Quantum cascade (QC) lasers are compact and versatile light sources suitable for a broad range of absorption spectroscopy based molecular sensing applications. However, for most of such sensing applications, single-mode operation of QC lasers is a prerequisite. Conventional single-mode QC lasers, e.g., distributed feedback (DFB)  or external cavity QC lasers , have much higher cost than multi-mode simple ridge QC lasers, mainly due to their complicated and demanding device fabrication or time-consuming system integration and alignment processes. In order to achieve more cost-effective single-mode QC lasers, we demonstrate a novel type of laser cavity design which consists of an asymmetric Mach-Zehnder (AMZ) interferometer structure monolithically integrated in a conventional Fabry-Perot (FP) cavity with simple ridge waveguide and as-cleaved facets. Strong wavelength selectivity is introduced by the properly designed AMZ interferometer whose transmission spectrum comprises equidistantly spaced narrow peaks, which in turn selects a specific FP mode associated with the entire laser cavity near the optical gain spectrum peak, effectively facilitating single-mode operation of the laser. Continuously wavelength-tunable single-mode operation of QC lasers is achieved in pulsed mode from 80 K to room temperature and in continuous-wave (CW) mode with high side-mode suppression ratio (SMSR) up to ∼35 dB. The observed spectral characteristics of the tested lasers are described with satisfying accuracy by our model developed for such cavity structures. The fabrication process for such AMZ interferometer type cavities is identical to that for simple ridge lasers, therefore providing a promising solution to achieving more cost-effective single-mode QC lasers.