from quantum mechanics to auto-mechanics

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