Abstract
Cavity ring-down spectroscopy is performed using a single-mode fibre loop as an optical cavity. The loop contains absorption cells for either liquids or gases, as well as an erbium-doped fibre amplifier. The fibre amplifier is gain-clamped using a second fibre-loop. Ring-down times can be tuned to permit several hundred roundtrips by adjusting the loss ratio between the two nested loops using a variable optical attenuator. Ring-down measurements were performed either in the time domain or using phase shift measurements.Acetylene gas was detected through a ro-vibrational absorption line of the ν1–ν3 combination band at 1532nm at a limit of detection of 50ppm. Liquid 1-octyne and 1-dodecyne were detected in a 1pL volume at detection limits of ∼20% dilution in dodecane through their C–H stretch vibrational overtone transition.
Highlights
Over recent years cavity ring-down spectroscopy (CRDS) and related cavity-enhanced absorption techniques have gained a large following
In CRDS light is coupled into a high finesse optical cavity and one measures the optical loss from the rate at which light escapes from the cavity
The ‘‘amplified fibre-CRD’’ technique will be applied to the detection of liquid 1-alkynes through their C-H overtone band and to gaseous acetylene through the m1 + m3 combination band
Summary
Over recent years cavity ring-down spectroscopy (CRDS) and related cavity-enhanced absorption techniques have gained a large following. Adding an absorbing sample into the cavity creates additional losses and the consequent decrease of the ring-down time allows for an accurate determination of the concentration of the sample [3]. Most CRD spectrometers are mirror-based and are designed for sensitive trace gas [4] or aerosol [5,6] absorption measurements with effective optical path lengths of several km to enhance detection [7]. Using liquids in a ‘‘conventional’’ cavity is complicated by the requirement to minimise losses due to the scattering of light at the interfaces of the liquid and sample containers, but excellent detection limits have been achieved in a variety of configurations [9]
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