Free-running frequency stability of mid-infrared quantum cascade lasers

The intrinsic frequency fluctuations of two single-mode quantum cascade (QC) distributed-feedback lasers operating continuously at a wavelength of 8.5 mum are reported. A Doppler-limited rovibrational resonance of nitrous oxide is used to transform the frequency noise into measurable intensity fluctuations. The QC lasers, along with recently improved current controllers, exhibit a free-running frequency stability of 150 kHz over a 15-ms time interval.