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OPTICAL FIBRES, 2, structure of optical fibre, OFS_FiberOptics_Figure3,…
OPTICAL FIBRES
Geometrical Properties
Basic Structure
Cladding: Located between Core and Coating
:check: Made of a dielectric material
:check: Reduces loss of light from the core into the surrounding air
:check: Reduces scattering loss at the surface of the core protects the fibre from absorbing surface contaminants
:check: Generally made of glass or plastic
Core: Surrounded by the cladding.
:check: A cylindrical rod of dielectric material
:check: Light propagates mainly along the core of the fibre
:check: Generally made of glass or plastic
Buffer/Coating: Outer layer.
:check: A layer of material used to protect the cladding from any environmental influences
:check: Material used for a buffer is a type of plastic
:check: Prevents scattering loss caused by microbends
Working Principle
:check: When a beam of light passes from one material to another with a different index of refraction, the beam is bent (or refracted) at the interface
:check: If the angle of incidence is greater than the critical angle for the interface, the light is reflected back into the incident medium without loss by a process known as total internal reflection
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:check: In order to ensure that rays entering the fibre end surface remain inside the core, the rays must approach the fibre end at angles less than the acceptance angle for the fibre
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Classification of Fibre
Fibre materials
Photonic crystal fibres (PCFs) are optical fibres that employ a microstructured arrangement of material in a background material of different refractive index. The background material is often undoped silica and a low index region is typically provided by air voids running along the length of the fibre.
Plastic-clad silica fibre (PCS) is an optical fibre that has a silica based core and a plastic cladding. PCS fibres in general have significantly lower performance characteristics, particularly higher transmission losses and lower bandwidths, than all glass fibres.
Refractive index profile
Single mode - The use of single-mode fibres in an optical transmission line fundamentally eliminates signal distortions due to modal delay, and thus single-mode fibres have now become the most widely used type of optical fibres in optical communications.
Step index - When an optical pulse is launched into a SI fibre, the propagation velocity of the pulse is highly dependent on the incident angle. This is problematic for high-speed optical communication as the incident pulse shape is degraded at the output.
Graded index - Graded index fibre was introduced in order to overcome this limitation of step index fibre. A graded index is an optical fibre whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fibre.
Core-cladding size
The function of the cladding is to provide a lower refractive index at the core interface in order to cause reflection within the core so that light waves are transmitted through the fibre.
Propagation path
The fact that the higher order modes travel farther in the glass core means that they have a greater likelihood of being scattered or absorbed, the two primary causes of attenuation in optical fibres. Therefore, the higher order modes will have greater attenuation than lower order modes, and a long length of fibre that was fully filled (all modes had the same power level launched into them) will have a lower amount of power in the higher order modes than will a short length of the same fibre.
Transmission Properties
Attenuation
:check: Signals lose strength as they are propagated through the fibre; beam attenuation
:check: Gives the transmission power loss in dB
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where Pin and Pout refer to input power and output power
:check: The attenuation of an optical fibre is wavelength dependent
Causes of attenuation/ Loss mechanism
:check: Absorption - Current manufacturing methods have reduced absorption caused by impurities to very low levels. Within the bandpass of transmission of the fibre, absorption losses are insignificant.
:check: Rayleigh Scattering - Microscopic-scale variations in the index of refraction of the core material can cause considerable scatter in the beam, leading to substantial losses of optical power. Rayleigh scattering is wavelength dependent and is less significant at longer wavelengths.
:check: Bending - Manufacturing methods can produce minute bends in the fibre geometry. Sometimes these bends will be great enough to cause the light within the core to hit the core/cladding interface at less than the critical angle so that light is lost into the cladding material. This also can occur when the fibre is bent in a tight radius. Bend sensitivity is usually expressed in terms of dB/km loss for a particular bend radius and wavelength.
Dispersion
:check: As the optical pulses travel the length of the fibre, they are broadened or lengthened in time; dispersion.
:check: Dispersion will result in pulse spreading and may lead to overlapping between adjacent pulses - information error occurs. Thus, dispersion sets a limit to the maximum link length (Lmax) possible for a certain bit rate (BR).
Types of dispersion
:check: Intermodal dispersion - Different modes propagate at different group velocities
:check: Intramodal or Chromatic dispersion - Material dispersion & Waveguide dispersion
:warning: Material dispersion - The index of refraction of the medium changes with wavelength
:warning: Waveguide dispersion - The index change across waveguide means that different wavelength have different delays
:check: Polarization mode dispersion - If waveguide is birefringent. Birefringent is a optical property of a material having a refractive index that depends on the polarization and propagation direction of light
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