Holographic couplers for a see-through application: compensation method and unexpected effects

Holographic systems for see-through applications have become a prominent topic in the optics and photonics community, with several display architectures already proposed and patented. In such systems, the holographic recording material plays a crucial role. Photopolymers are widely considered one of the most suitable materials for both transmission and reflection holograms. However, in both recording geometries, secondary interference patterns can arise due to Fresnel reflections, volume changes during exposure, and the post-cure tilt of the internal fringes. These effects, particularly material shrinkage, can significantly distort or even prevent proper image propagation through the system. In this work, we investigate and quantify the impact of secondary gratings and a shrinkage precompensation strategy with two different photopolymers, respectively. The specific values range from $$\\sim 1.5\\%$$ for Bayfol HX 200 and $$\\sim 4.8\\%$$ for the acrylate-based and low-toxicity Biophotopol. Furthermore, we design a recording geometry that compensates for these shrinkage effects, resulting in an optimized diffractive performance, which serve as a practical guideline for the fabrication of prism-free holographic couplers with predictable properties at the Bragg condition.