Systems and methods include receiving (S11) data for a plurality of elements associated with an optical network; determining (S12) an incremental noise penalty for each element of the plurality of elements based on the received data; and storing (S13) the incremental noise penalty for each element of the plurality of elements. The steps can further include determining (S14) Signal-to-Noise Ratio (SNR) across an optical path in the optical network by concatenating associated incremental noise penalties for each element in the optical path along with corrections. The present disclosure includes a fast, nonlinear estimation process with improved accuracy for low loss spans compared to traditional closed-form GN models, as well as a method to determine the coherent nonlinear penalty in an arbitrary concatenation of mixed heterogeneous fibers which is not considered by existing fast nonlinear interference calculation methods.