Fiber Connector Cleanliness

More than 75 percent of troubleshooting in optical networks results from dirty fiber connectors, a stunning fact first learned by high data rate equipment manufacturers, and later by transport installation teams in the telecom sector. Many in the cable industry may find this surprising, but the problem exists and is quickly becoming intolerable as fiber networks expand. Good news! The solution to this problem is simple, established and proven. Dirty little secret High data rate equipment manufacturers first discovered that dirt was a quiet killer of signal performance, a lesson that took years to sink in and was not addressed until massive waste had occurred. As high-speed equipment made its way into the field, telecom carriers took several years before discovering the same fact. Today, a large-scale effort is under way to implement a new discipline where first-level technicians consistently clean and inspect fiber-optic connectors before mating them together. Although these practices are difficult to ingrain, most companies continue to work through the process.

The same problem also affects fiber-optic connectors in cable networks. The industry must attack this problem now before it becomes a major source of network downtime, lost revenue and waste.

Fiber-optic connectors hold great promise for both performance and reliability. When cleaned and inspected properly during installation, they offer great advantages over copper alternatives and are often referred to as "lifetime connections." The effects of dirty connections can be difficult to detect during initial installation and test, but as traffic on a given fiber link increases, contaminated connectors will induce unacceptable loss. Contaminated connectors are hidden liabilities that become disruptive during turnup, or later as traffic increases. The problem The plague of dirty fiber connectors that beset equipment manufacturers in the late 1990s led to the establishment of a team of industry experts who performed practical research within a group called iNEMI (http://www.inemi.org). This research is now one pillar of a pending international standard that prescribes inspection procedures and pass/fail criteria for manufacturers and operators of fiber-optic networks (IEC-61300-3-35).

The problem is simple: Light cannot pass through dirt. Cleaning a connector so that it is 100 percent free of microscopic particles is difficult and expensive. So the team set out to determine the acceptable level of contamination and means for expressing this data in a manner practical for field implementation. A high-level summary follows – the entire body of work is too extensive to detail in this article.

First, a brief explanation of fiber optic connector architecture (Figure 1): The glass fiber strand is composed of an outer area, or "cladding," and an inner "core" area, each with a different refractive index. 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 body. The connectors are male; therefore, a female-to-female adapter joins them. Trapped microscopic dirt particles between two mated connectors prevent light from moving naturally down the fiber and block, absorb, scatter or reflect a portion back toward the source. Further, some of the trapped dirt becomes permanently buried or embedded in the glass and requires replacing or re-polishing of the connector (Figure 2). The cost of troubleshooting, asset damage and network downtime are exponentially higher when dirt is embedded into the fiber inside expensive network equipment, where replacing or re-polishing the fiber is not an option. The iNEMI team set out to discover the relationship between the amount of dirt and its location and the 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 (Figure 3). Further, large particles nearly anywhere in the innermost 200 microns were prone to breaking apart and spreading across the end face of the fiber. Therefore, 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 matings (Figure 4). Our understanding of contamination migration led to recommendations that large particles be eliminated within this entire area. A series of tables, specific to the fiber type, that provide pass/fail inspection criteria were produced (Table 1). Those tables are an essential component to the pending international standard and are core to successful deployment of modern fiber-optic cabling systems. Simple solution Take the following three factors together:

1. Dirt on the core produces massive signal degradation.

2. Large dirt particles away from the core can break apart and end up on the core after successive matings.

3. Dirt mated between connectors can become permanently buried or embedded in the glass of the fiber, making cleaning impossible.

Combining these three facts, the clear solution is to clean and inspect connectors before mating them. This proactive approach to cleaning and inspection starkly contrasts with common practice of many service providers today. Only a few companies have adopted proactive cleaning and inspection on a mass scale. Each of those companies has experienced massive reduction in troubleshooting of the physical layer and now enjoys the lower operating costs associated with such progressive change. The flow chart in Figure 5 best expresses the process. Implementation Effective implementation requires three key elements: process development, equipment selection and training. The preceding section primarily addressed process development; however, a successful strategy requires practical visual aids and training guides.

Selecting equipment used in this 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 (Figure 6). Focus on these elements when considering a fiber inspection microscope:

1. Ensure the microscope can inspect both male connectors and connectors located inside bulkhead adapters.

2. Choose video-based microscopes because of the inherent laser safety and potential advantages for accessibility.

3. Choose a microscope that offers use with multiple tips because technicians require a tip for each connector type encountered, including legacy connectors used on existing equipment in the headend. Test each tip for both ease of use (getting the fiber on the screen and focusing easily) and accessibility. Tip availability does not ensure it will work for every application. Consider difficult-to-reach connectors first and choose a system that will work in your worst-case applications.

4. Choose a display that fits your workflow. Options exist for handheld and PC-based displays.

5. Choose a vendor that offers automated software providing the user with pass/fail grading of the image, which greatly accelerates learning the process and improves the successful process implementation.

When selecting equipment for cleaning fiber, the landscape is murky, with multiple vendors claiming an advantage. Few users can wade through the jargon. At least one industry document prescribes a method for evaluating cleaning solutions (IPC-8497-1). 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. A brief guide to the options for cleaning tools follows.

Automated machines: Some vendors offer automated systems that generally have a high initial cost, but a low operating expense. These tend to make sense only in high volume installations and can be extremely valuable at such locations. They work equally well for patch cords or bulkhead cleaning and are unique in their ability to clean SFP/XFP transceivers. (See Figure 7.) 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. (See Figure 8.) Bulkhead cleaning: For cleaning connectors within bulkhead adapters, two categories of consumable products are available. The first products are specialized swabs resembling high-tech Q-tips, which are inexpensive, but must be thoroughly tested because they have a reputation for merely moving dirt and not removing it. Two vendors now offer bulkhead cleaners that use a tiny cleaning tape that advances across the fiber; these are gaining popularity. (See Figure 9.) Cleaning solvents: 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. Key considerations include:

1. Use solvents only when dry cleaning techniques are insufficient.

2. Using solvents in the bulkhead adapters can create problems; therefore, use only fast drying solvents in small amounts.

Training programs for successful implementation are a cornerstone of success. Contact the best suppliers of the equipment for expertise in guiding operators through this process. Comprehensive programs will include:

1. Establishing a training plan, which includes site identification and train-the-trainer opportunities

2. Developing and mastering presentation materials, a course syllabus and practical visual aids for field use

3. Classes offering hands-on experience for field technicians Putting it all together Proactive cleaning and inspection during fiber installation can save massive waste and headaches. The process is simple, although it requires action and education. Remember, while addressing this issue represents a new challenge for many in the cable industry, the processes and solutions are well-established and proven. Simple solutions exist for what could potentially become a massive problem.

Steve Lytle is general manager for the JDSU Communications Test and Measurement business segment. Reach him at [email protected].