Department of Mechanical Engineering Students: Seth Davies, Betsy Farris, Alex Mende, Constantino Tadiello, Joseph Williams Advisors: Jordan Rath and Dr. Azer Yalin

Research Motivation The laser sensor project is in support of the NASA ASCENDS Program (Active Sensing of CO 2 Emissions over Nights, Days, and Seasons) whose main goal is to develop a better understanding of the global spatial distributions of atmospheric carbon dioxide (CO 2 ). To do so, the ASCENDS program employs laser sensors from aircrafts to the ground. 2

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. 3

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 CO 2 distribution Compare recorded spectra to simulated spectra (HITRAN) 4

Constraints 90 centimeter sampling cell length No foreign particulates on the CRDS mirrors DAQ system with limited simultaneous sampling Budget of $3000 5

Distribution of Tropospheric CO 2 NASA’s Region of Interest 6

Introduction to Cavity Ring-Down Spectroscopy (CRDS) More CO 2 → Faster Decay → Smaller τ Less CO 2 → Slower Decay → Larger τ 7

Geometric Modeling/Design Summary Sampling Cell 8

Geometric Modeling/Design Summary Cutaway View of System 9

Geometric Modeling/Design Summary Fully Enclosed System 10

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 11

Optical Components Distributed Feedback Laser A near infrared (NIR) distributed feedback diode with a fiber-optic output Variable wavelength with a center wavelength of nm Temperature and current controlled Telecomm style component 12

Indium Gallium Arsenide (InGaAs) photodiode Light signal detection with gain adjustment from 626 V/A to 18.8X10 6 V/A Detection of very low light energy levels escaping the cavity during ring down Primary Detector 13

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 14

Mirrors % 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 15

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 16

Results Achieved 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 17

Data Points Achieved 18

Lineshapes 19

Broadening Contributions 20

Expected Results High Temperature → Larger Width Low Temperature → Smaller Width High Pressure → Larger Width Low Pressure → Smaller Width Experimental Results High Temperature → Smaller Width Low Temperature → Larger Width High Pressure → Larger Width Low Pressure → Smaller Width 21

Broadening Comparisons Temperature Broadening At fixed P (0.5 bar) Pressure Broadening At fixed T (-30° C) 22

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 CO 2 concentrations. 23

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 ( ) 24

QUESTIONS? 25