A Novel Hemispherical Retro-Modulator for Free-Space Optical Communication Links

We introduce a hemispherical retro-modulator in this work for use in passive free-space optical (FSO) communication links. It incorporates a high-refractive-index glass (S-LAH79) hemisphere for retroflection and a semi-insulating-InP (SI-InP) layer for modulation via electroabsorption. The electroabsorption is interpreted through a unified model of the Franz-Keldysh effect, on field-induced changes to absorption, and Einstein model, detailing band-edge characteristics of absorption. The model is fit to experimental data with good agreement and is used to optimize the SI-InP layer. The hemispherical retro-modulator is realized with a 600-
$\mu \text{m}$
-thick SI-InP layer to give strong retroflection and modulation, at a depth of roughly 34%. Such operation can enable improved performance in passive FSO communication links.