Photography measures how much light of different color hits the photographic film. However, light is also a wave, and is therefore characterized by the phase. Phase specifies the position of a point within the wave cycle and correlates to depth of information, meaning that recording the phase of light scattered by an object can retrieve its full 3D shape, which cannot be obtained with a simple photograph. This is the basis of optical holography, popularized by fancy holograms in sci-fi movies like Star Wars.
Holography is a technique based on the wave nature of light which allows the use of wave interference between the object beam and the coherent background. With the invention of intense coherent light sources (lasers) and their most recent technological advancements, optical holography has become a popular technique for three-dimensional (3D) imaging of macroscopic objects, security applications, and microscopic imaging.
Holography has been also recognized as a future data storing technology with unprecedented data storage capacity and ability to write and read a large number of data in a highly parallel manner. The enormous growth in demand for cloud storage has highlighted the need to rethink our storage systems from the media up. NAND flash and spinning hard disk drives are the mainstays of today’s warm cloud storage but are no longer improving capacity exponentially; in addition they face reliability and performance challenges due to the mechanical moving parts in hard disk drives and the declining endurance of flash cells.
While magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface of the recording medium, holographic data storage records information throughout the volume of the medium and is capable of recording multiple images in the same area utilizing light at different angles. Additionally, whereas magnetic and optical data storage records information a bit at a time in a linear fashion, holographic storage is capable of recording and reading millions of bits in parallel, enabling data transfer rates greater than those attained by traditional optical storage.
Holographic memory offers the possibility of storing 1 terabyte (TB) of data in a sugar-cube-sized crystal. A terabyte of data equals 1,000 gigabytes, 1 million megabytes or 1 trillion bytes. Data from more than 1,000 CDs could fit on a holographic memory system. Most computer hard drives only hold 10 to 40 GB of data, a small fraction of what a holographic memory system might hold.
Holographic data storage contains information using an optical interference pattern within a thick, photosensitive optical material. Light from a single laser beam is divided into two, or more, separate optical patterns of dark and light pixels. By adjusting the reference beam angle, wavelength, or media position, a multitude of holograms (theoretically, several thousands) can be stored on a single volume.
The stored data is read through the reproduction of the same reference beam used to create the hologram. The reference beam’s light is focused on the photosensitive material, illuminating the appropriate interference pattern, the light diffracts on the interference pattern, and projects the pattern onto a detector. The detector is capable of reading the data in parallel, over one million bits at once, resulting in the fast data transfer rate. Files on the holographic drive can be accessed in less than 0.2 seconds.
Holographic data storage can provide companies a method to preserve and archive information. The write-once, read many (WORM) approach to data storage would ensure content security, preventing the information from being overwritten or modified. Manufacturers believe this technology can provide safe storage for content without degradation for more than 50 years, far exceeding current data storage options.
Over the past decade, the Defense Advanced Research Projects Agency (DARPA) and high-tech giants IBM and Lucent’s Bell Labs have led the resurgence of holographic memory development. After more than 30 years of research and development, a desktop holographic storage system (HDSS) is close at hand. Early holographic data storage devices will have capacities of 125 GB and transfer rates of about 40 MB per second. Eventually, these devices could have storage capacities of 1 TB and data rates of more than 1 GB per second — fast enough to transfer an entire DVD movie in 30 seconds.
Holographic memory technology
Prototypes developed by Lucent and IBM differ slightly, but most holographic data storage systems (HDSS) are based on the same concept. Here are the basic components that are needed to construct an HDSS:
- Blue-green argon laser
- Beam splitters to spilt the laser beam
- Mirrors to direct the laser beams
- LCD panel (spatial light modulator)
- Lenses to focus the laser beams
- Lithium-niobate crystal or photopolymer
- Charge-coupled device (CCD) camera
When the blue-green argon laser is fired, a beam splitter creates two beams. One beam, called the object or signal beam, will go straight, bounce off one mirror and travel through a spatial-light modulator (SLM). An SLM is a liquid crystal display (LCD) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam around to the light-sensitive lithium-niobate crystal. Some systems use a photopolymer in place of the crystal. A second beam, called the reference beam, shoots out the side of the beam splitter and takes a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal — the data is stored as a hologram.
An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it. During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer.
The key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.
Although HDSS components are easier to come by today , there are still some technical problems that need to be worked out. For example, if too many pages are stored in one crystal, the strength of each hologram is diminished. If there are too many holograms stored on a crystal, and the reference laser used to retrieve a hologram is not shined at the precise angle, a hologram will pick up a lot of background from the other holograms stored around it. It is also a challenge to align all of these components in a low-cost system.
Magnonic Holographic Memory demostrated in 2015
Researchers at the University of California, Riverside Bourns College of Engineering and the Russian Academy of Sciences have demonstrated a new type of pattern recognition using a “magnonic” holographic memory device, intended to improve hardware for speech and image recognition. The device is based on patterns of sound and images that are encoded into the phase (timing) of spin waves, which are collective oscillations of spins in magnetic materials. Spin wave devices have a shorter wavelength than light, so they are more scalable. Spin-wave devices are also compatible with conventional electronic devices and can be integrated into a chip
The prototype eight-terminal device consists of a magnetic matrix with micro-antennas placed on the periphery of the matrix to excite and detect spin waves. The principle of operation is based on the effect of spin wave interference, which is similar to the operation of optical holographic devices. Input information is encoded in the phases of the spin waves generated on the edges of the magnonic matrix, while the output corresponds to the amplitude of the inductive voltage produced by the interfering spin waves on the other side of the matrix. The level of the output voltage depends on the combination of the input phases as well as on the internal structure of the magnonic matrix.
The work builds upon findings published last year by the researchers, who showed a 2-bit magnonic holographic memory device can recognize internal magnetic memory states via spin wave superposition. That work was recognized as a top 10 physics breakthrough by Physics World magazine.
The main challenge associated with magnonic holographic memory is the scaling of the operational wavelength, which requires the development of sub-micrometer scale elements for spin wave generation and detection.
Microsoft shows off holographic storage device in Sep 2020
Microsoft has announced Project HSD, an effort to build holographic storage devices for its Azure cloud service. While the idea of storing data throughout the volume of the medium (rather than on the surface as with magnetic and optical data storage devices) is not new, Microsoft believes that smartphone camera and display advances mean that reading data from HSDs is now much more scaleable. Additionally, by designing it for data centers, the company can create a product at the rack-scale, rather than having to worry about a smaller consumer-friendly size.
The company is using a lithium niobate (LiNBO3) crystal, with an added iron dopant. This brings an additional electron donor level and a deep trap state to the electronic energy levels of the LiNBO3. To store data, two green-light beams are used to illuminate the crystal – one a data-storage carrying beam, and the other a reference beam. Where the two beams cross an interference pattern is created. In the bright regions of the interference pattern, the crystal absorbs light causing electrons to be excited from the iron donor level to the conduction band where the electrons are free to move around the crystal lattice. These electrons then preferentially decay into the deep iron trap level where they remain trapped.
This results in a spatially varying distribution in electron density and its associated electric field that stores the data as a hologram. The data can then be read out by diffracting the reference beam off of the stored hologram, and capturing the data image on a camera. Exposing it to UV light excites electrons out of the deep iron trap level, where they then preferentially decay into the iron donor level, resetting the medium to be used all over again.
The company claims it has made several major HSD advances and that its system is 1.8x as dense as the best publicly known HSD research prototypes. This was partly made possible by turning to technologies that have advanced rapidly over the past decade, Microsoft CTO Mark Russinovich said at Ignite this week. “If we take a look at the kinds of technologies required to very efficiently implement holographic storage, we need very high resolution cameras. If you take a look at the cameras coming out of commodity smartphones today, they’re up at the resolutions in the tens of megapixels ranges that we need to commercialize a technology like this.”
Smartphones have also helped drive down the cost of display screens, useful for high resolution LCOS spatial light modulators. “We’ve been able to read and write with no mechanical seeks to unlock access rates that are comparable to hard disks,” said Russinovich. “We’ve also been able to leverage software computation via deep learning to be able to read out with high degrees of accuracy the data that’s been stored in the holographic storage.”
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