What the Standards say about bi-directional OTDR Testing

Viavi Solutions

Douglas Clague , February 13, 2023

Are your standards high enough to ensure quality fiber builds?
In the previous blog we saw that bi-directional (bi-dir) OTDR testing provides a number of advantages and lets you deal with issues arising from differences between fibers being spliced together (specifically difference in Modal Field Diameter – MFD) that result in false positives or false negatives on an OTDR test result and certification fails.

When to perform bi-dir testing or if it’s necessary in the first place is a tricky decision to make; luckily there are industry guidelines that help make that decision for you. Adhering to the relevant fiber test standards is critical and the only way to guarantee that a fiber link is up to specification and will perform as expected. Standards bodies such as IEC and ITU-T, lay out exactly what tests should be performed and detail how they should be implemented to correctly characterise every aspect and element of a fiber link.

This is what the standards say regarding correct evaluation of splice loss and end connectors:
Rec. ITU-T G.650.3 (08/2017)
“In practical engineering, unidirectional OTDR test results can be used to roughly judge the splice quality, but accurate splice loss measurement must be based on the bidirectional OTDR test.”
Rec. IEC 61280-4-2:2014
“In order to accurately measure the first and last connection for bi-directional averaging, one should keep the launch and tail cords in their initial measurement positions. Thus, the launch cord of the first direction becomes the tail cord of the opposite direction. This will ensure that identical optical fibres are mated so that the effects of mode field mismatch between the test cords and cabling can be averaged out.”
So, one of the reasons for carrying out bi-dir OTDR testing is so that you can get true loss measurements for splices. This is achieved by averaging the loss measurements taken in both directions (described in ITU-T G.650.3) and allows the removal of any excessive mismatches between fiber sections spliced along a link. And as you see the ITU-T group describes this as a “must”.
And don’t forget the extremities, i.e., the connectors at the ends, are just as likely to have issues as the splices and fiber sections in a link; some argue that they are the more susceptible to issues. The last thing you want is for a “certified” link to have problems at turn-up and then find out it was due to inadequate or incorrect test implementation and a situation that could have been avoided.

To an OTDR, an unmated connector looks like a cable end or a fiber break and is a highly reflective element. It simply reflects most of the light that hits it and because of that on an OTDR trace it looks like a big spike. With this blast of reflected light, it is impossible for an OTDR to tell you anything about it other than it’s an unterminated connector or a break.

To measure and qualify a connector it must be mated with another connector so that when an OTDR test is performed light passes through the connector end face into the end face of the receiving mated connector and receiving fiber, providing an optical linkage that allows losses to be measured for the end connector.
A fiber link that has been bi-dir OTDR tested but without the use of a receive fiber means you can perform bi-directional analysis for the link splices but only uni-dir for the end connectors (even though you have shot the fiber in both directions). This means you have not fully bi-directionally certified the link and as such it’s a non-standard certification.
And this is what the IEC standard is talking about when it mentions the use of launch and tail cords (launch and receive fibers): basically that you should be using both in order to perform the bi-dir OTDR analysis of the end connectors. And, critically, those test cords have to stay connected where they are — no switching of the launch receive fibers connected to the fiber under test. VIAVI works with many service providers and dark fiber providers to apply best practices and get the right results, which calls for bi-dir testing for new build or network expansions.
The takeaways here are that accurate splice loss measurement must be based on bi-directional OTDR testing, and always use launch/receive fibers to deliver certification that includes the end connectors to be fully compliant with the standards.


Douglas Clague is currently solutions marketing manager for fiber optic field solutions at VIAVI. Doug has over 20 years of experience in test and measurement with a primary focus on fiber optics and cable technologies, supporting the telecommunications industry. Prior to VIAVI, Doug held positions as manufacturing engineer, solutions engineer and business development manager. Doug has participated on numerous industry panels around fiber and cable technology trends. He attended Brunel University in London and graduated with an honors degree in electrical and electronic engineering.

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