Equalizer for an optical transmission system

An equaliser processes, in the electrical domain, a signal obtained from a path of an optical transmission system. The equaliser comprises N cascaded stages (where N ≥1). At least one of the stages comprises a cascade of a linear equalisation element and a non-linear equalisation element. The equaliser is able to compensate for both linear impairments, such as dispersion, and non-linear impairments. The cascaded linear and non-linear elements can simulate the effect of signal propagation through a fibre which has the opposite propagation parameters (e.g. attenuation, dispersion, non-linearity) to those of the propagation path experienced by a signal in the transmission system. The non-linear equalisation element can be a non-linear phase rotator which rotates phase of an input signal proportional to the squared modulus of the input signal amplitude. The linear equalisation element can comprise a digital filter, such as an Infinite Impulse Response (IIR) filter or a Finite Impulse Response (FIR) filter.