Frequency noise of free-running 4.6 μm distributed feedback quantum cascade lasers near room temperature

The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 <i>μ</i>m range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz²/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.