Coupling and waveguiding in photopolymers

Hydrophilic photoactive plastic matrices are investigated by holographic and wave-guiding techniques. The photoactive matrices are based on acrylate type monomers, which are dissolved together with xanthine dyes and other additives in poly(vinylalcohol). Dry plastic coatings are obtained by casting the aqueous polymer solutions on glass substrates. Photo-recording occurs in real-time and in-situ, without any post-exposure processing. This paper first describes the recording and diffraction properties of thick asymmetric holographic volume gratings. Such holographic gratings exhibit very high diffraction efficiencies (DEs) approaching 80 percent into free space. Here, they are considered for coupling into and coupling out of wave-guide layers at 850 nm. Our investigations indicated good wave-guiding properties in both, unexposed and exposed photopolymer layers. Grating couplers exhibited angular and spectral bandwidths of about 0.15 degrees and of about 1 to 2 nm, respectively. Coupling DEs of over 50 and de-coupling DEs reaching 6 percent were determined. The large difference between coupling and de-coupling DEs are explained by the losses due to an increase in angular divergence as a result of diffraction and modal dispersion.