A communication apparatus for correcting a situation of a spectrum inverted signal includes a channel estimation module and an equalization module. The channel estimation module determines a channel estimation parameter, and receives at least one frame signal to generate a convolution restored frame signal corresponding to the frame signal. The equalization module includes a first computation circuit and a second computation circuit. The first computation circuit receives the channel estimation parameter and the convolution restored frame signal to generate a transformation channel estimation parameter and a transformed convolution restored frame signal. The second computation circuit receives the transformed channel estimation parameter and the transformed convolution restored frame signal to generate an original frame signal corresponding to the frame signal. The first computation circuit further feeds back a transient original frame signal to the channel estimation module to update the channel estimation parameter.

Methods and systems are provided for wavelength shift elimination during spectral inversion in optical networks. The method includes receiving an input optical signal, and generating a combined optical signal by combining, by Bragg scattering, the input optical signal having an input wavelength with a first pump signal having a first wavelength. The method further includes converting the combined optical signal into an output optical signal, by phase-conjugation, using a second pump signal having a second wavelength. The output optical signal has the same wavelength as the input optical signal.

A method (10) of non-linear propagation impairment equalisation, the method comprising the steps of: a. receiving (12) communications traffic carried by an optical communications signal transmitted over an optical communications link; b. generating (14) a time dependent filter representation of a nonlinear time-variant impulse response of the inverse of the optical communications link; and c. applying (16) the time dependent filter representation to the received communications traffic to form non-linear propagation impairment equalised communications traffic. An optical communications link nonlinear propagation impairment equaliser and optical communications signal receiver apparatus are also provided.

The invention relates to an optical spectrum inverter, configured for counteracting phase distortion effects in an optical channel over a predefined frequency range, to an inverter node, configured for duplex operation in at least two wavelength channels, and to a method for counteracting phase distortion effects in an optical channel. The optical spectrum inverter comprises an optical receiver comprising a polarization diversity unit configured for polarization resolved detection, wherein the optical receiver is configured for receiving an optical input signal comprising a first wavelength span and for converting the optical input signal to an electrical signal, and an optical modulator connectable to the optical receiver and configured for modulating the electrical signal onto an optical signal, wherein the optical modulator comprises an optical filter which is configured for selecting a second wavelength span of the optical signal corresponding to an optical output side band of an inverted spectrum of the optical channel such that phase distortion effects are counteracted at least partly over the predefined frequency range. In this way, an optical spectrum inverter is provided which is simple and cost-effective to realize and allows higher optical power levels in conjunction with a higher capacity per optical channel and is also adapted for longer transmission distances.

The invention relates to an optical spectrum inverter, configured for counteracting phase distortion effects in an optical channel over a predefined frequency range, to an inverter node, configured for duplex operation in at least two wavelength channels, and to a method for counteracting phase distortion effects in an optical channel. The optical spectrum inverter comprises an optical receiver comprising a polarization diversity unit configured for polarization resolved detection, wherein the optical receiver is configured for receiving an optical input signal comprising a first wavelength span and for converting the optical input signal to an electrical signal, and an optical modulator connectable to the optical receiver and configured for modulating the electrical signal onto an optical signal, wherein the optical modulator comprises an optical filter which is configured for selecting a second wavelength span of the optical signal corresponding to an optical output side band of an inverted spectrum of the optical channel such that phase distortion effects are counteracted at least partly over the predefined frequency range. In this way, an optical spectrum inverter is provided which is simple and cost-effective to realize and allows higher optical power levels in conjunction with a higher capacity per optical channel and is also adapted for longer transmission distances.