Chapter 11: Connectors and Splices

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Chapter 11 Connectors and Splices
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Need for Connectors and Splices

• Fiber optic cables are made in limited lengths
• Typically 1 to 6 km
• Required at intermediate points between
  transmitter and receiver
• Entrance to buildings, wiring closets, etc.
• Fibers must be connected to the source in the
  transmitter and the detector in the receiver
• Fiber subsystems must be connected together

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Connector Requirements

• Low loss
• Easy installation
• Repeatability connected and disconnected many
  times
• Consistency no variation in loss
• Economical
• General loss requirements
• 0.2 dB or less for telecom and long haul
• 0.3 to 0.75 dB for LANs and intrabuilding
• 1 to 3 dB for low-cost applications (plastic
  fiber)

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Causes of Loss in Interconnections

• Intrinsic or fiber-related
• Extrinsic or connector-related
• System factors

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Intrinsic Loss Factors

• NA mismatch
• Core-diameter mismatch
• Cladding-diameter mismatch
• Concentricity
• Ellipticity

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Extrinsic Loss Factors
End Separation
Lateral Displacement
Angular Misalignment
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System Related Loss Factors

• Launch and receive conditions
• Short launch and short receive
• Short launch and long receive
• Long launch and short receive
• Long launch and long receive
• Conditions in fiber change until EMD
• Mode stripping in overfilled fibers at source
• Cladding modes will not couple into second fiber
  and will be lost

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Insertion Loss

• Method for specifying the performance of a
  connector or splice
• Power through a length of fiber is measured
• The fiber is cut in the center and a connector or
  splice is applied
• Power at the end of the fiber is again measured
• Insertion loss is

Where P2 is initial power measured and P1 is
power after connector is applied
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Additional Losses

• When two different types of fiber are connected,
  diameter- and NA-mismatch losses may be
  significant

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Additional Losses Example

• Transmitting fiber has a core diameter of 62.5
  ?m and an NA of 0.275
• Receiving fiber has a core diameter of 50 ?m and
  an NA of 0.20

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0.20
LossNA 10 log10
-2.8 dB
0.275
2
50
Lossdia 10 log10
-2.9 dB
62.5
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Loss in Single Mode Fibers

• Small cores in single mode fibers require tighter
  tolerances for connectors
• A gap of 10 times the core diameter results in a
  loss of 0.4 dB in a single mode fiber.

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Return Reflection Loss

• When two fibers are separated by an air gap,
  optical energy will be reflected back toward the
  source. (Fresnel reflections)
• In single mode fiber with flat end faces, the
  loss amounts to -11 dB.
• If 500 ?W is at the end of the fiber, about 40 ?W
  is reflected back towards the source
• Can cause instabilities in the laser diode source
• A rounded or angled polished fiber end can help
  minimize the return loss.

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Return Reflection Loss
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Fiber Termination

• A connector or splice is used to terminate the
  fiber.
• Fusion splice is permanent and has very few parts
  in the connection (if any at all)
• Connectors use a ferrule to hold the fiber in
  optical alignment.
• Ceramic ferrules are best, stainless steel and
  plastic are also used
• Fibers are typically epoxied into the connector
• After the epoxy cures, the fiber end is polished
  to a smooth end finish (rounded or angled)

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Connector Examples

• FC-style Connector
• First one to use 2.5mm ceramic ferrule
• Uses a threaded coupling nut
• Has a tunable key
• Available in both single mode and multimode
  versions.

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Connector Examples

• ST-style connector
• Designed by ATT for premises wiring in buildings
  and LANs
• Uses same 2.5mm ceramic ferrule as FC
• Employs a quick release bayonet coupling
• Insertion loss is around 0.3 dB
• Most popular connector style

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Connector Examples

• SC-style connector
• Name comes from subscriber connector (telecomm)
• Designed to be pull proof, ferrule is decoupled
  from the cable and connector housing
• Uses a push-pull engagement for mating (snaps
  into socket)
• Can easily be configured into multiple fiber
  connectors

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Splices

• Fusion Splice permanent solution
• Uses an electric arc to weld two fibers together
• Computer controlled alignment of fibers
• Losses on the order of 0.05 dB
• Virtually eliminate return reflections
• Mechanical Splice
• Easily applied in the field
• Losses on the order of 0.1 to 0.2 dB
• Can be reenterable (reused)

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Fiber Preparation

• End face of fiber should be square to the fiber
  but 1? to 2 ? off the perpendicular is OK
• End face of fiber should have smooth mirror like
  finish
• Fiber is cleaved using scribe-and-break method
• End face polishing is accomplished after the
  fiber is connectorized.
• Polishing is done is several steps with
  repeatedly finer polishing grits. (down to 1 ?m
  or 0.3 ?m)