In last month’s column, I discussed other uses for optical fiber beyond communications. I mentioned lighting, viewing and laser surgery. I also mentioned sensors, the topic I elaborate on this month.
One thing all techs learn about fiber optics is to minimize stress on the fibers. That means preventing cables from bending too tightly and getting crushed. Stressing the fibers causes attenuation, and it can also change the way light is transmitted in the fiber, which actually enables us to use the fibers in another way: as a stress-type sensor. Stress-type sensors can be used as microphones. Fibers can measure movement, which allows for the creation of fiber optic gyroscopes used in place of conventional gyros for navigation.
I won’t get into how most of these sensors work, especially gyros, because as one of my favorite science fiction authors, Isaac Asimov, said, “Sufficiently advanced technology is indistinguishable from magic.” Also, it would take too long.
My first experience with stress sensors was developing a fiber optic scale that could weigh vehicles on a roadway as they drove over it at a toll both. It turned out to be so sensitive that when buried under the ground, it would measure a person or animal crossing over it, which makes it a very sensitive intrusion alarm. One application for this technology was securing the periphery of classified government sites.
Another way to create an intrusion alarm is to weave the cable into a fence. Any stress on the fence can be detected and, with an instrument that works like an OTDR, located precisely. These types of sensors have been commercially available for decades.
A much more sophisticated sensor uses techniques to greatly increase the sensitivity of the fiber to stress, making it work like a microphone but much more responsive. An additional benefit is they can be strung out along a cable and provide many sensors in a line connected to one set of electronics. One big application for these is tracking submarines underwater. They can be towed behind a ship or submarine or permanently installed to guard a harbor or shoreline. They can also be used as seismic sensors for monitoring earthquakes and for oil and gas exploration.
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 and enables the sensor to measure very high voltages and currents (megavolts and amperes) while being electrically isolated by the fiber it is attached to. These have been used on high-voltage transmission lines for decades. Clamp-on devices are used for temporary measurements or sensors that 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, such as near corrosive or in high temperatures. Hazardous environments are an excellent application for fiber optic sensors where glass is impervious to most chemicals, high temperatures and electrical interference.
There are other sensors that use fiber to measure reflections or light reflected from a movable reflector that are less sensitive and less costly. They can be used as sensors for liquid level, push-button or limit switches and microphones. We once built a fiber optic microphone using nothing but two fibers (one source and one to catch the reflections) and a Mylar membrane. It was used in a high EMI environment to allow for conversations with workers in a testing lab.
I was recently introduced to a new application—sensing arc-flash events. A system in general use runs a bare fiber in locations where arc flash is a potential problem. When an event occurs some of the light is picked up by the fiber, sensed by a detector and then electronics create an alarm.
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 partly because not much of it involves traditional installations.
Some of the sensors used in intrusion alarms involve installing fiber, including splicing and termination. Several approved training schools teach techs how to install and handle the ruggedized cables used for oil and gas exploration. But not many would be interested in some of the underwater or hazardous environment work involved with fiber optic sensor installation.