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How Holographic Versatile Discs Work


The HVD System: Writing Data

A simplified HVD system consists of the following main components:

  • Blue or green laser (532-nm wavelength in the test system)
  • Beam splitter/merger
  • Mirrors
  • Spatial light modulator (SLM)
  • CMOS sensor
  • Photopolymer recording medium

The process of writing information onto an HVD begins with encoding the information into binary data to be stored in the SLM. These data are turned into ones and zeroes represented as opaque or translucent areas on a "page" -- this page is the image that the information beam is going to pass through.

 Page data stored as a hologram
Page data stored as a hologram
Photo courtesy Optware

Once the page of data is created, the next step is to fire a laser beam into a beam splitter to produce two identical beams. One of the beams is directed away from the SLM -- this beam becomes the reference beam. The other beam is directed toward the SLM and becomes the information beam. When the information beam passes through the SLM, portions of the light are blocked by the opaque areas of the page, and portions pass through the translucent areas. In this way, the information beam carries the image once it passes through the SLM.

When the reference beam and the information beam rejoin on the same axis, they create a pattern of light interference -- the holography data. This joint beam carries the interference pattern to the photopolymer disc and stores it there as a hologram.

A memory system isn't very useful if you can't access the data you've stored. In the next section, we'll find out how the HVD data-retrieval system works.