Optical sensors are promising alternatives to high-cost mass spectrometry and high-maintenance micro-scale sensing technologies in environmental monitoring applications. Optical frequency combs generating broadband radiation with a series of phase-locked evenly spaced modes have enabled new approaches to molecular spectroscopy. The mid-infrared molecular fingerprint spectral region is of great importance for spectroscopic sensing applications. Among various mid-infrared frequency comb generation approaches, quantum cascade laser frequency combs (QCL-FCs) attract growing interest as all-electrically-driven, chip-scale comb devices, providing high optical power, and, as for semiconductor sources, they have relatively broadband mid-infrared emission. This research delves into the development of QCL-FC-based spectroscopy systems, covering the entire journey from initial laboratory studies to real-world field deployments.

The first part of the thesis focuses on gaining a deeper understanding of and control over frequency tuning dynamics in QCL-FCs. A full characterization and separate control of two independent fundamental comb parameters, the repetition rate and the offset frequency, in rf-optimized mid-infrared QCL-FCs is demonstrated. When combined with an absolute optical frequency reference technique, these fully controlled QCL-FCs facilitate high-resolution, gapless dual-comb spectroscopy (DCS) across their entire 80 cm^-1 optical bandwidth. 

In the second part, we explore the design and system-level optimization of a reconfigurable mid-infrared dual-comb spectrometer using QCL-FCs. This technology holds promise for the simultaneous detection and localization of multiple hazardous chemical leaks in complex urban environments. With results from multiple in-field controlled experiments, the quality of the spectroscopic measurements conducted by the system and its real-world application impact gets validated.

In the final section, we introduce a moving-parts free, single frequency comb mid-infrared Fourier transform spectroscopy by combining an electronic repetition rate sweeping of a QCL-FC with a highly imbalanced interferometer. Compared to the DCS discussed in the first two sections, this technique uses only one QCL-FC which eliminates the need for two high-quality, well-matched QCL-FCs and high-bandwidth detection electronics required in DCS. This paves the way for a further simplified, cost-effective broadband spectroscopic system.

Adviser: Gerard Wysocki