OCAM6 Flashcards by Adrian Lee

What are the 3 layers of optical fibre?

1) Core - nearly pure silicon dioxide, transmission path for optical signals
2) Cladding - nearly pure silicon dioxide with some impurities to confirm light to core
3) Buffer/coating - durable resin to protect core and cladding from moisture and physical damage

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What the two basic configurations of optical fibre?

1) 250 micron coated fibre

2) 900 micron tight buffered fibre

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What are the two types of optical fibre cables (bundles)?

1) Tight buffered

2) Loose tube

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Properties of tight buffered cable

1) Multiple, colour coded 900micron optical fibres
2) Indoor applications
3) Must pass flame-retardant and smoke-inhibitor ratings from National Electrical Code (NEC)

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Properties of loose tube cable

1) Multiple, loose tubes with 250micron bare fibres
2) Outdoor application
3) Must be moisture resistant, UV resistant and temperature tolerant

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Properties of cordage cables

1) Typically used for connection between equipment and patch panels (short distance)
2) Only one fibre/buffer combination in a jacket
3) Can be simplex or duplex

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Properties of simplex cordage

1) single fibre with a tight buffer
2) aramid yarn strength member
3) PVC jacket
4) One way transmission

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Properties of duplex cordage

1) AKA zipcord

2) combines two simplex cords for two transmission

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Properties of Distribution cable

1) Multiple, tight buffered fibres bundled ins jacket to run through a building
2) not meant to be used outside office walls
3) Used for patch panels or communication closets

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Properties of breakout cable

1) Two or more simplex cables around a central strength member covered with outer jacket
2) Run through buildings but are connected individually

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Properties of ribbon cable

1) fibres bonded side by side or held in grooves in a single tight jacket
2) Save space as fibres not individually jacketed
3) Useful for high volume installs

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Properties of Armoured cable

1) Steel or aluminium jacket covered with polyethylene jacket
2) Can be run aerially, installed in ducts or placed underground

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Properties of submarine cable

1) steel wire to provide armour to several layers
2) Stainless steel tube for strength
3) Water blocking properties allowing 8000m depths

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What is the main differences between SMF and MMF?

The size of the core which affects the light carrying characteristics

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Properties of SMF

1) Most optical network applications
2) Suitable for long distances, high bandwidth applications
3) Core diameter is sized to be a small multiple of the wavelength range of the signal. This concentrates the light in a single path, reducing optical loss

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Types of SMF

1) Non-dispersion Shifter Fibre (NDSF) - ITU G.652
2) Dispersion Shifted Fibre (DSF) - ITU G.653
3) Non-zero Dispersion Shifted fibre (NZ-DSF) - ITU G.655

Properties of MMF

1) Applications of short distances
2) Larger core diameter, allowing for wavelengths to take multiple paths
3) Means wavelengths care reach destination at different times (modal dispersion) - okay for short, low bandwidth networks
4) Operate at 850 and 1300nm

Types of MMF

1) OM1 - premise applications
2) OM2 - premise applications
3) OM3 - premise applications, laser optimised (higher bit rate)
4) OM4 - premise applications, laser optimised, longer transmission rates
5) OM5

Types of full size connectors

1) Straight Tip (ST) - SMF & MMF, twist connection
2) Ferrule connector (FC) - SMF & MMF, screw on connection
3) Subscriber connector (SC) - SMF & MMF, snap in connection
All connectors have 2.5mm ferrule

Types of Small Form Factor (SFF) Connectors

1) Lucent connectors (LC) - SMF & MMF, push and latch connection
2) Mechanical Transfer Registered Jack (MT-RJ) - MMF, 2 fibres in one ferrule
3) Miniature Unit (MU) - SMF & MMF, push and pull connection
4) E2000 (LX.5) - SMF & MMF, automatic spring loaded protective cap
5) Multi-fibre Push-On (MPO) - SMF & MMF, push and pull connection, mass terminated in 4-72 ribbon cables

What is the Silica-based Glass fibre loss profile?

Defines the windows of low loss where communication is used in optical cable (850, 1310 and 1550nm). Also defines the ares called water peak where the OH ion causes fibre impurities giving high loss.

What factors contribute to optical loss?

1) Fibre intersections like connections

2) Anomalies in the fibre itself

Define optical attenuation

The loss of optical power as the light signal travels through the fibre, measured in dbs. Limits the length a signal can travel

Factors that cause attenuation

1) Absorption
2) Scattering
3) Bending losses
4) Fresnel reflection
5) Splices
6) Contamination in fibre connectors

What is absorption?

Absorption is when the glass of the optical fibre absorbs a portion of the light and converts it to heat. The amount that is absorbed depends on the wavelength and the molecular structure of the glass. Absorption cannot be controlled.

What is Rayleigh Scattering?

Is the effect when light hits a particle or impurity in the fibre causing the light to scatter in all directions. If the light scatters back towards the source, this is back scatter. Backscatter is approx 0.0001% of transmitted light The longer the fibre the likelier there will be more scattering.

What is a micro bend?

Microbending is when the fibre bends at a microscopic scale. It is not visible to the eye but appears as wrinkle on the fibre. This small wrinkle can change the path at which the light is suppose to propagate. This increases attenuation because low-order modes become coupled with high-order modes. The main cause for micro bends is temperature changes during the manufacturing.

What is a macro bend?

A macro bend is visible to the eye and is when the bend radius is larger than the fibre diameter. This can happen during install when bending the fibres too much or installed tie wrap to the fibres.

What is Fresnel reflections?

Fresnel reflection occur when connecting fibres with a connector results in an air gap. This air gap reflects the optical signal back into the fibre segment.

Factors that affect fresnel reflections?

1) Angle of incidence - straight on or 90 degrees reflect the most 2) Density difference of the materials - great the difference means more reflection 3) Quality of polish - finer polish, less reflectance

What is ORL?

Optical Return Loss is the total loss from reflected light and is measured in dbs. A +50db ORL is preferable but +30db is acceptable as the higher the value indicates less reflected power

What are the two types of splicing?

1) Fusion | 2) Mechanical

What is fusion splicing?

Fast, reliable, low loss fibre- fibre connection by creating a homogenous joint. The two ends of the fibres are melted together using an electric arc resulting in loss range 0.01-0.1db. It is practically non-reflective. However, if the cores are misaligned or of different sizes then some light may not enter the second fibre. this is the splice loss.

What is mechanical splicing?

Mechanical splicing is less reliable, not as cost effective and can result in higher losses. This is because it depends on the alignment of the core of the fibres, the wider the gap, the more loss. However, it is improving and is used for emergency restoration as it is fast, easy and although not cost effective is still inexpensive for one off restoration. Loss range of 0.05-0.2db

What is connector loss?

Connectors rely on the ability of the bulkhead to line up the two fibre ends and so light spreads as it comes out of the first core. Even when the gap is small, some light can be lost. Furthermore, contamination on the connectors can cause further loss. Cleaning and inspection of the fibres is very important as even microscopic dirt can hinder an optical link.