An apparatus for determining a signal distortion of an optical signal, the optical signal comprising a number of frequency channels (m), the signal distortion being introduced by an optical transmission channel having a number of transmission spans (i, k), the apparatus comprising: a provider being configured to provide a signal power of the optical signal for each frequency channel (m) and for each transmission span (i, k) to obtain a number of signal powers, to provide a chromatic dispersion, CD, value for each frequency channel (m) and for each transmission span (i, k) to obtain a number of CD values, each CD value indicating a CD of the optical signal, and to provide a differential group delay, DGD, value for each frequency channel (m) and for each transmission span (i, k) to obtain a number of DGD values, each DGD value indicating a polarization mode dispersion of the optical signal; a determiner being configured to determine a number of first distortion measures (κ) upon the basis of the CD values and the DGD values, each first distortion measure (κ) indicating a nonlinear signal distortion of the optical signal induced on a single transmission span (i, k), and to determine a number of second distortion measures (ρ) upon the basis of the CD values and the DGD values, each second distortion measure (ρ) indicating a correlation between nonlinear signal distortions of the optical signal induced on transmission span (i) and on transmission span (k); and a combiner being configured to combine the signal powers, the first distortion measures (κ), and the second distortion measures (ρ) to obtain the signal distortion of the optical signal; wherein the determiner is configured to determine the first distortion measures (κ) or the second distortion measures (ρ) according to the following equations: κ i = κ CD i = ∑ k = 1 i − 1 D k L k, DGD i = ∑ k = 1 i − 1 PMD k 2 L k = 〈 | E → CD i, DGD i nl − E → 0 | 2 〉 ρ m CD i, DGD i, CD k, DGD k = Re 〈 E → CD i, DGD i nl − E → 0 E → CD k, DGD k nl − E → 0 † 〉 / κ i κ k wherein κ denotes a first distortion measure, ρ denotes a second distortion measure, CD denotes a chromatic dispersion value, DGD denotes a differential group delay value, D denotes a chromatic dispersion coefficient, PMD denotes a polarization mode dispersion coefficient, L denotes a length of a transmission span, i denotes a transmission span index, k denotes a further transmission span index, m denotes a frequency channel index, E → CD i, DGD i nl denotes a distorted electrical field vector, E → CD k, DGD k nl denotes a further distorted electrical field vector, and E 0 denotes an undistorted electrical field vector. a provider being configured to provide a signal power of the optical signal for each frequency channel (m) and for each transmission span (i, k) to obtain a number of signal powers, to provide a chromatic dispersion, CD, value for each frequency channel (m) and for each transmission span (i, k) to obtain a number of CD values, each CD value indicating a CD of the optical signal, and to provide a differential group delay, DGD, value for each frequency channel (m) and for each transmission span (i, k) to obtain a number of DGD values, each DGD value indicating a polarization mode dispersion of the optical signal; a determiner being configured to determine a number of first distortion measures (κ) upon the basis of the CD values and the DGD values, each first distortion measure (κ) indicating a nonlinear signal distortion of the optical signal induced on a single transmission span (i, k), and to determine a number of second distortion measures (ρ) upon the basis of the CD values and the DGD values, each second distortion measure (ρ) indicating a correlation between nonlinear signal distortions of the optical signal induced on transmission span (i) and on transmission span (k); and a combiner being configured to combine the signal powers, the first distortion measures (κ), and the second distortion measures (ρ) to obtain the signal distortion of the optical signal; wherein the determiner is configured to determine the first distortion measures (κ) or the second distortion measures (ρ) according to the following equations: κ i = κ CD i = ∑ k = 1 i − 1 D k L k, DGD i = ∑ k = 1 i − 1 PMD k 2 L k = 〈 | E → CD i, DGD i nl − E → 0 | 2 〉 ρ m CD i, DGD i, CD k, DGD k = Re 〈 E → CD i, DGD i nl − E → 0 E → CD k, DGD k nl − E → 0 † 〉 / κ i κ k wherein κ denotes a first distortion measure, ρ denotes a second distortion measure, CD denotes a chromatic dispersion value, DGD denotes a differential group delay value, D denotes a chromatic dispersion coefficient, PMD denotes a polarization mode dispersion coefficient, L denotes a length of a transmission span, i denotes a transmission span index, k denotes a further transmission span index, m denotes a frequency channel index, E → CD i, DGD i nl denotes a distorted electrical field vector, E → CD k, DGD k nl denotes a further distorted electrical field vector, and E 0 denotes an undistorted electrical field vector.