*Communication* **Design, Fabrication and Characterization of an Adaptive Retroreflector (AR)**

**Freddie Santiago 1,\* , Carlos O. Font <sup>1</sup> , Sergio R. Restaino <sup>1</sup> , Syed N. Qadri <sup>1</sup> and Brett E. Bagwell <sup>2</sup>**


**\*** Correspondence: freddie.santiago@nrl.navy.mil

**Abstract:** Recent work at the U.S. Naval Research Laboratory studied atmospheric turbulence on dynamic links with the goal of developing an optical anemometer and turbulence characterization system for unmanned aerial vehicle (UAV) applications. Providing information on the degree of atmospheric turbulence, as well as wind information and scintillation, in a low size, weight and power (SWaP) system is key for the design of a system that is also capable of adapting quickly to changes in atmospheric conditions. The envisioned system consists of a bi-static dynamic link between a transmitter (Tx) and a receiver (Rx), relying on a small UAV. In a dynamic link, the propagation distance between the Tx/Rx changes rapidly. Due to SWaP constraints, a monostatic system is challenging for such configurations, so we explored a system in which the Tx/Rx is co-located on a mobile platform (UAV), which has a mounted retroreflector. Beam divergence control is key in such a system, both for finding the UAV (increased beam divergence at the Tx) and for signal optimization at the Rx. This led us to the concept of using adaptive/active elements to control the divergence at the Tx but also to the implementation of an adaptive/active retroreflector in which the return beam divergence can be controlled in order to optimize the signal at the Rx. This paper presents the design, fabrication and characterization of a low SWaP adaptive retroreflector.

**Keywords:** adaptive retroreflector; tunable lens; adaptive lens; polymer optics; divergence control; fluidic lens; tunable optics
