3D optical data storage is the term given to any form of optical data storage in which information can be recorded and/or read with three-dimensional resolution (as opposed to the two-dimensional resolution afforded, for example, by CD).[1][2]

This innovation has the potential to provide petabyte-level mass storage on DVD-sized discs (120mm). Data recording and readback are achieved by focusing lasers within the medium. However, because of the volumetric nature of the data structure, the laser light must travel through other data points before it reaches the point where reading or recording is desired. Therefore, some kind of nonlinearity is required to ensure that these other data points do not interfere with the addressing of the desired point.

No commercial product based on 3D optical data storage has yet arrived on the mass market, although several companies are actively developing the technology and claim that it may become available 'soon'.
3D Optical Storage
Today's optical storage is currently on 2-dimensional medium, e.g., the CD-ROM, DVD, Blu-Ray, etc. While these devices have steadily improved in storage capacity, they are still limited by the fact that data can be written in layers on the disc. 3D storage, on the other hand, isn't limited by the layers: by adding volume to the medium, the storage capacity grows by leaps and bounds. Currently there are two major types of 3D storage: so-called simple storage of data throughout he volume of the disk, and holographic storage.

3D Storage of Data: Localized-Bit
Localized-bit is an extension of standard disc storage: Data is not only available on the surface of the disc but throughout its volume. The laser then reads the data through the medium, instead of across the lands and pits of a traditional CD-ROM or DVD. As of the writing of this lesson, the laser can be used to focus on one bit at a time, or which can read several bits of data at the same time. The technology is still developing, however, and these advances will require changes in hardware in order to read these new types of discs.

The image below is a cross-section of a 3D optical disc and how data is accessed:

3d optical disc
While this method is viable, a solution with much greater storage and retrieval potential is holographic data storage.

Holographic Data Storage
A hologram is a three-dimensional image which is created as light beams (e.g., from a laser) merge together. Typically a laser beam is split into two paths: Data and reference beams are directed into the storage medium such as a crystal. While it might sound like something from Star Wars, holographic storage has very real possibilities and opportunities for implementation.

Storing and Retrieving Data
Data storage and retrieval is non-linear in a 3D system. Therefore, by using light at different angles, the data can be stored throughout the volume of the medium; multiple images can be recorded into the same data area. Millions of bits of data can be stored in parallel, which greatly increases the data transfer rate as compared to standard optical storage. The stored data is read by re-creating the same reference laser beam that created the hologram. As the reference beam's light is cast upon the disc, the light hits a detector and is refracted; this detector reads that data (and can read it in parallel with other data), up to one million bits at a time!