In an optical communications link, phase jitter of the signal caused by the combined effects of mode hopping in the optical source and intramodal distortion in the fiber. Note: Mode hopping causes random wavelength changes which in turn affect the group velocity, i.e., the propagation time. Over a long length of fiber, the cumulative effect is to create jitter, i.e., mode partition noise. The variation of group velocity creates the mode partition noise.