Quality
The quality of patch cords is governed by industry specifications. The two most common are EIA/TIA-568-B.3 and Telcordia GR-326-CORE. The 568-B.3 specification was written for commercial building applications, including office and campus environments, where both single-mode and multi-mode networks can be deployed. GR-326-CORE, part of Telcordia’s General Requirements series, was written to be consistent with the Telecommunications Act of 1996. It was intended for service provider markets, which predominantly involve long-haul, high-speed applications such as telecommunications and cable TV. In this market, the requirement is single-mode, so this specification has no provisions for multi-mode patch cords.
Each specification details a series of environmental and mechanical tests. There is overlap in the type of testing performed in GR-326-CORE and 568-B.3. Nearly all the individual tests in 568-B.3 have counterparts in GR-326-CORE, and these tests make up the majority of the section of GR-326 referred to as “Service Life” testing. The function of the Service Life tests is to simulate the stresses a connector may experience during its lifetime. GR-326-CORE also includes additional tests that have no equivalent in 568-B.3, known as “Reliability Tests,” designed to identify potential shortcomings in the design and materials of the connector across various service environments. Service Life testing is sequential, meaning the entire sample population is subjected to every test in a specified order. On the other hand, a new sample population can be submitted for each of the Reliability Tests, just as with each test in 568-B.3.
The testing in these specifications is not done on 100% of production. These tests are usually performed periodically over years to ensure that the components and manufacturing procedures can produce patch cords of the proper quality level.
Environmental Testing
Both specifications include a series of environmental tests, specifying the temperatures and conditions the connectors must endure for extended periods. As these are Service Life tests, GR-326-CORE requires the testing to be performed in a specific sequence. The purpose of these tests is not to ensure the patch cords can withstand prolonged exposure to 85°C or temperature fluctuations up to 125°C but to simulate the effects of aging in various environments. By subjecting the connector to extreme temperatures, the test is designed to cause expansion and contraction of all the different materials in the terminated connector. Epoxy, metal, ceramic, glass, and, in some cases, polymers intersect at a single point just behind the ferrule.
At higher temperatures, these materials will expand at their intrinsic rates, causing various strains on the components. If the test involves thermal cycling, where the temperature fluctuates over a wide range, the test becomes more extreme. Thermal cycling involves changing the ambient temperature of the connector by 125°C every 2 to 4 hours.
Heavy stresses and strains will be induced on each of the materials involved. This test will also expose any weaknesses in the termination. If the design and procedures are not optimal, this can lead to fiber movement or “pistoning” within the ferrule, or in extreme cases, fiber cracks or breakage.
Humidity testing is designed to introduce moisture into the system and, as the test is performed at elevated temperatures, simulate the effects of long-term exposure. Exposure to moisture or water can be very harmful to glass optical fiber, and prolonged exposure can cause the fiber to become brittle and more susceptible to stresses, a condition known as fiber “decay.”
Mechanical Testing
Several mechanical tests are required in these two primary specifications, including Flex Testing, Twist Testing, Proof Testing, Cable Retention, Impact Testing, Vibration Testing, Durability, and Transmission with an Applied Load. The detailed requirements vary slightly between the two specifications, but the general procedures and concepts remain the same.
Many of these mechanical tests confirm that the patch cord can survive the installation and maintenance performed. Testing such as Cable Retention, part of 568-B.3, ensures that the terminated patch cord can withstand the pulling forces incurred during installation. Proof Testing, the equivalent in GR-326-CORE, is similar to Cable Retention and also ensures the strength of the connector’s latching mechanism. Should the patch cord receive a sudden tug after installation, this test ensures that the patch cord will neither break nor pull out of the adapter. 568-B.3 has a separate test called “Strength of Coupling Mechanism” for this criterion.
The only other test that attempts to duplicate installation problems is Impact Testing. Impact Testing verifies that connectors are not damaged when dropped.
Flex, Twist, Vibration, and Transmission with an Applied Load tests are all performed to simulate stresses on the terminated cable and mated connector that could be incurred over the life of the connector. Connectors subjected to physical extremes are then optically verified to ensure the quality of the termination. In GR-326-CORE, many of these connectors are “pre-stressed” after concluding the environmental portion of the sequence. Weaknesses in components or termination procedures are often exposed during mechanical testing due to these stresses.
Durability testing is also designed to simulate the repeated use of a connector. This test involves continually inserting the connector into the adapter or jack. The test can potentially reveal any design and material flaws in the connector, such as portions of the latching mechanism that become heavily strained or flawed by frequent use.
Reliability Testing
The criteria for these tests are exclusive to Telcordia GR-326-CORE. The testing includes exposure to various environments, including additional environmental and exposure tests.
The additional environmental tests include extended versions of Thermal Life, Humidity, and Thermal Cycling. These tests, which run for 2000 hours each (83 days), further study the life of the connector across various service environments. Testing is non-sequential, so there is no cumulative effect. In Service Life environmental testing, the sample includes both pigtail assemblies and jumper assemblies, as defined by the specification. These extended tests are limited to jumper assemblies. The rationale for using jumper assemblies is to test for Temperature-Induced Cable Loss (TICL). TICL is caused by cable shrinkage from prolonged exposure to elevated temperatures followed by exposure to lower temperatures.
Many of the extruded compounds used in jacketing and buffering will shrink after exposure to elevated temperatures, causing micro-bending in the glass fibers and inducing excessive loss. The tests are monitored using a 1550nm source only, as micro-bending is more easily detected using longer wavelengths.
Exposure tests include Dust, Salt Fog, Airborne Contaminants, Ground Water Immersion, and Immersion/Corrosion. Dust can seriously impair optical performance. Particles that contaminate the end face can block optical signals and induce loss. Whether or not the dust particles find an exposed path to a ferrule end face is largely a matter of probability. Over time, dust particles will find their way to the optical connection if it is possible. While dust particles are not difficult to remove, the cleaning process involves disconnecting the connector, which not only stops transmission but also exposes the end face to additional contamination risk. This test involves intense exposure to dust of specified size particles to determine if there is a risk of any particle finding its way to the ferrule end faces.
Salt Fog (referred to as Salt Spray) guarantees patch cord performance in free-breathing enclosures near the ocean. This test exposes the connector to a high concentration of NaCl over an extended period. After the test, optical testing is performed, followed by a visual inspection to confirm no evidence of corrosion on the materials.
The Airborne Contaminants test guarantees the performance and material stability of connectors in outdoor applications with high pollution concentrations. The test repeatedly exposes mated and unmated connectors to various gases and inspects the connector optically and visually, as in the Salt Fog test. An assortment of volatile gases is used in a small chamber for 20 days to simulate prolonged exposure to these elements.
The materials are also verified in the Immersion/Corrosion test. This test has no optical requirements but involves prolonged submersion in uncontaminated water. This test, like Dust, Salt Fog, and Airborne Contaminants, involves both mated and unmated connectors. Mated connectors are checked for ferrule deformation by measuring the Radius of Curvature before and after the test and comparing the values. If the ferrule is not geometrically stable during this test, it could indicate a flaw in the zirconia material used in the ferrule. Unmated connectors are checked for Fiber Dissolution, which involves checking if the fiber core has recessed too far into the fiber cladding.
The final exposure test is Groundwater Immersion. This test verifies the product’s ability to withstand underground applications. The Immersion/Corrosion test strictly verifies the materials involved and uses de-ionized or distilled water. Connectors deployed in underground environments are much more likely to be exposed to contaminated mediums if their enclosures fail. During this test, the connector is exposed to a variety of chemicals found in sewage treatment and agricultural fertilization, among other applications, as well as biological mediums. These chemicals include ammonia, detergent, chlorine, and fuel. The presence of these chemicals can detrimentally affect the materials comprising the connector and adapter, reducing optical performance.
Biological contaminants include exposure to various organisms, including Streptococcus salivarius and Escherichia coli. The presence of these bacteria more accurately duplicates exposure to outdoor environments. These bacteria would also be attracted to and sustained by any growths on the connector over time, creating a potential health risk.
Reliability
Patch cord reliability is guaranteed not only by using quality components, manufacturing processes, and equipment, but also by adhering to a successful Quality Assurance program. While patch cords are typically tested 100% for insertion loss and return loss (if applicable), there are many other factors that need to be monitored to ensure the quality of the patch cord.
One of the most important factors is epoxy. Epoxies generally have a limited shelf life and working life, or “pot life.” This information is readily available from the manufacturer. It is absolutely necessary to verify and maintain these criteria during production. Expired epoxy must be discarded. Chemical changes that affect the cured properties of the epoxy can occur after its expiration date. This date can also depend on storage conditions, which must be observed.
Most epoxies used in fiber optic terminations are two-part epoxies, and while they cure at elevated temperatures, preliminary cross-linking begins upon mixing. Once this starts, the viscosity of the epoxy can begin to change, making application more difficult over time. The epoxy can become too thick to properly fill the ferrule and too viscous to allow the fiber to penetrate, leading to fiber breakage.
Mixing two-part epoxies introduces trapped air or “bubbles,” which are injected into the connector. This trapped air creates inconsistencies in the cured epoxy, leading to a high risk of mechanical failure. The amount of trapped air or bubbles must be minimized.
Many of the tools used in patch cord assembly also require periodic maintenance and have a limited tool life. This includes all stripping, cleaving, and crimping tools. Most stripping tools, whether hand tools or automated machines, can be damaged by the cable components, particularly the aramid yarn strength members. Buffer strippers will dull with prolonged use, increasing the likelihood of not cutting the buffer cleanly. This can lead to overstressing the fiber when the buffer is pulled off.
When a cleaving tool wears out and a clean score is not made, it is almost impossible to detect during manufacturing. However, the result could be uneven fiber breakage during the cleave, which may result in either breaking or cracking the fiber below the ferrule endface. In such cases, the connector will need to be scrapped. Even crimp tools require periodic maintenance to ensure consistent forces and dimensions. Crimp dies also tend to accumulate epoxy buildup, which can affect crimp dimensions and potentially damage the connector.
The integrity of incoming materials and manufacturing processes, once specified, must adhere to all appropriate guidelines and procedures. The importance of these materials has a strong influence on product reliability as well as product performance.
Performance
Understanding the performance of a patch cord is best achieved not only by understanding the importance of the parameters involved but also by recognizing the limitations of the final product. To draw conclusions about the specifications, technologies, and procedures for a patch cord, it is useful to model a “perfect patch cord” as defined below.
“Perfect Patch Cord”
A “perfect patch cord” is defined as one with near-zero insertion loss, which is the relative power loss caused by a mated pair of connectors. The performance of a “perfect patch cord” should be comparable to fiber splicing loss, which is around 0.02 dB. A “perfect patch cord” is defined to meet insertion loss <0.05 dB and return loss >58 dB.
To make a “perfect patch cord,” the best available fiber, ferrule, and test equipment are required. The model of the “perfect patch cord” will be made using single-mode components due to the tighter material requirements and specifications involved.
There are many parameters that can affect patch cord performance. To create a “perfect patch cord,” the endface geometry of the ferrule must be controlled, and a proper polishing process must be followed. The endface must also be kept clean, making the cleanliness of the production line and cleaning technique very important.
Loss is reduced by properly aligning the fiber cores within the ferrules of two mated patch cords. The primary factors that influence core alignment are ferrule inner diameter, ferrule concentricity, and ferrule outside diameter (OD). Identifying and controlling all relevant parameters are key to making a “perfect patch cord.”
The final fiber-core-to-ferrule-OD concentricity, or connector concentricity, is the vector sum of all the misalignments. This is one of the most important parameters in defining a “perfect patch cord.” A “perfect patch cord” must have sub-micron connector concentricity. Figure 1 relates insertion loss to connector concentricity.
The curve in Figure 1 shows the calculated insertion loss expected solely by connector concentricity. A simulation using the component parameters to estimate insertion loss matched well with the available data from measurements. The reference cable, especially the far end of the cable (Ref end), is always critical to measuring insertion loss and return loss of a connector on a PUT (patch cord under test).
Techniques applied to the “perfect patch cord” can be used in making reference cables. A reference cable used to test optical performance must be superior to the connectors on a patch cord being tested. The length of the reference cables and “perfect patch cords” should be long enough to avoid any effect of light intensity distribution across the fiber core on insertion loss. A length of at least three meters is recommended.
Polishing
Polishing is a very important step in making the “perfect patch cord.” The polishing process determines the condition and geometry of the connector endface. Polishing can be done one connector at a time by hand, or multiple connectors at once using a machine polisher.
Polishing involves grinding the endface with microscopic grits to remove excess epoxy and scratches from the fiber endface, as well as shaping the ferrule and glass. Both hand and machine polishing typically use water, which acts similarly to a cutting fluid.
Hand polishing is slow and results can be operator-dependent, while machine polishing offers consistency and speed. There are various fiber optic polishers available, and one cannot expect the same polishing quality across different manufacturers.
Most ceramic ferrules are pre-domed, meaning the endface is shaped to have an optimal radius of curvature (ROC) and the smallest possible apex offset (AO). In this case, the polishing grit must be hard enough to remove epoxy from the ferrule and scratches from the fiber endface, but not so hard that it significantly alters the ferrule geometry. If a ferrule is not pre-domed, proper geometry must be formed through extended polishing.
Polishing is done in multiple steps. The process is specified with polishing films, times, and pressures. The procedure typically consists of the following steps:
- Remove epoxy from the ferrule front surface.
- Form or maintain the dome shape.
- Polish the fiber surface to a shine.
Depending on the polisher and connectors, optimization of polishing pressure, time, and speed is necessary. The polishing film and rubber pad durometer used in each step must be properly selected.
After polishing, a microscope with at least 400x magnification is used to visually inspect for scratches and damage. A “perfect patch cord” should have no visible scratches on the fiber endface.
The number of scratches, particularly in certain regions of the fiber endface, is influenced by the polishing parameters and cleanliness. Scratches through the fiber core can not only affect optical performance but also damage other fiber endfaces they come into contact with. Therefore, scratches must be minimized in both count and size.
To ensure optimal performance, it is important to adhere not only to the polishing procedure but also to the cleaning procedures.
If the connector is single-mode, an interferometer is used to check endface geometry, including ROC, AO, and spherical height (SpH), as shown in Figure 2. Table 1 shows the Telcordia values for endface geometry after polishing. Although weak, some correlations have been noticed between apex offset and insertion loss variation, as well as between insertion loss and spherical height.
To properly test optical performance, the best available reference cable and a high-quality adapter should be used with a light source and meter. Once the fiber surface is clean, scratch-free, and confirmed to have endface geometry within specifications, insertion loss and return loss should fall within expected specifications.