Basis functions in spatial domain of Fourier coefficients (of order = 0, 1, and 12) , for even spectral function (a) and , for odd spectral function (b). The spectral functions are finite for the interval from 0 GHz to 1000 GHz for which the centre frequency = 500 GHz and bandwidth = 1000 GHz follow. The basis functions in the spatial domain are given by a sum or difference of two sinc functions, depending each on the optical path difference ,, and . Furthermore, the sinc functions are modulated by sine or cosine with and in the argument. The sinc functions for fixed become unity at , obviously increasing in .