A method for determining an in-band noise parameter on an optical signal-under-test propagating along an optical communication link and comprising a data-carrying signal contribution of any arbitrary degree of polarization and a noise contribution within an optical signal bandwidth, the method comprising: obtaining a test optical spectrum trace of said optical signal-under-test at a test point along said optical communication link, said test optical spectrum trace corresponding to a spectral range encompassing at least a portion of said optical signal bandwidth; mathematically discriminating (408, 410) said signal contribution from said noise contribution in said signal-under-test, within said optical signal bandwidth, using said test optical spectrum trace; and determining said in-band noise parameter on said optical signal-under-test at least from the mathematically discriminated noise contribution; characterized in that the method further comprises obtaining a reference optical spectrum trace of a reference signal comprising a signal contribution spectrally representative of the signal contribution of said signal-under-test, and a noise contribution which is at least approximately known over said optical signal bandwidth, said optical signal-under-test comprising one of a polarization-multiplexed data-carrying signal and a signal that is being polarization scrambled at a rate significantly faster than the electronic detection bandwidth of a spectral resolution means used to acquire said test optical spectrum trace; in that the method further comprises estimating a spectral shape trace of said signal contribution in said signal-under-test using said reference optical spectrum trace; and in that said signal contribution is mathematically discriminated (408, 410) from said noise contribution in said signal-under-test using said spectral shape trace and said test optical spectrum trace. obtaining a test optical spectrum trace of said optical signal-under-test at a test point along said optical communication link, said test optical spectrum trace corresponding to a spectral range encompassing at least a portion of said optical signal bandwidth; mathematically discriminating (408, 410) said signal contribution from said noise contribution in said signal-under-test, within said optical signal bandwidth, using said test optical spectrum trace; and determining said in-band noise parameter on said optical signal-under-test at least from the mathematically discriminated noise contribution; characterized in that the method further comprises obtaining a reference optical spectrum trace of a reference signal comprising a signal contribution spectrally representative of the signal contribution of said signal-under-test, and a noise contribution which is at least approximately known over said optical signal bandwidth, said optical signal-under-test comprising one of a polarization-multiplexed data-carrying signal and a signal that is being polarization scrambled at a rate significantly faster than the electronic detection bandwidth of a spectral resolution means used to acquire said test optical spectrum trace; in that the method further comprises estimating a spectral shape trace of said signal contribution in said signal-under-test using said reference optical spectrum trace; and in that said signal contribution is mathematically discriminated (408, 410) from said noise contribution in said signal-under-test using said spectral shape trace and said test optical spectrum trace.