ther forms of light communications has been the dispersion of light in the air and requirement of a line-of-sight between the transmitter and the receiver. A number of scientists contributed to the development of an optical cable within which light could be confined and transmitted to remote locations. These were based on the fact that light will be reflected within a medium if the incident angle is smaller than a certain value as shown in Figure 9.20. Early optical fibres however were hampered by impurities within the glass medium which absorbed or reflected light, leading to large signal loss. Scientists at Corning, a glass manufac- turing company, developed techniques that reduced signal loss in fibres to 17 dB/km in 1970. Further improvements have reduced path loss to less than 0.5 dB/km. A glass fibre structure is shown in Figure 9.21. The glass core is very thin and in the order of several micrometres. It is covered by several layers of protection made of plastic to ensure that the core is not broken. The resulting strand of fibre is less than 0.5 mm thick. Broadband Telecommunications Technologies and Management Cladding Acceptance Cladding Figure 9.20 Diagram of how light is confined within a glass fibre Jacket 400 um Buffer 250 um Cladding 125 um Figure 9.21 Structure of a glass fibre [16] Because of its very small diameter, special devices are needed to launch coherent, information modulated light into the fibre core. The light source needs to be monochromatic, and in many cases also coherent to ensure best transmission performance is obtained. A typical silica glass fibre exhibits different path losses at different wavelengths. Path loss is minimum in three regions of light 'colour' with wavelengths of 1.2, 1.3 and 1.5 um with losses of less than 1 dB/km. The equivalent bandwidth for signal transmission at these wavelengths is very wide: for example a system operating around the 1500 nm wavelength can use the follow- ing channel bandwidth: Assume upper band = 1530 nm, or 196078 GHz Assume lower band = 