We estimated experimentally the linewidth (0.28 MHz) of a distributed feedback quantum cascade laser emitting at 6.2 μm using optical feedbackinterferometry. The method is simply based on the analysis of the histogram of laser self-mixing fringe periods measured directly as voltage variation across the laser terminals. We assessed the optimal experimental conditions estimating the influence of the optical feedback strength on the interferometric phase noise and compared our results with those reported using conventional interferometric methods based on the analysis of the frequency noise power spectral density.