Passive Terahertz Wave Imaging with a Single Bolometer and with a Bolometer Array

Abstract

Detection of concealed threats like guns, knifes or explosives is of prime importance at airports and other public places. Passive millimetre wave (mmW) imagers operating at frequencies up to almost 100 GHz have been demonstrated for this application. However with increasing distance to the object under investigation they suffer from limited spatial resolution and sensitivity. In this paper we demonstrate passive indoor imaging up to a distance of 7 m. For the initial experiments a Silicon bolometer cooled to 4.2 K was used as detector. The bolometer has a NEP of ~1.5 10-13 W/Hz1/2. It is sensitive in the range from 90 GHz to 3 THz. Images were taken with a mechanical scanner which moves the field of view of the detector across the object under investigation. Although it was possible to take images of hidden objects even at a distance up to 7 m a Silicon bolometer is not the optimal choice for imaging, because it is not a background limited detector and its response time is too large for real time video rate imaging. Also it is not a simple task to implement it into large format focal plane arrays (FPAs). To increase the imaging speed, we implemented a focal-plane array of millimeter-wave superconducting transition-edge sensors red out via superconducting quantum interference device current sensors. This decreased the acquisition time significantly.