Transcript Slide 1

Introduction to Optical Fibre
Principles
Wavelength and Spectra
Wavelength:
Light can be characterised in terms of its wavelength
• Analogous to the frequency of a radio signal
•
The wavelength of light is expressed in microns or nanometers
The visible light spectrum ranges from ultraviolet to infra-red
Optical fibre systems operate in three IR windows around 800 nm,
1310 nm and 1550 nm
200
400
600
800
1000
1200
1400
Visible light
Fibre operating windows
Spectrum of light (wavelength in nanometers)
1600
1800
Advantages and Disadvantages
Advantages
Low attenuation, large bandwidth allowing long distance at high bit rates
Small physical size, low material cost
Cables can be made non-conducting, providing electrical isolation
Negligible crosstalk between fibres and high security, tapping is very difficult
Upgrade potential to higher bit rates is excellent
Disadvantages
Jointing fibre can be more difficult and expensive
Bare fibre is not as mechanically robust as copper wire
Fibres are not directly suited to multi-access use, alters nature of networks
Higher minimum bend radius by comparison with copper
Applications for Fibre in Buildings
Horizontal Cabling
Building Backbone
Most fibre is used in
campus and building
backbones
Horizontal cabling is
mainly copper at present
but may become fibre
Campus Backbone
How does Light Travel in a Fibre?
Optical Fibre
Transmitter
Electrical
output
signal
Receiver
Light ray trapped in
the core of the fibre
Electrical input
signal
Fibre Types
Three generic fibre types dominate the building cable market
Multimode is most popular but singlemode is now being installed more
frequently
Multimode is more tolerant of source and connector types
Singlemode offers the largest information capacity
Multimode fibre
Multimode fibre
Singlemode
fibre
62.5 micron
core diameter
50 micron core
diameter
8 micron core
diameter
125 microns
cladding
diameter
Decibels and Attenuation
Basic decibel power equation relates two absolute powers P1 and P2:
Power ratio in dB = 10 Log [P1/P2]
10
In a fibre or other component with an input power Pin and an output power
Pout the loss is given by:
Loss in dB = 10 Log [Pout/Pin]
10
By convention the attenuation in a fibre or other optical component is
specified as a positive figure, so that the above formula becomes:
Attenuation in dB = -10 Log [Pout/Pin]
10
Absolute power in Decibels
It is very useful to be able to specify in dB an absolute power in watts or
mW.
To do this the power P2 in the dB formula is fixed at some agreed reference
value, so the dB value always relates to this reference power level.
Allows for the easy calculation of power at any point in a system
Where the reference power is 1 mW the power in an optical signal with a
power level P is given in dBm as:
Power in dBm = 10 Log [P/1mW]
10
For example 2 mW is +3 dBm, 100 µW is -10 dBm and so on. Negative dBm
simply means less than 1 mw of power. 1 mW is 0 dBm
Watts to dBm Conversion Table
Power (watts)
Power (dBm)
1W
100 mW
10 mW
5 mW
2 mW
1 mW
500 mW
200 mW
100 mW
50 mW
10 mW
5 mW
1 mW
500 nW
100 nW
+30 dBm
+20 dBm
+10 dBm
+7 dBm
+3 dBm
0 dBm
-3 dBm
- 7 dBm
-10 dBm
-13 dBm
-20 dBm
-23 dBm
-30 dBm
-33 dBm
-40 dBm
Attenuation in Fibre:
Transmission Windows
Three
low loss transmission windows exist circa 850, 1320, 1550 nm
Earliest systems worked at 850 nm, latest systems at 1550.
1st window
circa 850 nm
Loss
dB/Km
2nd window
circa 1320 nm
3rd window
circa 1550 nm
10
1
Wavelength in nanometers
0.1
600
700
800
900
1000 1100 1200 1300 1400 1500 1600
Bending Loss in Fibres
At a bend the propagation conditions alter and light rays which would
propagate in a straight fibre are lost in the cladding.
Macrobending, for example due to tight bends
Microbending, due to microscopic fibre deformation, commonly caused by
poor cable design
Microbending is commonly
caused by poor cable design
Macrobending is commonly
caused by poor installation or
handling
Fibre Dispersion and
Bandwidth
Types of Optical Fibre
Three distinct types of optical fibre have developed
The three fibre types are:
• Step index fibre
Multimode
• Graded index fibre
fibres
• Singlemode fibre (also called monomode fibre)
Dispersion in an Optical Fibre
Fibre type influences so-called "Dispersion"
The higher the dispersion the lower the fibre bandwidth
Lower fibre bandwidths mean less information capacity
Modal Dispersion:
Reduced by using graded index fibre
Eliminated by using singlemode fibre
Material Dispersion:
Reduced by using Laser rather than LED sources
Reduced by operating close to 1320 nm
Multimode Fibre Bandwidth (I)
Combination of modal and material dispersion limits fibre bandwidth
Dispersion is rarely specified, bandwidth is more useful
Typically stated as MHz.km
For example ISO 11801 specifies 500 MHz.km for 50/125 µm fiber in
the 1300 nm window
Bandwidths range from about 200 MHz.km to 2000 MHz.km.
50/125 µm fibre will have higher bandwidth than 62.5/125 µm fibre
Multimode Fibre Bandwidth (II)
To find the bandwidth of a fibre span, divide the bandwidth in MHz.km by
the fibre span in km.
The longer the fibre span, the lower the overall bandwidth.
Example:
MHz.km
Assume a fibre bandwidth of 600
Fibre span = 1.5 km
0.9 km
250 m
Overall bandwidth = 375 MHz
Overall bandwidth = 666 MHz
Overall bandwidth = 2400 MHz
Multimode Fibre Bandwidth and Bit
Rate in LANs
Relationship between available bandwidth and maximum bit rate is
complex
For LANs and building cabling systems rule is (from standards):
Maximum bit rate in MB/s =
Fibre bandwidth in MHz.km
2 x Fibre span in km
Rule is very conservative, assumes zero dispersion penalty is required
For example for a 500 MHz.km over 2000 m the maximum bit rate is
125 MB/s
In practice use a fibre that exceed the standards for a given LAN to
ensure adequate bandwidth
Summary
Optical fibre systems utilise infared light in the range 700 nm to
1600 nm
Fibre has a number of significant advantages
Building fibre systems operate around 1320 nm
Multimode fibres suffer from modal and material dispersion
Material dispersion is minimised by operating near 1320 nm
Singlemode fibre eliminates material dispersion
Planning Fibre Systems:
Standards & Power Budgeting
in Local Area Networks
Relevant standards
Power budget definition
Power margins
Sample exercises
EN 50173: Functional Elements
EN 50173 Information technology Generic cabling systems
A number of functional elements are defined:
–Campus
Distributor (CD)
–Campus
Backbone Cable
–Building
Distributor (BD)
–Building
Backbone Cable
–Floor
Distributor (FD)
–Horizontal
Cable
–Transition
Point (optional) TP
–Telecommunications
Outlet (TO)
Krone
EIA/TIA 568-B and Fibre
EIA/TIA 568-B 2001 Commercial Building Telecommunications Wiring
Standard
•This is an American Standard
•International and European standards used this as their basis
•Recognises 62.5/125 micron fibre for horizontal cabling
•Recognises 62.5/125 micron fibre and singlemode fibre for backbones
•Section 12 of the standard covers fibre specs
•No longer specifies a particular connector type but sets minimum
standards the connector must meet
•Maximum mated pair connector attenuation is 0.75 dB
•Maximum splice loss for fusion or mechanical is 0.3 dB
•Different colour coding for multimode and singlemode connectors
Summary of EIA/TIA 568-B Fibre
Specifications
Specification
Horizontal 62.5/125
µm
Backbone
62.5/125 µm
Backbone
Singlemode
Atten @ 850 nm
3.75 dB/km
3.75 dB/km
-
Atten @ 1300 nm
1.5 dB/km
1.5 dB/km
0.5 dB/km (outside)
1.0 dB/km (inside)
Atten @ 1550 nm
-
-
0.5 dB/km (outside)
1.0 dB/km (inside)
Bandwidth (850
nm)
160 MHz.km
160 MHz.km
Not spec.
Bandwidth (1300
nm)
500 MHz.km
500 MHz.km
Not spec.
Fibres/cable
recommended
Minimum 2
fibres/cable
6-12 fibres/cable
6-12 fibres/cable
ISO 11801:2002
Information technology -- Generic cabling for customer premises
•
•
•
•
•
•
ISO/IEC 11801 specifies generic cabling for use within premises, which may comprise single or multiple
buildings on a campus. It covers balanced cabling and optical fibre cabling.
ISO/IEC 11801 is optimised for premises in which the maximum distance over which
telecommunications services can be distributed is 2000 m. The principles of this International Standard
may be applied to larger installations.
Cabling defined by this standard supports a wide range of services, including voice, data, text, image
and video.
This International Standard specifies directly or via reference the:
• structure and minimum configuration for generic cabling,
• interfaces at the telecommunications outlet (TO),
• performance requirements for individual cabling links and channels,
• implementation requirements and options,
• performance requirements for cabling components required for the maximum distances specified in
this standard,
• conformance requirements and verification procedures.
Safety (electrical safety and protection, fire, etc.) and Electromagnetic Compatibility (EMC)
requirements are outside the scope of this International Standard, and are covered by other standards
and by regulations. However, information given by this standard may be of assistance.
ISO/IEC 11801 has taken into account requirements specified in application standards listed in Annex F.
It refers to available International Standards for components and test methods where appropriate
Fibre Types in LANs
• According to ISO – 11801
– International Standards Organization
• OM1 fiber – 200/500 MHz.km OFL BW (in practice OM1
fibers are 62.5 μm fibers)
• OM2 fiber – 500/500 MHz.km OFL BW (in practice OM2
fibers are 50 μm fibers)
• OM3 fiber – Laser-optimized 50 mm fibers with 2000 MHz.km
EMB at 850 μm
Maximum Distances
• According to ISO 11801
• Maximum channel length varies between 300m to 2000m
depending on the application
• Specific applications are bandwidth limited at the channel
lengths shown in the standard document
• For example ATM running over a 50μm fiber
– ATM 155 Mbits/s @ 850nm 1000m
– ATM 622 Mbits/s @ 850nm 300m
– ATM 155 Mbits/s @ 1300nm 2000m
– ATM 622 Mbits/s @ 1300nm 330m
ISO 11801 Optical fibre
cable attenuation
Maximum cable attenuation dB/km
OM1, OM2 and OM3
Multimode
OS1 Single-mode
Wavelength
850 nm
1300 nm
1310 nm
1550 nm
Attenuation
3.5
1.5
1.0
1.0
ISO 11801:2002
Note:Attenuation is in dB/km
ISO 11801 Optical fibre
Channel Classes
• Class OF-300
– Supports applications to a minimum of 300m
• Class OF-500
– Supports applications to a minimum of 500m
• Class OF-2000
– Supports applications to a minimum of 2000m
ISO 11801 Optical fibre Channel
Attenuation
Channel Attenuation in dB
Channel
Multimode
Single-mode
850nm
1300nm
1310nm
1550nm
OF-300
2.55
1.95
1.80
1.80
OF-500
3.25
2.25
2.00
2.00
OF-2000
8.50
4.50
3.50
3.50
The channel attenuation shall not exceed the values shown in the table
above. The values are based on a total allocation of 1.5dB for connecting
hardware.
ISO 11801:2002
11801 Standards for Fibre
Joints in Buildings
For connectors maximum mated pair connector attenuation is 0.75 dB
Different colour coding for multimode and singlemode connectors
Maximum splice loss for fusion or mechanical is 0.3 dB
Mated pair of ST type
Optical Connectors
Building Cabling Connectors
and Standards
Presently the ST-compatible connector and SC-compatible connector are the most
commonly used connectors for termination.
ISO 11801 nolonger specifies a specific connector type but points to a minimum set of
specifications that an optical connector must meet.
The primary advantages of the SC connector are:
 It is a duplex connector, which allows for the management of polarity.
 It has been recommended by a large number of standards.
 Most SC connectors offer a pull-proof feature for patch cords.
Many small form factor connectors are now being widely used in the building cabling
market
ISO 11801 Multimode optical fibre modal
bandwidth
Minimum modal bandwidth MHz x km
Overfilled launch bandwidth
Wavelength
Optical fibre Core diameter
type
μm
OM1
50 or 62.5
Effective laser launch bandwidth
850 nm
1300 nm
850 nm
200
500
Not specified
OM2
50 or 62.5
500
500
Not specified
OM3
50
1500
500
2000
Note Effective laser launch bandwidth is assured using differential mode delay (DMD) as specified in
IEC/PAS 60793-1-49. Optical fibres that meet only the overfilled launch modal bandwidth may not
support some applications specified in Annex F.
ISO 11801:2002
FDDI
www.wildpackets.com/support/compendium/fddi/overview
http://www.cisco.com/en/US/docs/internetworking/technology/handbook/FDDI.html
Fiber Distributed Data Interface
•Standard published in 1987
•Uses a token passing protocol
like ‘Token Ring’
•Power budget is 11dB
•TX -20dBm, Rx -31dBm
•Dual Ring LAN
From CISCO
•Operate in opposite directions called ‘counter rotating’
•Primary Ring which is normally used ‘live’
•Secondary Ring which lies idle
•Can use single or multimode fibre
•SM – 60km, MM – 2km
Dual Ring
Station failure – see above
Cable failure – see above
From CISCO
•The primary reason for the dual ring feature of FDDI is for fault tolerance. If a
station is powered down, fails or a cable is damaged then the ring is automatically
wrapped on itself.
•Limited to one station or cable fault
Optical Bypass Switch
From CISCO
•Provides continuous dual ring operation if a device on the dual ring fails.
•Uses an optical switch to reroute the data
•Network does not enter the wrapped condition
Power Budgeting
Power Budget Definition
Power budget is the difference between:
ƒ
The minimum (worst case) transmitter output power
ƒ
The maximum (worst case) receiver input required
Power budget value is normally taken as worst case.
In practice a higher power budget will most likely exist but it cannot be relied
upon
Available power budget may be specified in advance, e.g for 62.5/125 fibre in
FDDI the power budget is 11 dB between transmitter and receiver
Power Budget (dB)
TRANSMITTE
R
Fibre, connectors and splices
RECEIVER
Launch Power
Fibre
LED/Laser Source
Launch power
Transmitter output power quoted in specifications is by convention the launch power.
Launch power is the optical power coupled into the fibre.
Launch power is less than the LED/Laser output power.
Calculation of launch power for a given LED/Laser and fibre is very complex.
Power Margin
Power margins are included for a number of reasons:
ƒ
To allow for ageing of sources and other components.
ƒ
To cater for extra splices, when cable repair is carried out.
ƒ
To allow for extra fibre, if rerouting is needed in the future.
ƒ
To allow for upgrades in the bit rate or advances in multiplexing.
Remember that the typical operating lifetime of a fibre system may be
as high as 20 years.
No fixed rules exist, but a minimum for the power margin would be 2
dB, while values rarely exceed 8-10 dB. (depends on system)
Sample Power Budget Calculation
(FDDI System)
Power budget calculation used to calculate power margin
Transmitter o/p power (dBm)
Receiver sensitivity (dBm)
Available power budget:
-18.5 dBm min, -14.0dBm max
-30 dBm min
11.5 dB using worst case value (>FDDI standard)
In most systems connectors are used
at the transmitter and receiver
terminals and at patchpanels.
Number of Connectors
Worst case Connector loss (dB)
Total connector loss (dB)
6
0.71
4.26
Fibre span (km)
Maximum Fibre loss (dB/Km)
Total fibre loss (dB)
2.0
1.5 dB at 1300 nm
3.0
Number of 3M Fibrlok mechanical splices
Worst case splice loss per splice (dB)
Total splice loss (dB)
10
0.19
1.9
Total loss:
9.16 dB
Power margin (dB)
2.34
Splices within patchpanels and
other splice closures
Answer
Sample Exercises
LAN Exercise 1
• The design for a building optical fibre link is as below.
Calculate the power budget using the ISO 11801 component
losses.
– Operates at 850nm
– Transmitter launch power
• Max -15dBm
• Min -18dBm
– Receiver Sensitivity
• Max -30dBm
• Min -28dBm
– 62.5/125 μm fibre
• 4 Lenghts, 500m, 300m, 150m and 800m.
– Connector pairs
• 2
– Splices
• 1
LAN Exercise 1, cont
• Calculate the bandwidth of the system.
• What improvements would be made to the system if
the operating wavelength is 1300nm.
LAN Exercise 2
• An optical link in a building and campus is to be the full
2000m length. Due to some restrictions the fibre must be
installed in a number of shorter lengths. Calculate what are
the minimum fibre lengths that can be installed if splices are
used and then if connectors are used. A power margin of 2dB
must be maintained. Note: we want to install the fibre in short
lengths to make the installation easier.
– Operates at 1300nm
– Transmitter launch power
• Max -8dBm
• Min -10dBm
– Receiver Sensitivity
• Max -30dBm
• Min -28dBm
LAN Exercise 3
• The FDDI link between locations shown below needs to be
extended and re-routed due to unforeseen building
alterations.
– The cable must be rerouted to avoid an obstruction
– The new cable pathway around the obstruction is approximately
150m long
– System is operating at 1300nm. Power budget is 11dB according to
FDDI standard
– Green circles are mated pair correctors
– X is a splice
• 1. Assuming all existing cable remains draw a new system
diagram and determine if the system will work using ISO
11801 losses.
• 2. Assuming new cable can be pulled in (replacing the whole
265m length) what is the improvement in the power budget
compared to one above.
1120m
TX
312m
265m
Obstruction
RX
158m
Specification and Other Issues
Component
Specification and
Selection
The Path from Specification to Completion
System
Specifications
System Design
and Optical
Design
In this section we are
concerned with some of the
issues which arise regarding
component selection,
installation and acceptance
testing
Component
Specification and
Selection
Installation
Completed
System
Commissioning
and Acceptance
Tests
Component Selection
Component
Comment
Transceivers
FDDI, Fibre channel etc.. Laser v LED
Fibre
Core size and multimode v singlemode
Cables
Construction and fibre count
Enclosures
Rack and patchpanels, cable management
Cable fixings
Tray types, outdoor ducts
Connectors
ST , SC or small form factor (SFF) connectors
Termination
method
Direct connection or fusion spliced v mechanical spliced pigtails
Adapters, pigtails, patchleads, fibre organisers etc..
Ancillary
Multimode Fibre Choices
Backbones can utilise multimode 50/125 µm, 62.5/125 µm or singlemode
fibre
50/125 µm fibre have a lower input power by comparison with 62.5/125 µm
fibre using the same LED transceiver: power budget impact
50/125 µm fibre has a larger bandwidth than 62.5/125 µm fibre, typically
60% larger.
62.5/125 µm fibre will support in excess of 1 Gb/s up to 300 m. 90% of all
building backbones are < 300 m long.
Coupling from LEDs into Multimode
fibres
Smaller core fibre
Larger core fibre
LED Source
Optical power coupled into the fibre depends on core diameter and
numerical aperture
Assume a 4.7 dB source coupling loss for the same LED source
into 50/125 µm fibre compared to a 62.5/125 µm fibre
Multimode V Singlemode Fibre
Choices
LED transceivers cannot be used with singlemode fibre
Singlemode uses Laser based transceivers, but will support all
backbone lengths at multi-Gb/s
Mix of multimode and singlemode possible,
Mix allows LED/multimode today with upgrade to Laser/singlemode
later without retrofit
Component Selection: Fibre Optic
Cables
Most effective method is to review installation and operating environment
Aids include the FIA guidelines "Fibre Optic Cable Selection Guide, Document
No. FIA/FCC/1/95
Other points to note are:
For direct burial and external duct installation loose tube cable means lower
fibre stress
Internal horizontal runs need flexible cables so tight jacket cables are the norm
Vertical runs need special care (see next overhead)
All fibres must be uniquely identifiable
Multimode and singlemode fibre may be accommodated in the same cable
Vertical Cabling
Vertical runs need care. Tight jacket cables tend to result in the uppermost fibre span
being loaded by cable weight, this favours loose tube
For tight jacket cables use short horizontal runs or cable loops to reduce fibre load
Loose tube cables has a problem with moisture protection gel oozing out of the cable
tubes under gravity in external vertical cable runs
Multimode and Singlemode Fibres in
Cables (I)
Multimode AND singlemode cables may be installed together
Singlemode is kept as dark fibre until used
Provides future upgrade path
Ratio of MM to SM fibres:
ƒ
Optimal ratio depends on forecasted customer needs
ƒ
Typically for customers forecasting gigabit applications the present advice is 30% singlemode
Cables may be separate or composite, choice depends on a number of
factors
Multimode and Singlemode Fibres in Cables (II)
Separate Cables:
MM and SM are segregate in two separate cables
Easier segregation, fewer installation errors
Ease of segregation is particularly important in outdoor applications
Occupies more physical space than a composite cable
Separate patchpanels can be used to avoid confusion
Composite Cables:
SM and MM share a single cable
Occupies significantly less space
May be more prone to installation errors,
May require single patchpanel, causes confusion
Limited availability and higher costs
Enclosure Specification and Selection
For enclosures selection is influenced by:
Environmental factors such as temperature and humidity as well as vibration and
moisture.
Mounting requirements: rack based or wall mounted
Location and access requirements. User interference, security
Ease of maintenance and repair. Future upgrade potential
Focas wall mounting splice enclosure
Focas 19" patchpanel
Cable Termination
In most building and campus
installations fibre cabling is installed
between patchpanels
Intermediate splices and
enclosures may be needed, where
a cable enters/leaves a building
At patchpanels a number of
termination options exist:
ƒ
Preconnectorised fibre pigtails fusion
spliced to incoming cable fibres
ƒ
Preconnectorised fibre pigtails
mechanically spliced to incoming
cable fibres
ƒ
19" rack patchpanel
Cable 2
Cable 1
Cable 3
19" rack patchpanel
19" rack patchpanel
Direct connectorisation of incoming
cable fibres
Connectors for patchcords to
transceivers or other fibres
Direct Connectorisation versus Spliced Pigtails
Economics:
ƒ
ƒ
Quickfit connector kits cost €1500 to over €3000, connectors cost about €5
Spliced pigtails involve the pigtail cost (€5) and the splice cost (€1-2 for mechanical but almost zero for
fusion).
Loss specification may influence decision. Splicing involves an extra "unneccessary"
loss by comparison with direct connectorisation
But preterminated pigtail connectors done in "ideal" factory conditions are likely to
show lower loss than those done in the field
AMP Lightcrimp Quickfit
Connectors
AMP Corelink
Mechanical Splices
Fusion Splicing versus Mechanical Splicing
Economics:
ƒ
Mechanical splices have low tooling costs, but each splice is more expensive (€1-2)
ƒ
Fusion splicing involves expensive equipment (€7K to €40K), but very low cost splices
ƒ
Organisations undertaking jointing infrequently should consider mechanical splicing
Loss specification may influence decision. Repeatable losses below 0.06 dB will require
fusion splicing
Installation conditions, labour costs etc.. greatly influence choice between fusion and
mechanical splicing. UK surveys have proved inconclusive
Northern Telecom Compact
Splicer
3M FibrLok II
Mechanical Splices
Pigtail Specification & Selection
Specification
Comment
Length
1 m typically but beyond 1.5 m excess fibre is untidy
Fibre
Multimode 50/125 or 62.5/125
Buffer
250 µm or 900 µm (blown fibres may be different)
Connector
ST or SC type (see connector specification & selection)
Colour code
Ideally a range of colour codes should be available, but not always so
Test Cert.
Test certificate should accompany all pigtails, stating factory insertion
loss test results
or
singlemode
Patchcord Specification & Selection
Specification
Comment
Length
Variable but 1-3 m is typical
Fibre
Multimode 50/125 or 62.5/125
Diameter
2.5 mm is typical but newer designs are smaller
Connector
ST or SC type (see connector specification & selection)
Duplex/simplex
Patchpanels normally use simplex, desktop-to- wall outlet use duplex.
Duplex at a patchpanel is tidier and less error prone
or
singlemode
Cable should indicate fibre spec (see above)
Markings
Test Cert.
Test certificate should accompany all patchcords, stating factory
insertion loss test results
Connector Specification & Selection
Applies to loose connectors and connectors on pigtails & patchleads
Specification
Comment
Type
SC is the industry standard but ST very common. Small Form Factor
(SFF) connectors are becoming more common
Ferrule
Plastic metal or ceramic. Ceramic gives the lowest loss, plastic is a
poor choice (high loss and susceptible to damage)
Polish
Not a big issue in building cabling
Strain Relief
Simple plastic strain relief on buffered fibres, more complex on
patchcord fibres
Directional coding and multimode/singlemode coding needed
Colour code