Novel modulation techniques for collinear holographic data encoding

We describe two novel modulation techniques for collinear holographic data encoding, employing a spatial light modulator (SLM) based on twisted nematic LCD. In the Fourier transform holographic storage system, the reference beam in the outside part and the object beam in the inside part are simultaneously modulated by one single SLM, with different modulation techniques. In one of the modulation methods, the reference beam is phase modulated with a circular blazed grating pattern, and then diffracted into the central part to interfere with the amplitude modulated object beam. Multiple holograms can be recorded on the same location with reference beams of different grating period. Another modulation method is to modulate both the reference beam and the object beam with pure phase modulation by the SLM. The binary ones are encoded with random phase shift from 0 to 2π, while the binary zeroes are encoded with a constant phase of 0. When the dc component of the spatial frequency generated by the binary zeroes is blocked, a homogeneous hologram will be obtained, and the amplitude object will be reconstructed directly. In this paper, both of the two modulation methods are performed theoretically and experimentally. From the experimental results, it can be seen that the blazed-grating modulation technique gets a higher efficiency, while pure phase modulation method can reconstruct the images with more uniform intensity. These techniques are demonstrated to be attractive for applications in data storage and encryption systems.