Assembly Handbook: Fiber Optics
The big challenge is converting voice, data or video electrons generated by computers and other devices into bits of light called photons, then transmitting those photons over long distances and converting them back into electrons in less time than it takes to blink an eye. An even bigger challenge is mass-producing the components that make it possible to do all that with speed, precision and accuracy.
Fiber optics technology uses microscopic strands of glass to transmit data. Light is emitted through a central core 0.005 inch in diameter that resembles a monofilament fishing line.
However, fiber optic communication requires more than just hair-thin strands of glass. A wide variety of optoelectronic components and photonic devices are necessary to generate, transmit, modulate, guide, amplify, switch and detect light. These tiny, complex devices are assembled into a package or module that couples the light into and out of fiber optic cable.
An optical signal originates from a modulated light source, such as a 980-nanometer diode pump laser, that feeds it into a strand of fiber. Multiplexers combine several wavelengths onto a single glass fiber so data can be transported more efficiently. Optical splitters divide the light traveling along a single transmission line into several new lines so that signal can be distributed to various points.
Because of attenuation, or the loss of energy light experiences as it travels through optical fiber, the signal needs to be amplified every 100 to 200 kilometers with amplifiers, repeaters and other devices. At the receiving end of the fiber, demultiplexers and receivers decode the optical signal and convert it back to electrons for use by computers and other equipment. In between are hundreds of connectors, filters, switches and many other fiber optic components.
Two classes of fiber optic components exist: active and passive. Active components consist of the semiconductor laser technology that is necessary to provide the light in a fiber optic network. These devices are generally easier to assemble. They require the integration of electronics and wiring into a package using traditional electronic assembly processes, such as soldering and die- and wire-bonding. Receivers, transmitters, modulators, amplifiers and switches are examples of active components.
Passive components operate on the light passing through the fiber and do not require power or electronics. Their function is to filter, divide or combine the light signals traveling through the optical fiber. These components are much more labor-intensive and costly to manufacture. Passive devices include couplers, filters, isolators and wavelength division multiplexers.
Assembly ChallengesMany different issues challenge photonic component assemblers, such as the often unpredictable nature of optical fiber, which makes it very difficult to work with. Because of the fiber manufacturing process, the core of the fiber is not in the same place each time. Optical fiber is also vulnerable to bending, kinking and crushing. Excessive handling of the delicate fiber opens the door to possible damage during the assembly process.
Assembling optical components requires extremely high precision, which is measured in nanometers. Tolerances often are in the submicrometer range. This tolerance is critical to ensure the reliability of the device. Any misalignment means reflected energy, which equates to light transmission loss and products that have to be scrapped. Most assembly is done in clean room environments because fiber optic components require precise, submicron adjustments throughout the manufacturing process.
Another challenge is the size of optical components, which tend to be very small. For example, a completely assembled thin-film filter wavelength division multiplexer is 5 millimeters wide and 30 millimeters long. Assembling this product requires six different epoxy and UV curing applications. Due to the difficult nature of assembly, manufacturing yields tend to be very low—20 percent to 30 percent is not uncommon.
A lack of standardized assembly and test procedures have also hindered assemblers and made it difficult to migrate toward high-volume production. No standards exist for many products, and there is a wide variation in package designs and configurations.
Several new initiatives are attempting to develop industrywide standards similar to those that exist in the semiconductor industry. One project is evaluating alternative attachment techniques and developing cost models. Another initiative is developing cleanliness standards and guidelines for fiber handling. These specifications are expected to identify gaps in the technology infrastructure, reduce assembly costs by up to 10 Arial current procedures and significantly reduce time to volume.
Packaging, which accounts for 60 percent to 90 percent of an optical component’s cost, will continue to be a major challenge and opportunity. However, manufacturers are experimenting with package designs that facilitate high-volume automated assembly.
For instance, components that use passive assembly allow engineers to pack a lot of functions into a very small space. In contrast, components that are manufactured using active assembly require more space around each optical element in order to grip and move them during the alignment process.
Preparing for ProductionFiber preparation is the first step in the photonic component assembly process. Before output fibers or pigtails can be attached to a device, 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 can be accomplished using hand tools and semiautomated or fully automated equipment. Protective coatings are typically stripped with blades that have a precision bore sized slightly larger than the fiber diameter. Heating systems can be used to soften the coatings so they can be easily removed.
Another key step is cleaning. If the fiber 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.
Fully automated fiber prep systems promise to improve this time-consuming process. For instance, one machine recently unveiled can strip, clean, cleave, splice and polish up to 40 fibers in a single pass in just 1 minute with no operator intervention. It replaces up to 15 operators as well as the associated benchtop tools.
The most challenging task in photonics assembly is the alignment and positioning of optical fibers and components. Submicron accuracy, fine resolution and high stability are required when coupling laser light with the core of an optical fiber. This delicate process is referred to as active alignment.
Traditional active alignment involves coupling a light source, such as a laser, with the input end of an optical fiber to determine the optimal position of the component and any light exiting at the output end of the fiber. The fibers are manipulated until the maximum possible light reaches the detector, at which point they are attached with adhesive, laser welding or solder. To achieve efficient coupling, alignment to better than 0.1 micron is necessary.
Manufacturers of lasers, amplifiers, connectors, filters, receivers, switches and other fiber optic components and modules are very concerned about the amount of signal loss that occurs across the component-to-fiber junction. Due to the extremely small size of optical fiber—8 or 9 microns for single-mode fiber cores, which is less than the width of a human hair—very high-resolution systems capable of producing nanometer-scale displacements are required to reduce losses across the optical junction.
Holding and handling optical components is a critical task. Because fiber is not very rigid, simply hanging on to it poses a challenge. If alignment of the optical fiber and component is off just slightly, the product will be useless.
NanopositioningA wide variety of manual, semiautomatic and automatic nanopositioning devices are available. Nanopositioning refers to precise motion and positioning with the accuracy of a fraction of a micrometer.
Fiber is measured in microns and wavelengths are measured in nanometers. A micrometer—also called a micron—is one millionth of a meter. A micron is equal to 10-6 meters. A nanometer is one billionth of a meter. A nanometer is 1,000 Arial smaller, or 10-9. One micron equals 0.00003937 inch, while 1 nanometer equals 0.00000003937 inch.
With 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.
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. 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.
Because optical fiber is so finicky, many photonics assembly processes require up to six degrees of freedom in as small a footprint as possible. Work envelopes vary depending on the type of device being assembled and the overall degree of automation required.
"Coarse" motion positioning is used to guide the optical fiber and component into the correct vicinity for final placement and assembly. There are coarse movements on the scale of tens of millimeters that may be required to enable the loading and unloading of parts. However, the "fine" stage of the component alignment process may only require motion on the scale of tens of microns.
Unlike many industrial automation systems, typical photonic assembly workcells are benchtop systems. Actuators and positioners have strokes less than 1 millimeter. Motion envelopes in some applications, such as assembly for fiber-to-device, are limited to hundreds of microns. Other applications, such as fiber Bragg grating, are in hundreds of millimeters to meters. In polishing and inspection applications, travels needed are in the 300-millimeter range.
Motors, stages, carriages, slides, tables and controls play a key role in nanopositioning systems. Most motion control devices are based on linear motor technology, piezo technology and rotary motor or ballscrew technology.
Joining TechniquesThe process of attaching an optical fiber to a component is commonly referred to as pigtailing. A small length of optical fiber is grabbed and its tip is encapsulated in a ferrule, which provides a metallic housing necessary for rigidity and joining purposes. Once optimum coupling efficiency is achieved, the component is attached.
Optical components are typically joined with adhesives, laser welding or solder. The most common way to join optical fibers and components is to use adhesives. Most are acrylic blends that match the optical quality of the fiber.
Adhesives are popular with assemblers because they are versatile, fill gaps on uneven surfaces, join dissimilar substrates and offer flexible adjustment time to position parts accurately. The adhesives are matched to the index of refraction of the fiber so energy is not lost in the connection interface.
As fiber optic manufacturers seek new ways to improve productivity and increase yields, they are paying more attention to adhesives and the way in which they are applied.
Thermally cured epoxies are widely used throughout the fiber optics industry. They offer reliable performance and are found in applications that range from bonding fibers to ferrules to backfilling connectors and collimators. Anaerobic adhesives are also used to assemble fiber optic components.
The bonding process can introduce large amounts of heat into the package, so minimizing any shift or movement is very important. Heat can cause outgassing, producing fumes that can cloud parts that need to be optically transparent. Ultraviolet curing methods have been implemented to address this issue.
Adhesives that cure quickly when exposed to an ultraviolet light source are being incorporated into a growing number of fiber optic component assembly processes. This cure on demand feature permits adjustment of parts after the adhesive is dispensed to obtain precise alignment.
Laser welding provides the fastest and most cost-efficient joining process. Compared with adhesive and solder attachment methods, it is more compatible with automation and high-volume production. Laser pulses focus through optical assemblies to specific points on the part surface with a high degree of accuracy and repeatability.
Typically, two vision-guided, pulsed Nd:YAG laser beams—one on each side of the component—weld components into place. This epoxy-free process is commonly used to assemble high-volume quantities of laser diode modules, such as the pump laser diode used in fiber optic amplifiers.
However, laser welding puts considerable limitations on package design. It requires that all components be housed in metal ferrules to protect optical fibers from the heat. Appropriate attention to the design of the joint and the execution of the weld must be taken to achieve a successful assembly.
Soldering is less expensive than welding, while offering some of the benefits of laser welding. Soldering is used to a lesser extent, mainly because it has not been proven in optical applications. Concerns exist over control of shrinkage and its inability to be controlled effectively.
Solder is used to join optical fibers parallel to substrates with surface-mounted electronics, such as lasers or sensors. In this process, the thin optical fiber is pulled out of alignment first by capillary forces during wetting of the solder on the fiber, and then by solder shrinkage during solidification and cooling.
Automation TrendsMore than 75 percent of fiber optic components are currently assembled manually in lab-like settings where rows of operators are equipped with hand-operated micrometers and other benchtop equipment. But, faced with growing product demand, rising labor costs, operator variability and inefficient yield rates, many manufacturers are searching for automated solutions to traditional manual assembly and test operations. The fiber optics industry is moving toward semiautomated and fully automated systems that can align and attach components in large volumes.
Automation allows component manufacturers to significantly boost throughput, reduce time to market and quickly increase capacity, while reducing their dependence on highly skilled assemblers. The trend toward automation also has the potential to lower the cost and improve the overall quality of fiber optic components.
In fact, the improvements can be dramatic. One major manufacturer that invested in automated assembly equipment increased productivity by 134 percent in splice stations used to produce optical amplifiers and by 1,600 percent in testing of couplers and amplifiers. An automated assembly cell can replace 10 to 20 operators and can save more than $4 million a year in scrap.
Flexible automation platforms for fiber optic component assembly applications include six-axis material handling robots, machine vision systems and precision stages for final fiber alignment. High-speed positioning tools equipped with both motorized and piezo stages enable assemblers to test, align and attach fiber optic components with greater speed than has been possible in the past.
Manufacturers are beginning to use semiautomated assembly cells and modular platforms that are batch loaded by operators. The next step will be to link those cells together into one continuous, in-line process that resembles the high-speed, flexible equipment used in the semiconductor industry.