Why Perform Bi-Directional Fiber Testing?

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Well, let’s start with why test at all?

With all the time, effort, and money (particularly the money) put into the deployment of dark fiber or construction of fiber networks, owners and investors want to make sure they are getting good quality fiber links out of these projects.

Everyone knows that some form of testing and certification is required when signing off and handing over these fiber build projects; it’s the only way to confirm that things have been done correctly and to gauge as best as possible that fiber links have been built to and adhere to the design specifications.

We all know the issues that can arise if things are not built to spec.

In the first instance, fibers just won’t be accepted, which will trigger rework and some sort of effort to revisit a site and fix/correct the issue. This is extra cost, most likely that has to be swallowed by the contractor (or contractor company); far better to avoid this by not having to go through that cycle of rejection, rework, re-certification, and so on by performing any corrective actions during the initial build and while you are still on site.

Or a fiber may pass certification but has issues that manifest at the point of activation which results in activation delays or early life fails. In this case, you have essentially built in future operational and maintenance costs.

In a way it’s a trade-off between the level or depth of testing you perform (and how it’s implemented) versus the potential future maintenance/operational costs and network lifespan. More upfront can mean less later.

The basic set of tests are Insertion Loss (IL) and Optical Return Loss (ORL). IL looks at the attenuation or loss across a fiber link and also confirms continuity. ORL looks at the amount of light backscattered and reflected back along a link. Network equipment vendors provide IL and ORL specifications or limits so that their optical transmission systems can perform correctly. High IL and ORL values typically result in signal degradation which causes unreliable transmission that manifests as higher levels of Bit Error Rates (BER). Tight IL/ORL budget networks, such as in FTTH/PON, and higher transmission bandwidth networks require precise insertion loss (IL) and ORL measurements. Usually, a link will have the same IL regardless of which direction it’s measured in, but a bi-directional (bi-dir) measurement will give you better accuracy of measurement. However, ORL may be different depending on direction of test; therefore a bi-dir measurement is mandatory.

While bi-dir IL and ORL are great tests to perform (and really a bare minimum) they only give you information about the link as a whole. What happens if you have perfect, minimum loss splices in all but one location, say there is a bad splice midway, but the link still passes IL and ORL tests? Or you have a good splice everywhere but a bend midway? The only way to characterize and gain insight into the condition of all splices or to check for bends is with an OTDR test. The most common way is testing from one end of a link — a uni-directional (uni-dir) test. But investing a little time and effort into doing bi-dir OTDR testing can help in a few ways and resolve issues that might cause fiber links to be failed when there is nothing wrong with them by eliminating false positives and false negatives occasionally thrown up by uni-dir OTDR testing.

What is a false positive/ negative?

A false positive is where an event on an OTDR trace looks like an increase, otherwise known as a gainer. Differences between fiber manufacturers or even manufacturing batches can lead to variances in the backscatter coefficient (from differences in Mode Field Diameter (MFD)) of a fiber and when spliced to another fiber results in a gainer. A false negative is an event that looks like it has too much loss, that could be a genuine issue such as a bad splice or it could be the same gainer but just measured/tested from the opposite direction. So how to get a pass or ok for a gainer event and verify if excessive loss is a genuine fault? The answer is bi-dir OTDR testing with bi-dir results analysis (averaging).

Bi-directional OTDR analysis
‘True’ splice loss is the average: (Event loss (A->B) + Event loss (B->A)) / 2

Think about it, what are your options when you get a splice failing due to excessive loss or what looks like a gainer? You re-splice and re-test and guess what you get exactly the same result. No matter how many times you re-splice you cannot get a pass. What now? Replace one of the fiber sections being sliced together? That’s a lot of effort and may not be practical and has no guarantee of success. Put the fiber into service and hope it works? Not exactly a professional solution and one that might land you with activation delays or penalties. Or just write off that fiber? Seems a bit wasteful!

Bi-dir OTDR testing allows you to average out these manufacturing/ backscattering/ measurement differences to give true event loss, helping you diagnose whether a splice, connector or section of fiber really is a problem and needs to be replaced, potentially saving you time and money or stopping you from abandoning a good fiber link.

In addition, bi-dir OTDR tests can also reveal events hidden by OTDR dead zones where events that are close together could be missed and shown as a single event: the reflected (or backscattered) light from the first event means that the light reflected by a nearby event, just after the first, is swamped or missed by the OTDR. Testing from the other end (far end) of the fiber link would reveal that second event so you have a more accurate view of what is in the real/actual fiber link.

The way to avoid these issues and costs is not just to perform a greater level/detail of test (i.e. full bi-directional IL, ORL and OTDR test) but to also pay attention to how those tests are implemented, whether the workflow is efficient (single vs dual test ports), what level of automation is applied to speed up test sequences (using the fiber under test for data exchange – setup and results), does it eliminate post-processing work and delays (on-board TrueBIDIR OTDR analysis).

It is possible to perform a more thorough bi-directional fiber certification in less time than traditional uni-directional tests and provide a higher degree of certainty that the fiber is of a better quality and will require less maintenance.

TestEquity are an approved UK partner for VIAVI
Content Source: Why Perform Bi-directional Fiber Testing? – VIAVI Perspectives (viavisolutions.com)