A monolithic coupling scheme with two active waveguides merging into a single waveguide is presented for a mid-infrared GaAs/AlGaAs quantum cascade laser. The rapid development of unipolar quantum cascade lasers (QCLs) creates a basis for a broad variety of technological applications. Farfield patterns of the identical laser with a truncated mixing region result in a different superposition of the two modes with even and odd symmetry. In conclusion, the fraction of the even mode increases with increasing coupling length. The obtained results allow for designing coherent QCL Y-junctions, which open new possibilities for monolithic interferometric sensing devices.