Bedford, Grant Richard; (2000) Investigating the effect detector geometry has on heterodyne to non-heterodyne signal ratio (HNHR) in a low-coherence tissue imaging interferometer. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A Monte Carlo simulation is used to simulate the emergent light distribution from a turbid media sample placed in the probe beam arm of a scanning low-coherence interferometer that is used to build-up voxelated images in three-dimensions. The sample is a simple three-layered model with an embedded cylinder which simulates the physical and optical properties of skin for near-infrared light passing through the sample. Coherent light from an input probe beam of variable profile and incidence angle is traced through the sample by means of ray-tracing, and any emergent beams are collected in a sample surface array which also logs any emergent Monte Carlo generated photons reaching the surface. At the surface, the heterodyne to nonheterodyne signal ratio (HNHR) measured by a user-defined variable geometry detector is calculated for each point upon a linear scan of the detector position across the sample surface. A coherence gate is set to image a voxel located at the uppermost point of the blood vessel-simulating cylinder. Monte Carlo simulations are performed for various input probe beam angles and probe beam profiles, and the HNHR analysed for various detector geometries by varying: the detector area, central detector axis angle and the acceptance angle at the detector. The Monte Carlo results confirm the benefit of confocal detection, and indicate that angular decoupling of the light delivery and detection systems can improve differential HNHR measurements at scan extremities by 25 to 30 dB provided that stratum comeum is either removed or the sample index matched to the imaging system. Also presented is the description of a prototype experimental low-coherence interferometer system and some of the results derived from it: at its best this system was able to measure reflected coherent signals with a dynamic range approximating 130 dB.

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