Precise and repeatable flow control in optical fiber manufacturing

Precision & repeatability in optical fiber manufacturing

optical fiber manufacturing producing a fiber bundle

Making the preform

The optical fiber manufacturing process begins with creating a preform. Layers of very pure glass are added to a cylinder and various doping gases in highly controlled amounts are used to deposit additional glass layers, with each new layer added to the base influencing the properties of the final optical fiber. The process is controlled via the burner flame: a rotameter or mass flow controller regulates fuel gases to maintain the optimal process temperature and conditions.

This preform construction process typically takes several hours, depending on the final size of the preform.

diagram showing a fiber draw tower with a preform becoming an optical fiberThe final preform consists of a core and cladding, each with its own refractive index. The differing indexes are what causes light to remain trapped within the core, due to a phenomenon know as total internal reflection. However, even slight imperfections or defects in the cladding can reduce the percentage of light transmitted or impair other cable properties.

Drawing the fiber

The preform is placed into a draw tower to produce the desired optical fiber. One end of the preform is heated, with shielding inert gases used to keep the heating element from burning up during the process.

Gravitational force causes a small drop of melted glass to begin to fall from the preform as it heats. This forms the beginning of the optical fiber, which is often <500 um in diameter. As the preform diameter may be several inches, draw towers are often multiple stories high (and often outdoors) to ensure there is enough space for the fiber to stretch from the preform during the multi-stage, carefully controlled drawing process.

Quality control & ribbon coating

After drawing the fiber, its thickness is measured and quality checked using purpose-built tools. At that point, a thin coating of a polymeric or acrylic layer may be applied to the outside of the glass, with the specific coating depending on the intended use for the optical fiber. The coating helps protects the pure glass from any harmful environmental conditions while preserving the properties of the fiber itself.

The flow of the coating is generally controlled using pressure regulation for this extrusion-like process. Given how thin the fiber is, the pressure control must be extremely accurate and repeatable over the length of the fiber for the coating to be consistent: even tiny amounts of pressure fluctuation can lead to microns of variation in the overall thickness, dramatically affecting the overall performance of the fiber.

The fiber is then cured, with UV or thermal curing occurring in an inert atmosphere.

Alicat’s role in optical fiber manufacturing

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Optical fiber manufacturers use Alicat mass flow and pressure controllers and meters in various of the preform manufacture and fiber drawing processes.

As the preform is created, mass flow controllers are used to control the burner and flame, controlling the fuel gases which heat up the preform and the deposition of each new layer of glass. During the fiber draw, mass flow controllers feed argon into the furnace area, where it shields the furnace element from burning during the process.

As fiber optic cables are produced at a rate of about 90 feet per second, rapid response of instrumentation is paramount to quality assurance. Flow and pressure instrumentation which control at 50 ms are needed to ensure that any external environmental fluctuations are accounted and compensated for, and to avoid any quality issues for the fiber.

Multivariate data collection means greater process insight

Alicat devices measure mass flow, volumetric flow, absolute pressure, and temperature simultaneously, and can log historical data. This enables manufacturers to determine any environmental changes which resulted in rejected batches, defects, or other quality issues in the process. This is particularly useful when manufacturing specialty fibers for high-energy applications, which require very specific optical properties.

This data is collected and can be monitored and integrated into other control systems in real-time. In case of a spike, zero-flow condition, or other warning trigger, the Alicat flow controller will send a signal to the PLC, which can shut down the line if necessary. Rather than producing feet or miles of unusable fiber, the tower operator can rapidly identify a problem, fix it, and return to full operation.

Compensating for changing environmental conditions

Local environmental conditions can drastically affect the optical fiber manufacturing process, and many control systems can’t respond fast enough to changes in environmental conditions to save a batch.

For example, manufacturers located in areas that experience heavy thunderstorms (causing significant changes in barometric pressure) will see a lot of inconsistent product is a storm front is rolling in. They may look at the weather forecast and choose not to begin the 12-hour process drawn process.

However, Alicat devices have a quick enough control and response time to overcome and compensate for those changing atmospheric conditions. This allows manufacturers to produce high quality fiber—regardless of weather—because mass flow measurements compensate for local pressure changes.

Alicat in glass melting and shaping

Container manufacturers use mass flow controllers to regulate the gas flow to their melt process. Mass flow control of flame and fuels is used in making fiberglass for structural components. Architectural glass requires precise temperature control in ovens hundreds of feet long to create a consistent glass product.

Before industrial automation, a technician with years of experience could determine the correct temperature by looking at the color of a flame, and make adjustments manually while walking up and down a line. It’s an art from – and transitioning it to a science required automation.

Digital mass flow controllers have allowed engineers to focus on developing optimal process conditions and monitoring parameters within these furnaces. Once the system is set up, it’s possible to know exactly what flow rates are needed to maintain the proper temperatures. Industrial protocols such as EtherNet/IP are high-speed industrial protocols which incorporate modern network architectures and the proven TCP/IP transmission protocol.

The end result is greater repeatability and efficiency in the optical fiber manufacturing process, leading to a much better product and fewer lost batches along the way.