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 Heath
Jim Fay
Aubrey Haudricourt
Larry Gupton
Basic Theme
Measurements with Lasers are:
 sensitive
 non-intrusive
 many different applications
 exciting (fun!!) to make!
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.
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
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
I0(n)
I(n)
Laser

Key components


Detector
the laser frequency
Coherent,
(wavelength) across an
monochromatic
energy transition
light source
 Detect absorption
Detector
Io  I(n )
s
Io
 Sweep
Example of a “Transition”
Probed
Oxygen A-band Spectrum
From Hitran 96 Database
Absorption Profile
Frequency molecule
 Line center shift velocity
 Signal strength  density
 Probe two transitions
simultaneously
strengths  temperature
Absorption Signal

n
Frequency, n 
Applications

Industrial


Environmental


monitoring velocity and temperature
measurements of atmospheric pollutants
from ppb to ppt
Scientific

lineshape profiles
Wavelength Modulation
Spectroscopy
Temperature
Controller
23.5oC
760.228
Mirror
Wavemeter
Current
Controller
Chamber filled with O2
Detector
DC +
Diode
Laser
External
Oscillator
Beam
Splitter
1 m cell
Lock-in
Amplifier
10kHz
to Lock-in
Amp.
10kHz
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
g
(
n
)
d
n

g
(
n
)
d
n

1

Lineshape Profiles-How do they
arise?
V.V. Old QM says discrete levels:
E3
E2
E1
E3 +/-  E3
E2 +/-  E2
E1 +/-  E1
Lineshape Profiles (Why bother?)

Pressure
Temperature
Collision Dynamics
Etc, etc

EVERYTHING is contained in profile



Lineshape profiles
Gaussian
Lineshape
 (n - n 0 )2 
1
exp  gD (n ) =

2
 n D
 n D 
1/ 2
 2 kTln 2 
n D = 2 
 no
2
 Mc 
n d
n D =
4 ln 2
Lorentzian Lineshape
gL (n ) =
n
2

 n  
2
2 (n - n o ) + 

 2  

Absorption Signal Profile
Theory
Experiment
m = 4.2, r = 0.03, y = /10, scoll = 1.7x10-15cm2
Third Harmonic Dectection
Second Harmonic Detection
0.7
Normalized Signal
0.5
0.3
0.1
-0.1
-0.3
-0.5
-0.7
760.280
760.275
Wavelength (nm)
760.270
760.265
760.260
760.255
760.250
760.245
760.240
760.280
760.275
760.270
760.265
760.260
760.255
760.250
760.245
760.240
Wavelength (nm)
Overlapping Lines
Second Harmonic: m = 2.1
Fourth Harmonic: m = 2.1
6.0E+8
2.0E+8
4.0E+8
1.5E+8
1.0E+8
2.0E+8
5.0E+7
0.0E+0
0.0E+0
-2.0E+8
-5.0E+7
-4.0E+8
-1.0E+8
-6.0E+8
-8.0E+8
-1.0E+9
-1.5E+8
line 1
line 2
both lines
-2.0E+8
-2.5E+8
-3.0E+8
line 1
line 2
both lines
Sixth Harmonic: m = 2.1
6.0E+7
4.0E+7
2.0E+7
0.0E+0
-2.0E+7
-4.0E+7
-6.0E+7
line 1
line 2
both lines
-8.0E+7
3.89998E+14 3.89999E+14 3.90000E+14 3.90001E+14 3.90002E+14
Frequency
Overlapping Lines
Fourth Harmonic: m/mo = 1.71
1.5E-5
Fourth Harmonic: m/mo = 2.01
2.0E-5
1.5E-5
1.0E-5
1.0E-5
5.0E-6
5.0E-6
0.0E+0
0.0E+0
-5.0E-6
-5.0E-6
-1.0E-5
-1.5E-5
-1.0E-5
-2.0E-5
-1.5E-5
-2.5E-5
Sixth Harmonic: m/mo = 1.71
Sixth Harmonic: m/mo = 2.01
2.5E-6
3.0E-6
2.0E-6
2.5E-6
1.5E-6
2.0E-6
1.5E-6
1.0E-6
1.0E-6
5.0E-7
5.0E-7
0.0E+0
0.0E+0
-5.0E-7
-5.0E-7
-1.0E-6
-1.0E-6
-1.5E-6
-1.5E-6
-2.0E-6

Mode
Hop
-2.0E-6


Mode
Hop

Null Measurement Technique
Seventh Harmonic Detection
4.0E-6
Change in signal = 38%
3.0E-6
Signal [volts]
2.0E-6
1.0E-6
0.0E+0
-1.0E-6
-2.0E-6
-3.0E-6
Line center shift
-4.0E-6
0.000304nm
760.280
760.275
760.270
760.265
760.260
760.255
760.250
Wavelength [nm]
TOPIC 2

Description of Optoacoustic
Measurements Follows
Basics of Optoacoustic
Measurements



Photons irradiate target
Energy converted to phonons
Phonon K E randomizes


Optoacoustic signal launched


This is heat generation
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
Sample
Laser
Driver
20MHz
piezoelectric
transducer
Pulsed
Laser
Wide-band
amplifier
Focusing
lens
Trigger out
Thin grease
layer
Trigger in
Computer for data
acquisition and
processing
GPIB
400MHz Digital
Scope
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)
Discontinuity
Back
(Grease)
layer
Front
layer
Incident
Laser Pulse
Grease for
acoustic
coupling
Pulse 1
Pulse 2
Pulse 3
Acoustic
signal
Pulse 4
Piezoelectric
transducer
1.0mm
0.5mm
Experiment
Setup – non contact detection
Pulsed
Laser
Laser
Driver
Pump
Acoustic
Wave
Photo
Diode
Probe
CW
Laser
Sample
KnifeEdge
Trigger
Computer for
data acquisition
and processing
GPIB
400MHz
Digital Scope
Wideband
Amplifier
Results
PVC sample (1.9mm)– Nd:YAG
(1064nm)
Probe beam size: 0.8mm
Signal Processing
Echo Separation by Fourier Transform Method
Time
1/T
Frequency
Signal Processing
Echo Separation by Fourier Transform Method
Direct Measurement
T = 6.06s
Fourier Transform
T = 6.130.31 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
=2.2103 m-1
c.f.
C2 layer
at 337nm
=5.8103 m-1
TOPIC 3

Description of Remote Sensing with
LIDAR Follows
Lidar for Atmospheric Studies
Grady 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
Sample Atmospheric LIDAR Return
Grady Koch, NASA Langley and ODU PhD Student
Zero Crossing at Line Center, used to
stabilize laser
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
0.3
error signal (volts)
0.2
0.1
0
-0.1
-0.2
-0.3
0.19
0.195
0.2
0.205
0.21
wavelength 2053.xxx nm
0.215
0.22
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
adder
100
Hz
C D
C+D
lock-in
amplifier
ref error
A-B
PZT driver
Labview
mod
out
out
Ho:Tm:YLF
laser
in
multipass cell
2 torr CO2
isolat
or
beam for
injection
seed
Figure 4.1: Layout of the spectroscopy and line stabilization experiments. Optical
pathe drawn as thicker lines.
Laser Line Stabilization
frequency fluctuation
Grady Koch, NASA LaRC and ODU PhD student
stabilazation engaged
27 MHz
absorption line
center
0
500
1000
1500
2000
2500
3000
3500
4000
frequency fluctuation
time (s)
absorption line center
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
215 MHz
0
500
1000
1500
2000
2500
3000
3500
4000
time (s)
Frequency fluctuations with (upper trace) and without (lower trace)
stabilization engaged. Fluctuations are measured by the error signal from