light. The
preferred receiver is a coherent receiver with a tunable local oscillator
laser. The receiver uses an electrical filter to remove the information
in the unwanted sideband.