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  • With Fiber Comes Miles-Long and Multiplexed Sensors

Photonics Spectra
Dec 2015
Fiber sensors can bring light into otherwise inaccessible places, and measure parameters such as vibration and strain. They can do so in a multiplexed way, prompting more and faster data, and providing a comprehensive view of what’s happening.

HANK HOGAN, CONTRIBUTING EDITOR, Contributing Editor, hank.hogan@photonics.com

How do you spot a landslide before it happens, or improve the efficiency of an airplane or rocket engine? Fiber may be the answer. Fiber sensors can transport interrogating light into otherwise unreachable places. They can also collect information along their entire length, thanks to the impact of the surrounding environment on light traveling through the fiber. They can measure vibration, strain, temperature, position and other parameters. What’s more, fiber sensing can potentially be done in a multiplexed way, the results of which are “more data, faster data and a better picture of what’s happening,” said Michael Heflin, CEO of fiber sensor interrogator maker Sensuron LLC of Austin, Texas.

Fiber sensors can be used in structural health monitoring in wind turbines and elsewhere.
Fiber sensors can be used in structural health monitoring in wind turbines and elsewhere.


These improvements are why the Photonics Sensor Consortium forecasts the fiber sensor market to grow from $585 million in 2013 to almost $1.05 billion in 2019. Heflin said the annual growth could be much greater than that in specific areas.

In some cases, a fiber optic amplifier is used along with fiber cables that convey light into and out of electronics-hostile environments.

“Typically you would use fiber optics for a harsh environment, maybe with a lot of moisture in the environment. Or heat. We have some fiber optic cables that can go up to 900 °F. Or maybe space is limited,” said Tom Corbett, photoelectric product manager at industrial sensor-maker Pepperl + Fuchs North America of Twinsburg, Ohio.

Blacksburg, Va.-based Prime Photonics LLC uses fiber to measure the tip clearance and vibration of gas turbine blades. Both parameters are important to the performance of airplane engines, but their measurements must be taken inside a running engine, which can be difficult. At Prime Photonics, the clearance and vibration measurements are collected by sending light through a fiber optic-connected probe, and the resulting reflections are analyzed to extract information.

Fiber sensors are used in spin testing for gas turbine development, detecting and measuring vibration of rotor blades.
Fiber sensors are used in spin testing for gas turbine development, detecting and measuring vibration of rotor blades. Photo courtesy of Prime Photonics.


Today, fiber probes are a test and evaluation tool for aircraft engines. In the future, though, things may be different. R&D efforts aim to extend the technology, said CEO Steve Poland, “so it can go on-wing and be part of a control system, or part of a health monitoring system.” Other sensors exploit the effects of the environment on fiber, a concept used in optical fiber sensors from Avago Technologies Ltd. The Singapore- and San Jose, Calif.-based company designs, develops and builds a broad range of optoelectronics.

One of Avago’s fiber sensors detects industrial arc flashes, which are similar to the familiar spark caused by plugging in a household light or appliance. However, they can be much more dangerous because thousands of volts are being switched.

Avago’s solution is an optical transceiver and a fiber with a transparent cladding. The transceiver monitors the fiber, looking for the telltale flash of light that indicates a problem. The fiber was specifically developed for this purpose, said Lothar Stoll, marketing manager of the Industrial Fiber Product Division at Avago.

“We invested a lot of R&D to explore light capturing in optical fibers and to optimize our new fiber,” he said.

An advantage of the company’s system is that it can monitor many different electrical switchgears. According to Stoll, this simplifies the system and offers a cost advantage compared to a traditional point sensor.

Another recently introduced Avago product involves fiber-strain sensors. A fixed-frequency, pulsed sinusoidal signal is sent through two optical fibers. One is subjected to strain by being attached to an object, while the other acts as a reference.

The phase differential correlates to the relative strain between the two. Such strain data could be used to monitor the load on a blade in a wind turbine, with pitch being changed to optimize the load and power output. The fiber-strain gauge also can be used for structural health monitoring.

While Sensuron’s fiber sensing technology goes back a decade, Heflin said the company is less than a year old, having spun off from 4DSP LLC in early 2015. Sensuron’s fiber sensing technology can monitor strain, temperature, load and vibration, as well as position in 2D and 3D space. Magnetic fields and the presence of chemicals can also be sensed.

Such capabilities mean that medical devices equipped with fiber sensors can determine the position of an instrument inside the body, as well as the temperature of the surrounding tissue. The sensors can also provide force feedback, allowing a surgeon to feel the tissue at the far end of the instrument.

Sensuron’s technology had its start in aerospace, and one of its applications now being investigated involves rockets. A launch scheduled this month will hopefully allow for capture of a critical piece of data regarding the cryogenic fuel that powers the rocket.

“The objective is to determine the actual fuel that is being left at the end of the burn, so they can actually reduce the amount of fuel they put on the rockets, therefore increasing the payload,” Heflin said. He added that Sensuron’s technology allows thousands of sensors to be created on a single fiber, and many fibers can be bundled together in a cable. Thus it’s possible to produce a mountain of data, which necessitates an equally hefty processing capability.

Looking forward, once sensing fiber is installed, it’s unlikely to be removed and replaced. Instead Heflin predicts the optical interrogators will be changed out as the technology evolves.

Another company commercializing fiber sensing technology is the startup Intelligent Fiber Optic Systems Corp. of Santa Clara, Calif. Behzad Moslehi, CEO and chief technical officer, said the company is focused on turning innovative technologies into products. The optics can be challenging because sensing, transport and signal processing are all done simultaneously. What’s more, all of the interrogating signal may be put to use.

Side-polished fiber arrays are used for chemical sensing and telecom applications.
Side-polished fiber arrays are used for chemical sensing and telecom applications. Photo courtesy of Intelligent Fiber Optic Systems.


“In sensors, you will be dealing with almost every property of light: wavelength, frequency, phase, amplitude, polarization, intensity, all of it,” Moslehi said.

He added that the company offers four sensor platforms. The first is based on fiber Bragg gratings that are written into the fiber core, with many hundreds placed on a single fiber strand. Each acts as a sensor, directly measuring strain and temperature. Indirectly, electric and magnetic fields, pressure, and chemicals can also be sensed with sophisticated instruments capturing the resulting slight shifts in color. That requirement is one reason why the company has developed its own high-speed and high-resolution optical interrogators.

A second platform is based on exploiting the scattering of light in fiber for temperature measurements in energy applications. A third platform depends on interferometry; it’s being used in a gyroscope, among other applications. The fourth platform involves fiber that has had its protective cladding selectively removed and replaced with a chemically sensitive material. The material reacts to a chemical or biochemical, yielding a fiber-based chemical sensor.

The capabilities and applications of fiber sensors are expanding, and costs are falling, Moslehi said. But the field could benefit from standardization and also could profit from a cut in the cost of specialty fibers, which are often used and can be tens of times more expensive than telecom fiber.

“We’d like to see the cable, the specialty fiber, prices dropping drastically,” Moslehi said.

Cost is also a concern for Luigi Zeni. An optoelectronics professor at the Second University of Naples in Italy, Zeni and his team are researching the use of fiber sensors for geologic hazard monitoring. For instance, the researchers are using fiber sensors for distributed strain measurements along a problematic cliff near Naples and in unstable soil near Potenza, Italy. The ultimate goal is to spot landslides before they happen.

The latest frontier of optical fibers, from long-haul data links to global geostructural health monitoring.
The latest frontier of optical fibers, from long-haul data links to global geostructural health monitoring. Photo courtesy of Luigi Zeni, Second University of Naples/Optosensing.


“By integrating a single fiber optic cable into soil or a geotechnical work, a large amount of accurate, spatially resolved data can be obtained,” Zeni said.

The researchers are using Brillouin optical time-domain analysis, in which two counter-propagating light waves travel through a fiber. The lower frequency pumps up the higher if the frequency offset between the two is within a particular range, as determined by the temperature of and strain on the fiber.

The method allows strain readings every meter along a total sensor run of up to 50 km, Zeni said. Thus, thousands of strain gauges can exist on a cliff, building or pipeline. The research and technology are being commercialized by Optosensing srl, a spinoff with which Zeni is associated.

When asked about challenges, Zeni listed three. The first is discriminating between strain and temperature. One solution, already found in commercial sensors, might be two parallel cables, one under strain and the other not. The second issue is the ability of fiber sensors to withstand their environments. This problem might be solved with plastic optical fibers, which are more robust than those made of glass.

Glass fiber optic cables with stainless steel sheathing
Glass fiber optic cables with stainless steel sheathing — capable of operation up to 900 °F — detect hot steel parts as they exit a blast furnace. Photo courtesy of Pepperl + Fuchs.


Cost is the third issue. Because nonfiber sensors exist, any new solution must not only work but also must give users a reason to switch. Clearly the expense associated with fibers, interrogators and other components is important, and progress in this area must be made to motivate large-scale adoption.

As Zeni said, “Cost is in some cases too high compared to traditional sensors.&rdquo


GLOSSARY
optoelectronics
A sub-field of photonics that pertains to an electronic device that responds to optical power, emits or modifies optical radiation, or utilizes optical radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer. Electro-optic often is used erroneously as a synonym.
interferometry
The study and utilization of interference phenomena, based on the wave properties of light.
cladding
The low-refractive-index material that surrounds the core of an optical fiber to contain core light while protecting against surface contaminant scattering. In all-glass fibers, the cladding is glass. In plastic-clad silica fibers, the plastic cladding also may serve as the coating.
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