Assembling Fiber Optics

Optical fiber is the backbone of today's digital economy. Global financial transactions, high-speed Internet access, online shopping, video gaming and other things that most people take for granted are possible because of thin strands of glass that transmit massive amounts of data every second.

While the technology has revolutionized telecommunications, fiber optics is also becoming more important in industries such as aerospace, medical devices, and oil and gas. And, as automotive engineers tackle issues related to autonomy and lightweighting, demand for optical fiber is expected to grow during the next decade.

Despite that increasing popularity, the process of cutting, stripping and assembling fiber optic components remains challenging. Engineers must address issues such as alignment and positioning, clarity, fiber preparation and outgassing.

Optical fibers are flexible, transparent cables made up of high-quality glass, plastic and silica that operate on the principle of total internal reflection of light. While single- and multimode fiber optics have a glass core, plastic optic fiber cables have a polymer core.

Light is emitted through hair-thin strands that resemble a monofilament fishing line.

A wide variety of optoelectronic components and photonic devices are necessary to generate, transmit, modulate, guide, amplify, switch and detect light. These tiny devices are assembled into a package or module that couples light into and out of fiber optic cable.

Optical fibers can transmit data at much faster speeds than aluminum or copper wire. Other benefits include smaller size and weight. Compared with copper, for example, optical fiber yields an average space savings of 25 percent and a weight savings of 50 percent. Optical fibers are also immune to electrical noise and can transmit data over longer distances than copper cable or wire.

However, fiber requires delicate handling and precise alignment, and it cannot be bent into complex shapes like traditional wiring. Optical fiber is not like copper wire, which can be misaligned and still transmit electricity. Due to its small size, extreme accuracy and small displacements are required to correctly align fiber.

According to Grandview Research, the global market for fiber optics totaled $7 billion in 2018. It's projected to grow at a rate of 5 percent annually from 2019 to 2025.

The average fiber-connected household generated 86 gigabytes of data per month in 2017. But, that's expected to surpass 260 gigabytes per month by 2022.

"Demand for fiber optic components is increasing, due to growing bandwidth requirements and increases in network demand," says Adam Houston, product manager for the optical solutions business unit of Molex LLC, a leading supplier of connectors, adapters and cable accessories. "Bandwidth is doubling every five years.

"Ten years ago, 10 to 40 gigabits per second was the big conversation," explains Houston. "Now, we’re preparing for 200- and 400-gigabit-per-second bandwidths."

To address that issue, Molex is promoting several new types of ceramic ferrule-based connectors that enable increased port density. For instance, CS connectors support next-generation QSFP-DD and OSFP transceivers with a doubling of port density. They feature reduced ferrule pitch and a nearly 50 percent higher density than traditional LC connectors.

"Density and low-loss drive the connector market today," says Tom Schiltz, director of product management at Molex. "As components continue to get smaller, packaging becomes a bigger issue. There's a balance between density, usability and performance."

"Think smaller, denser, faster and easier," says Robert Whitman, vice president of global market development for carrier networks at Corning Inc., the world's leading supplier of optical fiber. "With today's bandwidth requirements, network operators require smaller, denser cable designs to pack more capacity into smaller spaces. They also value designs that are easier and faster to install, repair and maintain so they can minimize costs while maximizing speed to market."

Corning recently unveiled an extreme-density cable called RocketRibbon. It delivers up to 3,456 fibers in the same diameter as existing 1,728-fiber central and stranded tube cables.

"In addition to the improved fiber density, a unique ribbon design makes the fibers within RocketRibbon cable easy to manage, identify and trace, which significantly improves install times and lowers ongoing maintenance costs," claims Whitman.

Data centers and the telecommunications industry continue to be the top markets for optical fiber, because it enables high-speed data transfer in both short- and long-range communications. Growing demand for cloud-based applications, video-on-demand services and 5G networks will drive future fiber optic applications.

However, demand for optical fiber is growing in other industries, such as aerospace, automotive, medical, and oil and gas. Although it has a reputation for being expensive and finicky to handle, recent advancements have made fiber optics technology more robust and easier to process.

"One of the challenges in getting engineers in industries outside of the telecommunications sector to adopt optical fiber is overcoming the fear that it's a delicate, brittle, hard-to-use and hard-to-terminate technology," says Bill Weeks, corporate technology fellow at TE Connectivity. "One common misperception is that if you touch the fiber it will break. While that might have been the case back in the 1970s, it certainly isn't today."

Because of growing interest in minimally invasive surgery, many new medical devices depend on fiber optics. The technology is used for light conduction and illumination, flexible bundling and laser delivery systems, such as endoscopy equipment.

Fiber optics is also popular in the oil and gas industry for down-hole applications, such as sensing pressure and temperature extremes.

In addition, the transportation equipment industry is bullish on fiber optics technology.

"Aerospace engineers are trying to eliminate complexity in both commercial and military aircraft," says Weeks. "They want to replace miles of parallel copper wiring. In addition to weight savings, they’re looking for products that are easier to install and repair, while also offering electromagnetic interference immunity and higher speeds.

"In addition to communication and in-flight entertainment, one growing application for fiber optics in the aerospace industry is sensing," explains Weeks. "Some manufacturers are looking at using fiber optics for monitoring things such as landing gear; fuselage and wing fatigue; and overheat detection in composite aero structures that contain embedded heaters for melting snow and ice."

"The aerospace and defense sector is also seeing an increased need for bandwidth," adds Scott Flint, director of aerospace and defense markets at Corning. "Data fusion, high definition real-time video, and multispectral and hyperspectral imaging data streams, are drivers across all domains—land, sea, air and space.

"In addition, reduced SWAP-C (size, weight and power–cost) solutions are being called for across these platforms," Flint points out. "When you consider the weight and form-factor of multiple copper cables vs. a single fiber optic cable, the latter allows for greater endurance across these platforms, because of less fuel consumption. It also can allow for additional hardware or sensors to be carried because of more available space."

In the automotive industry, Weeks says engineers are running out of space for wiring harnesses, especially with new autonomous vehicles that require numerous cameras, lidar, radar and other sensing devices. "Rather than using dedicated pairs of copper wire, fiber optics makes more sense in cars and trucks," Weeks points out.

"Automotive engineers are looking into fiber optics to address lightweighting and bandwidth issues," says Tom Marrapode, director of advanced technology development at Molex. "They are also eager to reduce the congestion and complexity of traditional wiring harnesses."

"Due to advances in automotive networks, bandwidth needs within a vehicle continue to grow," adds Mark Bradley, director of industrial optical networks at Corning. "While links within vehicles may be short (less than 15 meters), bandwidth consumption is expected to grow beyond 5 gigabytes per second in the next design cycle.

"One key driver of these data speeds is the distribution of uncompressed video, which can affect the performance response times of new safety systems," explains Bradley. "While copper cables (twisted pair or coax) can deliver higher bandwidths, tradeoffs must be considered, such as cable size, cable weight and cable noise immunity. Optical solutions can provide benefits to alleviate these constraints, while still supporting the growing bandwidth needs, just as they have in data centers."

Assembling fiber optic components is challenging. The flexible nature of fiber makes it different than handling rigid parts like aluminum or copper wire.

Before fibers can be attached to a connector or ferrule, they must be prepared. This process typically includes stripping the protective cladding and outer jacket that surround the optical fiber; cleaning any residue that remains; cleaving the fiber; and polishing the fiber endface to achieve an optical quality surface.

Fiber stripping is usually accomplished using hand tools or semiautomated benchtop equipment. Protective coatings are stripped away with blades or lasers. Heating systems are also used to soften the coatings so they can be easily removed.

"The big difference between optical fiber and copper wire is terminating connectors," says Weeks. "It's relatively simple to terminate a copper wire. But, attaching one or more fibers to a connector is a more involved process."

"The quality of connectors today is much better than in the past," adds Pete Doyon, vice president of product management at Schleuniger Inc. "However, connectors also keep getting smaller, which makes automation difficult. The most common size of connectors that we see used with our equipment is 125 microns. By comparison, single-mode fiber has a core size of just 9 microns. However, a multimode fiber has a larger core size.

"Demand for our fiber optic processing equipment has been steady, and it's growing," says Doyon. "Our most popular product is the FiberStrip 7030, which is a benchtop machine that strips the buffer and coating down to the bare glass. It strips fiber in a semiautomatic fashion without touching the glass fiber."

The motorized machine is designed for stripping single coated and buffered fibers. Stripping speed, heating time and heating temperature are adjustable.

"Our equipment is primarily used to mass-produce pigtails and jumpers," explains Doyon. "A pigtail is a short strand of cable that has one end attached to a connector; the other end gets spliced onto another cable. Lengths vary, but pigtails are typically made in 3-, 6-, 9-, 12- and 15-foot variants. Jumpers have connectors attached on both ends.

"Assembling pigtails and jumpers is usually not a high-speed operation; it's typically more about quality," Doyon points out. "The process takes an average of 20 seconds or more."

Because even a tiny speck of dust can completely block light, cleaning is important. If a fiber strand isn't cleaned correctly, it can result in bad fusion splices and inadequate bonds. Fiber is typically cleaned with isopropyl alcohol or an ultrasonic cleaner.

"Surface preparation plays a vital role in fiber optic assembly," says Venkat Nandivada, manager of technical support at Master Bond Inc., a leading supplier of adhesives for fiber optic assembly applications. "Parts must be clean and dry before applying adhesive. A product can be really good in terms of its optical and thermal properties, but if you don't have the right surface prep, bonding strength will be limited."

With fiber optic assembly requirements ranging from 10 microns down to submicron levels, precise alignment is critical to achieving accurate and consistent results. The major issue in joining fibers is getting the face of each fiber aligned and square to each other. Amplification of light signals is very important.

The primary function of nanopositioning in photonics assembly is to align transmitting and receiving components to minimize light loss in optical couplings. Assemblers need to carefully line up the core of the fiber.

Any misalignment will result in a loss of signal. Misalignment of only one-tenth of a micron will result in a 30 percent loss of light coupling.

Factors such as curvature of the fiber tip and the gripping position on a fiber pigtail add an extra degree of variability to the alignment process. Optical fiber also has an outer layer that consists of a protective cladding and buffer that must be stripped away before processing.

"Most of our customers use manual assembly processes," says Molex's Marrapode. "The assembly process involves jacket and fiber buffer removal, fiber insertion into the ceramic ferrule with epoxy and then polishing the end face."

"Engineers typically rely on epoxy to attach optical fibers to ferrules, because they need a stable interface over temperature and lifetime," explains Marrapode. "Quick-curing UV adhesives are also used for some applications—mostly in data centers that don't have harsh environmental requirements or shorter lifetimes."

"Adhesives are typically used to attach individual optical fibers or bundles of fibers to components," says Nandivada. "Common applications include bonding optical fibers into connectors, potting fiber bundles and sealing fibers into ferrules.

"Adhesives are also used to assemble fiber optic components such as amplifiers, filters, isolators, switches and transceivers," explains Nandivada. "Epoxy is frequently used for most assembly applications, but silicones, urethanes and UV curing systems are other options.

"We offer a wide variety of epoxies, ranging from chemistries that are more rigid with a low coefficient of thermal expansion to epoxies that are more flexible and feature excellent resistance to thermal cycling," notes Nandivada. "EP30-2 is our most popular two-part epoxy product. It's used for a wide variety of fiber optic assembly applications.

"UV curing is ideal for high-volume production applications that require throughput," Nandivada points out. "A one-part material that doesn't require mixing and measuring typically lends itself better to automation. These adhesives also cure extremely quickly when properly exposed to UV light.

"In some applications, however, silicones may be better, because they offer extremely low stress and good temperature resistance," says Nandivada.

Traditionally, automated handling and assembly of fiber optic components has been challenging. The flexible nature of optical fiber makes it more difficult than handling rigid parts such as aluminum or copper wire.

"The assembly of bundled optical fibers with plug connections, which are used in information and communication technology, is still difficult to automate today," says Marvin Berger, an engineer at the Fraunhofer Institute for Production Technology (IPT).

"In particular, next-generation polarization maintaining (PM) fibers require high precision manipulation of the fiber in at least 4 degrees of freedom," explains Berger. "Fibers with fixed polarization must be aligned with great precision in the connector, and their handling and bonding also require maximum accuracy.

"With single-mode fiber arrays, the precise arrangement of the individual fibers is crucial," notes Berger. "Up to 32 light-conducting fibers are mounted in several layers in one connector. Today, they are usually still glued manually and individually in the connector, since the correct alignment of each individual fiber determines whether the component can perform the desired data transmission."

However, engineers at Fraunhofer IPT and Aixemtec GmbH recently developed an automated method to handle the complex and costly task of assembling PM fiber arrays.

"The system automates all of the essential process steps for manufacturing the connectors, from storage and feeding of the fibers to rotary and translational alignment, gluing and hardening of individual fibers through to final assembly of the entire system into a linear fiber array," says Berger.

"It can already assemble connectors with up to 16 fiber connections autonomously," claims Berger. "With further development, we hope to increase the number of fibers processed and improve the handling of the nonrigid fibers, thus further accelerating the entire production process.

"The patented manipulation system is the world's first system that delivers the requirements for an automated PM-fiber array assembly," Berger points out. "And, the flexible assembly cell platform allows us to easily integrate additional hardware.

"Vision-based routines in combination with the developed hardware allows the alignment of single fibers with a repetition accuracy below 0.01 degree," says Berger. "The placement itself is done with machine accuracy (around 1µm). However, special care has been taken to optimize the fiber tacking on a v-groove element for fiber arrays.

"The single fibers are tacked with a special UV-curable adhesive on a v-groove," explains Berger. "With an optimized assembly process, the fiber tips are located close to each other afterwards.

"Currently, the machine has to be fed by an operator for each fiber array," adds Berger. "But, we are developing different steps to fully automate the machine setup to leave humans out of the process chain."