Digitally Enhanced Coherent Optical Metrology

Abstract

Light is an excellent tool for metrology due to its short wavelength and ability to offer sub-picometre displacement sensitivity. This thesis looks at techniques for taking the precision of laser interferometry and pairing this with high-performance digital signal processing to create flexible, software-defined instruments for optical metrology applications. Specifically, this thesis relies heavily on Digitally Enhanced Interferometry (DEI) which is a technique that exploits code-division multiple access modulation techniques to discriminate multiple interferometric signals at a single photodetector based on their individual time of flight.

We start by using DEI as a backbone to develop coherent random modulated Light Detection And Ranging (LiDAR). We present a detailed analysis of techniques to mitigate the effects of phase noise and Doppler-induced frequency offsets in both phase and amplitude modulated coherent random modulated continuous wave (RMCW) LiDAR. For amplitude modulation, the analysis focuses specifically on a technique which uses coherent dual-quadrature detection to enable a sum of squares calculation to remove the input signal's dependence on carrier phase and frequency for intensity modulation. This increases the correlation bandwidth of the matched-template filter to the bandwidth of the acquisition system, whilst also supporting the simultaneous measurement of relative radial velocity with unambiguous direction-of-travel. A combination of simulations and experiments demonstrate the sum of squares technique's ability to measure distance with consistently high SNR, more than 15 dB better than alternative techniques whilst operating in the presence of otherwise catastrophic phase noise and large frequency offsets. For phase modulation, a frequency correction algorithm using a dual-quadrature measurement of the light is used to provide a 35 dB improvement in correlation SNR in the presence of large phase noise from a broad-linewidth DFB laser and rotating Lambertian target. The technique is also demonstrated to provide an alternative direction measurement of unambiguous frequency shift of the light. Time-separated quadrature detection is also proposed and demonstrated experimentally, to simplify the optical setup to just electro-optic phase modulators.

Through outdoor range testing of the LiDAR sensor, a gap in current understanding of performance and false-alarm rates for phase-encoded random modulation continuous-wave (RMCW) LiDAR was identified. We present the derivation of a model which focuses on propagating the effects of relevant noise sources through the system to determine an analytical expression for the detection rate, expressed by the probability of detection. The model demonstrates that probability of detection depends only on three factors: i) the mean signal-to-noise ratio (SNR) of the measurement; ii) the measurement integration time; and iii) speckle-induced intensity noise. The predicted analytical relationship between measurement SNR and probability of detection was validated by numerical simulations and experimental demonstrations in both a controlled fiber channel and under fully-developed speckle conditions in an uncontrolled free-space channel.

Based on this coherent LiDAR work, an opportunity was identified to use a class of Pseudo-Random Bit Sequence (PRBS) to provide a step-change in the crosstalk suppression performance of DEI. We demonstrate digitally enhanced interferometry with better than 100 dB mean crosstalk suppression with Golay Complementary Pairs (GCPs) using a combination of numerical simulations and experiments. These results exceed previously reported crosstalk suppression using conventional maximal length sequences by more than 48 dB at a measurement bandwidth of 30 kHz.