Frequency noise mitigation in advanced interferometric fibre strain sensors

Abstract

Frequency noise greatly limits the resolution of high resolution fibre optic strain sensors. The experiments presented in this thesis provide an effective way of obtaining better strain resolution in systems limited by laser frequency noise. In this thesis two main methods are utilised; active frequency noise suppression through feedback control, and post processing noise removal. In chapters 2, 3, and 4, we explore different methods to reduce the frequency noise in lasers used to interrogate fibre interferometric strain sensing systems.

We investigate the use of local fibre lasers and their applicability to high resolution strain sensing. We use radio frequency read out techniques to provide a high resolution read out of strain and frequency fluctuations. Then using active feedback, we lock the laser to a stable Mach-Zehnder interferometer. We demonstrate, with active feedback, ultra-high strain resolutions of 14 f{u220A}/{u221A}Hz at infrasonic frequencies. To allow for more flexibility and ease of use, we used a field programmable gate arrays for digital locking and signal processing systems. We present an analysis of the potential performance of a digital feedback system using the Pound-Drever-Hall technique. In this analysis, we discuss the noise sources and their affect on a digital feedback loop. We then experimentally characterise and verify the theoretical analysis.

With this digital system we then explore the use of an absolute frequency reference. An interrogation system was designed and implemented that extends the range of an absolute sensing systems. Using this method, we are able to improve a free running system by up to 1.5 orders of magnitude whilst improving the range by a factor of 10 over standard pre-stabilised systems. We then propose and implement a frequency noise insensitive sensing system based on post processing techniques. We utilise time-delay interferometry, a technique developed for gravitational wave detection to eliminate frequency noise through post processing. The technique is combined with digitally-enhanced interferometry, and slightly modified to account for, and remove, several noise sources, including lead fibre phase noise, that is common to fibre optic strain sensors. The design and performance considerations are discussed and analysed, and a first time proof of concept experiment is reported. The design implemented has shown to significantly improve the performance of a frequency noise limited strain sensor.