B1-05.10 FIBRE OPTICS

Fibre Optics

• Fibre optics is an important topic in aviation due to its increasing use in aircraft systems.

Common Fibre Optic Technology Terms

• Numerical Aperture (NA): Defines which light will propagate through the fibre.
  • Light entering the core within the cone of acceptance is propagated by total internal reflection. Light outside this cone will not propagate.
• Attenuation: The loss of power during transit.
  • It's specified in decibels per kilometre (dB/km).
  • Commercially available fibres range from approximately 0.1 dB/km for single-mode fibre to 1000 dB/km for large-core plastic fibres.
• Dispersion: Phenomena that cause a broadening or spreading of light as it propagates through an optical fibre.

Optical Fibre Cables

• Light is visible through optical fibre even over many kilometers, with a thickness comparable to human hair.
• A real fibre optic cable is made from very pure glass.
  • The glass is drawn into a thin strand, coated in two layers of plastic (cladding and outer jacket).
  • Coating the glass in plastic creates the equivalent of a mirror, minimizing light losses by total internal reflection.
• Light bounces at shallow angles within the fibre.
• Analogue signals are converted to digital signals to send data.
  • A laser at one end switches on and off to send each data bit.
  • Modern fibre systems transmit billions of bits per second.
• Newer systems use multiple lasers with different colors/frequencies to fit multiple signals into the same fiber.
  • Modern fibre optic cables can carry signals up to 100 km, limited by attenuation.
  • Equipment huts are placed every 70-100 km to pick up and retransmit the signal.
• Laser safety: It is crucial to never look directly into a fibre optic cable or connector containing fibre optic terminations.

Optical Fibre Communications System Basics

• A fibre optic data link sends data through fibre optic components to an output device.
• Three basic functions:
  • Convert an electrical input signal to an optical signal.
  • Transmit the optical signal over an optical fibre.
  • Convert the optical signal back to an electrical signal.
• A fibre optic data link consists of three main components:
  • Transmitter
  • Optical Fibre
  • Receiver
• The electrical signal output should match the data input.

Optical fibre communications system components

• Transmitter: Converts the input signal to an optical signal by varying the current flow through the light source.
  • Two types of optical sources: LEDs and laser diodes.
  • The optical source launches the optical signal into the fibre.
• Receiver: Converts the optical signal back into an electrical signal.
  • An optical detector detects the optical signal.
  • The receiver amplifies and processes the signal without introducing noise or signal distortion.
  • Optical detectors can be a semiconductor Positive-Intrinsic-Negative (PIN) diode or an Avalanche Photodiode (APD).
• Fibre optic data link: Includes optical fibre cable and passive components (splices, couplers, connectors).
  • Passive components join fibre connections but can affect the data link's performance and introduce losses.
  • The optical signal can weaken and distort due to scattering, absorption, and dispersion.

Basic Structure of an Optical Fibre

• Basic Structure consists of three parts:
  • The Core
  • The Cladding
  • The Coating or Buffer.
• Coating or Buffer (Outer Jacket): A plastic layer protecting against physical damage and abrasions.
  • Prevents scattering losses from micro-bends when the fibre is on rough surfaces.
• Core: A cylindrical rod of dielectric material (non-conductive).
  • Light propagates mainly along the core, made of glass and surrounded by cladding.
• Cladding: A dielectric material (glass or plastic) surrounding the core.
  • Its refractive index is less than the core's.
• Functions of the cladding:
  • Reduces light loss from the core into the surrounding air.
  • Reduces scattering loss at the core's surface.
• Functions of the coating:
  • Prevents the fibre from absorbing surface contaminants
  • Adds mechanical strengt
• Optical signals extend partially into the cladding material. Low-order modes penetrate the cladding slightly, while high-order modes penetrate further.

How Fibre Optic Cable Functions

• Optical fibre cables propagate light signals down a fibreglass line by refraction off its side walls.
• Refraction transfers light from the source to receiver.
  • The angle of refraction depends on the wavelength of the light signal.
• Wavelengths used range from 600 to 1600 nm (infrared).

Fibre Optic Operational Behaviour

• Multiple light frequencies (or colors) can pass down the same cable simultaneously.
• Light from each signal source uses a different path due to differences in refraction angles based on frequencies.
• Receivers at the end are tuned to detect different frequencies, isolating data from the flashing optical signal & contain multiple sensors tuned to the frequency of the transmitters.
• Optical fibres are classified into two types:
  • Single-Mode Fibres
  • Multimode Fibres.
• Classified by the number of modes that propagate along the fibre, which depends on the core size.

Single-Mode Fibres

• The ARINC 802 Specification defines these fibres as having a diameter of 9 µm.
• Single-mode cable uses only one mode of transfer.
• It's a smaller diameter cable used in long-distance communication links.
• Single-mode fibres: Have lower signal loss Higher bandwidth Transfer more data due to low fibre dispersion (spreading of light).

Multimode Fibres

• Graded-index cable (multimode fibres) propagate more than one mode.
  • The number of modes depends on the core size and Numerical Aperture (NA).
• Multimode fibres can propagate over 100 modes.
  • Their larger core makes connections easier and core alignment less critical during splicing.
  • Multimode fibres permit the use of LEDs, while single-mode fibres use laser diodes.
  • LEDs are cheaper, less complex, and more durable.
• Disadvantages of Multimode Fibres:
  • Modal dispersion increases with the number of modes, meaning the modes arrive at the fibre end at slightly different times.

Fibre Optic Cable Losses

• Attenuation: Caused by absorption, scattering, and bending losses, reducing optical power.
  • Limits the distance an optical signal (pulse) can travel.
  • If the power is too low, the receiver cannot detect the pulse, causing an error.
• Absorption: A major cause of signal loss in fibre optics when optical power converts into another energy form (heat).
  • It results from imperfections in the fibre structure, impurities, and contamination.
• Scattering: Losses caused by the interaction of light with density fluctuations (higher and lower molecular density) within a fibre.
  • Light interacts with these areas and scatters in all directions.
• Bending: Causes attenuation, classified based on the bend's radius of curvature: micro-bend loss (microscopic bends) or macro-bend loss (sharp routing).
• Microbends: Microscopic bends of the fibre axis mainly occur when a fibre is cabled.
  • They are likened to dents in the cladding and core, making the core not smooth and linear, increasing microbend loss.
• Macrobends: Occur when the cable changes direction quickly with a small bend radius.
  • During installation, sharp bends cause macrobend losses and is a great source of loss when the radius of curvature is less than several centimetres.
  • Light propagating becomes high-order modes, then lost or radiated out of the fibre.
• Dispersion: Spreads the optical pulse as it travels along, reducing bandwidth/information capacity by making the receiver unable to distinguish between input pulses.
  • Effects of attenuation and dispersion increase with distance.
  • Dispersion loss occurs when modes of propagation take different paths, with slight refractive index variation with wavelength.

Fibre Optic Cable Handling Precautions

• Never bend fibres/cables below the manufacturer's specified minimum bend radius to avoid additional fibre loss or breakage.
• Installation precautions:
  • Never look directly into a fibre optic cable or connector with a fibre optic termination.
  • Avoid placing hard and heavy items on the cable.
  • Keep protective caps on unplugged fibre optic cable connectors.
  • Never pull cables tight or fasten them over sharp corners or cutting edges.
  • Clean fibre optic connectors before mating to avoid connection loss/damage.
  • Prevent the cable from becoming kinked/crushed during hardware installation.
  • Only trained, authorized personnel should install or repair fibre optic systems.
  • Dirty terminations significantly decreases data transfer performance and increases the risk of damage to the core.

Fibre Optic Terminations

• Critical aspect in fibre optic system design is how optical power is launched or coupled from one component to the next.
• Fibre optic connections enable the optical power transfer from one component to another to allow fibre optic systems to operate properly.
• A fibre optic data link may require:
  • A fibre optic splice.
  • Connector.
  • Coupler.

Fibre Optic Splices

• Makes a permanent connection by splicing optical fibres together by making a permanent joint between two fibres or groups of fibres.
  • Two types: mechanical and fusion splices.
  • Even with possible removal of some mechanic splices, they are intended to be permanent.
• 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.
• Splicing techniques should optimise performance and reduce loss, requiring proper fibre end preparation and alignment.
• Mechanical Splicing: A permanent connection alignment for an indefinite period without movement.
  • The amount of splice loss must be stable over time and unaffected by environmental/mechanical changes and include glass, plastic, metal and ceramic tubes and V-groove devices.
  • The tools to make the connections are relatively inexpensive, but the connections themselves are expensive.
• Fusion Splicing
  • Uses localised heat to melt or fuse fibre ends after proper fibre end preparation that include removing protective coatings, cleaving the fibre ends with a score-and-break, and inspecting the ends under a microscope.
  • Fusion splicing usually takes more time (personnel and equipment) to make the joint, as well as takes more time per successful splice.

Fibre Optic Connectors

• Permits coupling between two optical fibres or groups and allows for disconnects/reconnects without significant loss of transmission and must maintain fibre alignment.
• The coupling loss results stem from a variety of the loss mechanisms cited earlier.
• Two basic types of fibre optic connectors:
  • Butt-jointed (physical contact)
  • Expanded-beam (non-physical contact).
• Types of Fibre Connectors:
  • Butt-jointed connectors
  • Expanded-beam connectors
• Butt-jointed connectors align and bring prepared ends of two fibres into close contact.
  • The ends themselves may or may not touch, depending on the design. Single-fibre connections may be butt-jointed or expanded beam with two plugs and an adapter.
• Ferrule connectors use 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 ferrule (with the fibre inside it) may also be crimped onto the end of a wire conductor.
  • The fibre-end faces are polished until they are flush with the end of the ferrule to achieve low loss.
• The alignment of ferrules occurs when they are inserted into the alignment sleeve. The inside diameter of the alignment sleeve aligns the ferrules, which in turn align the fibres, held in place with a threaded outer shell or some other coupling mechanism with an accurate hole through the center. Metallic or ceramic connectors are used.
• Expanded-beam connectors use two lenses to expand/refocus light from the transmitting fibre into the receiving fibre and are plug-adapter-plug-type connections. Fibre separation & lateral misalignment are less critical with expanded-beam coupling. -Expanded-beam produces lower coupling loss but are more affected by angular misalignment making them more difficult to produce
• Present applications for expanded-beam connectors involve multi-fibre connections for printed circuit boards and other applications.

Fibre Optic Couplers and Remote Terminals

• Some fibre optic data links require more than simple point-to-point connections, instead requiring multi-port devices as a part of a more complex design.
• Systems requiring multiple ports would require fibre optic components capable of redistributing (combine or split) optical signals.
• A fibre optic coupler is a fibre optic component that allows optical signals redistribution from one fibre among two or more fibres and attenuate the signal more than a connector or splice. It can combine 2 or more signals into one also.
• Couplers are either active or passive devices where passive couplers redistribute the optical signal without optical-to-electrical conversion and active couplers are electrical with detectors and sources for input/output.
• Fibre Optic Coupler - Splitter: A passive device that splits the optical power from one cable into two or more cables.
  • The input may be evenly split to the outputs in a form known as a Y-coupler, or unevenly split to preferentially give power to one over the others such as a T-coupler or optical tap.
• Fibre Optic Coupler - Combiner: A passive device that combines the optical power of multiple input fibres into one output cable.
• Fibre Optic Coupler or X Coupler: A device combining splitter and combiner functions of the two inputs with two outputs together, another name being a 2x2 Coupler due to it's dual 2x2 capabilities.

Optical Fibre System Terminals

• The power launched into a fibre depends on the radiance (brightness) source -a measure of its optical power launching capability.
• Radiance is a power emitted in a direction/time per surface area unit.
• Other terms for fibre optic transmitter/receiver are Control Terminal and Remote Terminal.
• Terminals made of modular components generally manufactured with fibre pigtails or connectors; short OF with fixed source or detector to be completed during manufacturing.

Optical Fibre Data Bus for Aircraft System Advantages

• Lighter weight and smaller size: Using fibre in avionic systems saves weight/space.
• Reduction of crosstalk: No light interferes with the signal 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/waveguide at the same frequency/unit length
• Wide bandwidth – Bandwidths from 100 MHz up to 1 GHz can be obtained using LEDs and laser light sources, which allows for greater signal throughput
• Lower cost: Materials used to construct optical fibres are less expensive than copper.
• Safety: Hazards/short circuits/sparks eliminated.
• Corrosion resistance: Fibre material inert/minimised effects

Disadvantages of Fibre Optic Data Communication

• Stringent coupling requirements: Critical transmission that can present issues if displacement losses avoided by exacting jointing
• Special techniques and equipment: Must employ size and nature of the cable material to achieve coupling
• An ultra-clean environment: small-particle pollution avoided by meticulous termination precautions

Aircraft Applications of Optical Fibre

• During flight, aircraft avionics transmit and receive RF signals over coaxial cables; increases with EMI environment.
• Fibre is used in newer fuel measuring systems for a safer way fuel tanks' measurements
• Boeing 777 uses 11 ARINC 629 data busses & single optical fibre data bus routes data from aircraft systems using Airplane Information Management System.

Fibre Optic Data Bus

• Can overcome limitations of Coaxial Cables that are inherently lossy, with less RF signal bandwidth & can weigh more, by Simultaneous transmission of different signals
• Is ideal incorportating in Avionic systems
• Can handle capacity as a data as an RF communication data bus.
• Boeing 787 fibre optic backbone contains 110 individual fiber links/1.7 km cable - feeds displays, CCRs & DCCs using ARINC664

Further Use Examples

• Vehicle management systems including propulsion/flight controls; fibre-based tech were demonstrated (some with time logged) and for feedback links - sensors & optoelectronics (F/A-18).
• Sensors used - air data/temperature/pressure; Flight Control Position; Engine Power Lever Control Position; stick and rudder all previously monitored via variable differential transformers, or LVDTs.) - system “Fly By Light”.
• Optical fibre in AFCS primary flight controls of a helicopter.
• McDonnell Douglas has fibre optic systems on commercial and military aircraft, but it is evident installation and maintenance must be prioritised.

Flight Data Recording

• Cost effective for data recording systems installed/ used to upgrade older aircraft with
• Newer aircraft have grown 88+ regulated parameters (some FDRs can record 1000+ characteristics) with newer systems able to meet requirements.

Flight Data Recording Table

• Before FDAUs but after October ‘91 , 18 n/a items required and date of compliance
• after October ‘91 and before August ‘00 , 22 August ‘01
• After October ‘91 and before August ‘00, 34, August ‘01
• after after august ‘00 and before , 88, August ‘00.