Superresolution technology for reduction of the far-field diffraction spot size in the laser free-space communication system

Abstract

In the free-space laser communication there is sometimes a strong need for reduction of the diffraction spot size in the far field. In this paper, instead of the usage of the larger size aperture lens in the free-space laser communication system, we introduce diffractive superresolution technology to design and fabricate a cheap pure-phase plate for realizing the smaller spot size than the usual Airy spot size, which can decrease the weight and size of the emitting lens. We have calculated 2, 3, 4, 5 circulation zones for optimizing the highest energy compression (Strehl ratio) with the constraint of the First zero ratio value G=0.8. Numerical results show that the 2- or 3-circular zone pure-phase plate can yield the highest Strehl ratio ( S≈0.59) with the constraint of G=0.8, but the 4, 5 circular zone binary phase (0,π) plates are calculated to yield the result of S≈0.57 with G=0.8. We have fabricated 2- and 3-circular zone binary phase plate with binary optics technology. Finally, we have established an experimental system for simulation of the free-space laser communication to verify the advantage of the superresolution phase plate. Detailed experiments are presented.