Propagation of light in a SNAP fiber coupled to a tapered regular optical fiber. Courtesy OFS Laboratories. |
Optics
and photonics may one day revolutionize computer technology with the
promise of light-speed calculations. Storing light as memory, however,
requires devices known as microresonators, an emerging technology that
cannot yet meet the demands of computing. The solution, described in a
paper published today in the Optical Society’s (OSA) journal Optics Letters, may lie in combining light’s eerie quantum properties with a previously unknown quality of optical fiber.
Researchers from OFS Laboratories
in Somerset, N.J., have developed a precise and efficient way to create
microresonators by making nanoscale changes to the diameter of normal
optical fiber. These narrow sections are able to confine light, sending
it on a back-and-forth corkscrew path inside a length of optical fiber
and creating a microresonator.
Though
trapping light in this so-called “Whispering Gallery” mode is a
well-known phenomenon, the researchers have discovered a quick,
efficient, and accurate way to manufacture long chains of these new
microresonators, all based on a never-before-recognized characteristic
of optical fiber. This is a new technology path and an essential step
toward designing a practical optical computer, as described in the Optics Letters paper.
“Optical
computers, which use light particles—photons—in place of electrons to
process and store information, have the potential to be much faster than
today’s electronic computers,” said Misha Sumetsky, a researcher at OFS
Laboratories and lead author on the study. “Unfortunately,
manufacturing microresonators that meet the demands of optical computing
has been a long and, until now, unsuccessful pursuit.”
Microresonator design
Designing
a practical microresonator has been something of a “Holy Grail” on the
path to optical computers. The current microresonator manufacturing
technology is based on the well-established process of silicon
lithography, which etches 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, formerly part of the famous
Bell Labs, pursued a path that abandoned the silicon wafer in favor of
the silica strand of optical fiber.
In
conventional applications, optical fiber—a very pure form of
glass—uses the fundamental properties of light and refraction to keep
light from slipping out and diffusing. The core and cladding of optical
fiber have slightly different indexes of refraction, giving the fiber
the ability to bend the path of light without causing scattering. Light
traveling through the fiber bounces back and forth inside the core,
keeping it traveling along the fiber for many kilometers with very
little signal loss.
Coaxing light into a whispering gallery
This
sends the light careening through the fiber at extremely high speed,
but just as cars barreling down the highway sometimes get directed onto
“cloverleaf” off ramps, so too can light be coaxed from the fiber and
into a spiral path. Unlike cars, however, light doesn’t need to slow
down on the off ramp.
Illustration of SNAP microresonators formed by nanoscale variation of the optical fiber diameter. Courtesy OFS Laboratories. |
In
this case, the off ramp is created by narrowing the fiber to a small
diameter to coax the light out of the core and into a fiber aligned
perpendicularly and positioned very close to, or actually touching the
first. Because they are so close, and the original fiber narrows down to
a mere fraction of its original size, a portion of the light is able to
make a literal “quantum leap” to the other fiber. This is an effect
known as “evanescent coupling” and it enables an electromagnetic
wave—light—to connect (or couple) from one fiber to another.
The
light now finds itself not traveling down a straight path but rather
racing around the fiber surface in very tight circles. Even though the
light maintains its original pace within the glass, because it’s really
taking the long way around, corkscrewing along the new fiber’s surface,
it propagates down the fiber at a fraction of its original speed.
This
special redirection of light is known as the “Whispering Gallery”
effect, named after the phenomenon that takes place in certain
architectures, such as the St. Paul’s Cathedral in London and Grand
Central Station in New York, where someone whispering along the wall
would hear their whisper coming from behind them as the sound traveled
around the edge of the room and returned to its original spot.
Optical fiber microresonators
Sumetsky
and his colleagues were able to vary the optical fiber diameter by
several nanometers. They did this with unprecedented precision, on the
order of a hundredth of a nanometer.
This
dimpling or narrowing of the fiber effectively changes the properties
of the Whispering Gallery and has the effect that light traveling along
the surface of the fiber would turn around and head back the way it
came. If it were traveling between two of these narrowed portions of
fiber, the light would continue to resonate back and forth with very
little loss of signal. This is, in fact, the microresonator.
These
optical fiber microresonators currently are able to retain light two
orders of magnitude longer than lithographic microresonators—and the
researchers say it’s possible to push that number even higher.
If
sufficient number of optical fiber microresonators were coupled
together, again taking advantage of evanescent coupling, 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.
Manufacturing is a ‘SNAP’
It’s
possible to create these nanoscale changes to the radius of the fiber
by exploiting a property inherent in the fiber created during
manufacturing and discovered at OFS Laboratories several years ago.
Optical fiber is made by heating a much thicker rod of glass with a
precise chemical makeup and stretching and drawing it out into extremely
fine and flexible fibers. When the fiber is drawn out, the process
introduces certain tension, and this tension is frozen in, creating a
predetermined amount of stress.
The
researchers harnessed this fixed stress by directing a laser beam at
the fiber to heat it. By raising the fiber’s temperature, but keeping it
well below the melting point, it was possible to release this intrinsic
pressure, 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. This process enables a technology that the
researchers call Surface Nanoscale Axial Photonics (SNAP).
“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.”
Previous
attempts have been made at harnessing optical fiber for
microresonators, but these relied on 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 SNAP process ensures this accuracy and that
each microresonator is nearly identical.
This
is the crucial point the researchers believe will enable the technology
to move from laboratory studies to manufacturing. As long as the
optical fiber is produced under the same conditions, it will always
produce the same effect when heated, changing its properties in the same
precise manner.
“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,” Sumetsky said.
According
to the researchers, it’s possible these microresonators could be used
in specialized devices in about two to three years. However, their
greatest potential may be in pioneering optical computing and in
enabling fundamental physics research.
Surface nanoscale axial photonics: Robust fabrication of high quality factor microresonators
