MAGNETO-OPTICAL ENHANCEMENT FOR SPECTROSCOPIC METHODS: HIGH-FINESSE CAVITY SPECTROSCOPY AND DUAL COMB SPECTROSCOPY

Abstract

Faraday rotation and Zeeman splitting describe the rotation of the polarization plane and the attenuation of polarized light during an interaction between light and matter while a magnetic field is applied axially with respect to the propagation of light. In this thesis, methods of using this phenomenon to enhance spectroscopic sensitivity are modeled and compared. Also in this thesis, Faraday rotation is used to enhance the selectivity and sensitivity for cavity enhanced spectroscopy (CES) methods and dual comb spectroscopy (DCS). A theoretical basis is provided for Faraday rotation and Zeeman splitting which is used as a reference and for gas sample concentration retrieval. In addition, the theoretical model is used to compare methods and simulate spectroscopic methods. Experimental results using Faraday rotation enhancement are presented for wavelength modulation spectroscopy (WMS), cavity attenuated phase shift spectroscopy (CAPS), integrated cavity output spectroscopy (ICOS), and DCS enabling these systems to detect Faraday rotations on the order of 1e-6 rad/√Hz per meter. Dual comb spectroscopic measurements are presented in the Mid-IR (3 μm and 10 μm) using Fabry-Perot quantum cascade lasers (QCLs) as the comb source to cover up to 70 cm-1 with 200 modes. Characterization of QCLs exhibiting comb operation is presented as well. Also included is modeling of the broadband and simultaneous multi-species detection ability of DCS. Post-processing techniques are developed and experimentally compared in terms of computation time and sensitivity for the purpose of achieving realtime measurements with temporal resolutions of ~1 μs and minimum detectable absorptions ~1e-5/√Hz per meter. And finally, the design process is presented for a compact breath sensor for oxygen using laser spectroscopy.