Novel coherent-detection spatial-tracking system in free-space laser communications

Abstract

It is well known that coherent detection systems are less susceptible to background radiation and thermal noise than direct detection systems. However, with a diffraction-limited field of view, coherent detection spatial tracking systems have angular span restricted to less than one beam width. Moreover, the azimuth and elevation tracking errors will become coupled when the tracking error increases. In this paper, a novel coherent detection spatial tracking system is designed to overcome the above mentioned problems. The optics design of the new spatial tracking is based on a highly sensitive angular discriminator, which can be implemented using a Fabry-Perot Etalon. The post-detection electronic processor is mainly a phase demodulator. The performance of the new spatial tracking system is presented in terms of the standard deviation of the angular tracking error. It is also shown that two of the four photodetectors used for spatial tracking form a dual-detector coherent receiver for communications. In other words, spatial tracking and communications have been integrated into a single system.