  We propose a robust and broadband integrated terahertz (THz) coupler based on
the in-plane surface plasmon polaritons (SPPs) waveguides, conducted with the
quantum coherent control -- Stimulated Raman Adiabatic Passage (STIRAP). Our
coupler consists of two asymmetric specific curved corrugated metallic
structures working as the input and output SPPs waveguides, and one straight
corrugated metallic structure functioning as the middle SPPs waveguide. From
the theoretical and simulated results, we demonstrate that the SPPs can be
efficiently transfered from the input to the output waveguides. Our device is
robust against the perturbations of geometric parameters, and meanwhile it
manifests broadband performance (from 0.3 THz to 0.8 THz) with the high
transmission rate over 70$\%$. The in-plane THz coupler can largely simplify
the fabrication process, which will make contribution to develop compact and
robust integrated THz devices and promote the future applications in all
optical network and THz communications.
