In this thesis we deploy mid-infrared quantum cascade lasers (QCLs) in external linear cavity (EC) and external ring cavity (ERC) configurations, with the aim to develop the platform as a source for high resolution gas spectroscopy. Initially, the temporal evolution of pulsed ERC-QCL system is compared to that of standard Fabry Perot (FP) QCLs to provide insight into the role played by the additional external feedback provided by ECs and ERCs. Time-resolved spectral measurements show that external feedback promotes single longitudinal mode operation on a shorter timescale than is the case for FP-QCLs. A room temperature wavelength tunable external ring cavity QCL is presented which shows improved performance over its linear counterpart due to its ability to support unidirectional emission regimes. Spectral comparisons between linear and ring cavities show that these regimes lead to improved performance due to the suppression of spatial hole burning (SHB), an instability common to linear laser resonators. An active modulation scheme based on a number of bespoke power amplifiers and bias-tees is developed. These systems are used to produce mode-locked pulse trains in external cavity and external ring cavity QCLs by modulating the QCL gain medium's drive current at frequencies matching the cavity's round trip frequency. Spectral and temporal comparisons are made between these two cases. Asymmetric pulse propagation is observed in the case of the ERC-QCL which has been suggested as promising platform for ultrashort mode-locked mid-infrared pulses.