FOA Guide


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 weíve 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 Ė at least in California where the researcher was located Ė 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. Thereís 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.

Itís 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 donít do sensor work, in part because there isnít 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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