Why Bidirectional Fibre Testing In Links?

OTDR Testing

We know that some form of testing and certification is required when handing over fibre optic construction related projects. This is the only way to confirm that the fibre construction project has been completed correctly and to measure as accurately as possible whether the fibre link meets the design specification.

If the fibre fails to pass acceptance, this triggers rework and a return to site for fixing/correcting the problem, which will incur additional costs, and is therefore far better avoided by taking direct corrective action while on site during the initial build, without having to go through the cycle of rejection, rework, re-certification and so on.

Or the fibre may be certified through testing, but problems occur when the business goes live, leading to delays or premature failure. In such cases, this increases our operational and maintenance costs.

Therefore, in-depth testing of the fibre is a must, and should be dual-fibre bidirectional.

Basic tests include Insertion Loss (IL) and Optical Return Loss (ORL). IL looks at attenuation or loss on a fibre link and confirms continuity. ORL looks at the amount of light that is backscattered and reflected along a link.

Network equipment vendors provide IL and ORL specifications or limits so that their optical transport systems will operate correctly. High IL and ORL values often result in signal degradation that impairs transmission reliability, as evidenced by high levels of bit error rate (BER).

Accurate Insertion Loss (IL) and ORL measurements are required in networks with tight IL/ORL budgets, such as FTTH/PON, as well as in networks with higher transmission bandwidths.

Typically, the link will have the same IL regardless of the direction of measurement, however, the ORL may vary depending on the direction of test.

However, bi-directional IL and ORL are actually minimal tests that provide only basic information about the entire link. What happens if, say, there is a bad fusion halfway through, but the link still passes the IL and ORL tests? Or, a good splice everywhere except for a bend in the middle?

The only way to characterise and understand the condition of all the splices or to check for bends is OTDR testing.

The most common method is to test from one end of the link, i.e. unidirectional testing.

However, investing a little time and effort into bi-directional OTDR testing can help in several ways, and by eliminating the occasional false positives and misses that uni-directional OTDR testing produces, it can solve problems that can cause fibre optic links to fail when there are no problems at all.

What are false alarms/missed alarms?

False alarms are events on the OTDR rail that look like enhancements, also known as gains.

Differences between fibre manufacturers or even between production batches can cause variations in the fibre’s backscatter coefficient (from the Mode Field Diameter (MFD)), which when fused to another fibre will result in gain.

Missed events are events that appear to have too much loss, which can be a real problem such as a bad fusion splice, or they can be the same gain but just measured/tested from the opposite direction.

So how do you get a pass or normal state of a gainer event and verify that the excessive loss is a real fault? The answer is a bi-directional OTDR test with bi-directional results analysis (averaging).
False alarms and erroneous results from unidirectional OTDRs
Think about it, what are your options when a fusion splice is found to be failing through excessive loss or what looks like gain?

Even if you re-spliced and re-tested, you would still get the exact same results. No matter how many times you re-spliced, the test would not pass. What do you do now?

Replace one of the fibre segments? This requires a lot of effort, may be impractical, and has no guarantee of success.

Put the fibre into service by fluke and hope it works? This may result in delays or penalties. Or simply scrap the fibre? Seems a bit wasteful!

Bi-directional OTDR testing gives us a true picture of the difference in backscatter/measurements to give a true picture and help us diagnose if there is indeed a problem with a splice, connector or part of the fibre, and if it needs to be replaced, saving time and money or avoiding giving up on a good fibre link.

In addition, bi-directional OTDR testing can reveal events hidden by OTDR dead zones, where neighbouring events may be missed and appear as a single event: reflected (or backscattered) light from the first event means that light reflected from nearby events after the first event is flooded or missed by the OTDR.

Testing from the other end of the fibre link (the far end) will reveal the second event and therefore give a more accurate picture of what is happening in the real/actual fibre link.

The way to avoid these issues and costs is not only to perform higher level/more detailed tests (i.e., full bi-directional IL, ORL, and OTDR tests), but also to pay attention to how these tests are implemented and whether the workflow is effective (single test port vs. dual test port).

Echo Huang

Echo Huang is an expert wordsmith and marketing professional at Bonelinks with more than 8 years of experience in high technology businesses – fiber optics, IoT, and telecommunication. She is very glad to share industry knowledge and communicate with others.

More To Explore

Optical Transceiver

What Is An Eye Diagram?

Eye diagram, which is formed by overlapping the waveforms of each code element obtained from the scan due to the afterglow of the oscilloscope. It contains a wealth of information,

Read More »
Scroll to Top
Contact us