We introduce a novel approach to refractometry using a low coherence interferometer at multiple angles of incidence. We show that for plane parallel samples it is possible to measure their phase refractive index rather than the group index that is usually measured by interferometric methods. This is a significant development because it enables bulk refractive index measurement of scattering and soft samples, not relying on surface measurements that can be prone to error. Our technique is also noncontact and compatible with in situ refractive index measurements. Here, we demonstrate this new technique on a pure silica test piece and a highly scattering resin slab, comparing the results with standard critical angle refractometry.
Two significant figures of merit for optical coherence tomography (OCT) systems are the axial and transverse resolutions.Transverse resolution has been defined using the Rayleigh Criterion or from Gaussian beam optics. The axial resolution is generally defined in terms of the coherence length of a Gaussian shaped source. Whilst these definitions provide a useful mathematical reference they are somewhat abstracted from the three dimensional resolution that is encountered under practical imaging conditions. Therefore, we have developed a three-dimensional resolution target and measurement methodology that can be used to calibrate the three-dimensional resolution of OCT systems.
In this paper a novel method for determining refractive indices of a multi-layered samples using low coherence interferometry (LCI), developed at the National Physical Laboratory, UK, is introduced. Conventional Optical Coherence Tomography (OCT) utilises a lateral scanning optical probe beam to construct a depth resolved image of the sample under investigation. All interfaces are detected in optical path length, resulting in an image depending on the refractive index of all prior layers. This inherent ambiguity in optical and geometric path length reduces OCT images to purely qualitative ones. We have demonstrated that by optically probing the sample at multiple angles we can determine bulk refractive index of layers throughout plane parallel samples. This method improves upon current approaches of extracting refractive index parameters from multi-layered samples as no prior geometrical information is required of the sample and the phase index for each layer is obtained as opposed to the group index. Consequently the refractive index result for each layer is independent of the refractive index of surrounding layers. This technique also improves on conventional measurements, as it is less susceptible to error due to surface defects. This technique is easily implemented, and can easily be modified to obtain in situ measurements. Investigating a silica test piece and comparing the refractive index obtained by that of standard critical angle refractometry has validated the robustness of the technique.
In this study, we have chosen to implement a Monte Carlo simulation of an OCT system in order to investigate elements of the underlying physics of OCT images. Of particular interest is the signal decay primarily attributable to optical scattering, refractive index variations, including index matching, and how these compare with the influence of layer anisotropy.
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