Breaking the glass wall

Active monitoring with OTDR is an excellent way to protect fiber infrastructure. But with coherent OTDR, you can break the glass wall and extend that protection to the surrounding environment, enabling new applications.
Prayson Pate
Virtual lights over city

The power of active OTDR monitoring

Let’s start with the basics of OTDR. Telcos have long used portable OTDR test sets for on-demand measurement and diagnosis of fiber optic lines. The problem with this approach is that it’s event-driven and requires technicians to drive to the site before manually attaching the OTDR test set. 

  • You might start with an initial qualification or birth certificate of a new line. Drive, attach, test, remove. 
  • You get a call from a customer regarding an outage. Drive, attach, test, remove.
  • You splice in a new splitter for GPON. Drive, attach, test, remove.

You get the picture. But with active OTDR monitoring, you can continuously and automatically monitor your fiber infrastructure. 

  • Once a fiber is installed, you have an initial birth certificate – automatically.
  • You can see if there are degradations over time, e.g., increased loss due to water absorption – automatically.
  • You get an immediate alarm if there’s a break in the fiber and an indication of exactly where it occurred – even with GPON. Automatically, of course.

Adtran’s ALM active monitoring solution is effective, economical and available today. See here for more information.

But it works by monitoring optical amplitude, so there are limitations on what external events it can detect. To expand that range, we need to move to coherent OTDR.

Coherent OTDR extends protection

Coherent OTDR extends monitoring from just amplitude to also include phase. But why is that important? External stimuli like vibration, temperature and strain typically do not affect the amplitude of the reflected signal, so standard OTDR won’t see them. But they will cause changes to the optical phase, which coherent OTDR can detect. 

What types of monitoring applications does coherent OTDR enable? 

Smart grid surveillance

Let’s start with monitoring a power line by co-locating a fiber, as shown below.


Now, we can detect the heat from a fire, the strain of a downed tree or the vibrations from construction equipment. In each case, the coherent OTDR can provide an alarm BEFORE a service-affecting event occurs, enabling preventative action. We can send a drone out with a loudspeaker to tell the backhoe to back off, before it even starts digging!

Smart city traffic monitor

Another example is smart city traffic monitoring. With coherent OTDR, optical cables buried under or next to roadways can enable information on vehicle speeds, road smoothness, vehicle counts/congestion or other parameters – all without the need for active equipment on or near the roads. That information can then be passed on to other systems such as traffic control or public safety.


Plus many other applications

Other applications for coherent OTDR include:


Who would have thought that you can detect an intruder using an optical fiber?

Direct or coherent? Pick the one that’s right for you

Well, almost. Coherent OTDR isn’t here yet, but will be available from Adtran in 2025. In the meantime, you can get started with Adtran’s ALM today and begin protecting your infrastructure. Contact us to learn more about ALM or our future coherent OTDR products and applications. And you can register here to view a recording of our recent webinar “Good fibrations” on coherent OTDR.

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