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.