Optical fiber microresonators show promise as optical memory

Researchers at OFS Technologies Somerset, NJ, say they have developed a precise and efficient way to construct microresonators by making nanoscale changes to the diameter of standard optical fiber. The researchers say this is “an essential step toward designing a practical optical computer.”

Optical computers, which use photons in place of electrons to process and store information, have the potential to be much faster than today’s electronic computers. But it has proven difficult to make the optical equivalent of a memory chip, in spite of extensive research in this area.

Now the researchers say they have designed a new kind of optical memory that could be commercially available in the next few years. "We can faithfully reproduce these resonators. There’s a real, robust way of fabricating these, and this is the first paper that actually shows that," said Misha Sumetsky, a researcher at OFS Laboratories and lead author on the study.

Most approaches to making microresonators are based on silicon lithography, which is used to etch extremely precise features onto silicon wafers. For microresonators the most promising design appeared to be a long series of microscopic loops, which bottle up photons in whirling circles and then pass them from one ring to the next. The longer the chain, the longer the signal could be stored as memory. Unfortunately, even the most precise manufacturing processes still produce tiny imperfections in the rings. These bumps on the road slowly weaken the signal, attenuating the light and allowing the memory held in the buffer to fade away.

Sumetsky and his colleagues at OFS Laboratories abandoned the silicon wafer in favor of a silica strand of optical fiber. Optical fiber has incredibly low losses, allowing light to be held in the resonator for much longer without fading. To make a microresonator, the diameter of an optical fiber is varied to create microscale indentations in its diameter. Light traveling between two of these narrowed portions of fiber will continue to resonate back and forth with very little loss. Light is coupled into and out of the resonators with evanescent coupling.

If sufficient number of optical fiber microresonators can be coupled together, then any information contained in the light pulses could be stored long enough for computational purposes. The researchers have so far been able to couple 10 optical fiber microresonators, an important proof-of-concept step.

Other researchers have tried to make microresonators out of optical fiber, but relied upon polishing or melting the fiber to change its diameter. This produced very uneven results and could not achieve nanoscale dimensions. "To enable evanescent coupling, it’s vital that the circumference of the microresonators be controlled to sub-angstrom accuracy," the researchers contend. The new process developed by OFS, dubbed Surface Nanoscale Axial Photonics or SNAP, is capable of such accuracy and ensures that each microresonator is virtually identical.

It’s possible to create these nanoscale changes to the radius of the fiber by exploiting the inherent stress in the fiber created during manufacturing. The researchers directing a laser beam at the fiber to create localized heating. By raising the fiber’s temperature, but keeping it well below the melting point, it was possible to release this stress, changing the diameter and refractive index of the fiber without deforming it any further. As long as the fiber is produced under the same conditions and it is heated below the melting point, the same effect is always achieved.

“We heated it to a temperature lower than the melting temperature,” said Sumetsky. “This annealing allows us to change the radius in this nanoscale range. In the new system, the accuracy of the fiber radius variation is about 0.1 angstrom – orders of magnitude better than achieved before.”

According to the researchers, it’s possible these microresonators could be used in specialized devices in about two to three years. However, they say the greatest potential may be in pioneering optical computing and in enabling fundamental physics research.

The work was reported in Optics Letters.