The angular selectivity was analyzed theoretically and numerically in the holographic storage system with the spherical
wave as reference beam. The results show that angular selectivity relates with the crystal thickness and independent on z0.
Experiments performed with Zn:Fe:LiNbO<sub>3</sub> crystals also proved the prediction.
We have designed a compact holographic data storage (CHDS) system based on the principle of volume holographic storage and angular multiplexing, using a spherical wave as the reference beam. The CHDS system has large-capacity, high-speed and can retrieve data without changing it, which will provide a revolutionary memory technology. In the experiment, pages of information were recorded and retrieved with the same spherical reference beam. A new method of multiplexing, which is angular multiplexing, furthermore, was developed to maximize the storage capacity, data transfer rate, and minimize the system volume. The main difference between this new method and traditional ones is that we rotate the recording material instead of shifting it. A real experiment system was set up with a volume of 28×18×16cm<sup>3</sup> in our laboratory.
An iterative method designed by angle multiplexing holographic storage is introduced to determine exposure schedule for multiplexing storage in Zn(2mol.%):Fe(0.03wt.%):LiNbO<sub>3</sub> crystal. In this experiment, 60 holograms were recorded in the same location of the crystal with 2mJ/cm<sup>2</sup> incident exposure energy per hologram (0.7s exposure time at 2.8mW/cm<sup>2</sup> total incident intensity). And the self-correlation peaks of holograms were collected by CCD. We found that the cumulative grating strength of correlation peak versus exposure energy could be fitted to be a sixth-order polynomial, and made use of an iterative expression to calculate the exposure schedule for 60 holograms. The correlation recognition of 99 pictures with 16 levels gray is completed and the accuracy is 100%, while an appropriate exposure schedule is used to store the two-dimensional holograms.
Zn:Fe:LiNbO<sub>3</sub> crystal has shorter response time and higher damage resistance ability than Fe:LiNbO<sub>3</sub> crystal because of the doping of Zn ions, and the diffraction efficiency in Zn:Fe:LiNbO<sub>3</sub> is comparable to that in Fe:LiNbO<sub>3</sub> crystal. Transmission holographic recording geometry has been taken for its larger attainable dynamic range and sensitivity than that of the 90-degree geometry. Although there is relatively strong fanning effect in transmission geometry, Zn ions in the co-doped Zn:Fe:LiNbO<sub>3</sub> crystal can restrain the fanning effect effectively. In our experiment, 60 holograms were angularly multiplexed and 72 spots were spatially multiplexed without hologram fixing technology, and the holograms can be stored more than ten days. A storage density of 4Gbits/cm<sup>3</sup> has been obtained. The whole writing/reading process was accomplished automatically.
The infrared spectra of congruent LiNbO<sub>3</sub>:Fe:M (M= Mg<sup>2+</sup>, Zn<sup>2+</sup>, In<sup>3+</sup>, [Li]/[Nb]=0.946) crystals with 0.03wt% Fe<sub>2</sub>O<sub>3</sub> were measured by a FT-IR spectrometer from 3400cm<sup>-1</sup> to 3580cm<sup>-1</sup>. The spectra show that OH<sup>-</sup> absorption band of double-doped LiNbO<sub>3</sub>:Fe:M crystal locates at about 3482 cm<sup>-1</sup> as that of single-doped LiNbO<sub>3</sub>:Fe crystals, and it is not shifted to ultraviolet band until the concentrations of M exceeds its concentration threshold. The threshold of damage-resistant ions M is 6mol% Zn<sup>2+</sup>, 4mol% Mg<sup>2+</sup> and 3mol% In<sup>3+</sup>, respectively. The OH<sup>-</sup> absorption band of M-doped (>concentration threshold) LiNbO<sub>3</sub>:Fe crystals shifted to 3535, 3525 and 3508 cm<sup>-1</sup>, which results from the OH<sup>-</sup> stretching vibration in [M<sub>Nb</sub>]<sup>a-</sup>-OH<sup>-</sup>-[M<sub>Li</sub>]<sup>b+</sup> complexes. The influence of valence on the concentrate threshold is also discussed.