nonlinear technigues
TRANSCRIPT
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11. Non-linear optical techniques
n ro uc on
,
Polarization S ectrosco , PS
IR measurement (IRPS, IR-DWM)
(Stimulated Emission, SE)
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Diagnostic dilemma:
LIF g sens v y,
2D imaging,
S ontaneous techni ue sensitive to stron back round
CARS
One-point measurements
Low sensitivity
Need: A coherent techni ue with hi hsensitivity and 2D imaging possibility
Candidates:DFWM, PS and SE (one point)
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Nonlinear optics
Thus far, the induced polarization of molecules has been assumed
to de end linearl on the a lied electroma netic field. This is
however only valid for incident radiation of low intensity.
,
applied electromagnetic field:
.....321 PPPP
.....332210 EEEP
, w v
symmetry), the even order polarizations vanish
DFWM and PS are four-wave mixing processes based on
Joakim Bood
the nonlinear response via the third-order susceptility (in the same way as CARS.
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DFWM
, ,
2 2
DFWM P Pr
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a) b)
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1.0E+05
6.0E+04
8.0E+04
ty
[au]
2.0E+04
4.0E+04
ntensi
0.0E+00
305.0 305.2 305.7 306.2 306.6 307.1 307.6 308.0
Wavelength [nm]
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Beam splitter
Sheet-shapedpump beams
Opaque screen with aperture
Probe beamLens
Flame
gna o e e ec e
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y(counts)
(a.u.)
800090001000011000
12000
gnalintensit
htabo
veburne
3000
400050006000
Distance across the flame (a.u.)
Si
He0
1000
P. Ewart et al.
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DFWM application
P. Ewart et al.
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Coherent technique with high sensitivity (ppm)
2D imaging possible
Complex theory
Advanced procedures for laser beam alignement
Problem with background scattering
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Polarization spectroscopy
A pump beam induces an optical anisotropy (Birefringence and Dicroism),
which is measured as a chan e in the olarization of the robe beam
Laser
e ec or
Fresnelrhomb
Analyzer
prpJJJJt IIBNI220 )(
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Theory
Exp.
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Two dimensional ima in
A strong linearly polarized pump beam formed to a narrow
sheet of light crosses an unfocussed weaker probe beam in
the flame
The intersection volume imaged onto an image intensifiedCCD camera
ump eam magng ens Imaging lens CCD array
A erture
ro e eam
Flame
SIGNAL DISTRIBUTION IN ONE PLANE IN THE FLAMERECORDED
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Ima in of OH and NO in flames
Images of OH signal NO PS in a
distributions recorded in a
CH4/O2 flame
H2/N2O flame
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Two-photon PSexamplified by H atom detection
Conventional
approach
New approach
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0 r
kT
hcJE
BJ
Ir
JJJJ
t )(
)12(ln
,
,prpJJJJt IIBNI
220
)(
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ee or s ng e s o , poss e,
visualization
-
T x yk
B J B J I x y I x y
( , )( ) /
ln ( ) / ( ) ln ( , ) / ( , )
2
2 2 1 2 1
1 2
1 1 1 2 2 2 1 2
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2D temperature imaging
2D temperature maps can be extracted from signal distribution
images, which are recorded with the laser wavelength tuned to
resonance with two different rotational lines
Single pulse two-dimensional temperature visualization
Analyzerbeam
2 beam
Polarizer
CCD
-
diffraction grating for spatial separation of the two images image-intensified CCD camera for image recording
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2D temperature imaging
Challenges to achieve high single shot
precision:
Stable dye laser beamprofiles, or properreferencing
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g sens v y, ppm
Good spatial resolution by crossed laser beams
2D imaging possibilities
Two-photon (2D) experiments demonstrated
Rather com lex theor
Possible problems with pressure induced birefringences
(e.g. from windows in an engine)
Sensitivit limited b extinction ratio of olarizers
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Why measurements in the IR spectral
In the UV/vis onl a limited number of s ecies OHCH, NH, C2, NO, CH2O, ..) can be probed with
resonant LIF, DFWM, PS.
Many combustion important species CO2, CO, H2O,
2 , 2 2, 4 ,
pose no accessible single-photon electronictransition in the UV/visible, but have stronga sorp on n e m - n rare - m v a ro-vibrational transition.
Spatially and temporally resolved measurementsneeded
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IRPS/IRDFWM
Spectral interferences, especially in
combustion environments where many speciesexist in a narrow spectral range
Doing non-linear experiments with invisible
Probing sensitively many important
otherwise are inaccessible with non-intrusivespatially resolved methods
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Typical IRPS experimental setup
Laser system: DFM in LiNbO3 crystal, 1~3 mJ at 2-4 m, 0.03 cm-1
LN cooled InSb detector
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IRPS spectra ofCH4 and C2H6
1.93% of CH4 and 0.57% of C2H6 mixed with Ar at 1 atm pressure.
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Methane flame detection
(a)Q
J
Cold flow
P(4) P(3) P(2) P(1)
(b)
2 mm
(c)
. mm
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Detection of acetylene and methyl with
=, .
Excitation scan of P(24) and P(23)
C2H2 lines in a gas flow
Calculated IRPS spectra of hot
methane
at 1 mm abover the burner
at 2 mm abover the burner
at 3 mm abover the burner
at 5 mm abover the burner
, 2 2,
Methyl, CH3, detected
Li et al. 31st Comb Symp.
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IRPS detection if C H in soot flames
exc a on spec ra n ca ra on gas: a
and flames: b)
= 1.00, c)
= 1.50 and d)
= 2.50.
HCN t i fl i
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HCN measurements in flames using
Sun et al. 2010
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IRPS measurements: HCN release history of
solid fuel combustion/gasification
Sun et al. 2011
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IRPS measurements: HCN release history of
solid fuel combustion/gasification
3000
2000
2500Wood,1600K
Wood,1300K
n(ppm)
1000
1500
concent
rati
500HCN
0 20 40 60 80 100 120 140
Time (s)
ComparisonofHCNreleaseatdifferenttemperaturesforwoodgasification.Sun et al. 2011
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HCl measurements usin IRPSCH4 /O2 flame seeded with chloroform
Li et al. Opt. Lett 2008
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2D-IRPS
measurements
SpectrarecordedinSF6dopedCH4/airflames,E.r.=1.1,Mckennatypeburner.ThebluearrowsshowHFhotlines.
Sunetal.: Inpreparation
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Experimental set-up: 2D-IRPS
2D i i f HF
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2D imaging of HF
(a) Investigations of the spatial resolution of the imaging system
(b) Thermal radiation from the flame without spectral filter
(c) Photograph of the welding torch flame burning with CH4/O2 (=2) doped with 2% SF6(d) 2-D IRPS imaging of HF. The wavelength of laser focused on R(3) line of HF
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New experimental scheme for IR
DFWM experiments
Z.W. Sun, Z. S. Li, B. Li, M. Aldn and P. Ewart, Detection of C2H2 and HCl with mid-infrared
degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace
molecular species, Appl. Phys. B 98, 593-600 (2010)
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IR-DFWM experiments on C2H2
Investigation of detection limits
IR-DFWM spectrum of 510 ppm C2H2 in
a nitrogen gas flow. Partial assignments
of the spectral lines have been made
Thermometry using IR H O
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Thermometry using IR H2O
Comparison-CARSB2
J42 J22 J6
B3
Lfstrm, Krll and Aldn, Proc. Comb.
Symp. 1637 (1992).Courtesy: Sun and Li 2010
Stimulated Emission (SE)
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Stimulated Emission (SE)
Conceptual behaviour:
Two-photon UV excitation followed by
SE
SE
FilterDichroic
mirror
SE
Burner
Sti l t d E i i
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Stimulated Emission
Signal generated as a new beam
Very strong signal
mp e se -up
Minor species detection (N,C)
Disadvantages Difficult to model
Ma interfere with LIF
Low spatial resolution (?!)
Fl li ti f SE
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Flame application of SE
O atom detection
N atom detection
Increased spatial resolution using SE
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Increased spatial resolution using SE
Ph t h i l ff t ?
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Photochemical effects?