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Tech Breakfast: Fibre Optic Cabling
An introduction
[email protected] @IsItBroke on Twitter http://www.root6.com/author/phil
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.
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!)
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
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.
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
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.
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.
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.
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.
Poetry in fibre-cabling….
Quick and cheap fibre infrastructure fault-finding
• £10 pocket laser pointer shows bad splices or damaged cores very quickly
Quick and cheap fibre infrastructure fault-finding cont.
• £100 fibre microscope shows contaminated connectors – Cleetops is used to restore the condition.
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.
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
