Improvement to holographic digital data-storage systems with random and pseudorandom phase masks

Various effects associated with using a random phase mask in holographic data storage are analyzed. It is shown that the nonlinear nature of recording a phase hologram coupled with the highly nonuniform profile of the object beam is the major source of interpixel cross talk. It is also shown that, although the nonlinear effects are reduced by an increase in the reference-to-object beam ratio, the scattering noise is increased. Thus an optimal beam ratio must be used to maximize the signal-to-noise ratio. It is demonstrated as well that the interpixel interference noise can be suppressed effectively by use of a multilevel pseudorandom phase mask, which also significantly reduces the nonlinear noise. These findings are supported by experimental results that show the signal-to-noise ratio and efficiency are improved significantly by use of a six-level pseudorandom phase mask.