The 1960 invention of the laser (Light Amplification by Stimulated Emission of Radiation) completely revolutionized communication technology. The laser, which is a highly coherent source of light waves, can be used as an enormously high capacity carrier wave for information carrying signals (voice, data or video) transmitted through an optical wave guide, such as an optical fibre.
The basic principle involved in all long distance communication systems is multiplexing - simultaneous transmission of different messages over the same pathways.
Consider transmission of an individual human voice. The frequency band required for transmitting human voice extends from 200 Hz to 4000 Hz, i.e., the information contained in this frequency band can be transmitted in any band whose width is 3800 Hz, regardless of the region of the spectrum in which it is located. Higher frequency regions have far more room for communication channels, and hence, have a much greater potential capacity than the lower frequencies.
The frequency corresponding to the visible optical region at 600 nm is 5 X 1014 Hz, while that at a wavelength of 6 cm is 5 x 109 Hz. Thus, the communication capacity of visible light in an optical fibre is about 100,000 times greater than that of a typical microwave in a metallic conductor.
The most extensively used optical wave guide is the step-index optical fibre that has a cylindrical central glass or plastic core (of refractive index n1) and a cladding of the same material but slightly (about 1%) lower refractive index (n2). There is usually an outer coating of a plastic material to protect the fibre from the physical environment.
When light from the core (n1) is incident on the interface of the cladding (n2 < n1), the critical angle of incidence for total internal reflection is given by
θc = sin−1(n2/n1)
Thus in an optical fibre, the light ray is made to enter the core such that it hits the core-cladding interface at an angle θ1 > θc. The ray then gets guided through the core by repeated total internal reflections at the upper and lower core-cladding interfaces.
The entire energy of the wave in the core is reflected back, but there is a power flow along the interface in the cladding. Such a wave is called an evanescent wave, and is extensively used in integrated optics for the coupling the energy of a laser beam into a thin film wave guide.