Simplifying OTDRs

Christian Schillab, segment product manager at Fluke Networks, Europe sets out to simplify and make easy OTDRs for integrators and end-users.

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By  Christian Schillab Published  February 25, 2008

Christian Schillab, segment product manager at Fluke Networks, Europe sets out to simplify and make easy OTDRs for integrators and end-users.

Identifying bottlenecks is the strength of an OTDR. The OTDR sends a pulse of light into fibre and measures the light reflected back at each component as the light lost at that component.

Not long ago the yardstick for fibre optic cabling was the IEEE 100BASE-FX standard, which supported a bitrate of 100Mbps over a channel with an attenuation of 11dB. Today, in order for IEEE 10GBASE-S to support a transmission rate 100 times higher, the transmission channel must attenuate the light by no more than 2.6dB.

It is this tightening of requirements for the physical media that represents a challenge for all components used to build and test a transmission path.

A standards compliant connector may contribute up to 0.75dB (0.5dB typical) to the total loss. This would mean that if the user patches two fibre segments together there would be a total of four connectors, which could - even though each individual segment is compliant - result, in worst case, in a total loss of (4 x 0.75dB) 3dB.

This exceeds the loss budget left for the entire link, and with a negative allowance left for the fibre.

This is where new test methods are required. Installers who work with optical fibre are no doubt familiar with optical loss test sets (OLTS). Performing a loss length test with an OLTS is an essential part of fibre installation. Every link needs to be tested to ensure the link is within the loss limits.

But an OLTS will only show if a link has passed or failed. If it fails, it will not show you why it failed, or where. For these answers, an optical time domain reflectometer (OTDR) is the best option. Using an OTDR need not be complicated or confusing. Understanding a few basic concepts will make OTDR use as straightforward as using a copper certification tool.

Testing fibre links as defined by national (TIA-568B) and international (ISO-11801) standards includes the use of an OLTS. Recently updated standards which focus on test methods for installed fibre links, such as ISO 14763-3 and TIA TSB-140, now recommend the complementary use of an OTDR.

These new standards add the use of an OTDR to verify not just that the link has passed, but to ensure the quality of each installed component on the link. Two levels of testing are defined in these updated standards: basic (Tier 1) testing uses an OLTS and extended (Tier2) testing involves the use of an OTDR in addition to OLTS.

The following example should help demonstrate how an extended test regime can help to ensure consistent quality during installation. In the following we assume that the first connector of a link performs extremely well while the second on is poorly installed or contaminated.

The measurement with the OLTS will show that the link passed with a slim margin of (0.95 - 0.93 = 0.02dB) but does identify the second connector as the bottleneck.

Identifying bottlenecks is the strength of an OTDR. The OTDR sends a pulse of light into fibre and measures the light reflected back at each component as the light lost at that component. The same is true for back-scattered light along the length of the fibre.

Setup required

An OTDR can produce accurate, highly detailed measurements, if the correct setup of the OTDR and necessary accessories are employed effectively.

Recent versions of standards like ISO 14763-3 make an attempt to specify all the necessary elements for a correct measurement with an OTDR, thus eliminating common sources of measurement error.

These include:

(1) specifications for launch and receive fibres

(2) correct use of launch and receive fibres

(3) positioning the cursor for the correct reading of link, component and segment attenuation

(4) specifications for conditions under which it is vital to measure fibre in both direction.

3531 days ago
Dave Wilson

This is a good article simplifying the differences between OTDRs and OLTS. However I do take exception with "Often a project specific standard which is derived from the manufacturer's data sheet..." None of a project designer, network owner, manufacturer or even a training/educational group (i.e. BICSI) can publish a "standard". These are only the realm of organizations such as IEEE, ISO, EIA/TIA etc. Other bodies can create specifications, guidelines, and proceedures etc. but these cannot be referred to as standards. Thanx Dave Wilson

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