Q+A CENTER

Over the last decade, the amount of information to be processed in image-capture systems has increased dramatically. This leads to an increase of storage capacity as well, while there is the continuing need for reducing device size and cost. The devices presented in this article show an excellent way of realizing both aspects offering a high degree of usability.

High-speed CMOS image sensors produced by Cypress Semiconductor (Mechelen, Belgium) and specifically laid out for motion capture at high frame rates can be applied in holographic data storage systems. Their resolution ranges from 0.4 up to 3Mp and they run at 485 full frames per second. The sensor architecture is based on progressive scan and the outputs deliver digital serial LVDS. Operational speed ranges from 206Mbps DDR up to 540 Mbps DDR, thereby realizing a throughput rate of 0.2 to 1.45Gpix/sec. Image quality is at least 8 bit. The target application requires a 6T snapshot pixel design with high sensitivity, where sensitivity greatly depends on pixel size. The devices are realized in a 0.25µm process.

Data storage and data access usually are done by means of magnetic and optical devices. However, this has the disadvantage of reading from and writing to memory just single bits. Holographic data storage uses a volumetric approach. It therefore can access substantially more data units per operation. Holographic storage has existed for a long time, but the vastly increasing amount of data, along with lower material costs, has made it a valid option for high-speed data access. High-speed image sensors can read the data from the storage element—processing multiple bits simultaneously.

THE PRINCIPLE

Basically, holographic data storage has a write and a read mode. Its functionality is based on a photosensitive material using the optical principle of interference of light beams. Figures 1A and 1B show an example of writing to and reading from the media. A laser provides a light beam, which is split in a reference beam and a signal beam. The signal beam is merged with the data pattern and contains the data. The reference beam is used to interfere with the signal beam and write the data to the photosensitive material. The interference pattern of both beams is stored in the photosensitive material by changing the physical properties of the material.

Readout is done by using the reference beam in the photosensitive material exactly the same way as in the write operation. The beam is diffracted onto the photosensitive material by the content (its physical properties). The diffracted light beam is an exact recreation of the signal beam where the data is stored. The output is captured by an image sensor and the stored data are digitized.