Pulse WidthThe pulse width controls the amount of light injected
into a fiber.
A short pulse width enables high resolution and short dead zones
butless dynamic range.
A long pulse width enables high dynamic range but less
resolution andlarge dead zones.
Acquisition timeThis is the time during which the OTDR acquires
and averages data pointsfrom the fiber under test. Increasing the
acquisition time improves thedynamic range without affecting the
resolution and dead zones.
Index of Refraction (loR)The IoR converts the time, measured by
the OTDR, to distance, displayedon the trace.
Entering the appropriate value for the fiber under test will
ensure accu-rate length measurements of the fiber.
In order to obtain accurate measurements, connectors mustalways
be cleaned prior to OTDR testing!!
How to Configure the Main OTDR Settings
An OTDR detects, locates, and measures events on fiber links.
Itrequires access to only one end of the fiber.
Attenuation (also called fiber loss)Expressed in dB or dB/km,
this represents the loss or the rate of loss be-tween 2 points
along the fiber span.
Event LossDifference of optical power level before and after an
event,expressed in dB.
ReflectanceRatio of reflected power to incident power of an
event, expressed as a negativedB value.
ORL (Optical Return Loss)Ratio of the reflected power to the
incident power from a fiber optic linkor system, expressed as a
positive dB value.
What Does an OTDR Measure?
Pulse Generator 1
The optical time domain re-flectometer (OTDR) injectsan optical
pulse into oneend of the fiber and analyzesthe backscattered and
re-flected signal coming back.
An operator at one end of afiber span, can measure andlocalize
Attenuation, EventLoss, Reflectance and ORL.
OTDR Block Diagram
To learn more, visit www.jdsu.com/fibertestNote: Specifications,
terms, and conditions are subject to change without notice.
30149186 000 0907 OTDR.PO.FOP.TM.AE
Optical Test Unit for the ONMS
Understanding Optical Time Domain Reflectometry
1550 nm0.19 dB/km
Connection betweenthe OTDR and the patchcordor launch cable.
1310 nm0.33 dB/km
Front end reflective event
A mechanical splice mechanicallyaligns 2 fibers together,using a
self-contained assembly.Reflectance: ~ -55 dBInsertion loss: ~ 0.5
Launch cableUsing a launch cable allows thecharacterization of
the connector at theorigin of the link, by moving it outsidethe
dead zone of the OTDR connector.The last connector can also be
testedby using a receive cable.
A fusion splice thermallyfuses 2 fibers together,using a
splicing machine.Reflectance: noneInsertion loss: < 0.1 dB
A ghost is a unexpected eventresulting from a strong reflection
causingechoes on the trace.
When it appears, it is often seenafter the fiber
end.Reflectance: lower than echo sourceInsertion loss: none
A gainer is a splice gainthat appears when two fiberswith
different backscattercoefficients are spliced together.Reflectance:
noneInsertion Loss: small gain
A connector mechanically mates2 fibers togetherand creates a
reflective event. Reflectance:- Polished Connector: ~ -45 dB- Ultra
Polished Connector: ~ -55 dB- Angled Polished Connector: up to -65
dBInsertion loss: ~0.5 dB (Loss = 0.2 dB with a very good
Macro bending resultsfrom physical constraints on the
fiber.Bending loss is higher aswavelength increases.Therefore to
distinguish a bendfrom a splice,2 different wavelengths are
used.Reflectance: none (generally)Insertion loss: varies
The Attenuation Dead Zone (ADZ)is the minimum distance aftera
reflective event where anon-reflective event (splice)can be
measured (usually 0.5 dB).In this case, the events are more closely
spaced than the ADZ and shown as one event. ADZ can be reduced
using smaller pulse widths.
Attenuation Dead Zone
The Event Dead Zone (EDZ)is the minimum distance where
twoconsecutive unsaturated reflectiveevents can be
In this case, the events are more closelyspaced than the EDZ and
shown as one event.It can be reduced using smaller pulse
Event Dead Zone
Distance Range specifies the distance that the OTDR will display
on the X axis
Noise Level (RMS)
(SNR = 1)
Dynamic range determines theobservable length of the fiber
anddepends on the OTDR design and settings.
The injection level is the power levelin which the OTDR injects
light intothe fiber under test.
Poor launch conditions, resulting inlow injection levels, are
the primaryreason for reductions in dynamicrange, and therefore
accuracy of themeasurements.
Connector pair Connector pairMacro bend Fusion splice Fusion
splice Mechanical splice Connector pairs Fiber end
Launch cable Fiber link
Dynamic Rangeand Injection Level
OTDR Trace Analysis
A fiber end or break is when the fiber terminates.The end
reflection depends on the fiber endcleavage and it's
environment.Reflectance:- PC open to air: ~ -14 dB- APC open to
air: ~ -45 dBInsertion loss: high (generally)
Fiber end or break
Event is hidden
Event is hidden
4 5 6 7 8 9
Front end reflective event
Understanding Optical Time
Domain ReflectometryCompact Optical Test PlatformScalable
Optical Test Platform
Due to fiber backscatter coefficient mismatches, a splicecan
appear as a gain or as a loss depending on the testdirection.
Bi-directional analysis is used to minimize this
possiblemismatch by measuring the splice loss in both directionsand
averaging the result to obtain the true splice loss.
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