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A multimode interference(MMI)structure is designed to simplify the fabrication of quantum cascade laser(QCL)phase-locked arrays.The MMI geometry is optimized with a sufficient output channel distance to accommodate conven-tional photolithography and wet etching process by which power amplifier array is fabricated without using the complicated two-step etching-regrowth or dry etching technique.The far-field pattern with periodically modulated peaks reveals that the beams from the arrays are phase-locked.Furthermore,the frequency tuning performance of the MMI-based phase-locked arrays is studied using the Littrow-configuration external cavity structure.A wavelength tuning range of more than 60 cm-1 is demonstrated,which will eventually realize the high power,frequency tunable,large-scale phase-locked arrays,and their application in spectroscopy.