Absorption spectroscopy of gases and liquids is amongst the most widely used methods to measure molecular concentrations. It is used in various fields, amongst them are industrial leak testing, medical analysis and surgery, process control and monitoring. Trace gas analysis of low-mass molecules is preferably performed in the mid-IR wavelength region, where the line strength for many molecules are high. With the QCL, invented in 1994, this spectral range has a laser source that delivers sufficient output power in continuous-wave operation. The semiconductor laser is robust and operates in a wide temperature range. This work is dedicated to explore the capabilities of QCLs and improve their performance in the wavelength region from 3 to 26 mum. Our active region simulations are based on a Density Matrix model. The choice of basis wavefunctions is verified and a method to find the optimal injection barrier is presented. The influence of different interface roughness models is discussed. Our model agrees well with the full quantum Non-Equilibrium Green’s Function model and with experiments. In this thesis we explore the short wavelength boundary of QCLs. Lasing emission around 3.3 mum requires highly strained active region material. We investigate in detail active region designs, growth optimization, the impact of intervalley scattering and interface roughness. We present a device emitting at 3.4 mum with dissipation values of only 250 mW and threshold currents as low as 16 mA in pulsed operation. A boxcar experiment with a 5.6 ms long pulse shows stable spectral behaviour of DFB devices, an important requirement for spectroscopic applications. We perform genetic optimizations of devices in the range from 4 to 26 mum and investigate active region design parameters. The optimizations are performed on ”seed” designs of published devices and designs from our own group. The current record design in wallplug efficiency for 9 mum is optimized. The design was extracted from literature and processed along with the optimized structure. Comparing the measurements, we improve the slope efficiency from 1.9 to 2.5 W/A, the wallplug efficiency from 9 to 12 % and the dynamical range from 1.5 to 2.1. For all optimizations, the seed and optimized structures are compared, resulting in some common strategies for optimization. The active region designs are explored experimentally as single stacks and broadband designs. Heterogeneous stacking is discussed and application examples for DFB, external cavity and comb operation are shown. An attempt for spectral coverage of a full octave is presented. We show laser emission spanning from 1090 to 1960 cm-1 at 80 K
