Noise mitigation techniques for high-precision laser spectroscopy and integrated photonic chemical sensors

Abstract

In this dissertation, laser spectroscopy is utilized to monitor trace-gas species for environmental and health applications. Due to their non-invasive and in situ sensing capabilities, optical platforms are attractive for on-site, real-time diagnostics. Two main techniques are investigated: (i) Faraday rotation spectroscopy (FRS) and (ii) tunable diode laser spectroscopy (TDLS), where noise reduction techniques are implemented in both cases for precise and accurate quantification of analytes. A variety of sensing configurations are demonstrated, including benchtop laboratory sensors [Chapters 4, 6, 7], transportable extractive point sensors [Chapter 5], and on-chip integrated sensors [Chapter 7].

Chapters 4 to 6 demonstrate FRS for detection of paramagnetic molecules. Using a combination of phase-sensitive signal recovery, balanced-detection and polarizer angle optimization, these sensors consistently demonstrate near shot-noise limited performance with minimum fractional absorption ~ 102× beyond conventional TDLS. Given that FRS is an ideal platform for implementation of common noise reduction techniques, it presents a viable solution to precision spectroscopy of chemical radicals.

Chapter 7 contributes toward a new generation of integrated spectrometers, with the goal of scalable precision sensing nodes. To this end, we present examples of compact TDLS sensor modalities, including integrated sources for broadband, multi-heterodyne spectroscopy, and evanescent waveguide spectroscopy on a silicon-photonic chip. We conclude this dissertation with a vision of a fully integrated TDLS sensor node applicable for real-time sample quantification and localization.