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A coherent phase lock of a quantum-cascade-laser to a thulium-fiber frequency comb is exploited for the investigation of multi-line absorption spectra of ammonia at 9.1 μm with extremely high metrological quality.

A sum-frequency-generation scheme is exploited to refer the frequency of a quantum-cascade-laser at 4.3 μm to a near-IR frequency comb based on a dual-branch Er:fiber laser without interposition of any transfer oscillator.

To provide new determinations of the Boltzmann constant, k, which has been asked for by the International Committee for Weights and Measures concerning preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole, an iMERAPlus joint research project has coordinated the European activities in this field. In this major European research project the Boltzmann constant has been determined by various methods to support the new definition of the kelvin. The final results of the project are reviewed in this paper. Determinations of the Boltzmann constant k were achieved within the project by all three envisaged methods: acoustic gas thermometry, Doppler broadening technique, and dielectric constant gas thermometry. The results were exploited by the interdisciplinary Committee on Data for Science and Technology (CODATA) in their 2010 adjustment of recommended values for fundamental constants. As a result, the CODATA group recommended a value for k with a relative standard uncertainty about a factor of two smaller than the previous u(k)/k of 1.7×10−6.

A distributed feedback diode laser emitting at 760 nm has been used to detect weak absorption lines of molecular oxygen. Using wavelength-modulation spectroscopy and second-harmonic detection at audio frequencies, a minimum detectable absorbance of 10 −6 is achieved, with an effective bandwidth of 1 Hz. Such sensitivity allows us to observe lines of the 16 O 18 O and 16 O 17 O molecules at atmospheric pressure, in a natural-abundance sample. Highly sensitive detection of NO 2 has also been performed; in this case, the laser source is an AlGaAs diode laser at 742 nm.