Holographic Storage

Does Holographic Storage have a future?

Holographic Storage has been going replace magnetic disks for many years, but so far (2011), no viable commercial products have appeared. I think it would be reasonable to conclude that conventional holography is probably dead in the water.

Magnetic disks have two problems; at some point in time they will reach their maximum possible data density, and the rate at which data can be written and read from them is limited by their mechanical, serial heads.

Holographic Storage remains a tantalising possible way to resolve these problems, as in theory it can store data by the terabyte, and transfer megabytes of data in a single operation. However the problem seems to be that Holographic Storage is competing in the data archiving area, and is competing against relatively cheap tape systems that use well established technology. At the other end of the scale, Solid State storage is steadily coming down in price and is much faster than Holographic as it has no moving parts. I suspect that this means Holographic Storage will struggle to find an economic niche.

Why is Holographic Storage not working yet?

The components needed for holography are generally available, and reasonably low-cost. The technology includes Liquid Crystal Display (LCD) and Charged-Couple Device (CCD) camera chips, both of which have been around for some time. So why are there no HDS systems in production yet? Well, they are still almost here. The TLA (Three Letter Acronym) of the moment is HVD or Holographic Versatile Disc.

Some of the reasons why commercial products are not available yet are -

• Older holographic systems used a two beam approach where the reference beam and information beam were configured in two different optical axes. This made the optical system complex and so expensive.
• There are no agreed holographic recording media standards yet, which means no co-operation between manufacturers
• The industry has not decided yet if the holographic media should be compatible with other optical systems like CDs and DVDs.
• Holographic systems require an exact alignment of the recording plane and this makes bulk manufacture difficult.
• The production of a new holographic substrate requires substantial investments in new equipment, unless old technologies can be adapted.

General Holographic Storage Principles

In holographic data storage, an entire page of information is stored at once as an optical interference pattern within a thick, photosensitive optical material. This is done by intersecting two coherent laser beams within the storage material. The first, called the object beam, contains the information to be stored; the second, called the reference beam, is a simple light wave. When the two combine in an optical storage medium, they change the chemical or physical construction of that medium and so store the data.
If the storage medium is then illuminated with the reference beam again, the object data beam is produced. The combination of the two beams writes a complete 'page' of data into the crystal, and the single reference beam reads a complete page of data out of the crystal.

The photo sensitive material within which data is stored in a 1024*1024 bit array, called a page. Each element of the array is light-or-dark, or one-or-zero, so each page represents one Megabit of information. The entire page is processed in parallel, so an entire megabit is read in one operation.

As the hologram is three dimensional, several pages of information can be recorded in the same piece of material as long as they are distinguishable from one another. Two ways to do this are to change the angle between the object and reference wave or by changing the laser wavelength. Any particular data page can then be read out independently by illuminating the stored gratings with the reference wave that was used to store that page. In theory, a single crystal could store terabytes of information, and access time should be fast, because the laser beams can be moved rapidly without inertia, unlike the actuators in disk drives.

It should also be possible to find a particular page quickly too. If you combine a search pattern with an object beam and illuminate the crystal, all the reference beams that were used to store data will be produced. The reference beam with the highest intensity will be the one that most closely matches the search pattern.

Holographic Versatile Disk

This is an alternative architecture to the traditional Holographic storage and just may be the breakthrough that this technology has been waiting for. The reference beam and the data beam are combined together into a single 'light pencil', and that avoids the complex optics needed to maintain two light beams at a precise, separated angle. This is usually implemented by combining light from a green laser and a red laser.

The main advantages of HVD are that the laser beams are collinear, that is, they travel in the same axis, so they avoid the complex opticals required by traditional holography that are required to precisely align two laser beams that travel on different axis. The other advantage is that HVD uses a disk that is the same size and thickness as a standard DVD, wheras traditional holography uses much thicker disks.

HVD still uses the object beam and reference beam concept, but these are combined as passed through a thick recording layer sandwiched between two substrates and which includes a dichroic mirror, that is a mirror designed to pass one wavelength of light and reflect all the others. It reflects the blue-green light object beam that carrys the holography data but allows the red reference beam to pass through.

A laser light beam is fired into a beam splitter which produces two identical beams. One beam is then passed through a Spatial light modulator (SLM), which imposes a pattern on a light beam to represent data. At it's simplest, the foil that you use on an overhead projector is an SLM, it takes a data representation and converts it to a light pattern. This beam is now the information beam. The other beam is unmodulated and is the reference beam. The reference beam and the information beam are then joined back together on the same axis so that they create a light interference pattern that forms the holography data. The interference pattern is then shone into a photopolymer disk where it is stored as a hologram.

To read the data back, A light pattern that is identical to the reference beam is projected onto the hologram and that retrieves the light pattern corresponding to the data beam that is stored in the hologram. This beam is passed to a CMOS sensor which converts it back to the page data.

Older Hologrpahic Storage technology

The following components are required

A LASER which is split into two beams, a reference beam and an object beam. The interference pattern created by these two beams forms the hologram.

A Spatial Light Modulator (SLM) which is basically just a 1024 * 1024 array of light or dark squares. This array represents the data to be stored, and is usually implemented by a set of pixels on an LCD. An SLM can usually be refreshed at rates of about 1000 frames per second.

A Multiplexing Agent which is used to allow the laser beam to access different pages in the hologram.

A Storage Medium which is a photosensitive polymer that can store the light pattern as a hologram.

A Charge Coupled Device [CCD], an array of sensors which corresponds to the pixels on the SLM. The CCD is used to read the interference pattern from the reference beam, and so read the information from the hologram. The matrix construction of the CCD allows it to read all 1Mb of the data at once.

Some HDS components

The diagram is simplified (to fit with my artistic skills). The reference laser beam, and the multiplexing system are not shown.
As an example, when polarised laser light passes through a photo-addressable polymer (PAP) its chain-like molecules become aligned and stay like that even after the beam has been turned off. The holographic effect is created by shining two laser beams that are in phase onto the PAP. One of the beams, the data beam, falls first on an object which encodes the data, in this case a liquid-crystal display 'template'. This changes its phase. When the two beams meet on the polymer an interference pattern indicating the difference between their phases is etched into the substance. Then, by adjusting the angle of the beam slightly, an entirely new pattern can be recorded on the same substance without disrupting any of the information already recorded.
The PAP alignment can then be read by shining an unpolarised laser beam through the polymer. The beam picks up the pattern in the PAP, and it is then read by the CCD

Are there any Holographic Storage systems on the horizon?

The main problem with holographic storage is the expense required to tool up for a new technology, when the old technology, magnetic disks, still has growth potential. However, it appears that holography will no go away.

Optware Collinear Holographic Versatile Disc (HVD)

The holographic systems described so far all use two laser beams to create an interference pattern in the storage medium, and it can be difficult to maintain the required precise alignment between these two beams. Optware changes this by combining two laser beams into a single co-linear light beam. A green laser beam is used to carry page data, and a red laser beam for tracking and controlling the signal. The two beams are combined by a dichroic mirror then focused by a lens onto the polymer resin recording medium. The data is then stored as an interference pattern in the holographic storage medium. A shiny metallic surface on the bottom of the disk then reflects that data back up to the laser to be read.

The reference beam is needed to read the data back, but the data cannot be read unless the exact reference pattern in used. This means that the data on an HVD can be encrypted very easily, and very securely, as each two dimensional page can have a different encryption key. The HVD could also protect from piracy, as it would be very easy to mass produce copies from a master disk, but the copied disks could not be used for further copying.
Optware plan to ship 200 GB capacity disks sometime in 2007, with a roadmap to increase capacity to 1TB then 3.9TB on a CD size disk. They will also produce a 30GB credit card size disk for consumer products, and a 'consumer' disk holding 100GB in 2008. The eventual data transfer rate is planned to exceed 1 Gbits/sec or 125MB/s, about three times the initial InPhase Tapestry transfer rate.

InPhase Tapestry systems

InPhase has been just about to ship holographic storage for several years. They ran a public demonstration of their 'tapestry' holographic technology in April 2005 and actually display a list of resellers for the Tapestry 300r but none of them appear to actually sell it. The company apparently went out of business in 2010, even though their website is still active. They planned to release write-once WORM discs designed for fixed content archiving with a capacity of about 300 GB. Re-writable discs with 1.6 TB capacities are planned for future releases. InPhase claim that the tapestry system has a recording density of 200 Gigabits / square inch and can read data at 20 MB/s. Contrast this with traditional magnetic disk that has a recording density of 120 Mbpsi and the newer perpendicular recording disks with a density of 240Mbpsi. High performance tapes can read data at 120 MB/s or more. The reason why the tapestry read speed is relatively slow, is because the DVD has to stop while it writes to each sector. This architecture needs to change so that the disk is spinning continuously and the data transfer becomes 'bitwise'.
Tapestry uses a twin polymer implementation for the storage medium. The recording medium polymer is dissolved inside a solid matrix polymer; This 2-chemistry combination is a 1.5 mm thick recording material that this is sandwiched between two plastic plates; there is no metallic layer such as used in DVD storage. Data is stored by crossing two separate laser beams inside the polymer, which records pages of data. Individual pages can hold approximately 1 megabit, and multiple pages are recorded by varying the angle of incidence and wavelength of the reference beam. 252 'pages' are collected together into one 'book', and fifteen books or 3780 pages can all the stored in the same piece of recording material.
However the problem seems to me to be that Inphase are competing in the data archiving space against tape products, that are a cheaper, proven technology, so it looks like they are struggling to make a cost case.

Collosal Storage

Another possibility is the holographic systems being developed by Colossal Storage. They use ferroelectric binary-state molecules that can be in one of two states. These states can be read or changed with ultra-violet light and so nano-optical holographic storage devices can be created. This is essentially an atomic switch with the potential to hold petabytes in a square inch.

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