Some Laser Applications Research at ODU

Amin Dharamsi

Dept. of Electrical and Computer Engineering

Old Dominion University, Norfolk, VA

23529-0246

Presented at Graduate Seminar on

31 March 2000

All Credit Goes to Students(Only Current Students

Listed) Graduate Students

Audra Bullock (PhD)

Zibiao Wei (PhD)Jim Barrington (PhD)Shujun Yang (PhD)Grady Koch (PhD)Colleen Fitzgerald (MS)David Lockwood (MS)Ted Kuhn (PhD)M. Abdel Fattah (PhD)

Undergraduate Students

(Senior Project Team)

Ed HeathJim FayAubrey HaudricourtLarry Gupton

Basic Theme

Measurements with Lasers are: sensitive non-intrusive many different applications exciting (fun!!) to make!

A. M. Bullock and A. N. Dharamsi, "Investigation of Interference between Absorption Lines by Wavelength Modulation Spectroscopy", J. App. Phys. Vol. 84, 6929, December 1998.

A. N. Dharamsi, A. M. Bullock, and P. C. Shea, "Reduction of Fabry-Perot Fringing in Wavelength Modulation Spectroscopy Experiments", Applied Phys. Letts., Vol. 72, pp. 3118-3120, June 1998.

A. M. Bullock, A. N. Dharamsi, W. P. Chu and L. R. Poole, "Measurements of Absorption Line Wing Structure by Modulation Spectroscopy", App. Phys. Letts.; 70, 1195-1197, March 1997.

A. N. Dharamsi and A. M. Bullock, "Measurements of Density Fluctuations by Modulation Spectroscopy," Applied Physics Letters, Vol. 69, pp. 22-24, June 1996.

A. N. Dharamsi and A. M. Bullock, "Application of Wavelength Modulation Spectroscopy in Resolution of Pressure and Modulation Broadened Spectra", App. Phys. B, Lasers and Optics; 63, 283-292, November 1996.

A. N. Dharamsi and Y. Lu, "Sensitive Density-Fluctuation Measurements Using Wavelength - Modulation Spectroscopy with High-Order-Harmonic Detection," Applied Physics B., Lasers and Optics, Vol. 62, pp. 273-278, February 1996.

A. N. Dharamsi, "A Theory of Modulation Spectroscopy with Applications of Higher Harmonic Detection," J. Phys. D., Vol. 28, pp. 540-549, February 1996

Some Recent Sample Journal Publications Relating to Modulation Spectroscopy

Note: Audra Bullock,Ying Lu and Patrick Shea who are co-authors in the list below were graduate students in Dr. Dharamsi’s group.

Basic Principle of Techniques

shine laser photons monitor effects

how many photons absorbed? what wavelength absorbed? how much scattering occurred? how much Doppler Shifting? what happened to photons?

converted to phonons? what happened to phonons?

etc, etc

Techniques have several variants

Emission Spectroscopy

Raman Spectroscopy

Absorption Spectroscopy

Optoacoustic Spectroscopy etc, etc

TOPIC 1 Description of Modulation

Absorption Spectroscopy Follows

Basics of Absorption Spectroscopy

Key components Coherent,

monochromatic light source

Detector

I(

Laser Detector

so

o

I I( )

I

Sweep the laser frequency (wavelength) across an energy transition

Detect absorption

I0(

Example of a “Transition” Probed

Oxygen A-band Spectrum

From Hitran 96 DatabaseFrom Hitran 96 Database

Absorption Profile

Frequency,

Ab

sorp

tion

Sig

nal

Frequency molecule Line center shift velocity

Signal strength density Probe two transitions

simultaneously

strengths temperature

Applications

Industrial monitoring velocity and temperature

Environmental measurements of atmospheric

pollutants from ppb to ppt Scientific

lineshape profiles

Wavelength Modulation

Spectroscopy760.228

Temperature Controller

Current Controller

External Oscillator

23.5oCWavemeter

Mirror

Beam Splitter

Diode Laser

DetectorChamber filled with O2

Lock-in Amplifier

10kHz

DC +

10kHz

to Lock-in Amp.

1 m cell

Lineshape Profiles What are they?

How do they arise?

Why should we, as ENGINEERS, bother with them?

Lineshape Profiles-What are they?

Probability of absorption/emission in the interval and + d is

Hence

1)( dg

dg )(

Lineshape Profiles-How do they arise?

V.V. Old QM says discrete levels:

E 1

E3 +/- E3

E 2

E 3

E2 +/- E2

E1 +/- E1

Lineshape Profiles (Why bother?)

Pressure Temperature Collision Dynamics Etc, etc

EVERYTHING is contained in profile

Lineshape profiles

D

2

20

DD

) - ( - exp

1 = )(g

o2

2/1

D cM

2kTln22 =

2ln4 = d

D

Gaussian Lineshape

2 + ) - (2

= )(g2

2o

L

Lorentzian Lineshape

Absorption Signal ProfileTheoryExperiment

m = 4.2, r = 0.03, = /10, coll = 1.7x10-15cm2

Second Harmonic Detection

-0.7

-0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

760.240

760.245

760.250

760.255

760.260

760.265

760.270

760.275

760.280

Wavelength (nm)

Nor

mal

ized

Sig

nal

Third Harmonic Dectection

760.240

760.245

760.250

760.255

760.260

760.265

760.270

760.275

760.280

Wavelength (nm)

Overlapping Lines

Second Harmonic: m = 2.1

-1.0E+9

-8.0E+8

-6.0E+8

-4.0E+8

-2.0E+8

0.0E+0

2.0E+8

4.0E+8

6.0E+8

line 1line 2both lines

Fourth Harmonic: m = 2.1

-3.0E+8

-2.5E+8

-2.0E+8

-1.5E+8

-1.0E+8

-5.0E+7

0.0E+0

5.0E+7

1.0E+8

1.5E+8

2.0E+8

line 1line 2both lines

Sixth Harmonic: m = 2.1

-8.0E+7

-6.0E+7

-4.0E+7

-2.0E+7

0.0E+0

2.0E+7

4.0E+7

6.0E+7

3.89998E+14 3.89999E+14 3.90000E+14 3.90001E+14 3.90002E+14Frequency

line 1line 2both lines

Overlapping LinesFourth Harmonic: m/mo = 1.71

-1.5E-5

-1.0E-5

-5.0E-6

0.0E+0

5.0E-6

1.0E-5

1.5E-5Fourth Harmonic: m/mo = 2.01

-2.5E-5

-2.0E-5

-1.5E-5

-1.0E-5

-5.0E-6

0.0E+0

5.0E-6

1.0E-5

1.5E-5

2.0E-5

Sixth Harmonic: m/mo = 2.01

-2.0E-6

-1.5E-6

-1.0E-6

-5.0E-7

0.0E+0

5.0E-7

1.0E-6

1.5E-6

2.0E-6

2.5E-6

3.0E-6

ModeHop

Sixth Harmonic: m/mo = 1.71

-2.0E-6

-1.5E-6

-1.0E-6

-5.0E-7

0.0E+0

5.0E-7

1.0E-6

1.5E-6

2.0E-6

2.5E-6

ModeHop

Null Measurement Technique

Seventh Harmonic Detection

-4.0E-6

-3.0E-6

-2.0E-6

-1.0E-6

0.0E+0

1.0E-6

2.0E-6

3.0E-6

4.0E-6

76

0.2

50

76

0.2

55

76

0.2

60

76

0.2

65

76

0.2

70

76

0.2

75

76

0.2

80

Wavelength [nm]

Sig

na

l [v

olt

s]

Line center shift 0.000304nm

Change in signal = 38%

TOPIC 2 Description of Optoacoustic

Measurements Follows

Basics of Optoacoustic Measurements

Photons irradiate target Energy converted to phonons Phonon K E randomizes

This is heat generation Optoacoustic signal launched

Carries info on target and light source Signal measured and analyzed

Applications

• Probing of material properties

• Nondestructive evaluation

• In-situ real-time applications

• Biomedical applications

Experiment: contact detection

Laser Driver

Pulsed Laser

Wide-band amplifier

Computer for data acquisition and

processing

400MHz Digital Scope

Trigger outFocusing lens

Sample

20MHz piezoelectric transducer

Thin grease layer

Trigger in

GPIB

Z. Wei, S. Yang, A. N. Dharamsi, B.Hargrave "Applications of wavelet transforms in biomedical optoacoustics", Photonics West, 2000. Proceedings of the Society of Photo Instrumentation Engineers (SPIE) volume 3900- Paper Number Bio 3916-03.

Experiment Data Acquisition - LabVIEW

Modeling Contact detection – Comparison

Results PVC sample (1+0.5mm)– diode laser (880nm)

Acoustic signal

Discontinuity(Grease)

Frontlayer

IncidentLaser Pulse

Backlayer

Grease foracousticcoupling

Pulse 1

Pulse 2

Pulse 3

Pulse 4Piezoelectrictransducer

0.5mm1.0mm

Experiment

Setup – non contact detection

Laser Driver

PumpPhoto Diode

Acoustic Wave

Computer for data acquisition and processing

400MHz Digital Scope

GPIB

CW Laser

Sample Knife- Edge

Trigger

Probe

Pulsed Laser

Wideband Amplifier

Results PVC sample (1.9mm)– Nd:YAG (1064nm)

Probe beam size: 0.8mm

Frequency

1/T

Signal Processing Echo Separation by Fourier Transform Method

Time

Signal Processing Echo Separation by Fourier Transform Method

0 1 2 3 4 5 6 7 8 9 10-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

Time (us)

OA

sig

na

l (a

.u.)

0 0.5 1 1.5 2 2.5 30.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

FF

T M

ag

nu

tide

Frequency (MHz)

Direct MeasurementT = 6.06s

Fourier TransformT = 6.130.31 s

Optoacoustic Applications II Pulsed OA on Tissue Sample – Experiment

C2 layer on top C1 layer on top

Optoacoustic Applications II Pulsed OA on Tissue Sample – Measurement

C1 layer at 337nm=2.2103 m-

1

c.f.C2 layer at 337nm=5.8103 m-

1

TOPIC 3 Description of Remote Sensing

with LIDAR Follows

Lidar for Atmospheric StudiesGrady Koch, NASA Langley and ODU PhD Student

Light reflected from aerosols is collected by the telescope.

Selection of Wavelengths for Lidar

• Size of scattering particle- UV and visible wavelengths best for molecular scattering.- Infrared (1.5-10 mm) best for aerosol scattering.- Near infrared (0.7 to 1 mm) best for mixture of above.

•Eyesafety- Infrared more safe than visible or UV.

• Special Applications- Chemical detection (laser tuned to absorption features).- Wind detection (coherent lidar must generally be eyesafe).Modeling of atmospheric absorption is critical to preserving

range capability.Grady Koch, NASA Langley and ODU PhD Student

-5

0

5

10

15

20

25

30

0 2000 4000 6000 8000 10000 12000

SNR profile looking toward zenithData taken 9/14/97 6:55 PM local

Dallas/Fort Worth International Airport

Altitude (m)

detector recoversat 375 m range

cloud layer

atmospheric boundary layer

Sample Atmospheric LIDAR Return Grady Koch, NASA Langley and ODU PhD Student

Zero Crossing at Line Center, used to stabilize laser

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.19 0.195 0.2 0.205 0.21 0.215 0.22

erro

r si

gnal

(vo

lts)

wavelength 2053.xxx nm

C. M. Fitzgerald, G. J. Koch, A. M. Bullock, A.N. Dharamsi, "Wavelength modulation spectroscopy of water vapor and line center stabilization at 1.462 mm for lidar applications", In Laser Diodes and LEDs in Industrial, Measurement, Imaging, and Sensors Applications II; Testing, Packaging, and Reliability of Semiconductor Lasers V, Burnham, He. Linden, Wang, Editors, Proceedings of SPIE Vol. 3945, pp 98-105, (2000). - Paper Number OE 3945-A14

G. J. Koch, R.E. Davis, A.N. Dharamsi, M. Petros, and J.C. McCarthy, "Differential Absorption Measurements of Atmospheric Water Vapor with a Coherent Lidar at 2050.532 nm," 10th Conference on Coherent Laser Radar, Mt. Hood, OR, 1999.

LIDAR STABILIZATION BY WMS

lock-in amplifier

ref error A-B PZT driver

mod out

Labview

out in

adder

C D C+D

100 Hz

multipass cell2 torr CO2

Ho:Tm:YLFlaser

isolator

beam for injection seed

Figure 4.1: Layout of the spectroscopy and line stabilization experiments. Optical pathe drawn as thicker lines.

0 500 1000 1500 2000 2500 3000 3500 4000

freq

uenc

y fl

uctu

atio

n

time (s)

stabilazation engaged

27 MHz

absorption linecenter

0 500 1000 1500 2000 2500 3000 3500 4000

freq

uenc

y fl

uctu

atio

n

time (s)

215 MHz

absorption line center

Frequency fluctuations with (upper trace) and without (lower trace) stabilization engaged. Fluctuations are measured by the error signal from the lock-in amplifier.

Laser Line StabilizationGrady Koch, NASA LaRC and ODU PhD student

G. J. Koch, A. N. Dharamsi, C. M. Fitzgerald and J.

C. McCarthy, “Frequency

Stabilization of a Ho:Tm:YLF Laser to an Absorption Line of Carbon

Dioxide” Accepted for

publication in Applied Optics