Transmission of Video Signals by Fibre Optics
Principles 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.
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 1014 Hz.
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 above 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.
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 16.4.
Very small units of length are measured in ‘microns’. One micron is one millionth of a metre, therefore, 1 micron is 0.001 mm and 125 microns is 0.125 mm.
The optical fibre is made from a rod of highly purified silica called a “pre-form”. The pre-form is heated and drawn out into a thin fibre using highly specialised and accurate equipment. As the fibre is drawn, it is coated with a protective polymer layer known as the primary coating. At this stage the coated fibre is approximately 0.25 mm diameter and is flexible enough to be coiled on drums with a bend radius of not less than 5 cm. In most fibres in use today the diameter of the glass fibre itself is 125 microns/ 0.125 mm. This primary coated fibre is then used as the building block for assembly into optical fibre cable that provides the ruggedisation needed for everyday use.
The optical fibre itself has internal structure with the refractive index of the fibre varying across its diameter with all fibres having a lower refractive index on the surface than at the centre of the fibre. This variation in refractive index across the fibre diameter is the key to the transmission of light by the fibre. Remembering school physics experiments, when light passes from a high to low refractive index media e.g. glass to air, some of the light ray is reflected and some is refracted out of the high refractive index media. As the angle of the light ray to the surface gets shallower, there comes a point where all of the light is reflected and no light is refracted out of the media. This angle (to the normal) is called the Critical Angle above which all light is reflected; optical fibre transmission uses this effect to transmit light along the fibre.
In diagram 17.5, the optical fibre structure is assumed to consist of a high refractive index glass core surrounded by a low refractive index glass cladding. Light rays are incident on the fibre end from a light source entering the fibre core over a range of incident angles. Once in the fibre these rays can be considered to be travelling in straight lines until they meet a refractive index discontinuity. At this point, some of the ray is reflected back into the fibre core and the rest is refracted out of the core into the cladding glass. The reflected light ray then transits the fibre core until another reflection occurs and the refracted ray hits the cladding glass/protective polymer cladding interface and is absorbed or dispersed. As this is concerned with light propagation down the fibre length it is clear that the reflected ray is the one that we require for signal transmission, with the refracted ray simply reducing the transmitted light signal intensity.
Consider a continuum of light rays in the fibre core covering all possible angles of incidence to the core/cladding discontinuity, then it can be seen that all light rays with an angle of incidence above the critical angle will be reflected back into the fibre core. This is known as “total internal reflection”. Those rays with an angle of incidence below the critical angle will be partly reflected and partly refracted in the manner explained above. The light rays transit along the fibre by being reflected at each refractive index change that they encounter; in effect the rays bounce off of the sides of the fibre core.
After multiple reflections the rays with angles of incidence below the critical angle will have been reduced in intensity by refraction losses and do not contribute to the light, and hence signal, transmission process. In contrast, the rays with angles of incidence above the critical angle will not be reduced in intensity by refraction and it is these rays that enable fibre optic transmission to work. As the angle of incidence is measured with respect to the normal to the relevant surface it can be seen that the fibre could be bent and twisted and still allow light to be transmitted along its length. This ability of optical fibre to guide light along a non-linear path, just like and electrical conductor, is essential for its use in real world applications.
This range of rays may be traced back to their original coupling to the fibre core and we find that the transmitted rays are contained in a cone of angles as shown in diagram 17.5. In defining optical fibre parameters this acceptance cone is characterised by the cone half angle and the Sine of this half angle is known as the fibre Numerical Aperture – N.A.
This article is an extract from chapter 17 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.