Every network today includes fiber optic connectivity—whether it's an all-fiber outside plant (OSP) infrastructure, thousands of fiber links between equipment in the data center, or the fiber backbone in a LAN. However, sometimes even after the fiber cabling is installed, tested, commissioned, and properly supporting an application, things can go wrong. There are several scenarios that can cause a fiber link to not adequately support a given application or to be completely non-functioning. We put together the following 5 tips for troubleshooting your fiber network.
Tip #1 – Check Your Loss
Specific fiber applications have a maximum channel insertion loss and length requirement, and those requirements have gotten more stringent with higher speed applications. For example, 10GBASE-SR supports 10 Gig over 400 meters of OM4 multimode fiber with a maximum channel insertion loss of 2.9 dB. High-speed 100, 200, and 400 Gig applications are only supported over 100 meters of OM4 multimode fiber with a maximum channel insertion loss of 1.9 dB. Other applications are more complicated. Newer short-reach singlemode applications like 100GBASE-DR, 200GBASE-DR4, and 400GBASE-DR4 have different insertion loss maximums based on 
the number and reflectance of the connectors in the channel.
During the design phase of a project, loss budgets are calculated for each channel based on the loss and length of the fiber (typically about 3 dB/km for multimode and about 0.5 dB/km for singlemode) and the loss (and sometimes reflectance) of each connection point within the channel, including all connectors and splices. Unfortunately, various problems can crop up post installation that can cause the overall insertion loss to exceed an applications maximum requirement.
If a fiber link is not performing as expected, the first step is to check the insertion loss to make sure it still complies with the application requirement. Testing the loss of a fiber link is achieved using an optical power meter.
When choosing a power meter, it's important to choose the right detector type based on the application's fiber type, wavelength, and power. For example, a silicon detector type is only suitable for multimode fiber for wavelengths to 850nm, a Ge (germanium detector) can test both multimode and singlemode wavelengths from 600 to 1650nm, and an InGaAs (indium gallium arsenide detector) is the highest power option that is ideally suited for high-power singlemode applications such as long-haul links. You also need to consider connector type. While most power meters will support SC, LC, and ST connectors, if your network features MPO/MTP parallel optic applications, you will want a power meter that features an MPO/MTP interface. Some power meters will support only 12-fiber MPO/MTPs while others will support both 12-fiber and 16-fiber MPOs/MTPs. Similarly, if you're testing high-speed singlemode systems using wave division multiplexing (WDM) technology, you want power meter capable of quickly and easily checking loss on each wavelength.
Tip #2 – Locate Faults
Following installation, changes and reconfigurations can cause loss events or prevent a link from functioning altogether. Connectors may not be properly mated or misaligned and cables can become damaged. For example, bends that occur from handling the fiber can cause light signals to leak out of the fiber core. These could be macrobends caused by exceeding the cable bend radius or small microbends that overtime can cause the glass of the fiber to crack. Note that during installation (i.e., under tension), the minimum bend radius is 20X the diameter of the cable. After installation (i.e., under no tension), the minimum bend radius is 10X the diameter of the cable.
A simple way to locate faults is to use a visual fault locator (VFL). VFLs come in a variety of styles, from simple compact pen-style VFLs for checking individual fibers to more advanced solutions that check all fibers within an MPO/MTP arrayed fiber cable at once. All VFLs work by emitting a visible bright red laser beam of light down the fiber link that becomes visible at any break or major bend in the fiber where the light leaks out. It's also a great tool for verifying continuity or tracing a fiber link to locate the opposite end. VFLs however may not be ideal for locating faults in very long links, in environments where the cable isn't visible (e.g., behind walls, underground, etc.), or if the cable jacket on the cable doesn't allow the VFL laser light to penetrate (e.g., armored cabled).
Tip #3 – Inspect Fiber Endfaces
One of the most common causes of failures in a fiber link is a contaminated connector endface. Any dirt, dust, scratches, or pits on a fiber endface can impede light signals from having a clear path along a fiber link, causing insertion loss and reflectance that degrade network performance. Even if a fiber endface was clean at the time of installation, any time a fiber connector is handled during moves, adds, and changes, it is subject to contamination.
There are a variety of solutions for fiber endface inspection. Simple hand-held optical microscopes and cameras capture magnified fiber endface images (typically 200X or 400X) for analyzing defects, while more advanced intelligent inspection systems automatically analyze cleanliness based on industry standards such as IEC 61300-3-35 that uses specific grading criteria based on the number and size of defects and debris on a fiber endface to determine pass or fail. It's important to consider that manual inspection is always subject to human error—what one person sees versus another can vary based on skill, experience, eyesight, and even ambient lighting.
It's also important to consider the type of connector. When inspecting multi-fiber push-on (MPO) connectors such as the MTP used in parallel-optic applications that transmit and receive over multiple fiber, each endface within the connector needs to be inspected. The process is much faster using an intelligent MPO inspection solution specifically designed to automatically inspect each fiber of an MPO/MTP in a single step. Check out our recent blog on inspection for more information on the best way to inspect MPO/MTP connectors.
Tip #4 – Verify Polarity
In every fiber application, polarity ensures that transmit signal at one end of channel correspond to the receiver at the other end. If the polarity is wrong, transmit signals won't reach their corresponding receiver and the link won't function at all. Polarity problems can crop up during moves, adds, and changes if the wrong type of patch cord is used. It's especially tricky in multi-fiber MPO/MTP applications where multiple fibers at the transmit side of a link need to all correspond correctly to the receive side of the link. Since VFLs check for continuity, they can also be used for checking polarity. More advanced fiber test equipment can quickly verify polarity of installed MPO/MTP links.
As discussed in a previous blog, one of the easiest ways to prevent mishaps with polarity is to use connectivity that supports changing polarity in the field rather than having to purchase new components. For example, our MTP Elite Pro fiber patch cords offer the ability to change polarity from Type A to Type B and vice versa using a simple tool.
Tip #5 – Perform an OTDR Trace
While an optical power meter will tell you the loss, a VFL can help you find a fault and determine continuity, and fiber inspection equipment will determine if a contaminated fiber endface is the problem, sometimes the problem is still easy to identify and locate. When all else fails, the ultimate troubleshooting tool for fiber networks is the optical time domain reflectometers (OTDR). This is the tool that will do it all—you can calculate the overall loss and length of the channel and locate specific loss and reflectance events—from breaks and bends, to contaminated connectors and bad splices. An OTDR does this by providing a comprehensive trace of the link where shifts in the trace indicate specific events at specific locations. Reading an OTDR trace takes some expertise, but many OTDRs on the market today offer automatic measurement modes for more beginner users. Like an optical power meter, an OTDR should support the fiber type and wavelengths of your application.
The good news is that CablesPlus has the solution you need for all your fiber troubleshooting needs! From a wide range of VFLs and inspection equipment, to a complete line of fiber test equipment for calculating loss or performing more advanced OTDR troubleshooting. For help determining which option is best for your troubleshooting your fiber installations, just contact us at sales@cablesplususa.com or 866-678-5852.