Many novel technologies are being pursued in parallel towards accomplishing higher capacities per disk and higher data transfer rates. Several unconventional long-term optical data storage techniques promise data densities greater than 100 Gb/in2 and perhaps even exceeding Tb/in2. These include near field and solid immersion lens approaches, volumetric (multi-layer and holographic) storage, and probe storage techniques.
A solid immersion lens approach using MO media pursued by Terastor in the United States promises at least 100 Gb/in2 areal density. This technique relies on flying a small optical lens about 50 nm above the storage medium to achieve spot sizes smaller than the diffraction limit of light. Since the head is now lighter, this type of technology may lead to access times comparable with hard drives. Several Japanese companies are intrigued by the approach and are involved in Terastor's activities. Similar objectives are pursued by Quinta, a Seagate Company, where increasing amounts of optical technologies including optical fibers and fiber switches are used to reduce the size and weight of the head, which is non-flying ("far-field"), but still placed quite near to the disk medium.
Multi-layer (multi-value) storage is pursued both in Japan and the United States. In Japan the effort concentrates on increasing the number of storage layers in a PC based DVD disk. Some researchers also envision adapting multi-layer recording to MO media by simultaneously reading and computing the data on several layers. Both approaches, however, have limited scalability in the number of layers. In the United States, Call/Recall, Inc. is using a fluorescent disk medium to record and read hundreds of layers. Also in the United States, significant effort is being put into developing holographic storage, aiming for areal densities exceeding 100 Gb/in2. Companies both in the United States and Japan are exploring the use of parallel heads to speed up data transfer rates. Finally, both in Japan and in the United States, optically assisted probe techniques are being explored to achieve areal densities beyond a Tb/in2. Fig. 1.9 summarizes the author's projections for long-term trends in optical storage.
In summary, a fast growing removable data storage market governed by optical storage has resulted from substantial progress that has been made in optical disk storage techniques. These advances have come through a combination of laser wavelength reduction, increases in the objective lens numerical aperture, better ISI and cross-talk management, and coding improvements under the constant pull of new applications. Undoubtedly, emerging applications will pull optical storage techniques to reach new performance levels. There is room for advances in storage capacity, as transitions to blue lasers, near-field optical recording, and multi-layer systems will occur.
To further report and analyze Japan's potential role in this developing important market, in the following chapters, we present in more depth the status of optical storage in Japan from a business oriented perspective, and a review of the technologies being developed for the near as well as longer terms.
Fig. 1.9. Alternative optical storage technologies and their potential impact (speculative personal projections from the author).