B2-05A.10 Fibre Optics PDF

Fibre Optics (5.10) Learning Objectives 5.10 List fibre optic handling and installation precautions (S). 5.10.1 Describe the advantages and disadvantages of fibre optic data transmission over electrical wire propagation (Level 2). 5.10.2 Describe the fibre optic data bus (Level 2)....

Fibre Optics (5.10) Learning Objectives 5.10 List fibre optic handling and installation precautions (S). 5.10.1 Describe the advantages and disadvantages of fibre optic data transmission over electrical wire propagation (Level 2). 5.10.2 Describe the fibre optic data bus (Level 2). 5.10.3 Describe fibre optic related terms (Level 2). 5.10.4 Describe fibre optic connectors as well as mechanical and fusion splice terminations (Level 2). 5.10.5 Describe fibre optic couplers, control terminals, and remote terminals (Level 2). 5.10.6 Describe the application of fibre optics in aircraft systems (Level 2). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 333 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Technology Common Fibre Optic Technology Terms Numerical Aperture (NA) of the fibre defines which light will be propagated and which will not, or the light-gathering ability of the fibre. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated. A high NA gathers more light, but lowers the bandwidth. A lower NA increases bandwidth. NA has an important consequence. A large NA makes it easier to inject more light into a fibre, while a small NA tends to give the fibre a higher bandwidth. A large NA allows greater modal dispersion by allowing more modes in which light can travel. A smaller NA reduces dispersion by limiting the number of modes. Attenuation is loss of power. During transit, light pulses lose some of their energy. Attenuation for a fibre is specified in decibels per kilometre (dB/km). For commercially available fibres, attenuation ranges from approximately 0.1 dB/km for single-mode fibre to 1000 dB/km for large-core plastic fibres. Dispersion is a general term for those phenomena that cause a broadening or spreading of light as it propagates through an optical fibre. Optical Fibre Cables Imagine a long, flexible plastic pipe with the inside surface coated with a perfect mirror. If you look in one end of the pipe while several kilometres away at the other end, a torch shines into the pipe, you will see the light in the pipe. Because the interior of the pipe is a perfect mirror, the torch’s light will reflect off the sides (even though the pipe may curve and twist) and you will see it at the other end. If the torch were used to send Morse code, you could communicate through the pipe. This is the essence of a fibre optic cable. Aviation Australia Optic fibre is a glass cable so pure that light visible through it, even when many kilometres long with thickness comparable to a human hair 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 334 of 444 CASA Part 66 - Training Materials Only  A real fibre optic cable is therefore made from glass. The glass is incredibly pure so that even though it is several miles long, light can still make it through (imagine glass so transparent that a window several miles thick still looks clear). The glass is drawn into a very thin strand with a thickness comparable to that of a human hair. The glass strand is then coated in two layers of plastic (cladding and outer jacket). By coating the glass in plastic, you get the equivalent of a mirror around the glass strand. This mirror minimises light losses by total internal reflection, just like a perfect mirror coating on the inside of a tube does. Light travelling through the fibre bounces at shallow angles and stays within the fibre. To send data through a fibre optic cable, analogue signals are converted into digital signals. A laser at one end of the pipe switches on and off to send each data bit. Modern fibre systems with a single laser can transmit billions of bits per second – the laser can turn on and off several billions of times per second. Aviation Australia Fibre optic cables The newest systems use multiple lasers with different colours (different frequencies) to fit multiple signals into the same fibre. Modern fibre optic cables can carry a signal quite a distance – perhaps 100 km (limited by attenuation). On a long-distance line, there is an equipment hut every 70 to 100 km. The hut contains equipment that picks up and retransmits the signal down the next segment at full strength. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 335 of 444 CASA Part 66 - Training Materials Only  Optical Fibre Communications System A fibre optic data link sends data through fibre optic components to an output device. It has the following three basic functions: To convert an electrical input signal to an optical signal To send the optical signal over an optical fibre To convert the optical signal back to an electrical signal. A fibre optic data link consists of three main components; transmitter, optical fibre and receiver. A fibre optic data link needs a transmitter that can effectively convert an electrical input signal to an optical signal and launch the data-containing light down the optical fibre. It also needs a receiver that can effectively transform this optical signal back into its original form. The electrical signal provided as data output should exactly match the electrical signal provided as data input. Aviation Australia Optical fibre communications system components Transmitter – converts the input signal to an optical signal suitable for transmission. It does this by varying the current flow through the light source. The two types of optical sources are LEDs and laser diodes. The optical source launches the optical signal into the fibre. Receiver – converts the optical signal exiting the fibre back into an electrical signal. An optical detector detects the optical signal. The receiver should amplify and process the optical signal without introducing noise or signal distortion. Noise is any disturbance that obscures or reduces the quality of the signal. An optical detector can be either a semiconductor Positive-Intrinsic-Negative (PIN) diode or an Avalanche Photodiode (APD). 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 336 of 444 CASA Part 66 - Training Materials Only  Fibre optic data link – Optical fibre cable and also includes passive components like splices, couplers and connectors. Passive components are used to join fibre connections but they also affect the performance of the data link. These components can also introduce losses that prevent the link from operating. The optical signal can become progressively weakened and distorted due to scattering, absorption and dispersion mechanisms in the optical fibre cable. Basic Structure of an Optical Fibre The basic structure of an optical fibre consists of three parts: the core, the cladding and the coating or buffer. The coating (outer jacket) or buffer is a layer of material used to protect an optical fibre from physical damage. Material used for a buffer is a type of plastic. The buffer is elastic in nature and prevents abrasions. It also prevents the optical fibre from scattering losses caused by micro-bends, which occur when optical fibre is placed on a rough and distorted surface. Aviation Australia Basic structure of an optical fibre - coating, cladding, core The core is a cylindrical rod of dielectric material. Dielectric material conducts no electricity. Light propagates mainly along the core of the fibre, which is generally made of glass and surrounded by a layer of material called the cladding. Even though light will propagate along the fibre core without the layer of cladding material, the cladding does perform some necessary functions. The cladding layer is also made of a dielectric material, usually glass or plastic. The index of refraction of the cladding material is less than that of the core material. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 337 of 444 CASA Part 66 - Training Materials Only  The cladding performs the following functions: Reduces loss of light from the core into the surrounding air Reduces scattering loss at the surface of the core. The coating performs the following functions: Prevents the fibre from absorbing surface contaminants Adds mechanical strength. Optical signals are not confined to the core of the fibre. The modes extend partially into the cladding material. Low-order modes penetrate the cladding only slightly; however, high-order modes penetrate further into the cladding material. Aviation Australia Light path within optical fibre- total internal reflection is due to refraction 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 338 of 444 CASA Part 66 - Training Materials Only  How Fibre Optic Cable Functions Optical fibre cables propagate a light signal that travels down a fibreglass line by constant refraction off its side walls. Relevant Youtube link: Refraction and internal reflection The phenomenon of refraction is used to transfer the light from the source to the receiver. Radio waves refract when they leave a medium and travel through another medium of different density. For example, a stick dipped in water appears to bend. The angle of refraction depends on the wavelength of the signal, in this case light, being used. The illustration in the slide shows how light transfers down the cable. The wavelengths used in the optical fibre range are from 600 to 1600 nm, infrared, just below the visible light frequency in the electromagnetic spectrum. Aviation Australia Total internal reflection - light travels down core by constant refraction off side walls 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 339 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Operational Behaviour Light Wave Propagation Phenomenon of Refraction Several light frequencies (or colours, if it were visible) can be passed down the same cable simultaneously. Light from each signal source travels down the cable via differing paths because the difference in frequencies means the angle of refraction is different for each frequency. At the receiver end, receivers, which are individually tuned to detect the different frequencies isolate the data from the flashing optical signal. Fibre optic receivers can contain multiple sensors tuned to the frequency of the transmitters at the other end of the fibre optic cable. Aviation Australia Two modes of propagation; single-mode fibre or graded-index (multimode) fibre Optical fibres are characterised by their structure and their properties of transmission. Basically, optical fibres are classified into two types; single-mode and multimode fibres. As each name implies, optical fibres are classified by the number of modes that propagate along the fibre. The structure of the fibre can permit or restrict modes from propagating in a fibre. Both fibre types are manufactured with the same materials; the basic structural difference is the core size. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 340 of 444 CASA Part 66 - Training Materials Only  Single-Mode Fibres The ARINC 802 Specification defines single-mode fibres as having a diameter of 9 μm. Single-mode cable uses only one mode of transfer. It is a much smaller diameter cable used in long-distance communication links. Single-mode fibres have a lower signal loss and a higher information capacity (bandwidth) than multimode fibres. They can transfer higher amounts of data due to low fibre dispersion. Basically, dispersion is the spreading of light as light propagates along a fibre. Multimode Fibres As their name implies, graded-index cable (multimode fibres) propagate more than one mode. The number of modes propagated depends on the core size and Numerical Aperture (NA). Multimode fibres can propagate over 100 modes. Their larger core size makes it easier to make fibre connections. During fibre splicing, core-to-core alignment becomes less critical. Another advantage is that multimode fibres permit the use of LEDs, while single-mode fibres typically must use laser diodes. LEDs are cheaper, less complex and more durable, so they are preferred for most applications. Multimode fibres also have some disadvantages. As the number of modes increases, so does the effect of modal dispersion. Modal dispersion means that modes arrive at the fibre end at slightly different times. The graded-index cable has a core that is composed on concentric rings if glass whose refractive indices decrease from the centre axis. The purpose is to reduce model dispersion and thereby increase fibre bandwidth. This means that the velocity of propagation increases away from the centre (light wave travels faster near outer diameter of cable cross section, but slower straight through the centre). Although all modes have different paths, they traverse the cable length in about the same time. This type is used for short distances and high data transfer rates. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 341 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Cable Losses Attenuation In an optical fibre, attenuation is caused by absorption, scattering and bending losses. Attenuation is the loss of optical power as light travels along the fibre. It reduces the amount of optical power transmitted by the fibre and controls the distance an optical signal (pulse) can travel. Once the power of an optical pulse is reduced to the point that the receiver is unable to detect the pulse, an error occurs. Aviation Australia Attenuation is the loss of optical power as light travels along fibre Absorption Absorption within fibre optics is a major cause of signal loss in an optical fibre. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibres occurs because of imperfections in the fibre structure and the presence of impurities and contamination. Scattering Losses are caused by the interaction of light with density fluctuations within a fibre. Density changes are produced during manufacturing when regions of higher and lower molecular density are created (relative to the fibre’s average density). Light travelling through the fibre interacts with the high- or low-density areas and is then partially scattered in all directions. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 342 of 444 CASA Part 66 - Training Materials Only  Bending Bending the fibre also causes attenuation. Bending loss is classified according to the bend’s radius of curvature: micro-bend loss or macro-bend loss. Microbends Microbends are microscopic bends of the fibre axis that occur mainly when a fibre is cabled. They can be likened to dents in the cladding and core that make the core no longer smooth and linear – just as crushing a coaxial cable reduces its Radio Frequency (RF) handling capabilities. Uneven coating applications and improper cabling procedures increase microbend loss. Aviation Australia Microbends are small microscopic bends possibly caused by dents in cladding and fibre optic core 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 343 of 444 CASA Part 66 - Training Materials Only  Macrobends Macrobends refer to routing of the fibre optic cable and occur where the cable changes direction very quickly with a small bend radius (or high curvature) of the optic fibre relative to the diameter of the fibre. During installation, if fibres are bent too sharply, macrobend losses will occur. Macrobends become a great source of loss when the radius of curvature is less than several centimetres. Light propagating at the inner side of the bend travels a shorter distance than that on the outer side. This condition causes some of the light within the fibre to be converted to high-order modes, which are then lost or radiated out of the fibre. Aviation Australia If optical fibres are bent too sharply during installation or maintenance macrobend losses will occur. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 344 of 444 CASA Part 66 - Training Materials Only  Dispersion Dispersion spreads the optical pulse as it travels along the fibre. This spreading of the signal pulse reduces the system bandwidth or the information-carrying capacity of the fibre. Dispersion limits how fast information is transferred. An error occurs when the receiver is unable to distinguish between input pulses caused by the spreading of each pulse. The effects of attenuation and dispersion increase as the pulse travels the length of the fibre. Dispersion loss is caused when the various modes of propagation in the cable take different paths. A slight variation of the refractive index with wavelength occurs in the fibre. Dispersion loss also occurs when some of the light energy travels in the cladding. Aviation Australia Dispersion is when the optical pulse spreads or loses intensity as it travels along fibre optic core In addition to fibre attenuation and dispersion, other optical fibre properties affect system performance. Fibre properties such as modal noise, pulse broadening and polarisation can also reduce system performance. These properties are too complex to discuss as introductory level material. However, you should be aware that attenuation and dispersion are not the only fibre properties that affect performance. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 345 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Cable Handling and Installation Precautions Optical fibres or cables should never be bent at a radius of curvature below the value specified by the manufacturer. This values is called the minimum bend radius and if this precaution is not observed additional fibre loss will occur. Extremely sharp bends increase fibre loss and may lead to fibre breakage. The following precautions need to be observed when installing fibre optic systems: Don’t place hard and heavy items on the cable Always keep a protective cap or dust cap on unplugged fibre optic cable connectors Fibre optic cables should never be pulled tight or fastened over or through sharp corners or cutting edges Fibre optic connectors should always be cleaned before mating. Dirt in a fibre optic connection will significantly increase the connection loss and may damage the connector. Precautions must be taken so the cable does not become kinked or crushed during installation of the hardware. Only trained, authorised personnel should be allowed to install or repair fibre optic systems. Aviation Australia Clean connectors before mating as dirt significantly decreases data transfer performance and increases the risk of damage to the core 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 346 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Terminations Fibre Optic Splices In fibre optic system design, the launching or coupling of optical power from one component to the next is important. Fibre optic connections permit the transfer of optical power from one component to another and allow fibre optic systems to serve as more than just point-to-point data communication links. In fact, fibre optic data links are often of a more complex design than point-to- point data links. A system connection may require a fibre optic splice, connector or coupler. Fibre Optic Splice One type of system connection is a permanent connection made by splicing optical fibres together. A fibre optic splice makes a permanent joint between two fibres or two groups of fibres. There are two types of fibre optic splices: mechanical and fusion splices. Even though removal of some mechanical splices is possible, they are intended to be permanent. Optical fibre splice termination is a permanent joint between two fibres or two groups of fibres Connection losses can be limited by following proper procedures and by reducing mismatches between the connected fibres. This is done by procuring only fibres that meet stringent geometric and optical specifications. Poor fibre end preparation and poor fibre alignment are the main causes of connection losses. Another source of signal loss is differences in optical properties between the connected fibres, such as different numerical apertures, core and cladding diameters, and refractive index profiles. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 347 of 444 CASA Part 66 - Training Materials Only  Fibre-to-fibre connection loss is increased by the following sources: Fibre separation Lateral misalignment Angular misalignment Core and cladding diameter mismatch Numerical aperture (NA) mismatch – NA is the light-gathering ability of the fibre. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated Refractive index profile difference – Refractive index is a property of optical materials that relates to the speed of light in the material versus the speed of light in a vacuum Poor fibre end preparation. Ideally, optical signals coupled between fibre optic components are transmitted with no loss of light. However, there is always some type of imperfection present at fibre optic connections that causes loss. The amount of optical power lost at fibre optic connections is a major concern of system designers. Aviation Australia Coupling losses are minimised by following proper connection procedures and by reducing fibre mismatches between connected fibres 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 348 of 444 CASA Part 66 - Training Materials Only  Optical Fibre Termination Precautions Optical fibre termination tasks require specialist training and equipment and should only ever be carried out when approved by the manufacturer and performed by approved and qualified personnel. When working around personnel terminating fibres, the real dangers are related to the small scraps of glass cleaved off the ends of the fibres being terminated or spliced. These scraps are very dangerous! The cleaved ends are extremely sharp and can easily penetrate your skin. If they get into your eyes, they are very hard to flush out. If they are swallowed, serious medical consequences can result. Safety glasses are a must! Always follow these rules when working where fibre optic cables maintenance is being performed. 1. Wear safety glasses. 2. All scraps should be disposed of in a properly marked container. 3. Do not eat or drink anywhere near the work area. Cleaved ends of fibre are extremely sharp – easily penetrate skin and damage eyes 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 349 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Splices A fibre optic splice is a permanent fibre joint whose purpose is to establish an optical connection between two individual optical fibres. Fibre optic splices also permit repair of optical fibres damaged during installation, accident or stress. Mechanical and fusion splicing are two broad categories that describe fibre splicing techniques. Mechanical splice – Mechanical fixtures and materials perform fibre alignment and connection Fusion splice – Localised heat fuses or melts the ends of two optical fibres together. Each splicing technique seeks to optimise performance and reduce loss. Low-loss fibre splicing results from proper fibre end preparation and alignment. Mechanical Splicing A mechanical splice is a permanent connection made between two optical fibres. Mechanical splices hold the two optical fibres in alignment for an indefinite period without movement. The amount of splice loss is stable over time and unaffected by changes in environmental or mechanical conditions. Typical mechanical splices include glass, plastic, metal and ceramic tubes and V-groove devices. Materials that assist with mechanical splices include transparent adhesives and index-matching gels. Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibres. The tools to make mechanical splices are cheap, but the splices themselves are expensive. Aviation Australia Mechanical splice – mechanical fixtures and materials align fibre and perform connection 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 350 of 444 CASA Part 66 - Training Materials Only  Fusion Splicing Fusion splicing involves using localised heat to melt or fuse the ends of two optical fibres together. The splicing process begins by preparing each fibre end for fusion. Fusion splicing requires all protective coatings to be removed from the ends of each fibre. The fibre is then cleaved using the score-and-break method. The quality of each fibre end is inspected using a microscope. The fusion splicing apparatus consists of two fixtures on which the fibres are mounted and two electrodes in the centre. On older models, the fibres are placed in the tool and manually aligned, using fine adjustment knobs and an inspection microscope. Carbon dioxide (CO2) lasers, electric arcs or gas flames are typically used to heat the fibre ends fusing them together. Traditionally, fusion splicing takes longer and is more difficult when compared to equivalent mechanical splicing. However, modern automatic splicing tools automatically align the fibres and have ‘profiles’ to change/save the different fusion settings for different specifications of fibre. This allows for more consistent and repeatable splices in less time. They also automatically test the splice for losses. The development of these automated fusion-splicing machines has made fusion splicing the preferred technique used in commercial applications. Fusion splicing requires specialist equipment and training it uses localised heat to fuse or melt two optical fibre ends together 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 351 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Connectors A fibre optic connector is a device that permits coupling between two optical fibres or two groups of fibres that can be disconnected. The device must allow for repeated fibre disconnects/reconnects without significant loss of light transmission. Fibre optic connectors must maintain fibre alignment during numerous connections. Connector coupling loss results from the same loss mechanisms described earlier. Fibre optic connectors Butt-jointed (physical contact) and expanded-beam (non-physical contact) connectors are two basic types of fibre optic connectors. Butt-jointed connectors align and bring prepared ends of two fibres into close contact. End-faces of some butt-jointed connectors touch, but others do not, depending on the connector design. Single-fibre butt-jointed and expanded beam connectors normally consist of two plugs and an adapter (plug-adaptor-plug coupling device). Aviation Australia Butt type optical connector 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 352 of 444 CASA Part 66 - Training Materials Only  Ferrule connectors use two cylindrical plugs (ferrules), an alignment sleeve and sometimes axial springs to perform fibre alignment. Precision holes drilled or moulded through the centre of each ferrule allow for fibre insertion and alignment. When the fibre ends are inserted, an adhesive (normally epoxy resin) bonds the fibre inside the ferrule (the fibre remains inside the ferrule, which is like the pin crimped on the end of a wire). The fibre-end faces are polished until they are flush with the end of the ferrule to achieve a low-loss fibre connection. Fibre alignment occurs when the ferrules are inserted into the alignment sleeve. The inside diameter of the alignment sleeve aligns the ferrules, which in turn align the fibres. Ferrule connectors lock the ferrules into the alignment sleeve using a threaded outer shell or some other type of coupling mechanism. Fibre alignment depends on the creation of an accurate hole through the centre of the ferrule. Normally, ferrule connectors use ceramic or metal ferrules. Expanded-beam connectors use two lenses to first expand and then refocus the light from the transmitting fibre into the receiving fibre. They are normally plug-adapter-plug-type connections. Fibre separation and lateral misalignment are less critical in expanded-beam coupling than in butt- jointing. The same amount of fibre separation and lateral misalignment in expanded beam coupling produces a lower coupling loss than in butt-jointing. However, angular misalignment is more critical. The same amount of angular misalignment in expanded-beam coupling produces a higher loss than in butt-jointing. Expanded-beam connectors are also much harder to produce. Aviation Australia Expanded-beam connectors use two lenses expand and refocus light from transmitting fibre into receiving fibre Present applications for expanded-beam connectors include multi-fibre connections, edge connections for printed circuit boards and other applications. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 353 of 444 CASA Part 66 - Training Materials Only  Fibre Optic Couplers and Remote Terminals Fibre Optic Couplers Some fibre optic data links require more than simple point-to-point connections. These data links may be of a much more complex design that requires multi-port or other types of connections. In many cases, these types of systems require fibre optic components that can redistribute (combine or split) optical signals throughout the system. Aviation Australia Fibre optic couplers distributes or combines optical signals between fibres One type of fibre optic component that allows for the redistribution of optical signals is a fibre optic coupler. A fibre optic coupler is a device that can distribute the optical signal (power) from one fibre among two or more fibres or combine the optical signal from two or more fibres into single fibre. Fibre optic couplers attenuate the signal much more than a connector or splice because the input signal or signals are distributed to multiple output ports. Aviation Australia A fibre optic coupler 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 354 of 444 CASA Part 66 - Training Materials Only  Fibre optic couplers can be either active or passive devices. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without optical-to-electrical conversion. Active couplers are electronic devices that split or combine the signal electrically and use fibre optic detectors and sources for input and output. Fibre Optic Coupler – Splitter An optical splitter is a passive device that splits the optical power carried by a single input fibre into two output fibres. The input optical power is normally split evenly between the two output fibres. This type of optical splitter is known as a Y-coupler. However, an optical splitter may distribute the optical power carried by input power unevenly, such as by splitting most of the power from the input fibre to only one of the output fibres. Only a small amount of the power is coupled into the secondary output fibre. This type of optical splitter is known as a T-coupler or an optical tap. Aviation Australia Fibre optic couplers - splitter and combiner Fibre Optic Coupler – Combiner An optical combiner is a passive device that combines the optical power carried by two input fibres into a single output fibre. Fibre Optic Coupler – X Coupler An X coupler combines the functions of the optical splitter and combiner. It combines and divides the optical power from the two input fibres between the two output fibres like a breakout on a data bus, where several device signals come into a common point for redistribution. Another name for the X coupler is the 2 × 2 coupler. Aviation Australia X coupler – combines functions of optical splitter and combiner 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 355 of 444 CASA Part 66 - Training Materials Only  Star and Tree Multiport Couplers Star and tree couplers are multiport couplers (passive devices) that have more than two input or two output ports. A star coupler distributes optical power from more than two input ports among several output ports. Aviation Australia Star coupler distributes optical power from multiple input ports to several output ports A tree coupler splits optical power from one input fibre to more than two output fibres. A tree coupler may also be used to combine the optical power from more than two input fibres into a single output fibre. Star and tree couplers distribute the input power uniformly among the output fibres. Aviation Australia Tree coupler splits optical power from single input fibre to multiple output fibres 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 356 of 444 CASA Part 66 - Training Materials Only  Fibre optic couplers should prevent the transfer of optical power from one input fibre to another input fibre. Directional couplers are fibre optic couplers that prevent this transfer of power between input fibres. Many fibre optic couplers are also symmetrical. Symmetrical couplers transmit the same amount of power through the coupler when the input and output fibres are reversed. Optical Fibre System Terminals The amount of optical power launched into an optical fibre depends on the radiance of the optical source. An optical source's radiance, or brightness, is a measure of its optical power launching capability. Radiance is the amount of optical power emitted in a specific direction per unit time by a unit area of emitting surface. For most types of optical sources, only a fraction of the power emitted by the source is launched into the optical fibre. Alternative terms for a fibre optic transmitter and receiver are Control Terminal and Remote Terminal respectively. They are modular components. Fibre optic transmitters and receivers are devices that are generally manufactured with fibre pigtails or fibre optic connectors. A fibre pigtail is a short length of optical fibre (usually 1 m or less) that is permanently fixed to the optical source or detector. Manufacturers supply transmitters and receivers with pigtails and connectors because fibre coupling to sources and detectors must be completed during fabrication. Teledyne Microelectronic Technologies Fibre optic transmitters and receivers are modular components manufactured with fibre pigtails or fibre optic connectors 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 357 of 444 CASA Part 66 - Training Materials Only  Optical Sources and Fibre Optic Transmitters (Control Terminals) The fibre optic device responsible for converting an electrical analogue or digital signal into a corresponding optical signal is a fibre optic transmitter. It converts electrical signals into optical signals and launches the optical signals into an optical fibre. Fibre optic data link performance depends on the amount of optical power (light) launched into the optical fibre. Semiconductor optical sources suitable for fibre optic systems range from inexpensive LEDs to more expensive semiconductor lasers. Semiconductor LEDs and Laser Diodes (LDs) are the principal light sources used in fibre optics. The output devices from the transmitters generally fall into two categories: optical connectors and optical fibre pigtails. Optical pigtails are attached to the transmitter optical source. The optical source may couple to the output optical connector through an intermediate optical fibre. One end of the optical fibre is attached to the source. The other end terminates in the transmitter optical output connector. The optical source may also couple to the output optical connector without an intermediate optical fibre. The optical source is placed within the transmitter package to launch power directly into the fibre of the mating optical connector. In some cases, lenses are used to more efficiently couple light from the source to the mating optical connector. Fibre optic transmitters come in various sizes and shapes. The least complex transmitters are typically packaged in Transistor Outline (TO) cans or hybrid microcircuit modules in Dual Inline Packages (DIPs). Fibre optic transmitters converts electrical signal to optical signal 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 358 of 444 CASA Part 66 - Training Materials Only  Optical Detectors and Fibre Optic Receivers (Remote Terminals) The optical signals propagating in the fibre become weakened and distorted because of scattering, absorption and dispersion. The fibre optic device responsible for converting the weakened and distorted optical signal back to an electrical signal is a fibre optic receiver. A fibre optic receiver is an electro-optic device that accepts optical signals from an optical fibre and converts them into electrical signals. A typical fibre optic receiver consists of an optical detector, a low-noise amplifier and other circuitry used to produce the electrical output. Aviation Australia Optical receiver converts weakened, distorted optical signal back into electrical signal for processing The optical detector converts incoming optical signal into an electrical signal corresponding to the original electrical signal provided to the transmitter. Any variation of the signal at the receiver end is termed distortion – an undesirable characteristic. The receiver must replicate the original signal. The amplifier then amplifies the electrical signal to a level suitable for further signal processing. The optical detector is a transducer that converts an optical signal into an electrical signal by generating an electrical current proportional to the intensity of incident optical radiation. Optical detectors that meet many of these requirements and are suitable for fibre optic systems are semiconductor photodiodes. The optical fibre is coupled to semiconductor photodiodes in a similar way that optical sources are coupled to optical fibres. Semiconductor detectors are designed so that optical energy (photons) incident on the photo detector produce a current. This current is called a photocurrent. The simplest fibre optic receivers consist of only the optical detector and a load resistor. However, the output signal of these simple receivers is not in a suitable form for most types of interfacing circuitry. To produce a suitable signal, a preamplifier, a post amplifier and other circuitry are generally included in the receiver. The high-impedance preamplifier is generally used with a large load resistor to improve sensitivity. Fibre optic receivers come in packages like those for fibre optic transmitters. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 359 of 444 CASA Part 66 - Training Materials Only  Receiver typically houses a; preamplifier, post amplifier, and other circuitry to produce a quality electrical output signal 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 360 of 444 CASA Part 66 - Training Materials Only  Optical Fibre Data Bus for Aircraft Systems Advantages of Fibre Optic Data Communication Advantages of using optical fibre cables in communications are: Lighter weight and smaller size – Using optical fibre cables in avionic systems instead of copper wires saves substantial weight and space in aircraft. Reduction of crosstalk – No light, and therefore no signal, interferes with light in adjacent cables. Immunity to electromagnetic interference – EMI does not affect energy at light frequencies. Lower signal attenuation – Attenuation figures are approximately 1/100 that of a typical cable or waveguide at the same frequency over a unit length of line. Wide bandwidth – Bandwidths from 100 MHz up to 1 GHz can be obtained using LEDs and laser light sources. This allows greater signal throughput in a communication system. Lower cost – Materials used to construct optical fibres are much less expensive than copper. Safety – Hazards or short circuits and sparks are eliminated. Corrosion resistance – Fibre material is inert and therefore corrosion effects are minimised. Fibre and electrical cables terminated in a single connector 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 361 of 444 CASA Part 66 - Training Materials Only  Disadvantages of Fibre Optic Data Communication The advantages far outweigh the disadvantages, but some disadvantages of using optical fibre cable are: Stringent coupling requirements – Transmission of light from light source to fibre, through fibre cable splicing and fibre to the optical detector is critical. Exacting joints must be accomplished to avoid displacement losses. Special techniques and equipment – Specialised knowledge and equipment must be employed because of the size and nature of the fibre cable material to achieve effective coupling. An ultra-clean environment – Meticulous precautions must be taken when terminating to avoid small-particle pollution. A disadvantage of fibre optics systems is the stringent coupling requirements when splicing fibre cables 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 362 of 444 CASA Part 66 - Training Materials Only  Aircraft Applications of Optical Fibre Fibre Optic Data Bus During flight, aircraft avionics transmit and receive RF signals to and from antennas over coaxial cables. As the density and complexity of on-board avionics increases, the Electromagnetic Interference (EMI) environment degrades proportionally. Coaxial cables are inherently lossy, limiting the RF signal bandwidth while adding considerable weight. Fibre optic data busses can overcome these limitations. Using a form of multiplexing, simultaneous transmission of multiple signals (including RF) can be achieved over a single optical fibre. The relatively small size and weight of an optical fibre makes it ideal for incorporation in avionics systems, where weight and space are at a premium. With all the advantages of optical fibre, it has the capacity and preference to become an integral part of aircraft and act as the RF communication data bus between antennas and the cockpit. Aviation Australia Optical fibre is an ideal medium to replace coaxial cable between communication system transmitters and antennas Concerns arising from aeroplane crashes have caused fibre optics to be incorporated into newer fuel measuring systems. Ever since the crash of TWA Flight 800 was attributed to a spark in one of the fuel tanks, the search has been underway for safer ways to measure fuel tank quantities. The Boeing 777 has 11 ARINC 629 data busses and a single optical-fibre data bus to route data from aircraft systems through the Airplane Information Management System. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 363 of 444 CASA Part 66 - Training Materials Only  The Boeing 787 also has a fibre optic backbone, containing 110 individual fibre links and 1.7 km of fibre optic cable. Fibre feeds the flight deck displays, Common Computing Resources (CCR), and data concentrators. The 787 does not use the same physical protocol as the 777 instead, it uses ARINC 664. Fibre optic technology has been implemented within diverse areas of the aircraft vehicle management systems, including propulsion and flight controls. At least four different fibre-based technologies have been demonstrated in the laboratory, and some have accumulated flight hours while installed in technology demonstration aircraft. Hardware and software were developed for optical feedback links in the flight control system of an F/A-18 aircraft. Developments included passive optical sensors and optoelectronics to operate the sensors. Sensors with various methods of operation were obtained from different manufacturers and integrated with common optoelectronics. The sensors detected the following: Air Data Temperature, Air Data Pressure, Flight Control Position, stick and rudder position, and Engine Power Lever Control Position (all traditionally monitored by linear variable differential transformers, or LVDTs). NASA installed the integrated system for flight testing and selected the most successful sensor approaches for use in a follow-on program in which the sensors will not just be flown on the aircraft and their performance recorded, but also used in closing flight control loops (for feedback signals). This system was labeled Fly by Light. Development is underway using optical fibre as the basis of helicopter AFCS primary flight controls (carrying error signals to the servo actuators as opposed to just carrying feedback signals as detailed above with respect to the FA-18 AFCS). A triple-redundant fibre optics network is replacing the fly- by-wire primary flight controls on the Apache helicopter. McDonnell Douglas has developed, installed and tested several fibre optic systems on various commercial and military aircraft. It has become evident that problems associated with installation and maintenance of fibre optic systems are perhaps the greatest impediment remaining to their incorporation on aircraft. 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 364 of 444 CASA Part 66 - Training Materials Only  Flight Data Recording In avionics, fibre-optic-based systems provide cost-effective for instrument flight data recording (FDR) systems installed in new aircraft and used in upgrading older aircraft to meet the latest safety requirements. For aircraft manufactured since August 2000, regulated parameters that must be recorded have grown to a total of 88 however some FDRs have the capacity to record more than 1000 in flight characteristics. This monitoring requirement requires a lot of sensors and data movement transfer within the aircraft. In both upgrading existing systems and meeting the more stringent requirements of newer systems, fibre optics have proved they can provide data gathering capabilities that are flexible, affordable, simple and safe. Table demonstrating the increase in data gathering 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 365 of 444 CASA Part 66 - Training Materials Only  Fibre Optics Gyroscopes Aircraft navigation will be covered in future modules however a gyroscope is a major component of most aircraft navigation systems. The purpose of a gyroscope is to monitor aircraft movement in the pitch roll and yaw axis. These will also be discussed when learning about aircraft aerodynamics. The Fibre Optic Gyroscope (FOG) detects this motion or rotation using two beams in opposite directions into long spools of fibre optic cable. Optic sensors monitor the phase difference of the two beams compared after their travel through the spools of fibre. There are three fibre optic spools that detect the motion for the three aircraft axis. FOGs have no moving parts and provide very accurate navigation data to the aircraft navigation system. A fibre optic gyroscope monitors aircraft movement in the pitch roll and yaw axis 2024-11-05 B2-05a Digital Techniques / Electronic Instrument Systems Page 366 of 444 CASA Part 66 - Training Materials Only