Sample Chapter: 17 Transmission by fibre optics
From CCTV Information
Sample chapter from Mike Constant's book: The Principles & Practice of CCTV
Transmission by Fibre Optics
Principals of fibre optic transmission
Most people are familiar with the everyday use of light, X-rays, radio waves, microwaves, and Radar. All of these are actually examples of electromagnetic radiation, which is characterised by a radiation wavelength or oscillation frequency. Diagram 17.1 shows the electromagnetic spectrum with application areas identified. The 400 - 750 nm region of the spectrum is the region of visible light; this region is expanded in the lower part. The area of interest for fibre optic transmission extends from the red region of the spectrum out into the wavelengths much longer than those visible to the human eye, the infrared. Specific wavelengths used have been driven by the requirements of the fibre technology and by source and detector technologies. Particular wavelengths used are nominally 780nm, 850nm, 1310nm, and 1550nm.
Diagram 17. 1 The electromagnetic spectrum
The different parts of the spectrum have previously been described in terms of the wavelength. An alternative measurement is the frequency of the part being considered. Frequency is the number of crests of a wave that move past a given point in a given unit of time. The most common unit of frequency is the hertz (Hz), corresponding to one cycle per second. The frequency of a wave can be calculated by dividing the speed of the wave by the wavelength. Thus, in the electromagnetic spectrum, the wavelengths decrease as the frequencies increase, and vice versa.
For example, the wavelength of infrared light is 850 nm; the equivalent frequency is 3.5 x 1014Hz.
Diagram 17. 2 Bandwidth at Different Frequencies
Different frequencies have different bandwidths and the higher the frequency the wider is the bandwidth. The wider the bandwidth then the more information can be carried. Frequencies in the visible part of the spectrum offer a wider bandwidth, therefore they provide more space for the multiplicity of TV signals and reams of data that need to be transmitted.
Transmission by Light
In fibre optics, messages whether data or video are first converted from electrical impulses into pulses of light. This function is performed by a minute device that incorporates a laser chip or an LED (light emitting diode). The infrared light is switched on and off at incredibly high speeds, thereby creating the stream of light pulses. These are then focussed onto the end of the optical fibre. The lightwaves travel along the fibre to the receiving end. Here the light pulses are converted back into electrical pulses by a photodiode or avalanche photodiode.
Diagram 17. 3 Basics of Fibre Optic Transmission
Optical Fibre Structure and "Light Guiding"
An optical fibre is a complex strand of silica glass. A cross section of a typical fibre is shown in Diagram 17.4.
Very small units of length are measured in 'microns'. One micron is one millionth of a Metre, therefore, I micron is 0.001 mm and 125 microns is 0.125 mm.
Diagram 17. 4 Construction of single optical fibre