General Ideas of Optical Fibers
The transmission of light via a dielectric waveguide was first proposed and investigated at the beginning of the twentieth century. However, a transparent dielectric rod, typically of silica glass with a refractive index of around1.5, surrounded by air, proved to be an impractical waveguide due to its unsupported structure. Proposals for a clad dielectric rod was made in the mid 1950s inorder to overcome these problems. This structure is illustrated down below.
The figure shows a transparent core with a refractive index n1 surrounded by a transparent cladding of slightly lower refractive index n2. The cladding supports the waveguide structure and substantially reduces the radiation loss into the surrounding air. In essence, the light energy travels in both the core and cladding allowing the associated fields to decay to a negligible value at the cladding-air interface.
The invention of clad waveguide structure led to the first serious proposals by Kao and Hockham in 1966 to utilize optical fibers as a communication medium, even though they had losses in excess of 1000 dB km-1. These proposals stimulated tremendous efforts to reduce the attenuation by purification of the materials. This has resulted in improved conventional glass refining techniques giving fibers with losses of around 4.2 dB km-1. Progress in glass refining processes such as depositing vapor phase reagents to form silica has allowed fibers with losses well below 0.1 dB km-1 to be fabricated.
Most of this work was focused on the 0.8 - 0.9 micrometer wavelength band because the first generation optical sources fabricated from gallium aluminium arsenide alloys operated in this region. However, as silica fibers were studied in further detail it became apparent that transmission at longer wavelengths would result in lower losses and reduced signal dispersion. This produced a shift in optical fiber source and detector technology in order to provide operation at these longer wavelengths. Hence at longer wavelengths, especially around 1.55 micrometers, fibers with losses as low as 0.01 dB km-1 have been reported.
In order to appreciate the transmission mechanism of optical fibers with dimensions approximating to those of a human hair, it is necessary to consider the optical wave guiding of a cylindrical glass fiber. Such a fiber act as an open optical waveguide, which may be analyzed utilizing simple ray theory. However the concept of geometric optics are not sufficient when considering all types of optical fibers, and electromagnetic mode theory must be used to give a complete picture.
Types of Fiber Materials
A broad distinction may be made between glasses based on pure SiO2 and those derived from low softeing point glasses such as the sodium borosilicates, sodium calcium silicates and lead silicates. For convenience, we shall refer to this as silica fibers and glass fibers respectively. An obvious requirement of the material used is that it must be possible to vary the refractive index. Pure silica has a refractive index of 1.45 at 1 micrometer. B2O3 can be used to lower the refractive index, whilst other additives such as GeO2 raise it. Thus a typical fiber might consist of an SiO2 : GeO2 core with a pure SiO2 cladding. Glass fibers can be made with a relatively wide range of refractive indices, but control of the impurity content is more difficult than with silica.
Other types of fibers are possible using plastics. For example, fibers are available with silica cores and plastic cladding. This plastic coated silica (PCS) fibers are easy to manufacture; the fiber core may simply be drawn through a bath of a suitable polymer which is subsequently cured by heating to a higher temperature to provide a solid cladding. This process readily lends itself to the production of step index fibers with large core diameters where very little of the energy is carried in the cladding. Such fibers are attractive for medium distance, moderate bandwidth communication systems where cost is a major consideration. Typical losses are in the order of 10 dB km-1 .
Fiber Optic Communication
Communications using an optical carrier wave guided along a glass fiber has a number of extremely attractive features, several of which were apparent when the technique was originally conceived. Optical fiber communication has following merits.
- Enormous potential bandwidth
- Small size and weight
- Electrical isolation
- Immunity to interference and cross talk
- Signal security
- Low transmission losses
- Ruggedness and flexibility
- System reliability and ease of maintenance
- Potential low cost
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