Optical Loss Budget (Example 2)

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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?

Ptmax 5 dBm
Minimum Loss (dB) 8 dB
Optical Loss Budget Bmax Ptmin Prmin Bmin
Ptmax -Prmax
Prmax -3 dBm
Ptmin 0 dBm
Optical Power Level (dBm)
Maximum Loss (dB) 23 dB
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-100
FFL-050
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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
Its the single most important tester used in the
installation, maintenance troubleshooting of
fiber plant

• 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.

T-BERD 4000 FTTx / Access OTDR
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Background on Fiber Phenomena

• OTDR depends on two types of phenomena
• Rayleigh scattering
• Fresnel reflections.

Light reflection phenomenon Fresnel reflection
Rayleigh scattering and backscattering effect in
a fiber
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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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Type of Fiber and Wavelengths

• Single Mode (SM)
• 1310 1550nm are primary wavelengths used in SM
  OTDR measurements
• 1625nm is used in trouble-shooting when testing
  on active networks is needed
• Multimode (MM)
• 850 1300nm are dominant wavelengths used in MM
  transmission testing

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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

Connector orMechanical Splice
Fusion Splice or Macrobend
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What does an OTDR Measure ?

• 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

• 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

Attenuation (dB)
ORL of the defined section
Distance (km)
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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
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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

SC - PC
SC - APC

• The angle reduces the back-reflection of the
  connection.

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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
  -65dBInsertion 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 lt 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)

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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

5ns
1µs

• Short Pulse
• More Resolution
• Shorter Dead Zones
• Less Dynamic Range
• More Noise

• Long Pulse
• Less Resolution
• Wider Dead Zones
• More Dynamic Range
• Less Noise

100ns
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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.

5s
30s
20s
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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 _at_ 1310nm x 75 24.75dB 0.20dB/km _at_ 1550nm x 75 15dB
Connector Loss Typically 2 connectors per span 2 x 0.5dB each 1dB
Splice Loss Typically lt 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 23.5
Dynamic 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
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Fiber Characterization

• Step-by-step review