Graduation Year

2026

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Andreas Muller, Ph.D.

Committee Member

Sarath Withanachchi, Ph.D.

Committee Member

Martin Muschol, Ph.D.

Committee Member

Ashwin Parthasarathy, Ph.D.

Keywords

gas sensing, non-resonant, optical cavity

Abstract

The continual increase in the production and utilization of chemicals across a growing range of industrial, environmental, and medical applications necessitates accurate and efficient measurement of molecular composition. Trace gas detection is particularly critical in industrialized settings, where the identification of colorless and odorless gases is essential for ensuring operational safety and regulatory compliance. Recent advances in optical instrumentation have positioned spontaneous Raman scattering as a promising analytical technique for trace gas analysis, with multipass cavity enhancement providing a cost-effective pathway to high detection sensitivity. However, existing implementations remain confined to laboratory-scale optical table setups, limiting their applicability in field environments where portability and robustness are essential requirements. This dissertation presents the investigation, development, and characterization of a portable multipass cavity-enhanced Raman trace gas analyzer.

Spontaneous Raman gas spectroscopy typically relies on frequency-doubled infrared laser sources to deliver the high optical power and narrow linewidth required for trace-level chemical detection. The relatively low wall-plug efficiency and high optical complexity of these sources, however, present fundamental challenges for integration into field-deployable instruments. Furthermore, the frequency-doubling architecture prevents the utilization of intracavity circulating power for Raman signal enhancement. To address these limitations, a diode-pumped praseodymium-doped yttrium lithium fluoride (Pr:YLF) laser was investigated as an alternative narrow-band visible excitation source. When operated with an intracavity etalon and pumped with 3.1 W from a blue InGaN diode laser, the system achieved a linewidth of 0.5 cm−1, a green output power of 0.37 W, and an intracavity circulating power of 16 W. Trace detection at atmospheric pressure was demonstrated through the spectral resolution of overlapping N2/CO and CO2/N2O Raman bands in air, confirming the suitability of this architecture for high-resolution portable Raman spectroscopy.

While the Pr:YLF architecture represents the ideal future excitation source, a first generation portable multipass cavity Raman gas analyzer was designed, constructed, and validated utilizing the 443 nm pump diode directly. The instrument was integrated within a ruggedized Pelican case and incorporated a desiccating gas handling system and a room temperature CMOS-based spectrometer, providing simultaneous multi-species detection across the 500–4500 cm−1 spectral range. The system operates on battery power, enabling fully autonomous field deployment across uncontrolled environmental conditions.

The instrument was validated through the spatial and temporal mapping of trace hydrogen concentrations using the 4150 cm−1 vibrational band as the concentration measure. Measurements were performed across three environments—a controlled laboratory, an indoor atrium, and an open outdoor space—using an electrolyzer producing hydrogen at rates of up to 600 mL/min. Each measurement was completed within a total sampling time of under 5 minutes at a chamber pressure of 0.34 MPa. The instrument demonstrated a detection threshold of 63 ppb above the ambient hydrogen baseline concentration of 530 ppb and successfully mapped excess hydrogen concentrations at distances of up to 8 m from the source under outdoor conditions. These results establish the feasibility of the portable analyzer for real-world applications, including industrial hydrogen leak detection, geological hydrogen prospecting, and, prospectively, medical breath analysis.

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