--- Fibre Optic Transmission ---

For the purposes of this article the following conventions have been used. Fibre Optics is the technology of transmitting data along cables that consist of Optical Fibres.

--- Advantages Of Fibre Optics ---

In the field of communications in general, fibre optic technology has made tremendous advances. Not many years ago every connection had to be made in a sterile, strictly controlled environment. The cost per joint was enormous in relation to a commercial project. This has now changed and fibre optic links are now commercially viable. In fact, there are now many developments that would not have been possible without fibre optic technology. Optical fibre cables are replacing copper in many applications. Optical fibres are much smaller and lighter than copper, therefore easier and cheaper to install in long runs.

In the field of communications in general, fibre optic technology has made tremendous advances. Not many years ago every connection had to be made in a sterile, strictly controlled environment. The cost per joint was enormous in relation to a commercial project. This has now changed and fibre optic links are now commercially viable. In fact, there are now many developments that would not have been possible without fibre optic technology. Optical fibre cables are replacing copper in many applications. Optical fibres are much smaller and lighter than copper, therefore easier and cheaper to install in long runs.

Attenuation is a term that is frequently used in connection with losses in fibre optics. It is simply a method of describing transmission losses along a cable. Attenuate means to weaken or dilute, therefore the lower the attenuation the less the reduction in signal strength and quality. In relation to CCTV this is one of the main reasons for using fibre optic transmission. Video signals can now be transmitted without being boosted over previously unthinkable distances. Twenty to thirty kilometres over one continuous fibre is quite feasible.

--- Wavelength, Frequency And Bandwidth ---

The electromagnetic spectrum is reproduced below because fibre optic transmission uses light from a particular part of the 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 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 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.

The frequencies used in fibre optic transmissions are between 850 Nm and 1550 Nm.

--- Transmission By Light ---

An optical fibre is a rod of the finest purity glass that technology can produce. The part of the light spectrum that functions best with optical fibres is the infrared frequency. Coincidentally, this is the same range that is used for infrared illumination. 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 focused onto the end of the optical fibre. The lightwaves travel along the fibre to the receiving end. Here the pulses are converted back into electrical pulses by a piece of electronic equipment, strangely enough called a converter. Converters also function more efficiently when dealing with infrared light.

--- The Optical Fibre ---

There is one manufacturer that produces a fibre optic transmitter that will fit directly onto the camera. The unit only measures 50mmx30mmx30mm yet can transmit up to three kilometres without the need for set up or repeaters.

--- The Optical Fibre ---

An optical fibre is a solid rod of glass, finer than a strand of human hair. Even so, the fibre is extremely flexible. Ordinary glass would lose a signal within a few metres due to impurities scattering the light. The glass used in fibre optics is so pure that a solid block 35 Km thick would appear as pristine and clear as a window pane. The fibres are produced in extremely exacting manufacturing conditions from pure silica, which is a type of common sand.

--- Transmission Losses In Fibre ---

Signal loss or attenuation in coaxial and twisted pair is usually given as so many dB per metre or per 100 metres. With optical fibre it is given as dB per kilometre. A typical value for single-mode fibre would be 0.5dB per kilometre, which is 0.05 dB per 100 metres. The figure given for a typical coaxial cable, URM 79, is 3.31 dB per 100 metres. For instance, for a 6 dB loss (50%) the coaxial cable run would be 181 metres. For the same loss the optical fibre run would be 12 kilometres. This example is over simplified but it explains the significant advantage of fibre optic technology. The typical weight of a single fibre cable is about 10 Kg per kilometre. Again to reflect the different technology, coaxial cable is provided on drums of so many hundred metres. Optical fibre cable is provided on drums of so many kilometres.

--- Multiple Transmissions ---

Diagram 17 illustrated a simple one to one connection through an optical fibre link. There will be many occasions when it is required to transmit signals from more than one camera. One method would be to use a multiplexer at each end of a single link. The disadvantage of this method is that the pictures are not truly multiplexed. They are sent as a stream of consecutive frames from all the cameras. They can then only be decoded by the same type of multiplexer as the one encoding at the transmitting end. The ability to use the individual pictures is very restricted.

--- Transmission By Multicore Optical Fibres ---

There are two main methods of transmitting multiple live pictures by fibre optic technology. The first and most obvious is to use multicore cable. Here again the difference in technology is apparent. The outside diameter of a sixteen-fibre cable suitable for running in a duct is only 10 mm. It only weighs 70 Kg per kilometre. For these reasons running multiple video signals is very much easier using fibre optics than conventional coaxial. This is apart from the greater transmission distances possible. There are applications where the cable runs are within the capacity of coaxial cables. However, running sixteen coaxial cables is time consuming and takes up a lot of space in trunking and in ducts. This is especially so if the route is tortuous and difficult to access. A single, light, optical fibre cable may offer dividends in overall cost.

--- Transmission By Multiplexed Video ---

Mention was made earlier about the wide bandwidth available with frequencies in the infrared part of the spectrum. It is not important to understand the meaning of bandwidth to appreciate the application. The following example illustrates how advantage can be taken of this feature. A technique known as 'frequency division multiplexing' is employed to transmit multiple channels of video along a single optical fibre. This is achieved by allocating an individual carrier frequency to each separate video channel. The signal, having been modulated, occupies approximately 35 MHz and carriers are spaced 40 MHz apart. Diagram 20 shows the concept of frequency division multiplexing.

At the receiving end of the system demodulators are tuned to each of the individual carrier frequencies. These detect and recreate the original waveform of the video signal.

The big advantage of this system is that each individual video signal is recreated at the receiving end. These then could be shown on separate monitors or be connected into a matrix switching system and combined with other systems. It is possible to add audio and data channels into the combiner and transmit these over the same single fibre. Transmission distances of up to 35 kilometres are achievable along the one fibre without the need for repeaters. Distances of 50 - 60 kilometres are possible using special equipment.

If telemetry signals are required for the remote control of equipment then one additional fibre will be required per group of receivers. The telemetry signal will be transmitted through a separate optical transmitter and receiver. The type of telemetry considered should be discussed with the optical equipment supplier to ensure compatibility of components.

It was said that the optical transmitters use infrared wavelengths between 850 Nm and 1550 Nm. The frequency division multiplexer can be further extended by making use of different wavelengths of light. This is called 'wavelength division multiplexing' and follows similar principles.

It was said that the optical transmitters use infrared wavelengths between 850 Nm and 1550 Nm. The frequency division multiplexer can be further extended by making use of different wavelengths of light. This is called 'wavelength division multiplexing' and follows similar principles.