A monolithic coupling scheme for mid-infrared quantum cascade laser arrays is investigated with respect to brightness enhancement. The tree-shaped resonator enables parallel coupling of six laser elements into a single element by means of several Y-junctions. Phase-locking is observed on the basis of far field analysis, and leads to in-phase emission on both sides of the device. The experimental results match calculated far field profiles and demonstrate a high level of modal control when driven far above threshold. Whereas optical power measurements confirm negligible coupling losses, the slope efficiency is below the theoretically expected value, which is attributed to modal competition. Additional evaluation of near fields and spectral characteristics provides background on the modal dynamics of the sophisticated cavity and reveals limitations to coherent beam combining. The findings pave the way to improved coupling efficiency and brightness scaling of a single facet emitting compact quantum cascade laser array.