Nonlinear Technigues

7/29/2019 Nonlinear Technigues

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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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-


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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




, 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?

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Nonlinear Technigues