Fourier Imaging Based on Sub-harmonic Detection at 600 GHz

Abstract

Fourier imaging is an indirect imaging method that records the Fourier spectrum coherently in the focal plane of the imaging system and reconstructs the image using computational resources. To reduce the cost of Fourier imaging at THz bands, the 2nd sub-harmonic heterodyne detection based on a broad-band Si CMOS TeraFET detector is adopted here. Continuous-wave (CW) object radiation at 600 GHz and local-oscillator (LO) radiation at 300 GHz are selected for the validation. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of a single detector unit through the focal plane. A key to the successful application of 2nd sub-harmonic mixing in Fourier imaging is the introduction of dual substrate lenses for the illumination of the detector. The lens for the LO radiation is a novel type of composite wax/PTFE lens on the front side of the detector chip, and it embeds the detector’s wiring. The other lens is a common Si substrate lens for the backside coupling of the object wave to the detector. The coupling efficiency of the LO radiation is increased by 10 dB compared with the case without the wax-PTFE lens, which results in a 20-dB dynamic-range enhancement of the reconstructed image and a significant improvement of the spatial resolution.