Ultrafast laser writing unlocks futuristic data storage

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The new method recorded 6GB of data in a one-inch silica glass sample, with each square measuring just 8.8 x 8.8mm.

In this modern information age where smartphones grant each person the capability of generating numerous gigabytes of data with ease through the capturing of high-resolution photos and videos, it is becoming increasingly challenging to store the exponentially growing amount of data.

Current data storage technologies relied on, such as hard disk drives (HDDs), magnetic tape, and optical disks, are not growing fast enough to keep up with the vast amounts of digital data generated worldwide.

In addition, these storage technologies carry with them a number of disadvantages. For example, using numerous HDDs is not only very expensive, but they also exhibit high energy consumption and have a short lifespan lasting only several years. While magnetic tape is sometimes used as an alternative, it also exhibits a short lifespan, in addition to long average-case response times (increased latency when accessing data), which limits its application.

The short lifespan is particularly an issue as it means that, for example, if a petabyte of information is stored on tapes/disks, in only a few years’ time it will be required to undergo migration onto fresh tapes/disks to prevent the data being lost. With many petabytes of new data continuously being generated, the problem is getting exponentially worse.

Despite optical data storage being heralded as an energy-efficient solution with a longer lifetime, the traditional technology of CDs, DVDs and Blu-Rays only has a capacity of hundreds of gigabits per disk. In addition, this longer lifetime is still only around a decade, which doesn’t provide a permanent solution.

Encoding data in glass

Back in 2018 at the University of Southampton’s Future Photonics Hub industry day, Laser Systems Europe first learnt of a new storage technology under development that involved using ultrafast lasers to scribe digital information into silica glass. Dr Benn Thomsen, senior researcher at Microsoft Research, explained at the time how this scribed glass, which unless heated to an excess of a thousand degrees would likely last forever, could serve as an alternative to the current technologies used to store information. Microsoft Research has therefore been working with the University of Southampton and its Optoelectronics Research Centre (ORC) to develop the technology.

However, according to ORC researchers who recently published in Optica, writing the data fast enough into transparent materials with a high enough density for realworld application, has so far proven challenging.

In the paper, the researchers have described their consequential development of an energy-efficient laser writing method that is not only fast, but could also store around 500 terabytes of data on CD-sized silica discs, which would make them 10,000 times denser than Blu-Ray optical disc storage technology.

Laser writing of birefringence structures inside silica glass. (a) Schematic of laser writing setup. EOM = electro-optic modulator and QWP = quarter-wave plate. (b) Images of the slow axis azimuth of voxels written by 100 laser pulses with the energy of 30nJ at different repetition rates from 1 to 10MHz; the pulse duration and wavelength in (b) are 250fs and 515nm, respectively. Pseudo-colors (inset) indicate the local orientation of the slow axis.

The researchers’ new method involves using a femtosecond laser with a high repetition rate to create tiny pits containing a single nanolamella-like structure in silica glass, measuring just 500 by 50 nanometres. These high-density nanostructures can then be used for long-term optical data storage. The format is described as being five-dimensional (5D), encompassing two optical dimensions plus three spatial dimensions.

‘Individuals and organisations are generating ever-larger datasets, creating the desperate need for more efficient forms of data storage with a high capacity, low energy consumption and long lifetime,’ said doctoral researcher Yuhao Lei, from the ORC. ‘While cloud-based systems are designed more for temporary data, we believe that 5D data storage in glass could be useful for longer-term data storage for national archives, museums, libraries or private organisations.’

A Yb-doped Satsuma 515nm fibre laser from Amplitude, with a repetition rate of 10MHz and a pulse duration of 250fs, is used by the researchers to achieve writing speeds of 1,000,000 voxels per second, which is equivalent to recording about 225 kilobytes of data (over 100 pages of text) per second. Pulse energy modulation was achieved by the control of the electro-optic modulator in the laser. The beam was focused with a 0.60NA Olympus objective lens with an aberration correction collar 170µm below the surface of a silica glass substrate, which was mounted on an Aerotech XYZ linear air-bearing translation stage.

Rather than using the femtosecond laser to write directly in the glass, the researchers instead harness the light to produce an optical phenomenon known as near-field enhancement. Through their method, a single pulse is used to produce a circular nanovoid via a microexplosion, and then subsequent lower-energy pulses are used to elongate the shape of the nanovoid – via the near-field enhancement effect – to an anisotropic nanolamella-like structure. Using near-field enhancement to make the nanostructures this way minimises thermal damage, according to the researchers, which they say has been problematic for other approaches using high-repetition-rate lasers.

Because the nanostructures are anisotropic, they produce birefringence that can be characterised by the slow axis orientation (4th dimension, corresponding to the orientation of the nanolamella-like structure) and strength of retardance (5th dimension, defined by the size of nanostructure). As data is recorded into the glass, the slow axis orientation and strength of retardance can be controlled by the polarisation and intensity of light, respectively. The laser-induced birefringence, retardance and slow axis orientation, can then be quantitatively analysed using a manually controlled optical microscope equipped with a birefringence measurement system operating at 546nm wavelength.

‘This new approach improves the data writing speed to a practical level, so we can write tens of gigabytes of data in a reasonable time,’ said Lei. ‘The highly localised, precision nanostructures enable a higher data capacity because more voxels can be written in a unit volume. In addition, using pulsed light reduces the energy needed for writing.’

The researchers used their new method to write five gigabytes of text data onto a silica glass disc about the size of a conventional compact disc, with nearly 100 per cent readout accuracy. Each voxel contained four bits of information, and every two voxels corresponded to a text character. With the writing density available from the method, the disc would be able to hold 500 terabytes of data. With upgrades to the system that allow parallel writing, the researchers say it should be feasible to write this amount of data in about 60 days.

‘With the current system, we have the ability to preserve terabytes of data, which could be used, for example, to preserve information from a person’s DNA,’ said Peter Kazansky, leader of the research team.

He and his colleagues are now working to increase the writing speed of their method – it was noted in the Optica paper that MB/s data writing speed could be achieved using a 40MHz repetition rate femtosecond laser – and to make the technology usable outside the laboratory.

Faster ways of reading the data will also have to be developed to enable the researchers’ technique to be used in practical data storage applications. Using the manually controlled optical microscope, they currently achieve a readout rate in the order of bytes/s, however they explain that this could be improved to tens of megabytes/ second using both automatic polarisation imaging and more powerful decoding algorithms than those currently being used.

‘The multiplexed optical data storage with merits of high data density, low energy consumption, and long lifetime could open a new era in data storage technology,’ the researchers remark in their paper. They also anticipate that their new writing method could be used for fast nanostructuring in transparent materials for applications in 3D integrated optics and microfluidics for chemical and biology applications.

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