There are three basic steps to identify the loss event and restore the fiber connection.
- Identify the type and location of light loss (fault)
- Repair fault(s)
- Test the repaired fiber
Where to start?
1. A closer look
A good start is to eliminate “human error.” For example, if the problem occurs in a local area network, ask if anyone has recently disconnected, reconnected, or moved network equipment, especially personal computer equipment. Maybe the connector was contaminated when the fiber optic cable was reconnected to the PC, or it was incorrectly reconnected, or maybe not at all!
Also, ask if office furniture or equipment racks have been moved recently. Moving furniture may accidentally crush or break the cable, or cause the fiber optic connector to disconnect.
Finally, ask if any telephone company or other service technicians are currently working in the building.
If the answer to any of these questions is “yes,” then it provides you with a logical place to start troubleshooting. If the answer to all these questions is “no,” then it’s time to break down the testing tool to track down the problem.
2. Use tools
1) Power meter and light source
Power meter and light source or optical loss tester (OLTS) are essential test tools for fiber installers or technicians. This test equipment will show you the overall end-to-end loss of fiber optic operation. The measurement can be compared with the theoretical loss during operation, and can also be compared with the loss measured during system commissioning. When testing a single link or fiber optic network operation, power meter and light source testing are the most effective. However, you do need access to both ends of the fiber run to work. Some units have the ability to measure optical return loss (ORL) and can provide automatic two-way testing. Other possible features include the user’s ability to set pass/fail thresholds, which helps to quickly verify whether the fiber span is within specifications. Many software has the ability to store test results for later download to PC software for documentation and reports.
2) OTDR/Optical Time Domain Reflectometer
Optical Time Domain Reflectometer is a photoelectric testing instrument used to characterize optical fibers. The OTDR test locates the event and determines the amount of attenuation at any point along the fiber. OTDR uses the effects of Rayleigh scattering and Fresnel reflection to measure the characteristics of optical fibers. Let us discuss Rayleigh scattering and Fresnel reflection to understand what they are and how they happen.
3) Rayleigh Scattering
OTDR pulsed light collides with incoming particles (dopants and impurities) and produces Rayleigh scattering. This is called Rayleigh scattering. Some of the scattered light (about 0.0001%) is sent back in the opposite direction (from where it came from), which is called backscatter. In optical fiber cables, Rayleigh scattering dominates the loss characteristics due to its length.
4) Fresnel reflection
Fresnel reflection is a major type of reflection. When light travels in glass, it encounters different densities, which will change the speed or refractive index of light. Some light (up to 4%) is reflected back to the light source. The power reflected by Fresnel can reach 40,000 times that of backscatter. Fresnel reflections occur at connection points and mechanical joints.
Therefore, the OTDR sends an adjustable light pulse into the optical fiber. By measuring the time required for the reflection to return and the reflected power, the OTDR generates the footprint or trajectory profile of the fiber under test. The graph or “trajectory” displayed by the OTDR shows each loss event during the operation, including the location and size of each event. By comparing the current trace with the archived trace, you can quickly identify any new loss events or known loss events, such as connector matching points or splicing points, which have deteriorated over time. In order to perform testing and provide OTDR tracking, the OTDR only needs to access one end of the fiber running.
5) VFL/Visual Fault Locator
The Visual Fault Locator (VFL) injects visible laser light into the fiber link. Excessive bending or breaking of the optical fiber will cause the light to leak from the sheath of the optical fiber, thereby visually showing the location of the optical loss. This test device is suitable for cables without external cable sheaths, such as bare optical fibers or tightly buffered optical fibers. Problem connectors can sometimes be identified by VFL. This device allows you to actually see the light leaking from the fiber core, whether it is bare fiber or tightly buffered fiber.
VFL is most suitable for testing the continuity of the fiber, looking for fiber breaks in connectors or connectors or near the connectors, and checking the fiber management micro-bends in the patch. As a result, while a VFL can help you identify a specific loss event, it cannot tell you how much loss is accumulating in the fiber link. Once you find the problem, you have to fix it. A variety of options are available to you
6) Quick termination of the connector
This is low labor skills, minimum equipment requirements, and on-site installation of the connector. The connector has a factory-polished sleeve that contains a short fiber that has been epoxidized. Having the supply of quick termination connectors on hand will allow you to quickly replace any failed connectors in the network. You can also use these connectors without connecting the device.
3. Censored connector
Mechanical joint-This is a low labor skill, quick installation joint that connects two pieces of bare or tightly buffered fibers together. Most mechanical joints only require basic fiber stripping tools for assembly. When you are repairing broken fibers, mechanical splices are the best option, and you cannot use fusion splicing equipment.
4. Mechanical horror
Splicing-Splicing is the safest and best performance (lowest loss) method to connect two optical fibers. When you repair a broken fiber, splicing is the best choice, either in the span or in the splicing frame, because this method will produce the lowest possible loss and back reflection.
