We fabricated a multilayered medium by a laminating process with pressure-sensitive adhesives. It was possible to reduce the thickness variations of both photosensitive layers and transparent layers by applying laminating films. This method is easy to use to pile up many layers for a multilayered medium. We fabricated twenty recording layers and demonstrated the capability to record with significant reading results. The recorded data in each layer were read out without crosstalk. We evaluated the signal-to-noise ratio and crosstalk between neighboring layers. It was found that the signal-to-noise ratio of a multilayered medium was higher than 50 dB.

The recording of data in multiple layers, rather than a single layer, permits a significant increase in the capacity of optical data storage devices. However, focusing to the different layers introduces different amounts of depth-dependent aberrations. Variable aberration correction is therefore necessary to maintain diffraction-limited operation. We demonstrate the use of adaptive optics to predict and correct these aberrations for both the recording and read-out of such media.

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We analyzed the optimum structure of recording media for a reflection confocal readout system of three-dimensional optical memory. In this analysis, we discuss the overlapping regions of the coherent transfer function of the reflection confocal readout system and the spatial frequency distribution of recorded bits. The intensity of the readout signal is proportional to the multiplication of the three-dimensional distribution of the coherent transfer function of the reflection confocal system and the spatial frequency distribution of recorded bits. We optimized film thickness to increase the signal from recorded bits. It was found that the readout signal increased twentyfold by optimization of film thickness. In addition, we optimized the relative refractive index of recording media by light propagation analysis of multilayered media. In the case of optimal film thickness, the ratio of refractive indices between recording layers and transparent layers should be 1.01 for obtaining maximum reflection.

We fabricated disc-type multilayered media with several refractive indices of pressure-sensitive adhesives for dynamic layer detection. We modified a commercially available disc tester in order to obtain two types of signal; one is the confocal signal and the other is the astigmatic signal. The confocal signal provides sufficient signal to detect recording layers, but the astigmatic signal is not useful because the reflected signals from the internal layers are too small. The confocal detection also provides sufficient signal for rotating discs. We found that the intensities of the reflected signals of rotating multilayered media were of the same level as those of the fixed multilayered media. The reflections from the multilayered media can be used as focus-servo signals.