Impedance Matching Readouts - a New Family of Ultrasensitive Cavity Enhanced Spectroscopy Techniques
Creators: name: Dong, Jiahao
Publisher: name: The Australian National University
Year: 2020

description: Optical cavity enhanced spectroscopy (CES) is a well-established analytical research area, providing powerful and ultra-sensitive tools for the detection and quantification of trace gas concentrations by using optical cavities to effectively enhance the optical absorption. In this thesis, we will explore a family of new CES techniques named 'the cavity impedance matching readouts'. These new techniques all utilise the impedance matching condition of a resonant cavity for the extraction of the intra-cavity absorption, leading to spectrometry measurements of the absorption lines of interest. This family of new methods possesses three highly desirable advantages, namely laser intensity noise immunity, real time, continuous wave operation, and a portable and scalable setup. The most notable feature for the impedance matching readouts is that for the first time in CES research, these three highly desirable advantages are united together in one single system, making them promising candidates for ultra-high precision measurements of trace gas concentration. The impedance matching condition of a cavity is the central concept for this new family of techniques, so we will thoroughly investigate the mathematical derivations behind it, and show the exact relationship between the impedance matching readout and intra-cavity absorption. We will demonstrate the zero background nature of the impedance matching readout and how it leads to immunity to laser intensity noise. The three members of the new impedance matching readout techniques are, name, Cavity Enhanced Amplitude Modulated Laser Absorption Spectroscopy (CEAMLAS), Cavity Enhanced Spatial Mode Interference Spectroscopy (CESMIS) and Polarisation mode Impedance Matching Spectroscopy (PIMS) respectively. Each of the technique ultilises a different approach to generate an amplitude reference for interfering with the probe beam, including radio frequency modulation, higher order spatial mode, and non-degenerate polarisation mode. In this thesis we will individually investigate the conceptual operation and mathematical framework for all the three new techniques, and then present experimental demonstrations on measuring carbon dioxide absorption lines. The performance of these initial setups will be evaluated, together with the analysis of some potential noise sources in each technique that can shed light on potential improvements for future systems. Having a perfectly impedance matched cavity is the optimal operating condition for all these new techniques, but it was found to be challenging to achieve in practice. Laser intensity noise can still affect those measurements when it is not fully suppressed in the presence of impedance mismatch. We have identified two modifications that can be applied to overcome this practical challenge. The first one involves using extra mirrors to form an auxiliary cavity, which allows the impedance matching condition of the coupled cavity to be controllable. The other modification follows a background nulling approach, where the background readout from the impedance mismatch is compensated by a deliberately introduced background readout, practically achieving a zero measurement background even when the cavity is not impedance matched. To further explore the potential of these new techniques, we will introduce a powerful configuration using a double-pass cavity, where the first pass through the cavity provides passive filtering of the spatial mode and frequency mode, resulting in a much cleaner optical input for interrogating the cavity impedance matching readout in the second pass. We will use a double-pass PIMS system as an example to show the derivation of the absorption signal, and to demonstrate the experimental implementation of such configuration, showing a noise equivalent absorption level as low as 5*10^-10 per root Hz, which is comparable to, if not surpassing, many existing CES systems that are widely used around the world.