More Fiber Will Make Healthier Aerospace and Defense

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

In military and aerospace applications, improving size, weight, power and cost (SWaP-C) is critical. That’s why electronics are being replaced by fiber optics. Fiber is lighter than copper and is immune to electromagnetic interference, a significant plus. It can also provide high-bandwidth connections over long distances.

Additionally, fiber can form the basis of sensors to measure strain, temperature, and biological or chemical compounds. It’s also possible to transmit power over fiber, simplifying the setup of mobile and temporary installations.

Fiber is increasingly used in military and aerospace applications because it offers bandwidth, is lightweight, takes up little space, is power stingy and is immune to electromagnetic interference.

Both defense and aerospace now have a growing appetite for fiber, said Bruno Daffix, a spokesperson for Airbus Defence and Space GmbH — the Munich-based division of the Airbus Group is Europe’s leading defense and space enterprise. One of the company’s latest ventures is its laser communication- and geosynchronous satellite-based broadband Space-DataHighway.

In the case of the military, some of the reasons for this growing need are the same as those behind civilian adoption of fiber technology, Daffix said. “This trend is mainly driven by the need of increasingly larger broadband, besides the specific advantage for defense of avoiding electromagnetic interference.” He pointed to pictures and video taken by drones, aircraft or satellites that carry ever more sophisticated cameras, radar and other sensors. These produce higher and higher resolution imagery and video, so there must be more bandwidth to transmit and analyze this data.

A May 2016 forecast from San Jose, Calif.-based Cisco Systems put the compound annual growth rate in worldwide data traffic at 22 percent from 2015 to 2020. Video will grow even faster, accounting for 82 percent of data by 2020 versus 70 percent in 2015.

Integrating many optical components onto one chip could yield fiber optic solutions that satisfy stringent military and aerospace specifications. Courtesy of Technobis.

Meeting the needs

Fiber can help meet communication needs. For one thing, links can span kilometers, making it possible to tie distant locations together. Another is that higher bandwidth is possible, according to Shawn Esser, director of product marketing at Finisar Corp., based in Sunnyvale, Calif. The company makes optical communications products for customers worldwide.

Currently, copper is limited to 25-Gbps bandwidth for any reasonable communication application, Esser said. In contrast, Finisar and others are developing fiber optic transceivers with 400-Gbps data rates.

As for aerospace- and military-specific needs, these applications can see fiber optics-enabled SWaP-C reductions. For instance, Esser said, 24 optical fibers can fit within a single copper cable, and the fiber would weigh as much as 70 percent less. Power savings can be significant, depending on the application.

“Copper has high attenuation, which requires high transmitter power levels. Fiber is basically loss-less so only a small amount of power is used,” Esser said.

As for fiber’s lack of electromagnetic interference, that pays off in two ways. First, cables can be routed as needed, without concern for the effect on each other or nearby equipment. Second, signals cannot leak out and be picked up by an adversary, which is desirable in military settings.

There is another potential security benefit from using fiber, Esser said. Long-distance telecom links use coherent technology because this allows the highest transmission rates over the greatest spans through very complex encoding of the data onto the optical signal. This makes information, even if intercepted by tapping the fiber, difficult to decode. He noted that coherent technology is being evaluated as a way to prevent unwanted eavesdropping.

Fiber for the future

Defense and aerospace applications can put advances such as coherent technology developed for other areas to use, depending upon the situation. For large data centers, for instance, the military can deploy standard commercial fiber optic products. Another example is dense wavelength division multiplexing, a staple in telecom for years. This technique sends many optical signals each on its own color, or wavelength, down a single fiber. For telecom, this means greater bandwidth. But it also is useful in data analysis and test systems of interest to military and aerospace users.

The same almost direct porting from other areas is not typically possible for airborne, ground-based and seagoing vehicle systems, according to Esser. Often, fiber optic products intended for these must be modifed to withstand high shock, vibration and wide operating temperatures, as well as handling exposure to salt spray and other hazards.

“Sometimes these products only need minor modifications, but other times a large-scale development program is needed,” Esser said.

In addition to communication, it’s possible to create fiber-based sensors that directly measure strain and temperature through changes in optical characteristics of such structures as Bragg gratings. Besides these intrinsic sensors, extrinsic transducers can sense other environmental factors. A reporter molecule, for example, can react to the presence of a chemical or biological agent in the environment and cause a change in a fiber’s optical properties.

Optical fiber sensors can monitor the health of structures by measuring strain in vehicles or in the off-shore foundation project shown here. Courtesy of Com&Sens.

For both intrinsic and extrinsic sensors, light travels down a fiber. Interrogating devices look at the wavelength, amplitude, phase or other characteristics of the light. Shifts in these attributes correlate to changes in strain, temperature or other environmental factors of interest.

Such sensors enable structural health and state monitoring. This capability can improve aerodynamics and performance through, for example, the detection of nonvisible damage because of impacts or overloads. A study by NASA’s Armstrong Flight Research Center in Edwards, Calif., found that fiber optic strain sensors could permit orders of magnitude more measurement points for orders of magnitude less weight, as compared to traditional electronic approaches.

Technobis Fibre Technologies, a part of the Technobis Group, of Alkmaar, the Netherlands, has been developing and collaborating in aerospace projects for years, said Rolf Evenblij, program manager for the company’s aerospace, space and automotive testing. Technobis Fibre currently does so using photonic integrated circuits, which combine many optical elements into one chip and allow modulation and detection of light.

“Due to the many advantageous capabilities, the trend is emerging to prefer fiber optics over conventional electrical systems,” Evenblij said. He added that the emergence of miniature and reliable photonic integrated circuit technology enables aerospace complaint instrumentation, the lack of which has heretofore been a hindrance.

Using photons and not electrons also improves safety, he said. If there is a vaporized explosive gas, eliminating electrons reduces the chance for a spark and a combustion catastrophe.

At Belgium’s Ghent University, Jeroen Missinne, an engineering professor, has been investigating fiber-based Bragg grating sensors. The latest efforts focus on thin foils embedded in composites with fibers conveying the light in and out. The approach offers the ability to precisely control the orientation of the sensors, making it possible to determine strain in different directions.

Fiber precisely embedded in a foil enables sensing of strain in different directions. Courtesy of the University of Ghent.

The embedded foil-in-composites technique can put different materials and their varying characteristics to use, thereby solving problems.

“We are developing sensors that, for example, are able to discriminate between temperature and strain. That’s at the moment a huge issue for existing sensors,” Missinne said.

This research is being followed up by various companies, including Ghent University spin-off Com&Sens BVBA. One problem with light-based sensors is that the optical fiber tends to break off at the point where it enters or exits the composite. Com&Sens has a solution to this, according to Geert Luyckx, managing associate.

“You can just cut the composite with a diamond saw,” he said of their patent-pending method. “We can reconnect it and stabilize it for use to do the measurement inside the structure.”

That makes it possible to put in many and embedded sensors, the better to monitor a structure and its health. This could be useful in aerospace applications, where there has been great reluctance to put anything inside a composite because of concerns about structural integrity.

A final extension of fiber optics beyond communications involves power over fiber. This technology would make for a simpler setup of a cell tower, a temporary military installation or other locations.

Researchers transmitted power over fiber, potentially providing a simpler way to set up mobile installations. Shown here are the experimental setup and a sample image of the fiber used. Courtesy of Motoharu Matsuura/University of Electro-Communications.

Recently, a group at the University of Electro-Communications in Tokyo demonstrated a new technique that offers more than 100 times the power reported previously by other systems, said research leader Motoharu Matsuura, associate professor. The key was the creation of a fiber bundle with a high fill factor inside a cable. The bundle combines and divides the optical data and power feed at the input and output of the transmission line. Filling more of the available space with light-carrying fibers boosted transmission power. This required a thinner core than the typical 105-µm diameter of multimode fiber, but this core was larger than 10-µm, typical of conventional single-mode fiber, Matsuura said.

Optical fiber is nonconductive, corrosion-resistant and suitable for electromagnetically noisy environments, he added. That makes it attractive in certain settings.

“The main application is for future mobile communications based on optical fiber technologies,” Matsuura said. “However, we think that our data and power transmission systems have many applications because optical fiber is a unique power line, unlike a conventional electrical power line.”