Digitally Enhanced Interferometry for Precision Metrology

Abstract

In this thesis we employ Digitally Enhanced Interferometric techniques to manage, measure or reject spurious interference. The technique, by using digitally controlled spread-spectrum modulation, manipulates the coherence of an optical source, enabling selective decoding of optical fields based on time-of-flight. Through this thesis, we demonstrate the use of this technique across three scenarios. Firstly, in Part 1, we consider the application to quasi-distributed interferometric sensing in the form of a multi-point interferometric acoustic sensing array. Using Digitally Enhanced Heterodyne interferometry, we demonstrate simultaneous readout from four discrete in-line surfaces using a single measurement optical field. The demonstration characterises the displacement sensitivity of the measurement system, and proceeds to characterise the residual cross-coupling between the independent reflection surfaces. We further demonstrate some of the limitations of the digitally enhanced readout when considering equidistant targets, which has ramifications for multi-point sensing applications such as LIDAR. The second body of work, presented in Part 2, discuses the continued development of the Digitally Enhanced Fibre Frequency Reference. In this instance, we consider the coupling of coherent Rayleigh backscatter into the interferometric measurement. Using Digitally Enhanced Homodyne interferometry, we enable digital manipulation of the coherence length, thereby restricting the coupling of Rayleigh backscatter. The resultant phase noise is both modelled and measured, with good agreement between both. We further implement a method, using a Rubidium stabilised optical frequency comb, to measure the long term drift of the fibre frequency reference. From this we are able to infer the thermal performance of the interferometer isolation chambers, the therm-optic and thermo-elastic expansion of the interferometer. From these insights, we continue on to develop a nominal design for a future fibre frequency reference architecture. The third and final project; Part 3, considers the implementation of a Digitally Enhanced Homodyne interferometric readout to a Sagnac interferometer. The objective of this work was to demonstrate rejection of first order Rayleigh scattering, and was informed by the work on Rayleigh scattering in Part 2. As part of this project, we also demonstrate the first demonstration of a digital interferometric readout using an incoherent source, one which is significantly broader than the digital interferometric spread-spectrum modulation. We also present a first demonstration of sub 1 urad/rt(Hz) noise floor using a digital interferometric readout. Finally, we also surpass the theoretical Rayleigh backscatter induced phase noise floor by over an order of magnitude, demonstrating rejection of first order Rayleigh scattering. Show more