Courtesy : J. Courtois, R. Vallon, J. Soutadé, D. Henry,

Transcription

1 8 th European Symposium on Aerothermodynamics for Space Vehicles, 2-6 March 2015, Lisbon, Portugal Frequency Comb High Speed Broadband Spectroscopic Sensor To Probe Aerodynamic Flows Ajmal Mohamed, Jean-Pierre Faléni Courtesy : J. Courtois, R. Vallon, J. Soutadé, D. Henry,

2 Outline Context Needs, recall on absorption spectroscopy, Broadband laser : Supercontinuum and Frequency comb New set up at ONERA and first results Perspectives 2

3 Atmospheric reentry flows issues Need to deal with chemistry (species concentrations and states) and radiation Mars Sample Return Orbiter 3

4 Measurement techniques Many exist for equilibrium / low enthalpy flows (33 listed below) Methods for flow diagnostics FLOW OVERALL STRUCTURE visualisation methods surface flow pattern vortices and wakes shock waves expansions slip lines WALL QUANTITIES viscous coating laser sheet water tunnel interferometry schlieren, shadography EBF glow discharge FIELD QUANTITIES velocity, pressure, temperature intrusive methods multi-hole probes thermometric probes hot wire/hot film particle based methods LDV DGV (PDV) PIV But few can be transposed to high enthalpy Non equilibrium + chemistry investigate inside the molecule pressure skin friction heat transfer orifices + scanning system PSP floating element thin film, thin wire Preston tube thin oil film liquid crystals logarithmic law fitting thermocouples surface films infrared thermography thermosensitive paints species, density, temperature,... laser spectroscopic methods laser diode absorption Rayleigh scattering Raman scattering CARS, DLCARS LIF (PLIF) EBF and EBF excited X rays pseudo-spark electron gun 4

5 Spectroscopic methods : Molecular states representation Potential energy Electronic state Λ Electronic states of CO And normalisation (partition function) Q = 5 i e Ei - k BT Level transition corresponding to an absorption line Population n i on level i having energy E i n ΛvK = Total density n 0 e. Q - E kt Λ Λ elec e. Q - E kt v V vib e. Q - E kt J rot R v = 0 Internuclear distance, r v = 2 v = 1 v = 3 J = 1 J = 0 Vibrational states Rotational states

6 Simplest laser spectrospy technique : TDLAS (tunable diode laser spectroscopy) Line strength (specie specific) I ( ) 0 σ Absorbing Laser n 0 molecules L I I ( σ ) = ( σ ) = I I 0 0 ( σ ) e ( σ ) e χ f(p,t,v) S(T) n f(p,t,v) Λvk n L 0 L One level Equilibrium Line profile (thermo specific) Many measurements on f(p, T, V) in high enthalpy flows with diode lasers

7 Tunable diode laser absorption spectroscopy (DLAS) in windtunnels Diode Laser wavelength tunable NO Absorption line R(3/2) Core V Velocity: θ Temperature (translation) Species densities?? Flow σ = x σ V θ c cos( ) x Valid if : - flat profiles in core flow - thin boundary layers (Line of sight technique) Transmission (%) Exp. Best fit Doppler shifted line V = 5642 m/s T = 534 K Area > Density FWHM >Temperature σ > Velocity Wavenumber (cm -1 ) HEG run 375 : t ms Rest position line V = 0 m/s T = 826 K

8 Some DLAS results 8 Velocity (km/s) Temperature (K) [NO] ( molecules/cm 3 ) Measurements on NO in F4 arcjet DLAS temperature, velocity and [NO] F4 Run 762 condition III (50 M Pa; 15 M J/kg) E15 1E12 1E13 1E10 1E8 5% error bars 20% error bars 20% error bars Flow time (ms) DLAS E quilibrium Frozen [CO] molecules/cm 3 T(K) V(km/s) Measurements on CO in HEG Shock Tube 1x x DLAS temperature, velocity and [CO] HEG Shot 487,condition I (39 MPa; 21 MJ/kg), N 2 flow Flow starts in test section Al diaphragme Exp. (487) filtered Time after shock initiation (ms)

9 Lead Salt laser diodes spectral limitations 1.0 total coverage of a diode laser (20 cm -1 ) practical window for a measurement 1 cm Absorbance Rotational lines NO v: 0--> wavenumber (cm -1 )

10 But we need a specific diode for each specie H 2 O diode NO CO 'N 2 0' CO 2 spectral coverage S4 F4, HEG F4, HEG F4, SIMOUN 6,5 6 5,5 5 4,5 1,0 µm 0,8 NO CO Transmission 0,6 0,4 H 2 O CO 2 (ν3, Absorption x 0.1) 0,2 0, Wavenumber (cm -1 ) All 3 species for Martian flows 600 cm -1 10

11 Review of requirements to characterise non-equilibrium flows Need simultaneously : Broad spectral coverage (few hundreds of cm -1 ) High spectral resolution (less than 10-2 cm -1 ) Fast repetition rate (higher than khz) High speed broadband spectroscopy

12 Recent developments in lasers Supercontinuum and frequency comb laser Generation I Monochromatic pump Optical cavity long. or diam L Broad band But not ordered (ex : supercontinuum) Frequency Comb Equidistants peaks spaced FSR = 1/2L ou multiple λ Spectral broadening By a non_linear effect (laser, Kerr, OPO, Raman,..) in Wide band crystal (Ti:Sa; OPO) µresonateur Non linear fiber λ Mode-locking Acousto-optic Saturable absorber Kerr lensing Rotation of polarisation Gallery mode Example : Femtosecond fiber lasers 12

13 Typical Frequency Comb emission fram a femtosecond laser T Fourier time-bandwidth limit T. ω ~ 1 Few millions peaks 100 femtoseconds 300 cm -1 ω 13/ 42

14 DUAL COMB Lasers set up at ONERA Based on Menlo fiber lasers

15 Lasers with 2 channels Emission Super-K voie H1 (Puissance totale = 300 mw) SuperContinuum channel Frequency comb channel laser intensity (arbitrary units) E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1 C 2 H 2 CO 2 HCN CH λ(nm) H 2 O comb comb λ(nm) 15

16 Standard absorption spectroscopy with the frequency combs Using an optical spectrum analyser) all rotatation lines of a vibrational transition Intensité laser (A.U.) Intensité laser (A.U.) C 2 H 2 HCN CH 4 Laser Peigne de fréquences G λ (µm) Laser Peigne de fréquences G2 HCN Acetylene Intensité laser (A.U.) Intensité laser (A.U.) Absorbance Acquis cuve 300 mbar C 2 H 2 Laser Peigne de fréquences G λ (µm) Simul Hitran : 300 mbar C 2 H 2 (P1000mbar, 300 K, 10 cm) Intensité laser (A.U.) Intensité laser (A.U.) Absorbance Acquis cuve 100 mbar HCN Laser peigne fréquences G λ (µm) Simulation NIST 15 cm, 25 Torr CH 4 Intensité laser (A.U.) Absorbance Laser à peigne de fréquences G1 cuve 300 mbar CH Simul Hitran : 300 mbar CH λ (µm) 16

17 Most intense absorption lines of some molecules mid infrared 17

18 To mid-infrared : with difference frequency generation (DFG) between the supercontinuum and the comb Supercontinuum Comb 1.55µm Non linear Crystal (GaSe, ) MIR Comb 18

19 Broadband laser Two difficulties: 1. How to detect such a large band at high spectral resolution? Solutions Sophisticated grating (VIPA) Dual comb beating Fourier spectroscopy 2. Up to now these lasers emit in the visible or telecom (< 2 µm) domain where molecules absorption lines are weak New laser material Conversion to MidInfrared through DFG (difference frequency generation) 19

20 Dual comb optical beating : Fourier spectrum Beating with slighly different comb spacings Fourier transform 20

21 Dual Comb typical results on C 2 H Typical interferogram Information on envelope profile of the comb optical beating Inteferogram intensity (a.u.) Information on molecular absorption time (µs) 21 Real time (22 khz)

22 conclusion / Further developments - Dual comb being characterised in the laboratory - Tests to be performed on a flame (high pressure/density) then in low pressure flows 1. Comb stabilisation for better quantitative spectra 2. Transposition to the mid-infrared (2-11 µm) where most molecules have their fundamental and most intense absorption lines 3. Microresonator sources for miniature frequency comb laser sources in flight But not least : femtosecond pulse high peak power Lidar, non linear techniques (Raman, CARS) for local and stand-off measurements 22