A method for spectrally characterizing an optical signal propagating along an optical communication link , said optical signal comprising a data-carrying signal contribution modulated at a symbol frequency (f sb ) within a data-carrying signal bandwidth and a noise contribution, said method comprising: measuring an optical power spectrum (P sη (f)) of said optical signal over a spectral range within said data-carrying signal bandwidth, said measured optical power spectrum (P sη (f)) comprising a data-carrying signal power spectrum contribution (P s (f)) associated with said data-carrying signal contribution of said optical signal and a noise power spectrum contribution (P η (f)) associated with said noise contribution of said optical signal; determining , from measurement, a spectral correlation function (P̃(f•)) for a set of pairs of spectral components of said optical signal , the spectral components in each pair being spectrally separated from each other by said symbol frequency (f sb ), said set of pairs being respectively centered on a corresponding set of center frequencies over a center frequency range within said spectral range, said measured spectral correlation function (P̃(f•)) relating a correlation intensity between the spectral components of each pair to the center frequency of the pair over said center frequency range; and obtaining , using a processor , a solution representing said data-carrying signal power spectrum contribution (P s (f)) based on said measured optical power spectrum (P sη (f)) of said optical signal , such that a calculated spectral correlation function for pairs of spectral components spectrally separated by said symbol frequency (f sb ) in said solution representing said data-carrying signal power spectrum contribution matches said measured spectral correlation function, wherein obtaining said solution representing said data-carrying signal power spectrum contribution (P s (f)) comprises: providing a nonlinear regression model relating said measured optical power spectrum and said measured spectral correlation function (P̃(f•)); and using said nonlinear regression model to determine said solution representing said data-carrying signal power spectrum contribution (P s (f)). measuring an optical power spectrum (P sη (f)) of said optical signal over a spectral range within said data-carrying signal bandwidth, said measured optical power spectrum (P sη (f)) comprising a data-carrying signal power spectrum contribution (P s (f)) associated with said data-carrying signal contribution of said optical signal and a noise power spectrum contribution (P η (f)) associated with said noise contribution of said optical signal; determining , from measurement, a spectral correlation function (P̃(f•)) for a set of pairs of spectral components of said optical signal , the spectral components in each pair being spectrally separated from each other by said symbol frequency (f sb ), said set of pairs being respectively centered on a corresponding set of center frequencies over a center frequency range within said spectral range, said measured spectral correlation function (P̃(f•)) relating a correlation intensity between the spectral components of each pair to the center frequency of the pair over said center frequency range; and obtaining , using a processor , a solution representing said data-carrying signal power spectrum contribution (P s (f)) based on said measured optical power spectrum (P sη (f)) of said optical signal , such that a calculated spectral correlation function for pairs of spectral components spectrally separated by said symbol frequency (f sb ) in said solution representing said data-carrying signal power spectrum contribution matches said measured spectral correlation function, <EOS>