Optical Fibre Properties

Questions and Answers

What is the purpose of the precision holes in the ferrule?

To bond the fibre to the ferrule

To polish the fibre-end faces

To allow for fibre insertion and alignment (correct)

To lock the ferrules into the alignment sleeve

What is used to bond the fibre inside the ferrule?

Alignment sleeve

Ceramic ferrule

Metal ferrule

Epoxy resin (correct)

What is the purpose of polishing the fibre-end faces?

To lock the ferrules into the alignment sleeve

To bond the fibre to the ferrule

To align the fibres

To achieve a low-loss fibre connection (correct)

How do ferrule connectors lock the ferrules into the alignment sleeve?

Using a threaded outer shell Signup and view all the answers

What type of connectors use two lenses to expand and then refocus the light?

Expanded-beam connectors Signup and view all the answers

What is the primary material used to make the core of an optical fibre?

Glass Signup and view all the answers

What is more critical in expanded-beam coupling compared to butt-jointing?

Angular misalignment Signup and view all the answers

What is the advantage of expanded-beam coupling over butt-jointing?

Less critical fibre separation and lateral misalignment Signup and view all the answers

What is the main function of the cladding layer in an optical fibre?

To reduce loss of light from the core into the surrounding air Signup and view all the answers

Why do high-order modes penetrate further into the cladding material?

Because they have a longer wavelength Signup and view all the answers

What is a disadvantage of expanded-beam connectors?

Harder to produce Signup and view all the answers

What is the typical core size of single-mode fibres?

8 to 10 μm Signup and view all the answers

What phenomenon is used to transfer the light from the source to the receiver in an optical fibre?

Refraction Signup and view all the answers

What is the main advantage of multimode fibres over single-mode fibres?

Easier fibre connections and lower cost Signup and view all the answers

What is the purpose of the coating on an optical fibre?

To prevent the fibre from absorbing surface contaminants Signup and view all the answers

What is the effect of increasing the number of modes in multimode fibres?

Increased modal dispersion Signup and view all the answers

Why is the index of refraction of the cladding material less than that of the core material?

To facilitate total internal reflection Signup and view all the answers

What happens to the light signal as it travels down an optical fibre?

It is refracted off the side walls of the fibre Signup and view all the answers

What causes attenuation in an optical fibre?

Absorption, scattering and bending losses Signup and view all the answers

What is the primary reason single-mode fibres are used in long-distance communication links?

Higher information capacity and lower signal loss Signup and view all the answers

What is the primary function of total internal reflection in an optical fibre?

To facilitate the transfer of light from the source to the receiver Signup and view all the answers

What is the purpose of using laser diodes in single-mode fibres?

Because LEDs are not suitable for single-mode fibres Signup and view all the answers

What is fibre dispersion?

The spreading of light as it propagates along a fibre Signup and view all the answers

What is the advantage of single-mode fibres over multimode fibres?

Higher information capacity and lower signal loss Signup and view all the answers

What is the main difference between active and passive fibre optic couplers?

Optical-to-electrical conversion Signup and view all the answers

What is the purpose of an optical splitter?

To split the optical power carried by a single input fibre into two output fibres Signup and view all the answers

What is a Y-coupler?

An optical splitter that splits the input optical power evenly between two output fibres Signup and view all the answers

What is an X coupler?

A device that combines the functions of an optical splitter and combiner Signup and view all the answers

What is the function of an optical combiner?

To combine the optical power carried by two input fibres into a single output fibre Signup and view all the answers

What is a T-coupler?

An optical splitter that distributes optical power unevenly Signup and view all the answers

What is a characteristic of a passive fibre optic coupler?

It redistributes the optical signal without optical-to-electrical conversion Signup and view all the answers

What is the purpose of an optical tap?

To distribute the optical power carried by the input fibre unevenly Signup and view all the answers

What is a key advantage of optical fibers in avionics systems?

Relatively small size and weight Signup and view all the answers

What is the purpose of incorporating fiber optics in newer fuel measuring systems?

To reduce the risk of sparks Signup and view all the answers

How many individual fiber links does the Boeing 787's fiber optic backbone contain?

110 Signup and view all the answers

What is the name of the protocol used in the Boeing 787's fiber optic backbone?

ARINC 664 Signup and view all the answers

What is a feature of the fiber optic system in the F/A-18 aircraft?

Passive optical sensors Signup and view all the answers

Why are fiber optics used in aircraft?

To act as the RF communication data bus between antennas and the cockpit Signup and view all the answers

How many ARINC 629 data buses does the Boeing 777 have?

11 Signup and view all the answers

What is a benefit of fiber optic technology in aircraft?

It allows for safer ways to measure fuel tank quantities Signup and view all the answers

Study Notes

Light Propagation in Fibre Optic Cables

Light propagates mainly along the core of the fibre, which is typically made of glass and surrounded by a layer of cladding material.
The cladding layer performs two necessary functions: • Reduces loss of light from the core into the surrounding air • Reduces scattering loss at the surface of the core
The cladding material has a lower index of refraction than the core material.

Functions of Cladding and Coating

The coating performs two functions: • Prevents the fibre from absorbing surface contaminants • Adds mechanical strength
Optical signals are not confined to the core of the fibre; modes extend partially into the cladding material.

How Fibre Optic Cable Functions

Fibre optic cables propagate a light signal that travels down a fibreglass line by constant refraction off its side walls.
The phenomenon of refraction is used to transfer the light from the source to the receiver.

Types of Fibre Optic Cables

Single-Mode Fibres

Core size or diameter is small, typically around 8-10 μm.
Uses only one mode of transfer.
Has a lower signal loss and a higher information capacity (bandwidth) than multimode fibres.
Can transfer higher amounts of data due to low fibre dispersion.

Multimode Fibres

Propagates more than one mode.
Core size is larger, making it easier to make fibre connections.
Permits the use of LEDs, which are cheaper, less complex, and more durable than laser diodes.
Has some disadvantages, including modal dispersion, where modes arrive at the fibre end at slightly different times.

Fibre Optic Cable Losses

Attenuation is caused by absorption, scattering, and bending losses.
Attenuation is the loss of optical power as light travels along the fibre.

Fibre Connection and Alignment

Fibre ends are inserted into ferrules, which are bonded using an adhesive, typically epoxy resin.
Ferrules are aligned using a threaded outer shell or some other type of coupling mechanism.
Expanded-beam connectors use two lenses to expand and then refocus the light from the transmitting fibre into the receiving fibre.

Fibre Optic Couplers

Can be either active or passive devices.
Passive couplers redistribute 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

Splits the optical power carried by a single input fibre into two output fibres.
Can distribute optical power evenly or unevenly between output fibres.

Fibre Optic Coupler - Combiner

Combines the optical power carried by two input fibres into a single output fibre.

Fibre Optic Coupler - X Coupler

Combines the functions of the optical splitter and combiner.

Applications in Avionics

Fibre optic technology has been implemented in aircraft vehicle management systems, including propulsion and flight controls.
Has been used in the Boeing 777 and 787 aircraft for data transmission and fuel measuring systems.

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Description

Learn about the structure and properties of optical fibres, including the core and cladding materials and their functions.