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US 10,895,528 B2Granted
Method for measuring the concentration of trace gases by SCAR spectroscopy
AssigneeConsiglio Nazionale Delle Ricerche - Cnr
InventorsDavide Mazzotti, Pablo Cancio Pastor, Iacopo Galli, Paolo De Natale, Giovanni Giusfredi
Life of this patent
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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.
Claims30
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 a frequency of said electromagnetic radiation emitted by said laser source so as to fix it to a value νi within a range of frequencies [νmin, νmax] including the resonance frequency of said molecular transition ν0;
- fixing an intensity of said electromagnetic radiation in the cavity at a value much greater than a 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 νi 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, δνi) for the recorded decay signal which depends on the following parameters:
- B(δνi) is a detection background, with (δνi=νi−ν0);
- Ad(δνi) is an amplitude of the decay signal at the beginning of the decay event;
- γc(δνi) is a cavity decay rate due to non-resonant and non-saturable losses (empty cavity decay rate);
- γg(δνi) is contribution of the targeted molecular transition to the decay signal;
- g is a peak normalized line profile g(ν−νo,wR) centered at the molecular resonance frequency νo 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;
- Z
- o
- =
- CP
-
- (
- 0
- )
- CP
- s
- =
- A
- d
-
- Z
- 1
-
- U
- is the saturation parameter at the beginning of the decay event and at the frequency of the targeted molecular transition ν0, P(0) is the intracavity power at the beginning of the decay signal;
- and
- P
- s
- =
- π
-
-
- w
- 2
- 2
-
- I
- s
- is a saturation power, where w is the spot size radius of the laser beam, i.e. a radius for which the amplitude of the field is 1/e times that of the axis and Is the saturation intensity;
- replacing Z1Ug(ν−νo,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 Z1Ug(ν−νo,wR) in the fitting function S(t, δνi).
46 more elements
A method of ring-down spectroscopy in saturated-absorption condition, for measuring a first concentration of a target gas through a measurement of a spectrum of a molecular transition of said target gas, the target gas being in a mixture together with other gasses the method comprising the steps of:
- repeating for a number m of fixed frequencies νi spaced each other by a of a frequency step in a range of frequencies including a frequency of the molecular transition of the target gas, the gases in the mixture having l=0, . . . ,n, absorptions in the measured spectral range, where l=0 is a target absorption, the following steps:
- repeating d times at the same frequency the following steps:
- 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 a frequency of said electromagnetic radiation emitted by said laser source so as to fix it to a value νi within a range of frequencies [νmin, νmax] including said molecular transition ν0;
- fixing an intensity of said electromagnetic radiation in the cavity at a value much greater than a 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 νi 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;
- and
- recording a plurality of data representative of said output, obtaining a decay signal for the fixed frequency;
- collecting the d*m SCAR decay signals obtained;
- fitting at the same time the d*m decay signals with d*m fitting curves considering a fitting curve s(t;
- p(ν))=[S(t;
- p(νi)j)]j=1d*m for the recorded decay by signals which depends on the following parameters p(ν)=[p(νi)j]j=1d*m:
- p
-
- (
- v
- )
- =
- [
- p
-
- (
- v
- i
- )
- j
- ]
- j
- =
- 1
- d
- *
- m
- =
- [
- B
- j
- ,
- A
- d
- j
- ,
- γ
- c
- j
- ,
- [
- {
- γ
- l
-
- (
- δ
-
-
- v
- il
- )
- ,
- Z
- l
-
-
- 1
-
- U
-
- g
- _
- l
-
- (
- δ
-
-
- v
- il
- ,
- w
- Rl
- )
- }
- j
- ]
- l
- =
- 0
- n
- ]
- j
- =
- 1
- d
- *
- m
- ,
-
-
- i
- =
- 1
-
-
- …
-
-
- m
- [Bj(δνil)]j=1d*m are the detection backgrounds of the j=1 . . . d*m recorded SCAR decay signals;
- and δνil=νi−νl is the detuning of the i (=1 . . . m) scanned frequency of the laser radiation with respect to the resonance frequency of the l resonant transition,
- [Adj(δνil)]j=1d*m are amplitudes of the j=1 . . . d*m recorded SCAR decay signals,
- [γcj(δνil)]j=1d*m are cavity decay rates of the j=1 . . . d*m recorded SCAR decay signals due to non-resonant and non-saturated gas absorption losses,
- [[γl(δνil)j]l=0n]j=1d*m are arrays of the absorption decay rates of the l=0 . . . n saturated transitions at the δνil detuning for the j=1 . . . d*m recorded SCAR decays,
- [
- [
- {
- Z
- 1
-
- U
-
-
- l
-
- g
- _
- l
-
- (
- v
- i
- -
- v
- l
- ,
- w
- Rl
- )
- }
- j
- ]
- l
- =
- 0
- n
- ]
- j
- =
- 1
- d
- *
- m
- are arrays of the l=0 . . . n saturation parameters of the j=1 . . . d*m recorded SCAR decay signals;
- gl(νi−νl,wRl) is the peak normalized line profile centered at the molecular resonance frequency νl and wRl is the HWHM width of the l-resonance;
- Z
- o
- =
- CP
-
- (
- 0
- )
- CP
- s
- =
- A
- d
-
- Z
- 1
-
- U
- is a saturation parameter at the beginning of the decay event and at the frequency of the targeted molecular transition ν0, P(0) is an intracavity power at the beginning of the decay signal;
- and
- P
- s
- =
- π
-
-
- w
- 2
- 2
-
- I
- s
- is a saturation power, where w is a spot size radius of the laser beam, i.e. a radius for which a amplitude of the field is 1/e times that of the axis and Is the saturation intensity;
- dividing the above mentioned parameters in a first and a second group;
- and
- fitting said recorded data with a function S(t;
- p(ν))=[S(t;
- p(νi)j)]j=1d*m by
- keeping the first parameter group fixed and equal to a set of pre-determined values and performing a first fitting only considering the second group as free parameters in a first fitting step;
- and
- keeping the second parameter group fixed to a value given in the first fitting step and performing a second fitting considering only the second group as free parameters in a second fitting step.
194 more elements
Dependencies are inferred from the claim text.