The
Holographic Versatile Disc (
HVD) is an optical disc technology developed between April 2004 and mid-2008 that can store up to several terabytes of data on an optical disc 10 cm or 12 cm in diameter. The reduced radius reduces cost and materials used. It employs a technique known as collinear
holography, whereby a green and red
laser beam 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. A
red laser is used as the reference beam to read
servoinformation from a regular CD-style
aluminium layer near the bottom. Servoinformation 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 servoinformation is interspersed among 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.