Image Recognition Using Volume Holographic Correlator And Multiple-Spectrum Filtering

The fast developments of the internet and multimedia are now placing new demands on the storage systems that are capable of high capacity memories with short access times. Owing to the advantages in high storage capacity, fast data transfer rate, and short access time, volume holographic memories (VHM) are considered to meet the needs. With the rapid improvement of the VHM in recent years, pattern recognition that uses VHM is now being researched widely. This paper is aiming at image recognition based on angle-multiplexed volume holographic correlator.The principles of the volume holographic storage are firstly introduced in this paper. They are the recording, reconstruction, diffraction efficiency and angular selectivity of volume holograms. Also the optical correlator and pattern recognition are discussed.With the horizontal and vertical angular multiplexing technique, 1020 Chinese images, divided into 17 lines and 60 rows, are stored onto the same point of photorefractive crystal Fe:LiNbO3. One thousand and twenty correlation spots are obtained by illuminating the crystal with the background image.In this paper, a novel multiple-spectrum (MS) filtering is proposed. It can efficiently suppress the DC term and noise, get a higher-resolution description of the image for correlation, make the correlation spot so obvious and let the correlation peak become sharp. It highly improves the discrimination of the volume holographic correlator.Owning to the erasing effect, the correlation spots have not the same brightness if the holograms were exposed with the same times while recording. The correlation spot that related to the late recorded hologram is brighter than that related to the previous one. This problem can be solved by a subsequence exposure technique using in holograms receding.Nowadays the holograms thermal fixing is widely researched. With the MS filtering, we firstly studied the correlation output after the holograms were thermal-fixed. We found that after thermal fixing, the readout holograms and the image of background correlation became dark. But with the MS filtering, the correlation output still has obvious spots. And the correlation peak is sharp too.In the last part of the paper, we summarized our research works and briefly discussed the future study.