High-Resolution Molecular Spectroscopy With An Optical Frequency Comb

Abstract

Optical spectroscopy provides a window into the world of molecules and their environment by the absorption of electric dipole radiation with frequencies characteristic to each molecular species. The temperature, concentration, and pressures of molecules in a gas sample can theoretically be obtained through examination of optical absorption spectra. This is provided the spectrum is of high enough resolution and sufficient bandwidth that the complicated molecular absorption spectrum may be observed, particularly in cases with multiple molecular species present in a sample. The invention of a fully-stabilised optical frequency comb in recent decades has revolutionised molecular spectroscopy. It provides a near-ideal spectral interrogation source for the high-resolution study of molecules, combining absolute frequency accuracy, broad singleshot bandwidth, and dense spectral sampling. The comb light is contained within a single beam, and must be dispersed into its component frequencies in order for a molecular spectrum to be extracted. There are numerous methods to perform this, with the technique employed in this thesis utilising a dispersive spectrometer based on a virtually imaged phased array. The spectrometer spreads the comb light from a single beam into a two-dimensional array of its component frequencies, allowing the power of each comb frequency to be measured. This thesis details the development and construction of a virtually imaged phased array spectrometer system for use with an optical frequency comb. Additionally, code that extracts the traditional absorption spectrum from the two-dimensional arrays of frequencies produced by the spectrometer were developed and demonstrated, along with a model to extract physical parameters of molecules. The theoretical basis to model the characteristic absorption fingerprints is presented for each of the molecules examined in the course of this thesis (hydrogen cyanide, carbon dioxide, and acetylene), as well as the differences in spectra caused by changes to the pressure, temperature, and concentration of molecules in the sample. The results chapters walk through the development of the spectrometer into a reliable system capable of rapidly acquiring high-quality molecular spectra from which highly accurate and precise measurements of concentration and temperature were demonstrated. The capability of the system to easily differentiate between isotopologues of the same species in the same sample makes this spectrometer a powerful spectroscopic tool that, with further development, may find use in out-of-lab applications such as medical breath analysis and environmental monitoring. Additionally, the demonstrated capability to measure extremely high-resolution spectra beyond the resolution limit of the spectrometer may find use in measurements of the thermodynamic properties of molecules.