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…
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