A method for determining an in-band noise parameter on an input optical signal (P(λ)) having a data-carrying signal contribution (S(λ)) and a noise contribution (N(λ)) within an optical signal bandwidth, said signal contribution being at least partly polarized and said noise contribution being mostly unpolarized, the method comprising: acquiring, for a number n SOP of varied State-Of-Polarization analysis conditions of the input optical signal (P(λ)), n SOP polarization-analyzed optical spectrum traces (Pa(λ)), the distribution of said SOP analysis conditions being approximately known; mathematically discriminating said signal contribution from said noise contribution within said optical signal bandwidth using said polarization-analyzed optical spectrum traces (Pa(λ)), said mathematically discriminating comprising: obtaining a differential polarization response (S'(λ)) that is related to the optical spectrum of said signal contribution (S(λ)) by a constant of proportionality; estimating the constant of proportionality of a differential polarization response (S'(λ)) to the optical spectrum of said signal contribution (S(λ)) as a function of said number nsop; estimating the optical spectrum of said noise contribution N(λ) from said input optical signal (P(λ)), within said optical signal bandwidth using said constant of proportionality and said differential polarization response (S'(λ)); and determining said in-band noise parameter on said input optical signal from the mathematically discriminated noise contribution. acquiring, for a number n SOP of varied State-Of-Polarization analysis conditions of the input optical signal (P(λ)), n SOP polarization-analyzed optical spectrum traces (Pa(λ)), the distribution of said SOP analysis conditions being approximately known; mathematically discriminating said signal contribution from said noise contribution within said optical signal bandwidth using said polarization-analyzed optical spectrum traces (Pa(λ)), said mathematically discriminating comprising: obtaining a differential polarization response (S'(λ)) that is related to the optical spectrum of said signal contribution (S(λ)) by a constant of proportionality; estimating the constant of proportionality of a differential polarization response (S'(λ)) to the optical spectrum of said signal contribution (S(λ)) as a function of said number nsop; estimating the optical spectrum of said noise contribution N(λ) from said input optical signal (P(λ)), within said optical signal bandwidth using said constant of proportionality and said differential polarization response (S'(λ)); and obtaining a differential polarization response (S'(λ)) that is related to the optical spectrum of said signal contribution (S(λ)) by a constant of proportionality; estimating the constant of proportionality of a differential polarization response (S'(λ)) to the optical spectrum of said signal contribution (S(λ)) as a function of said number nsop; estimating the optical spectrum of said noise contribution N(λ) from said input optical signal (P(λ)), within said optical signal bandwidth using said constant of proportionality and said differential polarization response (S'(λ)); and determining said in-band noise parameter on said input optical signal from the mathematically discriminated noise contribution.