Low-Loss Coupling Between Two Single-Mode Optical Fibers With Different Mode-Field Diameters Using A Graded-Index Multimode Optical Fiber

We present a method for ultra-low-loss coupling between two single-mode optical fibers with different mode-field diameters using multimode interference in a graded-index multimode optical fiber. We perform a detailed analysis of the interference effects and show that the graded-index fiber can also be used as a beam expander or condenser. The results are important for devices in which optical fibers with different mode-field diameters are coupled in series, such as in ultra-short-pulse fiber ring lasers, or in optical fiber communication links. © 2011 Optical Society of America