Abstract
Terahertz technologies are attracting strong interest from high-end industrial fields, and particularly for non-destructive-testing purposes. Currently lacking compactness, integrability as well as adaptability for those implementations, the development and commercialisation of more efficient sources and detectors progressively ensure the transition toward applicative implementations, especially for real-time full-field imaging. In this work, a flexible illumination system, based on fast beam steering has been developed and characterized. Its primary goal is to suppress interferences induced by the coherence length of certain terahertz sources, spoiling terahertz images. The second goal is to ensure an enhanced signal-to-noise ratio on the detector side by the full use and optimized distribution of the available power. This system provides a homogeneous and adjustable illumination through a simplified setup to guarantee optimum real-time imaging capabilities, tailored to the sample under inspection. Working toward industrial implementations, different illumination process are conveniently assessed as a result of the versatility of this method.
Highlights
This technique has been investigated for laser projection application in order to average out what is called speckle in the visible range, an interference pattern induced by the diffusion of wavelength-sized particles [23]. This solution has been investigated in the terahertz range, demonstrating an improved illumination quality, but inducing a significant radiation power loss [24]. Another more robust solution to overcome this coherency issues consists in fast assessing the illumination beam position, using galvanometric beam steering to ensure averaging over imaging frames
We propose to further explore this solution at 2.5 THz and 3.78 THz, to limit the impact of the interference fringes on the image, and to implement an enhanced control over the illuminated area, better suiting the size and the opacity of inspected objects, for an improved versatility and adaptability in real-time imaging applications
The chosen relative phase and frequency pairs, and so γ Lissajous factor, are impacting the scanning pattern as well as the resulting homogeneity. To evaluate such illumination heterogeneity, which represents an important criterion for imaging quality, a calculation of the coefficient of variation on the illuminated area is used and defined as the ratio of the standard deviation σ by the mean value μ, cV = σ/μ
Summary
For more than a decade, terahertz imaging has demonstrated its ability to detect, localize or identify compounds inside optically opaque materials since some dielectric materials, like polymers or ceramics, have relatively low absorption coefficients in this part of the electromagnetic spectrum. Sensors 2020, 20, 3993 time-consuming to obtain a relevant graphical depiction [12], requiring several hours of measurements for a 2D image and up to days to complete the stack of images required for a 3D tomographic reconstruction [10] This tedious process is far from matching with the required frame-rate targeted for quality control or non-destructive testing applications in an industrial context. This solution has been investigated in the terahertz range, demonstrating an improved illumination quality, but inducing a significant radiation power loss [24] Another more robust solution to overcome this coherency issues consists in fast assessing the illumination beam position, using galvanometric beam steering to ensure averaging over imaging frames. We propose to further explore this solution at 2.5 THz and 3.78 THz, to limit the impact of the interference fringes on the image, and to implement an enhanced control over the illuminated area, better suiting the size and the opacity of inspected objects, for an improved versatility and adaptability in real-time imaging applications
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