Optical fibers serve multiple applications, from telecommunications, to medical and industrial. The following information are meant to be a short introduction into the different challenges associated with the manufacturing of specialty 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.
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 (numerical 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.
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 .
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.
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.
