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To obtain maximum power transfer efficiency from one outside guide to another in a three-guide coupler, the differences among the propagation constants of the three propagating modes must be equal, i.e., must equal . This is not the case when all three guides are identical. By increasing either the width or index of the center guide relative to the outside guides, the differences between the propagation constants can be made equal. This can result in a significant increase in power transfer from one outside guide to the other. With equal spacing between the propagation constants of the modes, overall power transfer efficiency is limited only by radiation losses at the input and output of the three-guide coupler, due to the imperfect match between the mode shape of the input and output guides and the shape of the linear combination of the three modes of the coupler which best matches it. For couplers with reasonable guide spacing to guide width ratios and reasonable confinement, this loss is usually small, and power transfer efficiencies for power transfer from one outside guide to another can approach 100 percent. For example, with correctly adjusted β's, power transfer efficiency in a three-guide coupler with can be as high as 99.8 percent. Even with , efficiencies can be as high as 97.8 percent.