High speed terahertz imaging based on optimized galvanometric illumination
Yuzhe Zhang, Ran Ning, Jie Zhao, Shufeng Lin, Lu Rong, Dayong Wang
The pursuit of high-resolution, high-fidelity, real-time imaging is receiving significant attention in terahertz community. In this study, a versatile illumination approach based on a dual-mirror galvanometer is proposed and optimized for terahertz full-field imaging and computed terahertz tomography. We analyzed the mechanism of galvanometric illumination and elucidated three main factors affecting its homogeneity properties. In this illumination module, the terahertz beam is deflected rapidly by the galvanometer which is driven by triangular voltage signals, and then focused by a self-designed aspherical f-θ lens to illuminate the object at an equal lateral scanning velocity. A homogeneous illumination field with a speckle contrast of 0.11 and isotropic imaging resolution is recorded by an array detector in the form of non-correlated accumulation in a single integration time. By virtue of leading illumination homogeneity and parallelism, a compact imaging system is built for 2D and 3D terahertz imaging with high imaging speed and fidelity.
Beam quality is a core issue in the field of terahertz full-field imaging. In this paper, we present a terahertz spatial filter consisting of two mounted ellipsoidal silicon lenses, which have the same eccentricity but unequal size, and an opening aperture on a thin gold layer between the lenses. At the frequency of 2.52 THz, the beam transmissivity and Gaussicity of the filtering system is investigated by simulation. COMSOL Multiphysics is used to conduct two-dimensional simulation experiments, aiming to find the appropriate aperture size and gold layer thickness, depending on the wavelength of incident light wave and numerical aperture of the ellipsoidal silicon lens. The filtering system is capable of filtering the non-Gaussian beam to a nearly fundamental Gaussian beam and achieves a high transmissivity. In terahertz full-field imaging, the system can not only obtain an imaging beam with uniform intensity distribution, but also reduce the beam energy loss caused by multiple surfaces. Besides, the lens system is applicable for a wide terahertz frequency range if the wavelength dependent part is properly scaled.
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