1. Fiber Optics 3 Losses and Testing
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Updated December 2015

2. Losses
Signal loss is of primary concern in fiber systems. The standards permit:
Multimode Fiber
3.5 dB/km (850nm) or 1.5 dB/km (1300 nm)
0.75 dB per connection
Singlemode Fiber
1.0 dB/km (1310 or 1550 nm) for inside plant
0.5 dB/km (1310 or 1550 nm) for outside plant
Standards for communication companies tend to be much more rigid.
Maximum acceptable loss of 0.02 to 0.05 dB per fusion splice is usual.

3. Losses in Fiber Losses include Scattering Absorption
Bends (macro and micro)
Chromatic Dispersion
Modal Dispersion
Waveguide Dispersion

4. Optical Emitters - LED LED (Light Emitting Diode), E-LED (Edge LED)
Low power
Inexpensive
Poorer switching characteristics
Wide wavelength range (+/- 15 to 25 nm)
Wide beam
Used with Multimode only
Image: digikey.com

5. Optical Emitters - Laser
Laser, VCSEL (Vertical Cavity Surface Emitting Laser), DFB (Distributed FeedBack), others...
High Power
Narrow wavelength range (+/- 2 nm or less)
Narrow beam
Shorter lifespan
Requires more careful planning (i.e. power settings, attenuators, etc)
Used with singlemode and some multimode
Attenuators are available if power to the receiver is too great.
Image:

6. Power and Wavelength Range
Laser
Power
LED
Wavelength

7. Scattering
Light collides with the elements that make up the glass and is scattered in a variety of directions. This is known as Rayleigh Scattering and is typically constant through the length of the glass.
Rayleigh Scattering is wavelength dependent. Generally, the longer the wavelength the less scattering there is.
Rayleigh Scattering is one of the major sources of loss in fiber.
Additionally, imperfections in glass cause scattering.
Core
Cladding

8. Rayleigh Scattering Rayleigh scattering occurs in nature:
The sky appears blue because the blue light is scattered in the atmosphere.
Sunsets and sunrises appear red because most of the blue spectrum of the sun’s light has been scattered through more atmosphere due to the sun’s low angle.

9. Absorption Absorption:
Imperfections. Foreign materials within the fiber core will absorb light.
Natural Absorption: Certain wavelengths react with the elements contained within the glass compound.
Absorption losses: light is converted to heat.
Core
Cladding

10. Transmission Windows
The silica glass used to make fiber has low loss regions where attenuation due to scattering and absorption is minimized.
Multimode Fiber Attenuation Windows
Attenuation
850 nm
1300 nm
1550nm
Wavelength

11. Wavelengths
Scattering and absorption are intrinsic to the glass used to make fiber optic strands.
Scattering and absorption cause losses over a range of wavelengths.
In Multimode fiber, transmission wavelengths with the fewest losses are:
850 nm
1350 nm
In Singlemode fiber, transmission wavelengths with the fewest losses are:
1310 nm
1550 nm

12. Dispersion
Dispersion occurs when a pulse of light is spread out over time. It limits the bandwidth of fiber.
Dispersion has many causes.
Basic dispersion includes:
Modal
Material
Waveguide

13. Modal Dispersion
As previously discussed, Modal dispersion occurs in multimode fiber where different rays of light will follow different paths within the fiber. Rays travelling a shorter distance will arrive sooner.
Modal dispersion can be reduced by the use of graded index fiber but cannot be completely eliminated.
Step Index Fiber
Input Signal
Output Signal

14. Chromatic (Material) Dispersion
Different wavelengths of light travel at different velocities within glass.
Prisms demonstrate wavelength dispersion. Longer wavelengths (infrared) travel faster than shorter wavelengths (ultraviolet). More optically dense materials increase this effect.
White Light
Core
Input Signal
Cladding
Output Signal

15. Chromatic (Material) Dispersion
Chromatic dispersion can be limited by using transmitters with narrow emission spectrum. Emitters with wider emission spectrum such as LEDs have more material dispersion than lasers with narrow emission spectrum.
Input Signal
Output Signal
Core
Cladding

16. Dispersion-Shifting
Even lasers have several wavelengths, and long-distances compound the problem with wavelength-based dispersion.
Interestingly, chromatic dispersion changes between 850nm where the longer wavelengths are faster, and 1550nm where the shorter wavelengths are faster. The zero dispersion point occurs at about 1300nm. Unfortunately there is greater attenuation at 1300nm.
With modifications to the glass compounds and configuration that make up the core, special fibers can be constructed that operate at the 1550nm range with a low dispersion:
Dispersion-Shifted Singlemode
Zero Dispersion-Shifted Singlemode
Non-Zero Dispersion-Shifted Singlemode

17. Worldwide Fiber System
Image
See:

18. Waveguide Dispersion
Waveguide dispersion occurs when light travels within the cladding layer of the fiber.
Predominantly a problem with singlemode fiber.
Advances in fiber manufacturing has reduced this problem.

19. Microbends
Microbends are small bends in the fiber. Microbends cause signal loss by changing the critical angle at the core- cladding boundary.
Core
Cladding

20. Causes of Microbends Common causes of Microbends: Cinched too tight.
A clamp, tie wrap or other mechanical attachment is too tight.
Storing or using a tight buffer in a cold environment.
The buffer and the fiber expand and contract at different rates, and the friction may cause microbends.
Excessive or improper pull.
If the buffer is pulled too much it may cause microbends when it retracts.

21. Macrobends
Macrobends are larger scale bends in the fiber that create loss.
Core
Cladding

22. Macrobends
Any bend in a fiber risks signal loss and fiber damage. Macrobends must be controlled during and after the installation phase.
The TIA/EIA 568C.1 states the following for minimum bend radii for fiber optics:
2 & 4 fiber horizontal:
25mm (1”) no load
50mm (2”) under load (50 ft-lb)
All backbone fiber
Follow manufacturer specifications
10 times the diameter, no load
15 times the diameter under load, Intrabuilding (within buildings)
20 times the diameter under load, Interbuilding (between buildings)

23. Causes of Macrobends
Macrobends may occur during or after the installation. Examples include:
Tangled patch cords
Cable is under physical pressure
Cable tray with weight of additional cable from above
Work Area
Computer pushed to the wall
Under carpet
Equipment Room
Patch cords hang unsupported
Small space behind the information outlet
Fiber in tray
Fiber in wall

24. Losses at Interface

25. Extrinsic Fiber Faults
Losses at the connector
End Separation
Lateral Misalignment
Angular Misalignment

26. Intrinsic Fiber Faults
Faults from fiber manufacturing errors
Concentricity
Core
Ellipticity
Diameter
Inconsistency

27. Extrinsic/Intrinsic Fiber Faults
Losses from fiber application
Numerical Aperture Mismatch
Core Mismatch
Contamination
Damage

28. Mismatched Core Attenuation
Examples of losses with mismatched fibers:
62.5/125 to 50/125: 1.9dB
62.5/125 to 10/125: 16dB
50/125 to 10/125: 14dB

29. Review Questions Review the Fiber errors on the previous slides.
How can these errors be minimized?
What are some “Best Practices” that may help prevent some of these errors?
Which errors cannot be easily corrected by the installer/user?

30. Testing Fiber

31. Test and Measurement All fiber installations must be tested.
There are a variety of tests that can be performed on cable:
Visual
Identification
Continuity
Attenuation
Performance
Length

32. Visual Checks Jacket markings and labels
Ensure the cables match the equipment and other cables (i.e /125 is not mixed with 50/125). All cables must be labelled.
Connectors, couplers and splices
Undamaged, clean and protected from future damage. Dust caps must be used on unused couplers and connectors. Use a microscope to check ferrule ends. Use an approved wipe to remove contaminants.
Installation and cable routing
No macrobends or other installation issues that may cause excessive losses or damage.

33. Continuity One of the simplest check of a cable is a continuity check.
Use a flashlight with multimode fiber
Use a visual fault locator. (Caution: very bright light)
Creates a visible red glow wherever light can escape the fiber.
End-to-end continuity check
Locate the end of the fiber
Locate breaks
Identify poor splices

34. Horizontal Measurement
The standards require attenuation measurement:
The measurement is based on 90 meters of cable and the loss of 2 connector pairs:
Telecommunications Outlet/Connector
Horizontal Cross-connect
Maximum loss should equal no more than 2.0 dB for multimode fiber
If a consolidation point is used, maximum loss is 2.75 dB.
Reference: TIA/EIA 568C
Link attenuation = Cable Attenuation + Connector Loss + Splice Loss

35. Backbone Measurement The standards require attenuation measurement:
The measurement is based on 300 meters of cable and the loss of 3 connector pairs:
Telecommunications Outlet/Connector
Interconnect in the Telecommunications Room
Cross Connect
Maximum loss should equal no more than 3.3 dB for multimode fiber
Reference: TIA/EIA 568C
Link attenuation = Cable Attenuation + Connector Loss + Splice Loss

36. Attenuation Measurement
A power meter and light source are used to measure attenuation. The equipment consists of:
Light Source
Power Meter
Couplers and Adapters
Launch cables
These devices are hand-held and battery operated.
Many of these devices are capable of measuring at 2 different wavelengths.
Meter designs are specific to core size.

37. Measurement Method
The TIA/EIAS 568C.1 references a separate standard called the ANSI/EIA/TIA for measurement procedures of fiber cables.
The standard specifies power loss measurement from one end only using a single launch cable.
Horizontal: test at either 850 nm or 1300 nm. The attenuation difference on a short length of horizontal cable will not be significant enough.
Backbone: test at both 850 nm and 1300 nm.

38. Measurement from Both Ends
Manufacturers and customers often request measurement from both ends.
Tests from both ends is more likely to detect a core sizing error
Some terminations may have more loss in one direction over another due to Fresnel reflections or imperfections.
Can identify record keeping errors.

39. Single Jumper Test 1-Calibrate unit with launch cable Meter Source
Cable Under Test
Launch Cable
Source
Meter
2-Test with launch cable at source
Coupler

40. Optional Double Jumper Test
In many cases the installer will prefer to use two launch cables instead of one.
Less wear-and-tear on the test equipment
Easier to keep the connectors at the equipment in a clean state
Easier to switch between connector types by adding the appropriate adapters

41. Optional Double Jumper Test
Source
Meter
1-Calibrate unit with one launch cable
Source
Meter
Launch Cable #1
Coupler
Launch Cable #2
2-Add the second launch cable. Do not recalibrate. Ensure extra loss is less than 0.75dB
Coupler
Coupler
Launch Cable #2
Launch Cable #1
3-Test the installed cable with the two launch cables and couplers
Cable Under Test
Meter
Source

42. OTDRs Optical Time Domain Reflectometer Operates somewhat like radar:
Launches pulses of light and times the returns from Raleigh Scattering and Fresnel reflections.
Provides a reference distance to the return losses.
Requires access to only one end of the fiber. Many need to be connectorized.
OTDRs are used to measure the length of a fiber. They also provide a profile of the loss over its length.
May be used to check the fiber on a spool of cable.
Used to check for poor connections

43. OTDR Characteristics
OTDRs measure the returned light levels over distance. As the distance increases, the returned losses are also decreased due to the Raleigh scattering, but these are expected to be relatively constant over the length of the fiber.
This scattering represents much of the loss of the fiber over that distance.
The chart produced by the OTDR indicates dB values of the returned signal over the distance of the fiber.
OTDRs indirectly indicate the level of light lost due to macro or microbends. This light is not reflected back but the level of backscatter after these loss points is reduced.

44. OTDR Characteristics
OTDRs have a “dead zone” and cannot measure short lengths of fiber. The device cannot measure the return losses while it is producing the sampling pulse. The length of the dead zone is tester-dependent.
Longer launch cables (“fiber test boxes”) are sometimes used with OTDRs to add distance between the tester and the cable under test.
OTDRs must be ranged properly. In order to take measurements, the OTDR must send out multiple test signals. The time between these test signals must be more than the time it takes for the last return.
Many OTDRs have range settings which will set the pulse repetition rate and may also change the output power of the test pulse.
More test pulses typically means better resolution.
Longer length setting typically means it will take more time to test the cable.

45. OTDR Sample Trace Fresnel Reflection Launch Dead Zone (End of Fiber)
dB Return
Distance
Reflection Echoes

46. OTDR Sample Trace Fresnel Reflection (End of Fiber)
Returns reduced after this point. Less signal getting through.
Launch Dead Zone
dB Return
Returns reduced after this point, and a Fresnel Reflection.
Distance
Reflection Echoes

47. OTDR and Standards The TIA/EIA 568C does not require OTDR tests
OTDRs can help identify the location of an error but are less effective at measuring end-to-end power.
OTDRs are expensive. Prices start at $20,000 for a basic unit and can climb to over $100,000.
OTDRs are required for long distance tests (outside of the TIA/EIA 568C standard).
Caution: OTDRs usually use laser light.
Warn others when testing
Do not test cable that is plugged into equipment.

48. Other Test Equipment Fiber Identifier Fiber Talksets
Bright light source
Used with non-destructive macrobending
Used in complex networks to identify fibers for splicing
Fiber Talksets
Communication via the fiber while identifying and testing other fibers. Feature often available for fiber testers.

49. Power Budget
Designing a fiber optic backbone outside of the EIA568C may require some special planning to ensure the proper amount of signal is present at the receiver end.
A power budget takes into account the launched power, the minimum (and maximum) power required at the receiver end and all of the losses in between.
Power Budgets are usually associated with long distance transmissions and with fiber, copper and wireless communications links.

50. Review Questions and Discussion
The instructor will now have some carefully chosen, intelligent and well-worded questions and discussion topics for you.
You will have the opportunity to demonstrate your skill, knowledge and high degree of attention at this time.
End Part 3
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