phtn1220 cabling, connectors, splices see hecht, chapters 8, 13

PHTN1220Cabling, Connectors, Splices

See Hecht, Chapters 8, 13.

Basic Reasons for Cabling

• Protection of fiber– Typical fiber can only withstand about 2 lb (1

kg of tension) before breaking.– Fiber also needs protection from crushing and

from water absorption.

• Ease of handling– Bare fiber is hair-thin.

FIGURE 8-1 Cross sections of four grades of cable, from light-duty indoor to deep-sea submarine cable (not to scale).

Jeff HechtUnderstanding Fiber Optics, 4e

Copyright ©2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Types of Cable

• Many different types• A few main categories

– Indoor cables• for patch cords, etc.

• for use in air ducts (plenums), above false ceilings, etc.

– Outdoor cables• strung between poles

• in ducts

• underwater

FIGURE 8-2 Breakout cable. (Courtesy of Corning Cable Systems)




Composite and All-Dielectric Cables

• All-dielectric cables contain no conductive material– Less likely to be struck by lightning– Avoid grounding problems, useful in explosive

environments, etc.

• Some cables have copper as well as fiber.– Can carry power for amplifiers or repeaters.– Can carry electrical as well as optical signals.

FIGURE 8-3 Composite cable contains both copper wires and fibers. (Courtesy of Corning Cable Systems)




Aerial Cables

• For stringing between poles

• Need strength members (steel, fiberglass, Kevlar) to prevent stressing the fiber

• Sometimes lashed to a messenger wire.

FIGURE 8-4 Aerial cable installations.




Submarine Cable

• Usually needs copper for provision of power to repeaters/amplifiers.

• Armor needed near shore to avoid anchor damage.

• Cable must be solid (no air pockets) to withstand pressure.

• Cable must be completely waterproof.

FIGURE 8-5 Fiber-optic submarine cable. (Courtesy of TyCom Ltd.)




Cable Structure

• Fiber must be protected from all stresses.

• Three main concepts in fiber protection– Tight Buffer– Loose Tube– Ribbon

Loose tube

• Buffered fiber loose in a tube containing air or in a gel.

• One tube may contain several fibers.

• Mainly used outdoors.

• Protects fiber from all stress including thermal stress.

• Not good for long vertical runs.

FIGURE 8-6




Tight buffer

• Buffered fiber enclosed in a protective sheath.

• Easier than loose tube to handle and connectorize.

• Better for vertical runs, as in riser cables.

• Mainly used indoors.

FIGURE 8-6 Tightly buffered and loose tube structures for cables.




Ribbon

• Similar to tight buffer but with multiple fibers.

FIGURE 8-7 Fibers in a ribbon cable.




High fiber counts possible

Courtesy Pirelli Cables

Fiber ribbons

Dielectric centralmember

Filled buffer tube

Water-blocking material

Outer strengthmembers

876 fibers

FIGURE 8-8 Stacking ribbons inside loose tubes can give very high fiber counts. (Courtesy of Pirelli Cable)

Strength Members

• Relieve stress on fiber.

• May include a central core and/or layers surrounding the fiber(s).

• Common materials are steel, fiberglass, aramid yarn (Kevlar).

FIGURE 8-9 Modular cable containing 216 fibers, with 12 in each of 18 loose tubes. (Courtesy of Corning Cable Systems)




Joining Fiber

• Connectors– Temporary– Removable

• Splices– Permanent– Can be broken but splice must then be redone

• Connectors typically have greater losses than splices.

Attenuation in Connectors and Splices

Typically greater for connectors than splices– Connectors on the order of 0.5 dB

– Splices about 0.1 dB or less

• Caused by several factors:– core overlap

– alignment of axes

– different numerical apertures

– spacing between fibers

– reflection at fiber ends

Overlap of Fiber Cores

• If cores are not perfectly aligned at a connector or splice, some light will be lost.

• Loss = total area / overlapping area• Example: overlapped area = 90%

• Loss = 1.111

• Loss in dB = 10 log 1.111 = 0.46 dB

• This is approximate: assumes light is evenly distributed throughout core.

Overlap of fiber cores

d

Cladding

Core

This light couples into cladding where it will leak out

Offset

The light enterscore and is transmitted

FIGURE 13-1 Offset fibers can cause loss.

Mismatched Cores

• When light goes from larger to smaller core diameter, some light is lost– For single-mode fiber, use mode field diameter

instead of core diameter

• Light can be lost between two sections of the same fiber type if the diameters vary– Diameter is not exact but has specified

tolerance.

Loss with Different Core Diameters

2

1

22

21

d

ddLoss

•Proportion of power remaining after mismatch isgiven by this equation

Example: different cores

• See Hecht p. 302• Let d1 = 8.9 m, d2 = 7.9 m• Proportion of power lost in the mismatch is

212.0

9.8

9.79.82

22

Example Continued

• The proportion of power remaining is

Pout/Pin = 1– 0.212 = 0.788

That is, the “gain” of the splice is 0.788.

The loss in decibels is found as described earlier (see p. 100)

dB 03.1788.0log10log10)dB(

in

out

P

PnAttenuatio

Other Fiber Mismatches

• From MM to SM fiber (much smaller core).– Loss about 17 dB

• From larger to smaller MM fiber.– Loss about 1.9 dB from 62.5 to 50 m core

• Fibers with asymmetrical or elliptical cores.

• Note: no light is lost when going from smaller to larger core.

FIGURE 13-2 Losses arise when cores are elliptical or off center.




Misaligned Fibers

• Loss due to some light not achieving total internal reflection

• For a given amount of misalignment, loss is greater with smaller N.A.

• Example: for a fiber with N.A. of 0.15, misalignment of 3 causes a loss of 1 dB.

FIGURE 13-3 Misaligned fiber axes cause losses.




Mismatched N.A.

• Loss occurs when going from larger to smaller N.A.

• Some light from first fiber will not propagate in the second.

• No loss going the other way.

Loss from NA mismatch

Acceptance angle of Input fiber

Light confined in first fiberLeaks out of second fiber

With smaller NA

Is larger than acceptanceAngle of output fiber

FIGURE 13-4 Mating fibers with different NAs can cause losses.

Loss due to NA Mismatch

2

1

2log10)dB(

NA

NALoss

Note missing minus sign in Hecht (p. 304).

Example of NA Mismatch

• Light goes from a fiber with NA = 0.3 to one with NA = 0.2. Calculate the loss due to the mismatch.

dB 5.33.0

2.0log10)dB(

2

Loss

Spacing Between Fibers

• Not applicable to splices.

• Connectors often include a small air gap between fiber ends to avoid scratching the fiber.

• Some light will spread out too far to reach the second fiber.

• Loss increases with NA.

FIGURE 13-5 End-separation loss.




Calculation of Spacing Loss

0

arcsintan2

2/log10(dB)

n

NAS

d

dLoss

•Note minus sign added to make loss positive.

•Note that if an index-matching gel is used to separate the fibers, instead of air, the loss goes to zero.

FIGURE 13-6 Loss caused by fiber spacing for three types of fiber, neglecting reflection loss. Note sign should be positive for loss. As written, this is gain.




End Reflection Loss

• Occurs at all interfaces where refraction indexes change.

• Not applicable to splices.

• If a connector has an air gap there will be two of these reflections. This adds a loss of approximately 0.32 dB.

Equation for Reflection Loss

2

1log10(dB)/nn

nnsurfaceLoss

fiber

fiber

Other Losses

• Improperly cleaved fibers.

• Scratches.

• Dust.

Reflections in Connectors

• Can affect laser sources.

• Angled connectors can be used to prevent this.

• Caution! Mating angled and straight connectors will cause damage! Very bad news when the angled connector is on an expensive laser source! Check first!

FIGURE 13-7 A simplified generic fiber connector with coupling receptacle or adapter.




Connector Parts and Functions

• Ferrule aligns fibers.• Polished fiber end aligned with end of ferrule.• Body has locking mechanism to hold connector in

place.• Strain relief

– Often cable held in place by epoxy or other glue.

Sleeves/Adaptors

• Join two connectors.

• Can mate connectors of different types.

• Can be cable- or bulkhead-mounted.

FIGURE 13-8 Connector panel. (Courtesy of Corning Cable Systems, Hickory, N.C.)




Standard Connectors

• Many types

• Screw on, bayonet or push on

• Single or multiple cables

• Standard or small form factor

FIGURE 13-9 SC connector, expanded and assembled. (Courtesy of AMP Inc.) A common push-on connector.




FIGURE 13-10 ST connector, expanded and assembled. (Courtesy of Corning Cable Systems, Hickory, N.C.). Bayonet type.




FC/PC ConnectorsScrew-on connectors.

Drawing of FC/PC Connector

FIGURE 13-11 Fixed shroud duplex (FSD) connector for FDDI network. (Courtesy of Corning Cable Systems, Hickory, N.C.)




FIGURE 13-12 MT ferrule holds a dozen fibers in parallel grooves.




FIGURE 13-13 Male MPO connector assembly (Courtesy of US Conec.)




FIGURE 13-14 Small form-factor connectors. (a) MT-RJ connector. (Courtesy of Corning Cable Systems)




(b) Duplex LC Connector

• Outside resembles RJ-45 phone jack

• Simplex or duplex

• 1.25 mm ferrule

• Photo from Lucent

FIGURE 13-14

Fiber Splicing

• Two basic types– Fusion splicers weld fibers together using an

electric arc.– Mechanical splicers butt fibers together, usually

with an index-matching gel to avoid air gaps.

FIGURE 13-15 Key components of a fiber splicer.




Fusion Splicer

Fusion Splicer

• Provides means to align fiber (microscope or camera/video display)

• Small arc pre-fuses fiber

• Some models can align fiber automatically for lowest loss.

• Fusing arc welds fibers.

• Some models can test fiber loss.

Fiber Splicing Enclosure

FIGURE 13-16 Capillary splice joins two fibers.




Mechanical Splices

FIGURE 13-17 Mass-splicing of 12-fiber ribbon in V-grooved plate.




Splice Housings

• Splices are weak• They may be protected with plastic coating

or jacket but they need additional protection• Splice enclosure usually has room for

several splices, protects them from stress and from the elements.

• Housing allows cable breakouts, suitable bend radius, etc.

FIGURE 13-18 Splices arrayed inside housing.




phtn1220 cabling, connectors, splices see hecht, chapters 8, 13

Documents

fiber optics

cable structure fiber

fiberoptic submarine

loose tube buffered

types of cable

composite cable

grades of cable

breakout cable