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In gas spectroscopy with distributed feedback quantum cascade lasers (DFB-QCLs), linewidth is an important measurement parameter. However, in standard usage, these lasers are used in pulsed operation, so that the laser linewidth can no longer be assumed to remain constant over the length of the pulse. For the instantaneous measurement of such dynamic linewidths, a suitable measurement technique was still lacking. Frequently, pulse lengths of 5 to 10 ns are used to obtain laser linewidths that will be suitable for gas measurements. In these experiments, standard photoconductive or photovoltaic mid-infrared detectors with bandwidths in the 1 MHz range are employed, so that the linewidth is averaged over the tuning range of the QCL. The linewidth averaging could be reduced by shorter pulses, but for pulses shorter than 5 ns, the Fourier limitation increases the linewidth again. We present a technique to measure the dynamic laser linewidth continuously within a laser pulse of 30 ns to several μs. Due to the use of high-bandwidth detectors working from 2 to 12 μm, laser linewidths can be determined much more accurately than with short pulses. Instantaneous linewidths of DFB-QCLs down to 60 MHz are obtained. This shows that QCLs are well suited for gas measurement in the mid-infrared. © 2004 American Institute of Physics.