Best practices for optical fiber inspection and cleaning ensure optimized network performance.
By Matt Brown
Fiber connectors are widely known as the weakest and most problematic points in a network. The more connections there are in a network, the greater the potential for interruption caused by improper handling during installation, operation, expansion and maintenance. The more information transferred per second, the less loss the system can handle, requiring tighter budgets on all network parameters. The more people served by a network, the greater the impact of a poorly performing, or failed, optical channel.
All of these factors make proper handling of the optical fiber connections more critical than ever. The recognition of the negative potential of poor fiber handling on network performance is bringing about the development and implementation of best practices for optical fiber inspection and cleaning.
Three P's of Efficient Fiber Connections
Network performance is optimized when the proper steps are taken to ensure low-loss fiber connections. The three basic principles necessary to achieve efficient fiber connections are:
1. Perfect core alignment.
2. Physical contact.
3. Pristine connector interface.
Today's connector design and production techniques have eliminated most of the challenges to achieving principles one and two. Number three-pristine connector interface-remains the biggest challenge to optimal network performance because it cannot be controlled by the manufacturer. The full potential for a low-loss connection is only realized when the technician ensures there is no contamination prior to connecting.
Number One Cause of Impaired Fiber Network Performance: End Face Contamination
Research indicates that more than 75 percent of physical network troubleshooting is a result of optical fiber connectors that are dirty or have been damaged by dirt. Light cannot pass through dirt or damaged fiber, so network performance is impaired. Figure 1 clearly shows the increase in loss resulting from core contamination.
That dirt impairs network performance was first discovered by high-bandwidth equipment manufacturers, and later by information transport systems (ITS) teams, which led to practical research within the industry by the International Electronics Manufacturing Initiative (iNEMI). This research by iNEMI is now one pillar of a pending international standard (IEC-61300-3-35) that will prescribe inspection procedures and pass-fail criteria for manufacturers and operators of optical fiber networks.
The iNEMI team set out to discover the relationship between the amount of dirt and its location and the resulting signal degradation it creates. Results of the research determined that dirt on the core dramatically affected signal performance, while dirt on the cladding had less predictable effects.
The research also showed that large particles nearly anywhere in the innermost 200 microns were prone to breaking apart and spreading across the end face. As a result, even when the core area is clean, if large particles exist on the cladding or inner ferrule, that dirt can "migrate" to the core after successive mating and affect signal performance.
This understanding of contamination migration led to iNEMI recommendations that large particles be eliminated within this entire area. See Table 1. A series of tables, specific to the fiber type, giving pass-fail inspection criteria were produced. Those tables are one of the essential components of the pending international standard and are core to the successful deployment of modern optical fiber cabling systems.
Optical Fiber Connectors are Especially Vulnerable to Contamination
To understand the potential negative effects of fiber contamination on network performance, it helps to better understand optical fiber connector architecture.
In an optical fiber connector, the glass fiber strand is composed of an outer area, or cladding, and an inner core area, each with a different refractive index. See Figure 2. The glass cladding serves to trap the light within the core but does not conduct light itself. The fiber is mounted within a round ceramic ferrule, which is then captured by a plastic connector body. The connectors are male; therefore, a female-to-female adapter joins them together.
When two connectors are mated, these microscopic dirt particles get trapped, preventing light from moving naturally down the fiber. The dirt blocks, scatters or reflects a portion of the light back toward the source. Due to the force exerted on the fiber during the mating process, some of the trapped dirt can actually become permanently buried or embedded, requiring replacing or repolishing the connector to restore it to optimal performance.
It is important to note that the costs of troubleshooting, asset damage and network downtime are exponentially higher when dirt is embedded in the fiber inside expensive network equipment where replacing or repolishing the fiber is not an option.
Proactive Inspection Is Superior to Reactive Inspection
It is important to visually inspect fiber connectors at every stage of handling before mating them. If you can catch contamination before mating (proactively) you can almost always clean it and eliminate the contamination.
If you wait to visually inspect fiber connectors during troubleshooting after a problem is detected (reactive), connectors and other equipment may have suffered permanent damage. This is because once mated, the dirt can embed in the fiber, making it uncleanable and permanently damaged and potentially damaging connectors that are mated to it.
Anyone who has worked with the physical layer of a network can understand the potential for connector contamination from dirt and the resulting need for inspection during network operation and troubleshooting. Unfortunately, most connectors are not inspected until problems are detected and damage has already occurred.
Damage caused by initial contamination can be avoided through the implementation of proactive inspection and cleaning processes. These practices are based on the following factors:
· The potential for contamination is always present, even in new components. Even the best clean manufacturing practices cannot prevent microscopic particles from entering sealed bags and under dust caps.
· Dirt particles on the core of the fiber produce massive signal degradation.
· Large dirt particles away from the core can break apart and end up on the core after successive mating.
· Dirt particles mated between connectors can become permanently buried or embedded in the glass of the fiber, making cleaning impossible.
These facts support the practice of proactive inspection of fiber connectors using a microscope designed specifically for this purpose at every stage of fiber handling- from component manufacturing, receiving and quality control to assembly, installation, system testing, troubleshooting and maintenance.
Indisputable ROI of Proactive Inspection Practices
This proactive approach to inspection adds time and costs to the network deployment process. As a result, it has not been common practice among installers and IT staff. However, the benefits clearly outweigh the costs, as evidenced by the massive reduction in trouble-shooting and lower operating costs experienced by the companies that have adopted proactive cleaning and inspection on a mass scale.
These operational benefits translate directly into business benefits. By reducing troubleshooting and network downtime, proactive inspection reduces maintenance costs and keeps the network active and users online. Because proactive inspection ensures that network components operate at their highest level of performance, signal and network performance is optimized. Finally, proactive inspection prevents network damage and ensures longevity of costly network equipment, protecting the technology investment.
Implementing Proactive Inspection Practices
Even with such clear benefits, systemwide change of this scale takes time to implement and requires effective process development, equipment selection and training.
A successful process development strategy should include a combination of hands-on training, practical visual aids and detailed training guides. Companies should look to sources of optical fiber inspection equipment for guidance and assistance with proactive inspection process development.
Selecting equipment used in the inspection process can be confusing because of the multiple sources available and the biases of each source. Successful users rely on field trials or pilot implementations to put each potential solution through its paces.
When selecting a microscope, consider these factors:
· Microscope should be able to inspect both male connectors and connectors located inside bulkhead adapters.
· Video-based microscopes provide inherent laser safety and potential accessibility advantages.
· A specific tip for each connector type encountered is required. Tips should be tried for ease of use (getting the fiber on the screen and focusing easily), as well as accessibility. Multiple tip availability does not ensure the microscope will work for every application. Consider difficult-to-reach connectors first and choose a system that will work in your worst-case applications.
· Different display options work best for different workflows. Options exist for hand-held and PC-based displays.
· Automated software that provides pass-fail grading of the image will greatly accelerate learning the process and improve chances for a successful process implementation.
Training programs are a cornerstone of successful implementations. Contact the best suppliers of the equipment for expertise in guiding operators through this process. Comprehensive programs will include:
· Establishing a training plan, which includes site identification and train-the-trainer opportunities.
· Developing and mastering presentation materials, a course syllabus and practical visual aids for field use.
· Classes offering hands-on experience for field technicians.
Inspection Best Practices
Follow the simple inspection process shown in Figure 3 to ensure fiber end faces are clean prior to mating connectors:
Step 1 Inspect: Use a probe microscope to inspect the fiber. If the fiber is dirty, go to step 2. If the fiber is clean, go to step 4.
Step 2 Clean: If the fiber is dirty, use a cleaning tool to clean the fiber end face.
Step 3 Inspect: Use a probe microscope to reinspect and confirm the fiber is clean. If the fiber is still dirty, go back to step 2. If the fiber is clean, go to step 4.
Step 4 Connect: If the fiber is clean, it is ready to connect.
Be sure to inspect both sides (patch cord "male" and bulkhead "female") of the fiber interconnect before connecting. Patch cords are easy to access and view compared with the fiber inside the bulkhead, which is frequently overlooked. The bulkhead side may only be half of the connection, but it is far more likely to be dirty. Inspecting both sides of the connection is the only way to ensure that it will be free of contamination and defects.
Multiple vendors claim the advantage when it comes to fiber cleaning equipment. Few users can wade through the jargon.
It is critical to understand that most real-world contamination is from airborne particulates. When comparing cleaning techniques, resist the urge to use hand and body oils to provide your baseline for contamination and cleaning effectiveness. The following is a brief guide to several cleaning tool options:
Automated Combination Inspection/Cleaning Systems
These dual-purpose systems can be very valuable in high-volume installation as the resulting lower operating expenses can definitely outweigh the initial high cost of these systems. These systems work equally well for patch cords or bulkhead cleaning and are unique in their ability to clean SFP/XFP transceivers.
For cleaning connectors within bulkhead adapters, two categories of consumable products are available. The first products are specialized swabs, which have a low purchase price but are relatively expensive on a per-cleaning basis. These must be thoroughly tested as they have a reputation for merely moving dirt and not removing it or actually adding debris. More advanced bulkhead cleaners, now offered by at least two vendors, use a tiny cleaning tape that advances across the fiber. These are quickly gaining popularity due to their superior performance, low per-cleaning cost and ability to clean both bulkhead connectors and patch cords.
Patch Cord Cleaning
For cleaning uninstalled connectors, solutions range from individual wipes and perforated wipes in small boxes to cassette cleaners. These should be tested for ease of use but are generally quite effective. The tape-based bulkhead cleaners shown above are also capable of cleaning patch cords.
Many of the wipes, swabs and bulkhead cleaners are offered with cleaning solvents to improve cleaning performance. In general, when chosen carefully and used properly, solvents are useful and positive elements in the cleaning process. However, it is best to use solvents only when dry cleaning techniques have failed. A common mistake is to saturate the bulkhead adapter when using a solvent, which leads to recontamination of the connector as the solvent dries. Ensure those used are fast drying and applied in small amounts. Dampen the cleaning tool with solvent, but do not saturate it.
With more people and locations to serve and more information to transfer at even greater speeds, today's networks require more fiber and fiber connectors than ever before. Particulate contamination is the number one source of troubleshooting in optical networks. Simple visual inspection of optical fiber connectors with a microscope is the only way to determine if connectors are clean before they are mated. To ensure optimal optical performance over the lifetime of the network, both end faces of all optical fiber connections must be proactively inspected, and cleaned if necessary, before mating at every stage of the network development process. Proactive inspection is simple, and the benefits-reduced downtime and troubleshooting, optimized signal performance and prevention of network damage-are great.
Reprinted with permission of BICSI News. www.bicsi.org