High Resolution Imaging using 325 GHz and 1.5 THz Transceivers

The Submillimeter-Wave Technology Laboratory (STL) at the University of Massachusetts Lowell has designed and constructed two high resolution terahertz imaging systems based on 325 GHz and 1.5 THz transceivers. While the systems were originally designed to acquire radar signatures of high fidelity scale models, their high sensitivity and high stability at 325 GHz and 1.5 THz make them novel systems for demonstrating the imagery capabilities of terahertz rays. Unlike many terahertz detection systems, the imaging system described here uses ultra stable CO2 pumped far-infrared (FIR) lasers to produce nominally 100 milliwatt continuous wave (CW) terahertz beams. Custom CO2 and FIR laser designs have produced systems with extremely stable power output and low phase drift. The imagery generated by these systems is formed by full beam illumination of the target of interest, followed by coherent processing of the backscattered signal. Several terahertz images will be presented, including high resolution imagery of a concealed weapon, detected through clothing from a distance of 25 meters for each system. An image collection time of less than three minutes produced 512 x 512 pixel image with 1 to 2 mm resolution. The overall systems, including the generation of the 325 GHz and 1.5 THz beams, heterodyne detection systems, target positioning apparatus, and image formation algorithms will be discussed.