Large depth-of-focus achievement based on an aspheric lens with a ring

Terahertz (THz) imaging technology has been widely studied because of its easy penetration of non-polar materials and low photon energy. To acquire a beam featuring both excellent transverse spatial resolution and a considerable depth-of-focus (DOF) to fulfill the demands of two-dimensional and three-dimensional THz imaging, this paper presents an aspheric lens with ring (ALR). The ALR has a controlled diameter of 50 mm, can be machined by 3D-printed technology, and does not need to use complex imaging optical paths to achieve the large DOF function. In a transmitted point-scan imaging system with a 140 GHz light source, the lens can achieve both a resolution of 6 mm and an effective DOF of 66.4 mm for objects greater than 27 mm from the lens surface.