Intrinsic linewidth of quantum cascade laser frequency combs

Optical frequency combs based on quantum cascade lasers have recently been demonstrated in the mid- and far-infrared spectral regions, opening the possibility for broadband, compact spectrometers. The successful operation of these systems will depend on understanding the frequency noise of these lasers, whose mode-locking dynamics leads to an almost constant optical power rather than pulse generation. We demonstrate that the four-wave mixing process-- responsible for comb formation--effectively correlates the quantum frequency noise of the individual comb modes. The plateau observed in the high-frequency portion of the noise spectrum is attributed to the quantum noise limit. This result proves that four-wave mixing introduces no additional frequency noise, showing that quantum cascade laser combs are well suited for high-resolution spectroscopy applications.