Tiny Circuits, Long Distances: Smaller Light Processing Devices for Fiber-optic Communication

Researchers at Michigan Tech have mapped a noise-reducing magneto-optical response
that occurs in fiber-optic communications, opening the door for new materials technologies.

Optical signals produced by laser sources are extensively used in fiber-optic communications,
which work by pulsing information packaged as light through cables, even at great
distances, from a transmitter to a receiver. Through this technology it is possible
to transmit telephone conversations, internet messages, and cable television images.
The great advantage of this technology over electrical signal transmission is its
bandwidth — namely, the amount of information that can be broadcast.

New research from a collaboration between Michigan Technological University and Argonne
National Laboratory further improves optical signal processing, which could lead to
the fabrication of even smaller fiber-optic devices.

The article, unveiling an unexpected mechanism in optical nonreciprocity — developed
by the research group of Miguel Levy, professor of physics at Michigan Tech — has
been published in the journal Optica. “Boosting Optical Nonreciprocity: Surface Reconstruction in Iron Garnets” explains the quantum and crystallographic origins of a novel surface effect in nonreciprocal
optics that improves the processing of optical signals.

Quiet Optical Signals

An optical component called the magneto-optic isolator appears ubiquitously in these
optical circuits. Its function is to protect the laser source — the place where light
is generated before transmission — from unwanted light that might be reflected back
from downstream. Any such light entering the laser cavity endangers the transmitted
signal because it creates the optical equivalent of noise.

“Optical isolators work on a very simple principle: light going in the forward direction
is allowed through; light going in the backwards direction is stopped,” Levy said.
“This appears to violate a physical principle called time reversal symmetry. The laws
of physics say that if you reverse the direction of time — if you travel backwards
in time — you end up exactly where you started. Therefore, the light going back should
end up inside the laser. But it doesn’t.

“Isolators achieve this feat by being magnetized. North and south magnetic poles in
the device do not switch places for light coming back. So forward and backward directions
actually look different to the traveling light. This phenomenon is called optical
nonreciprocity,” he said.

For Michigan Tech’s FEI 200kV Titan Themis Scanning Transmission Electron Microscope
(STEM) (one of only two Titans in the state of Michigan), all the world’s a stage.​

Optical isolators need to be miniaturized for on-chip integration into optical circuits,
a process similar to the integration of transistors into computer chips. But that
integration requires the development of materials technologies that can produce more
efficient optical isolators than presently available.

Recent work by Levy’s research group has demonstrated an order-of-magnitude improvement
in the physical effect responsible for isolator operation. This finding, observable
in nanoscale iron garnet films, opens up the possibility of much tinier devices. New
materials technology development of this effect hinges on understanding its quantum
basis.

The research group’s findings provide precisely this type of understanding. This work
was done in collaboration with physics graduate student Sushree Dash, Applied Chemical
and Morphological Analysis Laboratory staff engineer Pinaki Mukherjee and Argonne National Laboratory staff scientists Daniel Haskel and Richard Rosenberg.

The Optica article explains the role of the surface in the electronic transitions
responsible for the observed enhanced magneto-optic response. These were observed
with the help of Argonne’s Advanced Photon Source. Mapping the surface reconstruction underlying these effects was made possible through
the state-of-the-art scanning transmission electron microscope acquired by Michigan
Tech two years ago. The new understanding of magneto-optic response provides a powerful
tool for the further development of improved materials technologies to advance the
integration of nonreciprocal devices in optical circuits.

Michigan Technological University is a public research university, home to more than
7,000 students from 54 countries. Founded in 1885, the University offers more than
120 undergraduate and graduate degree programs in science and technology, engineering,
forestry, business and economics, health professions, humanities, mathematics, and
social sciences. Our campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway
and is just a few miles from Lake Superior.