A pair of light waves – one zipping clockwise the other
counterclockwise around a microscopic track – may hold the key to
creating the world’s smallest gyroscope: one a fraction of the
width of a human hair. By bringing this essential technology down
to an entirely new scale, a team of applied physicists hopes to
enable a new generation of phenomenally compact gyroscope-based
navigation systems, among other intriguing applications.
“We have found a new detection scheme that may lead to the
world's smallest gyroscope,” said Li Ge, a physicist at the
Graduate Center and Staten Island College, City University of New
York. “Though these so-called optical gyroscopes are not new, our
approach is remarkable both in its super-small size and potential
sensitivity.”
Ge and his colleagues – physicist Hui Cao and her student Raktim
Sarma, both at Yale University in New Haven, Connecticut – recently
published their results in The Optical Society’s (OSA) new
high-impact journal Optica.
More than creative learning toys, gyroscopes are indispensable
components in a number of technologies, including inertial guidance
systems, which monitor an object’s motion and orientation. Space
probes, satellites, and rockets continuously rely on these systems
for accurate flight control. But like so many other essential
pieces of aerospace technology, weight is a perennial problem.
According to NASA, it costs about $10,000 for every pound lifted
into orbit, so designing essential components that are smaller and
lighter is a constant struggle for engineers and project
managers.
If the size of an optical gyroscope is reduced to just a
fraction of a millimeter, as is presented in the new paper, it
could then be integrated into optical circuit boards, which are
similar to a conventional electric circuit board but use light to
carry information instead of electric currents. This could
drastically reduce the equipment cost in space missions, opening
the possibility for a new generation of micro-payloads.
Putting a New Spin on Light-powered Gyroscopes
Quite different from mechanical gyroscopes, which are currently
used on ships for stabilization and rockets for guidance, optical
gyroscopes have no moving parts. Instead, dual light waves race
around an optical cavity or fiber, constantly passing each other as
they travel in opposite directions.
Traditional mechanical gyroscopes use Newton’s laws of motion to
maintain stability and orientation. These same physics principles,
however, do not apply to light, so measuring motion requires
looking for telltale yet very subtle optical signals instead.
One such signal comes from the unusual property of light known
as the Sagnac effect, which – put simply – creates a measurable
interference pattern when light waves split and then recombine upon
leaving a spinning system. Commercial optical gyroscopes build on
this principle, with their sizes varying from that of a baseball to
a basketball. They could be made much smaller, but measuring
rotation would require a much greater level of sensitivity than is
currently available.
Making a Gyroscope Out of Light
Traditionally, engineers have used two approaches to make
optical gyroscopes, both based on the Sagnac effect. The first one
uses an optical cavity – an engineered structure on a crystal – to
confine light and the second one uses an optical fiber to guide
light.
The second approach has, to date, been most practical because
its sensitivity can be easily enhanced by using longer sections of
optical fiber (some up to five kilometers long). These lengths of
fiber would then be wrapped around an object about five centimeters
in diameter, achieving a more manageable size. Though this system
is sensitive to rotation, there are practical limits to how long
the fiber can be and how small it can be wrapped before the fiber
itself is damaged.
To go truly small, optical cavities seem to be the preferable
option, where the Sagnac effect manifests as a subtle color change.
The problem, however, has been that the sensitivity of this type of
optical gyroscopes degrades as the cavity gets smaller.
“This issue was the roadblock that has hindered scientists from
developing tiny optical gyroscopes,” noted Ge. “There have been
several attempts to get around this limitation, but they could not
get around the real problem, the Sagnac effect itself.”
The researchers were able to overcome this hurdle by using a
very different principle based on far-field emission. Rather than
directly measuring the color change of the light waves, the
researchers determined that they could measure the pattern the
light produced as it exited the cavity.
“That was our key innovation – finding a new signal with a much
improved sensitivity to rotation,” said Ge. “Optical gyroscopes
optimized to produce and detect this new signal, we found, could be
about 10 microns across – smaller than the cross section of a human
hair.”
The idea is similar to rotating an uncovered light bulb. You
can’t see any direct spinning, but on small scales, the act of
rotation itself causes a small but measurable relativistic effect –
slightly bending space in and around the light source. This then
almost imperceptibly distorts the pattern on the wall. If measured,
however, the speed of rotation can be calculated from the degree of
distorting.
Spinning the Gyroscope
To start the new optical gyroscope, light waves are first pumped
into the optical cavity. This naturally produces light waves
traveling in both clockwise and counterclockwise directions. This
behavior is similar to plucking a guitar string in the middle,
sending vibrations in both directions simultaneously.
By carefully designing the shape of the optical cavity, the
researchers were able to control where both waves would exit.
Normally, cavities are designed to trap light as long as possible.
Here, the researchers needed to balance the light trapping
properties of the cavity with the need for some light to escape to
create a far-field emission pattern. This pattern is observed by
placing a pair of camera-like detectors facing the cavity at
different angles that move along with the cavity. This allows them
to continuously monitor the pattern for distortions that would
reveal the speed of rotation.
Though this only reveals one plane of motion, multiple such
sensors at different orientations would be able to give a fully
three-dimensional picture of how the object is moving.
Next Steps and Technology Development
According to the researchers, further studies are needed to take
into consideration the possibility that many modes, or light paths,
exist simultaneously in the cavity. Their far-field emission
patterns may change in different ways, which causes a reduction of
the sensitivity to rotation. The researchers are currently working
on different methods to control this effect.
Paper: L. Ge, R. Sarma, and H. Cao, “Rotation-induced evolution
of far-field emission patterns of deformed microdisk cavities,”
Optica, 2, 4, 323-328 (2015)
http://www.opticsinfobase.org/optica/abstract.cfm?uri=optica-2-4-323
doi: http://dx.doi.org/10.1364/OPTICA.2.000323
EDITOR’S NOTE: Images and an advanced copy of the Optica paper
are available to members of the media upon request. Contact Kelly
Mack at optica@ecius.net or 202.296.2002.
About Optica
Optica is an open-access, online-only journal dedicated to the
rapid dissemination of high-impact peer-reviewed research across
the entire spectrum of optics and photonics. Published monthly by
The Optical Society (OSA), Optica provides a forum for pioneering
research to be swiftly accessed by the international community,
whether that research is theoretical or experimental, fundamental
or applied. Optica maintains a distinguished editorial board of
more than 20 associate editors from around the world and is
overseen by Editor-in-Chief Alex Gaeta of Cornell University. For
more information, visit http://optica.osa.org.
About OSA
Founded in 1916, The Optical Society (OSA) is the leading
professional organization for scientists, engineers, students and
entrepreneurs who fuel discoveries, shape real-life applications
and accelerate achievements in the science of light. Through
world-renowned publications, meetings and membership initiatives,
OSA provides quality research, inspired interactions and dedicated
resources for its extensive global network of optics and photonics
experts. OSA is a founding partner of the National Photonics
Initiative and the 2015 International Year of Light. For more
information, visit www.osa.org.
Environics CommunicationsKelly Mack,
202-296-2002optica@ecius.net