When coherent light is launched into a multimode optical fiber, a speckle pattern, namely, modal noise, can be observed at the exit face of the fiber as a result of the random interference of many different propagation modes. Changes in the source frequency can also cause modal noise variations in time at the exit face. The width of the frequency correlation function of a time varying modal noise is directly related to the modal dispersion of light transmission through a multimode optical fiber. In this paper, the frequency correlation functions for both step- and graded-index fibers are analyzed as a function of the source frequency difference. To derive the frequency correlation function, the phase of each mode in fibers is calculated by using the modal delay along the optical path of the geometrical light ray.

© 1989 Optical Society of America

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