Signal Processing - March 2016 - 15

using advanced filtering," Fedyanin
and private properties-are becoming
says. "Such a low bandwidth can be
Internet connected. A sensor technology
achieved, for example, by embedding
developed by a team of experimental
the actuation-transduction scheme in a
physicists led by Uwe Hartmann, a prophase-locked loop." Under laboratory
fessor at Saarland University, aims to
conditions, it is possible to efficiently
prevent unauthorized individuals from
perform measurements simply using a
gaining access to secure sites by raising
lock-in amplifier,
an alert as soon as
Fedyanin notes.
anyone attempts to
researchers worldwide
Fedyanin says that
climb over or cut
are currently implementing through a fence.
the biggest challenge
signal processing in
facing the researchers
The magnetic senvarious and frequently
is getting the sensor to
sors are integrated
work reliably and
into a thin cable that
innovative ways to help
accurately at room
can be added to virtucreate a new generation
temperature, which
ally any type of
of powerful sensors that
has so far proved eluperimeter fence. The
promise to improve lives. cable and fence are
sive. "Experimental
studies
and
then connected via a
optimization are required for developing
digital bus to a processing and analysis
practical devices," he says.
system that can rapidly calculate the preAlthough the research has already genercise location of any security breach.
ated some commercial interest, Fedyanin
"Any tiny vibration of the fence due to
says that the sensor itself remains several
intrusion can be detected by the magnetyears away from production. "We expect
ic field sensors in an accurate way
that such a device can appear on the market
through analyzing the data from all of
in five to ten years," Fedyanin predicts.
the sensors," says Saarland University
physicist Haibin Gao, a research team
member working on the project's sensor
A "smart" fence
technology. "Complex algorithms permit
As the IoT gradually weaves its way into
the discrimination of false alerts and
the fabric of everyday life, even property
identification of the intrusion category."
fences-such as types surrounding airThe "smart" fence is designed to
ports, nuclear power stations, industrial
detect a wide range of tampering
sites, and various other types of public

saarLand University.

waveguide and the cantilever experiences a ponderomotive force (i.e., a nonlinear force that a charged particle
experiences in an inhomogeneous oscillating electromagnetic field), which is
proportional to the gradient of the
squared electric field of the pump signal," Fedyanin says. Since the pump signal is sinusoidally modulated, it
produces a sinusoidal force on the cantilever. "This can be considered as a transduction process: the pump optical signal
is converted to mechanical oscillations
of the cantilever," Fedyanin explains.
"The amplitude of these oscillations
depends on the resonant frequency of
mechanical oscillations of the cantilever
and the quality factor of the cantilever."
The oscillating cantilever modulates
the intensity of the probe optical signal at
the frequency of the cantilever's mechanical oscillations. "The probe signal transmitted through the waveguide section
with the cantilever contains information
about the cantilever's vibrations and
noise," Fedyanin says. Thanks to the
strong interaction between the highly confined optical mode and nanobeam cantilever, the laser noise is typically lower than
the noise produced by the cantilever and
does not limit the sensitivity of the sensor." Finally, the probe signal is detected
by a photodetector, which converts the
optical signal into an electrical signal.
The cantilever's oscillations make it
possible to determine the chemical composition of the environment in which the
chip is placed. That is because the frequency of mechanical vibrations depends
not only on the materials' dimensions and
properties, but also on the mass of the
oscillatory system, which changes during
a chemical reaction between the cantilever and the environment. By placing different reagents on the cantilever,
researchers can make it react with specific
substances or even biological objects.
"The sensor sensitivity is high enough to
detect single proteins and DNA molecules," Fedyanin says.
"To achieve ultimate sensitivity and
single-molecule resolution, one needs
to maximize the signal-to-noise ratio by
reducing the effective electrical bandwidth of the photodetector and filtering
out the noise, which can be done by

figure 3. Fenced-in areas can be continuously, unobtrusively monitored with a sensor-powered
"smart" fence technology developed by a team of experimental physicists, led by Prof. Uwe Hartmann
(left) of Saarland University and coresearchers Uwe Schmitt (right) and Haibin Gao (front).
IEEE Signal Processing Magazine

March 2016

Table of Contents for the Digital Edition of Signal Processing - March 2016