A fiber fused coupler is one of the most popular passive components for wavelength multi/demultiplexing or branching/combining of optical signal. They are used to split optical signals between two fibers, or to combine optical signals from two fibers into one fiber. Fused couplers do suffer from some disadvantage. Multimode fused couplers are mode dependent. Single fused couplers only transmit one mode, so they do not suffer from mode dependency.
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