Fibre Optics

The use of fibre optics as light guidance allows a great modularity and flexibility in the setup of an optical measurement system.

Optical fibres can be made of many materials, such as plastic, glasses and silicates (SiO2). For high quality fibre optics, as used in spectroscopic applications, synthetic fused silica (amorphous silicon dioxide) is used, that can be intentionally doped with trace elements to adjust the optical properties of the glass.

The basic principle of light transport through an optical fibre is total internal reflection. This means that the light within the numerical aperture of a fibre (NA = input acceptance cone) will be reflected and transported through the fibre. The size of the numerical aperture depends on the materials used for core and cladding.

Two basic types of silica fibres can be distinguished - singlemode and multi-mode, depending on the propagation state of the light, travelling down the fibre.

For most spectroscopic applications multi-mode fibres are used. Multi-mode fibres can be divided into 2 subcategories, step-index and graded-index. A relatively large core and high NA allow light to be easily coupled into the fibre, which allows the use of relatively inexpensive termination techniques. Step-index fibres are mainly used in spectroscopic applications.

Graded-index multimode fibres have a refractive index gradually decreasing from the core out through the cladding. Since
the light travels faster in material with lower refractive index, the modal dispersion (amount of pulse-spreading) will be less. These graded-index fibres are mainly used in telecommunication applications, where bandwidth at long distance (2-15 km) plays an important role.

In the 'Fibre Optic Properties' and 'Probe Properties' sections below, some basic fibre optic components and properties are presented.