Fault finding and how to use an OTDR to its full potential.

In a certified fibre cable with 72 counts, our client complained that some fibres were not working. On retesting we discovered that some fibres were completely dark. The client assumed that the damage was performed on what we shall call as side A, as this was the only cabinet which was open to third parties.

We advised the client that we will investigate, the team was confident the issue would be on side A, we performed basic VFL and found nothing amiss! Luckily we always carry our EXFO OTDR for such investigations, we also always as a minimum use a launch fibre so that we are able to see the first ODF.

𝐀𝐬 𝐜𝐚𝐧 𝐛𝐞 𝐬𝐞𝐞𝐧 𝐢𝐧 𝐢𝐦𝐚𝐠𝐞 𝟏, we have 500m of our lunch fibre, event 2 marking the reflection on the ODF connector, the fibre cable length and then some odd reflections. This was not the expected result, in the meantime, a receive fibre was placed on side B and the test taken again.

𝐈𝐦𝐚𝐠𝐞 𝟐 gave us a much better picture, light manages to get all the way through to the other side as we see the reflection delimiting the ODF on side B (event 3) with a sudden drop seen across our receive fibre. What this tells us is that something is really bad on the far end. So we decided to shoot the OTDR facing the ODF on Side B, the result?

𝐈𝐦𝐚𝐠𝐞 𝟑, showing the clear drop in the cable performance as the light leaves the launch fibre. Following this a VFL was performed and we could see a break in the fibre with the light leaking.

OTDRs are great tools, often misused; consultants specify OTDR test for optical loss, this is bad, the power measurement of the OTDR is uncertain and the real reason consultants like it so much is that it gives a distance measurement. This distance measurement can be fooled by leaving huge loops of fibre within ODFs. OTDR should be used for advanced fault-finding in ‘short’ fibres and as a second test (following OLTS) to map OSP fibres containing splice joints and multiple events.

𝐃𝐨𝐧’𝐭 𝐭𝐚𝐤𝐞 𝐨𝐮𝐫 𝐰𝐨𝐫𝐝 𝐟𝐨𝐫 𝐢𝐭:

https://www.thefoa.org/tech/ref/testing/OTDR/OTDR.html

https://www.thefoa.org/tech/ref/testing/accuracy/accuracy.html

https://www.thefoa.org/tech/ref/testing/5ways/fiveways.html

𝐈𝐧𝐭𝐞𝐫𝐞𝐬𝐭𝐞𝐝 𝐢𝐧 𝐫𝐞𝐜𝐞𝐢𝐯𝐢𝐧𝐠 𝐦𝐨𝐫𝐞 𝐢𝐧𝐟𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧❓ Send us an email to telecomsales@tektraco.com 📩