Frequency noise of free-running room temperature quantum cascade lasers

Narrow-linewidth lasers exhibiting a high spectral purity have important applications in various fields, such as high-resolution spectroscopy, coherent optical communications or time and frequency metrology, to name a few. With a proper feedback loop for linewidth reduction, such lasers can be used as ultra-stable reference for low phase noise microwave generation in frequency metrology. Since mid-infrared quantum cascade lasers (QCLs) are expected to have a narrow intrinsic linewidth (due to a linewidth-enhancement factor α H close to 0), we have recently investigated the frequency noise properties of commercial free-running singlemode QCLs emitting in the 4.6-µm wavelength range and operated in CW mode near room temperature.