The authors have developed a hybrid system for coherent raster-scan imaging at 0.6THz. It combines a high-power electronic source (a multiplied Gunn emitter) with a femtosecond Ti:sapphire laser in order to achieve a high dynamic range via electro-optic detection. The single-scan dynamic range of 60db at a lock-in time constant of 10ms is sufficient to permit detection of scattered terahertz radiation in addition to specularly reflected/transmitted light. Active synchronization of the electronic source and the laser is not needed because of the remarkably low jitter between the two radiation sources.

Characterization of the topography of materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a tenth of the wavelength and more. In this case, THz radiation can provide an interesting alternative. We demonstrate heterodyne profilometry at 600 GHz as a method for the accurate determination of surface topography with an achievable expanded standard uncertainty of 0.5 mum.

We report on the realization of two active, fully electronic THz cameras, both operating at room temperature, but at different frequencies (645 GHz and 300 GHz, respectively). Active illumination together with the frequency-modulated-continuous-wave (FMCW) approach allows us to implement unique features such as phase-sensitive detection, suppression of spurious reflections and high resolution ranging. The measuring of the phase of the THz wave allows for sub-wavelength depth resolution while the ranging capabilities enable us to acquire an absolute depth map of objects under test with accuracy in the mm range. With both systems, we are able to acquire images with more than 55,000 pixels (phase and amplitude) in 9 seconds and the dynamic range exceeds 35 dB. The typical object distance is about 75-150 cm and the image size is in the order of hundreds of cm 2 suitable for stand off detection of concealed weapons.

No abstract

No abstract