When your fiber network encounters an unexpected performance issue, often the first instinct is to replace a connector, jumper, or a trunk in a haphazard way to restore service. While this may initially seem to be the fastest method, oftentimes it may be more costly and potentially more disruptive. Some may compare this to an inexperienced auto mechanic randomly installing parts in your car in an attempt to resolve a concern without proper diagnostic tools, all at your added expense. Like the auto analogy, when it comes to fiber networks, having the right tools and knowledge of what to look for can quickly identify the issue and allow you to properly correct or address the problem the right way, likely with less material and labor costs.
There are many brands and types of equipment on the market to perform this type of diagnosis, but when the problem occurs, scrambling to research what tools you need and how to use them can become quickly overwhelming. The Light Brigade offers many brands and types of testing, inspection, and troubleshooting tools, along with the proper skills training, to help. I am often asked, “what tools and equipment do I need to locate the problem.” While it might seem like an easy question to answer, it all depends on the kind of network, type of fiber, connector style, and in many cases how long the spans reach.
The OTDR may seem like the end-all, be-all tester for troubleshooting. In some cases that may be true, especially for identifying faults within the fiber span itself, but on shorter span networks, the OTDR may not be able to properly isolate the exact problem or location. The OTDR is just one tool that you should have in your arsenal for troubleshooting. Other recommended tools are a good light source and power meter, a visual fault locator (VFL), a live traffic identifier, video inspection units, and proper cleaning tools and materials.
Light Source / Power Meter (LSPM or OLTS)
Fiber type is a key factor in determining what equipment you need. When looking at a light source and power meter (LSPM) options, one must consider whether they will be testing single-mode or multimode and at what wavelengths are used. Selecting the wrong fiber type or wavelength range when ordering your equipment may yield you a very nice piece of equipment that is not capable of working with your network. When it comes to multimode, there are a multitude of possibilities ranging from legacy 62.5/125 (OM1) to the varieties of 50/125 ranging from the legacy OM2 to the newer laser optimized and SWDM capable multimode fibers called OM3, OM4, and OM5. Another consideration is the proper test leads or reference jumpers and the connector interface required to match not only the interface of the test equipment but also the connector type of the device or span under test. The launch and receive test leads must match the fiber type under test for accurate results.
Trouble Shooting Tools– Visual Fault Locator
Visual fault locators are also a great tool for visually identifying fiber breaks, problems at the backside of a connector, and in some cases to easily verify continuity and polarity. Most visual fault locators use a visible red laser, typically around 650nm wavelength. These are similar to what you may be familiar with in a simple laser pointer, but VFLs usually have a purpose-built housing that not only allows you to properly attach a connector, but also provide a continuous and pulsed option to ease identification.
As with a laser pointer, care must be taken to avoid direct eye exposure. There are various grades and quality levels of VFLs available. Looking for the cheapest, or highest power, or greatest achievable distance are not always the best criteria. There are many look-alikes available on various e-commerce platforms at a low price, but the lasers contained within may not be FDA approved and could pose a greater risk to you or your technicians. There are many reputable brands available that are specifically designed and intended for safe use in fiber optic troubleshooting.
Figure 1 - Fiber Identifiers
When troubleshooting a fiber network, it is a good practice to verify if there is any presence of live traffic. Identifying the presence of traffic allows you to confirm that the light source or transmitter on either end is turned off, indicating that it is safe to work on the link. In other cases, identifying the presence of live traffic allows you to identify the proper link, none of us want to fix something that is not broken. There are tools designed for this purpose and are fairly simple devices, but again the fiber type presents us with some barriers. Not all live traffic identifiers work with multimode fibers and in most cases, they are single-mode only devices. Choosing between single-mode and multimode is no longer the only factor when choosing a traffic identifier.
Over the past decade or so, the industry has seen a major shift toward bend-insensitive fibers. These fibers allow tighter bend radii than non-bend-insensitive varieties. Traffic identifiers operate by placing a controlled bend onto the fiber or cable and measure light leakage to detect the presence of signal and the direction that it is traveling. Placing a bend on bend-insensitive fibers will not allow the same amount of light leak and hence you may see a false negative. There are a few manufacturers that have figured out how to compensate, for this, with traffic identifiers specifically advertised as being able to be used with bend-insensitive fibers.
Normally bend-insensitive fibers are referred to by their ITU designation of G.657.A1, A2/B2, or B3, each representing a tighter allowable bend radius. Some manufacturers are producing historically non-bend-insensitive single-mode (G.652.D) fiber with bend-insensitive properties similar to G.657.A1 but maintain the G.652.D mode field diameter. Just because your fiber is classified as G.652.D does not automatically mean that a traffic identifier intended for non-bend-insensitive fibers will work.
Clean and Inspect
There have been many claims over the years that over 80% of all network fiber problems are the result of dirty connector end faces. Historically this was blamed on three factors: the cost of cleaning and inspection equipment, the time it adds to inspect and clean in the field, and the mindset of not believing that contamination that cannot be seen with the naked eye could impact performance. Over the years the cost of cleaning and inspection tooling has been reduced, and the quality has improved, but the implementation has had only marginal change.
Unfortunately, most contractors and installers recognize the value of the time and money when they are in a pinch to finish testing or turn over a project, and discover that they are spending twice the original labor estimate chasing dirt at the last minute. All of that could be easily eliminated with a clear inspection and cleaning regimen, along with the proper tools and equipment.
When it comes to visual inspection, legacy field inspection tools had been primarily hand-held, compound magnification, microscopes. The quality of optics and magnification values varied, but for the most part, even the worst was better than none. Over the years, digital probe type inspection devices have made their way into the market, though due to higher costs, were not initially mass adopted. The early versions were often tethered to external monitors or other pieces of test equipment and simply provided a digital view of the endface. They also allow for safer viewing as the risk of directly viewing a live signal was all but eliminated. Later evolutions introduced intelligent analysis software that allowed the user to inspect with an automated, objective, comparison to the industry standard IEC 61300-3-35 pass/fail criteria.
As we entered the COVID-19 era, and the associated precautions, it quickly became clear that video probe scopes were a more sanitary option as compared to hand-held units held to the eye. The latest versions inspect both patchcords and bulkhead connections, have an integrated screen or can be paired with a mobile device via WiFi or Bluetooth, and can store serialized results and images.
A good rule of thumb is to always inspect before you connect. Similarly, always inspect before cleaning. After all, there is no need to waste cleaning supplies on an already clean connector. Finally, remember that just because it is factory sealed does not mean that it is clean.
Figure 2 – Endface Inspection Flow Chart
When inspection of the endface shows that there is contamination or damage that violates the acceptable criteria, there are a few additional steps to consider. When scratches, pits, chips, or cracks are identified, the recommended practice is to replace the connector or assembly when possible. While in some cases it is possible to visually correct scratches and pits, many times field correction methods will alter the endface geometry and likely cause other performance issues. If the failing defect is contamination, the first step is to determine if it is fixed or loose contamination. In most cases, if it is a factory finished connector, any contamination present would be considered loose. If the connector is field polished, there is a chance that the contamination could be fixed or loose. Generally, fixed contamination will require a solvent to remove while most loose contamination can be completed with dry cleaning methods provided that high-quality cleaning tools and materials are used. Attributes to look for are lint-free, abrasion-free, and where possible static-reducing cleaning contact materials.
There are many mechanical advanced cleaners on the market such as “one-clicks” and “cassettes” that do an excellent job. Occasionally, defined “loose” contaminate may not be completely removed using dry cleaning methods. In those cases, it is recommended that a small amount of fiber optic cleaning solution be applied to the cleaning material and then wipe the connector across the wet section followed by the dry section, and inspect to verify proper removal. Due to varying environmental conditions such as humidity, the use of isopropyl alcohol to clean endfaces outside of a lab or manufacturing setting is not recommended.
Figure 3 – Defect Examples and Actions