ACCURATE COMB-ASSISTED CAVITY RING DOWN SPECTROSCOPY OF MOLECULAR HYDROGEN

Because molecular hydrogen is the simplest molecule, it is considered as the best candidate for a direct comparison of experiment against high level ab initio calculations, both in terms of transition frequencies and line strength. Unfortunately, this apparent simplicity is not only spoiled by the weakness of the transition, but also by its surprisingly complex line profile that hampers accurate parameters to be straightforwardly derived. To address that problem, we have recorded with unprecedented sensitivity pure H$_{2}$ Q(1) 2-0 and D$_{2}$ S(2) 2-0 transitions around 1.24 and 1.59 $\mu$m, respectively, down to a pressure of 100 Pa. A limit of detection of about 2$\times$10$^{-12}$ \wn was achieved with the two accurate comb-referenced cavity ring down spectrometers used. Effective parameters were determined for different line profiles (NGP, SDNGP, HTP), allowing line reproduction down to the noise level. The zero pressure parameters will be presented and discussed

OPTICAL FEEDBACK STABILIZED LASER CAVITY RING DOWN SPECTROSCOPY: FROM SATURATED SPECTROSCOPY TO ISOTOPIC RATIO.

Optical feedback frequency stabilized cavity ring-down spectroscopy is a very high resolution (sub kHz) and sensitive technique (2$\times$ 10$^{-12}$ \wn limit of detection). Both aspects will be emphasized through the two followingh applications: To illustrate the high resolution, Doppler-free saturated-absorption Lamb dips were measured at sub-Pa pressures on rovibrational lines of H$_{2}$O near 7180 \wn, The saturation of the considered lines was so high that at the early stage of the ring down, the cavity loss rate remained unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies were determined down to 100 Hz precision and kHz accuracy. To highlight the high precision and stability of the instrument, we will present the first optical absorption measurements of O-17 anomalies in CO$_{2}$ with a precision better than 10 ppm, matching the requirements for paleo-environmental applications

METROLOGY WITH AN OPTICAL FEEDBACK FREQUENCY STABILIZED CRDS

We will present a metrological application of our recently developed Optical Feedback Frequency Stabilized - Cavity Ring Down Spectrometer (OFFS-CRDS). This instrument, which ideally fits with an optical frequency comb for absolute frequency calibration, relies on the robust lock of a steady cavity ring down resonator against a highly stable, radiofrequency tuned optical source. At 1.6 $mu$m, over 7 nm, we demonstrate Lamb dip spectroscopy of CO$_2$ with line frequency retrieval at the kHz level, a dynamic in excess of 700,000 on the absorption scale and a detectivity of 4x10$^{-13}$cm$^{-1}$Hz$^{-1/2}$. Such an instrument nicely meets the requirements for the most demanding spectroscopy spanning from accurate isotopic ratio determination and very precise lineshape recordings to Boltzmann constant redefinition

FEED-FORWARD COHERENT LINK FROM A COMB TO A DIODE LASER : APPLICATION TO SATURATED CAVITY RING DOWN SPECTROSCOPY

We applied a feed-forward frequency control scheme to establish a phase-coherent link from an optical frequency comb to a distributed feedback (DFB) diode laser: This allowed us to exploit the full laser tuning range (up to 1 THz) with the linewidth and frequency accuracy of the comb modes. The approach relies on the combination of an RF single-sideband modulator (SSM) and of an electro-optical SSM, providing a correction bandwidth in excess of 10 MHz and a comb-referenced RF-driven agile tuning over several GHz. As a demonstration, we obtain a 0.3 THz cavity ring-down scan of the low-pressure methane absorption spectrum. The spectral resolution is 100 kHz, limited by the self-referenced comb, starting from a DFB diode linewidth of 3 MHz. To illustrate the spectral resolution, we obtain saturation dips for the 2\nub{3} R(6) methane multiplet at $\mu$bar pressure. Repeated measurements of the Lamb-dip positions provide a statistical uncertainty in the kHz range

First observation of electric-quadrupole infrared transitions in water vapour

Molecular absorption of infrared radiation is generally due to ro-vibrational electric-dipole transitions. Electric-quadrupole transitions may still occur, but they are typically a million times weaker than electric-dipole transitions, rendering their observation extremely challenging. In polyatomic or polar diatomic molecules, ro-vibrational quadrupole transitions have never been observed. Here, we report the first direct detection of quadrupole transitions in water vapor. The detected quadrupole lines have intensity largely above the standard dipole intensity cut-off of spectroscopic databases and thus are important for accurate atmospheric and astronomical remote sensing

Communication: Saturated CO 2

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