Holographic Versatile Disc (HVD) is an advanced optical disc technology still in the research stage which would greatly increase storage over Blu-ray and HD-DVD optical disc systems.
It employs a technique know as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. The blue-green laser reads data encoded as laser interference fringes from 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 monitor the position of the read head over the disc, similar to the head, track, and sector information on a conventional hard disk drive. 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 prevents interference from refraction of the blue-green laser off the servo data pits and is an advance over past holographic storage media, which either experienced too much interference, or lacked the servo data entirely, making them incompatible with current CD and DVD drive technology. These disks have the capacity to hold up to 3.9 TB’s of information (whoa!) , which is approximately eighty times the capacity of Blu-ray Disc. The HVD also has a transfer rate of 1 Gbit/s.
Source: Wikipedia
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