A comprehensive investigation of the frequency-noise spectral density of a free-running midinfrared quantum-cascade laser is presented for the first time. It provides direct evidence of the leveling of this noise down to a white-noise plateau, corresponding to an intrinsic linewidth of a few hundred hertz. The experiment is in agreement with the most recent theory on the fundamental mechanism of line broadening in quantum-cascade lasers, which provides a new insight into the Schawlow-Townes formula and predicts a narrowing beyond the limit set by the radiative lifetime of the upper level.

Radiocarbon ((14)C) concentrations at a 43 parts-per-quadrillion level are measured by using saturated-absorption cavity ringdown spectroscopy by exciting radiocarbon-dioxide ((14)C(16)O(2)) molecules at the 4.5 μm wavelength. The ultimate sensitivity limits of molecular trace gas sensing are pushed down to attobar pressures using a comb-assisted absorption spectroscopy setup. Such a result represents the lowest pressure ever detected for a gas of simple molecules. The unique sensitivity, the wide dynamic range, the compactness, and the relatively low cost of this table-top setup open new perspectives for ^{14}C-tracing applications, such as radiocarbon dating, biomedicine, or environmental and earth sciences. The detection of other very rare molecules can be pursued as well thanks to the wide and continuous mid-IR spectral coverage of the described setup.

We report on a novel approach to cavity ring-down spectroscopy with the sample gas in saturated-absorption regime. This technique allows us to decouple and simultaneously retrieve the empty-cavity background and absorption signal, by means of a theoretical model that we developed and tested. The high sensitivity and frequency precision for spectroscopic applications are exploited to measure, for the first time, the hyperfine structure of an excited vibrational state of 17O12C16O in natural abundance with an accuracy of a few parts in 10{-11}.

No abstract

No abstract

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 the most accurate measurement of the helium fine structure splitting. The 2 3 P 0 -2 3 P 1 energy splitting is 29 616 949.7 6 2.0 kHz. Laser saturation spectroscopy, heterodyne pure frequency determination, and fluorescence detection were combined in a novel experimental approach. The absence of external perturbing magnetic fields, used in earlier experiments, lends confidence to our determined value and allows us to discriminate between contradictory results previously reported. This result, when combined with expected advances in theory, should yield a new value of the fine structure a, which may help clarify a presently puzzling experimental situation. [S0031-9007(99)08416-1]

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