In addition to the phase uctuation induced by spontaneous emission, instantaneous carrier variations in semiconductor lasers generate coupling between optical gain and refractive index. This coupling between phase and amplitude of the electric field in the optical cavity is driven by the linewidth enhancement factor, which is responsible for the optical linewidth broadening, occurrence of nonlinearities or gain asymmetry, due to the curvature difierence between the conduction and valence bands. This key parameter typically takes values between 2 and 6 in interband lasers with quantum well or quantum dot active media. In quantum cascade lasers, since the lasing transition occurs between two subbands of the conduction band that have therefore similar curvatures, the linewidth enhancement factor was expected to be naught. However sub-threshold linewidth enhancement factor was measured taking values from -0.5 to 0.5 and the above-threshold linewidth enhancement factor at room temperature was found between 0.2 and 2.4. In this work, the linewidth enhancement factor of a mid-infrared quantum cascade laser emitting around 5.6 μm is measured using either the wavelength shift under optical feedback or self-mixing interferometry, resulting in values ranging from 0.8 to 3. Furthermore, a strong increase of the linewidth enhancement factor with the pump current was observed, that can be explained by a relatively large gain compression in such structures, of the order of 5 × 10-15 cm3.