A Study Of Thermal Fixing For Multiplexed Holograms

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.