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. 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 is then surrounded by a refractive sheath. In simple terms, the action of a waveguide can be partially understood by considering the rays down the fibre. In this manner it travels along the length of the fibre. It is also possible for the wave to follow a helical path down the guide. However, this view is too simple to explain all features of waveguide behaviour.