A method of monitoring optical parameters of a modulated optical signal comprises receiving a first optical power, Xi, of said optical signal for a first bandwidth, Bi, across the said optical signal and receiving a second optical power, Xj, of said optical signal for a second bandwidth, Bj, across the said optical signal. The method comprises obtaining a power spectral density description of said optical signal. The method comprises determining an optical signal to noise ratio of said optical signal. The optical signal to noise ratio depends on said first optical power, Xi, said second optical power, Xj, and said power spectral density description. The method also comprises generating a data signal indicative of said optical signal to noise ratio.