hi all,
The basic propagation equation that governs pulse evolution inside a single-mode fiber,considering expanding B to the third order of is given by (Agrawal, 2010)(see figure for equation).
Upon propagation in a linear lossless dispersive medium, a monochromatic plane wave of frequency traveling a distance z in the z direction undergoes a phase shift and the pulse will be broadened. Figure shows the propagation of an initially unchirped Gaussian pulse
through a fiber with anomalous dispersion (B2 = GVD < 0). The pulse remains Gaussian, but its width expands and it becomes chirped with a decreasing chirp parameter (downchirped).In this case the pulse has a decreasing instantaneous frequency.