Tech Breakfast: Fibre Optic Cabling
Transcript of Tech Breakfast: Fibre Optic Cabling
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Tech Breakfast: Fibre Optic Cabling
An introduction
[email protected] @IsItBroke on Twitter http://www.root6.com/author/phil
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Fibre optic cabling• Applications within Film & TV• Single mode vs. Multi mode – fundamentally different.• Tight buffered vs. loose tube cable• Pre-made vs. spliced cable• Connector types; LC, SC, ST, APC, MPO etc• Future developments.
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Applications within film & TV facilities
• Storage Area Networks• Network switch uplinks (backbone / “vertical segments” / End-Of-
Aisle topology)• Synchronous broadcast signals
– MADI, SDi, HDMI – KVM
• Long distance CWDM / DWDM (for December’s Tech Breakfast!)
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Single mode vs. multi mode cable• Single mode cable (“OS1” / “OS2” or “9µ/125µ”) is the original style of fibre
optic cable developed by BT and Corning in the 1970s• With a nine micron transmissive core the cable is able to contain the 1270 –
1690nm wavelengths efficiently.
V = number of modes k0 = wave number a = core’s radius (9µ vs. 50µ) n1 & n2 = refractive indices
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The cheap ‘n’ cheerful young pretender – Multi mode
• Whereas single-mode contains the light wave near-perfectly multi-mode cable relies on total internal reflection; the wavefront bounces down the fibre core as it hits the core/cladding interface.
• By clever use of interference modes many distinct waves can be launched down the cable and by using an interference detector laser-diode the modes can be recovered. All at 850nm typically.
• Limitations are length – 500 – 1,500m typically• Graded index fibre OM4 is the newest standard.
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A bit of multi-mode history• OM1 (lit. “Optical Multimode type 1”) ~ late ‘90s, obsolete
– 62.5µ core with 125µ cladding and VCSEL optimized glass • OM2 ~ early 2000’s; largely unused!
– 50µ core with 125µ cladding and VCSEL optimized glass • OM3 ~ 2007; still widespread.
– 50µ core with 125µ cladding and laser optimized glass • OM4 ~ 2009; the current standard.
– 50µ core with 125µ cladding and laser optimized graded-index glass
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Limitations of multi-mode optics
• Distance – due to “modal-dispersion” • Distances typically range from 150 – 650m (depending on
application) and can be calculated using the bandwidth-loss product. 10Gbits-1; typical max data rate in 2017.
• Graded Index OM4 fibre uses a transmissive core that varies it’s refractive-index across the diameter of the core so that the wavefront is better guided down the core.
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Some multi-mode gotchas• Encircled-flux refers to the effect of “overfilling” the
fibre when launching with cheap optics. The cladding carries an interfering signal.
• At multi-mode your link budget is single-figure dBs – at OM1 8dBs of loss is acceptable, at OM4 it should be sub 2dBs.
• If you mix & match 50µ and 62.5µ parts you loose ~2.5dBs at each junction.
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Live demo – Wesley “low-loss” Cyrus shows cable splicing• Pre-made patch cords are only really useful for local patching;
50m pre-mades are not good!• Tight-buffered cable (used to make patch cords) has worse
minimum-bend-radius vs. loose-tube cable. • Tight-buffered cable is not easily repaired when damaged.• Loose-tube cable spliced to pigtails is the better way.
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Cladding alignment vs. Core alignment splicing• Any manufacturing tolerance
issues in fibre show themselves in the position of the core within the cladding.
• Original machines used precisely milled v-grooves to give accurate “cladding alignment”
• Current gen machines (like the INNO IFS-15 Wes is using) use two cameras to accurately align the cores.
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Poetry in fibre-cabling….
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Quick and cheap fibre infrastructure fault-finding
• £10 pocket laser pointer shows bad splices or damaged cores very quickly
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Quick and cheap fibre infrastructure fault-finding cont.
• £100 fibre microscope shows contaminated connectors – Cleetops is used to restore the condition.
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The better technology – single mode fibre.
• In many ways very similar to multi-mode technology, same connectors, style of patch panel, cable construction etc.
• Works in an entirely different way – light is optimally constrained in a 9µ transmissive core so no need for total-internal-reflection with problems of modal dispersion etc.
• MUCH higher data rates and distances (80km without amplification)
• MUCH better signal-to-noise ratio; 28dBs with best optics.• Wider application in video and data.• Multiplexing is possible – more of that in the next session.
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Further reading• http://www.root6.com/tag/fibre/• http://philtechnicalblog.blogspot.co.uk/search/label/fibre• https://www.nexans.co.uk/eservice/UK-en_GB/navigate_221886/Cable_the_Future.html