Tracking Down Optical Loss

Tracking Down Optical Loss



There are three basic steps to identifying loss events and restoring a fiber link.

1. Identify the type and location of optical loss (fault(s))
2. Repair the fault(s)
3. Test the repaired fiber

Where to begin?

A good way to start, is to eliminate “human error”. For example, if the problem is in a LAN, ask if anyone has recently disconnected, reconnected or moved network equipment, especially PC’s. Perhaps the connector was contaminated when the fiber optic cable was reconnected to the PC, or maybe it was reconnected improperly or maybe even not at all!

Also ask whether office furniture or equipment racks have been moved recently. Moving furniture around can accidentally crimp or break a cable, or cause an optical disconnect at a connector.

Finally, ask whether there have been any telephone company or other service technicians working in the building recently.
If the answer to any of these questions is “yes”, it provides you with a logical place to begin troubleshooting. If the answer to all of these questions is “no” then it is time to break out your testing tools to track down the problem.

Some useful tools include:

1. Power Meter and Light Source 

– A Power Meter and Light Source or Optical Loss Test Set (OLTS) is an essential testing tool for the fiber optic installer or technician. This test equipment will show you the overall end to end loss of a fiber run. The measurement can be compared to either the theoretical loss for the run, or it can be compared to the loss measured when the system was commissioned. Power Meter and Light Source testing is most effective when testing individual links or runs of a fiber optic network. However, you do need access to both ends of a fiber run for this to work. Some units have the ability to measure Optical Return Loss (ORL) and are capable of providing automated Bi-Directional Testing. Other possible available features include the ability for the user to set Pass/Fail Thresholds which help to quickly verify if a fiber span is within specs. Many have the ability to store test results for later download into PC software for documentation and reporting purposes.

2. OTDR/Optical Time Domain Reflectometer 

– An Optical Time Domain Reflectometer is an electro-optical test instrument that is used to characterize optical fibers. OTDR testing locates events and determines the amount of attenuation at any point along the optical fiber. The OTDR uses the effects of Rayleigh Scattering and Fresnel Reflections to measure the characteristics of an optical fiber. Let’s talk about Rayleigh Scattering and Fresnel Reflections for a moment to understand what they are and how they happen.

  • Rayleigh Scattering

--When the pulsed light from an OTDR collides with particles (dopants and impurities) in the glass, the pulse of light will become scattered in all directions. This is called Rayleigh Scattering. Some of this scattered light (approximately 0.0001%) is sent back in the opposite direction (to where it came from), and it is called backscatter. The Rayleigh Scattering occurs along the entire length of the fiber and it is the major loss factor in an optical fiber cable.

  • Fresnel Reflections 

--Fresnel Reflections are a major type of reflection that occurs when light traveling in glass encounters a different density, which changes the light’s speed or Index of Refraction. Some of the light (up to 4%) is reflected back toward the light source. The power of a Fresnel Reflection can be up to 40,000 times greater than that of backscatter. Fresnel Reflections occur at connector points and mechanical splices.

The OTDR sends an adjustable pulse of light into a fiber. By measuring the time it takes for the reflections to return, as well as the power of the reflections, the OTDR produces a footprint or trace profile of the tested fiber. The graph or “trace” shown by the OTDR displays every loss event along the run, including both the location and magnitude of each event. By comparing a current trace to an archived trace, you can quickly identify any new loss events, or known loss events, such as a connector mating point or a splice point, that has deteriorated over time. The OTDR only needs access to one end of the fiber run in order to perform a test and provide an OTDR trace.

3. VFL/Visual Fault Locator

 – A Visual Fault Locator (VFL) injects visible laser light into a fiber optic link. Excessive bends or breaks in a fiber will cause the light to escape through the fiber’s jacket, giving a visual indication of where the optical loss is occurring. This test device is suitable for cables that don’t have an outer cable jacket, like bare fiber or tight buffered fiber. Problem connectors can sometimes be identified with a VFL. This device allows you to actually see the light that is leaking from the fiber core in either bare fiber or tight buffered fiber. A VFL is best suited for testing continuity in a fiber, looking for fiber breaks in or near connectors or splices, and for inspecting the fiber management in patch panels for micro bends. So, a VFL can help you locate a specific loss event, but it won't provide information about the cumulative losses in the fiber link.


Once you locate the fault, you need to repair it. There are several options available to you.

Quick Termination Connectors 

Bobtail Connectors

--These are low-labor-skill, minimal equipment needed, field installable connectors. Quick Term Connectors have a factory polished ferrule that contains a stub of fiber already epoxied inside. Having a supply of quick termination connectors on hand will allow you to quickly replace any failed connectors in a network. You can also use these connectors in situations where you do not have access to splicing equipment.

Mechanical Cleavers

--Mechanical Splice – These are low-labor-skill, fast to install splices for joining two pieces of bare or tight buffered fiber together. Most mechanical splices require only basic fiber stripping tools for assembly. Mechanical splices are the best choice when you are repairing a broken fiber, and you do not have access to fusion splicing equipment.

FIS Splice On Connectors

--Fusion Splice – Fusion splicing is the most secure and best performing (lowest loss) method of joining two fibers. Fusion splicing is the best choice when you are repairing a broken optical fiber, either mid-span or in a splice enclosure, because this method will yield the lowest possible loss and back reflections.

Learn More:

Fiber Optic Link Loss Budget

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