Coutinho, Ricardo Cesar; (2003) High sensitivity optical detection using temporal coherence interferogram phase changes. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

Most current detection and imaging systems rely on intensity differences between the object of interest and the background. With the recent advances in detector and computing technologies, sensitivities are reaching the background quantum fluctuation level, and performances tend to be limited by background clutter. In order to obtain further contrast enhancement, new dimensionality must be added to the detection process. This thesis studies a detection technique named Interferogram Phase Step Shift (IPSS), which relies on the coherence contrast between target and background to improve discrimination. The approach employs an interferometer to create a coherence profile, a narrowband optical filter to generate a feature in this profile, and an algorithm to locate this feature. By measuring the shift in this feature in path difference, incremental changes in coherence can be sensed. Unlike Fourier transform spectroscopy, it does not require a Fourier transform, and performs a much narrower path difference scan, generating less data for processing. Laboratory experiments with different simulated targets and experimental arrangements demonstrate very high sensitivities for coherent sources (e.g. -46 dB signal-to-clutter ratio for a laser), and high sensitivities for partially coherent sources (-30 dB for light from a monochromator). A system based on a CCD camera, with no moving parts, is also shown to yield high sensitivities. The experimental results compare favourably with most competing techniques reviewed. The performance of the proposed detection system was also characterised through the calculation of its Receiver Operating Characteristic (ROC) curves. A laser target with a signal-to-clutter ratio of -26 dB was detected with a probability of detection of 90%, and a probability of false alarm of 2.10-4. The thesis includes a theoretical model, which predicts the experimental results with good agreement. The theory also calculates the effects of target-to-filter bandwidth ratio and central wavelength offset, which was also confirmed to be in agreement with the experiments.

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