In this paper we present the development of mid-infrared GaAs/AlGaAs QCLs technology and discuss basic characteristics of lasers fabricated at the Institute of Electron Technology. We also show that reliable simulation methods which can deal with the complicated physical phenomena involved in the quantum cascade lasers operation are necessary to predict the behaviour of new structures and optimize their performance. The developed lasers show the record pulse powers of 6 W at 77 K and up to 50 mW at 300 K. This has been achieved by careful optimization of the epitaxial process and by applying a high reflectivity metallic coating to the back facet of the laser. The devices have been successfully used in prototype ammonia detection system working in ppb range.