1. I. Ventrillard-Courtillot, Th. Desbois, T. Foldes and D. Romanini
NO2 trace measurements by Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS)
I. Ventrillard-Courtillot, Th. Desbois,
T. Foldes and D. Romanini
Laboratoire de Spectrométrie Physique,
Grenoble, France

2. NO2 detection… Air-quality monitoring
Atmospheric chemistry (precursor of ozone)
NOx emission by the snow mantle
OF-CEAS advantages:
Detection limit < 10-9 cm-1
Spectral resolution: 150 MHz
High selectivity on small spectral region (~1 cm-1)
Time response <1s
Large dynamic: 3 orders of magnitude
Real time
Simple, compact and robust apparatus
NO2 :(from <100 pptv to >100 ppbv)
nitrated photolysis induced by intense UV radiation.
and more generally Nox (precursor of tropospheric ozone)
in traffic areas, rural areas
high production of NOx in snow would be due to a photolysis induced by intense UV radiation of the nitrates trapped in the snow
chemiluminscence : sensitive (sup PPB) but limited accuracy (Tens of ppb) interference with several species (ozone, HONO…) +sensitivity to environemental conditions (T, humidity)

3. OF-CEAS : Optical – Feedback CEAS
PZT
PDsignal
Variable
attenuator
PDref
Laser
translation
Feedback rate : λ/2 + polarizer
Feedback phase : PZT
V shaped optical cavity
=> Optical Feedback occurs only at resonances
J. Morville, S. Kassi, M. Chenevier, and D. Romanini, Appl. Phys. B, 80, 1027 (2005)
Patent WO

4. Effect of optical feedback
narrower spectrum
OF induces laser bandwidth narrowing & laser frequency locking to the cavity modes => optimal coupling
With optical feedback
locking range
No optical feedback
Laser frequency tuning 
Dnlaser ~ MHz
time
Laser spectrum T
Dncavity~ 10kHz
Cavity resonance
OF permitting optimal excitation of cavity resonances and temporary laser frequency locking to cavity resonances
that allows fast frequency scanning speed and hence high acquisition rate
x 10 !
Intensity
Cavity transmission

5. OF-CEAS: detection limits
Species
1Hz detection limit [ppb]
 [µm]
CO2 CO
H2S
C2H4
CH4
NH3
H2O
HCl HF
N2O
NO2
300 1
100 50
0.5 2 3
1.6
2.3
1.62
1.66 and 2.3
1.53 and 2.3
1.39
1.74
1.3
2.27
0.41
Applications :
- Environnement
- Industry
Medicine
Initially developped with DFB (IR)
Extended to ECDL (vis)
Airbones applications
Environnement : mesure de polluants
 Industrie: détection de fuites, qualité des gaz, rapports isotopiques,…
 Médical : diagnostic non invasif
Collaborations : E. Kerstel (Univ. Groningen NL), LSCE (Saclay, Fr), LGGE (Grenoble, Fr), CEA (Saclay, Fr), L. Gianfrani (Caserta, It)…
SARA : Business Unit (since 2007)

6. NO2 detection in the blue
410 nm
A. C. Vandaele et al , J. Geophys. Res., 107, 4348 (2002).
intense electronic transtions occurs in the visible
FT resolution 0.1cm-1 (150MHz=>0.005cm-1 : 20 fois plus resolu)
ECDL are compatible with OF-CEAS
I. Courtillot et al, Appl. Phy. B (2006)
V. Motto-Ross et al, Appl. Phy. B (2007)

7. NO2 portable OF-CEAS spectrometer
Temperature stabilized
Acoustic and mechanical insulation
65 cm
V-Shape Cavity :
- 3 Mirrors, 1m ROC
- R= % ( F =35 000)
- L = 2x50 cm
V cavity
50 cm
Insulated from acoustic and mechanical vibrations
Coil spring
for optimum stability the cavity is made of… V grooves where drilled
input gaz flow is obtained from dry nitrogen in which is injected a fraction of NO2 diluted sample
Teflon tubes
Gas circulation-system :
- Pressure controller (100 mbar)
Rotary vacuum pump
Calibrated NO2 samples :
NO2 standard (8.69 +/-0.43 ppm)
Dilution in pure dry N2 with 2 high precision flux controllers (1,2)
Allows dilution down to 30 ppb of NO2 total flux of 0.25 L/min)
Temperature stabilized
Pressure regulation: 100mbar
Flux: 0.1 – 0.3 L/mn N2
pump
NO2
(8.69 ppm +/-0.43 ppm)

8. Data processing (1) : From raw data to absolute linear absorption
reference
transmission
Cavity transmitted signal
NI USB card with labview software

9. Data processing (1) : involves both CEAS and CRDS
CEAS : Transmitted intensity measurements
at mode m ( Airy formula) :
(1)
CRDS :
Ring Down at mode k
(2)
CEAS measurements at each mode m,
normalized with CRDS performed at mode k :
NI USB card with labview software
(1) & (2) =>
where:

10. Data processing (2) : From  to NO2 concentration
Fit of a single spectrum (100ms)
Real-time multilinear fit :
Reference
NO2 spectrum
Baseline (polynomial)
absolute linear absorption
Real-time multilinear fit of the linear absorption adjusting the coefficients of
No spectra found in the literature
NO2 spectrum has to be previously saved by OF-CEAS with a calibrated sample
OF-CEAS spectral resolution: 150 MHz
Reference spectrum has to be acquired by OF-CEAS

11. Sensitivity Deduced from fit residuals:  ~ 2 - 4 x 10-10 cm-1
ppt
@100mbar
From cNO2 values:
 ~ ppb
Present limitation:
Stability of the signal (attributed to the optical setup)
Deduced from NO2 concentration measurements (0 to 2ppm) typical values:
residual unexpected OF
Standard deviation
Concentration values

12. Linearity Deviation < 10% Offset : 40ppb
Error bars correspond to standard deviation (20 ppb) Y scale) and are derived from flux controllers accuracy (X scale, 2% max).
Stability ~2% over 7hours
Deviation of 7% between measurements and expected values from calibrated samples
Deviation < 10%
Offset : 40ppb
Reproducibility over 8 hours ~ 5%
Wall effects to be minimized

13. Conclusions NO2 detection @100mbar:
Linearity and accuracy better than 10%
Sensitivity :
Limited by signal fluctuations: 3 ppb in 0.1s
A gain of a factor 10 seems achievable: 200ppt in a single scan (0.1s)
to conclude we can say that OFCEAS offers
sucessive injection of TEM00 modes yield a mode by mode spectrum separated by the FSR

14. Perspectives
Optics : Investigation of instability of the signal (etaloning effects, small change in the coupling efficiency… )
Gaz :
Special coating of tubes and cavity (Sulfinert®)
Concentrations samples below 100ppb (permeation tube)
Higher pressure studies: averaging over a small spectral region (renounce to spectral selectivity)
Metal passivation
Present limitations stability of the signal:
OF pendant le RD diferent d’un spectre à l’autre car le pzt bouge
Bcp plu ssensible au OF que DFB car taux de rtroaction beaucoup plus eleve ~20-50% contre 1%
In-situ measurements in remote areas, in the frame of studies on NOx emitted by the snow mantle.
Restek’s proprietary Sulfinert® passivation technique bonds an inert silica layer
into the surface of the stainless steel, preventing active compounds from reacting
with or adsorbing to the stainless steel.

16. Cavity Enhanced Absorption Spectroscopy (CEAS)L
Effective absorption length
~ L . F with F >
laser
photodiode
Gas sample
Mirror
R>99.99%
PD signal :
modified transmission
frequency
t(n) t0
time
CRDS
CEAS
CO (0.5 ppm)
CH4 (9 ppm)
Données breath analysis
Parler ici du fit multi composantes à partir des données HITRAN
Spectra self-calibrated by CRDS (Cavity Ring-Down Spectroscopy)

17. OF-CEAS : Cavity transmission signal
the frequency locking range can be adjust to nearly match an FSR
Frequency monitoring with a thick uncoated etalon (1 GHz)

18. OF-CEAS spectrometer for in-situ measurements
Quantitative and selective measurements
Free from calibration with a reference gaz sample (CRDS)
Response time < 1s
Analysed gaz sample : 18 cm3
Flux : 0.1 – 0.4 L/min
Pressure : mbar
Transfert de technologie, réalisation d’appareils : pour des labo ou transfert techno avec des partenariats industriel
30kg, rack + 1 pompe et un ordi
Spectroscopie par amplification resonante d’absorption
SARA : Business Unit (since 2007)
Collaborations : E. Kerstel (Univ. Groningen NL), LSCE (Saclay, Fr), LGGE (Grenoble, Fr), CEA (Saclay, Fr), L. Gianfrani (Caserta, It)…

19. OF-CEAS : L’appareil

20. OF-CEAS spectrometer for in-situ measurements
Collaborations : E. Kerstel (Univ. Groningen NL), LSCE (Saclay, Fr), LGGE (Grenoble, Fr), CEA (Saclay, Fr), L. Gianfrani (Caserta, It)…

21. OF-CEAS: gaz and detection limits
Species
1Hz detection limit [ppb]
 [µm]
Carbon dioxide CO2
300
1.6
Carbon monoxide CO 1
2.3
Hydrogen sulfide H2S
100
Ethylene C2H4 50
1.62
Methane CH4
0.5
1.66 and 2.3
Ammonia NH3 2
1.53 and 2.3
Water (vapor) H2O
1.39
Hydrochloric acid HCl
1.74
Hydrofluoric acid HF
1.3
Nitrous oxide N2O
2.27
Nitrogen dioxyde NO2 5
0.41
Applications :
- Environnement
- Industry
Medicine
Initially developped with DFB (IR)
Extended to ECDL (vis)
Environnement : mesure de polluants
 Industrie: détection de fuites, qualité des gaz, rapports isotopiques,…
 Médical : diagnostic non invasif
Collaborations : E. Kerstel (Univ. Groningen NL), LSCE (Saclay, Fr), LGGE (Grenoble, Fr), CEA (Saclay, Fr), L. Gianfrani (Caserta, It)…
SARA : Business Unit (since 2007)

22. NO2 portable OF-CEAS spectrometer
PD1
ECDL
PD2
PZT
l/2
V-cavity
Pump 2 1
NO2
8.69 ppm N2
inlet
outlet
Pressure controller