OH DETECTION USING OFF-AXIS INTEGRATED CAVITY OUTPUT SPECTROSCOPY (OA-ICOS) C. Lengignon 1 , W. Chen 1 , ∗ , E. Fertein 1 , C. Coeur 1 , D. Petitprez 2 1Université du Littoral Côte d’Opale, LPCA, 189A Av. Maurice Schumann-59140 Dunkerque, France (* chen@univ-littoral.fr ) 2Université des Sciences et Technologies de Lille, PC2A, 59655 Villeneuve d’Ascq Cedex
Motivations OA-ICOS applied to OH detection Why detect OH? Motivations OH plays a critical role in atmospheric chemistry due to its high Outline of talk reactivity with chemical species such as volatile organic compounds Introduction (VOCs) and greenhouse gases (GHGs): ICOS ICOS expr. Air quality impact Coupling Climate changes investigation Exp. Details Setup Calibration Normalisation Need an adapted system that allows : ASE Calibration Real time measurement (short OH life time ≤ 1 sec) Validation Amp. Stabilization High selectivity (interference-free from atmospheric H 2 O, CO 2 ) Results High sensitivity (low OH concentration 10 6 ∼ 10 8 OH.cm − 3 ) NEAS OA-ICOS perf. Conclusion & High spatial resolution (compact setup for in field measurements) Perspectives Thanks 2/18
Outline OA-ICOS applied to OH Introduction 1 detection Integrated Cavity Output Spectroscopy Motivations ICOS expression Outline of talk Off-Axis coupling to ICOS Introduction ICOS Experiment details 2 ICOS expr. Setup design Coupling Exp. Details Calibration Setup Normalisation Calibration ASE Normalisation ASE Calibration Calibration Validation Validation Improvement : Laser Amplitude Stabilization Amp. Stabilization Results NEAS Results and Outlook 3 OA-ICOS perf. Noise Equivalent Absorption Sensitivity Conclusion & Perspectives OA-ICOS system performances Thanks 3/18
Introduction Integrated Cavity Output Spectroscopy OA-ICOS In a typical Fabry-Perot cavity, the transmitted intensity, I T , is calculated applied to OH detection as the sum of the leaking radiations from Beer-Lambert law [1,2]. As Mie and Rayleigh scattering don’t occur in our case : ⇒ I = I 0 × e − N σ ( λ ) × L Motivations Outline of talk [1] A. O’Keefe, J. J. Scherer, J. B. Paul, Chem. Phys. Lett. 307, 343-349 (1999) [2] A. O’Keefe, Chem. Phys. Lett. 293, 331-336 (1998) Introduction ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives In a high finesse optical cavity, the light trapped inside can make a great Thanks number of round-trips between the cavity mirrors. 4/18
Introduction Integrated Cavity Output Spectroscopy expression OA-ICOS Intensity at cavity output is an infinite applied to OH detection sum (integration) of leaking radiations in- tensity at each round-trip : Motivations ⇒ I T ( σ ( ν )) = � i I i ( σ ( ν )) Outline of talk Introduction ICOS ICOS expr. Coupling Integrated Cavity Output Spectroscopy expression : Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives Thanks 5/18
Introduction Off-Axis coupling to ICOS Off-Axis ICOS OA-ICOS applied to OH detection An on-axis light injection will excite the fundamental TEM ( 0 , 0 ) modes, while high orders TEM ( m , n ) modes will be excited in the case of off-axis Motivations injection [3]. Outline of talk Introduction [3] H. Kogelnik, T. Li, Proceedings of the IEEE Vol. 54, N 10, 1312-1329 (1966) ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives Thanks Spectra SNR depends on the coupling to the cavity 6/18
Experiment details Setup design OA-ICOS applied to OH detection Motivations Outline of talk Introduction ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives Thanks 7/18
Experiment details Calibration : Normalisation OA-ICOS applied to OH Importance of offset level determination detection Motivations The laser frequency is scanned at Outline of talk a rate of 10 Hz with a peak-to- Introduction peak amplitude of 1.00 V, allowing a scan over 1 cm − 1 around 6965.1939 ICOS ICOS expr. cm − 1 to cross the OH transition Coupling line Q(2,5f) and the H 2 O lines a near Exp. Details Setup 6965.7 cm − 1 . Calibration Normalisation a The 946 ← 1037 transition of the 2 ν 1 band of H2O ASE at 6965.58 cm − 1 Calibration The 541 ← 532 transition of the n 1 + 2 ν 2 band of H2O at Validation 6965.80 cm − 1. Amp. Stabilization Results NEAS Normalised spectrum OA-ICOS perf. Conclusion & ⇒ I N = ( I 0 − I Off I − I Off − 1 ) / L Perspectives Thanks 8/18
Experiment details Amplified Spontaneous Emission (ASE) OA-ICOS applied to OH detection Motivations Outline of talk Introduction ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS ASE may pass through cavity adding an additional background offset in OA-ICOS perf. Conclusion & cavity output intensity Perspectives Thanks 9/18
Experiment details Calibration L OA-ICOS Calibration : Interaction pathlength determination ( L eff = 1 − R ) applied to OH detection The effective reflectivity is calculated from Voigt profile fit area : N H 2 O . S H 2 O ⇒ R = 1 − Motivations A Outline of talk Introduction ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives Thanks Normalized direct absorption signal of pure H 2 O vapor at different pressure 10/18
Experiment details Calibration : Validation Calibration result : I off choice validation OA-ICOS applied to OH detection Effective interaction pathlength from calibration : L eff = 1263 m Corresponding mirrors reflectivity : R = 99 . 96 % (compared to manu- Motivations facturer’s R ≥ 99 . 98 % ) Outline of talk Introduction ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. OA-ICOS absorption spectrum (1 − I / I 0 ) of pure H 2 O vapor at 0.75 mbar Conclusion & Perspectives (black). A simulation spectrum based on the Beer-lambert law is shown in Thanks red for comparison with a L eff = 1200 m . 11/18
Experiment details Further improvement : Laser Amplitude Stabilization OA-ICOS applied to OH detection Motivations Outline of talk Introduction ICOS Reduction of laser excess noise ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives Thanks 12/18
Experiment details Further improvement : Laser Amplitude Stabilization Fluctuation in probe light limits the sensitivity. OA-ICOS applied to OH Intensity fluctuations (temperature, current) : technical noise. detection The DFB laser power stabilization is implemented for reduction of laser excess noise. Motivations Outline of talk Introduction ICOS ICOS expr. Coupling Exp. Details Setup Calibration Normalisation ASE Calibration Validation Amp. Stabilization Results Results of the use of laser amplitude stabilization. Spectra recorded without NEAS (black) and with (red) power stabilization. OA-ICOS perf. Allan variance curves : laser amplitude stabilization ⇒ optimal averaging Conclusion & Perspectives time ≥ 200 s (red) , compared to 100 s without (black). Noise equivalent Thanks sensitivity enhanced by a factor of ∼ 5. 13/18
Results and Outlook Noise Equivalent Absorption Sensitivity (NEAS) OA-ICOS MDA (Minimum Detectable Absorption) per scan (MDA ps ) or per applied to OH detection point (MDA pp ) & NEAS are deduced from data acquisition rate and SNR [4]: Motivations √ n √ T scan ⇒ MDA ps = ( ∆ P P ) n Outline of talk Introduction MDA ps N pts & MDA pp = MDA ps ⇒ NEAS = L eff √ √ ICOS N pts ICOS expr. Coupling [4] E.J. Moyer et al., Appl. Phys. B 92, 467–474 (2008) Exp. Details Setup Where n is the number of scans averaged, T scan the time of a scan, Calibration Normalisation L eff the effective interaction pathlength and N pts the number of ASE Calibration points per scan. Validation Amp. Stabilization Results NEAS (cm − 1 × Hz − 1 / 2 ) System (1-R) (ppm) Pathlength (m) NEAS OA-ICOS perf. 1.1 × 10 − 8 With 725 689 Conclusion & Perspectives 6.7 × 10 − 8 Without 725 689 Thanks 14/18
Results and Outlook OA-ICOS system performances Performances OA-ICOS applied to OH detection OH detection using an OA-ICOS setup with high sensitivity 1 (1 × 10 − 10 cm − 1 /Hz 1 / 2 with an effective absorption path length Motivations of L eff ≃ 1 . 2 km ). Outline of talk 1 σ detection limit of 2.1 × 10 11 OH.cm − 3 achieved (signal-to- Introduction 2 ICOS noise ratio (SNR) of 345) ICOS expr. Coupling Laser amplitude stabilization implementation ⇒ improvement of 3 Exp. Details the laser instrument stabilization time, and of the NEAS by a Setup Calibration factor of ∼ 6. Normalisation ASE Calibration Validation Amp. Stabilization Results NEAS OA-ICOS perf. Conclusion & Perspectives Thanks 15/18
Recommend
More recommend
