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We report on saturated-absorption spectroscopy of H218O at 1.39 µm by means of a low-power distributed feedback diode laser. In the transit-time regime, we observed a 2.5 MHz wide line. This is the first narrow resonance in the spectral window around 1.4 µm. Possible applications in metrology are discussed. In addition, we propose some experimental schemes to measure the absolute frequency of H218O lines in this spectral region.

We performed saturated-absorption spectroscopy of H218O at 1.4 μm, using a low-power mirror-extended distributed-feedback diode laser. Although the rotational lines of the ν1+ν3 band investigated in this work had a small transition dipole moment, saturation was possible thanks to a short Fabry–Perot cavity with a finesse of about 500. We investigated the sub-Doppler spectrum of the 212→111 line for different sample gas pressures. A pressure-broadening coefficient of 32.9±0.7 MHz/Torr was measured. We also evaluated the saturation intensity as well as the transition dipole moment. To our knowledge, the observed Lamb dip represents the first narrow resonance at 1.4 μm.

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.

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.