Composite ferrules pass optical-fiber connector performance tests

Stephen Cook

Siecor

Ceramic ferrules face new competition as components made from strengthened polymers emerge as a cost-effective connector alternative.

Traditional optical-fiber connectors incorporate a ferrule made from a ceramic material manufactured to meet the very close tolerances required for these components. The fiber-optics industry relies on ceramic ferrules as a proven technology for connector performance. However, a recent innovation molds the connector ferrule out of strengthened polymers. One example is a glass-filled polymer that is stronger than a standard polymer and manufactured to meet the fiber-optic connector tolerances. This strengthened polymer is reliable and capable of maintaining the optical performance needed in private-network fiber-optic systems.

To verify the reliability of this material, Siecor performed extensive testing on composite ferrule connectors. The optical and mechanical parameters of ferrules are often evaluated as a sub-component of finished fiber-optic connectors. As sub-components, ferrules undergo the standard tests for field-installable fiber-optic connectors. Insertion loss, temperature cycling, durability, impact, and vibration are considered key tests for evaluating connector performance and reliability. Both the composite and the ceramic ferrule connectors met the test requirements.

Optical and mechanical performance

Reliable optical performance is essential for private-network systems. The best measure of a connector`s optical performance is insertion loss and temperature cycle testing.

Insertion loss is the measure of the total optical power decrease in a system, caused by the insertion of a component such as a mated connector pair. The loss is measured by using a light-emitting diode (LED) source and concatenating (linked together) jumpers. The standard requirement for average insertion loss is 0.5 dB. For this series of test samples, the average insertion loss is 0.24 dB for ceramic ferrules and 0.23 dB for composite ferrules (see Fig. 1).

Temperature cycling evaluates the effects of expansion and contraction on machined and molded parts that have very close tolerances. The connectors are subjected to test conditions cycling from -40C (-40F) to +75C (+167F) for 14 days, in accordance with ANSI/EIA/ TIA-455-3A FOTP-3A, "Procedure to Measure Temperature Cycling Effects on Optical Fibers, Optical Cable, and Other Passive Fiber-optic Components." The requirement for temperature cycling is a change of insertion loss 0.3 dB. For this test set, the average change in insertion loss was 0.05 dB for ceramic ferrules and 0.02 dB for composite ferrules (see Fig. 2). Insertion-loss measurements were taken every 15 min.

The ability to achieve multimode performance levels is associated with polished ceramic ferrule connectors. However, these same performance levels are readily attainable when using the same polishing techniques with composite ferrule connectors.

In private networks, the ability to reconfigure the network without degrading the signal is important. Any reconfiguration can exert mechanical stresses on the connector assemblies. The durability, impact, and vibration tests simulate multiple reconfigurations and evaluate the mechanical sensitivity of the connectors.

The durability test is a measure of the wear normally experienced as the connectors are repeatedly mated and unmated. In accordance with ANSI/EIA/TIA-455-21A FOTP-21, "Mating Durability for Fiber-optic Interconnecting Devices," the connectors are disconnected, then reconnected 500 times, with a cleaning after every 25th mating. Insertion-loss measurements are taken before and after each cleaning cycle. The requirement for connector durability is a change of insertion loss 0.2 dB. For this test series, the average change in insertion loss for both ceramic and composite ferrules was 0.01 dB (see Fig. 3).

The impact test measures the mechanical strength of the connector and simulates dropping the component during installation. In accordance with ANSI/EIA/TIA-455-2B FOTP-2, "Impact Test Measurements for Fiber-optic Devices," the connectors are dropped eight times from a height of 6 ft onto a concrete block. Insertion-loss measurements are taken before and after the test. The change of insertion loss required in the impact test is 0.2 dB. The average change in insertion loss was 0.02 dB for ceramic ferrules and 0.01 dB for composite ferrules (see Fig. 3).

The vibration test is designed to evaluate the tight tolerances experienced in mating two surfaces; any relative motion between components can generate debris in the connector`s parts that will affect optical transmission and can cause wear in the assembly. The connectors are subjected to test conditions in accordance with ANSI/EIA/TIA-455-11B FOTP-11, "Vibration Test Procedures for Fiber-optic Components and Cables." The vibration profile consists of an amplitude of 1.5 mm (peak-to-peak) with the frequency sweeping continuously between 10 and 55 Hz at a rate of 45 Hz per min for two hours in each of the assemblies` principal axis. Insertion-loss measurements are taken before and after the test. The vibration test requirement is a change of insertion loss 0.1 dB. For this test series, the average change in insertion loss for both ceramic and composite ferrules was 0 dB (see Fig. 3).

What test results indicate

The composite ferrule assemblies meet or exceed the optical requirements with no signs of breakage or degradation. The results show that the durability of the composite ferrules under the same conditions compares favorably with ceramic ferrules. Additional research produced varying results, but the composite ferrules always satisfied the optical-fiber connector test requirements.

The data-communications community is comfortable with the performance of ceramic ferrules. Today, however, vendors can produce composite ferrules that are more cost-effective and clearly meet the same performance criteria needed in the private-network multimode market.

The results of this study are not only true for the single-fiber/single-ferrule fiber-optic connectors. With the emergence of new dual-fiber/single-ferrule connectors, the same optical performance and reliability is expected.

The industry is starting to see the arrival of dual-fiber connectors with ferrules made from strengthened polymers. These composite ferrules provide excellent packing density, which allows the user to gain the maximum amount of fiber capacity in the smallest amount of rack space.

These small-form-factor connectors are starting to become the connector of choice in the fiber-optic industry. The strengthened polymers are a more cost-effective solution than the ceramic ferrule because the composite material is able to house two fibers. Based on the test and performance data of composite ferrules, it makes sense to use a composite material for these new connectors. u

Stephen Cook is an applications engineer at Siecor (Hickory, NC).