Producing Single Mode Fibers

The Single mode fiber is the most common fiber type by fiber length.

A producer of Single mode fibers (SMFs) faces many challenges. They include making an excellent product at competitive price reliably and in large volume.

Historic price of a single mode fiber
Historic price of a single mode fiber

The United Nations’ International Telecommunication Union (ITU) has specified a common set of minimum parameters for single mode fibers through recommendations: e.g. G.672 and G.657. Every producer aims to produce fibers that not only meet but exceed the specified values.

At the same time, standard SMFs have turned into a commodity, with the associated price pressure. Producers of SMFs are constantly working to reduce their overall production cost.

This can be done by:

  • Increased batch size: reduced down time of equipment, lower personnel need, lower share of beginning and end losses
  • changing draw speed
  • homogeneous preform outer diameter: prolong furnace life, stable draw process

The Heraeus’ RIC process has been the driving force in the industry to increased batch sizes and help lower single mode fiber production cost.
More information on the RIC® process

The volume of data transmitted over optical networks is constantly increasing. Operators need 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).

As the 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 strong.

Additionally as the pulse travels, it is losing 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 gets attenuated too low, to be detected. However, there are limits to the launching power. If it the power density get too large, nonlinear effects occur, that prevent a longer reach.

If 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 change between core and cladding the essence of an optical fiber, many fibers rely on a cladding with reduced refractive index. 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 strive 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.
More information on fused silica tubes and cylinders
More information on the RIC® process

Optical fibers

Another move to reduce the production cost is through the economy of scale. State of the art draw facilities produce more than 10 million fiber km; they process more than 300 tons of preforms per year.

Some producers rely on commercial preforms while others make the preform in-house.

The investment for equipment of 300 tons capacity is significant, and new entrants into SMF preform production are rare. However, it is possible to outsource over 90% of the volume, by outsourcing the cladding production. The fiber producer then only needs to produce the light guiding core rod. The jacketing is then done either as a separate process, that Heraeus offers as a service, or during fiber draw – in the Online RIC® process.
More information on cladding services
More information on the RIC® process

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