Multiplexed Holographic Storage In Doubly Doped LiNbO₃ Crystals

Volume holographic storing (VHS) is a promising technique for next generation data storage owning to its high storage density and high data transfer rates. Photorefractive materials (e.g., doped LiNbO3) have been considered as one kind of the most important materials for VHS with their advantages of wider dynamic range and commercial availability in large size. But in these normal cases, readout always results in the erasure of recorded information, known as volatility of photorefractive gratings. Although several techniques for fixing holograms, such as thermal fixing and electrical fixing, have been developed, it is difficult to implement these techniques in real time, and impossible for rapid optical refreshment of memory. These drawbacks affect the practical process of holographic read-write memories. By using doubly doped LiNbO3 crystals with deep-doped center and shallow-doped center, nonvolatile holographic storage can be achieved in an completely optical way.In such two-center holographic technique, sensitizing light of high photon energy (UV light) excites electrons of deep traps to shallow center via conduction band, so that the material is sensitive to the recording lights of low photon energy. During the recording with two coherent beams producing spatially modulated light pattern, the recording medium is sensitized simultaneously by UV light, so that the holograms are both stored in the shallow center and deep center. Since light of low photon energy is only able to erase the holograms recorded in the shallow center, and the holograms in the deep center are left that is insensitive to red light for further readout, thus the nonvolatile volume holographic storage is realized.We studied theoretically the inherent mechanisms of nonvolatile holographic storage in doubly doped LiNbO3:Fe:Cu crystals. The two-color recording and fixing processes are analytically discussed. Three-step recording scheme is made: firstly, illuminate the crystal with UV light to sensitize it for some time, then record holograms with two coherent beams as recording light and a homogeneous UV beam as sensitizing light ,finally fix the hologram with single homogeneous recording beam at the Bragg angle.We performed a series experiments to find out the optimization of two-color holographic recording in LiNbO₃:Fe:Cu crystals. The optimum parameters of...