Phased Array of Phased Arrays for Free Space Optical Communications1,2

An aperture composed of multiple small apertures has several advantages over a large continuous aperture when practical issues are included in the analysis. For example, a single bullet can easily produce a catastrophic failure of the laser communication link if all the optical gain for the transmitter (or receiver) is derived from a single large-diameter telescope. In addition to battlefield concerns, other naturally-occurring problems for large diameter optical systems exist such as turbulence and scintillation. A distributed aperture, composed of small segments (smaller than the coherence length along the beam path), provides a powerful means for mitigating turbulence effects, particularly if the sub-apertures are dynamic steering elements with independent phase adjustments. This type of capability is available using a phased array of phased arrays (PAPA) architecture, which also provides non-mechanical steering, single and multi-spot beam shaping, and high-power beam combining. This paper discusses some of the attributes and potential problems associated with the PAPA approach.