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The frequency-noise power spectral density of a room-temperature distributed-feedback quantum cascade laser emitting at λ = 4.36 μm has been measured. An intrinsic linewidth value of 260 Hz is retrieved, in reasonable agreement with theoretical calculations. A noise reduction of about a factor 200 in most of the frequency interval is also found, with respect to a cryogenic laser at the same wavelength. A quantitative treatment shows that it can be explained by a temperature-dependent mechanism governing the transport processes in resonant tunnelling devices. This confirms the predominant effect of the heterostructure in determining shape and magnitude of the frequency noise spectrum in QCLs.

We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250 kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1 kHz, and its frequency is stabilized with an absolute traceability of 2×10−12. This source has allowed the measurement of the absolute frequency of a CO2 molecular transition with an uncertainty of about 1 kHz.

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic\ud spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral\ud region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes\ud a distributed feedback quantum cascade laser emitting at 4.33 lm. We achieved a laser optical\ud power buildup factor of 500, which corresponds to an intracavity laser power of 0.75 W. CO2\ud has been selected as the target molecule for the I-QEPAS demonstration. We achieved a\ud detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise\ud equivalent absorption coefficient of 1.4108 cm1 and a normalized noise-equivalent absorption of\ud 3.21010 W cm1 Hz1/2