The single mode fiber is the most common fiber type by fiber length produced. A producer of single mode fibers (SMF) faces many challenges. They include making an excellent product at a competitive price, reliably, and in large volume.
The United Nations’ International Telecommunication Union (ITU) has specified a common set of minimum parameters for single mode fibers embodied in various standards: e.g. G.652 and G.657. Every producer aims to provide fibers that not only meet but exceed the specified values.
At the same time, standard SMF’s have become a commodity product, with associated price pressures. Producers of SMF’s are constantly working to reduce their overall production cost.
This cost reduction can be achieved by:
The Heraeus’ RIC® process has been the driving force in the industry to increase batch sizes and reduce production costs for single-mode fibers.
The volume of data transmitted over optical networks is constantly increasing, requiring operators to increase the capacity of their networks. Typically, this includes adding new channels (adding a new wavelength) or shrinking the time between light pulses (signaling faster).
Because a pulse broadens with the distance it travels due to dispersion, it begins to overlap with the neighboring pulse. The receiver needs to identify individual pulses, so the overlap should not be too large.
Additionally, as the pulse travels, it loses intensity due to attenuation - the signal gets weaker. The quality of the fiber link is determined by its signal to noise ratio. It is beneficial to reduce the noise, and/or increase the signal power. The higher the launching power, the longer the distance the signal can travel before it is attenuated too much to be detected. However, there are limits to the launching power. If it the power density is too high, nonlinear effects can occur that prevent a longer reach.
When new fiber networks are designed, fibers with reduced attenuation characteristics or with a larger core (that can cope with higher launching power) or with both properties can be selected.
Attenuation is influenced by a multitude of parameters. The purity of the glass is one parameter that influences attenuation. Fiber producers have reduced the germanium content in the core to reduce attenuation. As a refractive index difference between core and cladding is the essence of an optical fiber, these fibers rely on cladding with reduced refractive index relative to pure silica. Heraeus supports these designs with fluorine-doped tubes.
Additional factors that influence attenuation are the surface quality of potential interfaces in the preform (micro-bending loss) and fiber draw conditions (stress), to name a few. Heraeus supports the drive for lower attenuation by offering high purity fused silica tubes with an excellent surface quality for core rod production as well as precisely machined fused silica cylinders for the RIC® process.
With fiber-to-the-home applications and the move closer to the final antenna, fibers are increasingly placed in an open environment. Additionally, optical cables need to be easy to install. It is important that personnel accustomed to handling copper cables are aware of some restrictions with fiber optic cables. The most important difference is the sensitivity of the optical fiber to bending. If bent too tightly, the guided light leaves the core and no signal reaches the fiber end. Over the decades, optical fibers have improved the bending tolerance.
Finally, the so-called bend-insensitive fibers were developed and standardized (G.657). Depending on the subcategory, these fibers can be bent to a radius as small as 2.5 mm. To achieve the bending performance, a ring with lower refractive index is placed around the core. This so-called trench is created using fluorinated silica. The process to make a fluorinated-glass ring can be challenging and expensive. The Heraeus RIC® process, however, provides a simple and cost-effective solution for large production volumes. The RIC® process is simply modified by placing an additional tube of fluorinated silica inside the cylinder and then the core rod into the fluorinated tube. This process is the primary one used globally to produce bend-insensitive fibers.
While design of the cable is important in optimizing the use of duct space, another significant contribution is the development of 200 µm fiber. Compared to the standard of 250 µm, the lower diameter is achieved by reducing the thickness of the coating. However, fibers with a reduced coating are more sensitive to bending induced attenuation. This sensitivity is due not only to macro bending, but also micro bending that occurs during the cabling process.
The solution is, therefore, to use bend insensitive fibers (G.657). These fibers have trench around the core with a lower refractive index. This trench helps to keep the light confined in the core, even when the fiber is bent. To achieve the lower refractive index, fused silica is doped with fluorine.
It is very easy to realize the trench in the RIC® process. A tube of fluorine doped fused silica is inserted into the cylinder, surrounding the core. The general process of drawing fiber is then unchanged.