End users continually ask for tighter and tighter specifications on all properties of the fiber, specifically on geometrical tolerances which are dictated by the drawing process.
This technology has evolved over the past 50 years and continues to make remarkable strides. Single-mode fibers drawn to 125 mm can now be held to ±0.5 mm with general ease, and to 0.3 mm in certain cases. This represents a major improvement in the technology, and it has been carried over into the specialty industry where many different fiber diameters are drawn in smaller volumes.
The rise in active fibers has created many challenges on the fiber draw side as well, such as drawing octagonal and other shaped claddings to smaller and smaller tolerances. Improvements in the preform quality, feedback loops between the diameter gauges and draw equipment and draw furnaces themselves has created longer lengths, stable geometry across the draw and improved performance from the fiber itself.
Fiber optic draw towers come in many shapes and sizes and can be customized to meet almost any need for a manufacturer or research facility. Each tower will have different parts, but there are some standard parts for a draw process when using a preform.
Figure below displays a simple schematic of a draw tower. It consists of the preform feed mechanism, a draw furnace, diameter gauges, coating station followed by a curing unit (either thermal or UV) and another diameter gauge. It then goes through a capstan and then will be collected on a drum winder.
Maintaining the center line of the furnace to the capstan and then centering the coating stages and curing units will ensure concentric fiber to the coatings of choice. Optical fibers coatings are another field unto itself nowadays. Not only can acrylate coatings be applied in line during the process but polyimide, silicone, low index polymers, metals and others that extend either the temperature range of the fiber, improved mechanical reliability and optical performance.
Some specialty manufacturers produce fibers in line with extruded materials such as nylons, Teflon and Tefzel materials as a jacket material. This improves quality, yield and lead time as opposed to sending fibers to extrusion companies as a secondary process. The advancements in the coating industry have created more application space for specialty optical fibers.
The draw furnace is the essential part of this process. Typically, the draw furnaces consist of high-purity graphite heating elements which create resistive heating or induction furnaces with zirconia parts that can heat up to 2300 to 2400°C.
These devices can control to within ±0.2°C from the desired setpoint. In order to prevent any oxidation of the graphite elements, argon or other inert gases are sued within the furnace heat zones to combat that effect. The more recent and updated furnace models have also put additional features to limit the particulate size that can be produced. This is a key in producing highstrength fiber with long cut lengths after proof-testing. The size of these furnaces is also tailored based on the manufacturers’ needs and capabilities.
Single-mode preforms can be made extremely large, over 200 mm in diameter, and require draw furnaces as seen in Fig above. Larger furnaces such as that allow for large quantities of 125 mm fiber to be drawn >500 km at extremely fast speeds.
For optical fiber manufacturers that only draw a few products with only one coating type, this is a popular furnace type. However, for specialty manufacturers, smaller furnaces that have the capability to draw preforms ranging from 20 to 60 mm in diameter, shown in Fig. below, are preferred.
A large majority of these are resistive furnaces that use graphite components (elements, locking nuts and flow tubes) and can provide more flexibility with wider assortments of element sizes and flow tubers for subsequent fiber draws. Specialty preforms can be limited to smaller sizes based on the glass-making capabilities that are available so being able to draw a 20-mm preform and a 50-mm preform requires different elements and flow tubes to ensure that the gaps between the glass preform and flow tubes in the heating zone are consistent between various sizes. Otherwise, draw tower operators need to compensate with temperature and/or argon flows to maintain a consistent neck down region, which affects the fiber draw tension.
This is extremely important in single-mode fibers as the cutoff wavelength and attenuation can be affected by changes in draw tension. In many towers, an inline draw-tension monitor can be used to ensure that the appropriate tension is applied.
If it is out of range, either the draw speed or temperature can be modified to compensate. In multimode fibers, the draw tension is important but does not have as dramatic an impact as on single-mode or large-modearea (LMA) fibers that inherently contain only a few modes. High tensions can lead to increased losses due to attenuation which affects the fiber performance and needs to be maintained.
The draw process, while fundamentally simple, has many complexities. Ensuring that a continuous feedback loop mechanism is fundamental to producing and manufacturing a quality optical fiber. Non-contact diameter gauges can take hundreds to thousands of scans per second and relay them back to the PLCs of the draw tower.
There the speed of the capstan can be changed to constantly maintain the fiber diameter. Optionally, a caterpillar capstan can be installed in line with the drawing system, which is designed to pull glass rods, thin-walled capillaries, or sub-structured canes with minimal surface pressure. All these devices are aligned vertically along the drawing line in a mechanically stable tower that is usually higher than 4 m. Larger heights (e.g., >20 m) are necessary for high-speed fiber drawing  at drawing speeds of >1000 m/min in order to provide sufficient fiber cooling across the distance between the furnace outlet and coating applicators.
Appropriate fiber cooling is necessary to prevent overheating the coating material from contact of the coating material with a high-temperature fiber, which can result in improper wetting.