Optical Communications Receiver Array

One of the major challenges to free space laser communications and ladar is the impact of turbulence on beam propagation, one example of which is signal fading. These impacts can be exacerbated on airborne platforms by turbulence in the vicinity of the laser system aperture and the platform wake. There are a number of strategies to mitigate this, including adaptive optics, active flow control, and various dimensions of diversity: wavelength, polarization, temporal, and spatial diversity. In this paper we will discuss spatial diversity implemented in the focal region of optical telescopes. We will briefly compare this with other methods, describe results of requirements analysis of array features and optical configurations for various atmospheric turbulence states, and suggest several attractive configurations. We will also report on the design and test of one configuration, implemented in a prototype, and tested for noise performance, optical transmission, modulation bandwidth, and BER performance with our dynamic turbulence simulator. Early evidence shows significant BER improvements of several orders of magnitude at high turbulence fluctuation frequencies using this technique.