International Symposium on Optical Memory and Optical Data Storage Topical Meeting

This article gives a review of holographic data storage, and covers such topics as: history and development; technological issues; recording media; recording and recreation systems; future outlook and prospects.

Numerous types of DVD recording equipment have emerged on the market, and there is much demand for such optical storage systems with high speed and large capacity for digital high definition broadcasts. However, it becomes difficult to maintain data reliability with increasing recording speed and density, because the signal to noise ratio degrades and recording control of the phase changed mark becomes difficult. To improve the error rate of reproduced data, PRML (partial response maximum likelihood) decoding is expected to be used. We have developed a more advanced method of PRML, where the difference of the reproduced waveform is analyzed and the PRML decoder is compensated. In this paper we report the analysis of the reproduced waveform and the effect of the developed PRML algorithm.

Optilink has developed and presented a polarization holographic reading writing system using 1-2 /spl mu/m thick storage layer on a card form media. A storage density of 1.3 bit//spl mu/m/sup 2/ has been achieved without multiplexing with sparse code modulation and Fourier-filtering. Because of the use of a thin storage layer, the volume data density is extremely high in the system. Further enhancement of the areal density up to /spl sim/100 bit//spl mu/m/sup 2/ can be achieved in an improved new polarization holographic system using a multilayer media instead of the thin film. A 6f system for reading with a suitable second spatial filter allows separation of the signal from different layers. This technique combines the advantage of the thin film holography with the idea of the confocal filtering.

The accuracy of achieving each of the specified levels in multi-level optical disk media determines the maximum number of levels that can be used to write data. If the standard deviation of the written reflectivity levels is large, the levels need to be spaced further apart for accurate identification of the levels. If the reflectivity range can be increased, additional levels can be used. This relationship is expressed as a ratio between the standard deviation and the dynamic range (SDR). AgInSbTe materials show the ability to achieve very low SDR at low recording speeds (/spl sim/2-4 m/s) but most often show increasingly higher SDR at higher speeds (>5 m/s). We describe an InSbTe eutectic-based phase change material that can achieve a low SDR at high as well as low speeds (2-6 m/s).

High-density and high-speed have been the trend of developing next generation optical storage technologies. Rauch et al.[1] proposed a hybrid recording system called wave-guide hybrid advanced MEMS (WHAM) by using two kind of fields. One is an optical beam that controls the track width and the other is the magnetic field that controls bit length as shown in Fig. 1. The recorded mark is confined within the small cross-region and therefore the recording density is effectively increased. Kim et al.[2] further designed the module by integrating waveguide and two-dimensional toroidal optical elements into a substrate. It can be easily fabricated by MEMS technologies.

In high density optical disc systems, several methods have been presented which employ an object lens with NA 0.85. Two element objective lenses with NA 0.85 have been developed and tried for next generation optical disc systems. However, we are sure that a single lens is required which would help to achieve a longer working distance and a smaller diameter because the size and the cost will be major factors in the future of NA 0.85 lenses. Last year we produced a prototype single lens with NA 0.85. Here we introduce the design concept, and also report on the favorable performance of the new lens re-designed for mass production.

Recently, a Blu-ray Disc (BD) has been proposed as a rewritable optical disc, in which HDTV digital data can be recorded for two hours or more. It has a capacity of about 25 GB with a size of 120 mm in diameter. It employs a blue laser of 405 nm wavelength and an objective lens with an NA of 0.85 to reproduce through a transparent substrate which is about 0.1 mm-thick. We conducted experiments to study a read only memory (ROM) disc with the same capacity as BD. Then, we confirmed the feasibility of the dual layer ROM disc with density equivalent to 50 GB capacity, which can be read from one side. We report the experimental results on recording of the master and the results of the evaluations.

A high capacity 3D multi-layer optical data storage system is being developed at Call/Recall, Inc. A single beam two-photon recording technique is used to record data tracks and layers within a monolithic thick plastic disk (D.A. Parthenopoulos and P.M. Rentzepis, Science vol. 245, pp. 843-845, 1989; H. Zhang et al., Proc. SPIE vol. 4090, pp. 174-178, 2000). The recorded bits emit broadband fluorescence when excited by a laser beam within the absorption band of the written molecule. The recorded volume has no noticeable index change in the visible spectrum. The recorded bits are non-reflective for the readout beam and recording beam. These properties allow the recording and readout beams to access multiple layers in parallel (H. Zhang, 18th IEEE Symp. Mass Storage Systems, pp. 225-236, 2000; E.P. Walker et al., ODS 2002; E.P. Walker et al., ODS 2001, Proc. SPIE, 2001). The capacity of the two-photon 3D multi-layer optical data storage system is influenced by the recorded bit volume, track pitch and layer separation.

Due to the fast development of multi-media technology, the demand of larger recording capacity and faster data transfer rate media is getting higher. The increase of recording capacity can be achieved by reducing the wavelength of laser pick-up or increasing the numerical aperture of object lens. To speed up data transfer rate, fast crystallization phase change material is required. Eutectic Sb/sub 70/Te/sub 30/ fast-growth material is one of the promising candidates for high data transfer rate recording (H.J. Borg et al, ISOM/ODS'99, Proc. SPIE vol. 3864, p. 191, 1999; H.J. Borg et al, ISOM2000, p. 6, 2000). However, it also shows the disadvantage of poor thermal stability (H.J. Borg et al, Jpn. J. Appl. Phys. vol. 40, pp. 1592-1597, 2001). An effective way to solve this problem is doping specific elements into the Sb/sub 70/Te/sub 30/ fast-growth material. Germanium with high value of T/sub g//T/sub m/ is expected to play the role of thermal stabilizer (M.Okuda et al, Jpn. J. Appl. Phys. vol. 31, p. 466, 1992). In this paper, we quantitatively study the effects of Ge doping on the optical properties and crystallization characteristics of Sb/sub 70/Te/sub 30/ recording film.

We previously reported the performance of high density magneto-optical (MO) recording with NA 0.85 and blue laser using land/groove substrate (T. Miki et al, Proc. SPIE vol. 4342, pp. 220-227, 2002). In that report, we confirmed that the groove shape of the substrate improves the recording power tolerance using land/groove substrate. DWDD (domain wall displacement detection) MO (T. Shiratori et al, J. Magn. Soc. Jpn., vol. 22, supp. no. S2, pp. 47-50, 1988) is the most useful medium for achieving high linear density recording without using higher NA objective lenses. Land/groove recording using DWDD has already been reported (S. Kai et al, Jpn. J. Appl. Phys. vol. 38, pp. 1768-1773, 1999; G. Fujita et al, Tech. Dig. ISOM '01, pp. 84-85, 2001; T. Sakamoto et al, Tech. Dig. ODS 2001, pp. 73-75, 2001). Securing a wide recording power tolerance is very important to realize a high density optical storage system. From this point of view, we investigated DWDD MO with groove recording using blue laser and NA 0.60 optics. We optimized DWDD media for groove recording and improved the groove shape of the substrate to obtain wider recording power tolerance at higher linear density.