Optical Fibers

Optical fibers

Optical fibers serve multiple applications, from telecommunications, to medical and industrial. This text is meant to be a short introduction into the different challenges associated with the manufacturing of optical fibers.

Generally fibers are made by drawing a thin strand of glass from a specifically designed glass rod (called a preform). The fiber is coated with a protective acrylate layer and then tested for various properties.

How do optical fibers work?

Optical fiber with total internal reflection

Optical fibers consist of a light guiding core and a cladding. The core must have an increased refractive index compared to the surrounding cladding, so that the light is totally internally reflected at the interface between core and cladding, and therefore guided along the length of the fiber – with extremely low attenuation. The important property is the difference in refractive index and the NA (aperture) of the fiber (the angle under which light that enters one end of the fiber is still guided to the end).

There are uncounted possibilities for optical fiber designs and applications; from the large volume standardized telecommunication fiber to very individual designs that serve one customer in a specific application.

The largest volume of optical fibers is the single mode fiber (SMF) for telecommunication applications. These fibers have an outer diameter of 125 µm and their key properties are specified by the international telecommunications union (ITU). The core size is not specified but defined by other properties and typically in the range of 8µm.
More details on the challenges associated with the production of single mode fibers .

The second largest group of fibers are called multimode fibers (MMF). These fibers may also be used in the telecommunication’s field (e.g.: in datacenters) but also serve a multitude of other applications (e.g.: spectroscopy and medical applications). There are two groups of MMF standards. Both state an outer diameter of 125µm but they differ in the core diameter of either 50 µm or 65 µm.
More information on optical fibers for spectroscopy
More information on optical fibers for medical applications

Many applications require the transportation of light from one location to another that are not connected by the line of sight, or where it is impractical or dangerous to have an open beam. In this case an optical fiber is the ideal solution. It can be bent and moved, yet keeps the two locations connected. A few examples of these applications include medical and spectroscopic applications as well as material processing.
More information on optical fibers for spectroscopy
More information on optical fibers for medical applications
More information on optical fibers for material processing

For each application the right optical fiber is selected because of its transmission characteristics and the shape of the beam that exits the fiber. These two properties are defined by the refractive index profile, material composition and geometrical shape of a fiber. How these parameters influence the transmission is described more in detail below. While these three properties represent a high degree of freedom to realize the customer needs, a lot of knowledge is necessary to achieve the optimal solution.

Preform design

Fluosil Preform

A preform is a bigger solid version of a fiber. The fiber is drawn from the preform and should have all the properties the preform had. The minimum requirement of a preform is, that its center, that later forms the core of the fiber, is made of a glass that has a higher refractive index than the glass that makes up the cladding.

To make a long story short, the performance of the fiber is defined by the material composition, the geometry and the refractive index of the different layers. At Heraeus we have accumulated fused silica know-how and processes that we combine to realize your ideas.
More information on our fused silica know-how
More information on our production processes

Pure fused silica without any impurities has an excellent transmission over a broad spectral range. This transmission can be modified by dedicated doping but can also effected by unwanted impurities. Furthermore dedicated doping is used to modify the refractive index of the material.

For example a material with high hydroxyl content is the material of choice for transmission in the UV range. For transmission in infra-red wavelengths a material with low hydroxyl content is required.

Furthermore rare earth elements can be used for doping. By applying these elements in the core of a fiber it can act as amplifier for light. These fibers are called active fiber or laser fiber.
More information on laser fibers

For the transmission of light through a waveguide a two-layer structure is required. A core with a higher refractive index than the outer cladding. This can be achieved either by doping the core with elements which increase the refractive index. E.g. with Germanium like in telecommunication fibers. Or by doping the cladding with elements like Fluorine which lowers the refractive index.

The height of the refractive index step defines how confined the light is guided in the core and how many different modes (pathways of the light trough the fiber) can be guided.

The layer thickness of the cladding also influences the guiding properties as always some light penetrates into the cladding. If the layer is too thin some light gets lost. Especially if the fiber is bent.

In modern designs the refractive index profile shows several layers with different optical functions. E.g. to create a ring shape instead of a single spot or to create a pump cladding for laser fibers .

The geometry is another factor defining the transmission properties. Some examples how the transmission is influenced by the geometry are given below:

Shape
A square shape core in a multi-mode fiber will cause a mixing of the different light modes transmitted. Therefore the energy density across the cross section of the light spot of this fiber will be more homogenous.
For laser fibers often an unsymmetrical cladding for guiding the pump light is used. The broken symmetry suppresses helix modes and increases the pump efficiency.

Distances
The layer thickness within a fiber design defines if the light is guided or stripped off. A thin layer for example can be used to strip of some unwanted modes of the light.
In polarization maintaining fibers stress elements a positioned beside the core. These often boron doped elements have a different thermal expansion and introduce therefore a mechanical stress which modifies the transmission properties. The amount of stress is influenced by the distance of the stress elements from the core.

Typically preform production can be divided into two major production steps. The production of the light-guiding core and possibly a first cladding. This product is called a core rod. In a second step the cladding is produced either separately or directly on the core rod.

  • For large batch size, telecommunication fibers, the most common processes to produce the core of an optical fiber are called VAD, OVD and PCVD.
  • For smaller batch sizes MCVD and FCVD are used.
  • A solid glass rod is bought and used as is.

More information on our production processes

Fiber draw

Optical fibers are produced on a draw tower.

Optical fibers are produced on a draw tower. The tower can be as high as 30m and consists of a holding and feeding mechanism for the preform, a furnace, measurement devices, a coating apparatus, curing light sources and a take up spool.
The speed of the fiber draw depends on the preform, fiber type and available equipment. It can be a few meters per minute up to 2500 meters per minute.

More information on Heraeus curing light sources

Post draw processing

The geometric properties are controlled during the draw process. After drawing optical fibers are tested to verify all other properties are within the specification.

These properties can include any of the following: attenuation, macrobend attenuation, cutoff wavelength, mode field diameter, dispersion, polarization mode dispersion, tensile proof test, glass geometry (curl, cladding diameter, core-clad concentricity, & cladding non-circularity), and coating geometry (coating diameter & coating-cladding concentricity).

In single mode production, the first test typically is a strength test. Then Optical properties like attenuation are measured. For the next process steps, fibers are cut to predefined length.

If a cable contains multiple fibers, they usually have received an additional color coating to allow for easier identification. To color code and mark the optical fibers, UV curing paints are applied and cured within seconds with UV radition. Take advantage of the Heraeus UV experts to select the right UV solution tailored to your process.

Also typically a cabled fiber gets a connector at both ends and the end faces are polished. For other applications several fibers are fixed together in one connector to create a flexible fiber bundle.

Contact our Heraeus UV experts

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