Refractive index mapping of layered samples using optical coherence refractometry

Abstract

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.