Novel cavity-enhanced techniques for metrology

Abstract

Over the past a few decades, Cavity Enhanced Laser Spectroscopy (CES) has been developed and applied to a broad range of industrial and research fields. It uses the optical cavities to effectively enhance the absorption strength introduced by the presence of intra-cavity gas and provides a powerful tool for gas concentration measurement, pressure detection and other metrology measurements. In this thesis, we will explain a new cavity enhanced spectroscopy technique (named as PIMS) which by manipulating the polarisation state of light when it interacts with a resonant cavity, to probe the optical cavity impedance matching condition. The concept of this PIMS technique was introduced and discussed in multiple conferences and places. This project was initially in collaboration with an industrial partner and the focus is on the proof of concept demonstration. Thus different from purely scientific research, practical concerns, such as easy-to-use and commercial implementability, are the other factors which defined the research direction. Therefore, we aimed at an instrument which is compact in size, immune to laser intensity noise and capable of real time measurement. Incredibly, all of those requirements can be met with the PIMS techniques. More attractively, the PIMS by nature is modulation-free. Depending on the application, it is flexible and can be implemented with no optical modulator, which opens up the opportunities of absorption measurement at different wavelengths and different species. In this thesis, we will not only explain how PIMS works, but also emphasis on the effort and progress we have made throughout the PIMS development. Most of them are not limited to PIMS system, and, more importantly, can be applied to other cavity involved systems. Since gas molecule detections with low absorption strength or low concentration are always challenging, we strived to improve the sensitivity of the PIMS system by eliminating noise sources and improving the readout through post-processing. In this thesis, we explain the process of design and implement a laser frequency stabilisation system and feed-forward correction. Along with improving the readout, we propose a cavity configuration to reduce the beam-pointing error and discuss the scattering induced parasitic etalons present in our system, which is common to all free-space system. We demonstrate two different methods that address this etalon problem both actively and passively. The experimental result shows a reduction in etalon size by a factor of 600 in total, which indicates the efficiency and effectiveness of those two ideas. Combing those effort, we manage to probe the real-time absorption spectrum with the state of art Noise Equivalent Absorption of 3x10^(-13) cm-1 Hz-1/2 using a 1-meter-long equilateral triangle cavity and the cavity finesse of around 2000. This reaches the fundamental shot noise limit and indicates the capability of measuring gas molecules with extremely low concentrations, which fulfils both scientific interests and commercial requirements.