Overcoming the challenges of active THz/MM-wave imaging: an optics perspective

Abstract

Imaging in the Terahertz (THz) and millimeter-wave (mm-wave) bands offer advantages over doing do in other conventional bands, such as the visible, infrared (IR). The THz band ranges from 300 GHz to 3 THz or in wavelength, 1 mm to 100 μm. These longer wavelengths allows THz radiation to pass unobscured through some materials allowing for imaging hidden threats or defects within such materials. Going further, millimeter-waves cover the spectral band of 30 – 300 GHz, or 10 cm to 1 mm. In addition to passing through denser materials, they also have much less atmospheric absorption, thus are ideal for imaging in adverse weather conditions. In the THz/mm-wave, the greatest challenge to real-time active imaging was previously the lack of compact sensor arrays. INO has overcome this by optimizing its microbolometer focal plane array (originally developed for the infrared) for the longer wavelengths, covering both the THz and mm-wave bands. The remaining challenge for active imaging is how to obtain useful imagery using coherent sources. INO has been working on improving the quality of the illumination beam over the past few years, as well as designing high quality fast imaging optics. This paper will focus on the different techniques that have been tested across the THz and into the mm-wave bands in both transmission and reflection imaging modes. The impact on image quality will be demonstrated, and their implications to developing useful systems for different applications will be discussed.