Optimising Signal Isolation and Phase Measurement in Digitally Enhanced Interferometry
Digitally enhanced interferometry (DI) combines code division multiplexing with optical interferometry through the use of pseudo-random noise codes. It allows time-of-flight multiplexed precision sensing with interferometric sensitivity by using these codes to identify the signal of interest, while rejecting spurious noise sources and undesired signal channels. This thesis aims to optimise digitally enhanced interferometry by improving three key areas of this technique; the signal isolation in both heterodyne and homodyne versions of DI and differential phasemeter optimisation based on synthesised common mode noise suppression. In the first part, we address the limitations of the pseudo-random code known as the maximal length sequence (m-sequence). This code has been the most commonly used in DI demonstrates to date, and the autocorrelation of this code restricts signal isolation to 1/Lm, where Lm is the code length. Although longer codes improve channel isolation, the time to compute the code autocorrelation also increases, severely restricting available signal bandwidth. To overcome this, we implement a novel method called offset decoding, which demonstrates that linear combinations of parallel decoding operations at multiple delays can synthesise zero correlation for spurious signals and algebraically eliminate phase cross-talk. Experimentally, we achieve 70 dB of signal isolation, surpassing the standard implementation of the Digitally Enhanced Heterodyne Interferometry (DEHeI) system by more than 40 dB. In the second part, we investigate the limitations of the homodyne version of digitally enhanced interferometry (DEHoI). DEHoI utilises a four-level Quadrature Phase Shift Keying (QPSK) code generated from two m-sequences, which compromises the code autocorrelation. This degrades signal isolation and increases crosstalk. To improve DEHoI autocorrelation, we develop a hybrid QPSK code using a single m-sequence and a square wave. This approach recovers the simple m-sequence correlation. We perform a comparative measurement between the hybrid QPSK code and standard 4-level QPSK approach, demonstrating a factor of 2 improvement in the broadband noise floor. In the last part, we demonstrate a differential phasemeter which is designed to maximise common-mode noise suppression between two interferometric measurements. Common-mode noise suppression is a key tool to mitigate common noise sources between measurements, and often used in interferometry. We develop an open-loop phasemeter topology which minimises demodulation and phase recovery errors for differential optical interferometric measurements. Experimentally, we demonstrate a phase difference measurement with a sensitivity of 10^(-6) rad/ Hz^(0.5) and a common-mode noise rejection of 141 dB. This performance was achieved even when the individual signals comprising this measurement experience numerous cycle slips and cannot independently track phase. We further demonstrate the ability of this technique to perform these measurements under the noise dynamics of a broad linewidth diode laser. Overall, this thesis presents three advancements in digitally enhanced interferometry, offering improved signal isolation, simplified correlation properties, and improved differential phase tracking phasemeters.
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