holographic versatile disc

a holographic versatile disc is a memory storage device that uses an advanced optical disc technology known as collinear holography. it offers far greater storage capacity (up to 3.9 terabytes) and faster transfer rate compared to existing compact discs (CDs) and digital versatile disc (DVDs). even the touted next-generation optical disc formats Blue-ray (27GB) and high density-DVD (15GB) pale in comparison with HVDs.
collinear holography - an advanced technique that allows an image to be recorded in three dimensions-uses two lasers (one red and one blue-green), which are combined and made parallel into a single beam. the red laser is of similar wavelength that common DVD technologies are using, while the blue laser technology affords higher storage capacity and superior resolution.
in HVDs, the blue-green laser reads the data encoded as laser interference fringes from a holographic layer near the top of the disc, while the red laser is used to read the servo information from aluminum layer near the bottom. servo or "positional control" information is used to monitor the position of the red head over the disc, similar to the head, track, and sector information on a conventional hard disc drive.
the main advantage of HVD over other disc formats is the used of dichroic mirror layer - between the holographic data and the servo data - which reflects the blue-green laser while letting the red laser pass through. this prevents refraction interference of the blue-green laser (of servo data pits), making the HVD compatible with current CD and DVD drive technology.