As telecom demands grow, optical fibers will need to level up

Whether you’re reading this story on a cell phone, tablet, or laptop, it comes to you courtesy of fiber optics. Even if you’re reading it on paper, fiber optics were involved at some point.

The words and images you see on your electronic screens made a digital journey through the internet to reach you, transmitted in the form of light pulses flashing through long, skinny optical fibers. If you’re reading the print version of C&EN, the story traveled that way, too, from our offices to the printing company that creates our paper magazines.

“Nearly 500 million km of optical fibers are made per year, and hardly anyone realizes that today’s modern conveniences wouldn’t exist without them,” says John Ballato, a materials scientist at Clemson University who specializes in the glass fibers.

Optical fibers form the vast pipeline through which nearly all voice, video, and data communications fly almost instantaneously around the globe. The overwhelming majority of these hair-thin, flexible fibers are composed of just one material—silica glass. The extreme purity of silica fibers and the precision with which they’re made enable this otherwise common material to carry the world’s communications.

But as good as silica fibers are at transmitting light over long distances, which is the key to fast communications, they have limitations: they don’t perform well when pumped full of highly intense light, for instance. This shortcoming becomes a problem as internet and cellular service providers aim to send more and more high-definition videos and other data by pumping higher- and higher-intensity laser light through individual fibers, a trend that accompanies the jump to 5G (fifth-generation wireless technology) and future, higher-performing cellular networks.

The demand of telecommunications and other industries for optical fibers that perform ideally at higher light intensities is driving fiber specialists and materials scientists to expand beyond conventional silica fibers. But to expand, researchers first need to know how silica fibers’ composition, structure, and other factors affect their optical properties. Only then can scientists modify the fibers to improve their performance.

In principle, “these scientists should be able to draw freely from nature’s spice rack,” says Ballato, referring to the periodic table. The researchers can, theoretically, dope silica fibers with additional elements to boost their properties. In practice, only a limited number of components combine to produce materials with stellar optical properties, and only a small subset of those combinations can be formed into precision glass fibers. Part of the challenge in making new types of fibers comes from the sheer size of the fiber optics industry and the dominance of silica. Since the late 1970s, when the first optical fibers were put to commercial use, the telecommunications industry has installed more than 4 billion km of silica fibers across the world. That’s enough fiber to span the distance between Earth and Neptune.

By 2019, the global fiber optics market was worth roughly $4.3 billion. In large part because of the growing demand for internet access and telecom services, particularly in China and India, the fiber optics market is projected to reach nearly $7 billion by 2024, according to the industry analysis firm MarketsandMarkets.

It will be no small feat to tweak today’s silica fibers or perhaps move beyond them to some new material entirely.

Optical fiber history

The idea of transmitting light signals via glass fibers dates back to the mid-1960s, around the time lasers were invented. As scientists began experimenting with lasers, they recognized that the light beams could be pulsed to generate something like an optical Morse code. That kind of light signal, researchers realized, would be able to travel much faster and farther than an electrical signal in copper wire, which is how all telephone signals traveled at that time.

Optical signals offered other potential advantages over electrical signals in copper wire: depending on the medium used, multiple light signals could be transmitted simultaneously, each at its own frequency. And light signals would be free from electrical interference—for example, from power lines. But what material could transmit light over many kilometers without the signal fading and distorting, as needed for telecommunications?

Charles K. Kao, a physicist at Standard Telecommunication Laboratories and later at the Chinese University of Hong Kong, evaluated various materials and proposed that some kinds of glass fibers, especially ones made of fused silica, should be able to do the trick. The concept grew from an old medical practice of using short glass rods to focus light on a particular site inside a person’s body during surgery. In the mid-’60s, though, silica fibers weren’t pure enough to transmit light more than several centimeters without dimming. The problem, explains Stephanie L. Morris, a senior optical fiber engineer with Corning, a global supplier of optical fiber, is that the standard way of making glass—mixing powdered materials and heating them until they melt—leaves impurities and structural flaws that scatter light. Driven by the tantalizing potential of optical fibers, several companies and research outfits soon began tackling the problem of making highly pure silica fibers.

Spoiler alert: the problem was solved within a few years through vapor deposition chemistry and fiber engineering. And in 2009, Kao was awarded the Nobel Prize in Physics for “groundbreaking achievements concerning the transmission of light in fibers for optical communication.”

Confining the light

The secret to ensuring that light propagates down the multikilometer length of an optical fiber instead of leaking out lies in its solid cylindrical structure. It also lies in the chemical composition of both the fiber’s inner core and its outer coating, or cladding. Depending on what application the fiber is meant for, its core diameter may measure roughly 10–50 µm. Add the cladding, and the fiber can have diameters of up to a few hundred micrometers.

By doping optical fibers with various compounds, including oxides of germanium, aluminum, phosphorus, and boron, and in some cases by adding fluoride and other ions, manufacturers fine-tune the refractive index of the core and cladding. The refractive index is a measure of how fast light moves through a material. For anyone who has ever noticed that a spoon sitting in a glass of water appears bent at the air-water interface, you have the difference in refractive indices of air and water to thank for the phenomenon.

Fiber makers engineer their products to have a core with a high refractive index surrounded by a cladding with a lower refractive index. That arrangement causes a laser light beam pumped into the core to stay there, thanks to a condition known as total internal reflection. When the beam veers off course, it is reflected from the cladding back into the core. The laser light ping-pongs that way all the way down the fiber, transmitting its encoded messages.

After Kao’s work with optical fibers, telecom companies began experimenting with this powerful medium for sending communication signals. By the late 1970s, firms began routing phone traffic via small fiber-optic networks. Large-scale commercialization took off in the ’80s. Two developments since then helped propel the technology’s explosive growth. One advance helps light signals zip along over very long distances; the other, over short ones.

To keep light traveling as far as possible through silica, telecom companies transmit signals in the near-infrared range of the electromagnetic spectrum—for example, around 1,550 nm. That’s because silica is highly transparent—it doesn’t absorb light—in that range. Even so, as laser light at that wavelength races through today’s silica fibers to support a cross-country videoconference, tiny signal losses due to leakage and other processes eventually add up, sapping the signal strength. To counter the loss over long distances, telecom providers use specially doped fibers to boost the signal. For those applications, erbium-doped silica is the most common.

“Erbium is fantastic,” Clemson University’s Ballato says, “because it emits light at around 1,550 nm, exactly the range where silica is most transparent.” The doped fiber, when excited at select wavelengths, functions as a laser and boosts the weakened telecom signal. These signal-boosting fibers, which may be spliced into very long cables every 100 km or so, keep light levels high, even in cables lying on the ocean floor. Neodymium, thulium, ytterbium, and other rare-earth dopants function similarly at other wavelengths.

Light signals travel easily over long, straight stretches of fiber-optic cable. But when the cable reaches the inside of an office or apartment building, it’s often routed through tight spots and narrow corners, which can bend and kink the fiber, disrupt total internal reflection, and cause severe signal loss. To combat the problem of light traveling over these short distances, manufacturers redesigned traditional silica fibers by including an additional thin, flexible reflective layer surrounding the core. The team at Corning that pioneered the widely used advance was honored in 2017 by the American Chemical Society with its Heroes of Chemistry Award. (ACS publishes C&EN.)

Moving beyond silica

As mighty as tiny silica fibers are, they have shortcomings. The intensity of light pushed through optical fibers keeps climbing. Telecom companies boost levels to keep pace with growing demand for data-rich services. Makers of industrial and military gear do so to generate ever-more-powerful light beams for manufacturing and defense applications. “The fiber optics industry wants to drive incredibly intense light through a fiber the thickness of a hair,” Ballato says. “The problem is, the glass can only take so much light.”

At moderate light levels, a number of ever-present phenomena, including Raman and Brillouin light scattering, go largely unnoticed in optical fibers because they don’t cause trouble. At higher light intensity, however, these so-called nonlinear effects can sap the power of transmitted light, shift its wavelength, scramble the signal, and cause other fiber-optic headaches. These processes are quickly causing fiber performance to plateau.

Optics researchers have tended to skirt these problematic effects by designing fibers with complex internal structures and by using other engineering solutions. But that approach leads to costly products and, according to Ballato, does not address the root cause of the problem: intrinsic limitations of conventional silica glass.

So Ballato, Peter D. Dragic—an optics specialist at the University of Illinois at Urbana-Champaign—and coworkers at Lawrence Livermore National Laboratory undertook a comprehensive experimental and theoretical study to evaluate a large number of potential fiber materials, including the oxides of strontium, aluminum, barium, and phosphorus. The team assessed each material’s propensity for triggering five deleterious nonlinear effects and produced a materials road map showing the properties’ complex interplay caused by blending those materials in various proportions.

No single composition is free from all the harmful optical effects, Dragic says, and some compositions are unsuited for making glass fibers. Nonetheless, the road map can be used to assess trade-offs and design new types of fibers for demanding, high-power applications (Int. J. Appl. Glass Sci. 2017, DOI: 10.1111/ijag.12336).

Lessons learned

In the meantime, scientists working to make better optical fibers for telecommunications might learn lessons from those who need advanced fibers for other applications. For chemical sensing, spectroscopy, and other applications requiring light in the mid-infrared, or MIR (longer than about 2 µm, or 2,000 nm), silica doesn’t work; it absorbs the light. So researchers have been developing new types of fibers for these uses.

“Silica is wonderful, but it’s quite limited in where it works really well,” says University of Southampton’s Anna C. Peacock, a specialist in fibers and photonics.

Fibers made of fluoride glasses transmit light up to roughly 5.5 µm. Fluoride glass is a generic term, encompassing a large family of compounds with components from across the periodic table, explains Mohammed Saad, a senior scientist at Thorlabs, a multimillion-dollar optics and laser company based in New Jersey. “After decades of research, fluoride fibers were recently commercialized,” he adds, albeit on a small scale.

One of the new fibers is made from a fluorozirconate glass, containing a mix of zirconium fluoride (the major component) and the fluorides of barium, lanthanum, aluminum, and sodium. Indium fluoride fibers, which transmit further into the MIR, also hit the market recently. According to Saad, fluoride fibers, which are expensive compared with silica fibers and can be brittle, are used in medicine to carry laser beams to a surgery site, where they can cut through tissue. They are also used for IR spectroscopy to analyze gases emitted from hard-to-reach spots, including factory chimneys and engine exhaust ports.

Manufacturers have also recently begun supplying small quantities of fibers made of chalcogenides. These materials combine sulfur, selenium, and tellurium with arsenic and germanium to offer a broad IR spectral range, transmitting light from roughly 1 to 10 µm.

Polymer-based fibers—for example, ones with a poly(methyl methacrylate) core and fluoropolymer cladding—can tolerate more bending and flexing than standard silica fibers and are inexpensive. They are sometimes used in low-speed home networks. And because these fibers are easily synthesized in a lab, researchers use them regularly in basic and applied studies.

Rei Furukawa, a fiber engineer at the University of Electro-Communications, aims to use polymer fibers embedded in bridges and tunnels as low-cost sensors to monitor internal fatigue and gauge the integrity of those structures. “We are developing fiber-optic strain sensors that allow users to literally see stress with their own eyes instead of relying on equipment such as spectrum analyzers,” she says.

In a recent demonstration, her group made poly(dimethylsiloxane)-based fibers with rhodamine and coumarin dyes in the cladding and core, respectively. When the fiber is deformed, which would indicate cement fatigue, emission from coumarin excites the rhodamine, and the stress causes an easy-to-spot color change at the fiber tip (Fibers 2019, DOI: 10.3390/fib7050037).

The enormous size of the telecommunications industry means that switching to new fiber materials will be challenging. Fluorides, chalcogenides, and polymers aren’t going to replace standard silica optical fibers, because in this business, silica is king, and silica fibers are relatively inexpensive to manufacture and use. But the recent appearance on the fiber optics market of a few new materials, plus a flurry of activity with experimental fibers, may spur fiber manufacturers to redouble their efforts to look beyond the same old material.

The level of global interconnectivity that defines today’s high-tech world rests squarely on fiber optics. The world’s reliance on these light conduits will continue to grow. As society and industry demand even more from these little pieces of glass, materials specialists will be called on to keep improving them.