Speckle In Optical Fibers

With a sufficiently narrow source spectrum, and either fiber movement or source frequency shift, a changing speckle pattern exists at a multimode optical fiber output plane which results in signal power fluctuations known as "modal noise." For the case of a step-index optical fiber, we report the measurement and analysis of the speckle spatial frequency distribution as a function of fiber parameters, and the correlation of two speckle patterns as a function of source frequency difference. We show how knowledge of the frequency correlation function and the source spectrum can be used to determine the speckle contrast. Again for a step-index fiber, we report the analysis and measurement of the probability density function of the optical power received by a detector at the end of a fiber as a function of the speckle's contrast and including the possibility of spatial filtration of the speckle, as might occur at a misaligned connector.