Four wave mixing generally occurs when two or more different wavelengths from two or more sources are launched into the fibre, resulting in a new wavelength known as idler (different from the given wavelengths). Here in this paper the efficiency of the generation of idler and the power of idler will be numerically simulated for two wave fibre transmissions. From this simulation, a curve will be obtained between power of idler and wavelength separation between signal and pump source, which will be used to propose a power independent method for extraction of dispersion parameters of a fibre.