Aplanatic THz imaging


Optical systems for THz imaging frequently consist of readily available components - typically a single spherical or parabolic surface. For THz imaging applications, the range is short, due to limitations associated with atmospheric attenuation. For these applications, single spherical and parabolic surfaces are neither stigmatic, aplanatic nor Herschel. As a result, many THz imaging systems exhibit significant image degradation caused by primary aberrations. Further, the short range limitations frequently result in image degradation due to near violation of the paraxial assumption. For improved imaging, an aplanatic system is required. To achieve aplanatism, a minimum of two aspheric surfaces is required. Aplanatism requires stigmatic performance which dictates surfaces that are conic sections of revolution. A minimum of two are required to exhibit stigmatism and meet the sine condition. An improvement of receiver form factor allows for a decrease in optical image distance and an increase in system magnification factor. This significantly improves a number of THz imaging characteristics such as depth-of-field while maintaining the diffraction-limit resolution and reducing the primary objective diameter. Reduction of objective diameter reduces signal strength principally at the expense of specular reflections. This paper summarizes the results of the optical system design and its incorporation into THz imagers containing THz receivers with improved form factors. Efforts at incorporation of optical zoom will be presented.

Publication Title

Proceedings of SPIE - The International Society for Optical Engineering