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5D data storage and the future of optical technology

Researchers say revolutionary 5D optical storage could protect hundreds of terabytes of data for billions of years, far beyond the capabilities of current storage media.

In 2013, researchers from the University of Southampton demonstrated the ability to store a 300 KB digital copy of a text file in fused silica glass. Since then, they've been refining 5D data storage technology and demonstrating its potential for archiving large quantities of data, while slowly preparing it for commercial use.

The new storage medium, also referred to as 5D memory crystal, uses ultrafast laser technology to encode data into multiple layers of glass, achieving densities unprecedented in storage devices. 5D storage is expected to be able to survive billions of years, without being affected by the environmental factors that plague other media.

What's 5D data storage?

The idea of using ultrafast lasers to store data in transparent materials, such as fused silica, was introduced in 1996. But it took 17 years for researchers to show it could be done, offering a new and revolutionary approach to optical storage.

Instead of relying on reflective lines etched into plastic, like those in CDs and DVDs, 5D storage uses dots that are imprinted at multiple layers within fused silica. Each dot contains a series of pits, or gratings, that store the data. To achieve this, a highly refined laser emits short, intense pulses of light that etch the gratings into the glass. Each disk contains three layers of dots that are only five micrometers apart.

5D optical storage
A 5D storage silica disk that contains the United Nations Universal Declaration of Human Rights

The disks currently used for 5D data storage are 25 mm -- nearly an inch -- in diameter, but researchers believe that the technology can support storage media up to several meters in size. They're considering a standard based on disks that are 12 cm in diameter.

Researchers believe a silica disk has the potential to hold as much as 360 TB of data -- about 72 million photos -- for as many as 13.8 billion years.

Researchers believe a silica disk has the potential to hold as much as 360 TB of data -- about 72 million photos -- for as many as 13.8 billion years at temperatures as high as 190 degrees Celsius. Today's optical storage media is nowhere near those numbers. A CD can store only 700 MB of data, a DVD can store between 4.7 GB and 8.5 GB, and a Blu-ray disk can store 25 GB or more of data. Lifespan estimates for these disks vary widely, from two years to a hundred.

The more recent M-DISC promises to last longer. As with Blu-ray, M-DISC can store only about 25 GB, but it boasts a lifespan of as much as 1,000 years and is more resistant to environmental damage than other media.

Researchers have even shown data stability with the silica disks at 1,000 degrees Celsius. The disks are resistant to the type of scratching, disk rot and other damage that can affect traditional types of optical storage. They can withstand a direct impact of as much as a half-ton.

How 5D data storage works

Fused silica is a noncrystalline form of silicon dioxide, found in abundance in sand, quartz crystals and other material. Considered the purest glass available, it's a synthetic product that differs from traditional glass because it contains no other ingredients, making it well-suited for the precision required to write and read 5D data.

But fused silica is only part of the equation. Also integral to 5D data storage is the concept of femtosecond laser writing, an extremely fast and precise process of etching the glass on multiple layers. The laser generates pulses of light for extremely short periods that are measured in femtoseconds. A femtosecond is one-millionth of a nanosecond, or 10-15 of a second.

Femtosecond laser writing is used in refractive surgery for correcting vision. When used on silica glass to etch the tiny gratings into the glass, the laser creates self-assembled nanostructures, called nanogratings, that can be as narrow as 20 nanometers. A nanostructure's size ranges between microscopic and molecular. In 5D storage, the structures are the nanogratings etched into the glass and self-organized, or self-assembled, into a series of dots, with each nanograting capable of storing eight bits of data.

The specifics of nanograting are, of course, much more detailed and nuanced than described here. What's important, however, is that the nanogratings change the way light travels through glass, modifying its polarization, similar to how polarized sunglasses cut glare.

The polarization helps to explain the five-dimensional nature of 5D data storage. The first three dimensions are defined by the three layers of dots etched into the glass. The other two dimensions are determined by a phenomenon known as birefringence, an optical property that nanogratings produce.

Two factors characterize birefringence. The first is slow axis orientation, which coincides with the nanograting's orientation within the glass, and the second is strength of retardance, which coincides with the nanograting's size.

Both the orientation and size affect how light responds when it hits the nanogratings. Using polarization and intensity, the femtosecond laser beam independently controls orientation and retardance, making it possible to retrieve the data using quantitative birefringence measurements. It's the orientation and size that provide 5D storage's fourth and fifth dimensions, enabling the disks to achieve much greater densities than they could with three dimensions alone. Because the nanogratings can be read from different angles, each nanograting can store eight bits of data, rather than one.

Birefringence is an important consideration when writing data to the glass. During this process, polarization is used to control slow axis orientation, and light intensity is used to control the strength of retardance, resulting in unique patterns that represent the data. The data can then be read using an optical microscope in conjunction with a polarizer, making it possible to take advantage of all five dimensions when accessing the data.

5D storage takes off -- for real

Commercial applications of 5D storage have yet to be realized, in part because of the high cost of the specialized lasers. However, the technology could prove useful to organizations that want to preserve large collections of information, such as museums, libraries and archives. Researchers have already put major documents, such as Newton's Opticks, the Magna Carta and the United Nations Universal Declaration of Human Rights, on silica disks.

More recently, 5D data storage made headlines because it's in Elon Musk's Tesla Roadster, which is onboard the Falcon Heavy, a SpaceX rocket launched in February from the Kennedy Space Center in Florida. The rocket, car and 5D storage, which contains Isaac Asimov's Foundation series of science fiction books, are on an orbit around the sun, following a trajectory that's similar to Mars'.

Tesla Roadster
5D storage was in a Tesla Roadster sent into space in a SpaceX rocket earlier this year.

The Tesla experiment is one indication of all the enthusiasm behind 5D storage. Given all of that, the technology could bring optical storage into the 21st century and perhaps into the centuries to follow.

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