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The Importance of Optical Time Domain Reflectometers (OTDRs) in Telecommunication Networks Speaker / Author: M. Nel Co-author: A. van Brakel. Content. Introduction Why Fibre? Proudly South African Use of OTDRs in the field Basics of OTDRs Selecting an OTDR Traceability

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The Importance of Optical Time Domain Reflectometers (OTDRs) in Telecommunication NetworksSpeaker / Author: M. NelCo-author: A. van Brakel

  • Introduction
  • Why Fibre?
  • Proudly South African
  • Use of OTDRs in the field
  • Basics of OTDRs
  • Selecting an OTDR
  • Traceability
  • NMISA measurement capability
  • Rapid growth of the ICT industry led to a global change in how business is done
  • Fibre optic communication networks
    • Ensure fast and efficient communication in Southern Africa and with the rest of the world
  • Business sector:
    • Fast access to information leads to wide range of new business opportunities

Tourism sector:

Prospective tourists: travel destinations and information

Websites, telephonic enquiries or e-mail

Using the Voice-over-Internet Protocol (VoIP) via client programs such as Skype™



why fibre
Why Fibre?
  • Fibre optic networks transfer information across networks
    • Enormous potential for bandwidth and bit rate
    • Media types transmitted over fibre optic networks includes:

voice, data, image and video

  • More secure
  • No electromagnetic interference
  • Data can travel long distances
  • Provide high-speed broadband access via
    • fixed-line networks,
    • mast-to-mast e.g. cellular phone
very proud south africa
Very proud South Africa
  • 2.7 million spectators watching all 64 matches
  • 2.8 billion people watching the final
  • Information sent via Fibre optic networks
  • 22 750 hours of feed produced by HBS
  • Soccer being broadcast from South Africa

  • OTDRs mainly measure optical distance
    • provide a benchmark for installation
    • trouble-shooting of fibre optic networks
  • Creates a visual representation of the fibre under test
  • Manufacturers application notes available
use of otdrs in the field
Use of OTDRs in the field
  • Telecommunications industry
    • troubleshooting, verification and documentation
  • More OTDRs sold, at lower prices
  • Rugged and usually hand-held
  • Optical fibres installed
    • roadside, power lines, manufacturing plants, inside buildings
  • Technicians
    • Need training to interpret and identify
  • Advantage: access needed only to one end of the fibre
basic otdr block diagram

Front end connector

Pulse generator

Directional coupler

Laser diode

Avalanche photo


A / D



Signal processing and trace analysis

Basic OTDR block diagram
otdr basics
OTDR Basics
  • OTDR trace contains ‘events’
    • Splices
    • connections
    • breaks etc.
    • Signal processing is performed within the OTDR to analyse each event
    • Properties and location of the event, can be determined
typical otdr trace
Typical OTDR trace

Dead zone


End of fibre



Lead in fibre

Poor connection


selecting an otdr
Selecting an OTDR
  • Different OTDRs on the market
    • Size, accuracy, operating ranges etc.
  • OTDR specifications are fairly complex and most of them are trade-offs
  • Some to look out for:
    • dead zone specification, dynamic range, resolution and distance range
  • Buyers: need solid understanding of these parameters to make best applications-based decision
dynamic range
Dynamic range
  • Dynamic range determines the maximum optical loss
    • bigger dynamic range means longer distances
  • Losses in the fibre link would also limit the measured length
dead zones
  • Types of ‘dead-zones’:
    • attenuation dead zone
    • event dead-zone
  • Detector is temporarily saturated
  • Time translates into distance
    • the longer the detector takes to recover, the longer the dead-zone
    • short dead-zone important for detecting closely spaced events
  • Patch cord can be introduced at the start to move out of the initial dead-zone
guidance when using an otdr
Guidance when using an OTDR
  • Select the
    • pulse width, distance range and sampling points according to application requirements
  • Resolution: between 4 cm to a few metres
  • Pulse width
    • Long pulse width gives better SNR
    • Shorter pulse detect closer spaced events
  • Using a large number of sampling points, to maintain a good resolution
    • few sample points: event may not appear on the trace
future trends of otdr usage
Future trends of OTDR usage
  • OTDR manufacturers
    • Improve measurement resolution and dead zones
  • Increased dynamic range
  • OTDR operating wavelengths
    • 850 nm and 1300 nm (MM)
    • 1310 nm and 1550 nm (SM)
    • Most wavelengths: CWDM and DWDM
calibration of otdrs
Calibration of OTDRs
  • How do you know that your OTDR is in fact giving the correct length measurement?
    • Cost implication to customer and installer
  • Are your OTDR capable of measuring fibre links to achieve specification?
    • Losses tested against system specifications
  • OTDRs need to be calibrated and verified
    • Ensure accurate length and attenuation results
  • Calibration involves
    • Comparison with standards traceable to the SI units
    • Methods available e.g. IEC 61746:2009

Caesium clock

South African National Time Standard

Fibre attenuation standard from NPL

Time interval counter

OTDR loss scale calibration

Fibre optic cable, calibrated for


OTDR distance scale and location offset

nmisa measurement capability
NMISA measurement capability

Distance scale deviation and location offset

Calibrate fibre delay lines for time-of-flight at 850 nm, 1310nm, 1550 nm and 1625 nm

Loss scale

Loss measurements at 1310 nm and 1550 nm


Calibration of this parameter is still under investigation

  • 2010 FIFA World Cup™ presented a unique opportunity for South Africa’s ICT industry to
    • showcase technological achievements
    • world-class telecommunications infrastructure
  • Important role of OTDR in fibre optic network analysis
  • Importance of calibrating OTDRs
  • NMISA Measurement capabilities