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THz speckle suppression by Hadamard diffuser Host Publication: Finds and Results from the Swedish Cyprus Expedition: A Gender Perspective at the Medelhavsmuseet Authors: I. Jäger, G. Koers, L. Zhang, J. Stiens and R. Vounckx Publication Date: Sep. 2007
Abstract: In recent years the field of terahertz science and technology has expanded in biomedical, security and military relevant applications, because THz waves have the ability to penetrate closing and to image concealed weapons. An advantage of mm wave radiation over X ray is that it is non-ionizing, and there are not any knowing hazards for human health risk. Imaging an object with a coherent beam (all existing THz sources are coherent) is accompanied with speckle phenomenon. This problem is of special importance for THz imaging, because surface roughness is closer to the object dimension (and to the wavelength) as in optical imaging. Some techniques for dealing with speckle in coherent imaging systems utilize frequency and angle diversity to reduce the occurrence of alternating interference. We present results, illustrating speckle reduction in full W-band by means of phase diversity Hadamard solution for a speckle-free imaging with a coherent THz beam.
First the Hadamard principle has been proven by experimental conversion of the coherent sum of electrical THz fields into an incoherent sum of intensities. Then Hadamard diffusers of two different geometries have been machined tested. If the optical Hadamard phase patterns were imprinted by Trisnadi in a piece of fused silica by standard lithography the THz Hadamard mask requires for the fabrication any perforation of a dielectric sample. We fabricated our first transmission Hadamard mask by micro-drilling of a Teflon piece. The mask, working in reflection is a metallic surface with square pattern cells and precisely machined surface height differences. The measured data give results on reflection that are in excellent agreement with the theoretical statistical estimations.
Because W-band supports an imaging of concealed objects, providing both good spatial resolution and penetration, the applications of the developed element, such as medical, security and non-contact profile measurements, could be benefited from such an arrangement. External Link.
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