Department of Mechanical Engineering2012-2013

Students: Seth Davies, Betsy Farris, Alex Mende, Constantino Tadiello,

Joseph WilliamsAdvisors: Jordan Rath and Dr. Azer

Yalin

2

Research MotivationThe laser sensor project is in support of the NASA ASCENDS Program (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) whose main goal is to develop a better understanding of the global spatial distributions of atmospheric carbon dioxide (CO2). To do so, the ASCENDS program employs laser sensors from aircrafts to the ground.

3

Problem Statement• Validate the feasibility of using Cavity

Ring-Down Spectroscopy to measure absorption spectra at different temperatures and pressures representative of different altitudes within the troposphere.

4

Objectives• Simulate a temperature range from

-20°C to 20°C• Simulate a pressure range from 0.1

bar to 1 bar• Record spectral data at T, P

corresponding to NASA’s CO2 distribution

• Compare recorded spectra to simulated spectra (HITRAN)

5

Constraints• 90 centimeter sampling cell length• No foreign particulates on the CRDS

mirrors• DAQ system with limited

simultaneous sampling• Budget of $3000

6

Distribution of Tropospheric CO2NASA’s Region of Interest

7

Introduction to Cavity Ring-Down Spectroscopy (CRDS)

More CO2 → Faster Decay → Smaller τ

Less CO2 → Slower Decay → Larger τ

8

Geometric Modeling/Design

Summary

Sampling Cell

9

Geometric Modeling/Design

Summary

Cutaway View of System

10

Geometric Modeling/Design

Summary

Fully Enclosed System

11

Temperature and Pressure Measurement

• Sensors: Thermistor array and pressure transducer• Real-time display to assess

steady state conditions• Used as input for Voigt fit

profile• Graphical displays in LabVIEW

program

12

Optical ComponentsDistributed Feedback Laser

• A near infrared (NIR) distributed feedback diode with a fiber-optic output

• Variable wavelength with a center wavelength of 1571.11 nm

• Temperature and current controlled

• Telecomm style component

13

• Indium Gallium Arsenide (InGaAs) photodiode

• Light signal detection with gain adjustment from 626 V/A to 18.8X106 V/A

• Detection of very low light energy levels escaping the cavity during ring down

Primary Detector

14

Acousto-Optic Modulator• Optical switch allowing energy into

cavity• 40 MHz acoustic wave causes

diffraction of the beam into multiple paths

• Provides a means to rapidly block beam from atmospheric cell when a resonant condition is achieved

15

Mirrors• 99.997% reflective cavity mirrors

made of UV grade fused silica• Create an effective path length of

~100 km• Allow for low levels of light to escape

for detection

16

Lineshape Broadening• Measuring the spectral broadening at different T,

P will assist ASCENDS researchers with interpreting spectra from aircraft measurements

• Doppler (Gaussian) broadening, an inhomogeneous and inertial mechanism, depends on temperature

• Lorentzian broadening, a homogeneous mechanism, depends on pressure• Amount of broadening is characterized by full-width-at-half-maximum (FWHM) values

17

ResultsAchieved Conditions

• Temperature range achieved: -31°C to 23°C

• Pressure range achieved: 0.1 bar to 1.4 bar

• Proved feasibility of recreating extreme tropospheric conditions

• Recorded CRDS spectral data at the T, P points ( ) to the right

• Recorded spectra at multiple temperatures and fixed pressure and vice versa

18

Data Points Achieved

19

Lineshapes

20

Broadening Contributions

2

772.2

41

11

2

v

Gv

lW

v

l

G

W

WWe

WW

II

L

v

LG

L

21

Expected Results

High Temperature → Larger WidthLow Temperature → Smaller Width

High Pressure → Larger WidthLow Pressure → Smaller Width

Experimental Results

High Temperature → Smaller WidthLow Temperature → Larger Width

High Pressure → Larger WidthLow Pressure → Smaller Width

22

Broadening Comparisons

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

0.0

0.2

0.4

0.6

0.8

1.0

-4.3°C -20.7°C -30.9°C

Nor

mal

ized

Abs

orpt

ion

(cm

/cm

)

Relative Frequency (cm-1)

Temperature Broadening

At fixed P (0.5 bar)

Pressure Broadening

At fixed T (-30° C)

23

Conclusions• Accurate detection of carbon dioxide through CRDS in

order to extract meaningful spectroscopic data has been achieved.

• Pressure affected broadening much more than temperature over tropospheric ranges for this spectral line.

• The Whiting approximation provides an accurate description of the spectral shapes for these pressure and temperature ranges.

• The results of this study will aid NASA ASCENDS researchers

• CRDS can also provide viable technology for aircraft measurements of local CO2 concentrations.

24

Acknowledgements

The CSU NASA Laser Team sincerely thanks everyone for all the help we’ve had during this epic journey. A special thanks goes out to Dr. Azer Yalin, Jordan Rath, Adam Friss, Isaiah Franka, Brian Lee, and Joshua Taylor. We would also like to thank last year’s team for all of their hard work. Additionally, we would like to thank Dr. Tammy Donahue, Dr. Mitchell Stansloski, and Dan Pierson for giving us great advice along the way. We couldn’t have had the success we had without all of you. Thank you.

Sincerely,

The NASA Laser Team (2012-2013)

25

QUESTIONS?