Princeton University

School of Engineering & Applied Science

High-accuracy Laser Spectrometers for Wireless Trace-gas Sensor Networks

Clinton Smith
Engineering Quadrangle B327
Friday, October 25, 2013 - 10:00am to 11:30am

The subject of my dissertation is the development of a wireless sensor network composed of laser-based instruments for accurate, highly sensitive, and reliable long-term monitoring of environmental trace-gases. The dissertation focuses on the development of low-power instruments and calibration methods that ensure the reliability of long-term measurements of atmospheric gases. First the field deployment of a low-power, portable, wireless laser spectroscopic sensor node for atmospheric CO2 monitoring is demonstrated. The sensor node shows below one part-per-million measurement sensitivity of CO2 concentration changes. It was first used to measure top-soil respiration rates in a controlled laboratory environment and then on forest floors in the field. Next, new design solutions were implemented to improve the ruggedness of the shoe-box sized sensors so that their accuracy would be comparable to commercial trace-gas analyzers which consume greater than 10 times more power. Three optimized sensor nodes were then deployed around the Princeton Engineering Quad (E-Quad) as part of a long-term solar powered CO2 monitoring distributed wireless network. The three node network monitored CO2 in a grassy/woody courtyard, on top of the roof of E-Quad, and next to a road beside E-Quad. These works show that ultra-low powered vertical cavity surface emission laser (VCSEL) based sensor nodes can be placed in off-the-grid environments for autonomous distributed geographic monitoring of trace-gases in a manner which is impossible with current commercial techniques. Next, this dissertation covers two techniques that were developed for the real-time calibration of laser-based trace-gas measurements. The first technique used an in-line reference gas cell and employed wavelength modulation spectroscopy (WMS) at higher harmonics to simultaneously probe the sample and reference spectra. The second technique used spectral correlation of the signals from a revolving in-line reference cell to suppress sources of error. These techniques were designed for eventual inclusion as a real-time calibration source for field deployable trace-gas sensors and wireless sensor networks. Finally, this dissertation demonstrates the use of the injection current into an external cavity VCSEL laser to tune the cavity emission's self-oscillation frequency and show through simulation and experiment that the tuning is dependent on VCSEL birefringence change.