1. A method of fiber Kerr nonlinear noise estimation in an optical transmission system, the method comprising: recovering received symbols from a received optical signal; isolating a noise component of the received optical signal from the received symbols and from either training symbols or estimated transmitted symbols determined from the received symbols; estimating coefficients of a matrix based on cross-correlations between the isolated noise component and the fields of a triplet of received symbols or training symbols or estimated transmitted symbols; estimating doublet correlations of the product or the quotient of the isolated noise component and the field of a received symbol or of a training symbol or of an estimated transmitted symbol; and estimating one or more parameters related to nonlinear noise based on the estimated coefficients of the matrix and based on the estimated doublet correlations.