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CCTV Transmission Using Fibre Optics

1 INTRODUCTION The use of fibre optic transmission is now commonplace in telecommunications, data communications and broadcast quality television signal applications. In contrast the use of optical fibre as the transmission media in CCTV security and surveillance applications is a relatively recent development fuelled in the 1990`s by the need for the installation of extensive CCTV systems to combat crime, vandalism and terrorism. Conventionally cameras are linked to monitors over copper cable links using the lowest cost components available. As system size has increased the distance between cameras and monitors has also increased resulting in an unacceptable degradation of received video signal quality. i.e. received picture quality, for link distances over 100 - 150 m. This has meant that the use of optical fibre transmission has had to be considered even in this most cost conscious of applications. In these sections we will attempt to de-mystify fibre optic transmission as applied to CCTV system use. We will start by outlining why fibre optics should be used, go on to consider the basic elements of a fibre optic system and installation practice and finally outline the technology to extend CCTV systems from essentially local installations to extensive, distributed multi-channel signal transmission systems.

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2 WHY USE FIBRE OPTICS? The principle reasons for using optical fibre as the transmission media in CCTV applications are: • The maintenance of picture quality and control data integrity over extended

distances: This is the major reason for using fibre optics which have superior signal amplitude loss characteristics than copper cable. Typically co-axial cable attenuation at a signal frequency of 5 MHz can be 20 dB/km. In comparison fibre attenuation is between 0.3 and 3 dB/km meaning that fibre optic link distances of 60 km+ can be achieved, depending on the precise details of the application. In addition this low fibre signal attenuation is achieved over a very wide signal frequency range so that optical fibre can be used for the transmission of multiple video signals over long distances.

• Immunity to electromagnetic interference: Optical fibre transmits signals as light pulses rather than electrical pulses. This light transmission is unaffected by the presence of electro-magnetic fields. As a consequence optical fibre transmission can be used in applications where links are routed near electrical conductors and electrical machines. This includes applications such as railways, tramways, power generation and vehicle manufacture with welding machinery. In addition the fibre cable usually has a metal free construction so that there are no ground loop problems between terminal equipment and the cable will not transmit lightning pulses. This elimination of ground loops makes fibre cable the media of choice for inter building links of whatever distance.

• Security of Information and Operational Safety Unlike copper cables fibre cables do not radiate any signals as a consequence fibre cables are virtually immune from “tapping” and so the signal content is difficult to access for unauthorised parties. As there are no emissions from optical fibre cable there is no risk that a fibre installation will act as a ignition source. This means that fibre can be used in explosive atmospheres such as chemical and petro-chemical sites providing a truly “Intrinsically Safe” transmission path. Note however, that this Intrinsic Safety, would not extend to the electro-optic termination modems which would need to be safety certified and protected the same as any other electrical equipment.

• Efficient use of duct space. Optical fibre itself is very small, each glass fibre being only 0.125mm diameter. Protective sheathing is then applied in stages, depending on the application area, to make up the fibre into a usable cable. Typically resulting cable would have a diameter of 3mm for a single fibre core patchlead or 8mm for a 8 fibre cable suitable for internal or external use. In contrast 75 Ohm CT100 coaxial copper cable has a diameter of 6.5 mm. It can therefore be seen that the small size of fibre cable gives significant savings over copper where installation space is in short supply or where duct space is limited. Along with the small fibre cable size comes a weight saving both of which give savings in storage and transportation costs prior to installation.

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• Multi-channel capability and “Future Proofing”.

While most CCTV fibres today will be used to transmit one video signal and perhaps a control data signal, the user may wish to upgrade the system to support more camera and control channels. Any glass optical fibre used today is able to transmit multiple optical channels either by using different optical carrier “colours” i.e. wavelength division multiplexing or by increasing the signal frequency using electrical multiplexing techniques. The transmission media is hence “future proofed” and the link will need only additional terminal equipment to expand the link capacity.

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3 COMPONENTS OF A FIBRE OPTIC CCTV TRANSMISSION SYSTEM The sole purpose of the fibre optic link in a CCTV fibre optic transmission system is to transfer electrical signals between two remotely separated points, A and B, with no degradation in the transmitted signal quality. In this way the fibre optic link becomes transparent to the user. An analogous situation is with a telephone call where you want to be able to talk to another person anywhere as though they were standing next to you. The basic components of a CCTV fibre optic transmission system are as follows: • Electrical to Optical Converter (Transmitter) at the camera end of the link.

This unit takes the analogue 1 v peak to peak signal from the surveillance camera and converts it into a light signal that varies in proportion to the camera output signal. The light signal is generated by an LED (light emitting diode) or laser transmitter which is designed to couple a maximum of the generated light into an optical fibre.

• The optical transmission fibre and fibre cable. The optical fibre guides the light from the LED or laser transmitter with a minimum of loss to the monitor or matrix controller end of the link. The optical fibre itself is protected by a variety of sheathing materials to provide a cable construction appropriate to the specific application. The fibre cable is connected to the terminal equipment using de-mountable screw or bayonet fixing connectors.

• Optical to Electrical Converter (Receiver) at the monitor end of the link. This unit takes the optical signal from the optical fibre and converts it into an analogue electrical signal that is compatible with the monitor input requirements. The light to electrical conversion is carried out by a semiconductor detector which is called a photodiode, or an avalanche photodiode. Subsequent electronic circuitry regenerates the output signal. Products from the better quality manufacturers compensate for optical fibre losses and transmitter output intensity variation with time and temperature by providing automatic gain control to give a standard 1 v peak to peak output format as generated at the camera output.

• Control data and audio connections.

Cameras in CCTV installations are either fixed, viewing a specific scene, or movable, so that different scenes can be viewed under the direction of the operator who would be sited in the remote control room. In the case of fixed cameras then the fibre optic link is required to transmit video only information from the camera to monitor, this requires only a single fibre link for each camera to monitor path. In the case of a movable camera then a return signal must be provided from the control room to the camera usually over a second optical fibre. It is usual for these return control links to provide remote control of the camera PTZ - pan, tilt and zoom functions plus camera enclosure wash/wipe activation. If camera control is used then the fibre optic link interface electronics must be compatible with the protocols used by the controller manufacturer. These functions are transmitted over the return fibre link using a standard digital transmission format such as RS232, RS485/422, 20 mA current loop and most recently Echelon Lonworks FTT10A. In addition some controller manufacturers require a return data channel from the camera to confirm camera movement. This return data is usually

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encoded by the camera optical transmitter electronics and sent over the same fibre as the video signal. Help point and door entry installations require the transmission of two-way audio signals over the fibre link. Again optical transmitter and receiver units are available to provide this facility in addition to the video and control data links all over the same two fibres. It is also possible to provide all of these video, data and audio transmission functions over one fibre using different wavelength (colour) lights sources to transmit light in each direction. This technique is known as wavelength division multiplexing; it maximises the use of installed fibre cores but at the expense of more costly transmitters and receivers.

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4 OPTICAL FIBRE BASICS We are all 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. Fig 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 of Fig 1. The area of interest for fibre optic transmission extends from the red region of the spectrum out into the wavelengths just longer than those visible to the human eye , the infra-red. 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. Opical Fibre Structure and “Light Guiding”.

The optical fibre is made from a rod of highly purified silica called a “preform”. The preform 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 0.25mm 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/ 0.25 mm. This primary coated fibre is then used as the building block for assembly into optical fibre cable, which provides the ruggedisation needed for everyday use.

Wavelength

Hz 0 -Hz 1KHz

100Km

10K Hz

10Km

LW MW VHF UHF

1m

Microwave

1cm

10MHz 1GHz 10GHz

1mm 100um 10um 1um 100nm 10n 1nm 100pm 10pm

10 x 16 10 x 19 Hz

Low Frequency

DC

RadioTelevision

THE ELECTROMAGNETIC SPECTRUM

1.6um 1.5um 1.4um 1.3um 1.2um 1.1um 1um 900nm 800nm 700nm 600nm 500nm 400nm

Optical Fibre Transmission Range Visible Light

Infra - Red Ultra - Violet

10THz

Satellite

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An optical fibre is a complex strand of silica glass. A cross section of a typical fibre is shown in Fig 2. It can be seen from Fig 2 that 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 our 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 Fig 2 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 we are 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. If we 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 above 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

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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. We can trace this range of rays 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 Fig 2. 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.

Multi-mode and Single Mode Fibre

Whilst we can now see how light is transmitted along a fibre core there is another complication due to the wave nature of light. The result is that only certain transmission paths, or angles of incidence, can be supported and each path is called a “mode”. The quantity of modes available depends on the precise details of the fibre construction. Broadly we can classify fibre types into “multi-mode” and “single mode” fibres. Referring to Fig 2, today the vast majority of fibre produced has an outer diameter (of the glass fibre NOT// the polymer primary coating) of 125 microns with a core diameter of 50 or 62.5 microns for multi-mode fibre or 8 microns for single mode fibre. In addition the core of a multi-mode fibre is usually made up from a range of glass compositions to give a parabolic distribution of refractive index across the fibre core diameter, the highest refractive index is on the fibre axis. These graded index fibres are used to improve the transmission speed i.e. bandwidth, characteristics of the fibre. In contrast single mode fibre is almost universally made up from core material with a single refractive index resulting in a structure called “step index” for obvious reasons. The bandwidth capacity single mode fibre is not affected by the use of a step index geometry because only one transmission mode is supported. Early multi-mode fibre was made in step index form because this structure was the simplest to manufacture and gave higher fibre N.A. values thus allowing more efficient coupling of light from LED or laser light sources. Advances in LED and laser technology plus higher fibre bandwidth requirements, driven by data communications applications, have resulted in graded index structures now being the predominant fibre type used. The transmission characteristics of multi-mode fibres make them best suited for the majority of local site CCTV applications where transmission distances of up to 4 km are found. Single mode fibres are best suited to high speed, long distance applications such as telecommunications or CCTV in transportation applications.

Fibre Transmission Losses Optical fibre transmission characteristics are different for different light wavelengths. It is therefore necessary to choose the optimum wavelength for the required application. However, as a user of fibre optics in CCTV applications you will not need to worry about this choice because the terminal equipment manufacturer will have sorted this out for you. The two fibre parameters affected by wavelength are attenuation and bandwidth.

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Fibre Attenuation and Scattering

Light is transmitted along the core of an optical fibre by total internal reflection at the core cladding interfaces. Whilst these reflections are effectively loss free the interaction of the glass material and the light results in the intensity of the transmitted light being reduced as the transmission distance increases. This is equivalent to signal attenuation in a copper cable. These losses arise because of absorption of light by impurities in the fibre core and because of scattering of the light by core imperfections. The major impurity in silica based fibres has been hydroxyl (–OH) ions. Today fibre preforms are made using semiconductor type preparation techniques and achieving semiconductor levels of purity. As a result attenuation effects have been reduced to virtually negligible levels and scattering has become a much more important loss mechanism. These optical attenuation mechanisms are wavelength dependent. Fig 3 shows a typical optical fibre attenuation with wavelength. It can be seen that attenuation reduces as the wavelength increases. It can also be seen that there is a small bump in the curve at around 1400 nm/1.4 microns, this has been included in this particular graph because it is indicative of the effect of –OH impurities. In the early days of fibre development –OH impurity levels were high and the resulting fibre absorption showed a minimum at approximately 850 – 900 nm (0.85 – 0.9 microns). As technology has improved this attenuation minimum has moved out to wavelength bands around 1310 nm and 1550 nm. These bands are known respectively as the “First”, “Second” and “Third” transmission windows. Table 1 summarises optical fibre transmission properties for single mode and multimode fibre. The CCTV equipment manufacturer will select and specify the window that their products work over. The application requirements, costs of the semiconductor sources and the detectors that are used in optical transmitters and receivers govern the choice of operational window.

OPTICAL FIBRE ATTENUATION AS A FUNCTION OF WAVELENGTH

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Fibre Bandwidth Whilst the signal speed with distance capacity of optical fibre is vastly in excess of that achievable over a copper cable it still has some limitations. This is due to effects known as modal dispersion, material dispersion and chromatic dispersion. These effects are caused by geometric effects in the optical waveguide and by the speed of light varying as a function of refractive index and wavelength. Note that single mode fibre bandwidth is much higher than multimode and is expressed in units of dispersion ps/nm.km vales of 3.5 and 18 at 1300 and 1550nm respectively are typical. Referring to Fig 2 it can be seen that in a multi-mode fibre different modes, or rays, travel down the fibre over different paths and that these paths are of different lengths. At a given wavelength the speed of light is constant, for a fixed refractive index. And so, a light pulse which starts at the input of the fibre and fills all the transmission modes will arrive at the far end of the fibre over a spread of times due to different modes taking different times to transit the fibre. Graded index fibre attempts to minimise these modal dispersion effects by reducing the transit time difference between modes because the speed of light is higher in lower refractive index media, that is the light speeds up in the regions furthest away from the centre of the fibre core. Single mode fibre eliminates these material and modal dispersion effects by using a step index structure and using a core size that only supports a single transmission mode. These transit time effects are also wavelength dependant. The effect is known as chromatic dispersion and its significance lies in the fact that both LED and laser sources emit light over a range of wavelengths. If we now consider the passage of a one mode along a fibre, and remember that the speed of light is wavelength dependant, then it can be seen that the different wavelengths of an instantaneous light pulse will arrive at the end of the fibre at different times. This effect again gives a pulse distortion and it is present in both single mode and multimode fibre, although of less significance. The effect is minimised by using very narrow line but very expensive laser sources. These dispersion effects all contribute to limit the information carrying capacity of a fibre by limiting the fibre bandwidth. This is expressed in units of MHz.km for multimode fibre and can be assumed to be proportional to fibre length. Typical values are given in Table 1.

Table 1 - Optical Fibre Parameters

Core diameter Microns

Numerical Aperture

Attenuation dB/km Bandwidth MHz. km

@ 850nm @ 1300nm @ 1550nm @ 850nm @ 1300nm @ 1550nm

8 0.11 N/A 0.5 0.3 - - - 50 0.20 3.0 1.2 N/A 400 600 N/A

62.5 0.275 3.5 1.5 N/A 160 500 N/A

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Fibre bandwidth limitations are much more significant in data communications and telecommunications systems than in CCTV systems. In a baseband CCTV link the video signal bandwidth will not need to exceed 10 MHz, with 5.5 MHz actually being sufficient. Taking a worst case of 850nm operation over 62.5/125 multimode fibre with a bandwidth of 160 MHz.km gives a bandwidth limited link length of 160/10 = 16 kms. At this wavelength, with an attenuation of 3.5 dB/km, this would give a link attenuation of 3.5 x 16 dB = 56 dB, which is far in excess of usual equipment loss budgets of 12 dB for an LED based system.

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5 FIBRE OPTIC CABLE

Optical fibre by itself is too fragile to be used in real applications in the same way that one would not use copper conductors without protection. In fact cabling technologies used for copper conductors and for optical fibres are very similar. The major difference is that optical fibre is slightly elastic and copper cable is inelastic and this difference must be allowed for in cable design. Effectively, optical cable design must ensure that any tension is born by the cable, and not by the optical fibre. This is usually achieved by including a Kevlar strength member in the optical cable design; Kevlar being a high strength polymer. Cabling is about protecting the fibres from damage in use and presenting the fibres in a way that enables ease of handling and ease of jointing. The precise design of cable used depends on the details of the application and there are as many options on optical fibre cable design as there are for copper cable design. Predominantly CCTV work is concerned with transmission cable types for outdoor use and indoor use. The main differentiation between these types is that outdoor cable will be used in environments where it will come into contact with, or be immersed in, moisture whereas indoor cable is not designed to be water ingress proof. In construction the difference is that indoor cable is built up from layers of material tightly surrounding the optical fibre. This type of cable is called tight jacket cable. In contrast, in outdoor cable the optical fibre is contained in a plastic tube, which is often filled with a water repelling gel. The fibre can move within this tube. This type of construction is called loose tube cable. Cables are made up from multiple optical fibres in physical arrangements dependant on the intended application. Fibre counts in cables are commonly 1, 2, 4, 8, 12, 16, 24, 32 and up to 96 or beyond. Conventionally cables are circular but flat cables for under carpet applications and ribbon cables for automated fibre jointing are available. As a starting point in cable construction, the primary coated optical fibre is covered with a further plastic conformal coating, which has a diameter of 0.5 mm to 0.9 mm. This secondary coating is usually coloured so that colour coding of the fibre can be used in the cable construction. This secondary coated fibre looks very much like conventional plastic coated copper wire.

Figure 4

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Examples of typical tight jacket and loose tube cables are shown in Figs 4 and 5. In the tight jacket cable a Kevlar strength member is wound around the optical fibre core and a plastic sheath is moulded around this assembly. Typically for a single fibre cable (simplex cable) the overall diameter of the cable would be no more than 3 mm. In a 2 fibre cable (duplex cable) the fibres may be moulded in a circular or oval cross section cable or may be joined by a central web to give the so called “zip cord cable”. Higher fibre count cables use more complex geometry’s often with the fibres arranged around a central strength member core. In loose tube construction a basic gel filled tube may be used to accommodate up to 8 or 12 fibres and this tube then used as the building block for higher fibre count cables. In any structure these tubes are either arranged around a strength member for multi tube constructions or in the case of a single tube, low fibre count cable then, the strength member may be included in the tube or wound around it. The total assembly is then encased in an appropriate overall sheathing material. Both types of cable construction are completed with an over sheath construction which gives the required environmental protection. Plastic materials now used are usually LSOH materials, low smoke zero halogen, thus giving flame retardance as well as being environment friendly. It is also possible to surround the cable with a metallic tape or welded steel layer to give an armoured cable for protection against rodent and mechanical damage. Finally it is possible to include copper conductors in the cable construction to enable remote power feed along the cable whilst carrying signal traffic over the optical fibre. This hybrid construction does, of course, negate one of the advantages of fibre optics for CCTV systems; namely that fibres and cables are usually non-conducting so that they can be used to interconnect buildings where earth potentials may differ. Nor will dielectric cables conduct lighting strikes from a camera housing down to ground or control room equipment. Overall cable constructions are likely to be available that will be able to satisfy all environmental requirements of CCTV system installation whilst giving the optical fibre the required degree of environmental protection to ensure long life, fault free utilisation.

Figure 5

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6. Fibre Jointing and Cable Installation Techniques

In any fibre optic installation optical fibres will need to be joined either to each other or to terminal equipment. These joints can be either permanent or de-mountable depending on the circumstances. In making a joint between fibres, all techniques are concerned with aligning the core of one fibre to the core of another fibre. In this way there is a minimum of light lost when the optical signal passes between the fibre ends, and maintaining this alignment over the operational life of the joint becomes less problematical. In most CCTV applications, and indeed most other applications, the installer will be concerned with jointing similar fibre types i.e. 62.5/125 to 62.5/125 fibre. Today fibre production technology is so reproducible that in most circumstances fibre joints can be made which rely on aligning the fibre cladding glass on each side of the joint. However, in every case note that the joint is made with all plastic coatings stripped off of the glass fibre surface. This can be achieved with proprietary stripping tools, much like wire strippers, plus the use of chemical cleaners to ensure a clean, grease free surface. It is also necessary to cut the fibre using a proprietary fibre cleaving tool, so that a fibre end with a good surface finish is achieved. This cleaving is accomplished by scratching the fibre surface with a diamond scribe and then causing the fibre to break/cleave by applying a bending stress to the fibre. This operation is similar, but obviously much more precise, to scribing and breaking a sheet of glass. The operations to prepare a fibre for jointing are actually very straight-forward: - Cut cable to length. Strip cable sheathing back to fibre secondary coating. Remove fibre secondary and primary coating. Cleave fibre to length. Chemically clean bare fibre end. The bared fibre end is now ready for jointing using either permanent or de-mountable joints. In general permanent joints are used to join segments of cable that run over extended distances and de-mountable connections are used to join short segments of cable and terminal equipment.

Permanent Joints or Splices Typically a permanent joint is made when it is necessary to join long lengths of cable. This form joint is usually made by holding two bared fibre ends together in alignment grooves and then melting the joint area with an electric discharge. This technique is known as fusion splicing. Equipment for carrying out fusion splicing can be very expensive because precision handling of the fibre ends is required. This is particularly the case for single mode fibre where closer positional tolerance is required than for multi-mode fibre because of the smaller fibre core diameters. Fusion splicing gives the lowest connection loss of any jointing technique with joint losses as low as 0.1 dB being achievable with the best equipment. Following fusion splicing the welded joint has protective sheathing applied to give the joint mechanical stability. Typically a fusion splice would be used in jointing a multi-core cable in a cable run where longer cable lengths are required or where a cable is cut to breakout intermediate equipment access points.

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An alternative to fusion splicing is known as mechanical splicing. Here the two fibre ends are laid end to end in a precision V groove and the fibres align themselves to their outer diameter. An index matching fluid is put between the fibre ends to enhance the optical coupling between the fibres. The aligned fibres are then held in place with mechanical clamps or they are simply glued in place. As an example one manufacturers splice uses a V groove in a tubular format, the fibre ends are pushed in from each end of the tube and then when mated the ends are glued in place in the tube. These mechanical splices can achieve splice losses in the 0.25 - 0.5 dB range. Typically mechanical splices would be used to join pre-connected fibre tails to the transmission fibre with the joints being housed in a readily accessible jointing enclosure or splice tray close to the terminal equipment. De-mountable joints/Connectors De-mountable joints or connectors provide the method for attaching the transmission fibre to the terminal equipment so that the equipment may be installed or removed as required without breaking the fibre. This is the optical equivalent of the screw terminal electrical connector and in fact early connector designs were based on electrical connector designs. The majority of de-mountable optical connectors use precision ferrules to align the fibre ends. If we consider the case where we wish to join fibre to fibre, then the alignment process consists, simply, of putting the bared fibre end into a precision ferrule. i.e. A short tube which has a hole bored down its centre line, slightly larger diameter than the fibre cladding glass (and has an O/D of usually 2.5 mm) precisely centred with respect to the fibre hole. The fibre is glued into the ferrule, with a small length of fibre protruding from the ferrule end. The glue is cured in a small oven and the fibre end is then polished on an abrasive pad to be flush with the ferrule end. Today these ferrules are made from ceramic materials and it is virtually impossible to over-polish the fibre end. The polished ends of two fibred ferrules are then held together with a sprung metal alignment cage. A mechanical superstructure around the ferrules is then used to hold the alignment and enable de-mounting as required. These parts may hold the joint with either screw fittings or bayonet fittings. Spring loading in the connector shell ensures that the two ferrule ends are held tightly together but also ensure that the ends cannot be damaged by over-tightening. In this 2 fibre configuration the two halves of the connector fit into an “in-line” or “unitor” which holds the alignment sleeve. The in-line itself will have a mounting flange allowing it to be secured into whatever enclosure is being used. Common connector styles are SMA- screw fitting, STII- bayonet fitting and FC/PC -screw fitting. In CCTV and data communications applications the STII connector is predominant and is used for multi-mode fibre jointing. The SMA screw connector is now less common because of its worse connector performance. The FC/PC connector has come from telecommunications applications and is commonly used for 1300nm transmission wavelengths and for single mode fibre connection.

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Finally we need to provide a de-mountable connection to terminal equipment in order to connect the optical transmitters and receivers to the transmission fibre. The equipment manufacturer may have attached a fibre lead to their LED, laser or photodiode during manufacture. In this case the manufacturer will have terminated this flying lead with a connector. This connector, plus an in-line will be mounted on the terminal equipment in such a way that a connector on the transmission fibre can complete the linkage in the same way that we described fibre to fibre jointing above.

Alternatively the manufacturer will have assembled the optical component into a receptacle that has a half an in-line on one side and a holder for the LED/laser/detector package on the other. The terminated optical fibre is then simply attached to the other side in the usual way. In either case the equipment manufacturer will have ensured that the user needs only insert a terminated fibre into their equipment to complete the fibre link.

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7 System Implementations Fibre optics for CCTV applications are predominantly used in extended local installations linking cameras back to monitors with dedicated fibres for each link. A typical optical system layout is shown in Fig. 6. This example illustrates the main features of any fibre optic system, which are as follows: 1. The fibre optic link and its associated terminal equipment fit between the camera and

the associated monitor/controller and provide a transparent signal path i.e. the camera and controller do not know that the signals have been transmitted over fibre.

2. The camera output is a 1V peak to peak composite video signal. 3. Movable cameras have a telemetry receiver is mounted near to the camera movement

mechanism. This telemetry receiver connects to the system controller to provide control of the camera pan/tilt and zoom PTZ functions.

4. At the control end of the link camera selection and movement is looked after by the

system controller and video signal outputs from the controller are displayed on a local monitor(s).

5. Electrical to optical and optical to electrical converters provides the interfaces to the

optical fibre transmission fibre. 6. At the camera end of the link the E/O converter is usually a single channel unit

packaged in a small enclosure which can be conveniently mounted near to the camera or telemetry receiver. These E/O converters are not usually environmentally sealed and so need to be protected from the elements often mounting them in the telemetry receiver enclosure. In their most cost effective form a PTZ E/O converter will use two multimode fibres to give a uni-directional video connection plus a bi-directional control data

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channel. 7. As an alternative these control and video link functions can be carried over a single

fibre using optical transmission at two wavelengths, WDM - wavelength division multiplexing. These WDM links are more expensive than single wavelength links but they do save on fibre usage and they also can make the best use of a previously installed fibre infrastructure.

8. The E/O converter data interface must be compatible with that used by the system

controller; these are often non-standard. 9. Fixed cameras can use a miniature E/O transmitter, which can connect directly to the

camera BNC signal output. This link requires only one fibre. 10. The camera end E/O converter is connected to the transmission fibre through a patch

box. This patch box provides a point of termination for the transmission cable and so prevents strain and wear and tear being placed on the transmission cable when installing, servicing or moving the terminal equipment. Optical connections between the E/O converter and the patch box are made with duplex patchleads (which are short fibre cable lengths terminated at each end with an optical connector). The patch box will only be a relatively small enclosure because it will only need to provide connectivity for a few fibre cores.

11. At the control room end of the link fibres from a large number of cameras will be

concentrated. Equipment must therefore be packaged accordingly and most often this means the use of 19” rack mount units. E/O converters are manufactured in modular card format, which enables multiple video channels to be accommodated in a 19” cage. Typically one 3 U high rack can accept plug-in E/O converters for up to 30 video only channels or 10 video/data channels (or a mixture of both).

12. The fibre transmission cables are also handled in 19” rack enclosures because now we

will be organising many fibre cores. These enclosures are called patch panels and they again provide a physical buffer between the transmission cable and the terminal equipment. Here the cable will be bought into the rear of the patch panel via a compression gland and the fibre cores will be broken out into the secondary coated cores. These cores will then be terminated with connectors, which are then connected into in-line adaptors mounted through the front bulkhead of the patch panel enclosure. This termination may either be carried out by the direct attachment of connectors to the fibre tails or factory terminated connectors tails will be spliced to the transmission fibre cores. If splices are used then the splice enclosures will be mounted in clips on the patch panel base. Patchleads then connect the patch panel bulkhead connections to the E/O converter optical connections. Copper leads then complete the connections to the system controller and monitors.

As part of the cable installation the installer will have measured the installed cable loss, a function of position using a piece of test equipment called an OTDR (Optical Time Domain Reflectometer). This measurement serves to finger print the system and provides a point of reference for future system maintenance. It also provides the value of the end to end loss of each optical fibre used. The total loss must not exceed the optical margin specified by the equipment manufacturer, otherwise the transmitted picture quality may be impaired. In a correctly installed multimode system link lengths of 4 km for 850nm products and 8 km for 1300nm products are readily achieved.

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Finally, a WARNING LED’s and lasers are very reliable semiconductor devices. However, their output does change with time and temperature, in particular the variation with time is unpredictable. It is therefore necessary to accommodate this change of output in the receiver electronics. While the receiver gain can be set up on installation there can be no guarantee that this setting will be correct with time. The best equipment uses internal AGC (Automatic Gain Control) to continuously, and automatically adjust the gain to give the standard 1V peak to peak signal output. Equipment with an AGC costs more than equipment without, but this saving is obtained at the expense of the risk of regular adjustment of the link during its life.