Appnote: Precision & repeatability in fiber optic manufacturing

Precision & repeatability in fiber optic manufacturing

In the optical fiber industry, manufacturers’ needs center around precision and repeatability. It’s somewhat analogous to archery, where precision means hitting the bullseye. If you can hit the same point on the target every time, that’s precision. Repeatability is being able to demonstrate the same precision every time you walk up to the line and shoot. Let’s take a look at how optical fibers are manufactured, and where Alicat mass flow controllers and meters fit into the picture.

How optical fibers are manufactured

The fiber optic manufacturing process begins with the creation of a preform, where layers of very pure glass are added to a cylinder. Different types of doping gases in controlled amounts are used to deposit new glass layers. Every layer that is added to the base will give the end product a different property. A flame uses fuel gases, regulated by a mass flow controller, to maintain a certain temperature and ensure the process is running optimally. The final preform contains a core and cladding of differing refractive index, which causes the light to remain trapped within the core due to a phenomenon know as total internal reflection (TIR). Even slight imperfections in the cladding can result in defects that reduce the percentage of light transmitted, or impair other cable properties. This preform construction process typically takes place over the course of many hours, depending on the size of the resulting preform.

Once the preform has been created, it is then placed into a drawing tower to produce the desired optical fiber. As one end of the preform is heated, inert gases are used to keep the heating element from burning up during the process. Small drops of melted glass begin falling down, due to gravitational force, and form the beginning the thin (often ~ <500 um in diameter) optical fiber. Glass preforms typically have an extremely large diameter relative to the optical fiber being produced. Due to this, the drawing towers are often multiple stories tall so that there is enough space for the fiber to pass through the multi-stage, carefully controlled drawing process.

Next, the thickness is measured and quality checked with specialized purpose built tools. Depending on the end use of the product, a coating process may apply a very thin polymeric or acrylic layer on the outside of the glass. This coating helps to protect the pure glass from environmental conditions and preserve the important properties within the glass fiber itself. Pressure control regulates the flow of this liquid polymer. In an extrusion-like process, it coats the fiber. Pressure needs extremely precise and repeatable control to provide a consistent coating over the product. Even tiny amounts of pressure fluctuation can lead to microns of variation in the overall thickness, which dramatically affects the overall performance of the fiber. During the UV or thermal curing, the material is kept in an inert atmosphere to aid the curing process. Alicat mass flow or pressure controllers can control each of these essential steps.

Alicat’s role in fiber optic manufacturing

Fiber optic companies use Alicat mass flow controllers and meters in several different areas of the preform process and drawing process. During creation of the preform, Alicat mass flow controllers and meters are used for the burner control application, which controls the fuel gases which heat up the preform and help to control the deposition of each thin layer. Our fast, repeatable, precise, and easy-to-use mass flow controllers are used to facilitate the drawing process, where argon is fed into the furnace area to keep the furnace element from burning up.

Every one millisecond, signals from our measurement sensors go through our entire processor. Depending on the type of process and operating pressures, the controller will have a control response as low as 50 milliseconds. (In many cases we can help to tune that number down to sub-50 millisecond time-frames when needed.) For anyone who might have a hard time imagining how fast this is, it takes the average human 300 to 400 milliseconds (thousandths of a second) to blink their eyes. Not only does response time indicate reaching a new setpoint, but also responding to external pressure fluctuations that may throw off the properties of the fiber. To put this into perspective, our controllers take numerous measurements, sense any external fluctuations, and adjust to these changed conditions to maintain a steady flow in less than the blink of an eye. That’s how we’re able to maintain repeatibility and accuracy when a fiber is produced at 90 feet per second.

More data collection means more insight into the process

Specialty fibers for high energy applications need to have very specific optical properties, which are shaped by the density and mix of materials. Therefore, depending on the part of the process, engineers want the ability to see as many parameters as possible. With an Alicat, you not only get the mass flow measurement, but also absolute pressure, volumetric flow, and temperature information so that you can refer back to it later. From a quality control perspective, you can use these parameters to determine what changed in the process and correlate that change to a run that was rejected because of a defect.

Typically, the signals that provide this information go into some sort of a controller like a PLC, for interpretation. In case of a spike or a zero flow condition, the Alicat controller would send a signal to the PLC, which would then be able to shut down the line. Instead of having hundreds of feet or even miles of unusable product, it gives manufacturers the opportunity to identify that there’s a problem, get it fixed, and start the process again quickly. This one signal sent by the controller has the ability to save the manufacturer a significant amount of time and money.


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Compensating for changing environmental conditions

Local environmental conditions can drastically affect the optical fiber manufacturing process. Unfortunately, many control systems don’t have the ability to respond fast enough to changes in environmental conditions. For example, a particular manufacturer is located in an area that experiences heavy thunderstorms, which cause changes in barometric (atmospheric) pressure, and that causes an inconsistent product. With a storm in the neighborhood, they might decide not to even begin their 12-hour process, depending on the day’s forecast. Alicat can overcome and compensate for those changing atmospheric conditions and allow the manufacturer to produce high quality fiber—regardless of weather—because we measure flow in the context of pressure changes. This differential pressure technology, along with the response speed, will have you feeling confident in your product each and every day.

Other glass shaping processes

Mass flow instrumentation is used in many other glass processes besides fiber optics. Container manufacturers use mass flow controllers for regulating the gas flow to their melt process. There are also manufacturers that use mass flow to control the flame used to make fiberglass for structural components. Architectural glass is another huge consumer of mass flow controllers. They have an oven that’s hundreds of feet long, which requires precise temperature control to create a consistent glass product. (These are typically huge sheets of plate glass.) At any given zone of the oven, a specific temperature must be maintained with very precise flow control of fuel. Alicat mass flow controllers are deployed by architectural glass manufacturers to enable maintenance of the consistent temperature required by this process.

In the days before industrial automation was available, someone who gained years of expertise could determine the correct temperature just by looking at the color of the flame. They eventually learned how to make slight manual adjustments while walking down the line. What will the company do when those people retire? That’s where digital mass flow controllers come in—automation allows people to focus on developing optimal process conditions and monitoring parameters within the furnaces. Once everything is set up, it’s possible to know exactly what flow rates are needed to maintain the proper temperatures. The latest Alicat mass flow controllers and meters also use the latest industrial protocols, such as EtherNet/IP.  EtherNet/IP is a high speed industrial protocol that incorporates modern network architectures and the proven TCP/IP transmission protocol. Incorporating these various elements provides an automated process which is repeatable, provides useful data and can help identify why there may have been an inconsistency in the glass produced. The end result is greater repeatability, efficiency and a much better product. Contact Alicat today to see how we can help improve your process.