Unidirectional couplers are those which only coupler the incident wave ie. the wave given as input. Unidirectional coupling, in a system of two coupled oscillators, represents the limit when one of the oscillator oscillates while the other remains at equilibrium, e.g. does not move. This corresponds to the limit of one of the mass becoming infinite, in the case of mass-spring oscillators, or one of the strings becoming of infinite length, for pendulums.
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Unidirectional coupling Wikipedia(Text) CC BY-SA