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Optical Time Domain Reflectometer
Piotr Turowicz Poznan Supercomputing and
Networking Center piotrek_at_man.poznan.pl 9-10
October 2006
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Link Characterization Using the OTDROTDR General
Theory

• What is an optical time-domain reflectometer
  (OTDR)?
• Single-ended measurement tool
• Provides a detailed picture of section-by-section
  loss
• Operates by sending a high-power pulse of light
  down the fiber and measuring the light reflected
  back
• Uses the time it takes for individual reflections
  to return to determine the distance of each event
• Measures/characterizes
• Fiber attenuation
• Attenuation example (new G.652.C fibers)
• 0.33 dB/km at 1310 nm (0.35 dB/km for worst case)
• 0.21 dB/km at 1490 nm (0.27 dB/km for worst case)
• 0.19 dB/km at 1550 nm (0.25 dB/km for worst case)

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Link Characterization Using the OTDROTDR General
Theory

• Measures/characterizes
• Reflection and optical loss caused by every event
  in the link
• Connectors
• Splices
• Fiber ends
• Detectable faults
• Misalignments and mismatches
• Dirt on connector ferrules
• Fiber breaks
• Macrobends

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OTDR
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OTDR Basic Principles
OTDR Basic Principles An OTDR sends short pulses
of light into a fiber. Light scattering occurs in
the fiber due to discontinuities such as
connectors, splices, bends, and faults. An OTDR
then detects and analyzes the backscattered
signals. The signal strength is measured for
specific intervals of time and is used to
characterize events. The OTDR to calculate
distances as follows
Distance c/n t/2
c speed of light in a vacuum (2.998 x 108
m/s) t time delay from the launch of the pulse
to the reception of the pulse n index of
refraction of the fiber under test (as specified
by the manufacturer)
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OTDR Basic Principles
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OTDR Basic Principles
An OTDR uses the effects of Rayleigh scattering
and Fresnel reflection to measure the fiber's
condition, but the Fresnel reflection is tens of
thousands of times greater in power level than
the backscatter.
Rayleigh scattering occurs when a pulse travels
down the fiber and small variations in the
material, such as variations and discontinuities
in the index of refraction, cause light to be
scattered in all directions. However, the
phenomenon of small amounts of light being
reflected directly back toward the transmitter is
called backscattering.
Fresnel reflections occur when the light
traveling down the fiber encounters abrupt
changes in material density that may occur at
connections or breaks where an air gap exists. A
very large quantity of light is reflected, as
compared with the Rayleigh scattering. The
strength of the reflection depends on the degree
of change in the index of refraction.
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Link Characterization Using the OTDROTDR General
Theory

• Reflectometry theory
• The OTDR launches short light pulses (from 5 ns
  to 20 µs)
• Measuring the difference between the launching
  time and the time of arrival of the returned
  signal, it determines the distance between the
  launching point and the event.
• The OTDR uses the IOR of the fiber under test to
  accurately calculate the distance (speed of light
  in fiber is different than in the air)

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Link Characterization Using the OTDROTDR General
Theory

• Rayleigh backscattering
• Comes from the fibers natural reflectiveness
• The OTDR uses the Rayleigh backreflections to
  measure fiber attenuation (dB/km)
• Backreflection level around -75 dB
• Higher wavelengths are less attenuated by the
  Rayleigh backscattering

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Link Characterization Using the OTDROTDR General
Theory

• Fresnel backreflections
• Come from abrupt changes in the IOR (e.g.,
  glass/air)
• - Fiber breaks, mechanical splices, bulkheads,
  connectors
• Show as a spike on the OTDR trace
• UPC reflection is typically 55 dB APC is
  typically 65 dB (as per ITU)
• Fresnel reflections are approximately 20 000
  times higher than fibers backscattering level
• Create a dead zone after the reflection

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Link Characterization Using the OTDROTDR General
Theory

• Distance corresponds to the distance range of
  the fiber span to be tested according to the
  selected measurement units

• Pulse corresponds to the pulse width for the
  test. A longer pulse allows you to probe further
  along the fiber, but results in less resolution.
  A shorter pulse width provides higher resolution,
  but less distance range.

• Time corresponds to the acquisition duration
  (period during which results will be averaged).
  Generally, longer acquisition times generate
  cleaner traces (long-distance traces) because as
  the acquisition time increases, more of the noise
  is averaged out. This averaging increases the
  signal-to-noise ratio (SNR) and the OTDR's
  ability to detect small events.

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Link Characterization Using the OTDROTDR General
Theory
Simplified OTDR trace
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Link Characterization Using the OTDROTDR General
Theory

• Loss in fiber is wavelength-dependent

http//www.porta-optica.org
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Link Characterization Using the OTDR Limitations

• Event dead zone
• Dead zones only affect reflective events
• The event or reflective dead zone represents the
  minimum distance between the beginning of a
  reflective event and the point where a
  consecutive reflective event should clearly be
  localized.

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Link Characterization Using the OTDR Limitations

• Attenuation dead zone
• The attenuation or non-reflective dead zone is
  the minimum distance after which a consecutive
  reflective or non-reflective event and
  attenuation measurement can be made.

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Link Characterization Using the OTDR Merged
Events

• If the spacing between two events is shorter than
  the attenuation dead zone but longer than the
  event dead zone, the OTDR will show merged
  events.

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Pulse Width vs. Dead Zonesand Dynamic Range
Short pulses give a higher resolution but a
shorter dynamic range
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Echoes on OTDR Traces
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Echoes on OTDR Traces
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Link Characterization Using the OTDRTesting
Techniques

• Launch cables
• A launch cable is recommended if the user wants
  to characterize the first or last connector of an
  optical link.
• It allows the OTDR to have a power reference
  before and after the connector in order to
  characterize it.
• Standard available lengths vary from 200 meters
  to 1500 meters

http//www.porta-optica.org
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Link Characterization Using the OTDRTesting
Techniques
Without a pulse suppressor box
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Link Characterization Using the OTDRTesting
Techniques
Four-point events loss measurement
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Link Characterization Using the OTDRTesting
Techniques

• The least-square approximation (LSA) method

• The least-square approximation (LSA) method
  measures the attenuation (loss/distance) between
  two points by fitting a straight line to the
  backscatter data between markers A and B.
• The LSA attenuation corresponds to the difference
  in power (dB) measured between two points.

http//www.porta-optica.org
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Link Characterization Using the OTDRTesting
Techniques
Two-point sections loss measurement
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Link Characterization Using the OTDRTesting
Techniques
Two-point sections attenuation measurement
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Link Characterization Using the OTDRTesting
Techniques
Acquisition parameter settings
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Link Characterization Using the OTDRTesting
Techniques
To take acqusition just press START
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References
http//www.porta-optica.org