A method for determining a noise parameter of an input optical signal having a data-carrying signal contribution and a noise contribution within an optical signal bandwidth, the method comprising: obtaining at least two optical spectrum traces from said input optical signal, said optical spectrum traces being taken under different conditions such that they show different non-zero signal-to-noise ratios; mathematically discriminating said signal contribution from said noise contribution within said optical signal bandwidth based on a mathematical combination of said optical spectrum traces; determining an in-band noise level on said input optical signal from the discriminated noise contribution; and determining the noise parameter from the determined in-band noise level, the noise parameter being indicative of the noise contribution within the optical signal bandwidth; wherein the signal contribution and the noise contribution have at least one of different degrees of polarization and different states of polarization from one another and wherein said obtaining at least two optical spectrum traces comprises: producing at least two samples of the input optical signal, under different polarization analysis conditions, states of polarization of the two samples being arbitrary relative to said input optical signal such that the data-carrying contribution is not substantially suppressed on any of said at least two samples, said at least two samples exhibiting mutually different non-zero signal-to-noise ratios; and acquiring an optical spectrum of each one of the first and the second samples to obtain said optical spectrum traces; wherein said discriminating comprises: calculating, from said optical spectrum traces, a difference optical spectrum substantially indicative of said data-carrying signal contribution; and solving said noise contribution using said optical spectrum traces and said difference optical spectrum. obtaining at least two optical spectrum traces from said input optical signal, said optical spectrum traces being taken under different conditions such that they show different non-zero signal-to-noise ratios; mathematically discriminating said signal contribution from said noise contribution within said optical signal bandwidth based on a mathematical combination of said optical spectrum traces; determining an in-band noise level on said input optical signal from the discriminated noise contribution; and determining the noise parameter from the determined in-band noise level, the noise parameter being indicative of the noise contribution within the optical signal bandwidth; wherein the signal contribution and the noise contribution have at least one of different degrees of polarization and different states of polarization from one another and wherein said obtaining at least two optical spectrum traces comprises: producing at least two samples of the input optical signal, under different polarization analysis conditions, states of polarization of the two samples being arbitrary relative to said input optical signal such that the data-carrying contribution is not substantially suppressed on any of said at least two samples, said at least two samples exhibiting mutually different non-zero signal-to-noise ratios; and acquiring an optical spectrum of each one of the first and the second samples to obtain said optical spectrum traces; wherein said discriminating comprises: calculating, from said optical spectrum traces, a difference optical spectrum substantially indicative of said data-carrying signal contribution; and solving said noise contribution using said optical spectrum traces and said difference optical spectrum. producing at least two samples of the input optical signal, under different polarization analysis conditions, states of polarization of the two samples being arbitrary relative to said input optical signal such that the data-carrying contribution is not substantially suppressed on any of said at least two samples, said at least two samples exhibiting mutually different non-zero signal-to-noise ratios; and acquiring an optical spectrum of each one of the first and the second samples to obtain said optical spectrum traces; wherein said discriminating comprises: calculating, from said optical spectrum traces, a difference optical spectrum substantially indicative of said data-carrying signal contribution; and solving said noise contribution using said optical spectrum traces and said difference optical spectrum. calculating, from said optical spectrum traces, a difference optical spectrum substantially indicative of said data-carrying signal contribution; and solving said noise contribution using said optical spectrum traces and said difference optical spectrum.