How Holographic Storage Works
Posted: June 26, 2000
Written by: Tuan "Solace" Nguyen
Introduction
Do you ever find yourself asking how something works? Well, that’s what my How It Works series have been all about, giving you the inside info on how something works. From hard drives to monitors to optics to processors, I’m covering it all. But this time, I’m going to touch down on something that’s a little bit different. Something that hasn’t been released to the public yet will be in subject today. What could it be?
Today we’re going to be focusing on another type of storage technology. Last time it was opticals and before that it was hard drives. If you read my opticals articles you might remember in my conclusion I hinted at the continuation of the technology.
This is the second installment of how optical storage works. But I’m 100% sure that you won’t have one of these devices installed on your computer.
Has anyone seen my HoloCube? I dropped it somewhere and it has 47 ISO images on it, damn.
By now you may have guessed that the topic today is holographic storage, but before we dive straight into the technology, I will tell you how holography works.
Holograms
What is a hologram?
It is a light wave interference pattern recorded on photographic film (or other suitable surface) that can produce a 3-dimensional image when illuminated properly.
How is a hologram made?
A laser beam is split into two beams:
The width of the reference beam is increased conically by a lens or curved mirror and aimed directly at the film plate.
The width of the object beam is increased and aimed at the object. The object reflects some of the light on the holographic film-plate. The two beams cross each other forming an interference pattern on the film. This is the hologram. Laser light is needed because it is made of coherent waves (of same wavelength and phase).
Click to view a full size diagram
How is a hologram viewed?
When the hologram is illuminated from the original direction of the reference beam, a 3-dimensional image of the object appears where the object was originally. Some holograms must be viewed with laser or monochromatic (single color) light, and others with white light.
What are the main types of holograms?
Transmission Holograms: Viewable with laser light. They are made with both beams approaching the film from the same side.
Reflection (White Light) Holograms: Viewable with white light from a suitable source such as spotlight, flashlight, the sun, etc. They are made with the two beams approaching the holographic film from opposite sides.
Multiple channel holograms: Two or more images are visible from different angles. There are different types of these holograms: simple ones with 2, 3, or a few images each viewed from a different angle.
Multiplex: A large number of "flat" pictures of a subject viewed from different angles are combined into a single, 3-dimensional image of the object. This is also known as a Composed hologram.
Rainbow holograms: The same image appears in a different color when viewed from different angles.
Types of Holograms (cont.)
Real Image Holograms (H-2's)
These are usually reflection holograms made from a transmission original (H-1). The image dramatically projects in front of the plate toward the viewer. Most holograms in holography museums are of this type. The procedure for making them is quite elaborate and demands precise control of angles.
Mass-Produced Holograms
Embossed: Made by stamping on foiled backed Mylar film using a metal master (most common method).
Polymer: Made from light sensitive plastic. The Polaroid Corporation mass produces holograms by this method.
Dichromate: Very vivid holograms on jewelry, watches, etc. They are recorded on a light sensitive coating of gel that contains dichromate.
What can holography technology be used for?
Holographic Art: Holography museums, advertising, postage stamps, jewelry, etc.
Security from Forgery: credit cards, tickets, etc.
Optical Devices: Holographic lenses, diffraction gratings, etc. These are holograms in which the "object" is a mirror or a lens. A flat mirror as an object produces a diffraction grating. A lens or a concave mirror as the object produce a hologram that behaves like a lens. These holographic lenses are lighter than traditional lenses and mirrors and they can be designed to perform more specialized functions such as making the panel instruments of a car visible in the windshield for enhanced safety.
Holographic Interferometry: A very precise technique for measuring changes in the dimensions of an object. It is useful in industrial stress analysis, quality, control, etc.
Pattern Recognition: Uses electro-optical devices with computers to interpret what is "seen" by a machine. It is useful in military applications of lasers and holographic optical devices.
Medical Applications: Combining CAT scans into a 3-dimensional image; a multiplex. It’s useful as ultrasound holography, etc.
Other: Holographic computer memory storage, holographic microscopy, holographic radar, etc.
Holographic Storage
Well, we’ve all been waiting for this point -- how do we store information using holographic technology?
Well, like first principles, the storage technology is based on principles of holography; it will use the concept of beam interference patters caused by interacting laser beams.
The following is a diagram of how complex the assembly is to be able to use the technology.
IBM's Almaden Research Center has built a precision Photorefractive Information Storage Materials (PRISM) test stand for evaluating photosensitive samples. It also illustrates the fundamentals of a holographic storage system, as shown in the picture above
A blue-green argon laser similar to the one above is split into a reference and an object beam. The object beam, which is the carrier for the data is expanded to fully illuminate a spatial light modulator (SLM). What’s an SLM? It is an LCD panel that displays a page of binary data as a grid of pixels being either on or off.
PRISM
The object beam finally interacts with the reference beam inside a photosensitive crystal. The ensuing interference pattern -- the substance of the hologram -- gets stored as a web of varying optical characteristics inside this crystal. To read the data from the crystal, the reference beam merely illuminates the crystal. The stored interference pattern that was recorded with the two interacting beams diffracts the reference beam's light so that it reconstructs the checkerboard image of the light or dark pixels. In other words, the original beam will appear when the reference beams strikes the page in the crystal. The resulting image is then emitted upon a charge-coupled device (CCD) sensor array, and it instantly captures the entire digital page.
When reading out the data, the reference beam must strike the crystal at the same angle that's used in recording the page. This incident angle is extremely important and it can't vary by more than a fraction of a degree.
If the angle is off by even a very small amount, it will not reproduce the data beam and the data page image. This is obviously a bad thing if you are talking about data integrity and accuracy, but actually, it can be a good thing overall. Why? The apparent flaw in the recording process actually helps store more data and achieves higher transfer rates. It's how holographic storage achieves its high data densities. By changing either the angle of the reference beam or its frequency, you can write additional data pages in to the same volume of crystal.
However, as you keep recording more data pages slightly away from previous pages, the holograms will begin to appear dimmer and fogged up because their patterns must share the material's finite dynamic range and the data page is physically etched into the crystal. Eventually you will run out of space to store because the crystal has depleted all of its physical storage capacity, sort of like write once, read many media such as CD-R. If more data is continuously written, the data images become so dim that laser beam noise creeps into the read-out operation.
The dynamic range of the recording medium will determine how many pages it can hold with strong integrity. The PRISM project is currently examining limitations in a various photosensitive materials. Current tests are conducted using iron-doped lithium niobate, strontium barium niobate, or barium titanate crystals. IBM is also interested in using organic material to be the medium because of its dynamic ability to change data.
The PRISM project has stored up to 200 holograms composed of 37.5-KB data pages (640 by 480 bits resolution) into a crystal with less than 1 centimeter on each side, achieving a storage density of 48 MB per cubic cm. You must be thinking “48 MB!? We’ve been developing holographic technology to store that little?!” Well, yes and no. Yes because that is an achievement of the technology previously not possible, and no because soon the same size volume of crystal will hold up to 10GB. The incentive is a page data density of 10 GB per cubic cm, a bit smaller than a standard gambling die.
PRISM and Holographic Materials
For a hologram to work in this technology, the material (holographic medium) must meet certain requirements.
Excellent optical quality: A high resolution data page with as many as a million pixels encoding digital data must be imaged through the material and onto the detector array, pixel for pixel. This requires very good homogeneity, and optical quality surfaces.
High recording fidelity: The material must faithfully record the data beam amplitude so that this high quality image can be reconstructed when the data is read out.
High dynamic range: The larger the number of holograms that are recorded in a common volume of material, the weaker each hologram becomes; the signal strength scales as the inverse square of the number of holograms, and is limited ultimately by the ability of the material to respond to optical exposure with the refractive index modulation that records the holograms. The greater is the materials ability to respond, i.e. the greater its dynamic range, the more holograms can be recorded, and ultimately, the greater the density of data that can be stored.
Low scattered light: The ultimate lower limit to the strength of holograms that are useful for data storage is determined by noise from readout beam scattering. Thus, scattered light also limits storage density.
High sensitivity: To store data in the material at a reasonable data rate, the material should respond to the recording beams with high sensitivity.
Non volatile storage: The material should retain the stored hologram for a time consistent with a data storage application, and should do so in the presence of the light beams used to read the data. For write-once read-many storage, an irreversible material (such as a photopolymer) can be used, which provides stable recording once exposed. If a reversible material is chosen in order to implement erasable/re-writable data storage, the requirement for nonvolatility is in conflict with that for high sensitivity unless a nonlinear writing scheme, such as two-color gated recording is used.
IBM estimates that it will take a few more years to tweak and refine the technology enough to build small desktop HDSS units. It guesses that desktop HDSS units could be available by as early as 2003. Okay, obviously I was joking about my HoloCube if you haven’t noticed... heh. :)
HDSS hardware uses an acoustoptical (accu-whaa?) light deflector. This means a crystal whose refractive properties change according to sound waves traveling through it. To modify the beam angle, IBM thinks that an HDSS system can retrieve adjacent data pages in fewer than 100 microseconds. "Any convention al optical or magnetic storage unit will require some sort of mechanical means to access different data tracks, which takes on the order of milliseconds to accomplish," they say. "A gigabit-per-second data rate appears reasonable for holographic storage, and this should make it a cost-competitive leader with whatever exists."
Below is a diagram showing the core data page recording process of the technology:
Notice how the single beam is split into two.
Well folks, there you have it, the inside details of how this promising technology works. Personally, I can’t wait until this technology arrives on the street, I wouldn’t keep having to burn my uh, "backed up"... ISO images on to CDs. Anyhow, I hope you’ve learned something from this article. Feel free to e-mail me if you have any questions or comments.
This isn’t the end of the line for storage technology though. The next installment of my How It works series will cover something very interesting -- Atomic Resolution Storage. Until then.
References: Scientific American, Byte Resources, IBM Almaden Research Center, The Holography Handbook by Fred Unterseher and co-authors Ross Books 1987, The Complete Hologram Book by J. E. Kasper & S. A. Feller, Prentice-Hall, 1987, and Understanding Lasers, by Jeff Hecht, H. W. Sams and Co., 1988.
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