Research on long-distance, wide field-of-view and large depth-of-field terahertz imaging based on aspheric lens

• Aspheric lens for large depth-of-field terahertz imaging at long distances. • Depth-of-field extension is achieved by incident light with a wide field-of-view. • Farther from the element than the Bessel beam depth-of-field start point. • Fast imaging of large targets can be achieved with multipixel terahertz detectors. Terahertz (THz) imaging can serve as a versatile instrument to disclose contents of various packages or items hidden under clothing. When imaging a large target with a certain thickness at a long distance, the requirements for the imaging speed and imaging quality of the system are particularly worthy of attention. To tackle these problems and extend an assortment of application, this paper proposes a wide field-of-view (FOV) aspheric lens suitable for rapid imaging systems for extending the depth-of-field (DOF) of THz imaging at long distances. The designed aspherical lens with a diameter of 264 mm – made of high-density polyethylene (HDPE) and manufactured by machining – allows extending the DOF up to 85 mm by separately focusing incident light with different FOV angles, which not only allows fast THz imaging combined with linear array detector but also refuses the use of complicated imaging optical paths. A reflective one-dimensional (1D) scanning imaging system with a 0.1 THz source reveals that the lens can simultaneously achieve wavelength-level resolution and a DOF range of 85 mm for objects at a distance greater than 415 mm from the lens surface in a wide-field illumination system, in which the contrast of the obtained results can reach more than 15%. The observed results justify the application value of the lens in long-distance, wide-field THz imaging systems.