Details, Details

We've all heard stories about the space shuttle being delayed because of the failure of an inexpensive gasket. Such incidents always raise one obvious question: Why didn't they inspect that part? Certainly, someone must bear the economic burden caused by the failure. One can only imagine how much added expense is involved when the space shuttle is delayed by a day.

Likewise, the costs can be astounding when a high-speed optical network fails. The situation is bad enough, but it is compounded when a $20 part is found to be responsible for bringing down a multimillion-dollar network. Today's communication networks carry huge amounts of critical data, the value of which can be staggering. Even one hour of downtime can amount to thousands of dollarsof lost revenue.

But the actual cost of network downtime involves more than just revenue; it also includes loss of productivity, customer confidence and opportunities. Service providers should take every precaution to ensure that only high-quality components are used.

However, all too often, network technicians install faulty parts without even realizing it, especially when the part is viewed as a commodity product such as an optical connector. Optical connectors in today's networks are called upon to carry a much more demanding load than they were a few years ago, making quality a key issue. In light of these increased loads, service providers must make every effort to ensure that optical cables don't become the weakest link in the network chain.

The strain

Much optical network equipment has built-in performance monitoring, testing and automated error recovery. These network elements transmit and receive information through optical cable that has been tested at every stage of the manufacturing and installation process. The splices have been measured and tested, and each splice point has been placed in an enclosure that has been tested to protect the mechanical integrity of every fiber.

The fiber optic cable is connected to the network elements with an optical connector. The most widely used practice is to fusion-splice a factory-terminated pigtail onto the fiber optic cable. Occasionally, field terminations are used, but it is generally more cost-effective to use connectorized pigtails purchased from suppliers.

Industry watchers claim that network bandwidth is doubling every six to nine months. This rapid growth has several effects on the network. First, it means that more revenue can be generated over each fiber, which increases the value of the network. Estimates place the revenue generated by a single OC-12 at up to $4 million per year, or about $456 per hour. If an OC-48 network can carry four times more information than an OC-12 and an OC-192 can carry 16 times the information of an OC-12, then the hourly revenue stream could be as high as $1826 and $7305, respectively.

The downside to increased transmission speed is the additional stress it places on individual network elements, including optical connectors. Consider a highway designed to handle 35 mph traffic. Each curve would be engineered to assure that a car could safely traverse the highway at that speed. Each curve and hill would be limited to a certain radius and incline. Now imagine that the speed limit on that highway was increased to 65 mph. Some curves and hills on the highway could pose a serious safety threat if not adjusted to handle the faster traffic.

Likewise, network elements in today's optical systems must be upgraded to handle increased transmission rates, additional wavelengths and higher laser power. As an example, if longer wavelengths are transmitted, certain fiber bends in the network may have to be changed because longer wavelengths require higher bend radiuses. Components whose reliability previously was taken for granted become critical at high data rates, and this includes the optical connector.

Higher data rates equate to more light traveling through the connector, often at higher power, making the performance of back reflection, four-wave mixing and material absorption more critical. If reliability is a key goal, then operators need to ensure that appropriate quality and performance standards are met by all network elements, including optical cable assemblies.

The benchmark

Optical cable assemblies come in a range of quality and performance levels. The quality of a cable assembly is derived from multiple elements such as the type of individual components used, including ferrules, epoxy, housings and fiber; the termination process itself; the training level of production operators and technicians; and the number and types of performance tests and quality inspections. In addition, today's fiber optic terminating equipment ranges from manual to almost automatic. This combination of process, technical training and equipment leads to significant variability in performance and reliability.

Because of the need to control expenses, network operators often pressure suppliers to reduce cost. The result is a proliferation of small independent termination shops, multiple wide tolerance connector components and taking shortcuts on testing. What arises is that some poor-quality connectors find their way into networks, causing field failures.

Telcordia Technologies generated quality standards for optical cable assemblies. The current standard is GR-326, Issue 3, which outlines performance and quality standards for single-mode optical cable assemblies.

GR-326 specifies parameters such as insertion loss, endface geometry and endface quality.

The requirements outlined in GR-326 are substantial and complicated, making it nearly impossible for a customer to determine if a cable assembly meets all the specifications. For example, three endface geometry specifications are fiber height, radius of curvature and apex offset. The accepted method of testing conformance to those three specifications is with an interferometer, a piece of equipment that can cost up to $30,000. However, the information obtained from the interferometer is valuable in determining compliance to quality standards.

Endface geometry specifications are critical to the long-term reliability of the network. For example, if the fiber height is too great, mating two connectors together could result in a crashing together of the fiber, resulting in damage that can impede transmission and cause a network failure. If the fiber height is too small, or undercut, then physical contact will not be made, which can increase back reflection and lead to laser destabilization. Although these parameters are critical to the reliable operation of a network, it is difficult to determine if terminations are meeting those specifications without expensive equipment and time-consuming tests.

Another critical area is endface quality. Although the GR-326 specification requires endface inspection, there is evidence that a more rigorous endface quality requirement is needed. GR-326 requires inspection of the polished end of the ferrule under a magnification of 100 power for cracks, chips or scratches. Inspecting endfaces at a higher power can lead to discovering more defects. Pits and scratches on a fiber weaken the surface of the glass, decreasing its ability to withstand stress and environmental changes. In addition, pits and scratches serve as reservoirs for contaminants. These findings have serious ramifications for network reliability.

The best buy

Given the inconsistent quality of cable assemblies on the market, what can service providers do to improve or monitor the quality of cable assemblies before they reach the field technician? Testing each one is not a viable option. However, personnel can work closely with suppliers to assure that both parties understand the requirements.

For instance, high-quality cable assemblies use ceramic ferrules rather than glass or metal because ceramic ferrules provide the best performance and remain more consistent. Another consideration is the bore and tolerance in the ferrule. A connector using 125 mum bore will consistently have lower insertion loss than a connector using a 126 or 127 mum bore. The smaller bore is more difficult to work with and more expensive, which is why manufacturers sometimes prefer the larger size. It also is advisable for service providers to ask the supplier about the quality of each component used in cable assemblies.

Service providers should understand the termination process used by their suppliers. The procedure should be a repeatable step process that leads to consistent and determinable results. Terminating an optical fiber requires multiple polishing steps, much like the finishing of fine furniture. The first steps are designed to shape the end, while the final steps are meant to put a smooth, scratch-free finish on the surface. The equipment used in the process needs to hold the tight tolerances required for optimum optical performance.

It also is important to monitor the termination process by testing the relevant parameters. Service providers should know how the cable assemblies are tested and inspected. What tests are preformed on each individual cable assembly? How are the cable assemblies inspected? If lot or batch sampling is used, how is the number of units to be tested determined, how is the data tracked and how is the frequency of tests determined? Test equipment should be adequate to measure the most important performance parameters such as insertion loss, back reflection, apex offset, fiber height and radius of curvature. Adequate visual inspection should take place at an appropriate magnification.

Service providers should conduct their own on-site spot testing and inspection of cable assemblies to monitor ongoing quality. The results can be documented to note any changes over time. Sharing test results with the supplier is an excellent reminder to the supplier that customers are quality-conscious.

If spot inspections are difficult, another option is to occasionally have cable assemblies independently tested by an outside laboratory.

In the field

Some field technicians may think their efforts will make little or no difference in the quality of optical connectors installed into the network. However, they can take actions to ensure that the network has only high-quality terminations.

First, technicians can ensure that connectors and adapters are properly cleaned before installation. Just because a connector has a cap on it, don't assume it is clean. Caps are generally a molded part and often retain release agents. Connectors and adapters should be cleaned with acetone or high-grade alcohol and lint-free cleaning wipes or swabs. Alcohol that is less than 99% pure may leave an oily film. It also is important to use only clean wipes or swabs to avoid scratching the connector surface.

In addition, technicians can visually inspect every connector before installation. The most critical connector part to inspect is the endface. A technician should have a magnifier of at least 100 power to check for contamination, scratches, pits and chips. At a magnification of 100 power, the endface should appear perfectly smooth. Connectors with visible endface defects should not be placed into service. If there is any question regarding the quality of the endface, the connector should be rejected or further inspected with more capable equipment.

Third, technicians should take care that the cable is properly routed to avoid sharp bends, edges and kinks. Networks can only operate at high efficiency when the light is unimpeded. Care should be taken to ensure that the cable cannot be damaged or bent because of poor routing or because of future service situations.

For instance, if a bulkhead drawer is opened for service, the cable should transition smoothly without additional bending and kinks as the drawer is opened and closed.

Fourth, technicians can make sure cable assemblies are properly stored to avoid damage. Optical cable can be damaged if enough weight is placed on it. Also, cable assemblies should be stored so that contamination cannot gather on the endfaces. Protective caps should remain in place, and the entire assembly should be stored away from heat, moisture, cold, dust and sunlight.

Finally, technicians should keep track of all cable assemblies removed from going into service. Frequently, cables removed from service are not properly identified and either end up back in service or unjustifiably discarded. Often a first step in correcting a network problem is to replace optical cable assemblies, but normally the cable assemblies are not evaluated to determine if they were the cause of the problem.

Most service providers do not have adequate time or resources to constantly monitor the quality of cable assemblies, so it is important to purchase them from suppliers that use high-quality components, incorporate a proven process, test adequately and guarantee compliance.

Only one method can guarantee that every cable assembly meets critical quality and performance criteria and that is to test and inspect every single connec tor.

By investing in a high-quality optical connector from a reputable supplier, service providers can make great strides in keeping networks running smoothly.

Many suppliers of optical cable assemblies are familiar with GR-326, and several advertise that their cable assemblies meet GR-326 requirements. To assess the current state of the industry regarding these claims and the overall quality of cable assemblies in the marketplace, Telect had cable assemblies from several manufacturers tested by an independent testing laboratory. All purchased cables were GR-326-compliant, according to suppliers.

The goal was to evaluate each termination with respect to GR-326 for compliance to insertion loss, back reflection, fiber height, apex offset and radius of curvature. In addition, the endface of each connector was visually inspected with a high-power magnification system to evaluate the quality of the polished endface.

Test results from the independent laboratory indicated that not all terminations met the specifications and that endface quality is highly variable.