Holographic technique stresses interference

By Eric Smalley, Technology Research News

Getting a sharp image is important in holographic data storage, but it's hard to achieve because holograms are created by the subtle interference patterns of two intersecting laser beams.

A team of researchers from China has added a twist to a holographic data storage system developed at the California Institute of Technology that promises to increase the system's diffraction efficiency, allowing for higher resolution recordings.

The technique uses a usually unwanted form of interference to reinforce the recorded images.

Data holograms are made when a pattern representing the data is put into a laser beam. When the beam comes together with a second laser beam on a recording medium, the interference pattern they create is captured in the medium. A hologram is read by shining a laser on the medium at the same angle as the second recording laser. This reproduces the original data pattern.

The Caltech system uses lithium niobium oxide suffused with particles of iron and manganese as its recording medium. The system first records, then fixes a hologram to make it permanent.

When the second beam bounces off the data pattern captured in the medium during both recording and fixing, it reproduces the pattern in the same manner as data is read out. The researchers noticed that when the beam hit this inadvertently read out pattern, it created a new interference pattern that was a copy of the original pattern. This echo pattern was also captured in the recording medium.

Depending on the orientation of the recording medium's crystals, the new pattern was either in phase or 180 out of phase with the original pattern, said De'an Liu, a graduate student at the Shanghai Institute of Optics and Fine Mechanics.

If the new pattern is in phase it reinforces the original pattern and if it is out of phase it weakens the original pattern, he said. "The superposition of the recorded and the new [patterns effects] diffraction efficiency, accordingly called self-enhancement or self-depletion effect," said Liu.

A hologram that has a higher diffraction efficiency can store more data because it's pattern is more sharply defined, which means it can fit more bits into a given area.

The researchers measured the diffraction efficiency for the four possible combinations of the effect: self-enhanced recording and self-enhanced fixing, self-enhanced recording and self-depleted fixing, self-depleted recording and self-enhanced fixing, and self-depleted recording and self-depleted fixing.

"The results show that the combination of self-enhanced recording and self-enhanced fixing has the highest diffraction efficiency, and it is twice as large as the lowest one from the combination of self-depleted recording and self-depleted fixing," said Liu.

The self-enhanced holographic storage technique could be used in practical systems in two to three years, he said.

Liu's research colleagues were Liren Liu, Youwen Liu and Changhe Zhou. They published the research in the November 6, 2000 issue of Applied Physics Letters. The research was funded by the National Natural Science Foundation of China and the Chinese Academy of Sciences.

Timeline:   2-3 years
Funding:   Government
TRN Categories:   Data Storage Technology
Story Type:   News
Related Elements:  Technical paper, "Self-enhanced nonvolatile holographic storage in LiNbO3:Fe:Mn crystals," Applied physics Letters, November 6, 2000




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April 11, 2001

Page One

Glass mix sharpens holograms

Material bends microwaves backwards

Shaky chip makes for bug-eyed bots

Cold plastic gives electrons free ride

Holographic technique stresses interference

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