Some Laser Applications Research at ODU Amin Dharamsi Dept. of Electrical and Computer Engineering Old Dominion University, Norfolk, VA 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!

A. M. Bullock and A. N. Dharamsi, "Investigation of Interference between Absorption Lines by Wavelength Modulation Spectroscopy", J. App. Phys. Vol. 84, 6929, December 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 , June 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, , March A. N. Dharamsi and A. M. Bullock, "Measurements of Density Fluctuations by Modulation Spectroscopy," Applied Physics Letters, Vol. 69, pp , June 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, , November 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 , February A. N. Dharamsi, "A Theory of Modulation Spectroscopy with Applications of Higher Harmonic Detection," J. Phys. D., Vol. 28, pp , 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(  LaserDetector u Sweep the laser frequency (wavelength) across an energy transition u Detect absorption I0(I0(

Example of a “Transition” Probed

Oxygen A-band Spectrum From Hitran 96 Database

Absorption Profile  Frequency  molecule  Line center shift  velocity  Signal strength  density u 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 Spectroscopy Temperature Controller Current Controller External Oscillator 23.5 o C Wavemeter Mirror Beam Splitter Diode Laser DetectorChamber filled with O 2 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

Lineshape Profiles- How do they arise? V.V. Old QM says discrete levels: E 1 E 3 +/-  E 3 E 2 E 3 E 2 +/-  E 2 E 1 +/-  E 1

Lineshape Profiles (Why bother?) Pressure Temperature Collision Dynamics Etc, etc EVERYTHING is contained in profile

Lineshape profiles Gaussian Lineshape

Lorentzian Lineshape

Absorption Signal Profile Theory Experiment m = 4.2, r = 0.03,  =  /10,  coll = 1.7x cm 2

Overlapping Lines

Null Measurement Technique

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 out Focusing 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, Proceedings of the Society of Photo Instrumentation Engineers (SPIE) volume Paper Number Bio

Experiment Data Acquisition - LabVIEW

Modeling Contact detection – Comparison

Results PVC sample (1+0.5mm)– diode laser (880nm) Acoustic signal Discontinuity (Grease) Front layer Incident Laser Pulse Back layer Grease for acoustic coupling Pulse 1 Pulse 2 Pulse 3 Pulse 4 Piezoelectric transducer 0.5mm 1.0mm

Experiment Setup – non contact detection Laser Driver Pump Photo 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

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  10 3 m -1 c.f. C2 layer at 337nm  =5.8  10 3 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 ( 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 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 , (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 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 cell 2 torr CO 2 Ho:Tm:YLF laser isolat or beam for injection seed Figure 4.1: Layout of the spectroscopy and line stabilization experiments. Optical pathe drawn as thicker lines.

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 Stabilization Grady 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