Step 4: Fiber Optics Testing

May 31, 2022 11:58:45 AM / by Sander van Leeuwen

Today’s business and regulatory environment demands that marine operations, such as seismic exploration, ROVs, and defense applications, stretch the lifetime of marine cables to their absolute potential. In this series of blogs, we offer a 5-step plan for marine cable inspection, including:


In this blog, we will cover testing subsea cables containing fiber-optic lines. 


Testing the fiber-optic attenuation

After performing a visual inspection and testing the electrical conductor and insulation resistance of the cable, the next step is testing its fiber-optic attenuation. Any loss of data through the fiber indicates that it has failed at some point in the length of the cable or in the connections. When that happens, the crew on the vessel will try to determine where the fault is. If this is a connector issue they might get away with cleaning the contacts. If not, they will have to swap to a spare fiber.  

Seismic Guide image 6-2


Once the cable runs out of spare fibers, the crew will call the field service team here at DeRegt. At that point, we make plans for someone on the team to travel to the ship and perform an inspection and possible repair or re-termination.


However, if the vessel is equipped with an optical time-domain reflectometer (OTDR) the crew may be able to measure attenuation (loss of signal) and pinpoint the location of any failure inside the cable themselves.


Using an OTDR to test subsea fiber optic cables

The use of an OTDR is as follows. All DeRegt marine cables are fitted with contacts at both the sea end and the deck end. To measure the loss across the entire cable these contacts must be included. 


This means connecting a short fiber optic launch cable to both ends. This allows the OTDR to read the power losses across the complete span of the cable from sea end to deck end, including the contacts.


The OTDR screen will show each interruption in the light signal as an “event.” By adjusting the settings on the equipment, you can detect the optical power loss on every event, as shown in Fig. 1.


Fig. 1. A typical OTDR trace of a fiber without a fault. Each interruption in the power level is termed an “event.”


  • The red arrow shows the launch cable connected to the OTDR at the sea end of the umbilical cable.
  • The blue arrow is the event related to the launch cable and the umbilical – this is the sea end contact.
  • The green arrow is the umbilical cable itself.
  • The orange arrow is the deck end contact.
  • The violet arrow is the launch cable connected to the deck end.


For all these events the OTDR will give you full information about the status of the fiber. If a fiber is damaged somewhere in the middle, the OTDR will show an extra event at that point, giving you the exact distance to that location. With that information and long-distance guidance from our team, the vessel crew could decide to cut out a section and then re-terminate the cable. If the vessel is not capable of doing the re-termination, DeRegt field service will always be ready to travel to its location and have the cable back in operation as soon as possible.


Prevent downtime with more cable handling tips

Taking good care of marine fiber optic cable equipment helps prevent downtime and high costs. The best way to prevent damage to any cable is to follow some basic, yet important guidelines during reeling, installation, and deployment. At the same time, by performing regular inspections, you will help ensure your cable and terminations will remain operational during their full lifetimes. And more importantly, taking good care of your equipment and using it in the way it is intended will save on downtime. 


Practical Guide to extend Cable Lifetime

For some fresh recommendations on the best practices for handling and caring for subsea cables, download our Seismic Guide on Cable Handling.

Download your copy here


Topics: Seismic

Sander van Leeuwen

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