Optical Fibre Types

Optical fibres can be split in to two basic types, multimode and singlemode. In multimode fibres the light travels in multiple paths (modes) down the fibre, with each path reflecting off the internal boundary between the cladding and the core. The glass in the core of a multimode fibre has a graded index such that light passing down the centre of the core travels more slowly than light passing through the outer edges. This ensures that all of the different modes of light reach the far end of the cable at roughly the same time, thus minimising the modal dispersion, a limiting factor in the performance of multimode fibre.

Multimode light transmission diagram

In a singlemode fibre the light travels as a single beam down the core of the fibre, i.e. there is only a single mode of travel.

Singlemode Light Transmission Diagram

 

Optical fibres are defined by the ratio between the diameter of the core and the diameter of the cladding, expressed in microns (thousandths of a millimetre). There are two constructions of multimode fibre (50/125 and 62.5/125) and one construction of singlemode fibre (9/125).

MM & SM construction diagram

 

Fibre Cross Section Diagram

 

 

The glass fibres then have a coloured plastic primary coating that takes the overall diameter up to 250 microns. This provides protection and strength to the glass fibres and enables individual fibres to be identified at each end of a fibre optic cable.

 

 

 

Categories of Optical Fibre

 

The performance of multimode optical fibres are defined by their bandwidth and are categorised as follows:

MM Fibre Classes Table

Singlemode optical fibres are defined by their attenuation, with OS2 fibre (low water grade fibre) offering improved performance around the 1383 nm wavelength.

SM Fibre Classes Table

 

Optical Fibre Application Support

 

The distances over which different types of fibre can support applications can be seen in the following table:

Fibre Application Support Table

The International Standards Organisation (ISO) specifies a range of Optical Fibre Classes that define the maximum attenuation for a fibre optic channel of a range of lengths.

ISO Fibre Classes Table

Applications can then be mapped to these classes so that a user with a fibre channel with a known length and attenuation will be able to determine what applications can be supported over it.

 

Summary

 

The relative pros and cons of multimode and singlemode fibres can be summarised as follows:

Multimode Vs Singlemode Table