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Optical Loss Budget (Example 2)
1000BASE-ZX GBIC– Ptmax = 5 dBm
– Ptmin = 0 dBm
– Prmax = -3 dBm
– Prmin = -23 dBm
Questions:– Can you connect one GBIC to
another with only a patchcord?
– How can you ensure that the fiber system does not exceed the maximum loss?
Optical Loss Budget:Bmax = Ptmin – Prmin
Bmin = Ptmax -Prmax
Opt
ical
Pow
er L
evel
(dB
m)
Maximum Loss (dB) = 23 dB
Minimum Loss (dB) = 8 dB
Ptmax 5 dBm
Prmax -3 dBm
Ptmin 0 dBm
Prmin -23 dBm
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Test!
Basic Tests– Visual Fault Locator (VFL)– Optical Insertion Loss– Optical Power Levels
Advanced Tests– Optical Return Loss (ORL)– Optical Time Domain Reflectometer (OTDR)– Chromatic Dispersion (CD)– Polarization Mode Dispersion (PMD)– Optical Spectral Analysis (OSA)
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Visual Fault Locator
VFLs provide a visible red light source useful for identifying fiber locations, detecting faults due to bending or poor connectorization, and to confirming continuity.
VFL sources can be modulated in a number of formats to help identify the correct VFL (where a number of VFL tests may be performed).
FFL-050 FFL-100
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Advanced Tests
Optical Return Loss (ORL) Optical Time Domain Reflectometer (OTDR)
– Detect, locate, and measure events at any location on the fiber link
Fiber Characterization– Determines the services that the fiber can be carry– Basic tests plus:
• Chromatic Dispersion (CD)
• Polarization Mode Dispersion (PMD)
Optical Spectrum Analysis (OSA)– Spectral analysis for Wavelength Division Multiplexing (WDM)
systems
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Introduction to OTDR
It’s the single most important tester used in the installation, maintenance & troubleshooting of fiber plant
T-BERD 4000 FTTx / Access OTDR Most versatile of Fiber Test Tools Detect, locate and measure events at any location on the fiber link Identifies events & impairments (splices, bends, connectors, breaks) Provides physical distance to each event/ impairment Measures fiber attenuation loss of each event or impairment Provides reflectance / return loss values for each reflective event or impairment Manages the data collected and supports data reporting.
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Background on Fiber Phenomena
OTDR depends on two types of phenomena:- Rayleigh scattering - Fresnel reflections.
Rayleigh scattering and backscattering effect in a fiber
Light reflection phenomenon = Fresnel reflection
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How does it work ?
The OTDR injects a short pulse of light into one end of the fiber and analyzes the backscatter and reflected signal coming back
The received signal is then plotted into a backscatter X/Y display in dB vs. distance
Event analysis is then performed in order to populate the table of results.
OTDR Block Diagram Example of an OTDR trace
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Dynamic Range & Injection Level
Dynamic Range determines the observable length of the fiber & depends on the OTDR design and settings
Injection level is the power level in which the OTDR injects light into the fiber under test
Poor launch conditions, resulting in low injection levels, are the primary reason for reductions in dynamic range, and therefore
accuracy of the measurements
Effect of pulse width: the bigger the pulse, the more backscatter we receive
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What does an OTDR Measure ?
Distance– The OTDR measurement is based on “Time”:
The round trip time travel of each pulse sent down the fiber is measured. Knowing the speed of light in a vacuum and the index of refraction of the fiber glass, distance can then be calculated.
Fiber distance = Speed of light (vacuum) X time 2 x IOR
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What does an OTDR Measure ?
Attenuation (also called fiber loss)Expressed in dB or dB/km, this represents the loss, or rate of loss between two events along a fiber span
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What does an OTDR Measure ?
Event LossDifference in optical power level before and after an event, expressed in dB
Fusion Splice or Macrobend
Connector orMechanical Splice
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ReflectanceRatio of reflected power to incident power of an event, expressed as a negative dB value
The higher the reflectance, the more light reflected back, the worse the connection
A -50dB reflectance is better than -20dB value
What does an OTDR Measure ?
Typical reflectance values
Polished Connector ~ -45dB
Ultra-Polished Connector ~ -55dB
Angled Polished Connector ~ -65dB
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What does an OTDR Measure ?
Optical Return Loss (ORL)Measure of the amount of light that is reflected back from a feature: forward power to the reflected power. The bigger the number in dBs the less light is being reflected.
The OTDR is able to measure not only the total ORL of the link but also section ORL
Distance (km)
Att
enu
atio
n (
dB
)
ORL of the defined section
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Optical Return Loss (ORL)
Light reflected back to the source
PT: Output power of the light source
PAPC: Back-reflected power of APC connector
PPC: Back-reflected power of PC connector
PF: Backscattered power of fiber
PB: Total amount of back-reflected power
ORL (dB) = 10Log > 0)(B
T
P
P
PAPCPPC PAPC PAPC
PT
PF PF PF
Light Source
Photo-diode
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Effects of High ORL Values
All laser sources, especially distributed feedback lasers, are sensitive to optical reflection, which causes spectral fluctuation and, subsequently, power jitter. Return loss is a measure of the amount of reflection accruing in an optical system. A -45dB reflection is equivalent to 45dB return loss (ORL). A minimum of 45-50dB return loss is the industry standard for passive components to ensure normal system operation in singlemode fiber systems.
Increase in transmitter noise– Reducing the OSNR in analog video transmission– Increasing the BER in digital transmission systems
Increase in light source interference – Changes central wavelength and output power
Higher incidence of transmitter damage
The angle reduces the back-reflection of the connection.
SC - PC SC - APC
OTDR Events
How to interpret a trace
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How to interpret an OTDR Trace
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Front End Reflection
Connection between the OTDR and the patchcord or launch cable
Located at the extreme left edge of the trace
Reflectance: Polished Connector ~ -45dB Ultra-Polished Connector ~ -55dB Angled Polished Connector up to ~ -65dB
Insertion Loss: Unable to measure
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Dead Zones
Attenuation Dead Zone (ADZ) is the minimum distance after a reflective event that a non-reflective event can be measured (0.5dB)
In this case the two events are more closely spaced than the ADZ, and shown as one event
ADZ can be reduced using shorter pulse widths
Event Dead Zone (EDZ) is the minimum distance where 2 consecutive unsaturated reflective events can be distinguished
In this case the two events are more closely spaced than the EDZ, and shown as one event
EDZ can be reduced using shorter pulse widths
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Connector
A connector mechanically mates 2 fibers together and creates a reflective event
Reflectance:
Polished Connector ~ -45dB
Ultra-Polished Connector ~ -55dB
Angled Polished Connector up to ~ -65dB
Insertion Loss: ~ 0.5dB
(loss of ~0.2dB w/ very good connector)
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Fusion Splices
A Fusion Splice thermally fuses two fibers together using a splicing machine
Reflectance: None
Insertion Loss: < 0.1dB
A “Gainer” is a splice gain that appears when two fibers of different backscatter coefficients are spliced together (the higher coefficient being downstream)
Reflectance: None
Insertion Loss: Small gain
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Fusion Splices
Direction A-B Direction B-A
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Macrobend
Macrobending results from physical bending of the fiber.
Bending Losses are higher as wavelength increases.
Therefore to distinguish a bend from a splice, two wavelengths are used (typically 1310 & 1550nm)
Reflectance: None
Insertion Loss: Varies w/ degree of bend & wavelength
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Mechanical Splice
A Mechanical Splice mechanically aligns two fibers together using a self-contained assembly.
Reflectance: ~ -35dB
Insertion Loss: ~ 0.5dB
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Fiber End or Break
A Fiber End or Break occurs when the fiber terminates.
The end reflection depends on the fiber end cleavage and its environment.
Reflectance: PC open to air ~ -14dB
APC open to air ~ - 35dB
Insertion Loss: High (generally)
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Ghosts
A Ghost is an unexpected event resulting from a strong reflection causing “echos” on the trace
When it appears it often occurs after the fiber end.
It is always an exact duplicate distance from the incident reflection.
Reflectance: Lower than echo source
Insertion Loss: None
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Typical Attenuation Values
0.2 dB/km for singlemode fiber at 1550 nm 0.35 dB/km for singlemode fiber at 1310 nm 1 dB/km for multimode fiber at 1300 nm 3 dB/km for multimode fiber at 850 nm 0.05 dB for a fusion splice 0.3 dB for a mechanical splice 0.5 dB for a connector pair (FOTP-34) Splitters/monitor points (varys with component)
Best Practices with OTDRs
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Performing an OTDR Test
1. Inspect & Clean connector end faces (patch cords & bulkheads (including test instrument)
2. Set up instrument for test environment
3. Test
4. View trace/table of results
5. Store / Report Results
6. Further analysis optional (for advanced users)
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Key OTDR Setup Parameters for Manual Operation
Pulse Width– Controls the amount of light injected into the fiber– A short pulse width enables high resolution and short dead zones,
but limited dynamic range– A long pulse width enables high dynamic range but less resolution
and longer dead zones
Short Pulse:• More Resolution• Shorter Dead Zones• Less Dynamic Range• More Noise
5ns
1µs
100ns Long Pulse:• Less Resolution• Wider Dead Zones• More Dynamic Range• Less Noise
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5s 30s
20s
Key OTDR Setup Parameters for Manual Operation
Acquisition Time (Averaging)– Length of time the OTDR takes to acquire and average the data
points– Increasing acquisition time improves the dynamic range w/o
affecting the resolution or dead zones.
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Key OTDR Setup Parameters for Manual Operation
Index of Refraction (IOR)– The IOR converts time, measured by the OTDR, to
distance, which is displayed on the trace– Entering the appropriate value into the OTDR will
ensure accurate length measurements for the fiber.
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How to select the right OTDR Test Module
OTDR modules are primarily specified in terms of dynamic range
Select the optimum test module as follows: 1. Determine the longest span you will be testing w/ this
module
2. Determine the expected link loss budget this will translate to
3. Select the module by subtracting 6 dB from the rated dynamic range of the module (this is the range of the unit to view backscatter signal or measure a splice loss)
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Example: Link Loss / OTDR Module selection calculation
Calculation Factors
Link example & calculations
Longest span length 75km
Avg fiber span loss 0.33dB/km @ 1310nm x 75 = 24.75dB
0.20dB/km @ 1550nm x 75 = 15dB
Connector Loss Typically 2 connectors per span
2 x 0.5dB each = 1dB
Splice Loss Typically < 0.1dB per splice w/ 1 splice per 5 km of fiber
75 / 5 = 15 splices x 0.1dB each = 1.5dB
dB adjustment OTDR module DR
Recommend allowing 6 dB for splice loss measurement
1310nm 1550nm
dB dB
24.75 15
+ 1 + 1
+ 1.5 + 1.5
+ 6 + 6
= 33.25 = 23.5Dynamic Range requirement for Module
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Tools to Optimize OTDR testing
Launch Cable Using a launch cable allows the characterization of the connector at the origin of the link. This shifts the first connector outside the dead zone of the OTDR connector The last connector can also be measured by using a receive cable
About Launch Cables
Launch cables are typically 100 – 1,000 meters in length.
The length required depends upon the dead zone performance of the OTDR. A minimum 2x the attenuation dead zone length is recommended, although in practice, most are much longer
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TB6000/8000 OTDR Distance Chart
Fiber Characterization
Step-by-step review