Vibration non-sensitive lithographic system for writing individualized holograms for data storage and security applications

A vibration non-sensitive lithographic system for writing individual computer-generated micro-holograms into a polymer material for data storage and security application is described. The robust lithograph is the central element of the data storage system consisting of the calculation of a computer-generated micro-hologram (CGH), the writing process and the retrieval of the data stored. The writing process is based on the pattern of a CGH, which projects a data pattern with an optimized signal-to-noise ratio. The hologram, which has the size of one square millimeter, consists of one million dots with a diameter of one micrometer each. In less than one second the laser scanning lithograph transfers the calculated hologram into a polyethelen perephtalate foil (PET) using a high-power single-mode laser diode. A thermal process locally converts the semi-crystalline structure of the foil into an amorphous structure. This leads to a change of the refractive index of more than 0.1 without the need of a post-processing5. Because of the diffractive nature of the introduced optical pattern a relative position accuracy of better than 100 nm has to be accomplished. A high-speed differential optical position detection system guarantees the required position accuracy even in a vibration-afflicted industrial environment. A position deviation of the writing spot caused by vibration is detected by a close correlation between the laser spot in the storage material and a second scanning laser spot on top of a grid mask. The position information is combined with the individual hologram pattern by a fast free-programmable gate-array (FPGA) processor, which again controls the laser diode. The reaction time of 26 ns ensures an interference compensation of up to one MHz. The quality of the reconstructed micro-hologram allows the retrieval of up to one kByte of machine-readable information.