Method for measuring the concentration of trace gases by SCAR spectroscopy
Year: 2016
Authors: Mazzotti D., Cancio P., Galli I., De Natale P., Giusfredi G.
Autors Affiliation: Istituto Nazionale di Ottica (INO) – CNR
Abstract: The present invention is relative to a method of ring-down spectroscopy in saturated-absorption condition, for measuring a first concentration of a gas through a measurement of the spectrum of a molecular transition of said gas, the method comprising the steps of: • inserting said gas whose first concentration is to be measured in a resonant cavity comprising two or more reflecting mirrors arranged so as to form a closed optical path for an electromagnetic radiation emitted by a laser source; • tuning the frequency of said electromagnetic radiation emitted by said laser source so as to fix it to a value v, within a range of frequencies [vmin, vmax] including said molecular transition v0; • fixing the intensity of said electromagnetic radiation in the cavity at a value much greater than the saturation intensity Is of the molecular transition to be detected; • irradiating said gas by means of said electromagnetic radiation beam emitted by said laser source having said fixed frequency vi, and intensity in said resonant cavity; • coupling said electromagnetic radiation to said cavity so as to obtain a laser-cavity resonance condition; • changing the frequency of the electromagnetic radiation emitted by the laser so as to switch off the laser-cavity resonance; • detecting an electromagnetic radiation beam in output from said cavity after the laser-cavity resonance has been switched off; • recording a plurality of data representative of said output, obtaining a decay signal for the fixed frequency; • considering a fitting curve S[t, δvi) for the recorded decay signal which depends on the following parameters: – B(δvi) is the detection background, with (δνi = vi = v0); – Ad(δvi) is the amplitude of the decay signal at the beginning of the decay event; – yc(δvi) is the cavity decay rate due to non-resonant and non-saturable losses (empty cavity decay rate); – yg(δvi) is contribution of the targeted molecular transition to the decay signal; – g̅ is the peak normalized line profile g̅(v – vO, wR) centered at the molecular resonance frequency v0 and wR is the HWHM width of the resonance, and wR = wL for a Lorentzian shape, wR = wG for a Gaussian shape, wR = {wL, wG} for a Voigt shape; – (I) is the saturation parameter at the beginning of the decay event and at the frequency of the targeted molecular transition v0, P(0) is the intracavity power at the beginning of the decay signal; and (II) is the saturation power, where w is the spot size radius of the laser beam, i.e. the radius for which the amplitude of the field is lie times that of the axis and ls the saturation intensity; • replacing Z1Ug̅(v – v0,wR) in the function S(t, δνi) with a constant value Z1ueff = constant of a predetermined value; • fitting said recorded data with a function Srepl(t, δνi) in which Z1ueff = constant replaces Z1Ug̅(v – v0,wR) in tne fitting function S(t, δνi).
Journal/Review: JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS
KeyWords: radiocarbon; cavity ring-down; trace gas detection