Fiber Optics PDF

Aircraft Digital Computer Systems  FIBRE OPTICS Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances. FIBRE OPTICS  F...

Aircraft Digital Computer Systems  FIBRE OPTICS Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances. FIBRE OPTICS  FIBRE OPTICS Optical fibres have been widely used as a transmission medium for ground-based long-haul data communications and in local area networks (LANs) for many years; they are now being introduced into the latest passenger aircraft to satisfy the need for wideband networked avionic and cabin entertainment systems.  FIBRE OPTICS By virtue of their light weight, compact size and exceptionally wide bandwidth, optical fibres are ideally suited for use as a replacement for conventional copper network cabling. The technology is, however, relatively new in the civil aircraft industry and brings with it a whole new set of problems and challenges for those involved with aircraft operation and maintenance.  FIBRE OPTICS Bandwidth describes the maximum data transfer rate of a network or Internet connection. It measures how much data can be sent over a specific connection in a given amount of time. For example, a gigabit Ethernet connection has a bandwidth of 1,000 Mbps (125 megabytes per second). FIBRE OPTICS FIBRE OPTICS FIBRE OPTICS  FIBRE OPTICS ADVANTAGES AND DISADVANTAGES Optical fibres offer some very significant advantages over conventional copper cables. These include: optical fibres are lightweight and of small physical size; exceptionally wide bandwidth and very high data rates can be supported; relative freedom from electromagnetic interference;  FIBRE OPTICS ADVANTAGES AND DISADVANTAGES Optical fibres offer some very significant advantages over conventional copper cables. These include: relatively low values of attenuation within the medium; high reliability coupled with long operational life; electrical isolation and freedom from earth/  FIBRE OPTICS ADVANTAGES AND DISADVANTAGES The reduction in weight that results from the use of fibre optical cabling can yield significant fuel savings. Copper cabling is typically five times heavier than polymer optical fibre cabling and 15 times heavier than silica optical fibre. On a large, latest- generation aircraft with sophisticated avionics, the total saving in weight can be as much as 1,300 kg.  FIBRE OPTICS ADVANTAGES AND DISADVANTAGES There are very few disadvantages of optical fibres. They include: industry resistance to the introduction of new technology; need for a high degree of precision when fitting cables and connectors; concerns about the mechanical strength of fibres and the need to ensure that cable bends have a sufficiently large radius to minimise losses and the possibility of  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Essentially, an optical fibre consists of a cylindrical silica glass core surrounded by further glass cladding. The fibre acts as a channel (or waveguide) along which an electromagnetic wave can pass with very little loss.  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Fibre-optics are governed by the fundamental laws of reflection and refraction. For example, when a light wave passes from a medium of higher refractive index to one of lower refractive index, the wave is bent towards the normal. Conversely, when travelling from a medium of lower refractive index to one of higher refractive index, the wave will be bent away from the normal. FIBRE OPTICS  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Reflection involves a change in direction of waves when they bounce off a barrier. Refraction of waves involves a change in the direction of waves as they pass from one medium to another. Refraction, or the bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves.  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES The light in a fibre-optic travels through the core by constantly bouncing from the cladding, a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances.  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Total internal reflection, courtesy of www.wikipedia.com  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Optical fibres are manufactured by drawing silica glass from the molten state and they are thus of cylindrical construction. The more dense medium (the core) is surrounded by the less dense medium (the cladding). Provided the angle of incidence of the input wave is larger than the critical angle, the light wave will propagate inside the core by means of a series of total internal reflections.  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Silica Glass  FIBRE OPTICS PROPAGATION IN OPTICAL FIBRES Any light wave which travels along the core and meets the cladding at the critical angle of incidence will be totally internally reflected. Therefore light wave is propagated along the fiber core by a series of total internal reflections.  FIBRE OPTICS Attenuation The loss within an optical fibre arises from a number of causes, including: absorption, scattering in the core (due to non- homogeneity of the refractive index), scattering at the core/cladding boundary, and losses due to radiation at bends in the fibre. FIBRE OPTICS  FIBRE OPTICS Attenuation Whereas the attenuation coefficient of an optical fibre is largely dependent upon the quality and consistency of the glass used for the core and cladding, the attenuation of all optical fibres varies widely with wavelength.  FIBRE OPTICS Attenuation Monomode fibres are now a common feature of ground-based high-speed data communication systems and manufacturing techniques have been developed that ensure consistent and reliable products with low attenuation and wide operational bandwidths.  FIBRE OPTICS Attenuation However, since monomode fibres are significantly smaller in diameter than their multimode predecessors, a consistent and reliable means of cutting, surface preparation, alignment and interconnection is essential, and for this reason slower multimode fibres are still prevalent in current aircraft designs.  FIBRE OPTICS Comparison of multimode and monomode fibres FIBRE OPTICS FIBRE OPTICS  FIBRE OPTICS DISPERSION AND BANDWIDTH A simple one-way fibre-optic data link  FIBRE OPTICS PRACTICAL OPTICAL NETWORKS The Boeing 777 was the first commercial aircraft to enter production with an optical fibre-based LAN for onboard data communications. The system was originally developed in the 1980s and it comprised an avionics local area network (AVLAN) fitted in the flight deck and electrical equipment bay, together with a cabin local area network (CABLAN) fitted in the roof of the passenger cabin.  FIBRE OPTICS PRACTICAL OPTICAL NETWORKS These two fibre-optic networks conform to the ARINC 636 standard, which was adapted for avionics from the fibre distributed interface (FDDI) in order to provide a network capable of supporting data rates of up to 100 Mbps.  FIBRE OPTICS Fibre-optic cable construction The construction of a typical fibre-optic cable: five optical fibres and two filler strands; separator tape; aramid yarn strength member; an outer jacket.  FIBRE OPTICS Fibre-optic cable construction A typical fibre-optic cable  FIBRE OPTICS Fibre-optic cable construction  FIBRE OPTICS The cable has an overall diameter of about 0.2 inches and the individual optical fibre strands have a diameter of 140 μm (approximately 0.0055 inches). A protective buffer covers each fibre and protects it during manufacture, and increases mechanical strength and diameter in order to make handling and assembly easier. The buffers are coded in order to identify the fibres using colours (blue, red, green, yellow and white).  FIBRE OPTICS The filler strands are made from polyester and are approximately 0.035 inches in diameter. A polyester separator tape covers the group of five fibres and two filler strands. This tape is manufactured from low friction polyester and it serves to make the cable more flexible.  FIBRE OPTICS A layer of woven aramid (or Kevlar) yarn provides added mechanical strength and protection for the cable assembly. The outer thermoplastic jacket (usually purple in colour) is fitted to prevent moisture ingress and also to provide insulation.  FIBRE OPTICS Fibre-optic connectors The essential requirements for connectors used with optical fibres are that they should be: reliable robust precise and repeatable (even after numerous mating operations) suitable for installation without specialist tooling low loss FIBRE OPTICS A typical fibre-optic connector arrangement  FIBRE OPTICS Fibre-optic connectors While the loss exhibited by a connector may be quoted in absolute terms, it is often specified in terms of an equivalent length of optical fibre. If, for example, six connectors are used on a cable run and each connector has a loss of 0.5 dB, the total connector loss will be 3 dB. This is equivalent to several kilometres of low-loss fibre!  FIBRE OPTICS Fibre-optic connectors A typical fibre-optic cable connector arrangement is comprised of: alignment keys and grooves guide pins and cavities coloured alignment bands three start threads.  FIBRE OPTICS Fibre-optic connectors Each connector has alignment keys on the plug and matching alignment grooves on the receptacle. These are used to accurately align the connector optical components; the guide pins in the plug fit into cavities in the receptacle when the plug and receptacle connect. In order to ensure that the connector is not over-tightened (which may cause damage to the fibres) the pins of the plug are designed to provide a buffer stop against the bottom of the cavities in the  FIBRE OPTICS Fibre-optic connectors The plug and receptacle have ceramic contacts that are designed to make physical contact when properly connected (the light signal passes through the holes in the end of the ceramic contacts when they are in direct physical contact with each other).  FIBRE OPTICS Fibre-optic connectors The coupling nut on the plug barrel has a yellow band, while the receptacle barrel has a red and a yellow band. A correct connection is made when the red band on the receptacle is at least 50 per cent covered by the coupling nut. This position indicates an effective connection in which the optical fibres in the plug are aligned end-to-end with the fibre in the receptacle.  FIBRE OPTICS Fibre-optic connectors Three start threads on the plug and receptacle ensure a straight start when they join. The recessed receptacle components prevent damage from the plug if it strikes the receptacle at an angle. The plug and receptacle are automatically sealed in order to prevent the ingress of moisture and dust.  FIBRE OPTICS KEY POINT Care must be taken when assembling optical fibre connectors, both in terms of the correct alignment of the plugs and receptacles and cleanliness of the contact area (this is essential to ensure low loss and efficient coupling). Before attempting to examine the face or ceramic contacts of a connector arrangement it is essential to disconnect the cable from the equipment at both ends. The light from the optical fibre is invisible and can be intense enough to