Fast Readout In Holographic Storage Based On Photopolymer

Volume holographic storage technology, which has the advantage of large capacity and high transfer rate, becomes the most potential optical storage technology. The aim of this paper is to design a fast readout system for holographic memories based on photopolymer as the storage media to get the readout rate not lower than 100Mb/s.Deep research about the readout rate of the holographic storage memory has been conducted in this paper, which finds the readout rate is affected by different factors. In order to improve the readout rate, we can adopt the following methods, such as using sensitive and low-noise image detector, enhancing the storage density namely decreasing the distance of the neighboring holograms, decreasing the f-number of the Fourior lens in the system, increasing the power of the reference wave and the diffraction efficiency of the holograms, decreasing the angle between the reference wave and the normal of the hologram surface. The practical readout rate of the system is affected by the addressing and capture equipment. In order to improve the practical readout rate, we need increase the speed of the addressing equipment and decrease the distance of the neighboring holograms.In order to improve the readout rate of the volume holographic system, when the performance of the hardware is good enough, the most proper method is to enhance the storage density of the system. We investigated the storage density in terms of the number of holograms stored per unit length along the tracks. Storage density is decided by the Bragg selectivity, but the thickness of the photopolymer limits the Bragg selectivity. By studying the coupled-wave theory for absorption gratings, we found that, the higher the absorption is, the bigger the Bragg angle is.In order to achieve a readout rate of 100Mb/s under the conditions of our laboratory, a multiplexing interval of 150μm in the photopolymer is needed. Absorption of the photopolymer leads to Bragg widening, and the multiplexing distance increased in a certain degree. Experimental study showed that, when the theoretical value of the multiplexing interval is 81μm, the practical multiplexing interval is about 120μm. In order to restrain the crosstalk, we can record holograms using a multiplexing interval of 150μm in our optical configuration.Holograms are readout when the addressing equipment is moving, which needs a strict synchronization between the addressing and capture equipment. According to the moving curve of the linear stage and the frequency of the pulse generator, we designed the time schedule of the pulse laser, addressing equipment and capture equipment. In the experiment, 10 holograms were recorded in a self-developed photopolymer sample with a multiplexing interval of 150μm under the Spatioangular-multiplexed scheme, and then were read out by a designed control program.In order to increase the practicality of the holographic memory, we designed a miniaturized system by using an MCU (micro controller unit), and verified the feasibility of this system. In this miniaturized system, the CMOS detector is triggered by the MCU, and the frequency is matched with the velocity of the addressing equipment. We set a pulse sequence with frequency at 133.3 Hz by Keil C51 software, and measured the practical waiting time from the beginning to the first rising edge. According to the pulse frequency and the moving curve of the addressing linear stage, we set the time schedule of the addressing and capture equipment. Five holograms were recorded in a self-developed photopolymer sample and read out by this miniaturized system, which spent 56ms. Because of the necessary time of a program, the experiment achieved a readout rate of 89 holograms per second.