Hygroscopic Qualities of Kevlar: Though Kevlar has been used in telecommunication optical cables for years it was avoided by NASA due to its tendency to retain moisture and inability to meet the standard outgassing requirement. Lockheed Martin studied these moisture and contamination questions and reported that untreated Kevlar yields a total mass loss (TML) of 0.6% and a collected volatile condensable material value (CVCM) of less than 0.01. When treated with lubricants or polymers, as is very often the case, Kevlar outgasses beyond typical acceptance limits. Lockheed’s study also showed that Kevlar strength members which are exposed to a humid environment will quickly (after 2 hours or so) absorb moisture beyond the TML limits. They found that it will only wick six inches into a cable with exposed ends. Even if the Kevlar has been exposed to a humid environment during manufacture, if an extrusion process is used to apply the jacket, the absorbed moisture is completely driven away in process and no moisture remains trapped inside of the cable. Though no data was provided, a conditioning bake would likely drive off wicked moisture in the cable ends. The report suggests capping the ends of un-terminated cable and highlights the necessity to specify untreated Kevlar for use in space grade optical cable [2].

Though Kevlar’s ability to wick uncured epoxy out of a connector has not been documented, if it occurs, the results could cause failure of a termination. This behavior will be studied and documented as part of a future evaluation effort.

Teflon Impregnated Fiberglass Strength Members: A concern has been raised regarding the bondability of Teflon impregnated fiberglass strength members using epoxy when terminating to connectors. The concern is that Teflon is hard to bond to and that crimping on fiberglass causes the strength members to break. This issue is still in review.

Strength Member Weave: One of the leading advantages of the FC style single fiber connector is its "pull-proof" design. To make the connector pull proof the spring, which maintains the ferrule’s PC contact, is mechanically isolated from the connector shell. When one pulls on the back, no force is extended to the ferrule and the optical connection is maintained. This is an improvement over the ST design which, when one pulls on the back of the connector, allows separation of the mated ferrules. The lay of the strength members has been found to affect the independent movement of the cable around the stationary fiber (or fiber and tight buffer, depending on the cable construction and termination configuration). Strength members which lay in the cable in a woven configuration, similar to that used for electrical cable shielding, tend to be too tight to allow the necessary movement of the cable components in a "pull-proof" connector. A spiral or lateral lay has been found to allow sufficient movement, at least 1 mm. A jacket that is too tight can have the same effect. An ability to move the fiber (or fiber/tight buffer) at least 1 mm back into the cable must be part of the cable specification to enable use of the cable with a pull-proof connector.

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