Optical Time Domain Reflectometer
Piotr Turowicz
Poznan Supercomputing and Networking Center
piotrek@man.poznan.pl
9-10 October 2006
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Link Characterization Using the OTDR
OTDR 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 OTDR
OTDR General Theory
Measures/characterizes:
– Reflection and optical loss caused by every event in the link
• Connectors
• Splices
– Fiber ends
– Detectable faults
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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 OTDR
OTDR 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 OTDR
OTDR General Theory
• Rayleigh backscattering
• Comes from the fiber’s “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 OTDR
OTDR 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 fiber’s
backscattering level
• Create a “dead zone” after the reflection
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Link Characterization Using the OTDR
OTDR 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 OTDR
OTDR General Theory
Simplified OTDR trace
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Link Characterization Using the OTDR
OTDR 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.
http://www.porta-optica.org
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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.
http://www.porta-optica.org
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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”.
http://www.porta-optica.org
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Pulse Width vs. Dead Zones
and Dynamic Range
Short pulses give a higher resolution but a shorter dynamic range:
http://www.porta-optica.org
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Echoes on OTDR Traces
http://www.porta-optica.org
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Echoes on OTDR Traces
http://www.porta-optica.org
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Link Characterization Using the OTDR
Testing 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 OTDR
Testing Techniques
Without a pulse suppressor box
http://www.porta-optica.org
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Link Characterization Using the OTDR
Testing Techniques
Four-point events: loss measurement
http://www.porta-optica.org
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Link Characterization Using the OTDR
Testing 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 OTDR
Testing Techniques
Two-point sections: loss measurement
http://www.porta-optica.org
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Link Characterization Using the OTDR
Testing Techniques
Two-point sections: attenuation measurement
http://www.porta-optica.org
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Link Characterization Using the OTDR
Testing Techniques
Acquisition parameter settings
http://www.porta-optica.org
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Link Characterization Using the OTDR
Testing Techniques
To take acqusition just press START
http://www.porta-optica.org
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References
http://www.porta-optica.org
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