The useful compromise between resolution and penetration power of the submillimeter or terahertz (THz) spectral region has long made it attractive for a variety of imaging applications. However, many of the demonstrations of imaging in this spectral region have used strategically oriented targets, especially favorable concealment materials, proximate imaging geometries, etc. This paper reports the results of studies aimed at better understanding the phenomenology of targets, the impact of this phenomenology on various active and passive imaging strategies, and most importantly, the development of imaging strategies that do not require the aforementioned special circumstances. Particular attention is paid to the relationship between active and passive images, especially with respect to how they interact with the illumination-and detector-mode structures of various imaging scenarios. It is concluded that the very large dynamic range that can be obtained with active single-mode systems (including focal-plane arrays) can be used in system designs to overcome the deleterious effects that result from the dominance of specular reflections in single-mode active systems as well as to strategically orient targets to obtain recognition. This will aid in the development of a much more robust and generally useful imaging technology in this spectral region.

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This paper introduces a new concept for the local oscillator (LO) for the Photon Counting Chirped Amplitude Modulation Lidar (PC-CAML). Rather than using a radio-frequency (RF) analog LO applied electronically either in post-detection mixing or via opto-electronic mixing (OEM) at the detector, or applied via pre-detection mixing using an optical intensity modulator as in previous systems, the new method mixes the single-bit binary counts from the photon counting detector with a single-bit binary LO using an AND binary digital logic gate. This type of LO is called the Digital Logic Local Oscillator (DLLO), and the resulting PC-CAML system is a type of bitstream lidar called bitstream PC-CAML (patent pending).The key advantage of the DLLO in the bitstream PC-CAML is that it replaces bulky, power-hungry, and expensive wideband RF analog electronics with single-bit digital logic components that can be implemented in inexpensive silicon complementary metal-oxide-semiconductor (CMOS) read-out integrated circuits (ROICs) to make the bitstream PC-CAML with a DLLO more suitable for compact lidar-on-a-chip systems and lidar array receivers than previous PCCAML systems.This paper introduces the DLLO for bitstream PC-CAML concept, presents the initial signal-to-noise-ratio (SNR) theory with comparisons to Monte Carlo simulation results, and makes suggestions for future work on this concept.

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