Saturday, April 24, 2010

Holograms: More than just pretty pictures?

At this point, you may be wondering, “Why does MIT, a university known for driving innovation, care about and collect pretty 3D pictures?” Well, it turns out that holography has some pretty important real-world applications, such as data storage.

As you probably know because you’re reading this blog, we live in an age of information. Of course, this explosion of information wouldn’t be possible without ways to quickly store and transfer large quantities of data. So far, we’ve been relying on conventional optical storage technologies (e.g. CD’s, DVD’s, Blu-Ray) to handle this need. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand.

Top view of a CD and its spiral of bitsSide view of holographic disc

This is where holography comes in. Although optical storage technologies have improved by leaps and bounds since the advent of CD’s (DVD’s can hold 15 times more information than CD’s and Blu-Ray discs can hold 10 times more than DVD’s), they are still limited to recording information on the surface of a disc. They all use lasers to etch a spiral of individual bits (1’s and 0’s) onto the surface of a recording medium, but holographic data storage uses lasers to etch pages of bits throughout the entire volume of a recording medium. Theoretically, holographic discs can store 40 times more data than Blu-Ray discs can. How exactly is this done?

Recording hologram of SLM

In my first blog post, I went over the process of using holography to make images of physical objects but how would we go about using holography to store information that lives inside of a computer and that we can’t actually touch or see? Suppose that you want to store a movie onto a holographic disc. Your computer converts the data into a sequence of bits and sends this to a spatial light modulator (SLM). The SLM is a screen that arranges this sequence into a page of bits. Each bit is represented by either a black or white square (0 is black; 1 is white), so that the LCD screen looks like a checkerboard of black and white squares. Now that the data is rendered into a visible form, we can make a hologram of it. Once again, a laser beam is split into a reference beam and an object beam. The object beam goes through the SLM and comes out carrying the pattern of the image that was displayed on the SLM. The two beams eventually meet at some place within the recording medium (e.g. at the surface or in the middle), which records the interference pattern of the two beams. To read a page of data, you recreate the object beam by illuminating the recording medium with the reference beam. The recreated object beam hits a charged coupled device (CCD), which is a sensor that is connected to a computer. The computer takes a page of black and white boxes and converts them back into a string of bits and back into data.

Reading hologram

Note that the reference beam must approach the SLM from the exact same angle that was used to write a page of data; otherwise, the phase of the beam will be different and you will recreate the wrong object beam. Because you can recreate different object beams by varying the phase and wavelength of the reference beam, it is possible to store multiple holograms in the same volume of recording medium. You can also stack holograms on top of each other by using mirrors and lenses to specify the location of the interference pattern within the medium. So far, the company InPhase Technologies was able to cram 500 Gigabytes into one square inch of medium that was as thick as a CD. Based on this figure, a disk the size of a CD could hold up to about four Terabytes (one Terabyte = 1000 Gigabytes) of information. With this technology, we could fit the entire printed collection of the US Library of Congress onto three discs.

Searching hologram

Not only are holographic discs superior to CD’s, DVD’s, and Blu-Ray discs in terms of storage capacity, they also permit high data transfer rates. One page of data (equivalent to 125 Megabytes) can be read with a single flash of light, whereas CD players are limited to reading one bit (1/8th of a byte) per flash of light. Since multiple holograms stored in the same volume of medium can be read by altering the angle of the reading laser, high transfer rates are possible since the angle of a laser can be manipulated quickly without inertia, unlike a CD player which relies on a mechanical part to move the laser back and forth. Another advantage of holographic storage is that it can be searched extremely quickly. If you illuminate a disc with an object beam, you will reproduce the corresponding reference beam and its angle, which immediately identifies the page on which the information is stored. This means that database searches can be done using physics rather than software.

So if scientists already know how holographic data storage works and have even built a few prototypes, why haven’t we seen any of these systems on the market today? In this technology, lasers have to be directed and aligned very precisely. Any slight deviation would make it near impossible to store and retrieve error-free data. For this reason, the components of the system are extremely expensive to manufacture quickly and in large quantities. Another issue arises out of the ability to stack multiple holograms. When a computer attempts to read a disc’s data, the reference beam will reproduce its corresponding object beam, but will also produce a lot of noise from the other holograms that are stacked on top of it. The more holograms there are, the more noise there will be. Noise makes it difficult to retrieve correct data. It’s like trying to watch TV with poor picture quality. You might be able to discern a few shapes here and there, but you can’t be absolutely sure of what you’re seeing. Theoretically, thousands of holograms can be stacked in a disc as wide as a CD, but it doesn’t make sense to do that if you can’t retrieve the data. The developers of this technology need to figure out how to reduce the noise as much as possible before the product can be commercialized.

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