Holographic Storage

Does Holographic Storage have a future?

In 2011 I wrote 'Holographic Storage has been going to replace magnetic disks for many years, but so far, no viable commercial products have appeared. I think it would be reasonable to conclude that conventional holography is probably dead in the water, because the cost and density of flash storage has improved so much'. However, holographic storage just refuses to die.

In 2020 Microsoft announced that they were investigating using holographic storage to cppe with the vast storage demands they were seeing on their Azure Cloud infrastructure. They announced a partnership between their AI research labs and the Azure Cloud division to investigate storage solutions based on holography. No doubt further details will arrive in time. According to Microsoft
"Our interdisciplinary team comprises experts in physics, optics, machine learning, and storage systems; and our mission is to design mechanical-movement free, high-endurance cloud storage that is both performant and cost-effective. This has led to new research challenges and breakthroughs in areas ranging from materials to machine learning."

In early 2016, scientists at the University of Southampton announced that they have developed a storage system that looks a lot like holography, and it can retain data for billions of years. Imagine trying to collect on that warranty!

They can store 360 TB on a disc that can withstand up to 1,000°C and claim that the data can last 13.8 billion years at 190°C because the 5D glass discs protect that information within their structure, safe from bumps and scrapes. They see the technology being used for eternal data archiving; for organisations with big archives, such as national archives, museums and libraries.

They state that 'Data is recorded using ultrafast laser, producing extremely short and intense pulses of light. The file is written in three layers of nanostructured dots separated by five micrometres (one millionth of a metre). The self-assembled nanostructures change the way light travels through glass, modifying polarisation of light that can then be read by combination of optical microscope and a polariser, similar to that found in Polaroid sunglasses'.

Why is it 5D? A recording on a CD has 2 two dimensions using pits burned onto a plastic surface, where the pit, or absence of a pit is a binary digit. A DVD extends this to 3 dimensions by burning pits on multiple layers. 5D uses the optical property of birefringence to store more than once bit of data in a 'voxel', so extending the effective capacity to five dimensions. Light travels through transparent materials at different speeds, and so bends more or less, depending on a property called the refractive index. If this property is not fixed, but depends on the polarization and propagation direction of the light, then this is called birefringence. By using birefringence, each voxel can store eight bits of information, so extendng the capacity of a single 12cm-diameter disc to hundreds of terabytes.

The team are now looking for industry partners to further develop and commercialise this ground-breaking new technology.

In 2018 a team from Northeast Normal University in China is still working on holographic storage. They have created a new kind of film made of titanium dioxide impregnated with nanoparticles of silver. A laser writes information to the silver nanoparticles by changing their charge, and since different wavelengths of the laser light affect the particles differently, the data is stored as 3D holograms. The storage capacity of an 10 x 10 cm film that is just 620 nanometers thick is roughly 8.5 TB. The team says that data can be retrieved at speeds of up to 1 GB per second.

Why is Holographic Storage not working yet?

Holographic Storage was originally proposed to fix the problems with Magnetic disks; 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. Holography 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.

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 -

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.

2 Laser beam interference pattern

The photo sensitive material within which data is stored in a 1024*1024 bit array, called a page. pixel checkerboard 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.

Laser Hologram
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

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