Holographic Versatile Disc
The Holographic Versatile Disc (HVD) is an optical disc technology that, in the future, may hold up to 3.9 terabytes (TB) of information, although the current maximum is 250GB. It employs a technique known as collinear holography, whereby two lasers, one red and one green, are collimated in a single beam. The 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 as the reference beam and to read servo information from a regular CD-style aluminum 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 green laser while letting the red laser pass through. This prevents interference from refraction of the 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 discs have the capacity to hold up to 3.9 terabytes (TB) of information. The HVD also has a transfer rate of 1 Gbit/s (125 MB/s). Optware planned to release a 200 GB disc in early June 2006, and Maxell planned one for September 2006 with a capacity of 300 GB and transfer rate of 20 MB/s -- although HVD standards were approved and published on June 28, 2007, neither company has released an HVD as of March, 2009.
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