Fibre Optic Properties

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.

Fibre Optic Design

Core
For spectroscopic applications, generally, multi-mode step index silica fibres are used. These range in core thickness from 50 microns to 1 mm. The core is made out of pure silica. Other fibre cores with much higher absorption are made out of certain glass types or plastics.

First a distinction is made between silica with high or low OH content. Silica fibres with high OH (600-1000 PPM) are used in the UV/VIS wavelength range because of the low absorption in the UV. They are referred to as UV/VIS fibres. For Deep UV applications (below 230nm) special solarisation resistant fibres can be used.

The water content causes strong absorption peaks in the NIR wavelength range. In order to get good fibres for the NIR range, the “water” is removed from the silica. This results in low OH fibers (<2 PPM) with low absorption in the NIR. They are referred to as VIS/NIR fibers.
 

Cladding
In order to get the light guiding effect the core is cladded with a lower index of refraction material. For the highest quality fibres with the lowest absorption this is a fluorinedoped silica, the so-called silica-silica or all-silica fibres with a numerical aperture (NA) of 0.22.

Buffers
Without extra protection, fibres would easily break because of small scratches or other irregularities on the surface. Therefore another layer, the buffer, is added. This buffer also determines under what circumstances the fibre can be used. Temperature range, radiation, vacuum, chemical environment and bending are factors to be considered.

Polyimide buffers offer a wide temperature range (-100 to 400°C) and superior solvent resistance. Also, this material is non-flammable. Drawbacks are sensitivity to micro bending and the difficulty to remove it.

For extreme temperatures (-190 to 750°C), a gold buffer is used. Gold-coated fibres are virtually inert to all environments and make hermetically sealed high pressure feed through's possible. (See pressure feed through's). The same is true for aluminum buffers for temperatures from -190 up to about 500°C. Low outgassing makes them also excellent for use in vacuum.

Technical Details

Fibre Transmission