Connector quality: Are standards enough to eliminate the weak link?
Manufacturers claim their optical connectors meet industry standards, yet some network failures are still attributed to connector quality issues.
Arespected telecommunications service provider experienced random failures within its optical network after upgrading to OC-192 (10 Gbits/sec). The service provider placed the highest priority on determining the source and eventually concluded that poor-quality optical connectors caused the failures. Technicians discovered that each of the failed connectors had some form of surface defect. Determined to eliminate optical connectors as a cause of network failure, the service provider changed the quality requirements for all optical connectors used in its network. The most significant change in the specification was regarding visible defects on the endface, including the outer regions of the connector away from the core (light transmission area). The new specification rejected all optical connectors with any visible defects (viewed at 400x) on the endface. To guarantee that all suppliers conformed to the new specification, the service provider began inspecting every connector purchased. Only connectors that met the current industry standards for insertion loss, reflectance, and endface geometry and had no visible surface defects were placed into service. This approach worked. There have been no failures attributed to optical connectors since the new requirements were implemented well over a year ago.
Some service providers view optical connectors as a commodity, purchasing cable assemblies from the lowest-priced bidder then failing to ensure compliance to quality standards. This practice causes at least two problems: First, network managers do not know if connectors in their system meet commonly accepted industry quality standards, and second, manufacturers and suppliers are not held accountable to the standards they profess to meet.
Failing to ensure optical connectors meet quality standards is risky, considering the connector is the only place in the optical network where bare fiber can be exposed to the environment. Great care is exercised throughout the remainder of the network to enclose and protect every location where the optical cable is disturbed or opened (splices, transition points, etc.), yet the single place where bare glass can be exposed is often overlooked.
With today's networks carrying higher data rates and more optical power than ever before, connector quality is even more critical to network performance. Network failures due to optical connectors can be virtually eliminated by understanding a few key issues surrounding connector quality and implementing some basic practices.
Telcordia Technologies (formerly Bellcore) has developed specifications for network components with the goal of ensuring network reliability. The specifications developed for fiber-optic terminations are outlined in GR-326 and specify, among other things, optical performance and endface geometry. GR-326 defines acceptable ranges of insertion loss, reflectance, apex offset, fiber height, and radius of curvature for radius polished connectors.
Telecommunications providers generally assume fiber-optic terminations that meet the GR-326 specifications perform reliably in high-speed networks. Therefore, carriers that purchase connectors from a supplier advertising GR-326 conformance expect to avoid any problems related to connector quality. Recent evidence based on field failures and laboratory testing, however, indicates that this is not always the case.
To determine the level of industry compliance to GR-326, Telect purchased cable assemblies from five manufacturers that specified GR-326-compliant product. Fifteen singlemode jumpers (30 terminations) were purchased from each vendor and an independent testing laboratory tested all of the connectors. The connectors were tested for insertion loss, reflectance, apex offset, fiber height, and radius of curvature. The results were then tabulated and analyzed.
Figures 1 and 2 display the test results for fiber height and reflectance conformance. Based on these test results, there is a reasonable possibility that some of these cable assemblies do not meet the Telcordia GR-326 specification for fiber height and reflectance.
Using connectors that do not meet fiber height and reflectance specifications can negatively affect network performance. For example, a connector with a fiber height that is too high can damage the optical endface of the mated connector. A connector with the fiber height too low can fail to make physical contact with the mated connector, increasing reflectance and insertion loss. High reflectance interferes with optical transmission and can cause laser de-stabilization or shutdown.
Test results of the other performance and endface geometry specifications demonstrated similar examples of nonconforming connectors. If a connector fails one requirement, such as fiber height, the entire cable assembly is classified as not meeting the GR-326 specification, even if it passes all other parameters. Connectors must meet all performance and endface geometry specifications for the cable assembly to be deemed GR-326-compliant. The test results indicate that most manufacturers are compliant for some specifications but poor at meeting others. The percentage of cable assemblies that passed all GR-326 compliance requirements ranged from 100% for one manufacturer to as low as 40% for another; the rest fell somewhere in between.
What happens when GR-326 specifications are not met? Network performance can be adversely affected in multiple ways. A quick look at a few GR-326 specifications amply demonstrates this point. Table 1 provides an overview of the standard's specifications and the possible network effect when a connector does not comply.
The telecommunications service provider that upgraded to OC-192 purchased cable assemblies that, according to the manufacturer, met the Telcordia GR-326 specification. Yet, the carrier still had network failures due to poor connector quality. How is this possible?
One possibility is that perhaps not all connectors met the specification as advertised. Most manufacturers today use some form of statistical process control (SPC) to monitor manufacturing processes and determine if quality requirements are met. SPC analyzes data taken from random samples to determine the level of compliance. It provides reassurance that the manufacturing process is in control and that a high percentage of the product meets the specifications. SPC is particularly useful in lowering production costs, because it reduces the amount of testing or inspection required. However, it is only an indication, and cannot-nor does it claim to-assure that 100% of the product meets the specifications. Generally, several test samples in a row have to either fail or be outside of the normal "trend" for a manufacturing process to be considered out of control. In other words, today's most commonly used method for assuring quality does not guarantee that 100% of the product meets the relevant specification.
Another reason for network failures due to connectors is that GR-326 may not be comprehensive enough to ensure reliability. There are other characteristics under "specified" or "unspecified" in GR-326 that may reduce performance or cause failure. For example, certain types of defects on the optical surface of the connector may not hamper initial performance, particularly if the defects are not in the light-carrying area, but over a period of time and under certain environmental conditions, the defect may change, eventually resulting in a network failure. Defects on connector endfaces may simply be failures waiting to happen.
The mechanical strength and reliability of optical fiber is a concern in all aspects of fiber optics from manufacturing to installation and long-term reliability. Special care is required to protect glass from any disturbance that may cause damage. Small defects can result in big failures, much like a small rock chip that ultimately results in a big crack in an automobile windshield.
Cabling manufacturers have spent millions of dollars in research and testing to ensure cable designs fully protect the fiber, even under the most extreme conditions. Hardware manufacturers have spent millions to develop cable support and closure hardware that provides a protected and relaxed environment for optical fiber during manufacture, installation, and usage.
Optical-fiber companies take great care to ensure fiber is protected as soon as possible after manufacture. During the manufacturing process the fiber is coated with a buffer, a protective coating that is extruded over the glass as it flows from the drawing tower onto the winding spool. The purpose is to protect the fiber from any damage, including moisture in the air.
During installation of optical cable, any place that the optical fiber is exposed to air, such as a splice, is quickly coated to protect it from damage. The only possibility of exposed glass in an optical-communications system is at the connectors. Connectors use pressure to make a good connection, increasing the possibility that damage from any surface irregularity will occur. Therefore, it makes sense to pay particularly close attention to the quality of connector faces.
Surface defects on connectors can consist of pits, chips, scratches, surface irregularities, and roughness. The defects can occur in the core area, the contact area, or the epoxy ring around the outside of the fiber.
Figures 3 and 4 show two different connector endfaces after careful cleaning to remove dust and dirt with typical surface defects. Since the connector in Figure 3 has surface defects in the core area, there is a very real possibility that this connector will cause network problems. A connector with this type of surface imperfection may cause an increase in reflectance, damage to the adjoining connector face, or high insertion loss. Intuitive reasoning suggests that defects in the light-carrying area of a connector endface can cause transmission problems.
If the defect is not obstructing the core area and does not impede alignment, it may have no initial adverse affect. Figure 4 shows a connector endface with no visible defects in the core area, but with defects on other areas of the connector surface. Under pressure from the connector springs and over a period of time of varying temperatures, humidity levels, movement, and vibration, defects may grow to a magnitude that interferes with optical performance or at least become reservoirs for contaminants.
As time passes, changes in the connector endface are likely to occur because of varying environmental conditions. Surface defects by definition are interruptions in the smooth surface of the glass, and these interruptions can migrate under pressure. Even the movement of air on the bare surface of glass can cause surface imperfections to travel. Studies indicate that the tensile strength of fiber decreases when surface imperfections are exposed to the environment. Moreover, fiber defects can propagate when exposed to humidity, vibration, and temperature swings. The severity, nature, and time of failure due to surface defects are largely unpredictable, although several models have been developed that attempt to predict reliability. Because surface defects demonstrate the likelihood of changing, network failure is possible. Empirical data and scientific research imply that the elimination of surface defects minimizes network failures due to poor connector quality.
Surface defects can trap contaminants and become reservoirs of moisture or other types of contamination. Moisture is particularly harmful because of absorption, which reduces mechanical strength. Other types of contamination may be abrasive to the surface, causing scratches, pits, and chips in the endface material. One manufacturer has even suggested that certain contaminants have an affinity for the light-carrying area of the fiber. Any contamination in the optical path can injure the reliability of the transmission.
Evidence discovered through research and field experience suggests that communications providers should take steps to ensure optical connectors meet all critical parameters of Telcordia GR-326 and have no surface defects in the core or contact areas. There are several ways to achieve this level of quality.
Service providers can inspect and test every connector end for all relevant specifications. This testing requires inspection equipment capable of accurately quantifying each measurement. Equipment that is commonly used to verify these parameters includes interferometers for apex offset, fiber height, and radius of curvature; high magnification measuring microscopes or imaging systems for surface defects; calibrated light sources and power meters for insertion loss; and reflectance meters. In addition, technicians skilled in the handling and cleaning of fiber and the use of the test equipment are required. Thorough inspection will ensure each connector is of sufficient quality to perform reliably in the optical network.
Of course, there are drawbacks to this inspection process, including a large investment in test equipment, increased labor costs, and a delay in placing optical cables into service while the inspection takes place. However, consider what the costs are when an optical cable assembly fails. The average time to repair a network failure is estimated at about two hours. An OC-192 network can generate $7,300 an hour in revenue, so just one failed connector can cost upwards of $14,000 in lost revenue, not to mention the intangibles such as customer confidence and productivity.
But if inspecting each and every connector is not feasible, there is a simpler way to ensure connector quality. Purchase cable assemblies only from a supplier that guarantees compliance to all relevant specifications through quality practices and 100% inspection. Check to make sure the supplier purchases or manufactures precision parts, has excellent control of the manufacturing process, and performs staged inspection and measurements on 100% of the cable assemblies. Reputable suppliers will be more than happy to discuss these concerns with potential customers.
Following this practice has several advantages for the communications provider. Cable assemblies are ready to use immediately upon receipt since no onsite inspection is necessary. The over-all cost of installing high-quality cable assemblies is actually reduced due to quantity efficiencies at the manufacturing facility. The logistics of returns are avoided and the provider is more confident that any network problems are not connector-quality-related.
Communications providers that implement and maintain an on-going comprehensive program to ensure connector quality will be ahead in the long run. Either through their own inspection process or by working with a supplier that guarantees compliance to quality standards, service providers can make certain only high-quality optical connectors are used in their networks. While it may require some time, effort, and expense, installing only high-quality connectors is an investment in the long-term performance of the network that will pay for itself many times over.
Ken Ditto is product manager, fiber optics, for Telect Inc. (Liberty Lake, WA). His e-mail address is firstname.lastname@example.org.