Quantum cascade lasers (QCLs) are optical sources exploiting radiative intersubband transitions within the conduction band of semiconductor heterostructures. The opportunity given by the broad span of wavelengths that QCLs can achieve, from mid-infrared to terahertz, leads to a wide number of applications such as absorption spectroscopy, optical countermeasures and free-space communications requiring stable single-mode operation with a narrow linewidth and high output power. One of the parameters of paramount importance for studying the high-speed and nonlinear dynamical properties of QCLs is the linewidth enhancement factor (LEF). The LEF quanties the coupling between the gain and the refractive index of the QCL or, in a similar manner, the coupling between the phase and the amplitude of the electrical field. Prior work focused on experimental studies of the LEF for pump currents above threshold but without exceeding 12% of the threshold current at 283K and 56% of the threshold current at 82K. In this work, we use the Hakki-Paoli method6 to retrieve the LEF for current biases below threshold. We complement our ndings using the self-mixing interferometry technique to obtain LEFs for current biases up to more than 100% of the threshold current. These insets are meaningful to understand the behavior of QCLs, which exhibit a strongly temperature sensitive chaotic bubble when subject to external optical feedback.