We propose fast and robust beam coupling in a three waveguide directional coupler based on Lewis-Riesenfeld invariant theory. This method provides shortcuts to optical waveguide couplers based on coherent tunneling adiabatic passage of photons. The design and performance of directional couplers using the invariant-based inverse engineering are discussed. Beam coupling is achieved by engineering the evolution of the eigenmodes of the invariant. We show that the invariant-based inverse engineering scheme reduces coupler length as compared with the common adiabatic scheme. The device is robust against coupling coefficient variations and has broadband characteristics.
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