The stretching process requires a special fiber preform, which may be 1 meter in length, and stretched into fibers of hundreds of meters or even kilometers. In the design and manufacture of the preform, dopants and other elements are carefully placed so that the stretched fiber has the correct refractive index, and chemical, mechanical, and geometric characteristics. To achieve the required properties, the finished fibers must be drawn using high-precision procedures so that they can be baked into the preform. Specifically, the drawing process has the following goals:
- Obtain high-strength fibers; for example, meet tensile strength specifications;
- Reach geometric specifications-fiber outer diameter and some special fiber shapes;
- Apply and cure the fiber coating with suitable adhesion, thickness, concentricity; and mechanical properties;
If you think of the drawing tower as a system that converts preformed fibers into drawn fibers, it should be noted that it has several subsystems that need to achieve the above goals. The key subsystems include the preformed feed module, heat source, bottom winch, and coating device.
A correct temperature balance is achieved through these systems, which impact the viscosity of the glass and create tension. Other subsystems help avoid vibration and ensure that the bare fiber is not exposed to dust, moisture, and other contaminants.
1. Multi-layer precision positioning machine
During the drawing process, the preformed fiber is drawn vertically, while the reel is drawn horizontally. The multi-layer tower allows the fibers to cool down before applying the coating. The higher the tower, the faster the drawing speed. In special fiber factories, the height of the drawing tower is generally between 7 and 14 meters. The goal of telecommunications fiber optic manufacturers is to achieve mass production at higher speeds. They usually use attraction towers with a height of more than 30 or 40 meters.
Tie-rod tower manufacturers, such as Special Gas Controls, use prefabricated components to build tie-rod towers, allowing the height to be customized for specific facilities. These parts are processed, so the tower will be completely straight. Then use the laser alignment system to assemble the tower. In addition, rigidity is critical, which in turn means that the tower must be isolated from vibration from the building and underground. This requires a special foundation and special feet in Rata.
This structural requirement means that the production and drawings of the prefabs are done in different rooms or different buildings. But before transporting the preforms into the tower and starting the drawing process, there are mandatory preparation procedures: cleaning and flame polishing the preforms. After these steps, the preform is brought to the top of the tower to start the suction.
2. Pull down the tower subsystem from the top of the tower
The list of equipment on the drawing tower may vary depending on the type of fiber being made. For example, telecommunications fiber manufacturers may have housings or pipes that work with cooling gas to speed up the drawing speed. The verification system can be done at the bottom of the pull tower, or “offline” in another place.
The following table lists the main types of equipment commonly used by specialty fiber manufacturers. The equipment is listed on the Lata from top to bottom. In addition, for some equipment units, there are different suppliers and many choices. Examples include the choice of the furnace (heat source), measuring instruments with different ranges, and the choice of curing lamps and ovens in coating systems.
3. Special fiber raw material tower subsystem/equipment
- Preform-feed module
- Furnace
- Start the tractor unit
- Gas supply system (argon)
- HEPA filter
- Fiber diameter measurement
- Fiber Coating System
- UV and heat curing oven
- Coating diameter measurement
- Fiber optic centering control
- Fiber coating concentricity control
- Fiber tension measurement
- Winch/fiber puller
- Fiber Winding Machine/Drum Changing Machine
- Anti-fiber Tester/Rewinder
The Optical Fiber Center directly cooperates with SG Controls and its full range of tools for the stretching process of manufacturing optical fibers. You can contact Larry and Rick at FiberOpticCenter@focenter.com to inquire about the above process and any equipment.
1) Pre-formed feed module.
With careful control of speed and position, the preform is fed into the furnace. The equipment includes a motor with a screw drive, a chuck to hold the preform, and an x-y positioning system to center it above the furnace. The rate at which the preform is fed down into the furnace is determined by the drawing speed, the diameter of the preform, and the specified fiber diameter.
Once the stretching starts, the performance feed rate is usually not adjusted. The fiber diameter is fine-tuned with the drawing speed. The stretching tension is controlled by the furnace temperature. Tension is a key variable to be monitored and controlled during the stretching process. Our goal is to keep it unchanged. There is a tensiometer at the bottom of the tower, just above the winch.
2) Furnace
The preform descends into a hole in the top of the furnace, which is cylindrical with a vertical axis. There is a small hole on the bottom surface, and small diameter fibers come out of the small hole.
By changing the diameter of both holes, the operator can control the flow of gas in the furnace. The furnace uses high-voltage electrical components-usually graphite resistance units. In the past, zirconia induction furnaces were used, but now most specialty fiber manufacturers use graphite resistance furnaces.
When the suction is started, the green body is placed in the furnace. The stretching zone is heated to above 1900°C, where the glass softens and stretches, and teardrop-like water droplets pull down the fiber. The fibers pass through the under-neck area in the furnace and exit the bottom hole.
3) Start-up tractor assembly
Under the furnace, the dripping liquid is cut, and the fiber passes through a two-wheel tractor assembly, pulling the fiber downward, continuing to reduce the diameter. When the diameter reaches the correct specification, the fiber is transported to the coating system and along the bottom of the tower to the position of the winch. The winch takes over the traction, and the tractor under the furnace is pulled out. After that, the winch at the bottom controls the speed in a feedback loop with a diameter gauge.
Note: some drawing processes are started without the use of a tractor, and rely on manual processing and the skills of operators.
4) Furnace gas supply
The furnace’s operating temperature is close to 2000°C, but graphite can be oxidized and in some cases burns above 600 to 800°C. To avoid this, the inert gas argon flows through the furnace. The degradation of graphite elements may pollute the bare fibers and also affect the life of the furnace.
Controlled argon gas flow also helps prevent turbulence caused by gas flow around high-temperature components. As mentioned earlier, even the smallest vibrations must be avoided, so air turbulence in the furnace must be minimized. The argon gas passes through the furnace, and its iris helps to control the flow. There are several “trade tricks” to monitor and maintain the correct argon flow.
5) Air filtration
Due to the slow drawing speed of specialty fibers, uncoated fibers are usually cooled by air before coating. Larger telecommunications fiber factories have closed gas extraction towers for cooling the gas. Some operations also set up suction towers in the clean room. However, most specialty fiber manufacturers use clean ambient air—cleaned by HEPA filters installed behind the preforms and furnaces and in the lower part of the tower.
6) Fiber diameter measurement
The diameter of special fibers ranges from less than 50µm to more than 1000µm (1mm). Widely used particle sizes include 80, 125, and 400 μm. Drawing speed determines the diameter
There may be small variations in fiber diameter due to small changes in the temperature of the furnace, the inert gas flow, or other stretching conditions. To avoid this situation, the tower has a continuously operating diameter measurement system. The measured data is input into the diameter control loop, and the draft speed of the winch can be adjusted through the diameter control loop. It may be necessary to adjust the preform feed by means of a secondary control loop. These control loops use diameter measurement to make quick adjustments.
There are several companies that provide diameter measuring instruments for optical fiber trolley towers, which use lasers for precise measurement, reaching an accuracy of hundreds of microns. Some of these companies also provide instruments for measuring coating thickness, concentricity, and even air bubbles. The measurement system also provides a record of diameter data.
7) Coating system
Fiber coating is essential for protecting glass fibers, maintaining the mechanical properties of the fibers, and the optical properties of some special fibers. For example, some rare-earth-doped fibers have a low-index polymer coating that acts as a second cladding to help guide the mode from the light “pump” source.
Two layers of coating usually adhere to the glass-a soft inner layer and a hard outer layer. This means that the coating system must apply and cure two different resins. After the diameter measurement, the fiber enters the first mold or “cup” of the coating system, which applies the primary coating (inner layer). Some secondary coatings called “wet-on-wet” can be applied before the primary coating is cured. In the “wet-to-dry” coating system, the main coating passes through the thermal curing or UV curing system before entering the secondary coating module. In this case, there is another UV or thermal system to cure the secondary coating.
8) Concentricity measurement
The coated fiber must be geometrically measured-the outer diameter and the concentricity of the coating. Coated fiber diameter measurements also use laser-based instruments and may include a feedback loop for adjustment. The liquid flow force of the coating material in the mold helps to keep the fiber in the center of the mold. If the concentricity monitor shows a problem, it may be necessary to stop the extubation and start again. The concentricity of the fiber coating is very important to avoid the micro-bending loss because micro-bending loss can cause attenuation problems. For some special fibers with special-shaped glass cladding, it is also important to ensure that the coating covers all the glass correctly.
9) Capstan and take-up reels
As fibers are pulled from the preform’s end by the winch at the bottom, the drawing speed increases. In addition, there is a system that winds the fiber onto the storage reel with proper tension. Providing the correct fiber tension and drawing speed are controlled by both systems.
10) Proof tester and rewinder
Tensile strength is a key indicator of the finished fiber. This measurement is also important in fiber factories because tensile strength failure can be used as an indicator of problems that need to be resolved during performing and stretching. The verification test machine usually includes a pay-off machine, two winches that can control continuous tension testing, and a winder. The tensile strength problem is caused by defects on the glass surface, and the purpose of the verification test is to “remove” large defects.
The verification tester can be located near the main winch of the drawing tower or in a separate room. Some special fiber manufacturers also have rewinding systems, which can divide the preformed output into multiple rolls according to the length specified by the fiber user.
11) Other drawing tower equipment
With so many variations in the world of special fibers, some types require additional subsystems to implement or control specific functions. In the drawing process, circularly polarized fibers and other types of fibers may need to be spun or twisted. These processes can be completed by the spinner in the preformed chuck or the twisting or shaking device underneath. Stretching photonic crystal fibers with air gaps, voids, and other characteristics may require additional gas flow and gas pressure systems to control internal pressure and moisture.
4. Optimize the drawing process
If there are multiple subsystems on the tower, there will be multiple potential sources of failure or problems. In this case, the “fault” may be the inability to meet any specifications of the optical fiber. Or positively speaking, all subsystems must work together perfectly to get the most benefit from a prefab.
Due to the complexity of this goal, we noticed that different special fiber types have different preforms, which require different settings and adjustments to the tower. No two preforms are exactly the same. Each stretching process must start with rechecking the alignment and diameter settings-fine-tuning the stretching speed, temperature, and tension. If it is impossible to maintain the diameter and the tension of the roll, the feed under the preform must also be adjusted.
Other complications include maintenance and calibration of measuring instruments. Careful control of gas flow is another tricky process, which is necessary to prevent contaminants from entering the fibers.
