High-accuracy laser spectrometers for wireless trace-gas sensor networks

Abstract

The subject of this dissertation is the development of a wireless sensor network composed of instruments which employ both VCSELs and QCLs 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.

First the field deployment of a low-power, portable, wireless laser spectroscopic sensor node for atmospheric CO2 monitoring is demonstrated. The sensor node shows 0.14 ppmv Hz<super>-1/2</super> 1&sigma; measurement sensitivity of CO2 concentration changes. It was first used to measure top-soil respiration rates in the laboratory and on forest floors in the field.

Then after a long-term field deployment to further assess instrument performance, new design solutions were implemented to improve fringe-limited precision of the nodes to 4-7 ppmv against a 400 ppmv CO2 background, making their performance comparable to higher power consuming commercial trace-gas analyzers. Three optimized nodes were then deployed into mixed landscapes as part of a solar powered CO2 monitoring wireless network. The three node network monitored CO2 in a grassy/woody courtyard, on top of the roof of an engineering building, and next to a road in the Princeton area. These works show that ultra-low powered 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 a revolving in-line reference cell to suppress background and other non-spectroscopic signals. 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 CW injection current into a VCSEL in an external cavity configuration to tune the cavity emission's self-oscillation frequency and show through simulation and experiment that the tuning is dependent on VCSEL birefringence change.