A comprehensive investigation of the frequency-noise spectral density of afree-running mid-infrared quantum-cascade laser is presented for the firsttime. It provides direct evidence of the leveling of this noise down to a whitenoise plateau, corresponding to an intrinsic linewidth of a few hundred Hz. Theexperiment is in agreement with the most recent theory on the fundamentalmechanism of line broadening in quantum-cascade lasers, which provides a newinsight into the Schawlow-Townes formula and predicts a narrowing beyond thelimit set by the radiative lifetime of the upper level.
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