Passive Millimeter-Wave Imaging Using Optical Modulators
Human vision lies within the range of electromagnetic radiation most eŸectively emitted by the sun, approximately between the wavelengths of 400 nm and 700 nm. Other ranges of the electromagnetic spectrum, such as X-ray, infrared (IR), terahertz radiation, radio-frequency (RF), etc., have been used for applications where they provide a signi˜cant advantage over visual observation systems. For example, radar systems provide signi˜cantly larger range and resistance to atmospheric disturbances for remote tracking of aircra¥. IR systems are very eŸective at detecting and imaging scenes of varying temperatures or emissivities, enabling the creation of thermal imaging systems. However, the millimeter-wave (MMW) regime remains relatively unexplored despite of the unique ability of MMW radiation to penetrate fog, clouds, smoke, and thin dielectrics while maintaining high resolution. Many potential applications exist for remote aerial imaging of obscured ground scenes and for security scanning. ™e lack of development in these areas is mainly attributed to the complex nature of MMW sources and detectors. For detection of passive emissions at terrestrial temperatures, very high detection sensitivity is required for an imager since the native blackbody radiation at millimeter wavelengths decreases by a factor of ~108 from its peak value. To date, most imaging technologies have been limited by the cost of low-noise ampli˜ers (LNAs) and the physical limitations imposed by the diŸraction limit, which require bulky imagers to achieve even modest resolutions.