Paul Ewart
Oxford Institute for Laser Science
Combustion Physics and Nonlinear Optics Group
From quantum mechanics to auto-mechanics
Frontiers in Spectroscopy. Ohio State University, March 2004
Lecture Outline• Lecture 1: Linear and Nonlinear Optics
Nonlinear spectroscopic techniquesLasers for nonlinear spectroscopy
• Lecture 2: Basic theory of wave mixingCoherent signal generationSpectral simulation
• Lecture 3: Spectroscopy and diagnosticsHigh resolution spectroscopyCombustion diagnostics
Combustion DiagnosticsMeasurement required of:
• Flows (including 2 phases): Velocity, particle size etc.• Thermodynamic parameters: Temperature, Pressure, Density etc• Chemical properties: major and minor species, reaction rates etc
Measurement challenges:
• High temperature and pressure• Steep temperature and density gradients• Non-invasive probes• Scattering and luminous environments• Restricted optical access• Low concentrations of key species – ppm.• Space and time resolution • etc. !
Solution: Laser Spectroscopic TechniquesNonlinear
^
Nonlinear spectroscopy
• Coherent signal generation• Time and space resolution• Sensitive to trace quantities• High signal-to-noise ratio• Doppler-free spectral response• Species and state selective• Microscopic (molecular) and Macroscopic
parameter measurements
Four Wave Mixing techniques
CARS Coherent Anti-Stokes Raman Scattering
Coherent Anti-Stokes Raman Scattering, CARS: Narrowband
Pum p
Pum p
Stokes
Anti-StokesS ignal
M ovingG rating
p
p
p
AS
AS
S
S
Signal
Narrowband Stokes CAR S Spectrum
Coherent Anti-Stokes Raman Scattering, CARS: Broadband or Multiplex
Pum p
Pum p
Stokes
Anti-StokesS ignal
M ovingG rating
p
p
p
AS
AS
S
S
Signal
Broadband StokesBroadband CAR S Spectrum
Time resolved spectra and temperatures
DFWM Degenerate Four Wave Mixing
DFWM in oxy-acetylene flame
DFWM in flames: OH spectra
612.8 613.0 613.2 613.4 613.6 613.8 614.0 614.2
0
2
4
6
8DFWM scanned spectrum of OH
signal fit
Inte
nsity
(ar
b. u
nits
)
Dye laser wavelength / nm
Doppler-free DFWM spectra of OH in methane/air flame
306.59 306.60 306.61 306.62 306.630
2
4
R1(12)
R1(6)
experimental data fit
Inte
nsity
(arb
. unit
s)
Wavelength / nm
Boltzman plot for temperature determination
0 500 1000 1500 2000 2500 3000 3500 4000
data points least squares linear fit
R1(13)
R2(10)
R1(11)
R2(12)
R2(9)
R1(9)
R2(8)R1(7)
R1(6)R1(5)
R1(4)R1(3)
Log
(sign
al int
ensit
y)
Ground State Energy / cm-1
Multiplex DFWM spectroscopy in flames
1. Broad laser spectrum overlaps molecular resonances
2. Broadband FWM spectrum recorded on CCD camera
3. Theoretical spectrum fitted to find temperature.
C2 spectrum in oxy-acetylene flame
1
2
3
Broadband DFWM Spectroscopy of C2 in oxy-acetylene flame
LITGS: Laser Induced Thermal Gratings
Density Perturbation in LITGS
(1) Acoustic gratings interfering…Speed of sound Temperature
(2) Temperature grating… Decay by diffusion Pressure
LITGS Laser induced Thermal Grating Spectroscopy of OH in high pressure flame
• 5 nsec pulses at 308 nm excite Thermal Grating in OH
• cw Argon ion laser at 488 nm probes Thermal Grating
• Scattered LITGS signal records dynamics of grating up to 40 bar
• Signal intensity limited by intensity of cw probe laser 1 Watt
LITGS in OH in high pressure CH4/air flame
Lasers for Nonlinear Spectroscopy
Multiplex Spectroscopy• Broad, variable bandwidth• Frequency tunable• No mode structure
High Resolution Spectroscopy• High power – pulsed• Narrow linewidth – Single longitudinal mode, SLM• Wide SLM tuneability ~nm• UV, visible and IR wavelengths
• Fluctuation in relative intensity or phase of modes leads to fluctuation In relative intensity of scattered molecular spectrum i.e. noise• Noise limits precision of fitting theoretical to experimental spectrum
Lasers and mode structure
• Conventional lasers impose mode structure by standing wave resonator
• Modeless laser uses travelling wave with no resonant cavity – hence no modes
• Noise limited only by quantum fluctuations
Temperature measurement in firing si engineusing broadband CARS with modeless laser
• Pump laser: Frequency doubled single mode Nd:YAG
• Stokes laser: Modeless laser
• Low noise gives precise fit to theoretical CARS spectrum – precision of 3 – 5% resolves cycle-by-cycle variations
3950 4000 4050 4100 4150 4200
0.0
0.2
0.4
0.6
0.8
1.0In
tens
ity (
arb.
u.)
Raman Shift (cm -1)
Multiplex CARS Spectroscopy of H2 in CVD Plasmausing Modeless Laser
Plasma off. Room Temperature, 300 K. Single-shot spectrum
Multiplex CARS Spectroscopy of H2 in CVD Plasmausing Modeless Laser
3950 4000 4050 4100 4150 4200
0.0
0.2
0.4
0.6
0.8
1.0In
tens
ity (
arb.
u.)
Raman Shift (cm -1)
Plasma on. Temperature, 2340 K. Single-shot spectrum
500 1000 1500 2000 2500 30000
10
20
30
40
Freq
uenc
y
Tem perature (K)
S.D. ~ 7%
Candidate laser systems for high resolution nonlinear spectroscopy
• Pulse amplified cw dye lasers
• Pulsed Optical Parametric Oscillators
• Diode seeded Alexandrite lasers
• Diode seeded Modeless lasers
The Diode-Seeded Modeless Laser
• No cavity mode matching required: robust and stable seeding.• Linewidth of diode laser (< 2MHz): output is transform limited.• Tuning determined by SLM diode laser: tuning range ~ 10nm.• Dye selected to suit diode output: 630 - 850 nm.
• SIMPLE• NARROWBAND• WIDE TUNEABILITY• MODULAR DESIGN
High power DSML system
SLM Nd:YAGPump Laser
Seeded Modeless Laser
ExperimentalArea
Spectrum of STL output
Bandwidth of output: 165 MHz
Fabry-Perot interferogram
The Diode-Seeded Modeless Laser, DSML
• High power – pulsed
• Narrow linewidth – Single longitudinal mode SLM
• Wide SLM tunability
• UV, visible and IR wavelengths
30 mJ, 5 ns pulse: 6MWSLM linewidth: 165 MHz, 0.006 cm-1
10 nm SLM tuning range
315 – 425 nm, 635 ± 5 nm (650 / 670 / 690 etc)2.4 – 4.2 m by DFG
High Resolution DFWM SpectroscopyIn a low pressure flame
Pressure broadeningPower broadeningOH A-X (0,0) system
DFWM:Experimental Layout
Low-Pressure Burner
DFWM Pressure Broadening in OH methane/oxygen flame
