Frequency correlation function of modal noise in multimode optical fibers

After transmission of highly coherent light through a multimode optical fiber, it is possible to observe a speckle pattern, namely, a modal noise, at its exit face caused by the interference of many different propagation modes. Changes of the source frequency can also cause modal noise variations in time at the exit face. The statistics of the modal noise appearing at the exit face of a fiber can thus be used to study the properties of the fiber and source. Measurements have been made of the frequency correlation function of modal noise in multimode optical fiber not only at the exit face of the fiber but also at a nearby point. A geometrical approximation is used to derive the frequency correlation function at the detecting position for various detecting positions. Experimental results are given and compared with the theory.>