Practical realization of reflective metallic coatings for planar-integrated free-space optics

In planar-integrated free-space optics (PIFSO) communication takes place inside a planar transparent substrate that serves as the integration platform. Optical signals are kept on zigzag paths by means of surface-relief diffractive optical elements and reflective coatings. Due to the repeated reflections at the substrate surfaces the physical properties and the quality of a coating is crucial for a high interconnect efficiency of a PIFSO system. For batch production of experimental prototypes metallic coatings are usually used because the material processing is relatively simple and low-cost. Here we report about compound coatings made of silver and aluminium that combine the high reflectivity of the first material with the chemical passivity and the good adhesion to glass of the second one. Various layered coating compositions and lithography-based deposition techniques are discussed. We present measurement results taken from experimental samples of the respective coating types that confirm their expected optical and mechanical properties.