Thermally-fixed volume-phase holograms in lithium-niobate crystals for optical interconnects

Summary form only given. Volume holographic components enable free-space optical interconnects with outstanding wavelength and angular selectivity. An application of particular importance is dense wavelength-division multiplexing (WDM). From each hologram light of one WDM channel is diffracted. Orientations and spatial frequencies of the gratings determine the directions of the diffracted light beams. A free-space arrangement allows simultaneous demultiplexing of several channels. The light can be re-coupled into fibers by, for example, gradient-refractive-index (GRIN) lenses. Photorefractive iron-doped lithium niobate crystals (LiNbO/sub 3/) are used for recording and superposition of the required holograms. Inhomogeneous illumination builds up space-charge fields which modulate the refractive index via the electrooptic effect. Thermal fixing is employed to store the holograms permanently. The crystal is heated during or after recording, protons become mobile and compensate for the space-charge fields. After cooling down to room temperature, the fixed hologram can be developed by homogeneous illumination. We report on the realization of a two-channel demultiplexer by superposition of two thermally fixed volume phase reflection holograms in a LiNbO/sub 3/ crystal.