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?

Time BaseControl

Unit

Pulse Generator 1

2

LaserDiodes

SamplingADC

AveragingProcessing

PhotodiodeDetector

Coupler

Amplifier

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

OTU-8000

MTS-8000T-BERD 8000

Optical Test Unit for the ONMS

Understanding Optical Time Domain Reflectometry

1550 nm0.19 dB/km

Connection betweenthe OTDR and the patchcordor launch cable.

km

dB

1310 nm0.33 dB/km

Front end reflective event

Mechanical splice

Connector

A mechanical splice mechanicallyaligns 2 fibers together,using a self-contained assembly.Reflectance: ~ -55 dBInsertion loss: ~ 0.5 dB

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

Fusion splice

Loss

Loss

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

Ghost

Loss

Macro bend

Loss

Loss

1550 nm

1310 nm

A gainer is a splice gainthat appears when two fiberswith different backscattercoefficients are spliced together.Reflectance: noneInsertion Loss: small gain

Gainer

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 connector)

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 accordingto wavelength

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 distinguished.

In this case, the events are more closelyspaced than the EDZ and shown as one event.It can be reduced using smaller pulse widths.

Event Dead Zone

Noise

Distance Range specifies the distance that the OTDR will display on the X axis

Loss

dB

Noise Level (RMS)

InjectionLevel

km

(SNR = 1)

RMS DynamicRange

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

ADZ

> 0.5dB

EDZ

1.5 dB

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

Noise

Event is hidden

Event is hidden

(L) (L)

2

3

4 5 6 7 8 9

1

2

1

3

45

6

7

8

9

Front end reflective event

Understanding Optical Time

Domain ReflectometryCompact Optical Test PlatformScalable Optical Test Platform

MTS-6000T-BERD 6000

Splice loss

Splice gain

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.

Bi-directional Analysis

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