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Optical fibre waveguide

For many years it has been appreciated that the use of optical (light) waves as a carrier wave provides an enormous potential bandwidth. Optical carriers are in the region of Hz to Hz, i.e. three to six orders of magnitude higher than microwave frequencies. However, the atmosphere is a poor transmission medium for light waves. Optical communication only became a widespread option with the development of low-loss dielectric waveguide. In addition to the potential bandwidth, optical fibre communication offers a number of benefits:

The primary disadvantage of optical fibre are the technical difficulties associated with reliable and cheap connections, and the development of an optical circuit technology that can match the potential data-rates of the cables. The speed of these circuits, which are electronically controlled, is usually the limiting factor on the bit-rate. The difficulty of connection and high-cost of associated circuitry result in optical fibres being used only in very high bit-rate communication. There is considerable current debate as to whether optics will ever completely replace electronic technology. In addition, good phase control of an optical signal is extremely difficult. Optical communications are forced to use the comparatively crude method of ASK modulation.

Optical fibre is a waveguide. The fibre (in its simplest form) consists of a core of glass of one refractive index, and a cladding of a slightly lower refractive index (Figure gif). The fibre is then surrounded by a refractive sheath. Typical fibre dimensions are to diameter.

Figure:   The basic structure of a fibre optic waveguide

In simple terms, the action of a waveguide can be partially understood by considering the rays down the fibre. A light-wave entering the fibre is either refracted into the cladding, and attenuated, or is totally internally reflected at the core/cladding boundary. In this manner it travels along the length of the fibre. The maximum angle at which it may enter the guide and travel by total internal reflection is termed the acceptance angle (Figure gif). It is also possible for the wave to follow a helical path down the guide. These rays are called skew-rays.

Figure:   Waveguide action of an optical fibre

However, this view is too simple to explain all features of waveguide behaviour. In fact, it is not possible for the wave to take any ray down the guide. Only certain rays can be taken. These rays are called modes. For any particular frequency, there is a different ray. The modal action of a waveguide is a consequence of the wave nature of the radiation. A mono-mode fibre is a fibre that only has one acceptable ray-path per frequency. A multi-mode fibre has a number of possible rays that light of a particular frequency may take.

The attenuation of light in the guide has a number of sources. Absorption of light occurs in the glass and this decreases with frequency. Scattering of light from internal imperfections within the glass -- Rayleigh scattering -- increases with frequency. Waveguide imperfections account for low-level loss that is approximately constant with wavelength. Bending the waveguide changes the local angle of total internal reflection and loss increases through the walls. A combination of these effects results in a minimum absorption of about to in the to wavelength region. It is these wavelengths that are used for transmission. (See Figure gif.)

Figure:   Sketch graph showing contributions to net spectral loss is a glass core

In addition to attenuation, optical waveguides also suffer from dispersion. The dispersion has two sources. Due to the modal behaviour, a waveguide is an intrinsically dispersive device. Put simply, rays of different frequency travel on different paths having different lengths. Because the different frequencies travel different lengths they take different times. In addition to the waveguide dispersion, however, is the material dispersion. Glass is an intrinsically dispersive media. In single mode fibres the material dispersion dominates the waveguide dispersion.

The bandwidth of optical fibres is dominated by the dispersion. In fact, the bandwidth of individual fibres is actually much the same as high quality co-axial cable. It is ironic that the principle justification for optical communication, very large bandwidth, has not in practise been realised. However, it is possible to lay many hundreds of optical fibres in the same cable cross-section as a single co-axial cable.

Fibre optic cable is available in three basic forms:

  1. Stepped-index fibre. In this type of fibre, the core has a uniform refractive index throughout. This generally has a core diameter of to . This is a multi-mode fibre. (Figure gif.)

    Figure:   Stepped-index fibre

  2. Graded-index fibre. In this type of fibre, the core has a refractive index that gradually decreases as the distance from the centre of the fibre increases. This generally has a core diameter of . This is a multi-mode fibre. (Figure gif.)

    Figure:   Graded-index fibre

  3. Mono-mode fibre. As the name suggests, the distinguishing characteristic of this fibre is that allows only a single ray path. The radius of the core of this type of fibre is much less than that of the other two, however it does have a uniform refractive index. (Figure gif.)

    Figure:   Mono-mode fibre

From, 1 to 3, we find that the cost of production increases, the complexity of transmitter and receiver increases, while the dispersion decreases. This latter property change means that the mono-fibre also has the potential to provide greater bandwidth. As it becomes cheaper to produce mono-mode fibre technology, we will see an increased use of this type of optical fibre. Figure gif gives typical operational information for a mono-mode fibre.

Figure:   Operational information for a mono-mode fibre

next up previous
Next: The electromagnetic spectrum; Up: Communication channels Previous: Transmission lines

Saleem Bhatti
Tue Mar 7 14:17:59 GMT 1995