1. Introduction to Optical Fibre Principles

2. Wavelength and Spectra Spectrum of light (wavelength in nanometers)
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, nm and 1550 nm
200
400
600
800
1000
1200
1400
1600
1800
Visible light
Fibre operating windows
Spectrum of light (wavelength in nanometers)

3. Advantages and Disadvantages
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

4. 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

5. 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

6. 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
125 microns
cladding diameter
62.5 micron core diameter
50 micron core diameter
8 micron core diameter

7. 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

8. 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

9. Watts to dBm Conversion Table
Power (watts)
Power (dBm)
1 W
+30 dBm
100 mW
+20 dBm
10 mW
+10 dBm
5 mW
+7 dBm
2 mW
+3 dBm
1 mW
0 dBm
500 mW
-3 dBm
200 mW
- 7 dBm
100 mW
-10 dBm
50 mW
-13 dBm
10 mW
-20 dBm
5 mW
-23 dBm
1 mW
-30 dBm
500 nW
-33 dBm
100 nW
-40 dBm

10. 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
2nd window
circa 1320 nm
3rd window
circa 1550 nm
Loss
dB/Km 10 1
Wavelength in nanometers
0.1
600
700
800
900
1000
1100
1200
1300
1400
1500
1600

11. 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

12. Fibre Dispersion and Bandwidth

13. Types of Optical Fibre
Three distinct types of optical fibre have developed
The three fibre types are:
Step index fibre
Graded index fibre
Singlemode fibre (also called monomode fibre)
Multimode
fibres

14. 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

15. 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 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

16. 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: Assume a fibre bandwidth of 600 MHz.km
Overall bandwidth = 375 MHz
Fibre span = 1.5 km
Overall bandwidth = 666 MHz
0.9 km
250 m
Overall bandwidth = 2400 MHz

17. 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):
Fibre bandwidth in MHz.km
Maximum bit rate in MB/s =
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

18. Summary
Optical fibre systems utilise infared light in the range 700 nm to 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

19. Planning Fibre Systems:
Standards & Power Budgeting in Local Area Networks

20. Relevant standards
Power budget definition
Power margins
Sample exercises

21. EN 50173: Functional Elements
EN 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

22. EIA/TIA 568-B and Fibre
EIA/TIA 568-B 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

23. Summary of EIA/TIA 568-B Fibre Specifications
Horizontal 62.5/125 µm
Backbone 62.5/125 µm
Backbone
Singlemode
850 nm
3.75 dB/km - nm
1.5 dB/km
0.5 dB/km (outside)
1.0 dB/km (inside) nm
1.0 dB/km (inside)
Bandwidth (850 nm)
160 MHz.km
Not spec.
Bandwidth (1300 nm)
500 MHz.km
Fibres/cable recommended
Minimum 2 fibres/cable
6-12 fibres/cable

24. ISO 11801:2002
Information technology -- Generic cabling for customer premises
ISO/IEC 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 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 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

25. 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

26. 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 nm m
ATM nm m
ATM nm m
ATM nm 330m

27. 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
ISO 11801:2002
Note:Attenuation is in dB/km

28. 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

29. ISO 11801 Optical fibre Channel Attenuation Channel Attenuation in dB
Multimode
Single-mode
850nm
1300nm
1310nm
1550nm
OF-300
2.55
1.95
1.80
OF-500
3.25
2.25
2.00
OF-2000
8.50
4.50
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

30. 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

31. Building Cabling Connectors and Standards
Presently the ST-compatible connector and SC-compatible connector are the most commonly used connectors for termination.
ISO 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

32. ISO 11801 Multimode optical fibre modal bandwidth
Minimum modal bandwidth MHz x km
Overfilled launch bandwidth
Effective laser launch bandwidth
Wavelength
850 nm
1300 nm
Optical fibre type
Core diameter μm
OM1
50 or 62.5
200
500
Not specified
OM2
OM3 50
1500
2000
Note Effective laser launch bandwidth is assured using differential mode delay (DMD) as specified in IEC/PAS Optical fibres that meet only the overfilled launch modal bandwidth may not support some applications specified in Annex F.
ISO 11801:2002

33. FDDI www.wildpackets.com/support/compendium/fddi/overview

34. 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
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
From CISCO

35. Dual Ring Station failure – see above Cable failure – see above
From CISCO
Station failure – see above
Cable failure – see above
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

36. 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

37. Power Budgeting

38. Power Budget Definition
Power budget is the difference between:
The minimum (worst case) transmitter output power
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)
TRANSMITTER
RECEIVER
Fibre, connectors and splices

39. 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.

40. Power Margin Power margins are included for a number of reasons:
To allow for ageing of sources and other components.
To cater for extra splices, when cable repair is carried out.
To allow for extra fibre, if rerouting is needed in the future.
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)

41. Sample Power Budget Calculation (FDDI System)
Power budget calculation used to calculate power margin
Transmitter o/p power (dBm)
-18.5 dBm min, -14.0dBm max
Receiver sensitivity (dBm)
-30 dBm min
Available power budget:
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 6
Worst case Connector loss (dB)
0.71
Total connector loss (dB)
4.26
Fibre span (km)
2.0
Maximum Fibre loss (dB/Km)
1.5 dB at 1300 nm
Total fibre loss (dB)
3.0
Splices within patchpanels and other splice closures
Number of 3M Fibrlok mechanical splices 10
Worst case splice loss per splice (dB)
0.19
Total splice loss (dB)
1.9
Total loss:
9.16 dB
Power margin (dB)
2.34
Answer

42. Sample Exercises

43. LAN Exercise 1
The design for a building optical fibre link is as below. Calculate the power budget using the ISO 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

44. Calculate the bandwidth of the system.
LAN Exercise 1, cont
Calculate the bandwidth of the system.
What improvements would be made to the system if the operating wavelength is 1300nm.

45. 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

46. 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 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.

47. 1120m TX
312m
265m
Obstruction RX
158m

48. Specification and Other Issues

49. Component Specification and Selection

50. The Path from Specification to Completion
System Specifications
System Design and Optical Design
Component Specification and Selection
Installation
In this section we are concerned with some of the issues which arise regarding component selection, installation and acceptance testing
Commissioning and Acceptance Tests
Completed System

51. Component Selection Component Transceivers Fibre Cables Enclosures
Cable fixings
Connectors
Termination method
Ancillary
Comment
FDDI, Fibre channel etc.. Laser v LED
Core size and multimode v singlemode
Construction and fibre count
Rack and patchpanels, cable management
Tray types, outdoor ducts
ST , SC or small form factor (SFF) connectors
Direct connection or fusion spliced v mechanical spliced pigtails
Adapters, pigtails, patchleads, fibre organisers etc..

52. 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.

53. 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

54. 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

55. 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

56. 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

57. 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:
Optimal ratio depends on forecasted customer needs
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

58. 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

59. 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

60. Connectors for patchcords to transceivers or other fibres
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:
Preconnectorised fibre pigtails fusion spliced to incoming cable fibres
Preconnectorised fibre pigtails mechanically spliced to incoming cable fibres
Direct connectorisation of incoming cable fibres
19" rack patchpanel
Cable 1
Cable 2
Cable 3
19" rack patchpanel
19" rack patchpanel
Connectors for patchcords to transceivers or other fibres

61. Direct Connectorisation versus Spliced Pigtails
Economics:
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 Corelink Mechanical Splices
AMP Lightcrimp Quickfit Connectors

62. Fusion Splicing versus Mechanical Splicing
Economics:
Mechanical splices have low tooling costs, but each splice is more expensive (€1-2)
Fusion splicing involves expensive equipment (€7K to €40K), but very low cost splices
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

63. Pigtail Specification & Selection
Length
Fibre
Buffer
Connector
Colour code
Test Cert.
Comment
1 m typically but beyond 1.5 m excess fibre is untidy
Multimode 50/125 or 62.5/ or singlemode
250 µm or 900 µm (blown fibres may be different)
ST or SC type (see connector specification & selection)
Ideally a range of colour codes should be available, but not always so
Test certificate should accompany all pigtails, stating factory insertion loss test results

64. Patchcord Specification & Selection
Length
Fibre
Diameter
Connector
Duplex/simplex
Markings
Test Cert.
Comment
Variable but 1-3 m is typical
Multimode 50/125 or 62.5/ or singlemode
2.5 mm is typical but newer designs are smaller
ST or SC type (see connector specification & selection)
Patchpanels normally use simplex, desktop-to- wall outlet use duplex. Duplex at a patchpanel is tidier and less error prone
Cable should indicate fibre spec (see above)
Test certificate should accompany all patchcords, stating factory insertion loss test results

65. Connector Specification & Selection
Applies to loose connectors and connectors on pigtails & patchleads
Specification
Type
Ferrule
Polish
Strain Relief
Colour code
Comment
SC is the industry standard but ST very common. Small Form Factor (SFF) connectors are becoming more common
Plastic metal or ceramic. Ceramic gives the lowest loss, plastic is a poor choice (high loss and susceptible to damage)
Not a big issue in building cabling
Simple plastic strain relief on buffered fibres, more complex on patchcord fibres
Directional coding and multimode/singlemode coding needed