A detailed investigation of the nonlinear multimodal interference in a shortgraded-index multimode optical fiber is presented. The analysis is performedfor a specific device geometry, where the light is coupled in and out of themultimode fiber via single-mode fibers. The same device geometry was recentlyused to obtain ultra-low-loss coupling between two single-mode optical fiberswith very different mode-field diameters. Our results indicate the potentialapplication of this simple geometry for nonlinear devices, such as in nonlinearswitching, optical signal processing, or as saturable absorbers in mode-lockedfiber lasers. Saturable absorption in this all-fiber configuration is discussedand it is shown that it provides attractive properties that can potentially beused in high pulse energy mode-locked fiber lasers.
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