Holographic Versatile Disc
1. Green writing/reading laser (532nm)
2. Red positioning/addressing laser (650nm)
3. Hologram (data)
4. Polycarbon layer
5. Photopolymeric layer (data-containing layer)
6. Distans layers
7. Dichroic layer (reflecting green light)
8. Aluminium reflective layer (reflecting red light)
9. Transparent base
P. PIT
Holographic Versatile Disc (HVD) is the next generation of optical disc past Blu-ray and HD-DVD. It uses two lasers, one red and one blue-green, fired as a single beam. The blue-green laser reads data encoded as laser interference fringes off a holographic layer near the top of the disc while the red laser is used to read servo information from a regular CD-style aluminium layer near the bottom. Servo information is used to track where on the disc you are currently reading from, similar to the sector, head and segment information on a regular HDD. On a CD or DVD this servo information is interspersed amongst the data. A dichroic mirror layer between the holographic data and the servo data reflects the blue-green laser while letting the red laser pass through. This negates interference from refraction of the blue-green laser off the servo data pits and is a step up on past holographic storage media, which either experienced too much interference, or have lacked the servo data entirely, making them incompatible with current CD and DVD drive technology [1]. These disks have the capacity to hold up to 1 terabyte (TB) of information, which is approximately twenty times the capacity of Blu-ray Disc. The HVD also has a transfer rate of 1 Gbit/s. It is being developed by the Japanese company, Optware together with Fuji Photo and CMC Magnetics. These three companies, along with Nippon Paint, Pulstec Industrial and Toagosei, have formed an "HVD Alliance" to promote HVD standards [2].
Context
The books in the largest library in the world, the U.S. Library of Congress, contain about 20 terabytes of text. Therefore, it could be stored on 20 of these discs.