Formularization and simulation of Bragg selectivity of readout signals in angular-multiplexing holographic data storage.

Bragg selectivity of readout signals in angular-multiplexing holographic data storage was investigated. The effects of degrading factors, namely, volume change, refractive-index change, and positional change (tilt and rotation) of a hologram, and the effects of compensating variables, namely, wavelength shift and reference-beam-angle shift, on Bragg selectivity were evaluated. Deviation of wave vectors of recovered pixels of a hologram from the Bragg condition under degrading factors and compensating variables, namely, Bragg mismatch, Δσ, was mathematically derived. Approximating Δσ by using the first-order Maclaurin series with respect to degrading factors and compensating variables revealed their effects on Bragg selectivity. The extent to which wavelength and angle of reference beam should be shifted to compensate for the degrading factors were determined. Then, readout images were simulated under multiple degrading factors and compensating variables. These simulated images were found to agree well with the experimentally obtained ones, which reveals the validity of the formalization of Bragg selectivity.

Analysis of tolerances in polytopic-multiplexing holographic data storage.

A fast Fourier transformation (FFT)-based simulator was developed to analyze a recovered image of a hologram formed by polytopic multiplexing holographic data storage and to determine the positional tolerances of optical devices. By focusing the positional shift of 4f configuration devices (phase mask, SLM, polytopic filter, and camera), the number of FFT iterations was reduced, and the calculation speed was improved. Using the simulator, the positional tolerances of these devices were determined. Also, to validate the simulator, simulated readout images under defocuses of the phase mask and camera were compared with experimentally obtained images and found to agree well.

Interlayer crosstalk reduction of a multilayer Blu-ray Disc using a grating in a three beam optical system.

In multilayer optical discs, light reflected by out-of-focus layers, which we call interlayer crosstalk, causes the tracking error signal to fluctuate, making the readout signal unstable. We previously proposed a novel method to use a grating along the optical axis in the return path of a pickup to suppress the fluctuation of a differential push-pull (DPP) signal. We develop a pickup and evaluate its performance to stabilize the DPP signal experimentally. DPP signal fluctuation is suppressed to one-third (6% to 2%), and also satisfactory readout jitters (about 8%) are obtained for a triple-layer Blu-ray Disc (BD), which demonstrate the validity of this method to reduce interlayer crosstalk of multilayer optical discs.