Optical interferometry using comb-swept lasers has the advantage of efficiently reducing the acquisition bandwidth for high-speed and long-range detection. However, in general, the use of a comb-swept laser involves a critical limitation in that the absolute distance cannot be measured, and, thus, multiple layers cannot be distinguished when measuring each position. This is because of the distance ambiguity induced by optical aliasing, in which there is periodic repetition of the frequency of an interferometric signal owing to discrete spectral sweeping, which does not occur in conventional optical interferometry that uses a continuous swept laser. In this paper, we introduce an optical Vernier sampling method using a dual-comb-swept laser to measure the absolute distances in a multi-layer target. For this, we designed a new type of dual-comb-swept laser to include two different free spectral ranges (FSRs) in separated wavelength bands to provide a stable lasing condition. Using a principle similar to that of a Vernier caliper for length measurement, the two different FSRs can be used to recover a higher frequency of an optical interferometric signal to measure longer distances from different layers in a target. Using the dual-comb-swept laser in optical interferometry, we solved the optical aliasing issue and measured the absolute distances of three layers separated over 83 mm using a point-scanning imaging setup and the simultaneous absolute distance of the top surfaces separated over 45 mm using a full-field imaging setup at 14 and 8 times lower acquisition bandwidth than a conventional continuous swept laser that is based on optical interferometry.
Extended measurement ranges with coherent ranging necessitate an increase in the digitizer sampling rate. Subsampled coherent ranging can reduce the sampling rate, but it has been demonstrated in a short range below around 10 cm. Herein, programmable optical Vernier-sampled coherent ranging over several tens of meters with a reduced sampling rate using a newly developed stepped-frequency-swept laser (stepped-FSL) is proposed. The measurable range of three-dimensional subsampled coherent ranging can be extended by increasing the coherence length of the stepped-FSL. The proposed stepped-FSL sequentially emits a discrete frequency output by applying a programmable voltage signal to an electro-optic modulator. The free spectral range of the stepped-FSL output can be rapidly and akinetically adjusted by changing the number of steps in the programmable voltage signal. The proposed system realizes an extended measurable range (over 20 m) and a high repetition rate (200 kHz). A low sampling rate of 10 MS s −1 is sufficient for operation at a subsampled interference frequency of 1 MHz. This rate is an order of magnitude lower than that of the conventional coherent ranging using an FSL. Experimental results demonstrate that the programmable optical Vernier-sampled coherent ranging can be improved by leveraging novel optical subsampling with a stepped-FSL.
The development of scientific technology for art authentication have elicited multidimensional evidence to distinguish forgeries from original artworks. Here, we analyzed three-dimensional morphology of cracks that contain information such as painting features of artworks using an optical coherence tomography (OCT). The forgeries were produced by an expert from the original oil paintings with cracks that occur as paint drying, canvas aging, and physical damage. The parameters such as shape, width, and depth were compared based on cross-sectional images of original and fake cracks. The shapes of original cracks were a rectangular and an inverted triangle, but fake cracks were a relatively simple inverted triangle. The original cracks were as deep as the thickness of the upper layer and mostly have ‘thin/deep' or 'wide/shallow'. The fake cracks were observed as 'thin/shallow' or 'wide/deep'. This study will improve the understanding of the crack characteristics and promote development of techniques for art authenticity.
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