Abstract
Optical coherence tomography (OCT) is a promising tool for detecting micro channels, metal prints, defects and delaminations embedded in alumina and zirconia ceramic layers at hundreds of micrometers beneath surfaces. The effect of surface roughness and scattering of probing radiation within sample on OCT inspection is analyzed from the experimental and simulated OCT images of the ceramic samples with varying surface roughnesses and operating wavelengths. By Monte Carlo simulations of the OCT images in the mid-IR the optimal operating wavelength is found to be 4 µm for the alumina samples and 2 µm for the zirconia samples for achieving sufficient probing depth of about 1 mm. The effects of rough surfaces and dispersion on the detection of the embedded boundaries are discussed. Two types of image artefacts are found in OCT images due to multiple reflections between neighboring boundaries and inhomogeneity of refractive index.
Highlights
The development of optical coherence tomography (OCT) [1] has been very fast during the past decades, and its applications are spreading beyond the biomedical area to the industry [2]
The scattering coefficient and the anisotropy factor of the alumina and zirconia ceramic samples are shown in the figures below
We consider that the quantitative agreement is achieved only if the following three criteria are fulfilled: (a) the rear boundary of the top layer is only detectable using the 1.7 μm OCT when the surface roughness is small (σ≈30 nm); (b) nothing from the bottom layer can be detected; and (c) the slope of the OCT signal attenuation is the same
Summary
The development of optical coherence tomography (OCT) [1] has been very fast during the past decades, and its applications are spreading beyond the biomedical area to the industry [2]. OCT has been involved in dimensional measurement, art objects inspection, material characterization, fluid sensing, temperature distribution measurement, strain field measurement and etc. Many promising applications have been presented and published, most of which can be found in the abstract collection of the dedicated symposium on OCT for non-destructive testing [3]. Microchannels, metal prints, defects and delaminations can in these cases be embedded in the ceramic layers at hundreds of micrometers beneath surfaces. As a non-destructive technique OCT shows a high potential for detecting and characterizing these features despite the high scattering in these ceramic materials [4,5,6,7]. The surface quality of the ceramic layers and embedded structures may significantly affect the result of OCT detection [8]
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