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,