Transcript Slide 1 - The Ohio State University

Frontiers in Spectroscopy.
Ohio State University, March 2004
From quantum mechanics
to auto-mechanics
Oxford Institute
for Laser Science
Combustion Physics and
Nonlinear Optics Group
Paul Ewart
Lecture Outline
• Lecture 1: Linear and Nonlinear Optics
Nonlinear spectroscopic techniques
Lasers for nonlinear spectroscopy
• Lecture 2: Basic theory of wave mixing
Coherent signal generation
Spectral simulation
• Lecture 3: Spectroscopy and diagnostics
High resolution spectroscopy
Combustion diagnostics
Combustion Diagnostics
Measurement 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. !
Nonlinear
Solution: Laser Spectroscopic Techniques
^
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
Pump
p
S p
AS
Anti-Stokes
Signal
Pump
Stokes
Signal
Moving
Grating
CARS Spectrum
Narrowband Stokes
S
p
AS
Coherent Anti-Stokes Raman Scattering,
CARS: Broadband or Multiplex
Pump
p
S p
AS
Anti-Stokes
Signal
Pump
Stokes
Signal
Broadband
CARS Spectrum
Broadband Stokes
S
p
AS
Moving
Grating
Time resolved spectra
and temperatures
DFWM Degenerate Four Wave Mixing
DFWM in oxy-acetylene flame
DFWM in flames: OH spectra
8
DFWM scanned spectrum of OH
Intensity (arb. units)
6
signal
fit
4
2
0
612.8
613.0
613.2
613.4
613.6
Dye laser wavelength / nm
613.8
614.0
614.2
Doppler-free DFWM spectra of OH
in methane/air flame
R1(6)
Intensity (arb. units)
4
experimental data
fit
2
R1(12)
0
306.59
306.60
306.61
Wavelength / nm
306.62
306.63
Boltzman plot for temperature determination
R1(3)
R1(4)
R1(5)
R1(6)
Log (signal intensity)
R1(7)
data points
least squares linear fit
R2(8)
R2(9)
R1(9)
R2(10)
R1(11)
R2(12)
R1(13)
0
500
1000
1500
2000
2500
Ground State Energy / cm-1
3000
3500
4000
Multiplex DFWM spectroscopy in flames
1
3
2
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
Broadband DFWM
Spectroscopy of C2 in
oxy-acetylene flame
LITGS: Laser Induced Thermal Gratings
Density Perturbation in LITGS
N on -Staton ary , A cou stic Co mpo nen ts
Density Variation
(1) Acoustic gratings interfering…
Speed of sound  Temperature
0
50
100
150
Time (ns)
A cou stic Th ermal G ratin g
(2)
A
couTemperature
stic Env elop e
200
250
300
D ensity Variation
Stationary Component - Thermal Diffusion
grating…
Decay by diffusion  Pressure
0
50
100
150
200
250
300
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 engine
using 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
Multiplex CARS Spectroscopy of H2 in CVD Plasma
using Modeless Laser
1.0
Intensity (arb.u.)
0.8
Plasma off.
Room Temperature, 300 K.
Single-shot spectrum
0.6
0.4
0.2
0.0
3950
4000
4050
4100
-1
Raman Shift (cm )
4150
4200
Multiplex CARS Spectroscopy of H2 in CVD Plasma
using Modeless Laser
1.0
Plasma on.
Temperature, 2340 K.
Single-shot spectrum
40
0.6
30
Frequency
Intensity (arb.u.)
0.8
0.4
20
10
0
500
1000
1500
2000
2500
3000
Temperature (K)
0.2
S.D. ~ 7%
0.0
3950
4000
4050
4100
-1
Raman Shift (cm )
4150
4200
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.
SIMPLE
Linewidth
of diode laser (< 2MHz): output is transform limited.
NARROWBAND
Tuning
by SLM diode laser: tuning range ~ 10nm.
WIDE determined
TUNEABILITY
Dye
selected to
suit diode output: 630 - 850 nm.
MODULAR
DESIGN
High power DSML system
Experimental
Area
SLM
Nd:YAG
Pump
Laser
Seeded
Modeless Laser
Spectrum of STL output
Fabry-Perot interferogram
Bandwidth of output: 165 MHz
The Diode-Seeded
Modeless Laser, DSML
• High power – pulsed
30 mJ, 5 ns pulse: 6MW
• Narrow linewidth –
Single longitudinal
mode SLM
SLM linewidth:
165 MHz, 0.006 cm-1
• Wide SLM tunability
10 nm SLM tuning range
• UV, visible and
IR wavelengths
315 – 425 nm,
635 ± 5 nm (650 / 670 / 690 etc)
2.4 – 4.2 mm by DFG
High Resolution
DFWM Spectroscopy
In a low pressure flame
Pressure broadening
Power broadening
OH A-X (0,0) system
DFWM:
Experimental Layout
Low-Pressure Burner
DFWM Pressure Broadening in OH
methane/oxygen flame