【作者】 孟纲；

【导师】 江竹青； 陶世荃；

【作者基本信息】 北京工业大学 ， 光学， 2001， 硕士

【摘要】 光折变晶体是在大容量全息存储中应用最为广泛的一种介质。根据电荷的激发和输运机理，电荷场在晶体中形成电子光栅，以存储光信息。然而，这种机制也导致了光照下全息图的光擦除。因此，在光折变晶体中记录的全息光栅不能长时间保存。为了克服全息光栅读出过程的光擦除作用，提出了几种固定方法，包括电固定、热固定和光固定技术。其中，热固定是研究最多的、最有效的固定方法。热固定的过程包括两个步骤：固定和显影。在固定阶段，晶体中的离子在高温下运动，形成补偿电子光栅的离子光栅。在显影阶段，电子光栅被擦除，显现出离子光栅。 应用分批方法，将需要存储在晶体同一位置的所有全息图分为若干批次，每组在室温下记录，然后高温定影，当所有组次都记录、固定完成后，统一显影。因此，分批热固定包括下列过程：电子光栅被同一组中后续记录的光栅的光擦除和不同组次记录光栅时的光擦除；离子光栅在热固定时对同组内电子光栅的补偿和后续组次固定时对前组离子光栅的平滑作用。根据光擦除和离子光栅在后续组次热固定过程中的多次平滑作用，设计了测量组间擦除时间常数的分批热固定实验。根据在所有组次中的每组衍射效率，选择了双掺LiNbO₃晶体。实验结果表明：组间擦除时间常数远大于常规擦除时间常数。其原因是光致电子的运动被相应的离子光栅阻碍，离子光栅对电子光栅的屏蔽效应减慢了不同组次间的光擦除。通过对单次和分批热固定的实验测量了固定次数对图像质量的影响。结果证明多次热固定过程对图像质量的影响并不明显。最后，通过测量重构全息图的信噪比，拟和出暗保存时间。更多还原

【Abstract】 Photorefractive crystals are commonly used as a recording medium in volume holographic storage. According to the charge excitation and transport mechanism, the electronic charges constitute holograms in photorefractive crystals to store some desired optical information. However, the above mentioned mechanism similarly causes optical erasure of holograms in further illumination process. Therefore, the recorded holographic gratings are not permanently maintained in photorefractive crystals. In order to overcome such optical erasure to holograms on readout, several fixing methods developed, including electric fixing.. thermal fixing and optical nonvolatile readout techniques. Among them, thermal fixing is the most active and effective technique. The procedure of thermal fixing includes two steps: fixing and developing. In fixing stage, the ions in crystals move to form an ionic grating and neutralize the electronic grating at elevated temperature. In developing stage, a homogeneous illumination erases the electronic grating and brings out the ionic grating when cooling to room temperature. Using the batch method, all the holograms to be stored in one location of a crystal are divided into several batches, each batch is recorded at room temperature and followed by a fixing process at higher temperature fixed at higher temperature. After all batches are recorded and fixed, they are revealed in whole at room temperature. Therefore, the batch procedure of thermal fixing includes the following processes: optical erasure of electronic gratings both by subsequent recordings in the same batch and by recordings in subsequent batches; ionic compensation during thermal fixing in one batch; smoothing of ionic gratings during thermal fixing of subsequent batches; and revealing of fixed ionic gratings. Based on mechanism of both optical erasure and smoothing repeated in batch scheme, an experiment of thermal fixing for multiplexed holograms to measure the optical erasure time constant of inter-batch for given crystal samples was further designed. Inter-batch optical erasure time constant was fitted out for co-doped lithium niobate crystals according to the diffraction efficiency measured in every stage of all the batches. Experimental results indicate that inter- batch optical erasure time constant t~ is indeed much longer than ordinary erasure time constant tE. The reason is that the migration of light-induced electrons are hampered by Abstract corresponding ionic gratings, screening effect of ionic gratings on electronic ones, can slow optical erasure in different batches. The effect of the fixing number on the quality of holograms was measured through the experiments on both single-scheme and batch-scheme of thermal fixing. The results show that the repeated fixing processes have not observable effect on the quality of holograms. Finally, the dark storage time was fitted out via measuring signal-noise ratio of the reconstructed hologram.更多还原