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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 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.

Optical fibers for telecommunications

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 8 µm.
More details on the challenges associated with the production of optical fibers

The second largest group of fibers are called multi-mode 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 125 µm but they differ in the core diameter of either 50 µm or 65 µm.

Preform design

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. This is achieved by raising the refractive index (RI) of the core above the normal fused silica RI (e.g. by germanium doping), or by lowering the RI of the cladding compared to the undoped core (depressed cladding), which can be achieved by fluorine doping.

For telecommunications applications international standards define the basic properties of the fibers. The individual design of the fibers (meaning the changes in refractive across the fiber) is usually company specific.

Preform production

Typically preform production can be divided into two major production steps. The production of the light-guiding core and usually 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.

Core rod processes

For large batch size, telecommunication fibers, the most common processes to produce the core of an optical fiber are called VAD, OVD and PCVD.
More information on production processes

Cladding processes

The cladding is directly deposited onto the core by an OVD process, or the RIC process is used. The RIC® process was developed by Heraeus, it allows for high core rod yields, allows to outsource the cladding to Heraeus and therefore enables the customer to focus R&D on core rod and fiber draw topics.

Fiber draw

Optical fibers are produced on a draw tower. The tower can be as high as 30 m 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 for up to date telecommunication fiber production.

Post draw processing

The geometric properties (diameter, ovality) 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, cut-off 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 radiation. 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.

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