A method (400; 800) for determining a noise parameter characterizing an optical signal-under-test having a signal contribution, an Amplified Spontaneous Emission (ASE) noise contribution and a non- ASE optical noise contribution within an optical-signal bandwidth, wherein said signal contribution is polarized and wherein said non-ASE optical noise contribution comprises a depolarized-signal contribution at least partly imparted from a non-linear effect to be characterized, said method comprising: acquiring (402; 802) a plurality of optical spectrum traces of the signal-under-test, the plurality of optical spectrum traces including a pair of mutually-orthogonal polarization-analyzed optical spectrum traces ( P ⊥ (λ), P // (λ) ) for each of a number n SOP of varied state-of-polarization analysis conditions of the signal-under-test; discriminating (406; 806) at least said non-ASE optical noise contribution from said ASE-noise contribution and said signal contribution based on known different spectral-shape properties between said non-ASE optical noise contribution, said ASE-noise contribution and said signal contribution, wherein said discriminating is carried out by using the plurality of optical spectrum traces and an extrema trace obtained by: i) calculating a ratio of the mutually-orthogonal polarization-analyzed optical spectrum traces ( P ⊥ (λ), P // (λ) ) to obtain a ratio trace (r(λ)), for each said n SOP acquisitions; ii) determining an extrema trace corresponding to minimum values (r min (λ)) or maximum values (r max (λ)) of the calculated ratio traces; and determining (408; 812) said noise parameter characterizing said signal-under-test using at least one of the discriminated non-ASE optical noise contribution and the discriminated ASE-noise contribution. acquiring (402; 802) a plurality of optical spectrum traces of the signal-under-test, the plurality of optical spectrum traces including a pair of mutually-orthogonal polarization-analyzed optical spectrum traces ( P ⊥ (λ), P // (λ) ) for each of a number n SOP of varied state-of-polarization analysis conditions of the signal-under-test; discriminating (406; 806) at least said non-ASE optical noise contribution from said ASE-noise contribution and said signal contribution based on known different spectral-shape properties between said non-ASE optical noise contribution, said ASE-noise contribution and said signal contribution, wherein said discriminating is carried out by using the plurality of optical spectrum traces and an extrema trace obtained by: i) calculating a ratio of the mutually-orthogonal polarization-analyzed optical spectrum traces ( P ⊥ (λ), P // (λ) ) to obtain a ratio trace (r(λ)), for each said n SOP acquisitions; ii) determining an extrema trace corresponding to minimum values (r min (λ)) or maximum values (r max (λ)) of the calculated ratio traces; and i) calculating a ratio of the mutually-orthogonal polarization-analyzed optical spectrum traces ( P ⊥ (λ), P // (λ) ) to obtain a ratio trace (r(λ)), for each said n SOP acquisitions; ii) determining an extrema trace corresponding to minimum values (r min (λ)) or maximum values (r max (λ)) of the calculated ratio traces; and determining (408; 812) said noise parameter characterizing said signal-under-test using at least one of the discriminated non-ASE optical noise contribution and the discriminated ASE-noise contribution. <EOS>