Holography and the Basics of Holographic Storage
Holography is a technique that images objects in such a way that they appear three dimensional (3D) when viewed later. This is the basic method of generating holograms, which are increasingly used in a variety of fields, such as art, security, data storage. This article focuses on the possibilities of holographic data storage.
Why Is There a Need for Holographic Storage?
Archival storage is a necessary part of every enterprise’s ability to meet compliance requirements and address litigation issues; however, deep archiving is still a challenge. Tape is removable and securable, but arouses concerns with long-term reliability and readability.
How Does Holographic Storage Work?
In holographic storage, a laser source produces a light field that illuminates both the object and the recording medium. The scattered light from the object interferes with the reference light, creating a subsequent interference pattern that produces the image. Let us explore this in more depth.
Holographic storage works by storing a sequence of discrete data snapshots within the thickness of the medium. When a laser beam is split into two signals, the storage process begins. One beam is a reference signal, and the other beam (known as the data-carrying beam) passes through a device called a special light modulator (SLM). This acts as a fine shutter system, passing and blocking light at points corresponding to ones and zeroes.
The reference beam is then reflected to impose on the data-carrying beam within the medium. This produces a three-dimensional refraction pattern (the hologram) that is captured in the medium. Holographic storage employs circular media, much like a blank CD/DVD, that spins to accept data across a continuous spiral data bath. When the medium is written, data is read back with the reference beam illuminating the refraction.
In order for the interference pattern to be encoded with information about the object, the coherence length of the illuminating light is an important factor. Due to the long exposure times needed, the available power, wavelength stability and divergence of the beam are critical.
The coherence length of the laser becomes less critical when being used to copy/print a hologram and many lasers are suitable.
The three-dimensional aspect of data recording is a critical difference between holographic storage and standard CD/DVD recording. Traditional optical media employ a single laser beam to write data in two dimensions along a continuous spiral data bath. Conversely, prototype holographic storage products can save one million pixels at a time in discrete snapshots, also known as pages, which form microscopic cones through the thickness of the light-sensitive medium. In fact, a single disc can store over 4.4 million individual pages.
Lasers for Holographic Storage
The torus family lasers from Laser Quantum are ideal for all holography applications as the beams have long (>100 m) coherence lengths, divergence, and the mode stability that only active locking provides:
- torus 532 nm — Green Single Longitudinal Mode (SLM) laser with active mode locking
- torus 660 nm— Red SLM laser with active mode locking
Benefits of Holographic Storage
For deep archiving purposes, long-term media stability and reliability are compelling advantages of holographic storage, particularly over tape and discs which cannot ensure reliability over time. Holographic storage is portable, allowing the distribution of dense data that cannot be easily sent across other networks such as broadcast or high-definition video.
This method of storage is also appealing for short term backups and archives as companies are beginning to rely less and less on tape backups. Holographic storage attached to a virtual tape library could be a beneficial replacement to physical tapes.
Holographic storage capacity and performance is below current disc and tape systems; however, they compare favorably to existing optical storage devices.
Since holographic storage technology is only a niche in the storage market, production of more capable and cost-effective systems may take a while. However, the possible introduction of hybrid holographic media could speed up this process.
To request more information or a quotation for these or other Laser Quantum products, contact IL Photonics.