Ver. E 100607

Prepared & Presented By:FiberNext, LLC

3 Robinson Rd., Suite A3, Bow, NH 03304Ph: 603-226-2400 - www.fibernext.com

“Fiber Optics 201”A training guide for installing

fiber optic cabling systems in accordance withANSI/EIA/TIA & IEEE standards

Tier 2 Testing:The OTDR

OTDR Testing

OTDR (Optical Time Domain Reflectometer) technology is designedto provide a single ended test of any cable. Utilizing sophisticated

algorithms, the equipment is able to calculate exact length andapproximate loss of “events” along the cable span.

GN NetttestCMA4000

OTDR

1. Generates a baseline trace:A “visual” of the link.

2. Can identify and evaluatespecific events in the link.

3. Cable acceptance tool.4. Fault location tool.5. Excellent documentation

capabilities.6. Limited use in short length

networks. <50ft

OTDR Testing

Noyes M600 OTDR

Fault-Locate (Using an OTDR)

Work Area

TelecomOutlet

Equipment Room

HorizontalCross-connect

Telecom Room

NetworkEquipment

MainCross-connect

PC

OTDR

MM

LaunchCable

Assorted Troubleshooting Tools

Talk Set

VFI

OTDR

OTDR Types

Most common OTDRs use a “console” design allowing theuser to upgrade or swap between MM and SM modules.These offer similar analytical features to the lab quality

OTDRs, but are more rugged and field portable. Files can besaved to various media and later downloaded to a PC.

Fluke

OptiFiber

Wavetek MTS5100

OTDR Types

More common console OTDRs. Files can be saved tovarious media and later downloaded to a PC.

Exfo

FTB-150

Anritsu CMA5000

OTDR Types

Micro-OTDRs are the next generation of fast, economicaltest sets for field use. These models offer fewer features thanthe larger console design and are currently not upgradeable.Many manufacturers are focusing on development of these

types of OTDRs for size and weight reasons.

Noyes M100OTDR

Noyes M200OTDR

Noyes OFL200OTDR

ControlUnit

Laser Transmitter

Detector

LCD Display

SplitterFiber under Test

OTDR Functionality

Basic OTDR function

OTDRConnector

Physical cable plantas displayed on

OTDR screen

OTDR Trace Analysis

Network under test

Loss

Launch CablesLaunch cables vary from simple reel (or “ring style”) through larger

“lunch box” style suitcases. Most modern OTDRs don’t require alaunch suppression longer than 250-500’, but many older models

needed delay lines of 1000’ or more.

Using an OTDR w/ Launch CableUse of a launch cable assures the user that the front end connector of

the network will be accurately measured. If the launch cable is tooshort, the front end connector will be consumed in the deadzone.Likewise, a receive cable assures the technician that the far end

connector is not broken and the span has continuity.

Launch Cable

ReceiveCable

Cable under test

OTDR LaunchPort event

EndEvent

NoiseFloor

OTDR Trace Analysis

Connect the OTDR to a launch (suppression/reference)cable. The secondary end of the launch cable will beconnected to an access panel at one end of the fiber

optic span under test. Optionally, a receive cable can beattached at the far end.

OTDR

Launch Cable

Panel PanelSplice Closure

Receive Cable

OTDR Trace AnalysisP

ower

Loss

Distance Scale

Launch Cable Receive CableOTDR

Network Under Test

Splice

Most commonly, users manipulate two cursors, “A” and “B”, toillustrate what is referred to as “two point loss” on an OTDR result.

This can be used to show loss in a single event or in a group ofevents. These cursors can be individually moved left and right to

specific points on the result.

OTDR Trace Analysis

A B

OTDR Trace Analysis

BA

Pow

erLo

ss

Distance Scale

Use cursor/markers to isolate individual events, such asthe repair splice location (above)…

OTDR Trace Analysis

BA

Pow

erLo

ss

Distance Scale

…or the two point loss (attenuation) of an entirenetwork span (above)…

OTDR Trace Analysis

BA

Pow

erLo

ss

Distance Scale

…or the physical length of a fiber span (above).

OTDR Setup - RangeOTDRs have four basic setup requirements regardless of brand:

Range/Resolution, Pulse Width, Index of Refraction and Time (number ofaverages). If any of these settings contradicts another, the results will be

poor. The first one to consider is “Range” or distance of fiber to test.Many OTDRs have automatic length detection functions, but if the lengthis known, the user can set the range manually. The range setting should

be adjusted to no less than 1.5 to 2x the fiber span under test.

2975’ spanunder test

Set to>6000’

Too short: less thanlink length

Link

Can’t see entire link –unpredictable results

Good: about 1.5x to2x link length

Link

Good trace – cansee end of fiber.

Too long: much largerthan link length

Link

Trace is “squashed”into left side of display.

OTDR Setup - Range: Summary

OTDR Setup - Pulse WidthLonger pulse widths are used for longer range tests. As distanceincreases, pulse width must go up, otherwise traces will appear“noisy” and rough. Similarly, short traces will be inconclusive if longpulse widths are used (events may be missed or clipped). Long cablespan=longer pulse width, Short cable span= short pulse width

“Short”Fiber rununder test

>6500’

“Long”Fiber rununder test>10,000’

Too wide:

Can’t resolve events

About right:

Events can be seenand trace is smooth.

Too narrow:

Trace “disappears”into noise floor.

Link Link Link

Where is thisthis event?

OTDR Setup - Pulse Width: Summary

Index of Refraction (IOR)In review, the Index of Refraction is a way of measuring the speed of lightin a material. Light travels fastest in a vacuum, such as outer space. Theactual speed of light in a vacuum is 300,000 kilometers per second, or

186,000 miles per second. Index of Refraction is calculated by dividing thespeed of light in a vacuum by the speed of light in some other medium

(such as glass in the case of fiber optics!).

Medium Typical Index of Refraction Speed

VacuumAirWaterCladdingCore

1.00001.00031.331.461.48

Faster

Slower

Index of Refraction =Speed of Light in a VacuumSpeed of Light in a Medium

OTDR Setup – Index Of RefractionEach different optical glass fiber has a different refractive index

profile consistent with it’s type and manufacture process. TypicalG.652.B singlemode fiber from Draka has an index number of 1.467

@ 1310nm and 1.468 @ 1550nm. Note that the longer thewavelength, the faster the light travels through the core.

The user must set the OTDR to the proper GIR (Group Index ofRefraction). If the GIR is not set to the proper number, the OTDR

may overestimate or underestimate linear cable footage. Since theindex is a measure of the speed of light, if the GIR is not set

properly, the OTDR cannot calculate the proper footage.

If the actual index is not known, use the machine’s default or thefollowing guidelines:

MM 850nm – 1.496MM 1300nm – 1.491

(Corning SMF28e) SM 1310nm - 1.4677(Corning SMF28e) SM 1550nm – 1.4682

Index Of Refraction: SummaryAs discussed earlier, Index of Refraction is a measure of the

speed of light in a medium. If the Group Index of Refraction (GIR)setting in the OTDR does not match that of the fiber under test,

the results will show incorrect distances as a result.

10,000’ of fiberGIR 1.4677 @ 1310nm

GIR set at1.462

OTDR thinksFootage is 9,800’

LaunchCord

OTDR Averaging TimeAveraging time refers to how long the user allows the device to take

samples (a.k.a. how long the test “runs”). The longer thetesting/averaging time allowed, the better the result. Eventually, enoughdata is averaged for a good test and continuing to test won’t yield any

more of an accurate result.

LaunchCord

MUTOA

CorningOV1000

Too many

Trace is smooth butwaste of time.

About right:

Trace is smooth.

Too few:

Trace is noisy – noisefloor is too high.

Link Link Link

OTDR Setup - Averages: Summary

LSA lines are an effective method of getting more accurate testresults. Most OTDRs have loss estimation based on the simple 2-point

method, but use of LSAs obtain better accuracy through events bycalculating lead-in slope and tail-out slope. See below for an example:

OTDR Trace Analysis

A B

Lead in Area

Tail out Area