A new approach has been developed for quantum cascade lasers to improve beam quality. Using two carefully designed high-reflection coatings, the fundamental mode was selected to have the lowest threshold current and the horizontal far-field pattern has been greatly improved, in good agreement with simulations.Introduction: The quantum cascade laser (QCL) is a compact and powerful semiconductor light source in mid-and far-infrared wavelength ranges for applications such as chemical sensing and pollution monitoring. For an edge-emitting QCL, the far-field pattern is often multiple lobes because the higher-order transverse mode has the lowest threshold current owing to the smaller mirror loss [1]. The poor beam quality is not convenient for practical applications. To improve the far-field pattern, in the near-infrared region, Crump et al.[2] successfully shifted the lasing from the second-order mode to the fundamental mode by antireflection coating of Al 2 O 3 on the front facet of the device emitting at 975 nm. However, this approach is infeasible for the midinfrared QCLs owing to the difficulty of forming very thick Al 2 O 3 film.In this Letter, we report the design of two multilayer high-reflection coatings for the front and rear facets, respectively. By modifying the mirror loss of each mode, the fundamental mode can have the lowest threshold and the far-field pattern has been improved greatly.The active region of our device consists of 30 periods of strain-compensated In 0.68 Ga 0.32 As/In 0.37 Al 0.63 As well and barrier layers. The layer sequence and thickness are similar to those reported in [3]. Our device is 13 mm wide and 2 mm long. The emitting wavelength is 4.72 mm.