Remote sensing systems represent an important class of photonics solutions used in a variety of applications, including autonomous navigation, topographic mapping, atmospheric research, and agriculture. Laser-based spectroscopic sensors are a subgroup of remote sensing systems, and they have distinct advantages enabled by wavelength-specific, narrow-linewidth, and single-frequency laser sources. This subgroup includes open-path spectroscopic sensors, which have high sensitivity, high selectivity, high temporal resolution, and robust operation.

Despite their great practical importance, open-path spectroscopy sensors are limited in their sensitivity over long distances, due to low return signal. With their limitations, localizing, mapping, and quantifying emissions sources such as hazardous gas emissions and fugitive leaks has remained challenging. This dissertation is centered on the development of techniques and sensor architectures to alleviate the existing drawbacks associated with open-path spectroscopic sensors. The solutions outlined were applied to fugitive methane emissions localization and detection and have broader applications in photonics to fields such as free-space optical communications, light detection and ranging (LiDAR), and directed energy systems.
This work is divided into three main sections, defining system-level architectures that improve open-path spectroscopic sensor performance. The first section is devoted to the implementation of innovative receiver solutions that allow for better photon collection efficiencies with reduced complexity. This section details methods developed for active adaptation to dynamically varying sensor pathlength and for optimization of the telescope design to reduce the system complexity.

The second section focuses on the development of higher-power laser transmitters, which employ amplification and coherent beam combination while preserving the spectral properties of the narrow-linewidth seed lasers. Two alternative strategies for scaling the output optical powers and peak intensities are demonstrated: Raman amplification and coherent beam combining. Depending on the application requirements, these techniques can be used independently or in combination.

Finally, the third section is devoted to system-level integration and performance assessment of open-path spectroscopic sensing systems that combine the developed transceiver technologies. Throughout this work, multiple systems were developed, and their performances have been evaluated in the field for both stationary and drone-mounted retro-reflector sensing configurations. Field testing an open-path chirped laser dispersion spectroscopy system using a drone-mounted retroreflector and inversion methods resulted in localizing, mapping, and quantifying fugitive methane emissions to within 1 meter and 30% of the emission rate.

My work resulted in an innovative patent-pending architecture for open-path spectroscopy systems that have reduced system complexity and size, with comparable sensitivity and tracking performance to the field-tested chirped laser dispersion spectroscopy systems. The results define a path toward advanced laser-based remote sensing spectroscopic systems with enhanced reach, sensitivity, and spatial resolution.

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Adviser: Gerard Wysocki