Fiber
Optic Sensors
FOA
is mostly concerned with fiber optics used in
communications, but fiber has other applications also. It
can be used for sensors in a variety of different ways
that take advantage of its unique properties, often
creating sensors much more sensitive than sensors built in
traditional technologies.
Fiber
itself is a good sensor. Fiber is sensitive to
temperature, stress and strain (vibration and acoustics)
which sometimes allows using regular fiber optic cables
to be used as sensors. Special fibers with enhanced
sensitivity are used where regular fibers have
inadequate sensitivity or special physical parameters
need to be measured. Fiber can also be used to deliver
light to specialized sensors that can measure
temperature, velocity of a medium for flow measurement,
linear or angular motion and chemical composition. And
of course, fiber can be used to connect regular sensors
with electronics that produce digital readouts.
Here are some examples of how fiber is used in sensors.
Sensors Using Fiber Stress.
One thing all techs learn about fiber optics is to
minimize stress on the fibers. That means preventing
cables from bending too tightly or getting crushed. Stress
on the fibers causes attenuation, of course, but it can
also change the way light is transmitted in the fiber.
Stress type sensors can be used also as microphones.
An early application for simple stress sensors was a fiber
optic scale that was made by simply sandwiching a fiber
between two flat sheets of hard material. The sensitivity
was increased by imbedding microspheres in the fiber
coating so they would create stress points all along the
fiber. This scale was used to weigh vehicles on a roadway
as they drove over it at a toll booth.
The same sensor turned out to be so sensitive that when
buried in gravel under the ground it could measure a
person or even an animal crossing it, making it a very
sensitive intrusion alarm. One application tested was
securing the periphery of government sites. Another way to
create an intrusion alarm using fiber stress is to weave
the cable into a fence chain link fence. Any stress on the
fence can be detected and with an instrument that works
like an OTDR, located precisely. These types of fiber
optic sensors have been commercially available for
decades.
A much more sophisticated fiber sensor uses techniques to
greatly increase the sensitivity of the fiber to stress,
for example using changes in polarization, phase,
wavelength or time of flight in the fiber. These fibers
can be used to measure vibration making the fiber a
microphone but much more sensitive than conventional
microphones. An additional benefit is they can be string
out along a cable providing many sensors in a line
connected to one set of electronics. One big application
for specialized sensors is underseas where they are used
to detect and track submarines. They can be towed behind a
ship or submarine or installed permanently to guard a
harbor or shoreline. These types of sensors are also used
for seismic monitoring, for
oil and gas exploration and even embedding in concrete
structures like bridges or buildings to monitor stress or
predict failure.
Most
of what we’ve discussed are applications using fiber
optics that require installing special types of cables.
But even regular telecommunications cables have the
potential to provide sensor data that can be useful. For
years fiber optic testing techs have known that cables
installed along railways or roads could sense vibration
from passing trains or vehicles. One tech we know
discovered fiber characterization test data from aerial
cables correlated with wind speed.
More recently, tests have shown that regular fiber optic
cables used for communications can also be used as seismic
sensors for monitoring earthquakes. Cables in California
have been used for seismic monitoring as part of an early
warning system for earthquakes. Even submarine cables are
being used as sensors for mapping undersea
seismic
activity while still transmitting communications signals.
This
really interesting and useful application for using
installed telecom cable as a sensor was discovered by
Lawrence Berkeley National Laboratory. A researcher
there was studying thousands of miles of cable to use in
geohazard awareness, monitoring landslides, permafrost
slumps, sinkholes and other natural phenomena. This work
ended up discovering another useful application – at
least in California where the researcher was located –
monitoring earthquakes.
Instead of using seismometers, which are single-point
monitors and require expensive installation and
operation and are unfeasible in some locations like
urban areas, installed fiber optic cable can provide
seismic data. There’s more than 100,000 miles (160,000
km) of cables in the US, most of which have dark fibers
(spares) that can be fitted with laser interferometers
and used as earthquake monitors.
It’s not as easy as it might sound and a lot of research
still needs to be done to calibrate the cables as
sensors. Work is being done by researchers at UC
Berkeley, Stanford University and Lawrence Berkeley
National Laboratory at test sites in California to
gather data and develop systems to analyze the data to
produce useful information.
Fiber-Connected Sensors
Another type of sensor uses the fiber to transport light
to a sensor that changes transmission when exposed to
physical phenomena. One type uses crystals that vary with
electrical fields, allowing the sensor to measure very
high voltages and currents (millions of volts and amps)
while being electrically isolated by the fiber it is
attached to. These have been used by electrical utilities
on high voltage transmission lines for decades. Clamp-on
devices are used for temporary measurements or sensors can
be permanently installed on the transmission wires.
Fiber can be used to measure temperature and chemical
composition, especially in hazardous environments where
electrical currents are hazardous or the physical
conditions are not compatible with wires, e.g. corrosive
or high temperatures. Hazardous environments are an
excellent application for fiber optic sensors where glass
in impervious to most chemicals, high temperatures and
electrical interference. Some of these fiber optic sensors
involve attaching the sensor to the end of the fiber,
depositing a sensing compound on the end of the fiber or
chemically treating the fiber.
There are other sensors that use fiber to measure
reflections or light reflected from a moveable reflector
which are less sensitive but also less costly. They can be
used as sensors for liquid level, push-button or limit
switches or even microphones. A fiber optic microphone can
be built using nothing but two fibers (one source and one
to catch the reflections) and a plastic membrane. One like
this has been used in a high EMI environment to allow
conversations with workers in a testing lab.
Other Types Of Sensors
Another application involves worker safety , using an bare
fiber for sensing arc-flash events. A system is in general
use that runs a bare clear-coated fiber in locations where
arc-flash is a potential problem. The arc-flash creates a
very bright light flash. When an event occurs, the light
from the arc flash is picked up by the fiber, sensed by a
detector and electronics create an alarm and/or shut down
the high voltage system.
Fibers
can also measure movement allowing the creation of fiber
optic gyros used in place of conventional gyros for
navigation. A fiber with multiple loops has light injected
in each end and as the loops are rotated, the phase of the
light going in opposite directions will shift, showing the
direction and amount of rotation. This can be interpreted
just like a gyro compass used in aircraft navigation, but
the ruggedness of the fiber gyro makes it possible to use
in many other applications where mechanical gyros are
unusable.
Pros
And Cons
Cost has always been a problem for fiber optic sensor use.
Unless the unique characteristics of the fiber optic
sensor justify its cost, cheaper traditional sensors are
generally used. Most fiber contractors don’t do sensor
work, in part because there isn’t that much of it that
involves traditional installation. Some of the sensors
used in intrusion alarms involve installing fiber
including splicing and termination. Several of the
FOA-approved training schools teach techs how to install
and handle the ruggedized cables used for oil and gas
exploration. But not many of you would be interested in
some of the underwater or hazardous environment work
involved with fiber optic sensor installation.
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