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, which later forms the core of the fiber, is made of a glass with a higher refractive index than the glass that makes up the cladding.
In essence, the performance of the fiber depends on the material composition, the geometry and the refractive index of the different layers. At Heraeus we combine our accumulated fused silica know-how and processes that we combine to realize your ideas.
Pure fused silica without any impurities has excellent transmission over a broad spectral range. Doping can modify transmission but can also be effected by unwanted impurities. Furthermore, dedicated doping can 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, using rare earth elements for doping in the core of a fiber can amplify the light. These fibers are called active fiber or laser fiber.
Refractive index profile
To transmit light through a waveguide requires a two-layer structure first. 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 guided light is in the core and how many different modes (pathways of the light through the fiber) there are.
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 are given below:
A square shaped core in a multi-mode fiber will cause a mixing of the different light modes transmitted. Therefore the light energy density of the cross section of this fiber will be more homogenous.
Laser fibers often use an unsymmetrical cladding for guiding the pump light. The broken symmetry suppresses helix modes and increases the pump efficiency.
The layer thickness within a fiber design determines whether the light is guided or stripped off. A thin layer, for example, is useful for stripping some unwanted modes of light.
In polarization, fiber stress elements maintain a position 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 distance of the stress elements from the core influence the amount of stress.